extensive middle amazonian mantling of debris …extensive middle amazonian mantling of debris...

Icarus 260 (2015) 269–288

Contents lists available at ScienceDirect

Icarus

journal homepage: www.elsevier .com/locate / icarus

Extensive Middle Amazonian mantling of debris aprons and plainsin Deuteronilus Mensae, Mars: Implications for the recordof mid-latitude glaciation

http://dx.doi.org/10.1016/j.icarus.2015.06.0360019-1035/� 2015 Elsevier Inc. All rights reserved.

⇑ Corresponding author at: Department of Earth, Environmental and PlanetarySciences, Brown University, Box 1846, Providence, RI 02912, USA. Fax: +1 401 8633978.

E-mail address: [email protected] (D.M.H. Baker).

David M.H. Baker ⇑, James W. HeadDepartment of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA

a r t i c l e i n f o

Article history:Received 15 May 2015Revised 22 June 2015Accepted 30 June 2015Available online 8 July 2015

Keywords:MarsMars, surfaceMars, climateGeological processesIces

a b s t r a c t

The mid-latitudes of Mars are host to a record of recent episodes of accumulations of ice-rich materials.The record includes debris aprons, interpreted to be debris-covered glaciers, that may represent the pre-served remnants of a much more extensive ice sheet. We assessed the possibility of former glacial extentsby examining debris aprons and the surrounding plains in Deuteronilus Mensae. Geomorphic units andstratigraphic relationships were mapped and documented from Mars Reconnaissance Orbiter (MRO)Context (CTX) and High Resolution Imaging Science Experiment (HiRISE) camera images, and craterretention ages were estimated from crater size–frequency distributions. Three major units are observedwithin the study area: debris aprons, lower plains, and upper plains. Debris aprons exhibit characteristicstypical for these features documented elsewhere and in previous studies, including integrated flow lin-eations and patterns, convex-upward profiles, and knobby and brain terrain surface textures. A lowerbound on the age for debris aprons is estimated to be 0.9 Ga. Debris aprons are superposed on a lowerplains unit having a lower bound age of 3.3–3.5 Ga. A 50–100 m thick upper plains unit superposes bothdebris apron landforms and lower plains units and has a best-fit minimum age of 0.6 Ga. The upper plainsunit exhibits characteristics of atmospherically-emplaced mantle material, including fine-grained nature,sublimation textures, cyclic layering, draping character, and widespread spatial distribution. Fracturingand subsequent sublimation/erosion of upper plains on debris aprons has contributed to many of the sur-face textures on debris aprons. The upper plains unit has also been eroded from the lower plains and pla-teaus, evidenced by isolated blocks of upper plains in the interiors of craters and on the walls and tops ofplateaus. While no conclusive evidence diagnostic of former cold-based ice sheets are observed in theplains within the study region, such landforms and units may have been poorly developed or absent,as is often the case on Earth, and would have been covered and reworked by later mantling episodes.These observations suggest that emplacement of thick ice-rich mantle deposits extended at least to nearthe Early/Middle Amazonian boundary and overlapped with the waning stages of glaciation inDeuteronilus Mensae.

� 2015 Elsevier Inc. All rights reserved.

1. Introduction

The Amazonian period of Mars has been marked by a number ofmajor cycles of ice and dust deposition and accumulation (Neukumet al., 2004; Carr and Head, 2010). Among the most prominent fea-tures resulting from these depositional events are found in themid-latitudes (�30–50�), where ice can accumulate due to cyclesin orbital forcing, including excursions to obliquities that are

higher than the present �25� (Head et al., 2003, 2005; Laskaret al., 2004). Here, we focus on a class of features named lobatedebris aprons (called ‘‘debris aprons,’’ herein) which have beenattributed to the flow of thick accumulations ice-rich materials(Squyres, 1978, 1979; Lucchitta, 1984). Debris aprons are tens ofkilometers in lateral extent and hundreds of meters in thickness,exhibiting surface ridge and furrow patterns indicative ofice-rich, downslope flow. While there is likely to be variations inice and debris content, much evidence supports the formation ofthese features in a manner similar to glacial systems on Earth(Lucchitta, 1984; Head et al., 2005, 2006a, 2010). In particular, deb-ris aprons and similar classes of features are likely to be preservedcold-based debris-covered glaciers, consisting mostly of glacial ice

http://crossmark.crossref.org/dialog/?doi=10.1016/j.icarus.2015.06.036&domain=pdf

http://dx.doi.org/10.1016/j.icarus.2015.06.036

mailto:[email protected]


http://www.sciencedirect.com/science/journal/00191035

http://www.elsevier.com/locate/icarus

270 D.M.H. Baker, J.W. Head / Icarus 260 (2015) 269–288

covered by a protective supraglacial debris layer. Geomorphic evi-dence to support this interpretation includes: (1) initiation of flowlineations within alcoves in plateaus and massifs, (2) downslopeintegration and coalescence of flow lineations and patterns overtens of kilometers, and (3) convex-upward topographic profiles(Li et al., 2005; Head et al., 2005, 2006a, 2006b, 2010).Geophysical observations from the SHAllow RADar (SHARAD)instrument also support the interpretation that many debrisaprons consist of nearly pure ice bodies with little internal layering(Holt et al., 2008; Plaut et al., 2009).

Recent work suggests that debris aprons may represent the pre-served remnants of the retreat and down-wasting of a much moreregional ice sheet (e.g., Fastook et al., 2014). Evidence supporting amore extensive ice sheet, include: (1) possible glacial highstands,suggesting more regionally thick (�1 km) ice deposits (Dicksonet al., 2008, 2010), (2) integrated flow patterns that are consistentwith regional glacial landsystems (Head et al., 2010), (3) global cli-mate models that show broad regional snow and ice accumulationrather than more localized accumulations (Madeleine et al., 2009),and (4) integrated glacial flow models that are consistent withthick regional accumulations of ice and retreat to produce morelocalized ice-rich landforms (Fastook et al., 2011). Under this ‘‘re-gional ice-sheet collapse’’ model, thick accumulations of snowand ice in the mid-latitudes (Madeleine et al., 2009) promotedthe development of regional ice sheets that once extended ontothe plains and plateaus surrounding debris aprons (Fastook et al.,2011). As accumulation waned and ablation dominated, the icesheet retreated to more localized, alpine-style glaciation, the rem-nants of which are currently preserved as debris aprons. The glacialice forming debris aprons would have been protected from subli-mation by thick supraglacial debris built up as a sublimation lagof rock shed from plateau walls and possibly airfall material (e.g.,Fastook et al., 2014). Other scenarios proposed for the formationof debris aprons do not invoke formation through retreat of aregional ice sheet. Such scenarios suggest more local depositionof atmospheric ice and glacial flow, or alternatively, ice-assistedmass wasting of ancient terrain (e.g., Pierce and Crown, 2003;van Gasselt et al., 2010). Under these scenarios, glacial modifica-tion was more restricted to the margins of plateaus and massifsand is not predicted to have extended into the broader plains.

If the regional ice-sheet collapse model is correct, it is possiblethat retreat of the ice sheet may have left a unique geomorphic sig-nature on the plains surrounding the present extents of debrisaprons, which may be assessed and tested through careful geomor-phic analysis of the observed landforms. Although localized heat-ing events may have initiated short-lived melting episodes in theform of small fluvial channels (Dickson et al., 2009; Fassett et al.,2010), Amazonian glaciation on Mars is predicted to be largelycold-based. Model results (Fastook et al., 2014) show that debrisaprons could not have historically attained temperatures near orabove the ice melting point and retained their current shape. Asa result, any geomorphic evidence of former glacial maxima ismost likely to resemble cold-based glacial landsystems and notwet-based landsystems such as those associated with many tem-perate glaciers on Earth (Evans, 2003). Glacial retreat and down-wasting will therefore be largely driven by sublimation of ice. If asupply of debris from rockfall or atmospheric sources is available,cold-based glaciation can produce a number of depositional geo-morphic features as seen on Earth, including drop moraines, lateralmoraines, sublimation tills, and, if localized melting exists, ero-sional lateral melt-water channels (Evans, 2003; Swanger et al.,2010; Atkins, 2013). Other unique landforms can be produced fromthe retreat and deposition of glacial sediments over down-wasting,buried glacial ice. Such landforms include controlled and hum-mocky moraines, with the details of moraine topography being lar-gely related to the spatial pattern, distribution, and concentration

of englacial and supraglacial debris concentrations over stagnantglacial ice at the glacier’s terminus (Benn and Evans, 2010). If nodebris source is available, it is possible for ice to override andretreat from the landscape, leaving little geomorphic evidence thatit was once present (see discussion in Atkins, 2013). Thus, recog-nizing evidence of cold-based glaciation is challenging, even onEarth where detailed geological investigations can be conducted.Indeed, the lack of geomorphic footprint on the landscape has longeluded geologists from recognizing the full extent and character ofcold-based glacial events (Atkins, 2013).

To test the regional ice-sheet collapse model through assess-ment of possible landforms that may be indicative of cold-basedglaciation, we investigated the morphology and topography ofplains units distal to debris aprons in Deuteronilus Mensae(39.6�N to 50.2�N and 13.6�E to 35.4�E) using image andstereo-derived topography data from the Mars ReconnaissanceOrbiter (MRO) Context (CTX) (�6 m/pixel resolution) and HighResolution Imaging Science Experiment (HiRISE) (�0.25 m/pixel)cameras (Malin et al., 2007; McEwen et al., 2007) and MarsExpress High Resolution Stereo Camera (HRSC) (Jaumann et al.,2007; Gwinner et al., 2010). The results of the investigation indi-cate that landforms diagnostic of terrestrial cold-based glaciallandsystems are not easily recognized in the plains within thestudy region; it is possible that they may not have been originallywell-developed, as is common on Earth (Atkins, 2013). However,we observe a unique mantling unit (upper plains) that was depos-ited near the cessation of debris apron flow. This unique plains unitwas once more extensive in the past and has significantly modifiedthe surfaces of debris aprons and plains in broad areas across themid-latitudes. These results add to our understanding of the his-tory of ice deposition and re-distribution on Mars, having impor-tant implications for understanding the timing of glaciation onMars and the present and past climate history of the planet.

2. Geological setting – Deuteronilus Mensae

This work focuses on a portion of the Deuteronilus Mensaeregion of Mars along the dichotomy boundary, extending from41� to 46.5�N latitude and 20� to 36�E longitude (Fig. 1). The studyarea includes a well-documented region of the so-called ‘‘frettedterrain’’ (Sharp, 1973) consisting of dissected Noachian highlandterrain and forming isolated plateaus and massifs that decreasein topographic prominence toward the north (Fig. 1). To the south,the �1–2 km high dichotomy boundary escarpment separates theheavily cratered highlands from the smoother, less cratered low-lands. To the north is Lyot crater, which is a �215 km impact basinwith a Late Hesperian to Early Amazonian age (Werner, 2008;Dickson et al., 2009; Robbins et al., 2013). Ejecta, sculpture, andsecondary craters from the Lyot impact event are found withinthe study area (Robbins and Hynek, 2011) but have subsequentlybeen reworked by younger processes.

Much mapping and geomorphic analysis has been completed inthe Deuteronilus Mensae region of Mars. Tanaka et al. (2005) intheir geologic map of the northern plains of Mars mapped theregion at 1:15,000,000 scale. They identified several units in theregion, including Noachian cratered highland plateaus and largemesas, Hesperian–Noachian plains materials interpreted to be deb-ris mass wasted from ice-rich, friable Noachian material, andHesperian to Amazonian deposits resulting from ice-lubricatedmass-wasting deformation processes. Lyot crater ejecta were alsomapped in the northern and northeastern portions of the studyarea.

