Sequential Architecture in a Fluvial Succession: Sequence

S E Q U E N T I A L A R C H I T E C T U R E IN A F L U V I A L S U C C E S S I O N : S E Q U E N C E S T R A T I G R A P H Y IN T H E U P P E R C R E T A C E O U S M E S A V E R D E G R O U P , P R I C E CANYON, U T A H TORBEN OLSEN*, RON STEEL, KENT HOGSETH*, TORE SKAR**, ANDSIGNE-LINE ROE

GeologicalInstitute, Universityof Bergen, 5107 Bergen,Norway ABSrgACr: Five unconformit~bounded sequences have been documented within a 1300 m thick succession of predominantly fluvial Upper Cretaceous and Lower Tertiary deposits in the Price Canyon area of the westernmost part of the Book Cliffs, east-central Utah, U.S.A. This area experienced major tectonically inducedchanges in the paleogeography during deposition. The earliest fluvial deposits in the study area were formed in a relatively simple foreland basin setting in front of the Sevier Orogenic Belt, and contain more marine influence than previously recorded. Following a gradual termination of thrusting, sedimentation became increasingly controlled by uplift of the San Rafael Swell to the southeast. Our studies suggest that the fluvial systems in the upper part of the succession alterhated between being confined within a valley open towards the north and being more directly eastward flowing. On the basis of the observed sequences, an idealized model of alluvial sequences, whose internal architecturecan be related to a fall-rise-fall cycle of the strntigraphic base level, has been established. Upward or downward changes in the position of this base level dictate the creation and destruction of accommodation on the alluvial plain and therefore exert a direct control on the sequential architecture and sandstone body geometry of fluvial successions. In our model, the basal sequence boundary is overlain by an amalgamated fluvial sandstone sheet. The sheet is succeeded by a finingupward, more mudstone-rieh level with more isolated sandstone bodies, and this level m y culminate in a marine or lacustrine transgression. The upper part of the sequence may show a coarsening-upwardtrend heralding the next phase of base.levd fall and sequenee-boundaD' generation. Use of the suggested model has the potential to refine existing lithostratigraphic schemes and, given the higher resolution and more detailed correlation, may significantly improve lmleogeographic reconstructions and aid in prediction of potentially hydrocarbon-bearingreservoirs. A revised and relined lithas~afigraphy has been established on the basis of alluvial sequence analysis. The lower half of the succession forms the youngest part of a major eastward-progruding clastic tongue, the Mesaverde Group, and is Campanian. We divide this part into the Blackhawk, Castlegate, and Price River Formations. The rest of the succession is Maastrichtian to Paleocene and is referred to the North Horn Formation. This formation was deposited in an intermontane setting.

BACKGROUND AND AIMS

Recent advances in sequence stratigraphic concepts and applications have probed the linkage between the architecture of shoreline-nearshore successions and the key surfaces and architecture of time-equivalent alluvial successions along basin margins (e.g., McCabe and Shanley 1992; Shanley et al. 1992; Posamentier and James 1993; O'Byrne and Flint, in press; Shanley and McCabe 1994). This kind of work does more than simply provide a time-line framework between alluvial and adjacent shallow marine strata: it has the potential of giving new insight into the controls on the internal architecture of clastic wedges that fill basins. Shallow marine successions are characterized by sequence boundaries and marine flooding zones that coincide with the approximate minimum

