Sedimentary Geology 226 (2010) 29–41

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Sedimentary Geology

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Reconstruction of the Early Mérida, pre-LGM glaciation with comparison to LateGlacial Maximum till, northwestern Venezuelan Andes

William C. Mahaney a,⁎, Volli Kalm b, John Menzies c, Michael W. Milner d

a Quaternary Surveys, 26 Thornhill Ave., Thornhill, Ontario, Canada L4J 1J4b Institute of Ecology and Earth Science, Tartu University, Tartu 51014, Estoniac Departments of Earth Sciences & Geography, St. Catharines, Ontario, Canada L2S 3A1d Michael W. Milner Consulting, 182 Gough St., Toronto, Ontario, Canada

⁎ Corresponding author.E-mail addresses: arkose@rogers.com (W.C. Mahane

jmenzies@brocku.ca (J. Menzies), Michael.milner@symp

0037-0738/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.sedgeo.2010.02.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 October 2009Received in revised form 22 January 2010Accepted 15 February 2010Available online 24 February 2010

Communicated by M.R. Bennett

Keywords:Clast fabric analysisScanning Electron MicroscopyTill analysisMicromorphologyIcecap reconstruction

Valley glaciers have beenwell studied in the northwestern Venezuelan Andes over the last 20-year periodwithmost sourcing reconstructions limited to the Last Glacial Maximum (LGM) and confined to the mapping ofmoraine positions and the detailed analysis of pedostratigraphic sequences reaching back into the MiddleWisconsinan (Weichselian) Glaciation. Using bedding macrofeatures, fabric analysis (magnetic azimuths)with mirror images, clast inclination andmicrotextural evidence determined with the FE-SEM (Field EmissionScanning ElectronMicroscopy) and EDS (Energy Dispersive Spectroscopy), andmicromorphology of the PED5composite section of Early to Late Mérida age was analyzed to reconstruct the till succession and glacialdynamics. The fabric is used to reconstruct the build-up of valley ice in the lowerMucuñuque Valley, the icecapspreading over the upper catchments of neighbouring drainage basins toward the headwaters of theMucuchache Valley over a time frame estimated to be b90 ka and lasting to∼60 ka.Microtextural analysis wasinvoked to determine if physical differences on quartz and garnet mineral surfaces could be determinedbetween various diamictons such as subglacial, flow andmeltout tills, and outwash. Sample identification wasmade optically and with EDS. The evidence supports the theory that a substantial ice mass – a virtual icecap –

existed during the EarlyMérida stade of the last glaciationwith the icemass originating and decaying from theupper Mucuñuque Catchment south over some 40° of arc. This is the first evidence for an icecap complex inthe northwestern Andes. Comparison between Early and Late Mérida glacial styles indicates that, in theMucuñuque–Mucuchache corridor of the Mérida Andes glaciation in the LGM was catchment restricted withno evidence of icecap growth.

y), volli.kalm@ut.ee (V. Kalm),atico.ca (M.W. Milner).

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The glacial geology of the Mérida Andes has been studied bynumerous researchers for over a century starting with Sievers (1888)and Jahn (1925, 1931), continuingwithRoyo yGomez (1956, 1959) andSchubert (1970, 1972, 1974a,b, 1984), amongst others. All earlyworkersdescribed glacial features down to near 3000 ma.s.l. elevation andpostulated two stages of the last glaciation; an older stade reaching tob3000 m and a younger one reaching 3500–3600 ma.s.l. Recently,Mahaney et al. (2001a, 2001b, 2007, 2008)) interpreted the stratigraphyin several catchments in both the eastern and western ranges of theMérida Andes and documented the type section for the MéridaGlaciation, originally named by Schubert (Mahaney et al., 2001b).While the distribution of landforms and their stratigraphic settings hasbeen established for the Last Glacial Maximum (Mahaney and Kalm,1996;Mahaney et al., 2007, 2008, 2009) limited attention has been paid