More detailed geological maps of portions of the study areahave been produced by McGill (2002) [United States GeologicalSurvey (USGS) quadrangles 30332, 35332, 40332, and 45332 from

Fig. 1. Deuteronilus Mensae region showing the extent of the study area (thick black outline). The basemap is MOLA colored topography over MOLA hillshade in a sinusoidalprojection centered at 28.0�E longitude. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

D.M.H. Baker, J.W. Head / Icarus 260 (2015) 269–288 271

�37.5� to 47.5�N and �25� to 30�E] and Chuang and Crown (2009)[USGS Mars Transverse Mercator (MTM) quadrangles 35337,40337, and 45337 from �32.5� to 47.5�N and �19� to 25�E].McGill (2002) mapped at 1:500,000 scale based on Viking imagesat 40 and 100 m/pixel resolution, Mars Orbiter Laser Altimeter(MOLA) topography, and a limited number of Mars OrbiterCamera (MOC) Narrow Angle (NA) images. A number of plainsunits surrounding young debris aprons were mapped. AHesperian-aged ‘‘heavily grooved lowland unit’’ was observed tobe proximal to debris aprons, which exhibited pronouncedgrooves, especially where two aprons abutted. A separate ‘‘lobatelowland material’’ was mapped, exhibiting smooth, flat tops androunded, convex-upward edges and contacts with underlyingmaterials. MOLA profiles showed that the relief of the lobate low-land material was approximately 100 m, and crater counts yieldedan early Late Hesperian N(1) age. The origin of these lobate andgrooved materials was uncertain but was attributed to debris flowmaterial from an unknown source. McGill (2002) also described a‘‘smooth lowland plains material’’ that was the oldest plains mate-rial and covered most of the mapped lowland area. The origin ofthis smooth lowland plains material was described as ‘‘enigmatic,’’but was interpreted to be a mix of volcanic, eolian, and debris-flowdeposits.

Chuang and Crown (2009) mapped additional USGS MTM quad-rangles based on multiple datasets, including a full-resolutionViking Orbiter MTM base map at �50 m/pixel and data fromThermal Emission Imaging System (THEMIS) Infrared (IR) andVisible (VIS) cameras, MOLA digital elevation models (DEMs),HRSC and MOC NA images. Most of the plains units in the quadran-gles were mapped as Early to Late Hesperian ‘‘smooth plains’’material, interpreted to be eroded plateau material mixed and cov-ered with eolian deposits. A ‘‘ribbed texture’’ was observed in thesmooth plains and on debris apron material, which was suggestedto form from partial retreat of the debris apron margin or aprondeposition over an uneven pre-existing surface. Chuang andCrown (2009) also observed mantle deposits on debris aprons,polygonal mesas, plains, and plateaus surfaces, which were sug-gested to have been emplaced as airfall material and then partiallyremoved at a time contemporaneous or subsequent to the growing

debris aprons. These observations were described as being consis-tent with previous work examining the variety of surface texturesof debris aprons and mid-latitude mantling (Carr, 2001; Mustardet al., 2001; Head et al., 2003; Mangold, 2003; Pierce and Crown,2003; Chuang and Crown, 2005). The youngest of these mantlingdeposits (emplaced within the past hundreds of ka to few Ma), ter-med ‘‘latitude-dependent mantle,’’ is described as being a layer ormultiple layers of ice-cemented dust or other fines that is subse-quently dissected through sublimation of ice and liberation offines. The fines are then later reworked by eolian processes.

3. Methods

3.1. Geomorphic mapping

To elucidate the origin and formation of plains units inDeuteronilus Mensae and to assess possible evidence of former gla-cial extents, we mapped geomorphic units within the study area(Fig. 2). Mapping was conducted at the 1:50,000 scale withinESRI ArcMap 9.3 in a sinusoidal projection centered at 28.0�E andfrom a basemap mosaic of CTX images (Malin et al., 2007) coveringthe study area at �6 m/pixel resolution. Image datasets, includingCTX images, were processed and projected using USGS IntegratedSoftware for Imagers and Spectrometers (ISIS) software, version3.2.0. Although the study area has nearly complete CTX coverage,HRSC images (Jaumann et al., 2007) were used where CTX imageswere not present. HiRISE images at �25 cm/pixel resolution(McEwen et al., 2007) were used for detailed geomorphic analysis,including elucidating contacts between geomorphic units. Theresulting map and observations were used for determining theextent, characteristics, and stratigraphic relationships betweenunits within the study area.

Topographic analysis, including profile generation, was con-ducted from HRSC (Gwinner et al., 2010), CTX, and HiRISE digitalelevation models (DEMs) depending on the desired scale of mea-surements. The NASA Ames Stereo Pipeline (ASP) tools (Morattoet al., 2010) were used to process CTX and HiRISE stereo pairs intodigital elevation models that were projected and referenced to the

Fig. 2. Geomorphic map of the study area. Major units are described in the text. Boxes show the locations of subsequent figures. The image basemap is a Viking orbiter mosaicin a sinusoidal projection centered at 28.0�E longitude.


MOLA areoid. Although still appropriate for the purposes here,DEMs processed using the ASP are not as accurate as those gener-ated using BAE Systems SOCET SET software, currently in use bythe USGS Astrogeology Science Center.

3.2. Crater retention ages

Crater size–frequency distributions were measured on repre-sentative portions of the debris apron and plains units. Count areaswere determined from clipped portions of the geomorphic map.Diameters were measured from the rim crest of each crater usingthe CraterTools extension in ArcMap (Kneissl et al., 2011) andthe morphology of each crater was classified (see Section 5 for adiscussion of observed crater morphology). Care was taken toexclude obvious clusters or chains of secondary craters; areas con-fining these secondary craters were also excluded from the countarea. The crater diameters and count areas were then used to gen-erate size–frequency distributions, which were plotted and com-pared to the Hartmann (2005) isochron system to determine abest-fit age. Best-fit ages were calculated by iteratively assigningan isochron to the data and minimizing the least squares misfit.Bins were weighted when determining the best-fit by the inverseof the square of the number of craters in each bin. In addition,those bins at the largest crater sizes that were visually identifiedto follow a single isochron were used to determine the best fit, sim-ilar to the fit segment method described by Berman et al. (in press).The largest diameter bins were used to avoid the effects of removalof smaller crater diameters due to erosion and resurfacing. At leasttwo bins, and more typically three or more, were required to definea best-fit isochron. For each bin, ‘‘error bars’’ represent a 90% con-fidence interval calculated from an inverse cumulative gamma dis-tribution, which is more appropriate for small numbers of craters(<10) (Kreslavsky, 2007; Fassett and Head, 2008; Kreslavskyet al., 2015). The more typical root-N estimate will approximatethe inverse cumulative gamma distribution for larger numbers ofcraters. Uncertainties in ages are calculated as twice the standarderror in the isochron fit to the data.

4. Mapping results

A geomorphic map of units within the study area is presented inFig. 2. Three major units are mapped in the study area based ontopographic and textural characteristics, and are described below:

(1) debris apron material (da), (2) upper plains (pu), and (3) lowerplains (pl).

4.1. Debris apron material (da)

The detailed characteristics of many debris apron landformshave been well documented across the mid-latitude of Mars(Pierce and Crown, 2003; Chuang and Crown, 2005; Li et al.,2005; Morgan et al., 2009; Baker et al., 2010; Head et al., 2010).Similar to previous studies, debris apron landforms in the studyarea exhibit relatively sharp, lobate fronts and convex-upward pro-files where they are in contact with lower plains material (Fig. 3).Debris apron thicknesses are on the order of hundreds of metersand they can extend for tens of kilometers from the steep wallsof plateaus and massifs, although some occurrences are much lessextensive (Figs. 1 and 2; Levy et al., 2014; Karlsson et al., 2015). Forsimplicity, we also included occurrences of ‘‘concentric crater fill’’within our mapped debris apron unit. Concentric crater fill occurswithin impact craters and has been shown to be genetically relatedto lobate debris aprons (e.g., Levy et al., 2010).

In addition to their topographic characteristics, textures uniqueto the surfaces of debris apron landforms were used as a basis formapping their spatial extent (Fig. 2). We note that the mapped ‘‘de-bris apron material’’ is a surface unit that corresponds to the spatialextents of the debris apron landforms, which extend in the verticaldimension and consist of layers of ice and debris hundreds ofmeters in thickness. As discussed later, many of the textures withinthe debris apron material unit are interpreted to be a product ofdissection of mantling material younger than the ice and supragla-cial debris forming the main mass of the debris aprons.

The surface textures of debris aprons within the study area arediverse and include characteristics similar to those described forother apron surfaces (Mangold, 2003; Pierce and Crown, 2003;Chuang and Crown, 2005, 2009; Li et al., 2005; Levy et al., 2007,2009; Baker et al., 2010). The most dominant texture in the studyarea is a knobby or stippled texture meters to tens of meters inscale (Fig. 4a) that commonly includes patterns of ridges and fur-rows delineating downslope flow directions and interactions(e.g., Baker et al., 2010) similar to glacial systems on Earth (Headet al., 2010). Debris apron surfaces in the study area also exhibitclassic ‘‘brain terrain’’ textures at small scales common to debrisaprons and related features in the mid-latitudes (e.g., Levy et al.,2009).

Fig. 3. Relationship between upper plains (pu), ribbed upper plains [pu(r)], lower plains (pl), debris aprons (da), and plateau material (pt). See Fig. 2 for image context. Unitboundaries are shown as yellow dash-dot lines. (a) CTX image mosaic showing smooth upper plains with lobate margins superposed on lower plains (CTX imagesP16_007373_2248_XN_44N333W, P18_008019_2227_XI_42N333W, and P15_007017_2251_XN_45N333W). Debris apron material with a stippled texture is also superposedon lower plains. Inset boxes show the locations of (b) and (c). (b) Portion of lower plains, showing craters with interior fill (black arrows) and pedestal-type crater ejecta(white arrow). (c) Contact between upper plains and lower plains. The ribbed upper plains unit has circular fracture patterns and collapse features controlled by buried craters(black arrows). (d) HRSC topographic profile (DTM 1461) traversing the region included in (a) (T–T0 line). The upper plains is estimated to be 50–100 m thick, determined byprojecting the lower plains level (dashed line) underneath the unit. The debris apron has a distinct convex-upward margin with the lower plains and is hundreds of metersthick. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Major types of debris apron textures observed within the study area. (a) Small-scale brain terrain with flow lineations (left) and knobby terrain (right). (b) Transverseridges, suggesting eolian reworking.


Other textures on the surfaces of debris aprons are more com-plex. Often the knobby textures grade into dune-like transverseridges (Fig. 4b), indicating potential reworking of surface materialsby eolian processes. A ‘‘ribbed debris apron’’ unit [da(r)] is mapped

on debris apron surfaces (Fig. 2) that exhibits meters-scale knobbyand brain-terrain texture similar to the rest of the mapped debrisapron material (e.g., Fig. 4) but with additional complex patternsof connected troughs, pits, and platforms that dominant the

Fig. 5. Gradual transition from upper plains (pu) (right) to ribbed upper plains[pu(r)], ribbed debris apron material [da(r)], and debris apron material (left). SeeFig. 2 for image context. Unit boundaries are shown as yellow dashed lines; many ofthe contacts between units are gradational. Isolated patches of upper plains(arrows) are observed within ribbed debris apron material, kilometers from themargins of upper plains. CTX image P15_007017_2251_XN_45N333W. (For inter-pretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)


texture at hundreds of meters scale (Fig. 5). This ribbed debrisapron unit grades with the ribbed upper plains unit [pu(r)] foundin contact with the terminal margins of debris apron landforms(Fig. 5; see descriptions in Sections 4.2 and 4.4, below). A distinc-tion between the two units is found at the meters to tens of metersscale, with the ribbed debris aprons more resembling the rougherand knobby debris apron surfaces at this scale and ribbed upperplains resembling the smoother surfaces of the upper plains unit.In addition, the trough and platform patterns within ribbed debrisaprons are not as sharp and pronounced and have been heavily dis-sected and incorporated into the surrounding textures of debrisapron materials (Fig. 5). As discussed in Section 4.4, the gradationalnature of these units represents a progression of modification anddissection of upper plains, which have mantled a large portion ofthe study area.

4.2. Upper plains (pu)

An upper plains unit covers much of the intermediate areasbetween plateaus (pt) and surrounding debris apron materialsnorth of �43�N in Deuteronilus Mensae (Fig. 2). Areas of upperplains with dense fracture and trough patterns are mapped sepa-rately as ‘‘ribbed upper plains’’ (see Section 4.4). The upper plainsunit is relatively smooth at Viking resolution but is pockmarkedin CTX images with numerous muted impact craters, and depres-sions formed by surficial collapse (Figs. 3a, c and 6). Irregular andpolygonal fractures <20 m in width and isolated elongate pits withraised rims are also observed in upper plains (Figs. 6 and 7). Somefractures widen and form concentric and radial patterns, appearingto be controlled by underlying structures, such as impact cratersand knobs of older terrain (Figs. 3c and 7b). A topographic profileover one of these areas of concentric fractures within ribbed upperplains, interpreted to be a buried impact crater (Fig. 8d), shows thatthe upper plains is depressed more than 70 m with respect to thesurrounding surface topography. HiRISE images reveal that frac-tures within upper plains and ribbed upper plains typically havesmall pits and pit chains tens of meters in width along their length(Fig. 7d), similar to pits observed in mantling deposits in UtopiaPlanitia that have been interpreted to result from sublimation ofice-containing materials (e.g., Morgenstern et al., 2007). The upperplains is finely layered and has an absence of boulders >1 m in size(Fig. 7c), indicating that the unit consists largely of relativelyfine-grained materials and may have been deposited over multipleepisodes.

