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Controls on large-scale patterns of fluvial sandbody distributionin alluvial to coastal plain strata: Upper Cretaceous BlackhawkFormation, Wasatch Plateau, Central Utah, USA

GARY J. HAMPSON*, M. ROYHAN GANI� , HIRANYA SAHOO� ,ANDREAS RITTERSBACHER� ,§, NAWAZISH IRFAN*,1, ANDREW RANSON� ,THOMAS O. JEWELL*,2, NAHID D. S. GANI� , 3 , JOHN A. HOWELL� , SIMON J. BUCKLEY�& BRYAN BRACKEN–*Department of Earth Science and Engineering, Imperial College London, South Kensington Campus,London SW7 2AZ, UK. (E-mail: [email protected])�Department of Earth and Environmental Sciences, University of New Orleans, 2000 Lakeshore Drive,New Orleans, Louisiana 70148, USA�Uni CIPR, University of Bergen, P.O. Box 7810, 5020 Bergen, Norway§Department of Earth Science, University of Bergen, P.O. Box 7800, 5020 Bergen, Norway–Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, California 94583-0719, USA

Associate Editor – Andrea Moscariello

ABSTRACT

Current models of alluvial to coastal plain stratigraphy are concept-driven and

focus on relative sea-level as an allogenic control. These models are tested

herein using data from a large (ca 100 km long and 300 m thick), continuous

outcrop belt (Upper Cretaceous Blackhawk Formation, central Utah, USA).

Many channelized fluvial sandbodies in the Blackhawk Formation have a

multilateral and multistorey internal character, and they generally increase in

size and abundance (from ca 10% to ca 30% of the strata) from base to top of

the formation. These regional, low-resolution trends exhibit much local

variation, but are interpreted to reflect progressively decreasing tectonic

subsidence in the upper Blackhawk Formation and overlying Castlegate

Sandstone. The trend may also incorporate progressively more frequent

channel avulsion during deposition of the lower Blackhawk Formation.

Laterally extensive coal zones formed on the coastal plain during shallow-

marine transgressions, and define the high-resolution stratigraphic framework

of the lower Blackhawk Formation. Large (up to 25 m thick and 1 to 6 km

wide), multistorey, multilateral, fluvial channel-complex sandbodies that

overlie composite erosion surfaces occur at distinct stratigraphic levels, and

are interpreted as fluvial incised valley fills. Low amplitude (

offset stacking of a small proportion (

and in which allogenic controls are not indepen-dently constrained.

GEOLOGICAL CONTEXT

The non-marine Blackhawk Formation wasdeposited on the alluvial to coastal plain behindthe western shoreline of the Cretaceous WesternInterior Seaway, which extended from north tosouth across the North American continent (e.g.Kauffman & Caldwell, 1993; inset map in Fig. 1).The formation comprises an interbedded succes-sion of coals, mudstones and sandstones thatoverlie and interfinger to the east with shallow-marine sandstones of the Star Point Sandstoneand Blackhawk Formation (Fig. 2; Spieker &Reeside, 1925; Spieker, 1931; Young, 1955). TheStar Point Sandstone, Blackhawk Formation andoverlying Castlegate Sandstone form an eastward-thinning siliciclastic wedge of early and middleCampanian age (Fouch et al., 1983) that passesbasinward into the offshore deposits of theMancos Shale (Book Cliffs cross-section in Fig. 2)(Young, 1955). The sediment in this wedge waseroded and transported from the Sevier orogenic

belt to the west (e.g. Kauffman & Caldwell, 1993;inset map in Fig. 1), probably from the CanyonRange Culmination ca 80 km west of the studiedoutcrop belt (DeCelles & Coogan, 2006). Tectonicsubsidence of the Western Interior Seaway wasproduced by a combination of short-wavelength,thrust-sheet loading in the Sevier Orogen alongits western margin, and long-wavelength subsi-dence across the entire seaway driven by dynamictopography above the shallowly subducted Faral-lon plate (e.g., Kauffman & Caldwell, 1993; Liu &Nummedal, 2004; Liu et al., 2011). The study areais located close to the coeval Sevier orogenicthrust-sheet load (

AA

M U S O D D M

V

V

Sou

th/s

outh

-wes

tN

orth

/nor

th-e

ast

Wes

t/nor

th-w

est

Eas

t/sou

th-e

ast

Fig.2.Summary

stratigraphic

cross-sectionsthroughtheStarPointSandstone,BlackhawkForm

ationandlowerCastlegate

Sandstonein

theW

asatchPlateau

(left,orientedobliqueto

depositionalstrike;afterHampsonetal.,2011)andBookCliffs(right,orientedalongdepositionaldip;afterBalsley,1980;Hampson&

Howell,2005;Hampson,2010;andreferencestherein).Thecross-sectionsare

locatedin

Fig.1.Thetopoftheupperm

ost

StarPointSandstoneparasequenceis

usedasalocaldatum

indifferentpartsoftheW

asatchPlateaucross-section,whereasthetopsofthelowerCastlegate

SandstoneandCastlegate

Sandstoneare

usedaslocaldatum

surfacesin

theBookCliffscross-section.Documentedsequenceboundariesin

theBookCliffsare

labelledusingtheterm

inologyofHowell

&Flint(2003):ASB,AberdeenSequenceBoundary;KSB,KenilworthSequenceBoundary;lSSBanduSSB,lowerandupperSunnysideSequenceBoundaries;

lGSB

and

uGSB,lowerand

upperGrassy

SequenceBoundaries;

DSB,Desert

SequenceBoundary;CSB,Castlegate

SequenceBoundary.Shallow-m

arine

parasequencesare

numberedin

theStarPointSandstone(K

Sp070-010),in

theSpringCanyon(SC4-7),Aberdeen(A

1-4),Kenilworth(K

1-5),Sunnyside(S1-3),

Grassy(G

1-4)andDesert(D

1-2)Members

oftheBlackhawkForm

ation,andin

theCastlegate

Sandstone(C1-3).Coal-zonestratigraphyin

theW

asatchPlateauis

modifiedfrom

Hampsonetal.(2011),basedonthedata

ofSanchez&

Brown(1983,1986,1987),Floresetal.(1984),Sanchezetal.(1983a,b),Brownetal.

(1987),Sanchez&Ellis

(1990),Tabetetal.(1999),Dubieletal.(2000),Gloynetal.(2003)andQuicketal.(2005).Coal-seam

stratigraphyin

theBookCliffsis

takenfrom

Gloynetal.(2003),Hampsonetal.(2005)andDaviesetal.(2006).

Alluvial to coastal plain stratigraphic architecture 2229

� 2012 The Authors. Journal compilation � 2012 International Association of Sedimentologists, Sedimentology, 59, 2226–2258

lower Castlegate Sandstone represent approxi-mately 3Æ5 to 4Æ0 Myr in combination (althoughdetailed correlation of the stratigraphic compo-nents of the Castlegate Sandstone is uncertain,such that the lower Castlegate Sandstone mayhave a longer duration; Miall & Arush, 2001).Previous work on the non-marine Blackhawk

Formation has focused on characterizing the coalresources of its lower part, which are mined inthe Wasatch Plateau coalfield (Spieker, 1931;Doelling, 1972; Hayes & Sanchez, 1979; Sanchez& Hayes, 1979; Sanchez & Brown, 1983, 1986,1987; Sanchez et al., 1983a,b; Flores et al., 1984;Brown et al., 1987; Sanchez, 1990; Sanchez &Ellis, 1990; Tabet et al., 1999, 2009; Dubiel et al.,2000; Gloyn et al., 2003; Quick et al., 2005). Thiswork has established stratigraphic schemes forthe lower Blackhawk Formation over large areasof the southern and northern Wasatch Plateau,based on outcrop and subsurface mapping of coalseams. Coal flora indicates that the climate wasseasonal and warm temperate to subtropical(Parker, 1976). A variety of channelized andsheet-like sandbodies, interpreted as fluvial-channel, distributary-channel and crevasse-splaydeposits, are present throughout the BlackhawkFormation, which represents an overall alluvial tocoastal plain setting (Marley et al., 1979; Floreset al., 1984; Adams & Bhattacharya, 2005). Thelower Blackhawk Formation is also interpreted tocontain shoreline-parallel lagoons and associatedback-barrier peat mires (Flores et al., 1984),which lay behind a series of linear to moderatelylobate, north/south-trending shorelines compris-ing wave-dominated deltas, spits, barrier islandsand strandplains that are represented by the StarPoint Sandstone (Flores et al., 1984; Dubiel et al.,2000; Hampson et al., 2011). Alluvial to coastalplain deposits in the upper Blackhawk Formationcorrespond to the widely studied, approximatelynorth/south-trending, wave-dominated shore-lines of the various shallow-marine members ofthe Blackhawk Formation that crop out in theBook Cliffs (Fig. 2) (e.g. Balsley, 1980; Hampson &Howell, 2005).The shallow-marine Blackhawk Formation in

the Book Cliffs is interpreted to form a low-frequency highstand systems tract truncated by asequence boundary at the base of the Castlegate

