Journal of Sedimentary Research, 2007, v. 77, 324339

Research Article

DOI: 10.2110/jsr.2007.029

HIGH-RESOLUTION FACIES ANALYSES OF MUDSTONES: IMPLICATIONS FOR PALEOENVIRONMENTALAND SEQUENCE STRATIGRAPHIC INTERPRETATIONS OF OFFSHORE ANCIENT

MUD-DOMINATED SUCCESSIONS

JOE H. S. MACQUAKER,1 KEVIN G. TAYLOR,2 AND ROB L. GAWTHORPE11School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, U.K.

2Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Manchester, M1 5GD, U.K.

ABSTRACT: The extent of lithofacies variability in fine-grained sedimentary rocks is very poorly known in comparison to thatpresent in coarser grained clastic and carbonate successions. The absence of this information means that sediments present oncontinental shelves are rarely considered as integrated systems because fine-grained facies (such as shales) are mostly excludedfrom sophisticated, regional facies models. To shed light upon lithofacies and grain size in shale-dominated successions, so thatthey can be incorporated into shelf-wide depositional models, the Blackhawk Formation of the Mancos Shale (Campanian age),exposed in the Book Cliffs, Utah was investigated using combined field, whole-rock geochemical, optical, and electron opticalmethods. These sediments show systematic grain-size variations, are intensely bioturbated, and composed predominantly ofdetrital clays (mainly dioctahedral micas), quartz, and feldspar with minor pyrite and organic matter. They are organized intovery thin (, 10 mm), upward-fining, genetic beds; they exhibit both systematic lateral (103 m scale), and vertical (1022 to100 m scales) lithofacies variability. Preferentially cemented units occur close to sequence boundaries (unconformities). Thesecemented units typically contain a very different detrital assemblage (including significant quantities of chlorite) from the restof the succession and are located close to levels where there are marked stacking-pattern discontinuities.

Overall, lithofacies variability is interpreted in terms of deposition occurring on an oxic continental shelf, with the coarser-grained facies being deposited in proximal settings (offshore-transition zone), in contrast to the finer-grained units that weredeposited in more distal environments (offshore zone). Storms are interpreted to have been the dominant mechanism dispersingthe sediment. Once deposited, the surface layers of the sediment were intensely reworked by burrowing organisms. Preferentialpreservation of ichnogenera from mid- and lower tiers suggest that the depositional environment was energetic and erosioncommonly removed the surface sediment layers, prior to deposition of thin (1022 m) storm beds. The larger scale (100 m)upward-coarsening units that are composed of stacked beds, are interpreted to be parasequences.

Preferential cementation, at levels where large-scale stacking patterns change, is interpreted to occur at horizons where therewere breaks in sediment accumulation, and bacterial metabolic processes were able to supply sufficient solutes to filluncompacted pore space with cement. The presence of a very different detrital assemblage in these units suggests thatwinnowing occurred. Despite their relatively innocuous appearance in the field, these surfaces are likely to be significant bypasssurfaces, over which sediment was delivered to the deeper parts of the basin during times of relative lowstand of sea level.

INTRODUCTION AND AIMS

Architectural elements present within fine-grained siliciclastic succes-sions (shales), and controls on their lithofacies variability are poorlyunderstood, in comparison to our understanding of these processes incoarse clastic and carbonate successions (e.g., Bohacs 1998; Potter et al.2005). This lack of understanding exists because most investigations ofthis variability have concentrated on interpreting data obtained froma relatively narrow range of techniques, employing either whole-rockgeochemical (e.g., elemental analyses coupled with mineralogical deter-minations), component-specific geochemical (e.g., organic carbon con-tent, stable isotopes in carbonates), or paleontological methods. Despitebeing apparently fit-for-purpose, and their wide application bygeologists investigating facies variability in fine-grained sedimentarysuccessions, these datasets are actually less than ideal because keyinformation such as grain-size, origin of different components, and

sedimentary textures are typically not directly obtained using thesetechniques. The absence of these data inevitably mean that researchers areforced to make complex and subtle extrapolations, using sometimescontradictory proxies to interpret many of the fundamental controls onmudstone variability.

Fine-grained sediments, like the majority of sedimentary rocks,comprise three distinctive components in varying proportions (e.g.,Macquaker and Adams 2003). These components are derived fromsedimentary detritus entering the basin (allochthonous or detrital-derivedcomponents), organisms living within the water column as well as in thesurface sediment layers (autochthonous or productivity-derived compo-nents), and chemical (diagenesis-derived) components which precipitateeither in the water column, at the sedimentwater interface, or once thesediment has been buried. Once the various components had beenproduced, they were then available to be transported by mechanisms suchas storms, tides, and ice before being deposited in areas where

Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-324/$03.00

accommodation was available. After deposition, the sediment was theneither resuspended or colonized by burrowing organisms, prior to beingburied and experiencing a variety of diagenetic transformations. Here,early diagenetic processes were typically mediated by microbial metabolicprocesses prior to compaction, and by physicochemical processes deeperafter compaction. To obtain a comprehensive understanding of how thesedifferent processes interacted to produce an individual rock, it isnecessary to have information about bed and laminae thicknesses,descriptions of any depositional and diagenetic textures present, in-formation about bedding contacts, data on grain-size variability, andinformation about what processes generated the constituent grains. Once

these data have been obtained for an individual sample, it is then possibleto determine the controls on larger-scale temporal and spatial faciesvariability (architecture) by comparing genetically related samples withinthe context of a well-constrained stratigraphic framework.

Obtaining the crucial data to determine architectural and faciesvariability in coarse-grained sedimentary successions is relativelystraightforward (e.g., Hampson et al. 1999; Adams and Bhattacharya2005; Edwards et al. 2005). On their own, however, field-, geochemical-,and paleontological-based descriptions of fine-grained successions arerarely sufficient to perform a critical investigation of facies andarchitectural variability in fine-grained strata. In this setting difficultiesarise because fine-grained sediments exhibit relatively little obvious hand-specimen-scale variability and are highly susceptible to weathering,making it hard to observe primary fabrics in these rocks (e.g., Potter et al.1980). Moreover, relying on descriptions of individual units based uponfeatures that are readily obtainable in the field (such as color, beddingthicknesses, and weathering style) is commonly fraught with uncertainly.Uncertainties arise because these descriptors do not always varysystematically with key sedimentary parameters, such as grain-size,proportion of total organic carbon, proportion of calcareous phyto-plankton relative to diagenetic carbonate, and proportion of authigenic asopposed to detrital clay (e.g., Macquaker and Gawthorpe 1993). Tocompound these problems further, most field-based descriptions ofmudstones are vague about microtextures present, in as much as they arebeyond the resolution of even the most diligent geologists with handlenses!

Recent research on fine-grained sediments, combining field, optical,electron optical, and geochemical methods, utilizing systematic samplingtechniques and subsequent manufacture of unusually thin (20 mm)polished thin sections of the rocks, has revealed enormous amounts ofpreviously unavailable information (e.g., Potter et al. 1980; OBrien and

FIG. 1.Map showing location of the BookCliffs in Eastern Utah, and locations of sections ofMancos Shale that were logged and sampled.

FIG. 2. Stratigraphic setting of the studied succession of the Book Cliffs,Mancos Shale and Blackhawk Formation.

MANCOS SHALE SEDIMENTOLOGY AND STRATIGRAPHY 325J S R

FIG. 3. Field photographs showing outcropexpression of the studied Mancos Shale time-equivalent to the Grassy Member of the Black-hawk Formation, location of stratal surfaces(mapped in the field) and location of the sampledsections. A) Thompson Pass. B) Blaze Canyon.C) Coquina Wash. (SB/ts 5 sequence bound-ary/transgressive surface). (This figure is repro-duced in color in the digital version.)

326 J.H.S. MACQUAKER ET AL. J S R

Slatt 1990; Macquaker and Gawthorpe 1993; Macquaker 1994; Macqua-ker and Taylor 1996; Macquaker et al. 1998; Macquaker and Howell1999; Williams et al. 2001; Rohl et al. 2001; Schieber 2003; Macquakerand Keller 2005; Rohl and Schmid-Rohl 2005). With these methodsgeologists have been able to make direct observations of grain-size,determine the origin of individual components, and make microtexturalobservations of the rock without resorting to making unreliableextrapolations from proxy data (e.g., whole-rock geochemistry). Togetherthese direct observations, in conjunction with information obtained fromconventional paleontological and geochemical techniques, have allowedresearchers to significantly extend their understanding of architecturalelements present, and controls on lithofacies variability within siliciclasticmudstone successions. Such studies are important because . 65% (e.g.,Blatt 1970; Aplin et al. 1999) of the rock record at the Earths surface iscomposed of fine-grained sediments. Moreover, these rocks are majorsites of natural carbon sequestration, and their investigation providesinsight into processes occurring elsewhere on continental shelves. Theirstudy, for instance, enable predictions of facies variability downdip, onthe continental slope, and updip, in the middle and upper shoreface, to bemade.