The margins of upper plains are commonly lobate in profilewhen in contact with the surrounding lower plains (Fig. 3).Topographic measurements from profiles (Figs. 3d and 8) constrainthe thickness of upper plains to be a maximum of approximately50–100 m in the central portions of the study area. The upperplains unit thins and becomes more dissected to the east, wherethe unit is mapped as ‘‘hummocky upper plains’’ [pu(h)] and ‘‘dis-sected upper plains’’ [pu(d)] (Fig. 2). Hummocky upper plains con-sist of relatively smooth upper plains but with rounded hummocksfrom underlying topography (Fig. 9a). Isolated patches and dippinglayers of more continuous and thicker upper plains are commonlyobserved within hummocky upper plains (Fig. 9a). The dissectedupper plains unit exhibits patchy albedo with dense networks ofsmall-scale pitting and fracturing and with greater topographicvariability (Fig. 9b). The margins of the dissected upper plains unitgrades diffusely with lower plains. The upper plains unit is absentin the southern and western portion of the study areas (Fig. 2),with the exception of isolated patches within the interiors ofimpact craters and on the walls and tops of plateaus (seeSection 4.4).

The upper plains unit is equivalent to the ‘‘lobate lowland mate-rial’’ of McGill (2002) and portions of the ‘‘smooth plains’’ mappedby Chuang and Crown (2009) (Section 2). The unit also overlaps thelocation of portions of Lyot ejecta material mapped by Tanaka et al.(2005).

4.3. Lower plains (pl)

A lower plains unit is characterized by patchy textures andalbedos but generally has rougher small- and large-scale texturesthan upper plains with a greater number of knobs and erosionalremnants of older terrain (Fig. 3). Craters in the lower plains haverounded rims and appear less subdued compared with those inupper plains (Fig. 3b). Craters with raised rims often lack ejecta(Fig. 3a and b) and commonly have interior mounds or remnantblocks of layered fill (Fig. 3b). Other craters have ejecta that formplatforms raised above the surrounding terrain, similar to pedestalcraters in the mid-latitudes (Kadish et al., 2009; Schaefer et al.,2011). Both the pedestal-like crater ejecta and remnant layersinside craters are suggestive that a once more continuous layerof material has been stripped away from the upper surface of thelower plains.

The lower plains unit is equivalent to the ‘‘smooth lowlandplains material’’ of McGill (2002) and the majority of the ‘‘smoothplains’’ of Chuang and Crown (2009) (Section 2).

4.4. Stratigraphic relationships

The major mapped units (debris aprons, upper plains, and lowerplains) exhibit a number of stratigraphic relationships that help toplace them in a relative chronology of formation. These relation-ships, aided by crater size–frequency distributions (Section 5), helpto elucidate the origin and processes of formation of these unitswithin the region (Section 6).

4.4.1. Debris aprons and upper plainsWhere the upper plains unit is in contact with debris apron

materials, surface fractures and elongated pits within upper plainswiden and link to form large, patterned fractures and ridges andfurrows that are <500 m in width and extend for tens of kilometersin length (Figs. 6, 7a, and 10a). These patterns of ridges and furrowswithin upper plains are separately mapped as ‘‘ribbed upperplains’’ [pu(r)] in Fig. 2. Occurrences of ribbed upper plains are gen-erally restricted to the margins of the upper plains unit and toareas in contact with or in proximity to debris aprons (Fig. 2).The unit is morphologically equivalent to the ‘‘heavily grooved

Fig. 6. Relationship between debris aprons (da), ribbed debris aprons [da(r)], upper plains (pu), ribbed upper plains [pu(r)], lower plains (pl), and plateaus (pt). (a) CTX imagemosaic (see Fig. 2 for image context; CTX images D19_034721_2239_XI_43N334W, G20_025873_2240_XN_44N335W, P13_006160_2252_XN_45N334W,P17_007729_2250_XN_45N334W, P18_008019_2227_XI_42N333W, P18_008230_2250_XN_45N335W, and P20_008665_2239_XN_43N334W). The unit boundaries areoutlined with yellow dash-dot lines. The margins of upper plains, mapped as ribbed upper plains, show complex fracturing and trough formation, especially where multipledebris aprons merge. Inset boxes show the locations of additional figures described in detail. (b) HRSC DTM colored topography (DTM 1461) over the CTX image mosaic in (a).Also shown are outlines of a CTX DEM (image pair G03_019570_2241_XN_44N334W and G03_019504_2241_XN_44N334W) and HiRISE DEM (image pair ESP_019570_2240and ESP_019504_2240) used for topographic analysis. White solid lines show topographic transects used for profiles in Fig. 8. (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)


lowland unit’’ mapped by McGill (2002) and the occurrence of‘‘ribbed texture’’ in plains units mapped by Chuang and Crown(2009). Dense fracture patterns and troughs within ribbed upperplains can appear to be concentric to each other and to the lobatemargins of debris aprons (Fig. 7a). Parallel and concentric patternsof fractures and troughs appear most prominent and wellexpressed at the convergence of multiple debris aprons (Fig. 6).These observations suggest that debris apron landforms are con-trolling the patterns of fracturing in upper plains, perhaps by com-pressional stresses induced by downslope creep or flow of icetoward the termini of debris aprons.

Ridges within ribbed upper plains are highly fractured(Figs. 6 and 7a). These fractures widen toward the contact withdebris aprons to form troughs with raised rims (Figs. 6 and 7a).The troughs separate intermediate patches of upper plains thatare depressed with respect to the raised rims of the troughs(Figs. 7a, 8 and 10). Widening of the troughs and the presence ofdunes within the troughs (Figs. 7c and 11) suggest eolian erosionand reworking of upper plains material along fractures, possibly

aided by sublimation of contained ice (Mangold, 2003). This ero-sional progression produces isolated patches of upper plains thatcan appear to be raised above the adjacent debris apron surfaceand associated textures (Figs. 7c and 10).

Patches of upper plains are also found to seamlessly grade tex-turally and topographically with the dominant debris apron sur-face textures (Figs. 5, 10a and 11). Ribbed upper plainscommonly occur on convex-upward margins (Figs. 8a, b and 10b)that appear topographically continuous with the main debrisapron mass. In some cases, isolated patches of upper plains arelocated kilometers from the margins of the mapped upper plainsunit (Fig. 5). Most of these isolated patches appear dissected alongtheir perimeters, forming knobby and brain-terrain like texturestypical of debris apron surfaces (Levy et al., 2009). The ridge andfurrow patterns of ribbed upper plains are often observed to gradeinto ribbed debris apron textures (Figs. 2 and 5). Ribbed debrisapron surfaces are common and mapped throughout the studyregion (Fig. 2). A HiRISE DEM profile of the transition from ribbedupper plains to debris apron material (Fig. 10b) shows the varying

Fig. 7. Detailed CTX and HiRISE images of the characteristics of the upper plains unit (pu). (a) Ribbed upper plains [pu(r)] shows a set of concentric fracture patterns thatwiden and connect toward the contact with the debris apron (da) (CTX image D19_034721_2239_XI_43N334W). Inset boxes show the locations of (c) and (d). (b) Circular,concentric fracture and trough patterns form over buried impact craters within ribbed upper plains (CTX image P18_008019_2227_XI_42N333W). (c) Zoomed view of thecontact between ribbed upper plains and debris aprons (HiRISE image ESP_037556_2245). An isolated patch of upper plains is surrounded by the knobby texture of the debrisapron. A knob is forming out of the patch of upper plains at the southeastern end (vertical white arrow). Layering is observed in the wall of the upper plains (angled whitearrows). Dunes are also present at the base of the wall. (d) Zoomed view of fractures within ribbed upper plains. Fractures show pits and pit chains (black arrows), perhapsrelated to sublimation processes (HiRISE image ESP_035011_2240). The surface of the upper plains appears largely fine-grained and lacks boulders larger than what HiRISE isable to resolve (�1 m in size).


scales of roughness between these two units. The ribbed upperplains unit is rougher than the debris aprons over a length scaleof hundreds of meters, but the debris apron material is rougherthan the ribbed plains unit at a length scale of meters to tens ofmeters. Fig. 11 shows patches of upper plains hundreds of metersin length being dissected to form brain-terrain textures andknobby textures tens of meters in scale. The patches of upperplains exhibit pervasive polygonal fractures and scalloped marginsthat are at the same length-scales as the surrounding erosional tex-tures (Fig. 11). These observations indicate that polygonal fractur-ing within upper plains is likely to be a precursor to formation ofknobby terrain and brain terrain. This interpretation is consistentwith current hypotheses for brain-terrain formation (e.g., Levyet al., 2009), which include an initial process of thermal contractionpolygon formation that undergoes subsequent modification bysublimation and local stresses.

Large, quasi-circular collapse depressions (Figs. 6a, b and 8a) arealso observed within ribbed upper plains (Figs. 6b and 8a). Thesedepressions are kilometers in scale, are up to approximately100 m deep (Fig. 8a) and are typically outlined by fractures.These collapse features are morphologically different from collapseobserved over buried craters (Figs. 7b and 8d), suggesting a differ-ent controlling process.

The above documented stratigraphic relationships provide evi-dence that the upper plains unit is superposed on and youngerthan the debris apron landforms and that the surface materialsand textures of debris aprons, presently mapped as ‘‘debris apron

materials’’ in Fig. 2, are a product of dissection and modificationof upper plains and include patterns inherited from primary flowlineations. This is consistent with earlier work showing that debrisapron surfaces have been heavily modified by superposed man-tling units undergoing sublimation and eolian erosion (Mangold,2003; Berman et al., in press).

4.4.2. Upper plains, lower plains and plateausWhere in contact with lower plains, the upper plains unit exhi-

bits a sharp, irregular, lobate margin (Fig. 3). Ribbed upper plains incontact with lower plains are not as pervasively fractured or later-ally extensive as occurrences in contact with debris aprons (com-pare Figs. 3a and 6a). In addition, patches of upper plains areoften found in the interiors of lower plains craters near the marginsof upper plains (Fig. 3b).

Isolated patches of upper plains are also found on the walls andtops of plateaus and massifs throughout the study area (Figs. 6, 12and 13) and elsewhere in Deuteronilus Mensae (Carr, 2001). Theseisolated patches occur as sets of dipping layers abutting steeperslopes. The example shown in Fig. 12a is located on top of a plateauapproximately 400 m above the surrounding plains (Fig. 8c).Similar layers also occur on massifs in the surrounding plains atthe margins of upper plains (Fig. 12b). Typically a dune field offine-grained material occurs at the base of the dipping layers(Fig. 12), indicating eolian reworking of material that is beingeroded from the patches of upper plains. In one example(Fig. 13), layered blocks are observed to be eroding out of the more

Fig. 8. Topographic profiles showing relationships between debris aprons (da), upper plains (pu), ribbed upper plains [pu(r)], and plateaus (pt). Transect locations areidentified in Fig. 7b. (a) Debris apron large-scale topography grades with the upper plains (CTX DEM image pair G03_019570_2241_XN_44N334W andG03_019504_2241_XN_44N334W). A large collapse depression, �100 m deep, occurs within the upper plains. (b) A debris apron lacks the characteristic convex-upwardterminal margin observed elsewhere (Fig. 3) (same CTX DEM as in Fig. 8b). Inset profile shows ridges and troughs with raised rims (black arrows) and intermediate areas (redbars) within upper plains, showing tens of meters of vertical relief. (c) The upper plains unit is raised approximately 100 m above the surrounding debris apron surfaces (HRSCDTM 1461). Remnant patches of upper plains on the top of a plateau (Fig. 12a) are located approximately 400 m above the surrounding upper plains unit. (d) Profile traversinga buried impact crater (Figs. 6 and 7b), showing a depression up to 80 m in relief (HRSC DTM 1461). (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

Fig. 9. Examples of hummocky upper plains [pu(h)] and dissected upper plains [pu(d)] (see Fig. 2 for context locations). (a) Hummocky upper plains exhibits a smooth,muting, small-scale texture, but underlying topography is better expressed (CTX image P17_007676_2208_XN_40N327W). Isolated patches of upper plains are also observed.(b) Dissected upper plains is thinner than upper plains and exhibits extensive pitting and polygonal fracturing with patchy albedo (CTX imageP15_007030_2258_XI_45N327W).