Sandstone (Taylor & Lovell, 1995; Van Wagoner,1995; Howell & Flint, 2003). Multiple high-frequency sequence boundaries and flooding sur-faces are documented within the low-frequency,Blackhawk Formation highstand systems tract inthe Book Cliffs (Fig. 2) (e.g. Howell & Flint, 2003;Hampson, 2010 and references therein). Thereexists no comparable sequence stratigraphicinterpretation of the non-marine Blackhawk For-mation in the Wasatch Plateau, although previouswork has highlighted several key stratigraphicrelationships. Laterally extensive coal zones inthe lower Blackhawk Formation directly overlieshallow-marine tongues (parasequences) in theStar Point Sandstone and are associated with theup-dip pinchouts of these tongues (Dubiel et al.,2000). Over a relatively small (ca 10 · 10 km)portion of the outcrop belt, channelized sand-bodies are observed to be more abundant, thickerand laterally extensive towards the top of theBlackhawk Formation (Marley et al., 1979). Chan-nelized sandbodies in the uppermost BlackhawkFormationwere deposited by similar river systemsto those that formed the overlying CastlegateSandstone, but sandbodies in the latter are muchmore strongly amalgamated (Adams & Bhatta-charya, 2005). This last relationship supports theinterpretation in the Book Cliffs of a low-frequency sequence boundary at the base of theCastlegate Sandstone (e.g. Van Wagoner, 1995;Yoshida et al., 1996; Horton et al., 2004), withreduced accommodation above the sequenceboundary resulting in greatly decreased preserva-tion of non-channelized, fine-grained deposits inthe Castlegate Sandstone. However, the interpre-tation of a sequence boundary at the base of theCastlegate Sandstone is not unambiguous, partic-ularly in distal locations of the eastern Book Cliffs,where recent work suggests that fluvial deposits inthe Castlegate Sandstone may be time-equivalentto shallow-marine deposits of the uppermostBlackhawk Formation (Pattison, 2010).

DATASET AND METHODS

The Blackhawk Formation crops out along theeastern edge of the Wasatch Plateau in centralUtah in a nearly continuous, SSW/NNE-oriented

Fig. 3. Geological map of the eastern Wasatch Plateau (Fig. 1), showing the distribution of outcrop and well dataused in this study (after Witkind et al., 1987; Weiss et al., 1990; Witkind & Weiss, 1991; Dubiel et al., 2000; Doelling,2004). Inset map (top left) summarizes the exposure quality of sandbodies along the outcrop belt; uncolouredsections of the outcrop belt are unexposed, densely vegetated or not captured by cliff-face photographs. Measuredsections shown in Figs 6 and 14 are located.

2230 G. J. Hampson et al.


MPW



cliff face that exposes up to 300 m of alluvial tocoastal plain strata over a distance of ca 100 km(Fig. 1). Several WNW/ESE-trending canyons cutthrough the cliff face. The cliff face is orientedsubparallel to the regional depositional strike ofthe wave-dominated deltaic shorelines thatconstitute the underlying Star Point Sandstone(Flores et al., 1984; Dubiel et al., 2000; Hampsonet al., 2011) and the coeval shallow-marine Black-hawk Formation in the contiguous Book Cliffs(e.g. Balsley, 1980; Hampson & Howell, 2005 andreferences therein).Although the eastern edge of the Wasatch

Plateau affords continuous exposures of theBlackhawk Formation, many outcrops occur invertical cliff faces or steep slopes that are inac-cessible. The quality of exposure is generally

high, but varies along the outcrop belt. Sandbod-ies form prominent exposed ledges (>50% expo-sure) in approximately two-thirds of the outcropbelt, but surrounding mudstones and coals aretypically covered by scree or lightly vegetatedslopes (Figs 3 and 4). Given the large scale of theexposures and their limited accessibility, many ofthe interpretations and correlations presented inthis paper are based on oblique aerial photo-graphs, captured from a near-perpendicular posi-tion to the cliff faces using a light aeroplane as acamera platform. The photographs provide acontinuous view of strata along the main east-facing cliff face, except where canyons dissectthis face and small gaps (

Sandstone were collected from accessible WNW/ESE-trending canyons (Wattis Road, StraightCanyon and Link Canyon measured sections inFig. 3). Additional measured sections have beencollected through shorter intervals of the Black-hawk Formation. The measured sections recordlithology, grain size and sorting, sedimentarystructures, palaeocurrents, body fossils and tracefossils. Conventional facies analysis has beencarried out using these measured sections. Faciesassociations identified in the measured sectionswere then grouped into two lithological classes,thick (>3 m) sandbodies and surrounding fine-grained rocks (mudstones, thin sandstones andcoals), that can be identified in the oblique aerialphotographs.Sandbody distribution in inaccessible parts of

the outcrop belt, between the measured sections,was mapped from the oblique aerial photographsusing a four-step procedure. Firstly, simple logsover the Star Point–Blackhawk Formation–Castle-gate Sandstone interval were constructed from theoblique aerial photographs at locations marked byprominent topographic features, which are readilyidentified on topographic basemaps (Figs 3 and 4).Secondly, the thickness of the Star Point–Black-hawk Formation–Castlegate Sandstone interval ineach photographic log (typically several hundredmetres)wasmeasured from topographic basemaps.Thirdly, the relative thicknesses of the BlackhawkFormation and its constituent sandbodies andfine-grained intervals, compared with the totalStar Point–Blackhawk Formation–Castlegate Sand-stone interval thickness, were used to estimate theabsolute thickness and position of each sandbody.Fourthly, continuous and discontinuous sandbod-ies were traced in between the logs using theoblique aerial photographs. A total of 195 photo-graphic logswere constructed, with spacings of 0Æ2to 26Æ9 km along the cliff face and canyon walls,corresponding to straight-line spacings of 0Æ2 to6Æ0 km (Fig. 3).The simple photograph-based method outlined

above has enabled construction of a gross strati-graphic framework over the large study area (ca100 km · 15 km; Figs 2 and 3) at reasonable timeand cost. This method is most accurate for verticalcliff faces that are nearly linear in plan view andlack vegetation or scree cover. Along non-verticalcliff faces, the top of the cliff (for example, Castle-gate Sandstone) is set further back from the camerathan the base of the cliff (for example, Star PointSandstone), which introduces a parallax effect tothe thicknesses apparent in the aerial photographstaken close to normal to the outcrop. This effect

was reduced by scaling different sections of eachphotographic log according to correspondingheights (taken from topographic basemaps) atprominent benches or breaks in the cliff face. Clifffaces that are highly non-linear in plan view arealso poorly represented in the near-perpendicularaerial photographs, because some portions of thecliff face are captured only from oblique angles.Heavily vegetated or scree-covered cliff faces areuncommon (Fig. 3), but introduce uncertainty andpotential for bias in interpreting covered sectionsof the outcrop. To minimize the impact of thesevarious effects, quantitative data obtained via thephotograph-based method from non-vertical, non-linear cliff faces with heavy vegetation or screecover are disregarded.Data that allow highly accurate (ca 0Æ1 m) spatial