The main aims of this study are to enhance our understanding of thescale of architectural elements present, and to investigate the fundamentalcontrols that underpin temporal and spatial controls on lithofaciesvariability in mud-dominated portions of the sediment transport path onmarine continental shelves. To meet this aim we have investigatedlithofacies variability utilizing field relations, optical, electron-optical,and geochemical techniques in the Upper Cretaceous Mancos Shale(Grassy Member, Blackhawk Formation) (Fig. 1), exposed in the BookCliffs, Utah (Fig. 2) and interpreted these observations within a sequence-stratigraphic framework. The Mancos Shale was specifically chosen forthis research because its updip stratigraphy is well constrained and there

is evidence that dynamic bypass in this succession led to the formation ofdowndip lowstand shoreface deposits (e.g., Hampson et al. 1999).Moreover, the excellent exposure means that it is possible to identify,trace, and sample the same stratigraphic interval at different locationsalong the sediment transport path within the mud-dominated parts of thissuccession.

GEOLOGICAL AND STRATIGRAPHIC SETTING OF THE MANCOS SHALE

Tectonism during the early Cretaceous in western North Americaresulted in the development of a foreland basin and formation of theWestern Interior Seaway (e.g., Burchfiel et al. 1992). By Maastrichtiantimes, this epeiric sea linked the polar ocean and the subtropical Gulf ofMexico (Fig. 1). The Upper Cretaceous succession currently exposed inthe Book Cliffs was deposited along the western margin of this Seaway asa wedge of eastward-prograding siliciclastic sediment, derived from theunroofing of the Sevier Fold and Thrust Belt (Fouch et al. 1983) to thewest. Excellent exposures in the Book Cliffs allow detailed studies oflarge-scale geometry and stratal architecture, and many stratigraphic,sedimentological, and diagenetic studies have been published on thefluvial and shallow marine strata (e.g., Van Wagoner 1995; OByrne andFlint 1995; Kamola and Huntoon 1995; Hampson et al. 1999; Yoshida2000; Taylor et al. 2000; Taylor et al. 2002; Taylor et al. 2004; Miall andArush 2001; Taylor and Gawthorpe 2003; Pattison 2005).

In this paper we study the downdip strata that are time-equivalent tothe Grassy Member of the Blackhawk Formation (Fig. 2). The Black-hawk Formation is composed of tongues of coastal-plain, fluvial, andshoreface strata, which interfinger eastwards into the mudstone-dominated Mancos Shale (Fig. 2). Within individual members, sequenceboundaries, marine flooding surfaces, and transgressive surfaces can betraced for tens of kilometers where the sediments are exposed (Kamola

FIG. 4.Details of the three logged and sampled Mancos Shale successions, including sample numbers, mudstone lithofacies codes, sand + silt:clay ratios, and totalorganic carbon (TOC) data. For full data information, see Table 1.

MANCOS SHALE SEDIMENTOLOGY AND STRATIGRAPHY 327J S R

FIG. 5.A) Thin-section scan of a representative clay-rich mudstone (Coquina 06). This clay-rich mudstone is thin, relict-bedded, and partially bioturbated (top of bedmarked by dashed line). The thin relict beds overall fine upward, from being relatively silt enriched at their bases to silt depleted at their tops (annotated c). Thediminutive ichnogenera present include Phycosiphon isp. (arrowed), Planolites isp., as well as an indeterminate fauna. B) Low-power optical micrograph of Blaze 6 (clay-rich mudstone) illustrating detrital silt grains in a matrix dominated by clay, amorphous organic matter, and pyrite. Note the presence of an agglutinated foraminifer andthe presence of dark, clay-filled burrows towards the bottom right of the micrograph (cfb). These diminutive burrows are attributed to Phycosiphon isp. C) Backscatteredelectron micrograph of Coquina 6 (clay-rich mudstone) illustrating prominent agglutinated foraminifer in a matrix of clay with some fine-grained silt (composed mainlyof quartz [low g] and detrital feldspar [intermediate g]). The foraminfer test is composed of quartz grains. The chambers within the foraminifer are infilled with framboidal

328 J.H.S. MACQUAKER ET AL. J S R

and Huntoon 1995). The Mancos Shale is composed of very fine-grainedsandstone and mudstones deposited in a shallow, well-oxygenated openmarine shelf (Howell and Flint 2003a; 2003b). OByrne and Flint (1995)published a high-resolution sequence stratigraphic framework for theGrassy Member, in which they recognized progradational tongues ofcoastal-plain and shoreface strata with two sequence boundaries, markedby fluvial incision, in the upper part of the member. They did not,however, extend this framework into the time-equivalent Mancos Shale.Hampson et al. (1999), Pattison (2005), and Chan et al. (1991) variouslyrecognized the presence of tidally influenced river channels, fluvial-dominated delta fronts, and storm-influenced shoreface deposits withinthe Mancos Shale. They correlated these units variously to the Aberdeenand Kenilworth members of the Blackhawk Formation. Equivalentdeposits have not been identified in the Mancos Shale downdip of theGrassy Member.

METHODS

In order to meet the aims of this research, a part of the Mancos Shalethat is time-equivalent to the Grassy Member of the BlackhawkFormation exposed in the Book Cliffs, Utah (Figs. 1, 2) was studied.The rocks here are well exposed and allow direct correlation of majorstratal surfaces preserved within coastal plain and shoreface successionsupdip, with mudstone-dominated strata downdip. The availability ofa detailed stratigraphic framework for the Grassy Member (OByrne andFlint 1995), allowed us to place the time-equivalent Mancos Shale intoa wider stratigraphic context (Fig 2). Lateral correlations between theseunits were achieved through a combination of physical correlation alongthe exposure, binocular observation, and photomontage examination ofcliff faces. Detailed sedimentary logs were measured and samples of theMancos Shale collected (Figs. 2, 3) along a 20-km-long transect, orientedat a slightly oblique angle to the main paleosediment transport direction(oriented SE to ESE). Samples were collected from Thompson Pass(proximal location), Blaze Canyon (intermediate location path), andCoquina Wash (distal location). Overall approximately 90 samples wereobtained at vertical intervals of approximately 0.5 m from each of theselocalities (Fig. 4).

Unusually thin (20 mm), polished thin sections were prepared fromeach sample, in order to facilitate making detailed lithofacies descriptions.Initially, each thin section was scanned using a flat-bed scanner (Epson1250) equipped with a 35 mm slide illumination system to obtain detailsof 1022 to 1023 m-scale textures present. The sections were then examinedoptically at low power (Nikon Labophot Pol) to obtain details of the 1023

to 1024 m-scale textures, and then at higher power using backscatteredelectron imagery (JEOL 6400 equipped with a Link 4 Quadrant solid statebackscattered electron detector and Semifore digital framestore) to obtaindetails of the 1024 to 1025 m-scale textures. Where mineral identity wasnot immediately obvious on the basis of varying optical characteristicsand backscatter coefficients (g), identity was confirmed utilizing semi-quantitative energy-dispersive spectrometry (PGT with windowlessdetector). The scanning electron microscope was operated at 20 kV and2.0 nA, at a working distance of 15 mm. Using this combined optical andelectron optical dataset, the grain-size of the silt could be estimated andthe abundance of each component semi-quantified by comparison with

published grain abundance charts (e.g., Flugel 1982). Crucially, thesemethods allowed us to broadly estimate the proportion of silt and finesand present, as well as the proportion of accessory minerals and clay bydifference.

Total organic-carbon (TOC) analyses were performed by the Univer-sity of Newcastle (U.K.). The total C contents of untreated, powderedsamples were obtained using an induction furnace (Leco C/S analyzer).Once the total C data had been obtained, a split of each sample wastreated with 6N HCl to remove all carbonate prior to determining thequantity of residual carbon in the acid-treated samples using the samefurnace. Organic-carbon contents were then determined by difference.The organic-carbon contents have a precision of 6 0.2%.

RESULTS

Systematic analyses of the thin sections revealed mainly the followingfine-grained lithofacies (nomenclature after Macquaker and Adams2003):

a) clay-rich mudstones (Fig. 5A, B, C);

b) silt-bearing, clay-rich mudstones (Fig. 5D, E, F);

c) very fine sand- and silt-bearing, clay-rich mudstones (Fig. 6A, B, C);

d) very fine sand-, silt- and clay-bearing mudstones (Fig. 6D, E, F);

e) fine-grained muddy sandstones (Fig. 7A, B, C);

f) carbonate-cement-rich mudstones (Fig. 7D, E, F);

g) silt- and clay-bearing, carbonate-cement-rich mudstones;

h) fine sand- and silt-bearing, carbonate cement-rich mudstones.

Descriptions of each sample are summarized in Table 1. Individualunits are either thin-bedded, or relict-bedded (, 10 mm), or intenselybioturbated (e.g., Figs. 5A, D, 6D, 7A). Where identifiable, individualunits have erosional bases (Fig. 6A), fine upward, and have bioturbatedtops (e.g., Figs. 6A, 7D). Typically the bases of these upward-fining unitsare composed of muddy sandstones and very fine sand-, silt-, and clay-bearing mudstones, in contrast to their tops, which are composed of silt-bearing, clay-rich mudstones and clay-rich mudstones (Figs. 6D, 7D). Inaddition, dewatering has disrupted some of the original depositionaltextures (Fig. 6A).

The diminutive ichnofauna present comprise an assemblage ofPhycosiphon isp., Planolites isp., and Terebellina isp., (e.g., Figs. 5A, D,6A) in the fine-grained units and Palaeophycus isp., Rhizocorallium isp.,Teichinus isp., Chondrites isp., and indistinct Thallisanoides isp. (Figs. 6D,7A) in the coarser-grained units. Escape traces, which completely disruptall the laminae within beds, are present in some units (e.g., Fig. 6A).