Fig. 10. HiRISE view of the transition from ribbed upper plains [pu(r)] to debris apron material (da) (see Fig. 6 for location; HiRISE image ESP_019570_2240). Interconnectedtroughs with raised rims within ribbed upper plains isolate patches of upper plains, which are being dissected to form debris apron textures. Solid line shows the transect forthe profile given in (b). Blue and red colors and asterisks correspond to those shown in the profile. (b) HiRISE DEM topographic profile (see (a) for location; image pairESP_019570_2240 and ESP_019504_2240). Troughs (blue segments) and patches of upper plains (red segments) are separated by raised rims (asterisks) and have relief onorder of tens of meters over hundreds of meters in length. Stippled texture near the margin of debris apron material (black segment) has meters-scale roughness over tens ofmeters length scale. Elevation values are relative to the lowest elevation point in the profile. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

Fig. 11. Fractured patches of upper plains with scalloped margins (arrows) arebeing dissected into knobby and ‘‘brain-terrain’’ type textures typical for debrisapron surfaces (see Fig. 2 for context location). Fracture patterns within upperplains are at a similar length scale as the produced textures. HiRISE imagePSP_005738_2245.


continuous upper plains surrounding an isolated massif. Patches ofupper plains at this location are also being dissected into brain ter-rain textures (Fig. 13). Similar occurrences of dipping layers within

Fig. 12. Isolated remnants of upper plains are observed as sets of dipping layers (white ar(a) Remnants of upper plains on top of a plateau (pt), 400 m above the surrounding plainlayers. Dune fields are common at the base of the layers. Inset shows a zoom-in of one oupper plains [pu(r)] and debris apron material (da) (HiRISE image ESP_035011_2240). A

Deuteronilus Mensae were described by Carr (2001) from MarsOrbiter Camera (MOC) NA images. Carr (2001) noted that the lay-ered remnants are much thicker and appear distinct from latitudedependent mantle (e.g., Mustard et al., 2001; Head et al., 2003).

Collectively, the above observations are evidence that the upperplains unit was once more laterally extensive in the past, coveringdebris apron landforms, portions of the lower plains and both thetops and walls of plateaus. The occurrences of remnants of upperplains at multiple elevations suggest that the process leading tothe unit’s emplacement must have also occurred at multiple eleva-tions. We also observe these erosional remnants of upper plainsoutside of the study area and on the plateaus south of the dichot-omy boundary (a particularly good example is found at 36.02�N,21.25�E).

4.5. Mapping summary

Geomorphic mapping and detailed stratigraphic relationshipsshow that a unique upper plains unit covers much of the interme-diate areas between plateaus and debris aprons within the studyarea. This upper plains unit is stratigraphically younger than boththe lower plains and debris apron landforms. The contacts between

rows) on the walls and tops of plateaus and massifs (see Fig. 6 for context locations).s (HiRISE image ESP_028339_2245). One set of dipping layers has over ten distinct

f the remnants. (b) Dipping layers at the base of a massif (pt) and adjacent to ribbeddune field is observed at the base of the layers.

Fig. 13. (a) A massif (pt) surrounded by the upper plains unit (pu) (see Fig. 2 forimage context; HiRISE image PSP_007732_2245). White arrows show locations ofisolated patches of upper plains. Inset box shows the location of Fig. 13b. (b)Zoomed area of the contact between upper plains and the massif. Upper plains isbeing dissected at the margins of the massif, forming isolated, layered remnants ofupper plains (white arrows), similar to other areas in the study area (Fig. 12). Upperplains are also observed to be dissecting into brain-terrain textures, as observed atother locations (Fig. 11).

Fig. 14. Areas of debris aprons (da), upper plains (pu), and lower plains (pl) used fordetermining crater size–frequency distributions (Fig. 15). Counted craters areshown as red outlines. Image is a Viking orbiter mosaic and is in sinusoidalprojection centered at 28�E longitude. (For interpretation of the references to colorin this figure legend, the reader is referred to the web version of this article.)


upper plains and debris apron surfaces demonstrate that dissectionof upper plains from the tops of debris apron landforms is con-tributing to their many surface textures. In this way, the mappeddebris apron surface materials and the upper plains, includingthe mapped ribbed textures, represent various forms of preserva-tion and modification of the upper plains unit. Further, remnantpatches of upper plains located within the interior of craters inlower plains and on debris aprons and plateaus, indicate that upperplains was once more widespread across the region and has subse-quently been eroded from these surfaces.

Although it is possible that the plains units may be glaciallyrelated, no conclusive evidence of glacial landform assemblagesthat may be diagnostic of vertical and lateral retreat of a former

cold-based ice sheet is observed in the plains surrounding debrisaprons within the study area. Such landforms whenwell-expressed on Earth, could include drop moraines, lateral mor-aines, sublimation tills, and lateral melt-water channels (Evans,2003; Atkins, 2013). As is typical for cold-based glaciers on Earth,which can grow and retreat over landscapes with little basal ero-sion or deposition (Atkins, 2013), it is possible that glacial landsys-tems were poorly developed within the study region, or perhapsunique landforms did not develop at all or are too subtle to be rec-ognized with the techniques used here. Sublimation tills, whichcould be emplaced through sublimation of ice and accumulationof residual dust/debris, are probably most likely on Mars, howeverconclusively recognizing these units and uniquely associatingthem with former ice sheets is difficult. Such evidence may havealso been covered and reworked by subsequent emplacement ofyounger mantle units. It is also possible that stagnant blocks ofice from a retreating ice sheet are still buried beneath the upperplains. Large-scale collapse depressions within upper plains(Figs. 6 and 8) may be evidence of sublimation of such ice blocksat depth.

5. Crater retention ages

To further assess the relative timing of emplacement and mod-ification of units within the study area, we measured crater size–frequency distributions and calculated crater retention ages fol-lowing the methods outlined in Section 3.2 for three representativeportions of debris apron material, upper plains, and lower plainsunits. Count areas with outlines of counted craters are shown inFig. 14. Crater size–frequency distributions are presented inFig. 15. Due to the consistent morphology and lateral continuityof the units, we assume that these crater size–frequency distribu-tions are representative of the mapped units as a whole, althoughslight variations in actual ages may exist due to local variations inthe process of emplacement. Such age variations, if they exist, arelikely to be smaller than the inherent uncertainties due to craterstatistics and local erosional and resurfacing processes affectingcrater survivability.


Craters were classified by their morphology depending on theunit (Fig. 16). On debris aprons, upper plains, and lower plains,we distinguished between fresh bowl-shaped craters and degradedbowl-shaped craters, and craters exhibiting more complex interiormorphologies. On debris aprons, these complex morphologiesinclude ‘‘ring-mold’’ type craters (Kress and Head, 2008) and‘‘filled’’ craters (Berman et al., in press) (Fig. 16a). These types ofcraters have circular to irregular rims, complex interior morpholo-gies, including small mounds or platforms, and preserve the

Fig. 15. Crater size–frequency distributions based on the Hartmann (2005) isochron systfor the crater populations. Gray line segments show the boundaries for the geologic epocAmazonian), 1.01 Ga (Middle Amazonian/Early Amazonian), 3.22 Ga (Early Amazonian/LLate Noachian), 3.85 Ga (Late Noachian/Middle Noachian), and 3.96 (Middle Noachiadistribution that does not follow a single isochron. An isochron fit to the two largest bi(Kress and Head, 2008) or filled craters (Berman et al., in press) (orange squares) dominsizes. Degraded craters (red triangles) follow a 0.1 Ga best-fit isochron (red line) and frplains: All crater types (black circles) approximately follow a 3.3 Ga best-fit isochron (thindistribution of all crater types above 250 m in diameter. Craters superposed on upper plaiCraters larger than 1 km fall on a 3.5 Ga best-fit isochron, while craters smaller than 1 kmaprons (all craters, Fig. 15a), upper plains (superposed craters, Fig. 15b), and lower plaiplains is also plotted (purple line). The distributions of craters smaller than 1 km for all thfor the units, and may point toward a genetic link in formation or similar characteristic tifigure legend, the reader is referred to the web version of this article.)

meters-scale texture of the surrounding debris apron surface(Fig. 16a). On upper plains, we identified a number of buried cra-ters with subdued topography and often with circular fracture pat-terns (Fig. 16b). Buried craters also have complex interiormorphologies where the overlying upper plains unit is interpretedto have collapsed, resulting in an interior depression with concen-tric patterns of ridges, fractures, and pits. Craters interpreted to besuperposed on upper plains lack the fracture patterns, and oftenhave circular, subdued rims and can have small mounds of interior

em. See Table 1 for a listing of summary statistics and best-fits (with uncertainties)hs on Mars based on Werner and Tanaka (2011): 0.27 Ga (Early Amazonian/Middleate Hesperian), 3.36 Ga (Late Hesperian/Early Hesperian), 3.55 Ga (Early Hesperian/n/Early Noachian). (a) Debris aprons: All crater types (blue diamonds) produce ans (blue line) yields a minimum age of 0.9 Ga for debris aprons. Ring-mold cratersate the large crater size fraction of all craters but are deficient at the smaller crateresh craters (green circles) follow a 0.01 Ga best-fit isochron (green line). (b) Upperblack line) at the largest crater sizes. Buried craters (red triangles) closely follow thens (purple diamonds) follow a 0.6 Ga best-fit isochron (purple line). (c) Lower plains:define a 0.7 Ga best-fit isochron. (d) All units: The distributions of craters for debris

ns (all craters, Fig. 15c) are plotted for comparison. The best-fit isochron for upperree units overlap near the 1 Ga isochron. This indicates similar crater retention agesme-scales of crater survivability. (For interpretation of the references to color in this

Fig. 16. Portions of mapped units showing the various crater types used for crater-size frequency distributions. Counted craters are shown as red outlines; the smallestcraters were not included in the count and ambiguous circular to irregular pits were also excluded from the count. (a) Debris apron material, showing ‘‘ring-mold’’ or ‘‘filled’’craters (black arrows). The remaining craters are of the degraded type or, less common, fresh craters (white arrow). CTX image P13_006239_2240_XN_44N330W. (b) Upperplains, showing examples of buried craters (black arrows) with circular fracture and ridge patterns with interior depressions. The remaining craters, commonly with subduedrims, are interpreted to be superposed on the upper plains. CTX image P04_002481_2241_XN_44N332W. (c) Lower plains, with craters exhibiting interior mounds andlayered fill. CTX image P16_007373_2248_XN_44N333W. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisarticle.)

Table 1Crater counting statistics and best-fit ages for different crater populations on debris apron material, upper plains, and lower plains. See Fig. 14 for location of count areas.

Unit Area, km2 N > 88 m N, Fit D, Fit range (km) Best-fit age (Ga) Epoch

Debris aprons 4810All craters 563 25 0.50–1.0 0.9 (+2.7,�0.9) MARing-mold/filled craters 231 – – – –Degraded craters 246 27 0.25–0.50 0.1 (+0.001,�0.001) LAFresh craters 86 39 0.16–0.35 0.01 (+0.01,�0.01) LA

Upper plains 1899All craters 995 36 0.50–2.0 3.3 (+0.4,�2.1) LHBuried craters 134 10 0.71–2.0 3.3 (+0.5,�2.3) LHSuperposed craters 861 74 0.25–1.0 0.6 (+0.5,�0.3) MA

Lower plains 3032All craters 1300 – – – –Small craters 1297 142 0.25–1.0 0.7 (+0.3,�0.2) MALarge craters 3 3 1.0–2.82 3.5 (+0.03,�0.03) EH

Notes:N > 88 m: number of craters greater than 88 m used for crater size–frequency distributions in Fig. 15.N, Fit: number of craters used for determining best-fit ages.D, Fit Range: crater diameter bin range used for determining best-fit ages.Best-fit ages are presented to two significant figures if >1.0 Ga, and to one significant figure if <1.0 Ga. Parentheses give uncertainties in the best-fit ages, which are presentedas two times the standard error in the fit.Epoch (based on Werner and Tanaka, 2011): EH = Early Hesperian, LH = Late Hesperian, MA = Middle Amazonian, LA = Late Amazonian.–: best-fit age not determined for crater population.


fill (Fig. 16b). While not used for estimating the crater retentionage of upper plains, buried craters can be used to provide a lowerbound on the age of the underlying unit. Numerous circular toirregularly shaped pits were located within portions of the countarea of upper plains (Fig. 16b) that are most likely due to collapseand degradation of the unit. We took care to only measure featuresclearly identified as impact craters; ambiguous features were notincluded in the measurements. All crater types, excluding possiblesecondary craters, were counted on the lower plains to yield alower bound on the age of the unit (Fig. 16c). Plots comparingthe crater size–frequency distributions for craters greater than88 m in diameter on debris aprons, upper plains, and lower plainsto the Hartmann (2005) isochrons are given in Fig. 15. The countareas, number of craters, and best-fit ages for each unit are sum-marized in Table 1.