to the early stade of the last glaciation, the evidence forwhich has eitherbeen eradicated by erosion or buried by younger ice advances. In oneselected area described herein (PED5 section), clast fabric and grainanalysis is used to determine if glaciers were limited to catchments or ifovertopping of drainage divides might indicate the presence of discreteicecaps or valley glaciation with narrow cross valley axes and henceconstricted flow. The PED5 section, in the eastern Andes, provides newevidence for the growth anddemiseof an ice cap over the northernflankof the easternMérida Andes during the early stade of the last glaciation.The question of age controls for the growth of the Early Mérida stade isimpossible to ascertain but is assumed to be less than Isotope Stage 5 orapproximately b90 ka. In the case of the PED5 section, theminimumageof Early Mérida till is known to be ∼60 ka BP by AMS dating (Mahaneyet al., 2001c; Dirszowsky et al., 2005).

2. Regional overview

The Sierra Nevada de Mérida and its northern branch the Sierra deSanto Domingo were both glaciated during the two stades of theMérida Glaciation (see Mahaney et al., 2001a). The latter stade of this

Fig. 1. Location of the PED5 section in the Eastern Range of the Mérida Cordillera, northwestern Venezuela. The leading edge of a thin wedge of LGM ice in the upper fan structure ofPED5 is marked.

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glaciation culminated around 18 ka, ending about 13 ka as indicatedby radiocarbon and correlation with the Cordillera Oriental ofColombia (Schubert, 1974b; Schubert and Valastro, 1974). Character-ized by two advances, the Mérida Glaciation (equivalent to theWisconsinan, Weichselian) began with a poorly constrained earlystade that left either till buried by the LGM (as at the PED5 locality) ordispersed patches of glacial diamicton eroded by runoff. While theLGM dominates there are only a few drainage basins where the earlystade can be studied, either buried under Late Mérida till andassociated deposits or on the surface. One site is at La Zerpa (northeastquadrant of the insetmap in Fig. 1) where ice fromVictoria, Aguila andLa Zerpa drainage basins coalesced and flowed northeast along theSanto Domingo River. The second locality is at La Canoa in theWesternMérida Andeswhere the type section of theMérida Glaciationwas firstdocumented byMahaney et al. (2001a). In both areas it is only possibleto study surface moraine features as deep excavations similar to thePED5 section are not available.

The Mucuñuque and Mucuchache catchments shown on Fig. 1 arenearly linear fault-controlled basins with headwaters on the westernslopes of PicoMucuñuque (4609 m a.s.l). Trending NW, the valleys aremarked by a number of transverse bedrock bars punctuated aboutevery 100–150 m by drops of ∼100 m elevation. The MucuñuqueValley debouches into Lago Mucubaji at 3600 m a.s.l. whereas theMucuchache empties into the Rio Chama and ultimately into LakeMaracaibo (Fig. 1). Prominent bogs are found at all levels from4000 ma.s.l., where the valley bottoms are narrowest at∼150 m, to thelower reaches where most valleys widen to ∼250 m. Waterfalls arepresent at major bedrock bars with the most prominent one (30-mdrop) present at the 3700-m contour in the Mucuñuque Basin. Thebedrock consists of felsic gneiss throughout with minor outcrops ofgranite, rendering the lithology nearly uniform, a situation in whichmineral variation in tills does not assist in identifying source areas.Minor variations in spessartine garnet seen at other sections north ofPED5 were not observed in samples described herein.