resolution have been acquired by an oblique-view,helicopter-borne Light Detection and Ranging(LIDAR) system over two sections of the outcropbelt (Rittersbacher et al., in press) (Fig. 3). Thesystem is mounted to allow near-perpendicularcapture of the cliff faces, as opposed to conven-tional airborne LIDAR that is mounted to cover anadir, or near-vertical field of view, and incorpo-rates a high-resolution digital camera, enabling thecreation of textured three-dimensional (3D) out-crop models (Buckley et al., 2008). The northerndataset (ca 5 km · 2 km in area) covers a non-linear part of the cliff face that cannot be appro-priately analysed using the photograph-basedmethod outlined above. The southern dataset (ca3 km · 1 km in area) covers a near-vertical, near-linear, sparsely vegetated section of cliff face,which allows the photograph-based method to becompared with its LIDAR-based equivalent(Rittersbacher et al., in press) (Fig. 5A). Thiscomparison suggests that large (>3 m thick,>60 m wide) sandbodies are consistently identi-fied by both methods, although the photograph-basedmethodmayunder-estimateorover-estimatethe thickness of such sandbodies by several metresrelative to LIDAR-based measurements (Fig. 5B).Discrepancies in sandbodywidthsmeasured alongthe cliff face (Fig. 5C) principally reflect differ-ences in interpretation of scree-coveredparts of thecliff face, which are independent of the data-collection method. The photograph-based methodresults in small, systematic discrepancies in thevertical positions of sandbodies in the BlackhawkFormation (

thecomparisonsuggests that thephotograph-basedmethodisnotsufficientlyaccurate torobustlymakesmallmeasurements (for example, sandbody thick-ness; Fig. 5B), but it is adequate to make largemeasurementswith reasonableaccuracy (for exam-ple, sandbody position; Fig. 5D).In summary, the three principal strengths of the

dataset are: (i) its large scale; (ii) the high degree ofexposure continuity; and (iii) the proximity of thestudy area to extensively-documented shallow-marine strata. Its weaknesses are: (i) the limitedaccessibility of much of the study area, whichimposes logistical and cost constraints on datacollection; and (ii) the two-dimensional (2D)nature of much of the outcrop belt, with 3D controlprovided locally by WNW/ESE-trending canyon

systems. Aspects of 3D stratigraphic architecture,including the plan-view geometry of many sand-bodies, are poorly constrained if they occur at ascale smaller than the spacing of the cliff faces andcanyon walls. Wells drilled behind the outcropbelt for coal mining (e.g., Dubiel et al., 2000) andfor hydrocarbon exploration and production pro-vide some additional 3D control (Fig. 3).

FACIES ASSOCIATIONS

Four facies associations have been identified inthe Blackhawk Formation and overlying lowerCastlegate Sandstone (Table 1). In combination,these associations are interpreted to represent two

E

S

S

S

S

S

V

V L

LL

L

S

A B

C D

Fig. 5. Comparison of measurements based on oblique aerial photographs and helicopter-borne LIDAR data col-lected from: (A) a near-vertical, near-linear, sparsely vegetated section of cliff face south of Straight Canyon (Fig. 3);(B) sandbody thickness; (C) sandbody width; and (D) vertical position of sandbodies above the base of the BlackhawkFormation. LIDAR-based measurements are more accurate, which has greater influence on small measurements (forexample, sandbody thickness; Fig. 5B) than large measurements (for example, sandbody position; Fig. 5D). Differentinterpretations of scree-covered parts of the cliff face account for several large discrepancies in measurements ofsandbody width (Fig. 5C), while the low resolution of topographic basemaps used to calibrate photograph-basedmeasurements causes a small, systematic over-estimation of sandbody position from these data (Fig. 5D).



Table

1.

Summary

sedim

entologyoffaciesassociationsin

theBlackhawkForm

ationandCastlegate

Sandstone.Intensity

ofbioturbationis

describedusingthe

bioturbationindex(BI)schemeofTaylor&Goldring(1993).W

eatheringcharacterisusedto

interpretfaciessu

ccessionsin

photographic

logs(forexample,F

ig.4).

Facies

association

Lithologyand

sedim

entary

structures

Ichnology

Process

interpretation

Weathering

characteristics

Marginal-marineback-barrier

Tidal-inlet?

channelfills

(TIC)

Channelizedheterolithic

bodies

(<8m

thickand<700m

wide)

comprisingfineto

coarse-grained

sandstonewithcarbonaceous

mudstoneinterbeds.

Troughandtabular

cross-beds,

planarlaminationand

current-ripple

cross-lamination.

Inclinedheterolithic

strata

with

mudstonedrapes.

Discontinuous

shell-hash

andoyster-sh

ell

lags

Absentto

moderate

bioturbation

(BI=0to

4);

low-diversitytrace

fossil

assemblage

(Ophiomorpha,

Thalassinoides,

Paleophycuand

Planolites)

Interm

ittentmigrationofsandy

dunesandripples,

alternating

withdepositionofmudfrom

susp

ension,within

laterally

accretingpointbars

developed

adjacentto

meanderingchannels.

Largevariationsin

flow

velocity

across

theentire

heightofpoint

bars.Influxesofbrackishto

marine

water

Laterally

discontinuousbodies

ofgrey-brownsh

ale

withresistant,

inclinedledgesof

yellow-greyoroff-

whitesandstone

Lagoonal(L)

Carbonaceousmudstonewithrare

bedsofvery

fineto

upperfine-grained

sandstone.Parallellamination

andwave-ripple

cross-lamination.

Low

diversityfaunaofbrackish-w

ater

bivalves(forexample,oysters)

Absentto

moderate

bioturbation(BI=0to

4);low-diversitytrace

fossil

assemblage

(Thalassinoidesand

Planolites)

Depositionfrom

susp

ension,with

episodic

wavereworking.

Brackishsalinity

Soft,grey-brownsh

ale

locallycontaining

resistantledgesof

yellow-grey

sandstone

Alluvialto

coastalplain

Fluvial

sandbodies(F)

Channelizedsandbodies(25m

thick

and50to

6000m

wide)comprising

very

fineto

coarse-grainedsandstone.

Troughandtabularcross-beds,

planar-

parallellaminationandsoft-sedim

ent

folding.Architecturalelements

insandbodiesincludesandstonestrata

inclinedparallelorperpendicularto

elementmargins,

near-horizontal

sandstonestrata

andchannelized

mudstones.

Most

sandbodieshave

internallycomplexgeometries,

with

multilateraland/ormultistorey

stackingofarchitecturalelements

Bioturbationtypically

absent(BI=0),

althoughrare

bored

logs( T

eredolites,

BI=1)maybe

present.Some

sandbodieshave

root-penetratedtops

Migrationofsandydunesand

barform

swithin

andadjacentto

channels.Architecturalelements

record

downstream

andlateral

accretionofbarform

s,active

(sandstone-dominated)fillingof

channels

andpassive

(fine-grained)fillingof

abandonedchannels.Multilateral

andmultistory

stackingof

architecturalelements

records

preservationofchannelbeltsand

complexes

Laterally

discontinuous,

resistant,

cliff-form

ing,

yellow-greyor

off-w

hitesandstone.



gross environments, marginal-marine back-barrierand alluvial to coastal plain.

Marginal-marine back-barrier faciesassociations

Two facies associations (L and TIC; Table 1)generally occur in close association as a minor(

n = 3

n = 3

n = 1

n = 1

n = 1

n = 3

n = 1

n = 1

n = 1

n = 3

n = 1

n = 2

n = 1

n = 5

n = 2

n = 1

n = 2

n = 2

n = 1

n = 3

n = 2

n = 1

n = 2

n = 3

TT

FWCDMEBCRPPS

M

FA

LT

U



they may represent tidally influenced fluvialchannels where observed in other stratigraphiccontexts (for example, 61 to 68 m in Fig. 6).

Alluvial to coastal plain facies associations

Two additional facies associations (F and AF;Table 1) occur as the major (>90%) component ofthe lower, coal-prone part of the BlackhawkFormation, and they are the sole (100%) compo-nent of the upper, coal-poor part of the Black-hawk Formation and the Castlegate Sandstone.