The very fine-grained sand and silt fractions are composed predomi-nantly of quartz with smaller proportions of detrital feldspar (albite andplagioclase) and dolomite (Figs. 5C, F, 6C, F, 7C, F). In most samples thefine-grained matrix is a mixture of illite, mixed layer illitesmectite, andamorphous organic matter, with minor framboidal pyrite (grain-size range10 to 30 mm), and macerated woody material (Figs. 5E, F, 6C, E).

Total organic-carbon (TOC) values average 1.3% (range: 0.2 to 2.0%),with the silt-bearing clay-rich mudstones and clay-rich mudstones beingthe most enriched (average 1.6%) compared to the coarser-grained unitsand cement-rich facies (average of 1.2% and 0.7%, respectively) (Table 1).

r

pyrite (high g). D) Thin-section scan of Coquina 25, a representative silt-bearing clay-rich mudstone. This unit is intensely bioturbated by a combination of Phycosiphonisp. (arrowed), Palaeophycus isp. (Pa), and Planolites isp. Most of the original lamina and some of the early Phycosiphon isp. burrows have been completely disrupted bylater burrowing activity. E) Low-power optical micrograph of Blaze 5 (silt-bearing clay-rich mudstone) illustrating silt grains and an agglutinated foraminifer in a matrixdominated by clay, amorphous organic matter and pyrite. Note the presence of clay-filled burrows and macerated woody material (arrowed w) in the matrix. F)Backscattered electron micrograph of Coquina 22 (silt-bearing clay-rich mudstone) illustrating prominent agglutinated foraminifer in a matrix of clay, organic matter,and fine-grained silt (composed of quartz and detrital feldspar). The chambers within the foraminifer are infilled with framboidal pyrite (very high g). Note the presence ofmacerated woody material in the matrix (very low g, arrowed w). (This figure is reproduced in color in the digital version.)

MANCOS SHALE SEDIMENTOLOGY AND STRATIGRAPHY 329J S R

FIG. 6. A) Thin-section scan of a representative sand- and silt-bearing, clay-rich mudstone (Thompson Pass 8). This sample is a relict-bedded and partiallybioturbated. The ichnofauna includes Rhizocorallium isp. (Rh), Planolites isp. (Pl), and Phycosiphon isp. (arrowed Ph). An escape trace (dashed outline ET) disrupts oneof the beds. Individual beds have erosional lower contacts (arrow to short dashed lines) and fine upward from being sand-rich at their bases to clay- and silt-enrichedtowards their tops. Dewatering has further disrupted some of the erosional contact. B) Low-power optical micrograph of Coquina 9 (sand- and silt-bearing, clay-richmudstone) from the top part of an upward-fining unit. Note the presence of clay-rich burrow fills. Scattered, partially crushed agglutinated formaminifers are present inthe matrix. The foraminifer tests are infilled by pyrite. C) Backscattered electron micrograph of the basal portion of an upward-fining unit from Thompson Pass 19 (sand-and silt-bearing, clay-rich mudstone). This micrograph illustrates very fine sand grains (composed mainly of quartz with minor feldspar) in a matrix of clay, organic

330 J.H.S. MACQUAKER ET AL. J S R

Nearly all samples contain agglutinated foraminifers, includingorganisms with uniserial, biserial, trochospiral, and planispiral involutemorphologies (e.g., Figs. 5B, C, E, F, 6B). These foraminifers are quitewell preserved, scattered throughout the matrix, and their tests arecomposed predominantly of quartz grains (e.g., Fig. 5C, F). The majorityof the shelter porosity preserved within their tests has been infilled byframboidal pyrite (e.g., Fig. 5B, C, F); some, however, has been infilledby authigenic kaolinite.

The composition of the detrital components in the coarse-grained,carbonate-cemented strata is very different from that of the enclosingsediments. In particular, these levels contain much greater proportions offine-sand-size detrital clay grains, variously composed of chlorite andmuscovite (Fig. 7E, F). Much of this coarse, detrital clay fractioncontains authigenic kaolinite intergrowths (Fig. 7F).

Within each of the three studied successions, there is systematic verticallithofacies variability on a 100 m scale. For instance, at Blaze Canyonsuccessive samples overall fine upward between depths 0 m and 2.0 m(Fig. 4). They also exhibit upward-coarsening trends on both 1 to 3 m(small) scales and 5 to 10 m (large) scales (Figs. 4, 8). Additionally, small-scale upward-coarsening units (e.g., from 5.0 to 8.5 m at Thompson Pass,from 20.0 to 22.5 m at Blaze Canyon (illustrated in Fig. 8A, B, C), andfrom 8.0 to 11.0 m at Coquina Wash) are separated from one and anotherby noticeably finer-grained intervals (e.g., 8.5 m at Thompson, 22.5 m atBlaze, and 11.0 m at Coquina Wash) (Fig. 4). To complicate mattersfurther, the small-scale upward-coarsening units systematically stack intooverall large-scale upward-coarsening successions. These large-scaleupward-coarsening successions are illustrated in Figure 4. (e.g., depths3.0 and 8.0 m at Coquina Wash, depths 10.0 and 19.0 m at Blaze Canyon,and depths 8.5 and 13.0 m at Thompson Pass).

Concretionary, cemented units are present either at the tops of theselarge-scale upward-coarsening successions (e.g., 19 m at Thompson Passand 10.5 m at Coquina Wash), or at horizons where there are significantgrain-size changes (e.g., 2.0 m at the Blaze Canyon location, where thereis abrupt coarsening). In the latter settings, units that contain abundantcoarse detrital clays abruptly overlie silt-bearing clay-rich mudstones(Fig. 4). Most of these cemented units have high (. 70%) minus-cementporosities (Table 1) and are composed predominantly of siderite, ferroancalcite, and nonferroan calcite cements. Granule-size apatite micro-concretions are also present associated with these units (Fig. 7E ).

DISCUSSION AND INTERPRETATION

The overall paleogeographic setting, predominant muddy grain-size,and presence of trace fossils such as Phycosiphon isp., and Planolites isp.,coupled with the presence of pyrite, indicate that this region of theMancos Shale was originally deposited on the distal parts of a clastic-dominated, marine continental shelf (see also Fouch et al. 1983; Miall andArush 2001; Howell and Flint 2003a).

Clastic Components

The fine-grained detrital clays, which constitute the bulk of sedimentdelivered to the study area, were probably derived from reworking ofupdip soil profiles, having originally formed in overbank environments by

weathering processes. The climate at this time is interpreted to have beenhumid (Van Wagoner 1995), because abundant coals and deeppaleoweathering profiles are present in coeval successions updip (VanWagoner 1995; Taylor and Gawthorpe 2003). In contrast, the coarser-grained detrital quartz and feldspar components are interpreted to be theresiduum of weathering, and are probably derived ultimately from theunroofing of the Sevier Fold and Thrust Belt. In addition to siliciclasticdetritus, these sediments also contain some macerated woody material.This woody debris is likely to be the remains of higher plants that wereliving originally in the catchment and transported into the basin followingtheir demise. Detrital, nonferroan dolomite is a dominant mineral influvial, coastal-plain, and shoreface sandstones in the Book Cliffsuccessions updip from the Mancos Shale (Taylor et al. 2000) and inother Upper Cretaceous successions on the western margin of theWestern Interior Seaway (e.g., Crossey and Larsen 1992; McKay et al.1995). It is likely that this dolomite was derived from the erosion ofdolomite platform strata in the Sevier Fold and Thrust Belt.

Although intense burrowing has destroyed many of the originaldepositional fabrics, residual small-scale upward fining is preserved insome units (Fig. 6A). These upward-fining strata are mostly less than10 mm thick, and are typically composed of very fine muddy sandstonesand coarser grained mudstones at their bases and bioturbated silt-bearingclay-rich mudstones towards their tops (e.g., Fig. 7D). Goldring et al.(1991) and Macquaker and Taylor (1996) described similar structuresfrom analogous paleogeographic settings (e.g., Cleveland IronstoneFormation, Staithes Sandstone Formation, exposed on the N.E. coastof England) that they informally called lam-scram (laminated andscrambled) units. These authors argued that lam-scram units wereproduced by infaunal organisms colonizing the tops of distal stormdeposits between storm events. Given the similarity of the structuresobserved here in the Mancos Shale to those from other locations, a similarinterpretation for their origin is invoked.

The presence of bioturbation in the tops of these units indicates thatthere was sufficient time between storm events for an infaunal communityto colonize the sediment surface. This colonization overprinted theoriginal depositional laminae, close to the sedimentwater interface, witha burrowed fabric, leaving the laminae in deeper tiers undisturbed. Giventhis interpretation, the upward-fining lam-scram units conform to thedefinition of genetic beds (sensu Campbell 1967), because they are theproducts of individual depositional events, separated by breaks insedimentation, rather than being component parts of individual de-positional units. Many field-based geologists would describe these unitsas being laminated, because their parting spacings are , 10 mm, butgiven their origin as individual depositional events, we believe that it ismore appropriate to describe them as thin beds. This observation alsomeans that, unsurprisingly, beds in this part of the Mancos Shale aremuch thinner than coeval units deposited in marginal and shallow marinesettings updip (e.g., OByrne and Flint 1995).