3. Methods

A sectionwas dug to depth and cut back into the cliff face to between0.5 and 0.75 m to expose fresh moist sediment unaffected bydewatering processes. Freeze thaw processes are unimportant in thetropical mountains up to 5000 m as the frost penetration in drysediment reaches only a fewcmand inwet sediment freezingoccurs to adepth of ∼2 cm when the ambient air temperature reaches −10 °C.Experimental temperature data for similar sediment is available inChapter 12 of Mahaney (1990). Facies were described and sedimentsamples collected for laboratory analysis including particle size, SEManalysis, and sediment chemistry. Particle size analysis was completedby hydrometer following procedures documented by Day (1965).Microscopic analysis includes observation by petrographic microscope(Menzies et al., 2006) and Field Emission Scanning ElectronMicroscope(FE-SEM) and Energy-Dispersive Spectrometry (EDS) (Mahaney, 2002).Preparation of samples for SEM analysis follows procedures outlined byVortisch et al (1987) Interpretation of SEM microfabrics is based onKrinsely and Doornkamp (1973), Mahaney (2002) and Strand et al.(2003). Fabric analysis of 50 clasts at each stationwas completedusing aBrunton compass and all azimuths are magnetic. Preparation ofpaleosols for AMS dating, the oldest dates providing minimum ageson the Early Mérida Till, was described previously by Mahaney et al.(2001c).

4. Results

4.1. Sections

Unequivocal evidence for pre-LGMMérida Glaciation had to awaitdetailed field mapping and stratigraphic analysis at the base of the

PED5 section (Fig. 1 for location; Fig. 2 for site details; Fig. 3 forstratigraphy) and radiocarbon dating of a suite of paleosols formed inglaciolacustrine sediments which are among the most ancientmaterials dated at Isotrace Laboratories at the University of Toronto(Mahaney et al., 2001c). The chronostratigraphy of the mid to upperPED5 section placed a minimum age on the glaciolacustrine sequenceat ∼60 ka; thus, while a maximum age for the older tills and outwashburied at the base of the PED5 section eludes us still, theminimum ageargues for an Early Mérida stratigraphic window.

4.2. The tills

Differentiation of glacial diamicton facies (tills) – lodgement,deformation, meltout and flowtill – was made partly on the basis offield texture (later amplifiedbyparticle size analysis), presence/absenceofweak stratification, clast size and debris-richness. Reliance on particlesize is only somewhat useful in distinguishing between tills asillustrated by units A and C (meltout vs. subglacial till) where slightdifferences in silt content occur. Even the two flowtills (units B and D)vary considerably in silt content. The presence of varying degrees ofstratification in units A, B and D markedly contrast the absence of it inunits C and E despite some similarities in particle size distributionsacross all facies. Clast size and debris richness are prime indicators ofindividual facies with the gradation from granule (2–4 mm) to pebble/cobble (∼250 mm) size increasing from unit A (smallest), and B and D(mid size) to C and E (largest),with the latter unit containing somewhatsmaller clasts than arepresent in C. Thehigh frequencyof granules in themeltout till and large clast size and high frequency in the subglacialtill precluded the collection of multiple samples for micromorphologyanalysis. Only flowtill D and thin deformation beds in C providedmicrosites where boxes for micromorphological analyses could becollected and only after several attempts to find microsites wherepebbles did not interfere with collection.

4.3. Particle size

Particle size distributions were analyzed to determine uniformity/contrasts within the sediments recorded in the section. In the PED5Section unit A, outwash beds contain about the same particle sizedistribution as the base of unit B above, a meltout till. Texturally, lowerunit B is sand. Within the flowtill at PED5, sand dominates within thelower member, diminishing considerably in the upper member as siltand clay increase, possibly a response to excessive glacial grinding andabrasion (Mahaney, 2002). Texturally, unit B grades from sand in thelower member to sandy silt in the upper member. Evidence of grindingin theupper bedsmay relate to reducedporewater content as discussedbelow.

The lodgement till in unit C (partly deformed in places) dominateswith sand throughout the lower, middle and upper members (seevan der Meer et al., 2003; Menzies et al., 2006 for deformation tillcharacteristics). The deformation in C is inferred from small contortedstratification making up about b15 percent of the exposed face.Texturally unit C is similar to the subglacial unit in E with the uppermember showing∼70% silt, possibly the result of glacial grinding duringthe late stage of the earlyMéridaGlaciation. The absence of ductile shearin diamictonsC andE, possibly argues for a lodgement designation. Clastfabric strength in units C and E, with the presence ofmainly slider clastsand the absence of a multi-modal orientation argues further for slowmoving ice (Lian et al., 2003).