5.1. Debris apron material

From the distribution of all crater types on debris aprons, abest-fit age of 0.9 (+2.7, �0.9) Ga is measured from an isochron

fit to the two largest diameter bins (Fig. 15a, Table 1). There arelarge uncertainties in this age, as only two bins are fit and due tothe large uncertainties in the largest bin size. The data points forall crater diameters do not follow a single isochron, as there is sig-nificant roll-off of the distribution for diameters less than 500 m(Fig. 15a). This roll-off is likely to be due to removal of craters ofsmall diameters by erosion and cover by younger mantling units,as evidenced by debris apron surface textures (Section 4.1).Ring-mold (Kress and Head, 2008) or filled craters (Berman et al.,in press) contribute measurably to the large size range of all cra-ters. Based on the clear removal of small crater sizes from debrisaprons, the 0.9 Ga best-fit age is considered to be a lower boundfor the debris apron surface material. The true age of the debrisapron landform is likely to be older than can be currently resolvedfrom crater statistics on these surfaces.

For comparison with another debris apron surface withinDeuteronilus Mensae, we classified crater morphologies in a simi-lar manner as Berman et al. (in press), yielding consistent cratersize–frequency distributions. Berman et al. (in press) examinedthe distributions for degraded and fresh crater populations and


identified what they interpreted to be resurfacing ages at approx-imately 0.2 Ga and 0.01 Ga due to emplacement of younger man-tling units. In Fig. 15a, degraded bowl-shaped craters between250 m and 500 m in diameter on our mapped debris aprons followa best-fit isochron of 0.1 (+0.001, �0.001) Ga. Fresh bowl-shapedcraters between 125 m and 354 m in diameter follow a best-fit iso-chron of 0.01 (+0.01, �0.01) Ga (Table 1). These ages likely repre-sent a characteristic period of time over which craters in thesesize ranges and morphologies survive on surfaces within the studyarea due to continual deposition and erosion of mantle throughoutthe Amazonian of Mars. As suggested by Berman et al. (in press)and previous workers, multiple major episodes of mantling likelyoccurred throughout the Amazonian period, with the last episodesoccurring within the past 10 Ma.

5.2. Upper plains

Craters counted on upper plains are separated into those buriedby upper plains and those superposed on the unit(Figs. 15b and 16b). Buried craters dominate the distribution ofall craters for diameters larger than approximately 375 m. An iso-chron fit to all craters (buried and superposed) larger than 500 min diameter yields a minimum age of 3.3 (+0.4, �2.1) Ga. This istaken to be a lower bound on the age of the unit underlying theupper plains. It is a lower bound, as some buried craters are almostcertainly not recognizable and cannot be counted due to lack ofsurface expression in the overlying upper plains. Craters super-posed on upper plains follow a 0.6 (+0.5, �0.3) Ga best-fit isochronfor diameters between 250 m and 1 km (Fig. 15b). The upper plainsunit appears to better retain small craters between 250 m and500 m than debris apron surfaces, as inferred from the absenceof a reduction of the frequency, or roll-off, of craters within thisdiameter range (Fig. 15b).

5.3. Lower plains

All craters measured on the lower plains show a kink in the dis-tribution at crater diameters of around 1 km (Fig. 15c). This kink isinterpreted to represent resurfacing of the lower plains (e.g.,Hartmann et al., 1981), which has affected craters less than 1 kmin diameter. Interpreting possible kinks in crater size–frequencydistributions must be viewed with caution, as they can occur ran-domly and may not be connected to a specific geologic process act-ing on the crater population. However, as discussed in Section 6, aresurfacing interpretation for the lower plains crater distribution isconsistent with our mapping, which shows that the upper plainsunit was emplaced on and then removed from the lower plains.As such, two separate best-fit ages were measured. An isochronfit to craters between 250 m and 1 km yields a minimum age of0.7 (+0.3, �0.2) Ga. For craters between 1 and 2.8 km, a best-fitage of 3.5 (+0.03, �0.03) Ga was measured. However, there is largeuncertainty associated with this age, as it is a result of a fit to onlytwo bins, representing only three craters. The older 3.5 Ga age isinterpreted to be a lower bound on the age of emplacement oflower plains, which is consistent with the 3.3 Ga minimum ageof the (lower) plains underlying the upper plains unit. The youngerage of 0.7 Ga is interpreted to be a possible lower bound on thetiming of resurfacing of the lower plains by later mantlingepisodes.

5.4. Comparison of ages

Fig. 15d plots the crater-size frequency distributions definingthe crater retention ages for debris aprons (all craters), upperplains (superposed craters), and lower plains (all craters). Thesethree units appear to converge at a single isochron slightly younger

than 1 Ga for diameters less than 1 km. For comparison, cratercounts on plateau surfaces within the study area by previous work-ers (e.g., Joseph et al., 2011) suggest an age of between 3 and 3.5 Gabased on craters >500 m, with a significant reduction in the fre-quency of crater sizes below 500 m.

If the crater retention ages for the mapped units representlower bounds on the timing of distinct geological episodes in thestudy region, they indicate that the apparent cessation of debrisapron flow, the emplacement of upper plains, and resurfacing ofthe lower plains all occurred contemporaneously or within a shorttime-span and are potentially genetically related. This is supportedby mapping and stratigraphic relationships, which show thatupper plains is superposed on both debris apron landforms andlower plains and has undergone significant erosion and modifica-tion subsequent to its emplacement on these units. Alternatively,the convergence of the crater-size frequency distributions near1 Ga (Fig. 15d) may represent similar timescales for crater survivalon these units, which may be a function of the cumulative effectsof repeated emplacement of ice-rich mantle units, with upperplains being one of many such episodes of varying vertical and lat-eral extent. Under this scenario, recognizing distinct geological epi-sodes is difficult and masked by the survivability of small craterdiameters. In either case, as we discuss further in Section 6, we col-lectively interpret many of the observations as resulting fromresurfacing of much of the study area by a relatively thick mantlingmaterial, which is currently preserved as the upper plains unit.

6. Discussion

6.1. Mantle characteristics and lateral extent of upper plains

The mapping results discussed in Section 4 show that a 50–100 m thick upper plains unit is superposed on both debris apronlandforms and surrounding lower plains and is widespread withinthe study area north of approximately 43�N (Fig. 2). The grada-tional and erosional progression in textures between upper plainsand mapped debris apron surface materials suggest that erosionand modification of upper plains have contributed significantlyto the current surface morphology of debris aprons within thestudy area. The modification of debris apron surfaces by upperplains is also supported by their similar best-fit ages of near 1 Ga(Fig. 15d, Table 1). These observations are consistent with previouswork that invoked erosion and modification of ice-rich mantlingunits on the surfaces of debris aprons to explain their observed tex-tures. Mangold (2003) suggested that fractures within the mantlewould facilitate subsequent eolian erosion of fines and/or sublima-tion of ice from the unit. Fracture patterns were suggested to havebeen controlled by the ridge and furrow flow patterns of the under-lying debris apron surface. Under this formation scenario, the flowpatterns are a primary feature of the debris aprons that wereformed within supraglacial debris during glacial flow prior toemplacement of superposing mantle. Subsequent dissection andmodification of the superposing mantle is largely controlled bythe primary debris apron topography. Consistent with mantlingmaterial, HiRISE observations show that the upper plains consistsof fines with few boulders; small pits (Fig. 7d) and widening offractures (Fig. 7a) within upper plains are also similar to sublima-tion textures observed elsewhere on Mars (e.g., Morgenstern et al.,2007).

The upper plains unit appears to also have been modified bycompressional stresses, as evidenced by numerous fracture andridge patterns that are most prominent at the margins and conver-gence of debris aprons (Figs. 2 and 6). We interpret these fracturepatterns to have resulted from stresses induced by limited down-slope flow or creep of ice toward the margins of debris aprons.


While the preserved circular crater shapes of ring-mold craters ondebris apron surfaces do not support significant differential surfaceflow of ice and shearing at the scale of craters within the past 1 Ga,limited down-gradient flow or creep of ice near the emplacementtime of upper plains (minimum estimate of �0.6–1 Ga) may haveyielded the requisite stresses to induce fracturing within upperplains without significantly shearing debris apron surface features.Subsequent eolian erosion and possible sublimation of pore ice atfracture planes may explain the widening of fractures into troughswithin the mapped ribbed upper plains (Fig. 7a). Ice-facilitatedcreep of the debris apron mass could have also contributed to frac-turing of upper plains superposing debris apron landforms. Thesefractures would have facilitated subsequent erosion of the unitand formation of textures in a manner similar to that proposedby Mangold (2003). Polygonal fracturing, including thermal con-traction cracks within upper plains, as well as primary debrisaprons topography, appear to have been important in formationof debris apron textures (Fig. 11 and Levy et al., 2009).

Multiple lines of evidence indicate that upper plains materialwas more widespread within the study area. Isolated, remnant lay-ers of upper plains are found within the interior of craters on lowerplains (Fig. 3b). Crater size–frequency distributions of lower plains(Fig. 15c) indicate that this unit may have been resurfaced near thetime of emplacement of upper plains (Fig. 15d). The resurfacing ageof lower plains could have plausibly resulted from subsequentdownwasting and reworking of the upper plains unit over portionsof the lower plains, which would have covered and reduced theapparent populations of craters less than 1 km in diameter. Largercraters would have been less affected, yielding the older 3.3–3.5 Ga minimum age of the lower plains (Fig. 15c). Alternatively,the near coincident best-fit ages of plains units may represent acharacteristic time-scale of crater survivability on these surfaces,caused by the cumulative effects of repeated emplacement ofice-rich mantle layers throughout the Amazonian. In this case, theages of both upper and lower plains are minimum ages and moreaccurately represent the ability of these surfaces to retain cratersof diameters on order of hundreds of meters rather than distinctemplacement or resurfacing episodes.

In addition, remnant layers of upper plains on the side walls andtops of plateaus and massifs throughout the study area (Figs. 12and 13) support the interpretation that the upper plains unit wasonce more laterally extensive, covering multiple older units andat multiple elevations. In addition, the laterally continuous layer-ing within the upper plains and its remnants (Figs. 7c, 12, and13) indicate that the upper plains unit may have been emplacedover multiple episodes. The relative timing between layers and ori-gin of the layering is not clear, but they may represent depositionalepisodes controlled by relatively short (tens of thousands of years)cycles in orbital forcing, which is known to have controlled the dis-tribution and deposition of water ice on Mars (e.g., Head et al.,2003; Madeleine et al., 2014). Layering is also observed withinthe thinner and younger latitude dependent mantle unit that isubiquitous in the mid-latitudes of Mars (Mustard et al., 2001;Head et al., 2003; Schon et al., 2009), and in the walls at the edgesof pedestal craters (Kadish and Head, 2011). Previous work hasshown that the latitude dependent mantle was atmosphericallyemplaced as an ice and dust deposit and that its internal layeringrepresents multiple episodes of deposition controlled by cycles inthe recent spin-axis obliquity of Mars (e.g., Head et al., 2003;Schon et al., 2009; Madeleine et al., 2014).

6.2. Formation scenario

To summarize our findings and interpretations, we describe aformation scenario for units within the study area below and inFig. 17.

6.2.1. Plateaus and lower plainsThe plateaus are remnants of fretting of ancient Noachian mate-

rials. The fretting process across the dichotomy boundary on Marsis not entirely clear (Kochel and Peake, 1984; Watters et al., 2007)but has involved extensive faulting and fluvial, mass wasting, andice-facilitated processes. A lower bound on the emplacement of thelower plains is taken to be approximately 3.3–3.5 Ga (Early–LateHesperian); the original emplacement age is likely to be olderdue to crater obliteration at these crater sizes (e.g., Irwin et al.,2013). The origin of the lower plains is unclear, but the presenceof wrinkle ridge systems across Deuteronilus Mensae indicate thatlower plains is likely to be Hesperian-aged volcanic materials(Tanaka et al., 2005). Later eolian deposits have modified andresurfaced the lower plains.