Biogenic Components

In addition to detrital components, some autochthonous, biogenicmaterials are also present. These include agglutinated foraminifers andamorphous organic matter. Foraminifers are relatively well preserved andparticularly common in the finer-grained lithofacies. The diversity of

r

matter (note the macerated woody detritus [arrowed]), and pyrite. Kaolinite (low g) fills some of the intergranular porosity (k). D) Thin-section scan of a representativebioturbated very fine sand-, silt-, and clay-bearing mudstone (Blaze 20). This sample is intensely bioturbated by Planolites isp., and an indeterminate community ofburrowing organisms. Most traces of the depositional laminae have been disrupted. E) Low-power optical micrograph (Blaze 20) of the contact between a mud-filledburrow and more sandy host sediment in a fine sand-, silt-, and clay-bearing mudstone. F) Backscattered electron micrograph of Blaze 24 (fine sand-, silt-, and clay-bearing mudstone) illustrating very fine sand grains in a matrix of clay, organic matter, and silt. The sand-size grains are made of quartz (low g). and feldspar(intermediate g). Pyrite framboids (high g. (arrowed). (This figure is reproduced in color in the digital version.)

MANCOS SHALE SEDIMENTOLOGY AND STRATIGRAPHY 331J S R

FIG. 7.A) Thin-section scan of a representative bioturbated silt-bearing, very fine-grained muddy sandstone (Thompson Pass 06). The burrowing fauna is dominatedby Chondrites isp. B) Low-power optical micrograph of part of a mud-filled burrow within this fine-grained muddy sandstone. C) Backscattered electron micrograph ofThompson Pass 06 illustrating very fine-grained muddy sandstone. Individual sand grains are composed of quartz (low g). and feldspar (intermediate g). A pervasivequartz cement infills the intergranular porosity. D) A thin-section scan of a representative upward-fining, partially bioturbated (dominated by Planolites isp. arrowed),carbonate-cement-rich mudstone (Coquina Wash 29). Faint residual laminae (arrowed) are preserved towards the bottom of this unit. E) Low-power optical micrographof a representative fine sand- and silt-bearing (comprising grains of quartz and altered detrital chlorite, arrowed), carbonate-cement-rich mudstone (Blaze 4). Thisphotomicrograph was taken close to the margin of the cemented zone. The cements in the matrix comprise authigenic apatite (a) micro-concretions enclosed by zoned

332 J.H.S. MACQUAKER ET AL. J S R

forms present in these units suggests that the environment was favorableand they represent an equilibrium community. There is no evidence tosuggest that these organisms were either simply colonizing the sedimentsurface or utilizing an exaerobic metabolic mechanism (i.e., able totolerate episodic anoxia). It is possible, however, that they were able totolerate slightly lower than normal marine pore-water oxygen concentra-tions and were utilizing dysaerobic respiratory mechanisms.

Once deposited, the sediment was disrupted by diminutive burrowingorganisms. Much of this infauna has an indeterminate affinity. Planolitesisp., Phycosiphon isp., Chondrites isp., and Terebellina isp., however, havebeen identified in the more fine-grained, clay-rich portions of beds, whilediffuse Thallasinoides isp., Palaeophycus isp., Teichichus isp., andRhizocorallium isp. are present in the coarse-grained facies at the basesof beds. There is also evidence of a variety of crosscutting relations, withThallasinoides isp. and Planolites isp. being interpreted as occupying themid- to upper layers, Phycosiphon isp. and Palaeophycus isp. living inmid-tiers, and Chondrites isp. colonizing the deep tiers (e.g., Ekdale andBromley 1991; Goldring et al. 1991). The presence of this tiering alsoindicates that sediment supply was episodic, and that there was sufficienttime for burrowing organisms to develop a stable community in thesediment between episodes of sediment delivery. That Phycosiphon isp.traces are so common, have colonized mud, and directly underlie erosionsurfaces (e.g., Fig. 6A) suggests that erosion may have removed both themixed-layer close to the sedimentwater interface and the upper burrowtier prior to deposition of the overlying unit. These data suggest that theorganism responsible for producing Phycosiphon isp. was an opportunis-tic colonizer of muddy sediment (see also Goldring et al. 1991). Overall,the presence of this infauna indicates that the substrate, at least in themid- and higher sediment tiers, was oxic and firm (rather than soupy) atthe time of deposition. The presence of Chondrites isp., a putativesymbiont (e.g., Bromley 1996), in deeper tiers of the more muddy unitssuggests, however, that a sulfidicoxic interface may have existed in thepore waters fairly close to the sediment surface. Such near-surfacesulfidicoxic interfaces commonly occur within Recent organic-rich shelfsediments (e.g., Aller et al. 1986; Chanton et al. 1987; Canfield et al.1993).

Variability of Vertical Stacking Patterns

Analyses of successive samples from all of the sampled sites reveallarger-scale stacking variability (Fig. 8). While it is tempting to interpretthe 1 to 3 m thick upward-coarsening strata as thick beds, this simplisticinterpretation is likely to be wrong, because individual genetic beds hereare thin (typically , 10 mm thick; see above). Given this observation, webelieve the 1 to 3 m thick upward-coarsening units are stacked beds (orbedsets) (sensu Campbell 1967). In this context, the existence of muchfiner-grained strata between individual upward-coarsening packages issignificant, in as much as their presence suggests that the length of thesediment transport path increased significantly over these intervals. Thesimplest explanation for all these observations is that that water depthsrapidly deepened at these levels, although updip diversion of clasticsediment supply, perhaps in response to channel switching, is possible.Given the significant role that changes in relative sea level are known toexert on updip facies variability elsewhere in the Mid-Cretaceous Seaway(e.g., Van Wagoner 1995), however, we prefer the first of these twoexplanations. We, therefore, interpret these upward-coarsening strata tobe the distal expression of parasequences, separated from one another by

much finer-grained units, i.e., marine flooding surfaces (cf. Van Wagoneret al. 1990). The fact that so many of the exposures are weathered,coupled with the subtle expression of much of this variability in the field,means that we are unable to distinguish unequivocally between thevarious environmental processes (namely tectonically driven subsidenceand eustatic sea level) that might have forced these grain-size changes(compare Miall and Arush 2001 with Howell and Flint 2003b). With thiscaveat in mind, and bearing in mind the widespread and pervasive natureof the observed stratal architectures, it is likely that regional controls wereoperating to control lithofacies variability. We therefore prefer the optionthat eustatic sea-level change was probably a significant driver on faciesvariability on this shelf.

Further investigation of successive 1 to 3 m upward-coarsening units(Fig. 4) indicates they too stack into overall upward-coarsening packages.Typically each of these larger-scale packages is 5 to 10 m thick and iscomposed of three or four individual parasequences that individuallycoarsen upward. Using analogous logic that allowed us to interpret thesmaller upward-coarsening units as parasequences, then these large-scaleupward-coarsening units are interpreted to be prograding parasequencesets (cf. Van Wagoner et al. 1990).

In offshore marine settings, thin overall, upward-fining successions areunlikely be parasequences because in these environments these unitstypically coarsen upward (e.g., Van Wagoner et al. 1990). Given theupward fining observed here, our best estimate is that these units area retrogradationally stacked succession of thin parasequences (each, 0.5 m thick) that overall fine upward. The sampling strategy that weadopted in this study, however, was not at sufficient vertical resolution todemonstrate this pattern unequivocally.

Diagenetic Components

Preferentially cemented units occur noticeably at those surfacescorrelated to amalgamated sequence boundaries and transgressivesurfaces updip. In the Mancos Shale these surfaces mark either levelswhere large-scale stacking patterns change or where there are disconti-nuities in stacking patterns. In addition to these significant features, thesecemented units also possess distinct compositional and textural proper-ties. For instance, their detrital mineralogies are very different from thesurrounding mudstones, and their pore spaces have been infilled withdiagenetic ferroan carbonate (variously siderite, ferroan calcite, and/orferroan dolomite) and sulfide (pyrite) cements. Petrographic evidence,such as high minus-cement porosity, indicates that cementation tookplace early and certainly prior to significant compaction. The survivingcement compositions suggest that they precipitated in response toincreased porewater bicarbonate, iron, and sulfide activities, probablyas a consequence of bacterially mediated metabolic processes (e.g.,Macquaker and Taylor 1996; Taylor and Macquaker 2000). Certainly,ferroan carbonate and pyrite cements are well-known products ofbacterial sulfate reduction, iron (III) reduction, and methanogenesis inanalogous (e.g., Coleman 1985; Curtis et al. 1986; Raiswell 1988; Taylorand Curtis 1995) and closely related depositional environments (Chan1992; Taylor et al. 2002). The existence of high minus-cement porosities,and the pre-compaction textures, indicates that significant volumes ofcement were precipitated early in the pore space. This evidence suggeststhat the pores in these units were either subjected to prolonged input fromthe products of bacterial metabolic activity or associated with enhancedfluid flow in the subsurface, to facilitate such large volumes of pore space

r

ferroan carbonates. F) Backscattered electron micrograph of a representative carbonate cement-rich mudstone (Coquina 8) illustrating very fine-grained sand and silt(composed of quartz (intermediate g), K-feldspar, and altered chlorite grains arrowed) in a matrix of fine-grained clay (low g). Zoned, ferroan carbonate cements haveinfilled much of the intergranular porosity (high g). Minor framboidal pyrite is also present (very high g). (This figure is reproduced in color in the digital version.)