DescriptionPlanar-parallel laminated, trough and tabularcross-bedded, very fine to coarse-grained sand-stones occur in laterally discontinuous, erosion-ally based, channelized bodies of variabledimensions (up to 25 m thick and 50 to 6000 mwide; facies association F) (Figs 6, 7A and B).Channelized sandbodies typically have complexinternal architectures formed by the lateral and/or vertical stacking of multiple architecturalelements (Fig. 8), each of which comprisessandstone strata inclined parallel to architec-tural-element margins and perpendicular to meanpalaeocurrent direction (Fig. 7A), sandstonestrata inclined perpendicular to architectural-element and parallel to mean palaeocurrentdirection (Fig. 7B), near-horizontal sandstonestrata that lap onto channelized erosion surfacesat architectural-element margins, or rare chan-nelized mudstones. Lags of wood fragmentscommonly line the base of channelized sand-bodies and their component architectural ele-ments (Fig. 7D), and soft-sediment folding iscommon (Fig. 7E). Channelized sandbodieseither erode directly into shallow-marine para-sequences of the Star Point Sandstone, or theyare encased in root-penetrated fine-grainedsuccessions (facies association AF) in the Black-hawk Formation.Root-penetrated fine-grained successions (facies

association AF) comprise carbonaceous mud-stones that contain palaeosols, including coals,and minor, interbedded, very fine to medium-grained sandstones. These deposits lie adjacent to,and typically encase, channelized sandbodies(facies association F) (Fig. 6). Sandstones withinthe facies association occur as thin (

Intervening mudstones record deposition fromsuspension during smaller, more frequent riverfloods. Isolated roots within the facies associationrecord periods of short-lived emergence and plantcolonization of the floodplain, whereas coal

seams and other palaeosols represent hiatuses inclastic deposition. Although coals are presentthroughout the Blackhawk Formation (Fig. 2),they are more common in its lower part in theWasatch Plateau, indicating better development

A

B

C

F G H

D E



and/or preservation of peat mires in a beltadjacent to the shoreline (Flores et al., 1984).

Distinction between facies associations indifferent data types

The four facies associations outlined above andsummarized in Table 1 can be clearly distin-guished in measured sections, but not inoblique aerial photographs or geophysical welllogs. In interpretations derived solely from thesedata, channelized sandstones belonging to twofacies associations (TIC and F; Table 1) andfine-grained deposits belonging to two faciesassociations (L and AF; Table 1) and includingfine-grained channel fills are grouped together(for example, Fig. 4B). Thus, photograph-basedinterpretations of the Blackhawk Formationalluvial to coastal plain deposits are essentiallyof lithology rather than of facies association.

STRATIGRAPHIC FRAMEWORK ANDSANDBODY DISTRIBUTION

The base of the Blackhawk Formation is definedby a series of shallow-marine parasequences inthe Star Point Sandstone (Flores et al., 1984;Dubiel et al., 2000; Hampson et al., 2011).These parasequences trend oblique to the out-crop belt, and are vertically stacked such thatthe base of the Blackhawk Formation becomesyounger from south to north (for example, theareal extent of the Blackhawk Formation in-creases successively from Fig. 9A to H). Thus,the base of the Blackhawk Formation climbsstratigraphically to the north (Flores et al., 1984;Dubiel et al., 2000; Hampson et al., 2011). Thetop of the Blackhawk Formation is defined byits contact with the overlying Castlegate Sand-

stone. The Blackhawk Formation has a rela-tively uniform thickness over the outcrop belt,such that the base of the Castlegate Sandstoneappears to step upwards from south to north inthe same manner as the base of the BlackhawkFormation. However, the nature of apparentstratigraphic climb of the Blackhawk–Castlegateboundary is unresolved, because the upperBlackhawk Formation lacks marker beds (forexample, coal seams) that would constrainstratal geometries, such as erosional truncationand angular divergence in the interval.

Coal zones and stratigraphic subdivision of thelower Blackhawk Formation

The stratigraphic framework of the lower part ofthe Blackhawk Formation is defined by a series oflaterally continuous coal zones, each containingone or more closely spaced coal seams (Fig. 2).Coal zones have been mapped previously by theUS Geological Survey using a combination ofmine records, boreholes and outcrop data inadjacent and overlapping segments of theWasatch Plateau (Hayes & Sanchez, 1979;Sanchez & Hayes, 1979; Sanchez & Brown, 1983,1986, 1987; Sanchez et al., 1983a,b; Brown et al.,1987; Sanchez & Ellis, 1990; Sanchez, 1990;Dubiel et al., 2000) that form the basis of recentestimates of remaining coal resources by the UtahGeological Survey (Tabet et al., 1999, 2009; Gloynet al., 2003; Quick et al., 2005).Historically, names were assigned to each coal

zone within a particular mine, which resulted insome inconsistent use of stratigraphic nomencla-ture between the various mines; the same namewas applied locally to different coal zones, anddifferent names were assigned locally to the samecoal zone (Flores et al., 1984; Dubiel et al., 2000).Such inconsistencies were mostly resolved by the

Fig. 7. Photographs illustrating alluvial to coastal plain deposits (Table 1). Fluvial sandbody facies association: (A)palaeocurrent-perpendicular view of a channelized sandbody containing lateral accretion surfaces; (B) palaeocurrent-parallel view of a channelized sandbody containing a large cross-set with several foresets containing small super-imposed cross-sets of the same orientation; these cross-strata are interpreted to record a barform with superimposed,downstream-accreting dunes; (C) log and (D) Teredolites-bored log, indicating marine influence, within lag liningerosional base of channelized sandbody; and (E) large soft-sediment fold within sandbody. Person for scale is ca 1.8 mtall. Aggradational floodplain facies association: (F) stacked sheet sandstones and root-penetrated siltstones; (G)current-ripple cross-lamination within sheet sandstone; and (H) cross-bedded and current-ripple cross-laminatedsandstone sheet penetrated by deep roots and capped by pedogenically mottled siltstones and a thin coal. Photograph(A) is fromwest of Link Canyon (from ca 90 to 100 m above the top of the Star Point Sandstone; Fig. 10C), photographs(C)/(D), (E) and (H) are from Link Canyon (respectively from ca 215 m, 235 m and 131 m in Fig. 6; Figs 2, 3 and 10C),photograph (B) is from Straight Canyon (from ca 207 m above the top of the Star Point Sandstone and ca 55 m belowthe base of the Castlegate Sandstone; Figs 2 and 3), and photographs (F) and (G) are from Ferron Creek (respectivelyfrom ca 0 to 12 m and 16 m below the base of the Castlegate Sandstone; Figs 2 and 3). Tape measure with centimetremarkings for scale in photograph (D) and pencil for scale (15 cm long) in photographs (G) and (H).



A B

Fig.8.(A

)Uninterpretedand(B)interpretedphotopanelofachannelizedfluvialsandbodyhighlightingmajorinternalerosionsu

rfacesthatboundchannel

storeysandchannelbelts.Thesandbodyhasamultilateral,multistoreyinternalarchitecture,andis

locatedin

theupperBlackhawkForm

ationatLinkCanyon

(ca229to

243m

inFig.6;Fig.10E).



work of the US and Utah Geological Surveys,although one remains in the data-poor, north-western part of the study area where outcrop issparse and exposure is poor. In this area, thecorrelations of Sanchez (1990) are followed.Sanchez (1990) interpreted the Flat Canyon and

Lower O’Connor coal zones to overlie, respec-tively, the shallow-marine sandstones of thePanther Tongue and Storrs Tongue in up-diplocations, rather than to correspond to the lowerpart of the Spring Canyon Member (Tabet et al.,1999; Gloyn et al., 2003) (Fig. 2). In addition, the

Thick (>1·2 m) coal seam













A B C

D E F



Axel Andersen and Cottonwood coal zones havebeen grouped together. Brown et al. (1987)mapped these zones at two closely spaced(1·2 m) coal seam