The diamicton in D is mainly sandwith little variation between thelower and upper members and is slightly stratified with minor sortingthroughout. The higher sand content compared with unit B mayrepresent higher pore water discharge within the glacier. In fact, itmay also represent liquified supraglacial till released onto the beds ofunit C near the terminus of the ice.

Fig. 2. A. Lowermost leg of the PED5 section showing a succession of lacustrine deposits complete with interbedded paleosols dating from ∼30 ka to ∼60 ka (Mahaney et al., 2001c).These paleosols have a Spodosolic (podzols) signature indicating a reducedmean annual temperature (MAT) compatible with recent pollen reconstructions. Prior to ∼60 ka periodicfluctuations of the edge of the EarlyMérida ice produced a succession of outwash, meltout, flowtill and lodgement till. After withdrawal of the EarlyMérida Glacier the highland basinfilled with water producing a succession of glaciofluvial gravels and till of LGM age. B. View from the south looking across the Pedregal Sandur toward the PED5 section.

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4.4. Clast fabric

The section was subjected to clast fabric analysis, where practical, tocollect clast orientation. Sandy beds or beds with skeletons of wholerock were avoided, the focus of attention placed on pebble beds withenough fine matrix material to constrain the a-axis orientation to givean approximation of the glacial source. Particle size curves were

analyzed for each sampling station to insure the degree of sorting (ifany) and pebbles were scrutinized for the presence of striations. Onaverage, pebbles with striated surfaces had azimuths that closelyapproximate to clast orientation; while about 10% of clasts containedcross striae with about half that number showing weathered crossstriations indicating they probably emanated from pre-Mérida icemovement.

Fig. 3. PED5 Section showing grain size distributions down section and till fabric for deposits overlying the lowermost outwash bed.

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The multi-station fabric for PED5 is shown in Fig. 3. Above theoutwash beds (unit A) at the base of the section, a glacial diamict,possibly meltout, in unit B shows a quadric-bimodal fabric centered on

around 325o azimuth which is consistent with an approximation of theregional slope at the site. Muchmore informative, perhaps, is the fabricwithin unit C, the first possible subglacial till in the sequence, which

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indicates a narrowly constrained bimodal fabric at base of 20o azimuth,with ice spillingout of thepresentQda. El Caballo, or the lowerMucubajiValley. The main north-northeast fabric probably represents the mainice stream. The highly constrained nature of the 045° vector in the samebed possibly argues for a glaciotectonic or syndeformational originpossibly due to compression during the growth of the icecap or possiblyfrom dewatering effects. The middle bed in unit C indicates a shift to045o with an ice source farther to the southeast on the divide betweenboth valleys. The upper unit C records a slight shift to the north of 10degrees of azimuth to about 35o as the main flow vector (Fig 3).

Unit D in the sequence is a flowtill which suggests warming near theglacier margin either subglacial or supraglacial or passive ice movementand once again orientation, as at unit B, is along the regional slope centeredonorabout325–340oazimuth. ThemajordifferencebetweenunitsBandDis in the tightly constrained azimuthal direction recorded in D compared

Fig. 4. SEM of outwash (Unit A). A. Represented subangular and subround sands in unit A; Bwith partially distorted lattice possibly inherited from the Boconó Fault; D. Subangular quagrooves, percussion apron and minor preweathered surface to left; F. Subround quartz with

with the unconstrained azimuthal readings in unit B, presumably resultingfrommeltout and increased water release.

Unit E, the upper diamictic unit in the sequence, records an eastwardshift in orientation to 55o azimuth in the lower beds suggesting a shiftor growth of ice along the high cordillera between Qda. Mucubaji andQda. Mucuchache depicting the growth of an icecap along the crest ofthe range, with ice overriding major watershed divides between thevalleys that lie northeast of the study site.

Unit F, a thin diamict bracketed within the 16–17 m depth of PED5yields a bimodal fabric with the primary clast direction orientednorth–south and a secondary vector oriented northwest–southeast.While undated, this fabric is thought to have resulted from an advanceof ice at the beginning of the LGM (∼25 ka), sourcing from theMucuchache Valley to the south and emplaced by an ice spur thattopped the right lateral moraine in Mucuchache Valley.