MANCOS SHALE SEDIMENTOLOGY AND STRATIGRAPHY 333J S R

TA

BL

E1.

S

emiq

uant

itat

ive

desc

ript

ions

ofea

chsa

mpl

ean

alyz

ed.

Sam

ple

Bri

efD

escr

ipti

on

Fin

esa

nd

Sil

tC

lay

Nan

no

-p

lan

kto

nA

uth

igen

icca

rbo

nat

eF

ora

min

ifer

mat

eria

lP

yrit

eV

isib

leo

mA

uth

igen

icC

lays

TO

C

Cla

y-ri

chm

udst

ones

%%

%%

%%

%%

%%

Bla

ze13

Cla

y-ri

chm

ud

sto

ne

wit

hT

erre

bell

ina

isp

.an

dP

hyco

siph

onis

p.

0.0

7.5

89.5

0.0

0.0

1.0

1.0

0.5

0.5

1.6

Bla

ze06

Bio

turb

ated

(Pla

noli

tes

isp

.,T

erre

bell

ina

isp

.)cl

ay-r

ich

mu

dst

on

ew

ith

asm

all

pro

po

rtio

no

fve

ryfi

ne

silt

.1.

07.

588

.50.

00.

01.

01.

00.

50.

51.

3

Bla

ze03

Bio

turb

ated

(Phy

cosi

phon

isp

.)cl

ay-r

ich

mu

dst

on

e2.

57.

587

.50.

00.

01.

01.

00.

50.

01.

1C

oq

uin

a06

Rel

ict-

bed

ded

and

bio

turb

ated

(Phy

cosi

phon

isp

.,an

dP

lano

lite

sis

p.)

clay

-ric

hm

ud

sto

ne

0.0

7.5

88.0

0.0

0.0

2.0

1.0

1.0

0.5

1.8

Co

qu

ina

18B

iotu

rbat

ed(P

lano

lite

sis

p.)

clay

-ric

hm

ud

sto

ne

wit

hso

me

silt

2.5

7.5

87.0

0.0

0.0

1.0

1.0

0.5

0.5

1.5

Co

qu

ina

23B

iotu

rbat

ed(P

lano

lite

sis

p.,

and

Phy

cosi

phon

isp

.)cl

ay-r

ich

mu

dst

on

e0.

07.

589

.50.

00.

01.

01.

00.

50.

52.

1C

oq

uin

a27

Bio

turb

ated

(Phy

cosi

phon

isp

.,P

lano

lite

sis

p.,

and

Cho

ndri

tes

isp

.)cl

ay-r

ich

mu

dst

on

ew

ith

asm

all

pro

po

rtio

no

fsi

lt0.

07.

589

.50.

00.

01.

01.

00.

50.

51.

8

Co

qu

ina

28B

iotu

rbat

ed(P

hyco

siph

onis

p.,

Pla

noli

tes

isp

.,an

dT

ereb

elli

nais

p.)

clay

-ric

hm

ud

sto

ne

wit

ha

smal

lp

rop

ort

ion

of

silt

.0.

07.

589

.50.

00.

01.

01.

00.

50.

51.

7

Sil

t-be

arin

g,cl

ay-r

ich

mud

ston

esB

laze

28B

iotu

rbat

ed(P

hyco

siph

onis

p.,

and

Ter

rebe

llin

ais

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e5.

010

.080

.50.

00.

02.

01.

01.

00.

5B

laze

27B

iotu

rbat

ed(P

hyco

siph

onis

p.,

Pla

noli

tes

isp

.,an

dT

erre

bell

ina

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

7.5

12.5

76.5

0.0

0.0

1.0

1.0

1.0

0.5

Bla

ze15

Bio

turb

ated

(Pla

noli

tes

isp

.,an

dP

hyco

siph

onis

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e.7.

515

.075

.00.

00.

01.

01.

00.

50.

01.

3B

laze

11B

iotu

rbat

ed(P

hyco

siph

onis

p.,

Pla

noli

tes

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

2.5

10.0

85.0

0.0

0.0

1.0

1.0

0.5

0.0

Bla

ze10

Sil

t-b

eari

ng,

clay

-ric

hm

ud

sto

ne

7.5

15.0

75.0

0.0

0.0

1.0

1.0

0.5

0.0

Bla

ze09

Bio

turb

ated

(Pal

aeop

hycu

sis

p.,

Phy

cosi

phon

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne.

5.0

12.5

78.0

0.0

2.0

1.0

1.0

0.5

0.0

Bla

ze08

Bio

turb

ated

(Pla

noli

tes

isp

,P

hyco

siph

onis

p.)

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e.2.

510

.085

.00.

00.

01.

01.

00.

50.

01.

7B

laze

07B

iotu

rbat

ed(P

hyco

siph

onis

p.,

Ter

rebe

llin

ais

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e5.

020

.072

.50.

00.

01.

01.

00.

50.

01.

3B

laze

05B

iotu

rbat

ed(P

hyco

siph

onis

p.,

Ter

rebe

llin

ais

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e.2.

012

.083

.00.

00.

01.

01.

00.

50.

51.

1B

laze

02B

iotu

rbat

ed(P

lano

lite

sis

p.,

Phy

cosi

phon

isp

.,T

erre

bell

ina

isp

.)si

lt-b

eari

ng,

clay

-ric

hm

ud

sto

nes

5.0

10.0

83.0

0.0

0.0

0.5

1.0

0.5

0.0

1.4

Co

qu

ina

01B

iotu

rbat

ed(P

hyco

siph

onis

p.,

Pla

noli

tes

isp

.,T

ereb

elli

nais

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e5.

025

.067

.00.

00.

01.

01.

00.

50.

5C

oq

uin

a02

Bio

turb

ated

(Pla

noli

tes

isp

.,P

hyco

siph

onis

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e2.

520

.073

.50.

00.

01.

51.

01.

00.

51.

9C

oq

uin

a03

Bio

turb

ated

(Pla

noli

tes

isp

.,P

hyco

siph

onis

p.,

Cho

ndri

tes

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

7.5

15.0

74.0

0.0

0.0

1.0

1.0

1.0

0.5

1.8

Co

qu

ina

04B

iotu

rbat

ed(T

ereb

elli

nais

p.,

Phy

cosi

phon

isp

.,C

hond

rite

sis

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

ew

ith

abu

nd

ant

wo

od

yfr

agm

ents

.5.

015

.076

.50.

00.

01.

01.

01.

00.

51.

8

Co

qu

ina

05B

iotu

rbat

ed(P

hyco

siph

onis

p.,

Ter

ebel

lina

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne.

2.5

10.0

84.5

0.0

0.0

1.0

1.0

1.0

0.0

1.7

Co

qu

ina

07B

iotu

rbat

ed(T

ereb

elli

nais

p.,

Phy

cosi

pon

isp

.,P

lano

lite

sis

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e1.

010

.085

.50.

00.

01.

01.

01.

00.

51.

7C

oq

uin

a10

Bio

turb

ated

(Pla

noli

tes

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

2.5

10.0

84.0

0.0

0.0

1.0

1.0

1.0

0.5

1.4

Co

qu

ina

11B

iotu

rbat

ed(P

lano

lite

sis

p.,

Phy

cosi

phon

isp

.,T

ereb

elli

nais

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e5.

012

.579

.00.

00.

01.

01.

01.

00.

51.

3C

oq

uin

a16

Bio

turb

ated

,(T

ereb

elli

nais

p.,

Phy

cosi

phon

isp

.,P

lano

lite

sis

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e5.

012

.579

.00.

00.

01.

01.

01.

00.

51.

9C

oq

uin

a19

Bio

turb

ated

,(P

hyco

siph

onis

p.,

Pla

noli

tes

isp

.)si

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne

5.0

10.0

81.5

0.0

0.0

1.0

1.5

0.5

0.5

1.7

Co

qu

ina

21B

iotu

rbat

ed(P

hyco

siph

onis

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e5.

015

.077

.00.

00.

01.

01.

00.

50.

51.

4C

oq

uin

a22

Bio

turb

ated

(Ter

ebel

lina

isp

.,P

hyco

siph

onis

p.

Pla

noli

tes

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

.1.

012

.583

.50.

00.

01.

01.

00.

50.

51.

8C

oq

uin

a24

Bio

turb

ated

(Phy

cosi

phon

isp

.,P

lano

lite

sis

p.)

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e0.

010

.087

.00.

00.

01.

01.

00.

50.

52.

0C

oq

uin

a25

Bio

turb

ated

(Pla

noli

tes

isp

.,P

hyco

siph

onis

p.,

Pal

eoph

ycus

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne.

5.0

10.0

82.0

0.0

0.0

1.0

1.0

1.0

0.0

Co

qu

ina

26B

iotu

rbat

ed(P

hyco

siph

onis

p.,

Pla

noli

tes

isp

.)si

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne

1.0

10.0

86.0

0.0

0.0

1.0

1.0

1.0

0.0

Th

om

pso

nP

ass

24B

iotu

rbat

ed(P

alae

ophy

cus

isp

.,P

lano

lite

sis

p.,

Cho

ndri

tes

isp

.)si

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne

2.5

20.0

74.5

0.0

0.0

1.0

1.0

0.5

0.5

1.6

Th

om

pso

nP

ass

23B

iotu

rbat

ed(C

hond

rite

sis

p.,

Phy

cosi

pon

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

0.0

15.0

82.0

0.0

0.0

1.0

1.0

0.5

0.5

1.8

Th

om

pso

nP

ass

22B

iotu

rbat

ed(P

lano

lite

sis

p.,

Phy

cosi

phon

isp

.,C

hond

rite

sis

p.)