U

T

M

Z



G H

Fig. 9. Maps showing the extent of mineable coal (>1.2 m thick) in selected coal zones and associated shorelinedisplacements during parasequence-bounding transgressions (Fig. 2): (A) Upper Last Chance coal zone (Quick et al.,2005) and transgression between parasequences KSp050 and KSp040 (Hampson et al., 2011); (B) Knight and FlatCanyon coal zones (Tabet et al., 1999; Gloyn et al., 2003; Quick et al., 2005) and transgression between parase-quences KSp040 and KSp030 (after Sanchez, 1990; Hampson et al., 2011); (C) Accord Lakes and Lower O’Connorcoal zones (Tabet et al., 1999; Gloyn et al., 2003; Quick et al., 2005) and transgressions between parasequences KSp-030 and KSp020 (after Sanchez, 1990; Hampson et al., 2011) and between parasequences KSp020 and KSp010 (afterHampson et al., 2011); (D) Axel Andersen, Cottonwood and Spring Canyon coal zones (Tabet et al., 1999; Gloynet al., 2003; Quick et al., 2005) and transgressions between parasequences KSp010 and SC4 (after Hampson &Storms, 2003; Hampson et al., 2011) and between parasequences SC4 and SC5 (after Kamola & Van Wagoner, 1995;Hampson & Storms, 2003); (E) Blind Canyon and Hardscrabble coal zones (Kamola & VanWagoner, 1995; Tabet et al.,1999; Gloyn et al., 2003; Quick et al., 2005) and transgression between parasequences SC5 and SC6 (Kamola & VanWagoner, 1995); (F) Bear Canyon and Helper coal zones (Kamola & Van Wagoner, 1995; Tabet et al., 1999; Gloynet al., 2003) and transgression between parasequences SC7 and A0 (after Sanchez & Ellis, 1990; Kamola & VanWagoner, 1995; Charvin et al., 2010); (G) Castlegate A and Wattis coal zones (Tabet et al., 1999; Gloyn et al., 2003)and transgression between parasequences A1 and A2 (after Kamola & Huntoon, 1995; Charvin et al., 2010); and (H)Bob Wright, McKinnon and Castlegate B coal zones (Tabet et al., 1999; Gloyn et al., 2003) and transgression betweenparasequences A2 and A3 (Kamola & Huntoon, 1995). The extent of each transgression is taken from the down-dippinchout of foreshore and upper-shoreface deposits in the underlying parasequence and their up-dip pinchout in theoverlying parasequence.



developed during transgressions, each of whichdisplaced the shoreline from its maximum regres-sive extent in the underlying parasequence to itsmaximum landward position at the up-dip pinch-out of the overlying parasequence. Their inferredtransgressive stratigraphic context is consistentwith an interpretation of coal zones recordinghiatuses or significant reductions in clastic sed-imentation synchronous with a rising groundwater table (cf. Bohacs & Suter, 1997). Two coalzones are interpreted to split down-dip, with theseam split containing a shallow-marine para-sequence (Accord Lakes and Axel Andersen–Cottonwood–Spring Canyon coal zones; Fig. 2).By implication, these coal zones record twoperiods of transgression and an interveningregression. Davies et al. (2006) document similarstratigraphic relationships in the Sunnyside Coalof the Book Cliffs (Fig. 2), with transgressive andregressive components of this coal seam having adistinctive petrographic expression. A series ofmaps showing the extent of selected coal zones(after Tabet et al., 1999; Gloyn et al., 2003; Quicket al., 2005) and their associated transgressiveshoreline displacements (after Sanchez, 1990;Sanchez & Ellis, 1990; Kamola & Huntoon,1995; Kamola & Van Wagoner, 1995; Dubielet al., 2000; Hampson & Storms, 2003; Charvinet al., 2010; Hampson et al., 2011) is presented inFig. 9. The extent of each transgression is takenfrom the down-dip pinchout of foreshore andupper-shoreface deposits in the underlying para-sequence and their up-dip pinchout in the over-lying parasequence (cf. method to determineextent of regression in Kamola & Huntoon,1995). In some cases, coal zones are partiallydeveloped and preserved beneath the basal trans-gressive surface of an overlying parasequence(Fig. 9C, F and G), indicating that shorelineretreat was accompanied by aggradation andpreservation of coastal-plain deposits (i.e. accre-tionary transgressive shoreline trajectory, sensuHelland-Hansen & Martinsen, 1996). Typically,thick (>1Æ2 m) coals accumulated in somewhatpatchy, shoreline-parallel belts that extend for 8to 42 km palaeolandward of the up-dip pinchoutof an overlying parasequence (Fig. 9). Similarrelationships have been noted in other coal-bearing coastal-plain strata in the CretaceousWestern Interior, particularly where coals aredeveloped in association with vertically stackedshallow-marine parasequences that record long-term aggradation (e.g. Ryer, 1981; Cross, 1988), asin the Star Point Formation and time-equivalentlower Blackhawk Formation (Fig. 2). The wide-

spread extent of the coal zones defines a high-resolution stratigraphic framework in the lowerBlackhawk Formation.

Stratigraphic subdivision of the upperBlackhawk Formation

The upper part of the Blackhawk Formationlacks mappable coal zones, bentonites or otherstratigraphic marker beds in the Wasatch Pla-teau (Fig. 2). Consequently, this interval cannotbe robustly subdivided using such markers.Instead, the upper part of the BlackhawkFormation has been split into three intervalsby projecting two coal zones from the BookCliffs (Kenilworth–Castlegate D and Rock Can-yon coal zones; Fig. 2) into the Wasatch Plateau.This projection is relatively closely constrainedin the northern Wasatch Plateau, where se-quence stratigraphic surfaces have been tenta-tively correlated into the non-marine BlackhawkFormation in the area around Price Canyon(Fig. 2) (e.g. Hampson et al., 2005), but becomesincreasingly uncertain further south. Strati-graphic intervals bounded by coal zones main-tain a uniform thickness in the lowerBlackhawk Formation (Fig. 2); if stratigraphicintervals in the upper Blackhawk Formationalso have a uniform thickness across theWasatch Plateau, then the base of the CastlegateSandstone represents an angular unconformitythat cuts down ca 200 m towards the south overa distance of 120 km (Fig. 2) (cf. Quick et al.,2005). Alternatively, these stratigraphic intervalsmay thin and their boundaries converge towardsthe south, such that the base of the CastlegateSandstone is marked by either a less pro-nounced angular unconformity or a non-angulardisconformity. In the analysis of overall sand-body distribution below, it is assumed thatstratigraphic intervals in the upper BlackhawkFormation have a uniform thickness across theWasatch Plateau (for example, Fig. 10). Althoughthe three-fold subdivision of the upper Black-hawk Formation in this study is poorly con-strained and its bounding coal seams are chosensomewhat arbitrarily, it does allow gross trendsin stratigraphic architecture to be identified andcharacterized.

General trends in sandbody distribution

Channelized fluvial and tidal-inlet sandbodies(facies associations F and TIC; Table 1) and fine-grained floodplain and lagoonal deposits (facies



associations AF and L; Table 1) were interpretedin each of the measured sections and photo-graphic logs, and then traced out along the clifffaces and canyon walls in the context of thestratigraphic framework described above (Fig. 2).Figure 10 presents the resulting sandbody distri-butions and stratigraphic architectures in cross-sectional panels along seven well-exposed partsof the cliff line (sandbody exposure of >50% inFig. 3). Only sparse sandbody-mapping andpalaeocurrent data are available to constrain theorientation and 3D geometry of the fluvial sand-bodies, which remain uncertain. However, theavailable data (for example, palaeocurrents inFig. 6) indicate a broad spread around meanpalaeoflow towards the east, perpendicular toregional shoreline trends (Fig. 9).Figure 11 presents maps showing lateral varia-

tions in the proportion of channelized sandstoneat photographic logs along the cliff line (Fig. 3),which corresponds to net to gross ratio in tightgas reservoirs in the nearby Uinta and Piceancebasins (e.g. Johnson, 1989; Johnson & Roberts,2003). The maps are constructed for four strati-graphic intervals of the Blackhawk Formation:(i) a coal-prone interval whose upper boundary ismarked by the Bear Canyon coal zone and thebase of the shallow-marine Aberdeen Member(Figs 2, 9F and 10); (ii) a coal-poor intervalbounded below by the Bear Canyon coal zoneand above by the projected Kenilworth–Castle-gate D coal zone (Figs 2 and 10); (iii) a coal-poorinterval bounded below by the projected Kenil-worth–Castlegate D coal zone and above by theprojected Rock Canyon coal (Figs 2 and 10); and(iv) a coal-poor interval bounded below by theprojected Rock Canyon coal and above bythe Castlegate Sandstone (Figs 2 and 10). Chan-nelized sandstone proportions of 25% and 65%correspond to the thresholds around which