. v-shaped percussion scars embedded in the upper right grain of A; C. Subround quartzrtz carrying inherited glacial crushing microfeatures; E. Angular quartz with radiatingpercussion fractures and soft weathered microfeatures.

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4.5. Scanning Electron Microscopy

4.5.1. Angularity/roundness variationsA testable hypothesis for degrees of angularity might predict

greater angularity in subglacial till where greater crushing is expected,grading down through other till types to outwash. An approximationof angularity vs. roundness using the Cailleux Index places theoutwash and a possible meltout till in the subangular mode, thesubglacial deformation in Unit C in high angular mode compared withUnit E. The possible flowtill in Unit D while containing some subroundgrains exhibits a subset of approximately 25% subangular grains.

4.5.2. Evidence of water transportEvidence ofwater transport is decidedlymoremarked, as expected,

in the outwash of Unit A (Fig. 4) and in the flowtill of Unit D than in theother tills. Waterborne grains in the outwash and in tills with higher

Fig. 5. SEM of subglacial till (Unit C). A. Deep grooves in quartz with minor etching on left sho(arrow) juxtaposed with severe fractured area; C. Angular quartz with deep grooves andpercussion cracks; E. Angular quartz with representative grooves, conchoidal cracks and m

fluidity and hence lower viscosity also carry v-shaped percussioncracks presumably from collision during turbulence. The tendency forchattermarks, first described by Folk (1975) to increase in flowtill andoutwash relative to deformation/lodgement till is also notable andmay relate to mechanical collision followed by weathering asdiscussed later.

4.5.3. Glacial crushing effectsOne of the most uncommon of crushing features observed on b5%

of the grains studied are chattermark trails described in Fig. 6 from thepossible subglacial till. In determination of glacial crushing it isimportant to try to distinguish severity of grain fracturing and abrasionfrom subglacial diamictons where it might be expected to be mostcommon in contrast to other tills and outwash units where wateraction might facilitate abrasion and rounding over fracturing process-es. As expected, virtually all glacial suites of grains show the hallmarks

ulder and minor new growth of quartz on top; B. Enlargement of new growth of quartzv-shaped percussion cracks; D. Enlargement of glacial grooves and minor v-shaped

inor v-shaped percussion cracks.

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of glacial crushing, including very infrequent conchoidal and subpar-allel fractures, glacial grooves, large block dislocation, glacial steps ofvarious kinds, and crescentic gouges to name the most commonobserved. Perhaps most strikingly the low percentage of glacialcrushing microfeatures observed on the grains studied is remarkable.Approximately50%of the subglacial till sample surfaces (Figs. 5, 6 and7)are covered with crushing microfeatures plus occasional v-shapedpercussion scars, the latter considered the hallmark of collision inwater(Mahaney, 2002). The non-subglacial till types and outwash exhibitcrushing microfeatures over considerably less of the grain surface orapproximately 25–30%.

4.5.4. Microtextures indicative of ice thicknessIf the frequency of microtextures is indicative of ice thickness as

suggested in other studies (Mahaney, 1995, 2002) the data indicatethat ice thickness might be on the order of b200 m. Distance oftransport, which in this instance is b6 km, also controls the degree towhich grains may impact one another, collisions that inducevibrations producing microfractures.

4.5.5. Abrasion microfeaturesAbrasion microfeatures leading to edge rounding are decidedly

more apparent in the outwash of Unit A and the flowtill of Unit D.Occasional bulbous edges were encountered very infrequently andmay relate to previous wind abrasion.

4.5.6. Age relationships and diagenetic effectsGrain surfaces studied by SEM show variations of fresh surfaces

among the various suites of glacial sediment studied. Fracture facesthat may relate to mechanical detachment from bedrock are commonthroughout in very low frequencies of occurrence (see Figs. 8B, C, 5C,

Fig. 6. SEM of subglacial deformation till (Unit E). A. Subround quartz with high frequencyquartz; D. Enlargement showing streams of percussion cracks (arrows).