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e2.

515

.078

.50.

00.

02.

01.

00.

50.

51.

9

Th

om

pso

nP

ass

18B

iotu

rbat

edsi

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

5.0

25.0

67.5

0.0

0.0

0.5

1.0

0.5

0.5

Th

om

pso

nP

ass

16B

iotu

rbat

edsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne

5.0

25.0

67.5

0.0

0.0

0.5

1.0

0.5

0.5

Th

om

pso

nP

ass1

4B

iotu

rbat

ed(P

alae

ophy

cus

isp

.C

hond

rite

sis

p.)

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e.5.

025

.067

.50.

00.

00.

51.

00.

50.

51.

6

334 J.H.S. MACQUAKER ET AL. J S R

Sam

ple

Bri

efD

escr

ipti

on

Fin

esa

nd

Sil

tC

lay

Nan

no

-p

lan

kto

nA

uth

igen

icca

rbo

nat

eF

ora

min

ifer

mat

eria

lP

yrit

eV

isib

leo

mA

uth

igen

icC

lays

TO

C

Th

om

pso

nP

ass

13B

iotu

rbat

ed(P

lano

lite

sis

p.,

Cho

ndri

tes

isp

.,P

alae

ophy

cus

isp

.,T

eich

ichn

usis

p.)

silt

bea

rin

gcl

ay-r

ich

mu

dst

on

e.C

hond

rite

sis

p.gr

ow

ing

wit

hin

oth

erb

urr

ow

fill

s.2.

520

.075

.50.

00.

00.

01.

00.

50.

51.

1

Th

om

pso

nP

ass

07B

iotu

rbat

ed(P

lano

lite

sis

p.,

Cho

ndri

tes

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

5.0

15.0

77.0

0.0

0.0

1.0

1.0

0.5

0.5

1.6

Th

om

pso

nP

ass

05B

iotu

rbat

edsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne

1.0

15.0

82.5

0.0

0.0

0.0

1.0

0.5

0.0

Th

om

pso

nP

ass

04B

iotu

rbat

ed(P

lano

lites

isp

.,P

hyco

siph

onis

p.)

silt

and

do

lom

ite

cem

ent,

bea

rin

gcl

ay-r

ich

mu

dst

on

e5.

020

.063

.50.

010

.00.

01.

00.

50.

0

Th

om

pso

nP

ass

03B

iotu

rbat

ed(P

lano

lite

sis

p.,

Cho

ndri

tes

isp

.)si

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

2.5

10.0

85.0

0.0

0.0

1.0

1.0

0.5

0.0

1.8

Th

om

pso

nP

ass

02B

iotu

rbat

ed(P

lano

lites

isp

.,P

alae

ophy

cus

isp

.,an

dC

hond

rite

sis

p.)

silt

bea

rin

g,cl

ay-r

ich

mu

dst

on

e7.

515

.075

.50.

00.

00.

51.

00.

50.

02.

0

San

d-an

dsi

lt-b

eari

ng,

clay

-ric

hm

udst

ones

Bla

ze26

Bio

turb

ated

(Phy

cosi

phon

isp

.)fi

ne

san

d-

and

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e15

.010

.072

.00.

00.

01.

01.

01.

00.

0B

laze

23B

iotu

rbat

ed,

very

fin

esa

nd

-an

dsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne.

15.0

20.0

63.0

0.0

0.0

0.5

1.0

0.5

0.0

1.3

Bla

ze22

Bio

turb

ated

(Pal

aeop

hycu

sis

p.)

fin

ean

dve

ryfi

ne

san

d-

and

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e.30

.035

.033

.50.

00.

00.

01.

00.

50.

0B

laze

18B

iotu

rbat

ed(P

hyco

siph

onis

p.)

,ve

ryfi

ne

san

d-

and

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e.20

.025

.053

.50.

00.

00.

01.

00.

50.

01.

1B

laze

17B

iotu

rbat

ed(P

hyco

siph

onis

p.

and

Ter

rebe

llin

ais

p.)

very

fin

esa

nd

-an

dsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne.

15.0

20.0

63.0

0.0

0.0

0.5

1.0

0.5

0.0

1.3

Bla

ze16

Bio

turb

ated

(Phy

cosi

phon

isp

.,P

lano

lite

sis

p.)

very

fin

esa

nd

and

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

es10

.015

.072

.00.

00.

01.

01.

01.

00.

01.

4

Bla

ze14

Bio

turb

ated

(Phy

cosi

phon

isp

.,P

alae

ophy

cus

isp

.,T

eich

ichu

sis

p.,

and

Pla

noli

tes

isp

.)ve

ryfi

ne

san

d-

and

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

es10

.020

.067

.50.

00.

01.

01.

00.

50.

01.

4

Bla

ze12

Th

inan

dre

lict

-bed

ded

,p

arti

ally

bio

turb

ated

(Pla

noli

tes

isp

.,P

hyco

siph

onis

p.)

very

fin

esa

nd

-an

dsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne.

10.0

15.0

72.5

0.0

0.0

0.5

1.0

1.0

0.0

1.5

Bla

ze01

Bio

turb

ated

(Pla

noli

tes

isp

.,P

hyco

siph

onis

p.)

very

fin

esa

nd

-an

dsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne

15.0

10.0

73.0

0.0

0.0

0.5

1.0

0.5

0.0

1.0

Co

qu

ina

09B

iotu

rbat

ed(T

ereb

elli

nais

p.,

Phy

cosi

phon

isp

.,an

dP

lano

liti

esis

p.)

very

fin

esa

nd

-an

dsi

lt-

bea

rin

gcl

ay-r

ich

mu

dst

on

e10

.015

.071

.50.

00.

01.

01.

01.

00.

51.

3

Co

qu

ina

12B

iotu

rbat

ed(P

lano

lites

isp

.,P

hyco

siph

onis

p.)

very

fin

esa

nd

-an

dsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne.

10.0

15.0

71.5

0.0

0.0

1.0

1.0

1.0

0.5

1.8

Co

qu

ina

17B

iotu

rbat

ed(T

ereb

elli

nais

p.,

and

Pla

noli

tes

isp

.)si

lt-

and

very

fin

esa

nd

-bea

rin

gcl

ay-r

ich

mu

dst

on

e10

.015

.072

.00.

00.

01.

01.

00.

50.

51.

6

Co

qu

ina

20B

iotu

rbat

edsi

lt-

and

fin

esa

nd

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e10

.020

.067

.00.

00.

01.

01.

00.

50.

51.

2T

ho

mp

son

Pas

s21

Bio

turb

ated

very

fin

esa

nd

-an

dsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne

25.0

15.0

57.5

0.0

0.0

0.5

1.0

0.5

0.5

1.4

Th

om

pso

nP

ass

19B

iotu

rbat

edve

ryfi

ne

san

d-

and

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e10

.030

.057

.50.

00.

00.

51.

00.

50.

50.

9

Th

om

pso

nP

ass

15B

iotu

rbat

ed(R

hizo

cora

lliu

mis

p.,

Pla

noli

tes

isp

.,an

dP

alae

ophy

cus

isp

.)ve

ryfi

ne

san

d-

and

silt

-b

eari

ng

clay

-ric

hm

ud

sto

ne

10.0

30.0

58.0

0.0

0.0

0.0

1.0

0.5

0.5

0.6

Th

om

pso

nP

ass

12B

iotu

rbat

ed(P

hyco

siph

onis

p.,

and

Pla

noli

tes

isp

.)ve

ryfi

ne

san

d-

and

silt

-bea

rin

g,cl

ay-r

ich

mu

dst

on

e10

.020

.067

.00.

00.

00.

51.

01.

00.

51.

1

Th

om

pso

nP

ass

11R

elic

t-b

edd

ed,

bio

turb

ated

(Cho

ndri

tes

isp

.,an

dT

ecic

hnus

isp

.)sa

nd

-an

dsi

lt-b

eari

ng,

clay

-ric

hm

ud

sto

ne.

Ind

ivid

ual

bed

su

pw

ard

-fin

ear

ed

isru

pte

db

yd

ewat

erin

g.S

om

em

acer

ated

wo

od

15.0

30.0

51.0

0.0

0.0

0.5

0.5

2.5

0.5

1.3

Th

om

pso

nP

ass

09B

iotu

rbat

ed(T

ereb

elli

nais

p.?

Tei

chic

nus

isp

.,P

lano

lite

sis

p.,

and

Cho

ndri

tes

isp

.)ve

ryfi

ne

san

d-

and

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e10

.030

.058

.00.

00.

00.

01.

00.

50.

51.

0

Th

om

pso

nP

ass

08R

elic

t-b

edd

ed,

par

tial

lyb

iotu

rbat

ed(R

hizo

cora

lliu

mis

p.,

Cho

ndri

tes

isp

.,an

dP

lano

lite

sis

p.)

san

d-

and

silt

-bea

rin

gcl

ay-r

ich

mu

dst

on

e.B

eds

hav

eer

osi

on

allo

wer

con

tact

san

du

pw

ard

-fin

e.10

.020

.068

.00.