randomly distributed channelized sandbodiesare connected in 2D cross-sections and 3D vol-umes, respectively (King, 1990; Larue & Hovadik,2006).Despite the clear stratigraphic organization of

shallow-marine deposits in the Book Cliffs intoparasequences with distinctive stacking patterns(Fig. 2), there is strikingly little organization in thethe spatial distribution of channelized sandbodiesin coeval alluvial to coastal plain strata. In thecoal-prone, lower Blackhawk Formation (Fig. 10),channelized sandbodies tend to occur in strati-graphic packages 10 to 30 m thick that are boundedby laterally extensive coal zones developed dur-ing transgression (cf. shallow-marine parase-quences). However there is little apparentstratigraphic organization in sandbody size, abun-dance or spacing either within or between thesecoal-zone-bounded stratigraphic packages, withthe exception of large (up to 25 m thick and 1 to6 km wide) multistorey, multilateral channel-complex sandbodies that occur at distincthorizons and are discussed further below. Thecoal-poor, upper Blackhawk Formation (Fig. 10)also lacks laterally consistent stratigraphicorganization at 10 to 30 m vertical scale (cf.coal-zone-bounded stratigraphic packages in thelower Blackhawk Formation) and at larger scales.This observation suggests that relative sea-level,which controlled shallow-marine stratigraphicorganization in the Blackhawk Formation (e.g.Kamola & Huntoon, 1995; Howell & Flint, 2003;Hampson, 2010), exerted only minor influence onthe organization of coeval alluvial to coastal plainstrata in the upper Blackhawk Formation, despiteevidence for marine influence in the form ofTeredolites-bored logs in sandbodies within thesestrata (Figs 6 and 7D).Overall, the size and abundance of channelized

sandbodies tend to increase from the base of the

Fig. 10. (A) to (G) Panels showing stratigraphy and facies architecture in the Blackhawk Formation mapped alongseven well-exposed, near-linear sections of the main cliff line along the eastern edge of the Wasatch Plateau (Fig. 3).Each panel uses a different local datum, at the top of a shallow-marine parasequence in the underlying Star PointSandstone. Different parasequences (numbered from oldest, KSp050, to youngest KSp010, after Hampson et al.,2011) are used in panels (A) to (G); older parasequences are used in the south [for example, the KSp050 parasequencein panel (A)] and younger parasequences in the north [for example, the KSp010 parasequence in panel (G)], reflectingthe diachronous, northward-younging character of the Star Point Sandstone. The local datum for each panel ismarked by laterally extensive foreshore deposits, which define an approximately palaeohorizontal surface at the topof the selected parasequence. Only the upper part of the Star Point Sandstone is shown in each panel (see Hampsonet al., 2011 for a fuller treatment of Star Point Sandstone stratigraphy). Correlateable coal zones (Fig. 2) are shown(after Sanchez & Brown, 1983, 1986, 1987; Sanchez et al., 1983a,b). The projected positions of the Bear Canyon,Kenilworth–Castlegate D and Rock Canyon coal zones, which are used to subdivide the Blackhawk Formation intofour gross intervals (Fig. 11), are shown. Measured sections in Figs 6 and 14, cliff-face photographs in Figs 4, 7A, 8and 12, and the stratigraphic level of the sandbody map in Fig. 13 are located. Additional measured sections (Figs 2and 3) lie in between the panels.



Blackhawk Formation to its top, although there ismuch local variability in this general trend (forexample, the opposite vertical trends are apparentin Fig. 10G). The proportion of channelizedsandbodies is ca 10% in the lower BlackhawkFormation (Fig. 11A) and increases to ca 30% inthe upper Blackhawk (Fig. 11D). It is worthnoting that most of the channelized sandbodieshave complex internal architectures which indi-cate that they represent channel belts and com-plexes composed of multiple laterally andvertically stacked storeys (Fig. 8; Table 1). With-out detailed analysis of the internal architectureof the sandbodies, it is not possible to distinguishthe impact on channelized sandbody size ofvariations in storey dimensions (a proxy for riverchannel morphology) or lateral and vertical amal-gamation of storeys (a proxy for river channelmigration). Thus, it is not yet clear to what extentthese two effects give rise to the general trends ofupward-increasing size and abundance of chan-nelized sandbodies.

Motifs in stratigraphic architecture

The stratigraphic architectures in the cross-sectional panels (Fig. 10) contain three recurring

motifs at a scale that is larger than the lateral andvertical stacking of architectural elements withinindividual channelized sandbodies (Fig. 8) andsmaller than the formation-scale trends in sizeand abundance of channelized sandbodies de-scribed above. These three motifs are describedand interpreted below.

Deep erosion at the base of the BlackhawkFormationDeep (up to 18 m) but narrow (

AB

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Fig.12.(A

),(C)and(E)Uninterpretedand(B),(D

)and(F)interpretedcliff-facephotographsillustratingstratigraphic

architecturalmotifs

inthecoastalplain

deposits

oftheBlackhawkForm

ation:(A

)and(B)channelizedmudstonedeeply

erodedinto

underlyingsh

allow

marinesandstonesoftheStarPointForm

ation

(cliff

facenorth-east

ofConvulsion

Canyon;Figs3and

10C);

(C)and

(D)multistorey,multilateralchannelcomplexconfined

byadeep

composite

erosion

surface,interpretedasanincisedvalleyfill(clifffacesouth

ofHuntingtonCreek;Figs3and13);and(E)and(F)seriesofverticallystackedandlaterallyoffset

channelizedsandbodies,withchannelaxesinterpretedto

beorientednear-perpendicularto

thecliffface(clifffacenorth-east

ofRockCanyon;Figs3and10F).

Keyasin

Fig.4.



have resulted from: (i) regressive erosion bydeltaic distributary channels; (ii) forced regres-sive erosion at the base of incised valleys; and/or (iii) transgressive erosion by tidal inletchannels. The narrow width, simple geometryand uniform character of the mudstone-filledchannels imply that an incised-valley origin isunlikely. Moreover, erosion was not confined tothe stratigraphic level at the top of any parti-cular parasequence. Tidal inlet bodies aregenerally sandstone-dominated and may containpronounced lateral accretion (e.g. Kumar &Sanders, 1974; Hayes, 1980; Siringan & Anderson,1993); sandbodies of this type are documentedin the lowermost Blackhawk Formation(Hampson et al., 2011) and correspond to theshallow, broad channelized sandbodies noted inthe cliff-face photographs. The low width/depthratios of the mudstone-filled channels (apparentwidth to depth ratios of 8 : 1 to 160 : 1 in clifffaces) are consistent with a deltaic distributaryorigin (Reynolds, 1999; Gibling, 2006). In par-ticular, the distal reach of a river, where it isdirectly influenced by the receiving basin

(‘backwater reach’ sensu Parker et al., 2008),may be characterized by rapid flow decelerationthat limits bedload transport, causing starvationof sand-grade sediment and localized incisionin downstream locations (e.g. Petter, 2010).