9B and 7C). New growth of quartz in Fig. 5A-enlargement in Fig. 5B isanomalous and may predate glacial transport. All etching micro-features appear to be inherited from post-depositional chemicalweathering with no occurrences of preweathered grain surfacesmodified by renewed crushing (Mahaney, 2002).

4.6. Thin section analysis

4.6.1. Ped 5-Unit DThis diamicton unit, is a brown (10YR4/6), lightly weathered,

matrix supported sediment. The thin section contains a series ofpoorly developed clay bands (tile structures see van der Meer et al.,2003) as seen horizontally across the centre of the photomicrograph.There are many short distance lineations indicative of short readjust-ments – tensile repositioning – as the sediment was deformed. Thissediment has all the appearance of a glacial diamicton and is mostrepresentative of a till that has suffered slope movement either afteroriginal deposition or possibly syndepositionally — thus it is likely aflowtill (Fig. 10).

4.6.2. Ped 5-Unit CRepresentative of a subglacial till in Unit E, but differing from Unit

D in that it exhibits a degree of homogenization possibly due to highstrain deformation. The sample was taken from a pocket of large clast-free sediment, one of the few microsites where collection waspossible. The sediment is again a till with locally distinctive‘marbleized’ units, typical of intense deformation. The sediment alsohas been over-printed by a later deformation phase as shown bycross-cutting short-distance lineations. Given the similarity to Ped 5-unit D, it is possible that part of this till has been deformed during itsinitial emplacement, subsequently deformed a second time possibly

of percussion cracks; B. Enlargement showing arc-shaped steps (arrows); C. Subround

Fig. 7. SEM of subglacial deformation till (unit F). A. General frame of angular and subangular quartz; B. Very angular quartz with sharp edges and minor etching on left shoulder;C. Angular quartz with subparallel fractures and minor etched surfaces; D. Flat, tabular quartz with minor skin fractures; E. subround quartz with broken right lower edge, minorpercussion and conchoidal fractures; F. Enlargement showing chattermark stream and subparallel fractures.

37W.C. Mahaney et al. / Sedimentary Geology 226 (2010) 29–41

due to surface mass movement caused by overburden emplacement.There is a lack of any evidence of clay banding (till structures). The∼40 m thickness of overlying sediment in the PED5 section may havecontributed to normal stress loading leading to subsequent secondarydeformation (Fig. 11).

5. Discussion

Clast fabric data suggest a quadric-modal orientation in unit Bshowing possible reorientation of clasts upon meltout release. Thelower beds of the till show a similar particle size distributioncompared with other till units above but the upper beds in unit Bshow a large increase in silt suggesting active glacial grinding anddecreased water movement of clastic debris. Because intense grindingis thought to occur under relatively dry to low pore water conditionswith excessive point pressure needed for crushing it might be that thesilt content was produced up-glacier where pore water might be low.

With increasing pore water carrying the pressure dry grinding isexpected to decrease. Tropical glaciers today are almost always at thepressure melting point so it is impossible to know what temperatureprofile, and hence, pore water levels existed in these glaciers duringcolder stages of the Last Glaciation.

At the base of unit C, a bimodal fabric with a source at 020o alsoshows a tightly constrained spike northeast to southwest that ispossibly indicative of a glacial tectonically-induced clast reorientationor dewatering effects.

In the mid-unit C the locus of ice activity shifts to near 45o

indicating a thickening ice crest with the main ice mass moving about20o of azimuth, an orientation that steadies, becoming more tightlyconstrained in the upper beds of the unit. However, the upper beds ofunit C are higher in silt, possibly indicating increased subglacialgrinding, and also possibly followed by more water movement as theglacier at this location deposits a flowtill package (unit D). The flowtillin unit D demarcates clast orientation along a regional slope to the

Fig. 8. SEM of flowtill (Unit B). A. Predominantly subangular grains throughout in A–C; B. minor etched surface; C. Micro block breakage in places with minor new growth of quartz;D. Enlargement of micro block breakage area and new growth of quartz (arrow).