6.2.2. Regional ice sheetAlthough our mapping did not recognize landforms diagnostic

of a former regional ice sheet in the study area—which is not fullyunexpected given its probable cold-based style of glaciation(Atkins, 2013; Fastook et al., 2014)—other geological evidenceand modeling suggests that Amazonian glaciation associated withformation of debris aprons may have been more extensive in thepast (see discussion in Section 1; Madeleine et al., 2009; Morganand Head, 2009; Dickson et al., 2008, 2010; Fastook et al., 2011,2014). If the region was in net accumulation (Madeleine et al.,2009), an ice-sheet may have extended both laterally and verticallyto cover the plateaus and plains (Fig. 17a; Fastook et al., 2014).Episodes of more dust-rich accumulation from enhanced dust loft-ing or possibly volcanic tephra (Wilson and Head, 2009; Kerberet al., 2012) would have led to thin layers of fine-grained materialson the surfaces of the ice-sheet and possibly internal layering(Fig. 17a).

6.2.3. Ice-sheet retreatIn our formation model, if correct, as the glacial system transi-

tioned to net ablation due to reduction in accumulation rates, theice-sheet down-wasted mainly through sublimation (Fig. 17b).Downwasting led to thickening of a surface dust-rich sublimationtill layer. Continued downwasting also exposed the bedrock scarpsof high-standing plateaus.

The ice sheet eventually retreated to more plateau-type glacia-tion near the present extent of debris aprons (Fig. 17c). Stagnantblocks of glacial ice are common at the margins of retreating gla-ciers on Earth, and may have existed and been preserved onMars by a thick insulating layer of sublimation till. Thermal cyclingand erosion of the exposed plateau scarps contributed to rockfallsand rockslides on top of the glacial ice, as seen in the McMurdo DryValleys (Swanger et al., 2010; Kowalewski et al., 2011; Marchantet al., 2013; MacKay et al., 2014). Through this process, a rockysupraglacial till developed and was integrated with the dust-richsublimation lag produced from initial downwasting of theice-sheet. Down-gradient flow of ice transported this till towardthe margins of the debris aprons and developed the glacier-likesurface ridge and furrow flow patterns characteristic of debrisaprons on Mars and debris-covered glaciers on Earth (MacKayet al., 2014). Craters may have been efficiently erased on the glaciersurface during this stage due to continual transport of ice and deb-ris and possible differential shearing of supraglacial morphology.

6.2.4. Emplacement and modification of mantlingAs ice/snow accumulation ceased and the supraglacial debris

became sufficiently thick through sublimation and plateau scarpretreat (Fastook et al., 2014), the present configuration of debrisaprons was largely obtained. The lack of sheared craters on the pre-sent surfaces of debris aprons indicates that substantial differentialflow within debris aprons may have ceased at least by 1 Ga, and

Fig. 17. Schematic formation scenario for glaciation and units within the study region. See Section 6 in the text for a detailed description of the scenario. The upper plains unitis interpreted as a thick mantling unit that covered debris apron landforms and lower plains in the waning stages and downwasting of mid-latitude glaciation. Modification ofthe upper plains unit occurred at the margins by limited down-gradient flow of ice, and collapse depressions may be indicative of buried glacial ice bodies at depth.


probably earlier. Near this time, a thick mantling layer of atmo-spheric dust and ice was deposited over most of DeuteronilusMensae, which is now preserved in its coherent, relative undis-sected form, as the upper plains unit. This mantling unit

superposed the primary flow lineations within the supraglacial tillof debris aprons. The thickness of the upper plains would havebeen spatially variable but was near 100 m. The entire thicknessof the upper plains was emplaced over multiple cycles, producing


internal layering. These cycles were likely to have been tied toobliquity variations, which modulated mid-latitude accumulationsof ice and dust (e.g., Madeleine et al., 2014).

Limited down-gradient flow or creep of the debris apron icemass led to compressional stresses within upper plains that weregreatest near the margins of the debris aprons and where multipledebris aprons converged (Fig. 17e). These stresses produced thefractured ridge and furrow patterns of the ribbed upper plains(Fig. 17e). On the surfaces of the debris apron landforms, fracturingof the upper plains mantle unit was controlled by a combination ofthe underlying surface topography (Mangold, 2003) and limitedcreep/flow of the debris aprons. Polygonal thermal contractioncracks (e.g., Levy et al., 2009) were also formed more recently inthe top layers of upper plains. This fracturing facilitated subse-quent sublimation and erosion of the upper plains mantle unit,forming the present surface textures on debris apron surfaces, asmapped as debris-apron materials (Fig. 17f).

It is possible that large-scale collapse features within the pre-sent extent of upper plains (Figs. 6 and 8a) were produced fromsublimation of possible stagnant ice bodies underlying the unit(Fig. 17e). In addition, erosion and sublimation of upper plainsfrom the tops of plateaus and over lower plains (Fig. 17f) led toremnant patches of the unit throughout the region. The presentspatial configuration of upper plains was controlled by variationsin the original thickness of the unit, topographic location and inso-lation geometries creating protective or erosional microclimates(e.g., Marchant and Head, 2007; Berman et al., in press).

Fig. 18. Units in other regions of the northern and southern mid-latitudes that exhibitanalogous units mapped in the study area {plateau (pt), debris apron material (da), ribbedMargin of debris apron in Protonilus Mensae, centered at 39.91�N, 47.58�E (CTX image G0and ribbed debris apron texture in Mareotis Fossae, centered at 48.87�N, 70.81�W (surrounding Hellas Montes resembling upper plains and ribbed upper plains, centeredFigs. 3, 5, 6, 7, 10a, and 13. (d) Dipping layers near Hellas Montes, resemblingB12_014438_1362_XN_43S258W). Compare with Figs. 12 and 13.

6.3. Mantling units elsewhere on Mars

Our preliminary observations indicate that units similar in mor-phology and stratigraphic relationships to upper plains occur inother northern and southern mid-latitude regions of Mars.(Fig. 18). In the northern hemisphere, a unit similar to ribbed upperplains occurs in Protonilus Mensae (Fig. 18a, 39.91�N, 47.58�E) anddebris aprons exhibit a ribbed debris apron texture in MareotisFossae (Fig. 18b, 48.87�N, 70.81�W). In the southern hemispherenear Hellas Montes, debris aprons appear covered by a smooth unitsimilar to upper plains, with circular fracture patterns and widen-ing troughs (Fig. 18c, 37.66�S, 95.29�E). Isolated blocks of dippinglayers on the walls of massifs near Hellas Montes are abundant(Fig. 18d, 43.08�S, 101.92�E), and are similar to isolated patchesof upper plains in Deuteronilus Mensae (Fig. 12). One block inthe Hellas Montes example exhibits over ten distinct layers(Fig. 18d). The above observations indicate that the process thatemplaced upper plains was not restricted to DeuteronilusMensae and occurred in both the northern and southernmid-latitudes and across different regions. The broad,mid-latitude spatial extent of upper plains is consistent with aclimate-driven, atmospherically derived, ice-rich mantle originfor upper plains, similar to other mantling units on Mars.

The upper plains mantling unit is older, thicker, and morpho-logically distinct from the young (<10 Ma) latitude dependentmantle that drapes a large portion of the mid-latitudes of Mars,including Deuteronilus Mensae (Mustard et al., 2001; Kreslavsky

similar characteristics to those in Deuteronilus Mensae. Features are labeled withdebris apron material [da(r)], upper plains (pu), and ribbed upper plains [pu(r)]}. (a)1_018567_2210_XN_41N312W). Compare with Figs. 5, 6, and 10b. (b) Debris apronCTX image F05_037876_2305_XN_50N071W). Compare with Fig. 5. (c) Material

at 37.66�S, 95.29�E (CTX image P15_006738_1416_XI_38S264W). Compare withisolated patches of upper plains, centered at 43.08�S, 101.92�E (CTX image


and Head, 2000, 2002; Head et al., 2003; Milliken et al., 2003).However, both mantle units are fine-grained, exhibit sublimationtextures, and are comprised of multiple layers, suggesting that theyboth may have been deposited through similar mantling processes,differing only in their age and thickness of deposition. If correct,our observations of the upper plains unit indicate that mantlingdeposits similar to the more recent latitude dependent mantleextended back to at least the Middle/Early Amazonian, with thickerdeposits perhaps being a product of longer obliquity cycles or netaccumulations over multiple depositional cycles.

Mantle units similar in thickness and certain morphologies toupper plains occur in Utopia Planitia (Morgenstern et al., 2007;LeFort et al., 2009). Like the upper plains, the Utopia unit is rela-tively smooth, polygonally fractured, has circular fractures overburied craters, and has thicknesses of tens of meters. Remnantsof the Utopia mantle unit are also found within craters in the plainswhere the mantle has been removed (Morgenstern et al., 2007;Soare et al., 2013). In contrast to the upper plains, the Utopia unitexhibits scalloped depressions, pervasive sublimation pits, andmargins that are sharp (not lobate or rounded) when in contactwith the surrounding plains. Tanaka et al. (2005) established aLate Amazonian age for the Utopia mantle unit (Morgensternet al., 2007), which is younger than the Middle Amazonian mini-mum age determined for upper plains.

Thick ice-rich mantling layers have also been important in theformation of pedestal craters on Mars (e.g., Kadish et al., 2009;Kadish and Head, 2014). Pedestal craters are thought to form fromimpacts into a volatile-rich layer tens to hundreds of meters inthickness (Kadish et al., 2009, 2010). Ejecta and induration fromthe impact event armors the volatile-rich layer surrounding theimpact crater (Wrobel et al., 2006), protecting it from subsequenterosion. Kadish and Head (2014) measured a range of crater reten-tion ages for individual pedestal surfaces, ranging from approxi-mately 1 Ma to 3.6 Ga, with a median of 140 Ma and with 70% ofthe ages less than 250 Ma. The plotted crater size–frequency distri-butions (e.g., Fig. 4 of Kadish and Head, 2014), however, show sig-nificant down-turn in the frequency of small crater sizes,indicating younger resurfacing or erosional processes. At leastsome of the best-fit ages for the pedestal surfaces from Kadishand Head (2014) are therefore likely to be minimum estimates,with slightly older ages more likely. Given that the upper plainsunit in Deuteronilus Mensae exhibits a similar minimum age offormation and similar thicknesses and morphologies as the unitsforming pedestals, it is possible that upper plains may be a pre-served remnant of a pedestal-forming layer that was once morewidespread. For example, small pedestal-type craters are foundin the lower plains where the upper plains have been removed(Fig. 3b). Pedestals throughout the mid-latitude are also observedto overlap each other (Kadish et al., 2010) and exhibit continuouslayering (Kadish and Head, 2011), indicating that the mantle layersforming pedestals were emplaced over multiple periods of deposi-tion. This observation emphasizes the point that emplacement ofupper plains represents only one of multiple repeated episodes ofmantle deposition on Mars, which has varied in thickness and dis-tribution with time.

7. Conclusions

If mid-latitude glaciation on Mars was more extensive in thepast, then it is possible that evidence of this glaciation may be pre-served as distinct landform assemblages where present glacial fea-tures are currently observed. We conducted geomorphic mappingof debris aprons and plains units within a portion of theDeuteronilus Mensae region of Mars to assess evidence of formerglacial extents and to elucidate the formation of surrounding plains

units. Landforms diagnostic of the retreat of a more extensivecold-based ice sheet were not observed in the plains surroundingdebris aprons. However, this observation does not preclude theexistence of past ice sheets. As discussed, other geomorphic evi-dence and modeling suggests that former ice sheets in themid-latitudes of Mars were likely. Further, as in cold-based glacia-tion on Earth (e.g., Atkins, 2013), depositional and erosional evi-dence of such glaciation may have been very limited in itsexpression and challenging to uniquely recognize. Depositionalfeatures would have been limited by the supply and source of deb-ris, including atmospheric dust and/or tephra or much more lim-ited supplies of rockfall from exposed high-standing topography.

Based on our mapping, a unique ‘‘upper plains’’ mantling unit isobserved that was emplaced approximately 0.6–1 Ga (minimumage) and was once more widespread across Deuteronilus Mensaeand other regions in the mid-latitudes of Mars. The unit is up to100 m in thickness, appears to be a mixture of dust and ice, anddrapes and modifies debris apron landforms and older plains units,contributing to their surface textures and modifying their craterretention ages. It is possible that any glacial landforms in the plainssurrounding debris aprons may be masked by this and other latermantling episodes. Possible remnants of glacial ice, marking retreatof a more extensive ice sheet, may be buried beneath the upperplains, as suggested by the presence of large collapse featureswithin the unit that may result from sublimation of ice blocks atdepth. Such possibility of buried ice should be further tested withSHARAD radar observations over these deposits.

Further, on the basis of observed layering, the emplacement ofthe upper plains appears to have been cyclic, possibly controlledby changes in orbital forcing over the past 1 Ga on Mars. The lay-ered characteristics are similar to the younger and thinner latitudedependent mantle that was emplaced <10 Ma. Deposition of icymantle units within the mid-latitude of Mars thus appears to havebeen repeated multiple times in the Middle to Late Amazonian,marking the transition from ice-rich accumulations of glacial iceto more dust-rich icy mixtures.