Multistorey, multilateral channel complexesconfined by deep composite erosion surfacesLarge (up to 25 m thick and 1 to 6 km wide)multistorey, multilateral channel-complex sand-bodies that are bounded at their base by com-posite erosion surfaces occur at distinctstratigraphic horizons, particularly in the lowerBlackhawk Formation (Fig. 12C and D). In somecases, several such sandbodies are mapped toerode down from the same stratigraphic level(Figs 10G, 12C and D) and form a network(Fig. 13A), implying the development of a sub-regional, high-relief surface of fluvial erosion.Measured sections through these large channel-complex sandbodies indicate that locally thereis increasing preservation of architectural ele-ments from their bases to their tops, althoughthere is no difference in the dimensions or

M

Mn = 2

n = 3

n=17

n=21

PWell

LTC z

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D

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P

U

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Fig. 13. (A) Map showing the extent of a multistorey, multilateral channel complex confined by a deep compositeerosion surface, based on cliff face exposures (Figs 10G, 12C and D), coal mine data (fig. 14 in Mayo et al., 2003) andpalaeocurrents from measured sections (for example, Fig. 14). The complex is interpreted as an incised valleynetwork. (B) Map illustrating the regional palaeogeographic context of the channel complex during maximumregression of SC4 parasequence (after Kamola & Van Wagoner, 1995; Hampson & Howell, 2005).



lithological character of the architectural ele-ments (Fig. 14).Three lines of evidence suggest that these

large multistorey, multilateral sandbodies areincised valley fills, based on the diagnosticcriteria of Zaitlin et al. (1994) and dimensionaldata for various types of channelized sandbody(Reynolds, 1999; Gibling, 2006): (i) the deepincision and widespread extent of compositeerosion surfaces at sandbody bases; (ii) theirmultistorey and multilateral character, withincreasing-upward preservation of architecturalelements recording an upward increase inaccommodation during valley infilling; and(iii) their large dimensions, which are greaterthan those of other channelized sandbodies inthe Blackhawk Formation but are consistentwith incised valley fills. However, three otherdiagnostic criteria of incised valley fills (Zaitlinet al., 1994) are lacking: (i) basinward shifts of

facies across the base of the sandbodies cannotbe demonstrated; (ii) there is no evidencewithin the sandbodies to support an upwardchange from fluvial to estuarine deposition; and(iii) interfluves characterized by well-developedpalaeosols have not yet been documented. Thepresent authors favour the interpretation of thesandbodies as either incised valley fills, whichwere initiated during falling relative sea-level,in locations beyond the up-dip limit of estua-rine deposition, or as fluvial valley fills formedby an upstream control such as climaticallydriven variability in fluvial discharge. Theformer interpretation is more likely for largemultistorey, multilateral sandbodies in the low-er part of the Blackhawk Formation (Figs 12C, Dand 13A), which were developed close to coevalshorelines in the shallow-marine Spring CanyonMember (for example, Fig. 13B). The two mostlikely candidates for incised valley fills underliethe Blind Canyon and Bear Canyon coal zones(Figs 10G, 13 and 14), and correspond respec-tively to the SC5 and SC7 parasequences of theSpring Canyon Member (Fig. 2); sequenceboundaries have not been previously interpretedat these two stratigraphic levels in the BookCliffs outcrops (Kamola & Van Wagoner, 1995),which provide only limited exposure alongdepositional strike (3 to 5 km). Sequence bound-aries of comparable vertical spacing have alsobeen interpreted at the base of similar multi-storey, multilateral channel-complex sandbodiesin the broadly coeval Masuk Formation of theHenry Mountains Syncline, to the south of theWasatch Plateau (Corbett et al., 2011).

Vertically stacked and laterally offset(‘clustered’) channelized sandbodiesThe upper Blackhawk Formation locally con-tains clusters of vertically stacked and laterallyoffset channelized sandbodies that are not con-fined by a ‘master’ composite erosion surface(Fig. 12E and F). The complexes defined bythese stacked, clustered sandbodies do notoccur at distinct stratigraphic intervals or indistinct palaeogeographic regions (Figs 10F, 12Eand F). Each of the channelized sandbodieswithin such complexes can be traced laterallyinto aggradational floodplain or lagoonal depos-its (facies associations AF and L; Table 1), butthe detailed nature of these transitions has notyet been documented. The complexes are rare,containing

contain apparently isolated channelized sand-bodies with a variety of multilateral and/ormultistorey internal architectures (Figs 10F, 12Eand F).In the absence of an apparent stratigraphic or

palaeogeographic control on their distribution,complexes of vertically stacked and laterallyoffset channelized sandbodies were probablyformed by large-scale avulsion of the BlackhawkFormation river systems (cf. Hajek et al., 2010).The architectures are interpreted to result fromclustering of successive channel-belt sandbodiesin an alluvial to coastal plain depocentre, untilthis region was sufficiently elevated aboveadjacent parts of the alluvial to coastal plainto cause regional avulsion of the entire channelsystem to a new depocentre (Mackey & Bridge,1995; Jerolmack & Paola, 2007). This mechanismis consistent with the coal-poor, alluvial toupper coastal plain setting of the upper Black-hawk Formation. The occurrence of verticallystacked and laterally offset channelized sand-bodies in strata that also contain isolated chan-nelized sandbodies implies that channel beltswere episodically active on aggrading parts ofthe floodplain adjacent to locations character-ized by sandbody clustering.

DISCUSSION: CONTROLS ONSTRATIGRAPHIC ARCHITECTURE

In the preceding section, allogenic controls andautogenic processes have been interpreted forsome components of the Blackhawk Formationstratigraphic framework and the architecturalmotifs that recur within it: (i) laterally continuouscoal zones in the lower Blackhawk Formationdeveloped during transgressions that are ex-pressed as flooding surfaces within coeval shal-low-marine strata; (ii) the size and abundance ofchannelized sandbodies generally increases up-wards within the Blackhawk Formation, implyinggreater lateral and vertical amalgamation of chan-nel-belt sandbodies, and possiblewidening and/ordeepening of channel belts, in the upper Black-hawk Formation (although significant local varia-tions occur; for example, Fig. 10G); (iii) with asmall number of exceptions, channelized sand-bodies in the Blackhawk Formation display littleapparent stratigraphic organization at scales smal-ler than the regional trend noted above, indicatingthat high-frequency variations in allogenic con-trols do not dominate stratigraphic architecture;(iv) anomalously large multistorey, multilateral

channel-complex sandbodies that locally overliedeep, composite erosion surfaces in the lowerBlackhawk Formation are interpreted as fluvialincised valley fills; and (v) clusters of verticallystacked and laterally offset channelized sand-bodies that occur locally in the upper BlackhawkFormation are tentatively interpreted to haveformed by a large-scale avulsion process(es).Figure 15 portrays the stratigraphic framework ofthe Blackhawk Formation, and large-scale patternsof fluvial sandbody distribution within the forma-tion. Aspects of stratigraphic architecture thatdrive these interpretations of allogenic controlsand autogenic processes are shown schematically.Key parameters that constrain the regional contextof the Blackhawk Formation outcrops are alsoshown. The distance from the coeval shoreline isplotted for the southern and northern limits of theoutcrop belt,measuredperpendicular to thedown-dip pinchout of foreshore and upper-shorefacedeposits mapped in specific shallow-marine para-sequences (KSp050,KSp040,KSp030,KSp020 andKSp010 parasequences, Hampson et al., 2011;SC4, SC5 and SC7 parasequences, Kamola & VanWagoner, 1995; A1 andA4 parasequences, Kamola& Huntoon, 1995; K1 and K4 parasequences,Taylor & Lovell, 1995; C3? parasequence, Hamp-son, 2010; Fig. 2). The distance from the sedimentsource is taken from structural restorations of theSevier fold and thrust belt (DeCelles & Coogan,2006), with sediment interpreted to have beenderived from the Canyon Range and SantaquinCulminations (Horton et al., 2004; DeCelles &Coogan, 2006) (Fig. 16). Sediment accumulationrates are shown for specific stratigraphic intervals(equivalent to parasequences KSP020-SC7, A1-K1,K2-K4, K5-S1 and C1-C3? in Fig. 2), based on theirthickness and interpreted duration. The latter isassociated with large uncertainty. Sediment accu-mulation rates are a proxy for tectonic subsidencerates. Interpretive block diagrams based on theseintegrated data are presented in Fig. 16.