00.

00.

01.

00.

50.

50.

9

Th

om

pso

nP

ass

01B

iotu

rbat

ed(P

lano

lite

sis

p.)

very

fin

esa

nd

-an

dsi

lt-b

eari

ng

clay

-ric

hm

ud

sto

ne

10.0

20.0

68.5

0.0

0.0

0.0

1.0

0.5

0.0

1.9

San

d-,

silt

-,an

dcl

ay-b

eari

ngm

udst

ones

Bla

ze25

bB

iotu

rbat

ed,

fin

ean

dve

ryfi

ne

san

d-,

silt

-an

dcl

ay-b

eari

ng

mu

dst

on

e30

.035

.033

.00.

00.

00.

01.

00.

50.

5B

laze

25a

Bio

turb

ated

,fi

ne

and

very

fin

esa

nd

-,si

lt-

and

clay

-bea

rin

gm

ud

sto

ne

20.0

30.0

48.5

0.0

0.0

0.0

1.0

0.5

0.0

TA

BL

E1.

C

onti

nued

.

MANCOS SHALE SEDIMENTOLOGY AND STRATIGRAPHY 335J S R

Sam

ple

Bri

efD

escr

ipti

on

Fin

esa

nd

Sil

tC

lay

Nan

no

-p

lan

kto

nA

uth

igen

icca

rbo

nat

eF

ora

min

ifer

mat

eria

lP

yrit

eV

isib

leo

mA

uth

igen

icC

lays

TO

C

Bla

ze25

Bio

turb

ated

,fi

ne

and

very

fin

esa

nd

-,si

lt-

and

clay

-bea

rin

gm

ud

sto

ne

25.0

35.0

38.5

0.0

0.0

0.0

1.0

0.5

0.0

Bla

ze24

Bio

turb

ated

very

fin

esa

nd

-,si

lt-

and

clay

-bea

rin

gm

ud

sto

nes

20.0

30.0

48.5

0.0

0.0

0.0

1.0

0.5

0.0

Bla

ze20

Bio

turb

ated

(Phy

cosi

phon

isp

.,an

dP

lano

lites

isp

.)ve

ryfi

ne

san

d-,

silt

-an

dcl

ay-b

eari

ng

mu

dst

on

es25

.030

.043

.50.

00.

00.

01.

00.

50.

01.

2B

laze

19B

iotu

rbat

ed(P

hyco

siph

onis

p.)

very

fin

esa

nd

-,si

lt-

and

clay

-bea

rin

gm

ud

sto

ne

25.0

30.0

43.5

0.0

0.0

0.0

1.0

0.5

0.0

0.9

Fin

e-gr

aine

dm

uddy

sand

ston

eT

ho

mp

son

Pas

s06

Bio

turb

ated

(Cho

ndri

tes

isp

.)si

lt-a

nd

clay

-bea

rin

g,ve

ryfi

ne-

grai

ned

mu

dd

ysa

nd

sto

ne.

60.0

20.0

18.0

0.0

0.0

0.0

1.0

0.5

0.5

0.2

Car

bona

te-c

emen

t-ri

chm

udst

ones

Bla

ze04

Pel

lete

dfi

ne

san

d-

and

silt

-bea

rin

g,si

der

ite

cem

ent-

rich

mu

dst

on

e20

.015

.04.

00.

055

.00.

00.

50.

55.

00.

6C

oq

uin

a8

Bio

turb

ated

(Pla

noli

tes

isp

.an

dP

hyco

siph

onis

p.)

carb

on

ate

cem

ent-

rich

mu

dst

on

es2.

55.

09.

00.

080

.00.

01.

00.

52.

01.

0C

oq

uin

a13

Car

bo

nat

ece

men

t-ri

chm

ud

sto

ne

0.0

2.5

6.0

0.0

90.0

0.0

0.5

0.5

0.5

0.3

Co

qu

ina

14C

arb

on

ate

cem

ent-

rich

mu

dst

on

e1.

05.

08.

00.

085

.00.

00.

50.

50.

00.

6C

oq

uin

a15

Bio

turb

ated

silt

-an

dcl

ay-b

eari

ng

carb

on

ate

cem

ent-

rich

mu

dst

on

e0.

010

.018

.50.

070

.00.

01.

00.

50.

00.

7C

oq

uin

a29

Th

in,

reli

ct-b

edd

ed,

par

tial

lyb

iotu

rbat

ed(P

lano

lite

sis

p.)

,ca

rbo

nat

e-ce

men

t-ri

chm

ud

sto

ne.

Ind

iviu

dal

bed

su

pw

ard

-fin

ean

dh

ave

bio

turb

ated

top

s.0.

57.

57.

50.

080

.02.

01.

01.

00.

51.

3

Th

om

pso

nP

ass

20B

iotu

rbat

ed(C

hond

rite

sis

p.)

fin

e-gr

ain

edsa

nd

-an

dsi

lt-b

eari

ng,

carb

on

ate

cem

ent-

rich

mu

dst

on

e20

.010

.00.

10.

069

.90.

00.

00.

00.

00.

6

Th

om

pso

nP

ass

10B

iotu

rbat

ed(C

hond

rite

sis

p.,

and

Pla

noli

tes

isp

.)si

lt-b

eari

ng

carb

on

ate-

cem

ent-

rich

mu

dst

on

e1.

010

.05.

00.

083

.50.

00.

50.

00.

00.

4

TA

BL

E1.

C

onti

nued

.

FIG. 8.Low-power optical micrographs illustrating small-scale upwardcoarsening at Blaze Canyon between sample depths 20.0 and 22.0 m (Fig. 4).The proportion of sand and silt increases from the bottom to the top samples. A)Bioturbated, fine sand-, silt-, and clay-bearing mudstone with a sand + silt/clayratio of 1.6 (Blaze 25). B) Bioturbated, fine sand-, silt-, and clay-bearing mudstonewith a sand + silt/clay ratio of 1.0 (Blaze 24). C) Bioturbated, fine sand-, and silt-bearing clay-rich mudstone with a sand + silt/clay ratio of 0.6 (Blaze 23). (Thisfigure is reproduced in color in the digital version.)

336 J.H.S. MACQUAKER ET AL. J S R

being infilled with cement. If these cements were indeed precipitating inresponse to bacterial metabolic activity, then it is most likely that thesesurfaces were located close to the sedimentwater interface for prolongedperiods (e.g., Raiswell 1987; Macquaker and Taylor 1996; Taylor et al.2000; Taylor and Macquaker 2000). Together, these factors stronglysuggest that at these levels cementation was indeed intimately associatedwith long breaks in sediment accumulation. Of course the presence ofcemented mudstones at major stratal surfaces, and in particularassociated with sequence boundaries and transgressive surfaces, has alsobeen documented elsewhere (e.g., Macquaker and Taylor 1996; Taylorand Macquaker 2000; Taylor et al. 2002). In these settings, as with theMancos Shale, the significant controlling element at these levels is thatthere was enough time to transport sufficient solutes derived from themetabolic processes of bacteria to the precipitation sites to fill most of theuncompacted pore space with cement.

Processes on Stratal Surfaces

Given that these cemented mudstones are located at sequenceboundaries, their very different detrital mineralogical composition,compared with the rest of the succession, is significant (these units, inaddition to containing an assemblage comprising quartz, feldspars, fine-grained detrital dioctahedral micas, and detrital dolomite also contain,detrital-grained muscovite and abundant chlorite). The existence of somuch coarse, dense, clay detritus at these levels suggests that dynamicbypass was also occurring on these surfaces. Under these conditions, lessdense and finer detritus (e.g., soil-derived dioctadedral micas and quartzsilt) was carried farther down the sediment transport path, leavinga coarse, dense, winnowed lag on these surfaces. Of course, it is possiblethat this pattern may have resulted from a brief provenance shift at thislevel. Such an interpretation, however, does seem unnecessarily complex,in as much as bypass processes are commonly invoked as having occurredat sequence boundaries, both in other mudstone successions (seeMacquaker 1994; Bohacs 1998) generally, and here in the Mancos Shale(OByrne and Flint 1995; Hampson et al. 1999), although not previouslyrecognized at this level.

Interestingly, at the Thompson Pass and Coquina Wash localities thecemented units at the Lower Grassy Sequence Boundary overlie anupward coarsening succession, whereas at Blaze Canyon they overlie anupward fining succession (Fig. 4). The presence of upward fining at theBlaze Canyon location below this level suggests that erosion has removedthe underlying highstand systems tract at this particular locality. At BlazeCanyon the existence of significant erosion and dynamic bypass at thislevel is not obvious from field investigations alone. This confirms thegeneral observations that mud-on-mud erosion surfaces, sequenceboundaries, and bypass surfaces are cryptic and difficult to observe insuch settings, unless they have been preferentially cemented. No doubtsimilar surfaces, with associated erosion, are common in analogousdepositional settings elsewhere, and sedimentologists should look out forthem.