Acknowledgments

Thank you to Jay Dickson and Caleb Fassett for assistance withdata processing and crater counting methods. We thank DanBerman and an anonymous reviewer for detailed reviews thathelped to improve the quality of the manuscript. We gratefullyacknowledge support from the NASA Mars Data AnalysisProgram Grant NNX11A181G to JWH and from JPL Grant1237163 for participation of JWH in the High Resolution StereoCamera Team on the Mars Express Mission.

References

Atkins, C.B., 2013. Geomorphological evidence of cold-based glacier activity inSouth Victoria Land, Antarctica. In: Hambrey, M.J., Barker, P.F., Barrett, P.J.,Bowman, V., Davies, B., Smellie, J.L., Tranter, M. (Eds.), AntarcticPalaeoenvironments and Earth-Surface Processes, vol. 381. Geological Society,London, Special Publications, http://dx.doi.org/10.1144/SP381.18.

Baker, D.M.H., Head, J.W., Marchant, D.R., 2010. Flow patterns of lobate debrisaprons and lineated valley fill north of Ismeniae Fossae, Mars: Evidence forextensive mid-latitude glaciation in the Late Amazonian. Icarus 207, 186–209.http://dx.doi.org/10.1016/j.icarus.2009.11.017.

Benn, D.I., Evans, D.J.A., 2010. Glaciers and Glaciation, second ed. Hodder Education,London, p. 816.

Berman, D.C., Crown, D.A., Joseph, E.C.S., 2015. Formation and mantling ages oflobate debris aprons on Mars: Insights from categorized crater counts. Planet.Space Sci., http://dx.doi.org/10.1016/j.pss.2015.03.013 (in press).

Carr, M.H., 2001. Mars Global Surveyor observations of martian fretted terrain. J.Geophys. Res. 106, 23571–23593. http://dx.doi.org/10.1029/2000JE001316.

Carr, M.H., Head III, J.W., 2010. Geologic history of Mars. Earth Planet. Sci. Lett. 294,185–203. http://dx.doi.org/10.1016/j.epsl.2009.06.042.

Chuang, F.C., Crown, D.A., 2005. Surface characteristics and degradational history ofdebris aprons in the Tempe Terra/Mareotis fossae region of Mars. Icarus 179,24–42. http://dx.doi.org/10.1016/j.icarus.2005.05.014.

http://dx.doi.org/10.1144/SP381.18


http://refhub.elsevier.com/S0019-1035(15)00288-2/h0015


http://dx.doi.org/10.1016/j.pss.2015.03.013

http://dx.doi.org/10.1029/2000JE001316

http://dx.doi.org/10.1016/j.epsl.2009.06.042



Chuang, F.C., Crown, D.A., 2009. Geologic Map of MTM 35337, 40337, and 45337Quadrangles, Deuteronilus Mensae Region of Mars: U.S. Geological SurveyScientific Investigations Map 3079.

Dickson, J.L., Head, J.W., Marchant, D.R., 2008. Late Amazonian glaciation at thedichotomy boundary on Mars: Evidence for glacial thickness maxima andmultiple glacial phases. Geology 36, 411–414. http://dx.doi.org/10.1130/G24382A.1.

Dickson, J.L., Fassett, C.I., Head, J.W., 2009. Amazonian-aged fluvial valley systems ina climatic microenvironment on Mars: Melting of ice deposits on the interior ofLyot Crater. Geophys. Res. Lett. 36, L08201. http://dx.doi.org/10.1029/2009GL037472.

Dickson, J.L., Head, J.W., Marchant, D.R., 2010. Kilometer-thick ice accumulation andglaciation in the northern mid-latitudes of Mars: Evidence for crater-fillingevents in the Late Amazonian at the Phlegra Montes. Earth Planet. Sci. Lett. 294,332–342. http://dx.doi.org/10.1016/j.epsl.2009.08.031.

Evans, D.J.A., 2003. Glacial Landsystems. Edward Arnold, London, p. 544.Fassett, C.I., Head III, J.W., 2008. The timing of martian valley network activity:

Constraints from buffered crater counting. Icarus 195, 61–89. http://dx.doi.org/10.1016/j.icarus.2007.12.009.

Fassett, C.I. et al., 2010. Supraglacial and proglacial valleys on Amazonian Mars.Icarus 208, 86–100. http://dx.doi.org/10.1016/j.icarus.2010.02.021.

Fastook, J.L. et al., 2011. Evidence for Amazonian northern mid-latitude regionalglacial landsystems on Mars: Glacial flow models using GCM-driven climateresults and comparisons to geological observations. Icarus 216, 23–39. http://dx.doi.org/10.1016/j.icarus.2011.07.018.

Fastook, J.L., Head, J.W., Marchant, D.R., 2014. Formation of lobate debris aprons onMars: Assessment of regional ice sheet collapse and debris-cover armoring.Icarus 228, 54–63. http://dx.doi.org/10.1016/j.icarus.2013.09.025.

Gwinner, K. et al., 2010. Topography of Mars from global mapping by HRSC high-resolution digital terrain models and orthoimages: Characteristics andperformance. Earth Planet. Sci. Lett. 294, 506–519. http://dx.doi.org/10.1016/j.epsl.2009.11.007.

Hartmann, W.K., 2005. Martian cratering 8: Isochron refinement and thechronology of Mars. Icarus 174, 294–320. http://dx.doi.org/10.1016/j.icarus.2004.11.023.

Hartmann, W.K. et al., 1981, Chronology of planetary volcanism by comparativestudies of planetary cratering. In: Basaltic Volcanism on the Terrestrial Planets,Basaltic Volcanism Study Project, pp. 1050–1129.

Head, J.W. et al., 2003. Recent ice ages on Mars. Nature 426, 797–802. http://dx.doi.org/10.1038/nature02114.

Head, J.W. et al., 2005. Tropical to mid-latitude snow and ice accumulation, flow andglaciation on Mars. Nature 434, 346–351. http://dx.doi.org/10.1038/nature03359.

Head, J.W. et al., 2006a. Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for Late Amazonian obliquity-driven climatechange. Earth Planet. Sci. Lett. 241, 663–671. http://dx.doi.org/10.1016/j.epsl.2005.11.016.

Head, J.W. et al., 2006b. Modification of the dichotomy boundary on Mars byAmazonian mid-latitude regional glaciation. Geophys. Res. Lett. 33, L08S03.http://dx.doi.org/10.1029/2005GL024360.

Head, J.W. et al., 2010. Northern mid-latitude glaciation in the Late Amazonianperiod of Mars: Criteria for the recognition of debris-covered glacier and valleyglacier landsystem deposits. Earth Planet. Sci. Lett. 294, 306–320. http://dx.doi.org/10.1016/j.epsl.2009.06.041.

Holt, J.W. et al., 2008. Radar sounding evidence for buried glaciers in the southernmid-latitudes of Mars. Science 322, 1235–1238. http://dx.doi.org/10.1126/science.1164246.

Irwin, R.P., Tanaka, K.L., Robbins, S.J., 2013. Distribution of Early, Middle, and LateNoachian cratered surfaces in the martian highlands: Implications forresurfacing events and processes. J. Geophys. Res. 118, 278–291. http://dx.doi.org/10.1002/jgre.20053.

Jaumann, R. et al., 2007. The high-resolution stereo camera (HRSC) experiment onMars Express: Instrument aspects and experiment conduct from interplanetarycruise through the nominal mission. Planet. Space Sci. 55, 928–952. http://dx.doi.org/10.1016/j.pss.2006.12.003.

Joseph, E.C.S., Crown, D.A., Berman, D.C., 2011. Using CTX-based crater size-frequency distributions to refine the geologic history of Deuteronilus Mensae,Mars. Lunar Planet. Sci. 42. Abstract 1206.

Kadish, S.J., Head, J.W., 2011. Preservation of layered paleodeposits in high-latitudepedestal craters on Mars. Icarus 213, 443–450. http://dx.doi.org/10.1016/j.icarus.2011.03.029.

Kadish, S.J., Head, J.W., 2014. The ages of pedestal craters on Mars: Evidence for alate-Amazonian extended period of episodic emplacement of decameters-thickmid-latitude ice deposits. Planet. Space Sci. 91, 91–100. http://dx.doi.org/10.1016/j.pss.2013.12.003.

Kadish, S.J., Barlow, N.G., Head, J.W., 2009. Latitude dependence of martian pedestalcraters: Evidence for a sublimation-driven formation mechanism. J. Geophys.Res. 114, E10001. http://dx.doi.org/10.1029/2008JE003318.

Kadish, S.J., Head, J.W., Barlow, N.G., 2010. Pedestal crater heights on Mars: A proxyfor the thicknesses of past, ice-rich, Amazonian deposits. Icarus 210, 92–101.http://dx.doi.org/10.1016/j.icarus.2010.06.021.

Karlsson, N.B., Schmidt, L.S., Hvidberg, C.S., 2015. Volume of martian midlatitudeglaciers from radar observations and ice flow modeling. Geophys. Res. Lett. 42.http://dx.doi.org/10.1002/2015GL063219, 2015GL063219.

Kerber, L. et al., 2012. The dispersal of pyroclasts from ancient explosive volcanoeson Mars: Implications for the friable layered deposits. Icarus 219, 358–381.http://dx.doi.org/10.1016/j.icarus.2012.03.016.

Kneissl, T., van Gasselt, S., Neukum, G., 2011. Map-projection-independent cratersize–frequency determination in GIS environments—New software tool forArcGIS. Planet. Space Sci. 59, 1243–1254. http://dx.doi.org/10.1016/j.pss.2010.03.015.

Kochel, R.C., Peake, R.T., 1984. Quantification of waste morphology in martianfretted terrain. J. Geophys. Res. 89, C336–C350. http://dx.doi.org/10.1029/JB089iS01p0C336.

Kowalewski, D.E. et al., 2011. Modeling vapor diffusion within cold and drysupraglacial tills of Antarctica: Implications for the preservation of ancient ice.Geomorphology 126, 159–173. http://dx.doi.org/10.1016/j.geomorph.2010.11.001.

Kreslavsky, M.A., 2007. Statistical characterization of spatial distribution of impactcraters: Implications to present-day cratering rate on Mars. SeventhInternational Conference on Mars, Abstract 3325.

Kreslavsky, M.A., Head, J.W., 2000. Kilometer-scale roughness of Mars: Results fromMOLA data analysis. J. Geophys. Res. 105 (E11), 26695–26711.

Kreslavsky, M.A., Head III, J.W., 2002. Mars: Nature and evolution of young latitude-dependent water–ice-rich mantle. Geophys. Res. Lett. 29 (15), 1719. http://dx.doi.org/10.1029/2002GL01539.

Kreslavsky, M.A., Ivanov, M.A., Head, J.W., 2015. The resurfacing history of Venus:Constraints from buffered crater densities. Icarus 250, 438–450. http://dx.doi.org/10.1016/j.icarus.2014.12.024.

Kress, A.M., Head, J.W., 2008. Ring-mold craters in lineated valley fill and lobatedebris aprons on Mars: Evidence for subsurface glacial ice. Geophys. Res. Lett.35, L23206. http://dx.doi.org/10.1029/2008GL035501.

Laskar, J. et al., 2004. Long term evolution and chaotic diffusion of the insolationquantities of Mars. Icarus 170, 343–364. http://dx.doi.org/10.1016/j.icarus.2004.04.005.

Lefort, A. et al., 2009. Observations of periglacial landforms in Utopia Planitia withthe High Resolution Imaging Science Experiment (HiRISE). J. Geophys. Res. 114,E04005. http://dx.doi.org/10.1029/2008JE003264.

Levy, J.S., Head, J.W., Marchant, D.R., 2007. Lineated valley fill and lobate debrisapron stratigraphy in Nilosyrtis Mensae, Mars: Evidence for phases of glacialmodification of the dichotomy boundary. J. Geophys. Res. 112, E08004. http://dx.doi.org/10.1029/2006JE002852.

Levy, J.S., Head, J.W., Marchant, D.R., 2009. Concentric crater fill in Utopia Planitia:History and interaction between glacial ‘‘brain terrain’’ and periglacial mantleprocesses. Icarus 202, 462–476. http://dx.doi.org/10.1016/j.icarus.2009.02.018.

Levy, J., Head, J.W., Marchant, D.R., 2010. Concentric crater fill in the northern mid-latitudes of Mars: Formation processes and relationships to similar landforms ofglacial origin. Icarus 209, 390–404. http://dx.doi.org/10.1016/j.icarus.2010.03.036.