High-resolution patterns within theBlackhawk Formation

Analysis of the data synthesized in Fig. 15 sug-gests that relative sea-level exerted a significantcontrol on stratigraphic architecture for onlyseveral tens of kilometres palaeolandward of thecoeval shoreline (up to 50 km palaeolandward ofthe up-dip pinchout of shallow-marine parase-quences), based on the distribution of coal zonesand candidate incised valley fills. Autogenicprocesses, in the form of apparent stratigraphic



disorder and sandbody clustering, predominatedin more upstream locations. The upstream limitof relative sea-level influence on stratigraphicarchitecture in the Blackhawk Formation iscomparable to the distance from the shoreline tothe upstream limit of coastal onlap in well-documented, modern coastal plains (40 to 400 km;table 1 in Blum & Törnqvist, 2000). However, thelatter were also lengthened seaward for largedistances during late Pleistocene relative sea-level lowstands (50 to 800 km; Table 1 in Blum &Törnqvist, 2000), in response to high-frequency,glacio-eustatic sea-level falls with amplitudes ofca 100 m. In contrast, the Blackhawk Formationcoastal plains were lengthened by much shorterdistances (9 to 80 km; progradational componentsof shoreline trajectory in fig. 13 of Hampson et al.,2011 and fig. 14 of Hampson, 2010), indicatingmuch lower amplitudes of high-frequency rela-tive sea-level variation (

Low-resolution trends from base to top of theBlackhawk Formation

The overall upward increase in the size andabundance of channelized sandbodies in the

Blackhawk Formation can be attributed to a num-ber ofmechanisms thatmayhave operated over the3.5 to 4.0 Myr duration of the Blackhawk Forma-tion and lower Castlegate Sandstone. Firstly, thistrend may represent gradually decreasing tectonic

A

B

C

A C

M

FAWO

A C

R

AA

C

S

F

Fig. 16. Block diagrams illustrating schematic, source to shoreline regional context, and interpreted controls onalluvial to coastal plain stratigraphic architectures and large-scale patterns of sandbody distribution for selectedstratigraphic intervals: (A) lower Blackhawk Formation (regression of SC7 shoreline, below Bear Canyon and Helpercoal zones; Figs 2, 9F, 10 and 15); (B) upper Blackhawk Formation (regression of K1 shoreline, below Kenilworth andCastlegate D coal zones; Figs 2, 10 and 15) and (C) lower Castlegate Sandstone (Figs 2, 10 and 15). Shorelinepalaeogeographies are based on data from the Book Cliffs (Kamola & Van Wagoner, 1995; Taylor & Lovell, 1995;Hampson, 2010), proximal facies distributions from patchy outcrops to the west of the Wasatch Plateau (Lawton,1986; Robinson & Slingerland, 1998), and tectonic subsidence patterns on data from the Wasatch Plateau (this paper)and Book Cliffs (Hampson, 2010). Active thrust faults (after Horton et al., 2004; DeCelles & Coogan, 2006) andreconstructed topography of the Sevier fold and thrust belt (DeCelles & Coogan, 2006) are shown. The present authorsinfer that fluvial sediment was, in part, routed between the northern tip of the Paxton thrust and the southern tip ofthe Charleston-Nebo thrust system, near the north-western limit of the study area (Edwards et al., 2005; Hampsonet al., 2011). Fluvial channel-belt sandbodies are not drawn to scale.



subsidence during deposition of the BlackhawkFormation, with an abrupt apparent decrease intectonic subsidence across the unconformablebase of the Castlegate Sandstone (e.g. Adams &Bhattacharya, 2005). This interpretation assumesthat river avulsion rate remained relatively uni-form through deposition, such that the abundanceof fluvial sandbodies is directly proportional to therate at which accommodation was generated bytectonic subsidence. Secondly, the rate of riveravulsion may have gradually increased duringdeposition of the Blackhawk Formation, with anabrupt apparent increase in avulsion rate acrossthe base of the Castlegate Sandstone (cf. Heller &Paola, 1996). Thirdly, the upward increase in thesize and abundance of channelized sandbodiesmay record the progressive advance of a fluvialsystem characterized in its upstream part by largeand well-connected, sandstone-dominated feederchannels that become progressively smaller, lessabundant (or less sand-rich) and more poorlyconnected downstream (cf. Castlegate Sandstoneriver system portrayed in fig. 51 of Van Wagoner,1995). Downstream decreases in channel size andsand content are common, but not exclusive,features of fluvial fans (e.g. Friend, 1978) anddistributive fluvial systems (e.g. Hartley et al.,2010), although channels become more abundantdownstream in such systems. These three mecha-nisms are not mutually exclusive, and may haveoperated in combination.The data herein indicate that sediment accumu-

lation rates progressively decreased in the middleto upper Blackhawk Formation and the lowerCastlegate Sandstone (Fig. 15), suggesting thattectonic subsidence slowed during deposition ofthis interval. This interpretation of a decreasingtectonic subsidence rate for this interval is sharedby all previous workers, and is considered to haveforced overall progradation of the Blackhawk–Castlegate clastic wedge (e.g. Taylor & Lovell,1995; Howell & Flint, 2003; Hampson, 2010). Thedecreasing subsidence rate was not accompaniedby a significant change in fluvial style across thebase of the Castlegate Sandstone (Adams & Bhat-tacharya, 2005), which appears to contradict theinterpretation of a basinward shift of a fluvial fan ordistributive fluvial system across this surface. Thefirst-order stratigraphic patterns in the upper partof the Blackhawk Formation and the CastlegateSandstone can be attributed simply to decreasingtectonic subsidence rate (Fig. 16B and C).In the lower to middle Blackhawk Formation,

sediment accumulation rates progressively in-creased (Fig. 15), although these rates are poorly

constrained by available age data (e.g. Taylor &Lovell, 1995 and Howell & Flint, 2003 interprettectonic subsidence rates to have progressivelydecreased during deposition of these same strata).The overall upward increase in channelized-sandbody size in this interval can perhaps bebest explained by the episodic advance of wave-dominated delta plains in which distributarychannels were rare and became wider upstream(Fig. 16A). This interpretation is consistent withthe occurrence of deep, narrow, mudstone-filledchannels, interpreted as the sand-starved ‘back-water reach’ of distributary channels, at the baseof the Blackhawk Formation. However, this inter-pretation cannot account for the upward increasein channelized-sandbody abundance in the lowerBlackhawk Formation; an upward increase inavulsion frequency must be invoked instead, iftectonic subsidence rate increased during depo-sition of this interval.

CONCLUSIONS

Alluvial to coastal plain stratigraphic architectureand large-scale patterns of fluvial sandbody dis-tribution have been analysed using a large out-crop dataset from a prograding clastic wedge, theearly to middle Campanian Blackhawk Forma-tion, exposed in the Wasatch Plateau, centralUtah, USA. The dataset integrates oblique aerialphotographs, Light Detection and Ranging (LI-DAR) data, measured sections and coal-resourceliterature collected from a large (ca 100 km),nearly continuous cliff-face section aligned ob-lique to depositional strike. Coastal-plain strati-graphic architecture can be related to its time-equivalent, shallow-marine counterpart in thecontiguous Book Cliffs, which are oriented ob-lique to depositional dip.In general, the low-resolution stratigraphic

architecture of the Blackhawk Formation ismarked by an upward increase in the size andabundance of channelized fluvial sandbodies,many of which exhibit complex internal architec-tures resulting from multilateral and multistoreystacking of architectural elements. The overallproportion of channelized sandbodies varies fromca 10% in the lower Blackhawk Formation to ca30% in the upper Blackhawk Formation, but thereis much localized variability around this trend.The upward increase in sandbody size and abun-dance coincides with a decrease in tectonic subsi-dence in the upper Blackhawk Formation andoverlying lower Castlegate Sandstone, which



forced overall progradation of the clastic wedgeand its constituent shallow-marine strata. Tectonicsubsidence rates are poorly constrained in thelower part of the Blackhawk Formation, whichmay have been deposited during conditions ofprogressively more rapid tectonic subsidence; inthis case, the upward increase in sandbody abun-dance requires increasing frequency of channelavulsion through time. There are no clear large-scale palaeogeographic trends in the size or distri-bution of channelized sandbodies.The high-resolution stratigraphic framework of

the lower Blackhawk Formation is defined by aseries of laterally extensive coal zones, whichformed on the coastal plain (up to 50 km from thecoeval shoreline), generally during shorelinetransgression. These coal-bearing strata containlarge (up to 25 m thick and 1 to 6 kmwide), multi-storey,multilateral, fluvial channel-complex sand-bodies that overlie composite erosion surfacesmapped at distinct stratigraphic levels. The largesandbodies are interpreted as fluvial incised valleyfills. Other sandbodies in the lower BlackhawkFormation are significantly smaller and lack appar-ent stratigraphic organization. Relative sea-levelvariations of modest amplitude (

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