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north–northwest as in unit B, although azimuthally more tightlyconstrained. Particle size changes are slight when compared with thesubglacial till below possibly suggesting fluidization of the materialwas unaccompanied by changes in particle size.

The upper subglacial till (unit E) maintains a similar particle sizeratio in the lower and middle beds with increasing to high amounts ofsilt near the truncated upper limit of the deposit. Clast orientation in thelower beds of unit E indicates continued growth of an ice cap over therange extending from Pico Mucuñuque to the upper slopes ofMucuchache, 8 km to the south. A sharp clast azimuth orientationvariation in the upper beds of unit E, may be symptomatic of demise ofthis icecapwith the locus of ice activity centered again overMucuñuque.The Early Mérida stade ends as it started with a mean ice flow vectorcentered on 015o, similar to the starting point in the base of unit C.

Because the meltout till indicates contrasting flow vectors fromsouth to near east it is likely the early stage of ice growth at the base ofthe section was sourced out of the Mucuchache Valley into the broaddepression between Mucuchache and El Caballo Valley. Thereafter,clast orientation depicts ice coming in from the northeast or fromeither El Caballo or Mucuñuque valleys.

The upper till of LGM age, sandwiched between the 16 and 17 mlevels within a blanket of glaciofluvial sediment, yields a consistentbimodal fabric indicating themain ice vector was south to north, mostprobably, given the existence of lateral moraines in all main valleys,the glacier originated out of the Mucuchache Valley, overtopping theright lateral moraine in the catchment. While the age of this LGMdeposit is poorly constrained it must be less than ∼30 ka and morethan ∼12 ka, based on radiocarbon dated beds below and above at thetop of the upper glaciofluvial deposit in the section Mahaney et al.(2001c).

What is most startling is the thinness of glacial beds as shown inFig. 3, especially at the base of the section. In the lower section, tillthickness is barely 2.5 m; with the added outwash it is only 3.5 m in

thickness, the entire mass deposited in an estimated 30 k yrs. Withwater issuing out from the lower outwash deposit excavation of theentire thickness of the deposit was not possible. Hence, the reporteddepth is a minimum. Still, the overall thickness of till units andoutwash is thin given the 5–7 km transport distances involved and the30 k time slot for emplacement. Also, ice/water erosion may haveremoved some sediment during oscillations of the Early Mérida ice,leaving only a minimal record as recorded herein.

The thinness of the upper LGM till body (b1 m; unit F in Fig. 3)argues for a narrowly constrained time of deposition or depositionover a longer time frame followed by erosion from the leading edge ofan ice spur emanating out of the nearby Mucuchache Valley, an eventpreceded and followed by deposition of a massive glaciofluvial fan, avirtual sandur (Mahaney and Kalm, 1996). The actual surface form ofthis till sheet is hidden by overlying glaciofluvial sediment but itappears related to the presence of two shallow moraines 0.4 and0.6 km, respectively, lying off to the southeast; both deposits areconsidered to be recessional moraines deposited by retreat of LGM iceduring the waning stages of the LGM.

Pebble fabric data previously analyzed by Dowdeswell and Sharp(1986) are broadly correlative with results obtained here. Their basalice fabrics are similar to our subglacial till fabric measurements withsome pebbles in their sections showing wider azimuthal variations byabout 20° of arc, and with their deformed till clast inclinations muchgreater than reported herein. However, ourmeltout till sample is closeto examples they report. In fact our meltout till buried beneath theflowtill is broadly similar to facies variations reported in front of theMatanuska Glacier in Alaska (Lawson et al., 2002).