Levy, J.S. et al., 2014. Sequestered glacial ice contribution to the global Martianwater budget: Geometric constraints on the volume of remnant, midlatitudedebris-covered glaciers. J. Geophys. Res. Planets 119. http://dx.doi.org/10.1002/2014JE004685, 2014JE004685.

Li, H., Robinson, M.S., Jurdy, D.M., 2005. Origin of martian northern hemispheremid-latitude lobate debris aprons. Icarus 176, 382–394. http://dx.doi.org/10.1016/j.icarus.2005.02.011.

Lucchitta, B.K., 1984. Ice and debris in the Fretted Terrain, Mars. J. Geophys. Res. 89,B409–B418. http://dx.doi.org/10.1029/JB089iS02p0B409.

Mackay, S.L. et al., 2014. Cold-based debris-covered glaciers: Evaluating theirpotential as climate archives through studies of ground-penetrating radar andsurface morphology. J. Geophys. Res. Earth Surf. 119. http://dx.doi.org/10.1002/2014JF003178, 2014JF003178.

Madeleine, J.-B. et al., 2009. Amazonian northern mid-latitude glaciation on Mars: Aproposed climate scenario. Icarus 203, 390–405. http://dx.doi.org/10.1016/j.icarus.2009.04.037.

Madeleine, J.-B. et al., 2014. Recent Ice Ages On Mars: The role of radiatively activeclouds and cloud microphysics. Geophys. Res. Lett. 41, 4873–4879. http://dx.doi.org/10.1002/2014GL059861.

Malin, M.C. et al., 2007. Context Camera Investigation on board the MarsReconnaissance Orbiter. J. Geophys. Res. 112, E05S04. http://dx.doi.org/10.1029/2006JE002808.

Mangold, N., 2003. Geomorphic analysis of lobate debris aprons on Mars at MarsOrbiter Camera scale: Evidence for ice sublimation initiated by fractures. J.Geophys. Res. 108, 8021. http://dx.doi.org/10.1029/2002JE001885.

Marchant, D.R., Head III, J.W., 2007. Antarctic dry valleys: Microclimate zonation,variable geomorphic processes, and implications for assessing climate changeon Mars. Icarus 192, 187–222. http://dx.doi.org/10.1016/j.icarus.2007.06.018.

Marchant, D.R., Mackay, S.L., Lamp, J.L., Hayden, A.T., Head, J.W., 2013. A review ofgeomorphic processes and landforms in the Dry Valleys of southern VictoriaLand: implications for evaluating climate change and ice-sheet stability. In:Hambrey, M.J., Barker, P.F., Barrett, P.J., Bowman, V., Davies, B., Smellie, J.L.,Tranter, M. (Eds.), Antarctic Palaeoenvironments and Earth-Surface Processes,vol. 381. Geological Society, London, Special Publications, pp. 319–352. http://dx.doi.org/10.1144/SP381.10.

McEwen, A.S. et al., 2007. Mars Reconnaissance Orbiter’s High Resolution ImagingScience Experiment (HiRISE). J. Geophys. Res. 112, E05S02. http://dx.doi.org/10.1029/2005JE002605.

McGill, G.E., 2002. Geologic map transecting the highland-lowland boundary zone,Arabia Terra, Mars: Quadrangles 30332, 35332, 40332, and 45332: U.S.Geological Survey Geologic Investigations Series I-2746.

Milliken, R.E., Mustard, J.F., Goldsby, D.L., 2003. Viscous flow features on the surfaceof Mars: Observations from high-resolution Mars Orbiter Camera (MOC)images. J. Geophys. Res. 108, 5057. http://dx.doi.org/10.1029/2002JE002005.

http://dx.doi.org/10.1130/G24382A.1

http://dx.doi.org/10.1130/G24382A.1

http://dx.doi.org/10.1029/2009GL037472

http://dx.doi.org/10.1029/2009GL037472













http://dx.doi.org/10.1038/nature02114





http://dx.doi.org/10.1029/2005GL024360



http://dx.doi.org/10.1126/science.1164246

http://dx.doi.org/10.1126/science.1164246

http://dx.doi.org/10.1002/jgre.20053

http://dx.doi.org/10.1002/jgre.20053










http://dx.doi.org/10.1029/2008JE003318


http://dx.doi.org/10.1002/2015GL063219




http://dx.doi.org/10.1029/JB089iS01p0C336

http://dx.doi.org/10.1029/JB089iS01p0C336

http://dx.doi.org/10.1016/j.geomorph.2010.11.001




http://dx.doi.org/10.1029/2002GL01539

http://dx.doi.org/10.1029/2002GL01539



http://dx.doi.org/10.1029/2008GL035501



http://dx.doi.org/10.1029/2008JE003264

http://dx.doi.org/10.1029/2006JE002852

http://dx.doi.org/10.1029/2006JE002852




http://dx.doi.org/10.1002/2014JE004685

http://dx.doi.org/10.1002/2014JE004685



http://dx.doi.org/10.1029/JB089iS02p0B409

http://dx.doi.org/10.1002/2014JF003178

http://dx.doi.org/10.1002/2014JF003178



http://dx.doi.org/10.1002/2014GL059861

http://dx.doi.org/10.1002/2014GL059861

http://dx.doi.org/10.1029/2006JE002808

http://dx.doi.org/10.1029/2006JE002808

http://dx.doi.org/10.1029/2002JE001885




http://dx.doi.org/10.1029/2005JE002605

http://dx.doi.org/10.1029/2005JE002605

http://dx.doi.org/10.1029/2002JE002005


Moratto, Z.M., Broxton, M.J., Beyer, R.A., Lundy, M., Husmann, K., 2010. Ames stereopipeline, NASA’s Open Source Automated Stereogrammetry. Lunar Planet. Sci.41. Abstract 2364.

Morgan, G.A., Head III, J.W., 2009. Sinton crater, Mars: Evidence for impact into aplateau icefield and melting to produce valley networks at the Hesperian–Amazonian boundary. Icarus 202, 39–59. http://dx.doi.org/10.1016/j.icarus.2009.02.025.

Morgan, G.A., Head III, J.W., Marchant, D.R., 2009. Lineated valley fill (LVF) andlobate debris aprons (LDA) in the Deuteronilus Mensae northern dichotomyboundary region, Mars: Constraints on the extent, age and episodicity ofAmazonian glacial events. Icarus 202, 22–38. http://dx.doi.org/10.1016/j.icarus.2009.02.017.

Morgenstern, A. et al., 2007. Deposition and degradation of a volatile-rich layer inUtopia Planitia and implications for climate history on Mars. J. Geophys. Res.112, E06010. http://dx.doi.org/10.1029/2006JE002869.

Mustard, J.F., Cooper, C.D., Rifkin, M.K., 2001. Evidence for recent climate change onMars from the identification of youthful near-surface ground ice. Nature 412,411–414. http://dx.doi.org/10.1038/35086515.

Neukum, G. et al., 2004. Recent and episodic volcanic and glacial activity on Marsrevealed by the High Resolution Stereo Camera. Nature 432, 971–979. http://dx.doi.org/10.1038/nature03231.

Pierce, T.L., Crown, D.A., 2003. Morphologic and topographic analyses of debrisaprons in the eastern Hellas region, Mars. Icarus 163, 46–65. http://dx.doi.org/10.1016/S0019-1035(03)00046-0.

Plaut, J.J. et al., 2009. Radar evidence for ice in lobate debris aprons in the mid-northern latitudes of Mars. Geophys. Res. Lett. 36, L02203. http://dx.doi.org/10.1029/2008GL036379.

Robbins, S.J., Hynek, B.M., 2011. Distant secondary craters from Lyot crater, Mars,and implications for surface ages of planetary bodies. Geophys. Res. Lett. 38,L05201. http://dx.doi.org/10.1029/2010GL046450.

Robbins, S.J. et al., 2013. Large impact crater histories of Mars: The effect of differentmodel crater age techniques. Icarus 225, 173–184. http://dx.doi.org/10.1016/j.icarus.2013.03.019.

Schaefer, E.I., Head, J.W., Kadish, S.J., 2011. Vaduz, an unusual fresh crater on Mars:Evidence for impact into a recent ice-rich mantle. Geophys. Res. Lett. 38,L07201. http://dx.doi.org/10.1029/2010GL046605.

Schon, S.C., Head, J.W., Milliken, R.E., 2009. A recent ice age on Mars: Evidence forclimate oscillations from regional layering in mid-latitude mantling deposits.Geophys. Res. Lett. 36, L15202. http://dx.doi.org/10.1029/2009GL038554.

Sharp, R.P., 1973. Mars: Fretted and chaotic terrains. J. Geophys. Res. 78, 4073–4083. http://dx.doi.org/10.1029/JB078i020p04073.

Soare, R.J. et al., 2013. Sub-kilometre (intra-crater) mounds in Utopia Planitia, Mars:character, occurrence and possible formation hypotheses. Icarus 225, 982–991.http://dx.doi.org/10.1016/j.icarus.2012.06.003.

Squyres, S.W., 1978. Martian fretted terrain: Flow of erosional debris. Icarus 34,600–613. http://dx.doi.org/10.1016/0019-1035(78)90048-9.

Squyres, S.W., 1979. The distribution of lobate debris aprons and similarflows on Mars. J. Geophys. Res. 84, 8087–8096. http://dx.doi.org/10.1029/JB084iB14p08087.

Swanger, K.M. et al., 2010. Viscous flow lobes in central Taylor Valley, Antarctica:Origin as remnant buried glacial ice. Geomorphology 120, 174–185. http://dx.doi.org/10.1016/j.geomorph.2010.03.024.

Tanaka, K.L., Skinner, Jr., J.A., Hare, T.M., 2005. Geologic Map of the Northern Plainsof Mars. U.S. Geological Survey Scientific Investigations Map 2888.

van Gasselt, S., Hauber, E., Neukum, G., 2010. Lineated valley fill at the martiandichotomy boundary: Nature and history of degradation. J. Geophys. Res. 115,E08003. http://dx.doi.org/10.1029/2009JE003336.

Watters, T.R., McGovern, P.J., Irwin III, R.P., 2007. Hemispheres apart: The crustaldichotomy on Mars. Ann. Rev. Earth Planet. Sci. 35, 621–652. http://dx.doi.org/10.1146/annurev.earth.35.031306.140220.

Werner, S.C., 2008. The early martian evolution—Constraints from basin formationages. Icarus 195, 45–60. http://dx.doi.org/10.1016/j.icarus.2007.12.008.

Werner, S.C., Tanaka, K.L., 2011. Redefinition of the crater-density and absolute-ageboundaries for the chronostratigraphic system of Mars. Icarus 215, 603–607.http://dx.doi.org/10.1016/j.icarus.2011.07.024.

Wilson, L., Head, J.W., 2009. Tephra deposition on glaciers and ice sheets on Mars:Influence on ice survival, debris content and flow behavior. J. Volc. Geotherm.Res. 185, 290–297. http://dx.doi.org/10.1016/j.jvolgeores.2008.10.003.

Wrobel, K., Schultz, P., Crawford, D., 2006. An atmospheric blast/thermal model forthe formation of high-latitude pedestal craters. Meteorit. Planet. Sci. 41, 1539–1550. http://dx.doi.org/10.1111/j.1945-5100.2006.tb00434.x.








http://dx.doi.org/10.1029/2006JE002869

http://dx.doi.org/10.1038/35086515



http://dx.doi.org/10.1016/S0019-1035(03)00046-0

http://dx.doi.org/10.1016/S0019-1035(03)00046-0

http://dx.doi.org/10.1029/2008GL036379

http://dx.doi.org/10.1029/2008GL036379

http://dx.doi.org/10.1029/2010GL046450



http://dx.doi.org/10.1029/2010GL046605

http://dx.doi.org/10.1029/2009GL038554

http://dx.doi.org/10.1029/JB078i020p04073


http://dx.doi.org/10.1016/0019-1035(78)90048-9

http://dx.doi.org/10.1029/JB084iB14p08087

http://dx.doi.org/10.1029/JB084iB14p08087



http://dx.doi.org/10.1029/2009JE003336

http://dx.doi.org/10.1146/annurev.earth.35.031306.140220

http://dx.doi.org/10.1146/annurev.earth.35.031306.140220



http://dx.doi.org/10.1016/j.jvolgeores.2008.10.003

http://dx.doi.org/10.1111/j.1945-5100.2006.tb00434.x

extensive middle amazonian mantling of debris …extensive middle amazonian mantling of debris...

Documents