Inferred maximum flooding and marine flooding surfaces here, unlikesome other mudstone-dominated successions (e.g., the ClevelandIronstone Formation in northeast England see Macquaker and Taylor1996), are marked neither by preferential cementation nor by any unusualenrichment in organic carbon contents. Instead, units at these levels areclay-rich mudstones at Coquina Wash and Blaze Canyon and silt-bearingclay-rich mudstones at Thompson Pass. In this part of the Mancos Shalethe maximum flooding surfaces are therefore cryptic. This suggests thatalthough there may have been a short break in supply of detrital sedimentat these levels, the breaks were not of sufficient duration to allow (becausethe rate of supply of solutes from bacterial metabolic processes were tooslow) significant volumes of cement to precipitate in the uncompactedsediment pore spaces and a preferentially cemented layer to develop.

Spatial and Temporal Variability

The lateral variability observed from relatively coarse muddysandstones and sand- and silt-bearing clay-rich mudstones, with aninfaunal assemblage comprising Palaeophycus isp., Chondrites isp.,Planolites isp., and Rhizocorallium isp. at Thompson Pass, through tofiner grained silt-bearing clay-rich mudstones, and clay-rich mudstones,with an infaunal assemblage comprising Phycosiphon isp., Planolites isp.,and Terebellina isp. at Coquina Wash, suggests a significant distal shift indepositional environments, from lower shoreface in the more proximallocations (Thompson Pass), to offshore-transition zones in the moredistal locations (Coquina Wash) (see also Goldring et al. 1991; Taylorand Gawthorpe 2003; Macquaker and Taylor 1996). This lateralvariability is consistent with the vertical variability and suggests thatthe sediments in this succession are indeed genetically related and that it isreasonable to use sequence stratigraphic principles to predict theirvariability.

Most of the succession, with the exception of the carbonate-cementedhorizons, contains between 1 and 2% total organic carbon. The mostdistal samples from Coquina Wash contain, on average, the most TOC(1.5%). Typically, the finest-grained facies, i.e., clay-rich mudstones andsilt-bearing clay-rich mudstones, are the most enriched in organic matter,although the lithofacies TOC trends at any one location are not thatsignificant. The absence of unusually high organic-matter contents isunsurprising given the extent of infaunal colonization of the sediment. Itsrelative abundance in the finest-grained mudstone facies does suggest thatburial efficiencies were maximized in these locations and/or some form oforganic matter adsorption on to clays was occurring (compare withTyson 1995; Kennedy et al. 2002; Tyson 2005) to result in the sedimentbeing enriched in organics. Certainly, there is no evidence of enrichmentof organics being a response to the existence of bottom-water anoxia inthis part of the succession.

Wider Implications

This work suggests that during deposition of the Mancos Shale thedistal continental shelf was a dynamic environment and strongly tied tomore proximal shoreface settings. Assuming that these observations arerepresentative of other shelf settings then:

N Distal shelf environments are energetic, even at times of relatively highsea-level stands, and are not low-energy systems where sediment inputis derived mainly from pelagic rain. In these settings the sedimentwater interface is subject to significant reworking and sediment inputsfrom a number of sources.

N The data from this study reinforce the fact that the whole shelfdepositionaldiagenetic system in marginal marine systems is closelyintegrated and it is an artificial device to separate it on the basis ofrock type. This observation has major implications for those seekingto model sediments on continental shelves from source to sink.

N Despite the fact that the sedimentwater interface in these environ-ments was dynamic and the surface sediment layers were burrowed,significant quantities of organic matter were still preserved. Thisemphasizes the fact that bottom-water anoxia is not a prerequisite fororganic-matter preservation and implies that efficiencies of organic-matter burial are probably controlled by rates of sediment accumu-lation.

CONCLUSIONS

Combined field observations and optical, electron optical, andgeochemical analyses reveal a great deal of subtle vertical and spatiallithofacies variability in the Mancos Shale that is not readily apparentfrom field studies alone. By use of these techniques eight lithofacies can be

MANCOS SHALE SEDIMENTOLOGY AND STRATIGRAPHY 337J S R

identified readily based on different proportions of components derivedfrom detrital, productivity, and chemical processes that each contains.These are: a) clay-rich mudstones, b) silt-bearing, clay-rich mudstones, c)very fine sand- and silt-bearing, clay-rich mudstones, d) very fine sand-,silt- and clay-bearing mudstones, e) fine-grained muddy sandstones, f)carbonate-cement-rich mudstones, g) silt- and clay-bearing, carbonate-cement-rich mudstones, and h) fine-sand- and silt-bearing, carbonatecement-rich mudstones.

Textural analyses of the thin sections reveal that individual genetic bedsare mostly very thin (, 10 mm) and were probably the distal products ofstorm events. There was sufficient time for the sediment, once deposited,to be extensively colonized by a diminutive macrofauna prior to the nextstorm event. The presence of this infauna indicates that the porewatersclose to the sedimentwater interface were oxic. The dominance of mid-to lower-tier ichnogenera at the tops of these thin beds suggests that themixed-layer and upper faunal tiers were reworked prior to deposition ofthe overlying storm units. The absence of these upper tiers suggests thatconditions at the sedimentwater interface were at least episodicallyerosive, and indicates that this outer-shelf environment was much moredynamic than low-resolution field observations suggest.

In spite of not being obvious in the field, analyses of successive verticalsamples from any one location indicate that individual beds stacksystematically into parasequences (1 to 3 m thick) and that theparasequences themselves stack into systems tracts (5 to 10 m thick).This larger-scale variability also manifests itself spatially, in as much asthe samples from Coquina Wash are overall significantly finer grainedthan those from the same interval at Thompson Pass. Changingaccommodation availability in response to widespread changes in relativesea level and sediment-dispersal patterns are likely to be responsible forthe substantial, yet largely cryptic (at hand specimen scales), lithofaciesvariability preserved in these mudstones.

The presence of early pre-compaction cements (mainly iron-richcarbonates with minor apatite) at the sequence boundaries is interpretedto indicate that these particular units were subject to prolongedbacterially mediated and predominantly anaerobic, diagenetic processes(Fe-reduction, sulfate reduction, and methanogenesis). Moreover, theexistence of stacking pattern discontinuities below some of thesecemented horizons (e.g., at the Lower Grassy Sequence Boundary atBlaze) and the presence of a completely different coarse-grained detritalassemblage at these levels (dominated by detrital chlorite and muscovite),suggests that erosion and dynamic bypass were occurring at these levels,prior to the sediment being cemented during the subsequent stillstand andfollowing transgression. The variability observed downdip reinforces thesequence stratigraphic interpretations that have already been made updip,and provides concrete evidence of the dynamic bypass necessary togenerate the lowstand fans of OByrne and Flint (1996). Theseobservations also vividly demonstrate that field studies alone do nothave sufficient resolution to describe the facies variability present in thesesuccessions and that the distal parts of continental shelves may bedynamic environments.

These data indicate that in the field the most easily recognizablearchitectural elements in this part of the Mancos Shale are the 5 to 10 mthick units which are interpreted variously to be parasequence sets andsystems tracts in addition to the cemented sequence boundaries overwhich bypass occurred. So, while it is not easy to do conventionalsequence stratigraphic analyses using field investigations alone, it ispossible to perform these analyses using combined optical, electronoptical, geochemical, and field data, because these provide details of thecryptic variability occurring at individual bed and bedset scales.Moreover, when these high-resolution studies are done in the context ofa well defined stratigraphic framework, they confirm existing interpreta-tions of updip and downdip facies variability and reinforce the fact thatsediments on distal parts of continental shelves form part of a continuum

of sediments that exist in marginal marine environments. Together, theseobservations suggest that high-resolution studies of mudstones havea bright future as part of continuing research to investigate the controlson variability of these sediments as researchers continue to investigatesediments from their sources to their ultimate sinks.

ACKNOWLEDGMENTS

We would like to thank Kevin Bohacs, Majorie Chan, and Associate EditorRichard Yuretich for thoughtful reviews of this paper. We also wish toacknowledge the remarkable support we get from Harri Williams and DavidOliver, who made the polished thin sections of these clay-rich sediments thatultimately make this type of work possible. The School of Earth,Environmental and Atmospheric Sciences at the University of Manchesterand the Department of Environmental and Geographical Sciences at theManchester Metropolitan University are also thanked for their continuinglogistic support. Finally, where would the Journal be without the support ofJohn Southard, Colin North, Melissa Lester (and of course Kitty Milliken),our paper is much better for your input, thank you.

REFERENCES

ADAMS, M.M., AND BHATTACHARYA, J.P., 2005, No change in fluvial style acrossa sequence boundary, Cretaceous Blackhawk and Castlegate Formations of CentralUtah, U.S.A.: Journal of Sedimentary Research, v. 75, p. 10381051.

ALLER, R.C., MACKIN, J.E., AND COX, R.T., 1986, Diagenesis of Fe and S in Amazoninner shelf muds: apparent dominance of Fe reduction and implications for the genesisof ironstones: Continental Shelf Research, v. 6, p. 263289.

APLIN, A.C., FLEET, A.J., AND MACQUAKER, J.H.S., 1999, Muds and mudstones: physicaland fluid flow properties in mudstones at a basin scale, in Aplin, A., Fleet, A., andMacquaker, J., eds., Muds and Mudstones: Physical and Fluid-Flow Properties:Geological Society of London, Special Publication 158, p. 17.

BLATT, H., 1970, Determination of mean sediment thickness in the crust: a sedimento-logical method: Geological Society of America, Bulletin, v. 8