The interpretation of clast fabric analyses reported here are at oddswith the interpretation of clast fabric as a discriminator of glacialfacies as reported by Bennett et al. (1999), a synthesis of 111 clastfabric analyses reported elsewhere. The conclusion by Hambrey et al.that clast fabric alone cannot be used to discriminate glacigenic facies

Fig. 9. SEM of flowtill (Unit D). A. Subround quartz with minor etching; B. Angular quartz with sharp edges and minor grooves; C. Subround quartz with block fractures, deep grooveand released wedges; D. Cracked quartz with deep grooves, broken edge and numerous v-shaped percussion cracks; E. Subround quartz with scale cracks, subparallel and conchoidalfractures; F. Subround quartz with deep grooves, crescentic gouges and chattermarks (arrow).

39W.C. Mahaney et al. / Sedimentary Geology 226 (2010) 29–41

seems in need of reinterpretation given the fabric data given here,especially since it is supported my SEM and particle size databases.The fabric and other sedimentary data reported herein clearly indicatethe growth and demise of Early Mérida ice in the northwestern Andes.Admittedly, results from one section (PED5) posit the possibility of‘chance encounters’ but offer a standard against which future workmight be tested if/when other sections of similar agemight be located.

The chattermark microfeatures observed in the upper subglacialtill have been ascribed to glacial crushing (Krinsely and Doornkamp,1973) and to weathering (Mahaney, 2002), but recent experimentalwork (Peterknecht and Tietz, in press) showed that collision ofparticles by mechanical impulses produces lattice disruption, theseverity of which depends upon mass/energy relationships. Chatter-marks are seen to develop, first frommechanically induced fracturingfrom abrasion, and second, from scratching during transport followedthereafter by dissolution due to chemical weathering. In this instancethe occurrence in subglacial till indicates this is where mechanicalaction would be most intense.

6. Conclusions

The clast fabric analysis, taken from a partially age-constrained tillstack, dating from the early stade of the last glaciation in the Andes,amongst other evidence appears to show a change in the locus of iceactivity varying over 40° of arc over ∼30 k years. The initial onset ofglaciation is unknown but inferred from other ages documented else-where to be approximately ∼90 ka; the end of the early phase to be∼60 ka, all radiometrically dated higher up in the PED5 Section. The earlyphase of glacier growth began as expected on the highest peak in thenorthern part of the Mérida Andes on Pico Mucuñuque (4609 ma.s.l.),the locus of ice thickness spreading slowly southward about ∼8 km, areversal taking place near termination of the stadial event.

Clast fabric analysis of the till in the upper part of a large sandursection seems to show that ice did not always invade this high Andeanbasin from the north-northeast to northeast, but during the LGMmayhave advanced from the south or south-southeast out of theMucuchache Valley, an adjoining catchment.

Fig. 10. T-section of Unit D, flowtill showing evidence of short distance lineations and poorly developed clay bands. The clay content is only slightly elevated compared to the T-section in Fig. 11 (subglacial deformation till).

Fig. 11. T-section in Unit C, subglacial till showing evidence of homogenization, possibly the result of high strain deformation. Marbleization is indicative of significant deformation asseen in other examples worldwide.

40 W.C. Mahaney et al. / Sedimentary Geology 226 (2010) 29–41

41W.C. Mahaney et al. / Sedimentary Geology 226 (2010) 29–41

Microtexture analysis of quartz grains in all till types indicate onlyslight differences in rounding and abrasion. Water transport is foundin all till but with a higher percentage frequency in the meltout tillrelative to subglacial till and flowtills. Variations in angularity vs.roundness range from only slight increases in subglacial till to flowand meltout tills. The overall conclusion is that while tropical ice wasclimatically forced during a drawdown of colder temperatures duringice age perturbations water transport left a considerable imprint ontransported grains.

The thinness of till/outwash deposits in the northwesternVenezuelan Andes is similar to well documented moraines elsewherein the tropical mountains, especially on Mt. Kenya (Mahaney, 1990).

Acknowledgements

This research was funded by Quaternary Surveys, Toronto. Financialsupport to VK by Estonian Research Foundation No. 0182530s03is acknowledged. Critical comment on an draft ms by Jaap van derMeer (University of London) and an anonymous reviewer are gratefullyappreciated. The SEMmosaics were prepared by Barbara Kapran (YorkUniversity).

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