(or falling) and maximum rates of sea-level rise during sediment accumulation. The recognition of major erosional and other key surfaces in alluvial successions suggests that these can be linked up with time-equivalent surfaces in more basinward settings. Such an approach is useful when evaluating the relative roles of the component controls on base level in forming the architectural details of clastic wedges. Likewise, the location and shah of potentially hydrocarbon-bearing fluvial reservoirs can be better predicted. The present work deals with the sequential architecture of the proximal reaches of one of the most studied and frequently visited clastic-wedge systems, the Upper Cretaceous Mesaverde Group of the Western Interior Basin, USA (Fig. 1). It also includes studies of the overlying Lower Tertiary intermontane North Horn Formation. The study area (c. 100 km2) is centered on Price Canyon in the westernmost part of the Book Cliffs, eastcentral Utah (Fig. 2), and includes a succession of predominantly fluvial deposits c. 1300 m thick. The succession spans from the base of the nonmarine part of the Blackhawk Formation through the Castlegate and Price River Formations before terminating in mixed lacustrine and fluvial deposits of the North Horn Formation. Within the Castlegate Formation we record previously undocumented marine influence. We also recognize marine influence in a zone c. 20-50 m above the top of the Castlegate. The facies within this zone correlate with the continental to shallow marine Farter Formation from the area east of the San Rafael Swell (Fisher el al. 1960; Lawton, written communication 1993). The overlying Price River Formation is characterized by several widespread erosion surfaces and also shows large variations in sand content and inferred fluvial style. It has on this basis been divided into 5 new lithostratigraphic members (Fig. 3). In addition, one new member has been defined in the North Horn Formation (Fig. 3). Fluvial deposition in the Price Canyon area was strongly influenced by tectonics, although no major faults can be seen within the study area. The basal Mesaverde clastic wedge is largely the product of infilling of a major foreland basin created by eastward-directed thrusting to the west in the Sevier Orogenic Belt (Fig. 1). The general causal relationship between the development of the geometry of the clastic wedge and the crustal deformation within the thrust belt has been well documented (Beaumont 1981; Jordan et al. 1988). The details and the controls &the internal architecture of the wedge, however, with its numerous regressive and transgressive components, are less well documented and less well understood. Also, the timing and importance of various block uplifts (Lammide phase of deformation), which influenced the upper part of the succession, is poorly constrained. Our purpose here is to describe briefly the rocks exposed in the area and to interpret their depositional environment. Further, we wish to demonstrate how the combination of sequence stratigraphic concepts, precise photogrammetric mapping of the canyon sides, a genetic subdivision of the alluvial units, and more detailed sedimentological studies can be used in a discussion of base-level changes and the causes of varying alluvial stacking patterns within thick continental successions. On this basis we define a general alluvial sequence, whose internal architecture can be related to a fall-rise-fall cycle of the stratigraphic base level.

* Present address: Statoil, N-4035 Stavanger,Norway. ** Present address: Vfije Universiteit,Facultyof Earth Sciences,De Boelelaan 1085, NI-1081 HV Amsterdam,The Netherlands. JOORN^LOrSeDluer,rrARvResrarc,, VoL.B65, No. 2, MAY,1995, P. 265-280 Copyright © 1995,SEPM (Societyfor SedimentaryGeology) 1073-1318/95/0B65-265/$03.0C

SEQUENCE STRATIGRAPHYIN FLUVIALDEPOSITS

The stratigraphic response to increase or decrease in the amount of accommodation is a key issue in sequence stratigraphic models (e.g., Vail

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Fio. 1.-Regional cross sectionshowingthe geologicaldevelopmentof the Mesaverde Group. (Modifiedfrom Armstrong1968.) et al. 1977; Posamentier el al. 1988; Posamentier and Vail 1988; Van Wagoner et al. 1990). The importance of accommodation in determining the architecture of fluvial deposits has also been stressed by Shanley and McCabc (1993), who additionally believe that varying accommodation is generally caused by base-level changes. The use of base-level rise and fall as an explanatory framework for fluvial deposition and preservation is, however, hampered by the ongoing discussion around the term "base level", especially the differencebetween geomorphologic and stratigraphic base level (e.g., Sehumm 1993, Shanley and McCabe 1994). We prefer the ideas of Bah'ell (1917), Sloss (1962), and Wheeler (I 964). Sloss (1962) expressed stratigraphic base level as a surface "above which a particle cannot come to rest and below which deposition and burial is possible". Within a given reach of the fluvial system, base-level fall is marked by erosion or nondeposition and baselevel rise by deposition. This definition is thus not sea-level dependent, and can be applied to alluvial areas far removed from any coeval shoreline (marine or lacustrine). The above definition of base level thus implies that the base-level surface is the upper limit of accommodation. Use of "equilibrium profile" or "graded stream profile" has been suggested as a more proper name for the upper limit of accommodation in the fluvial realm (e.g., Schumm 1993; Posamentier and James 1993). This usage makes the term base level

essentially equal to sea level and hence redundant. In addition, it implies that streams really are graded or in equilibrium, but graded streams are by definition neither erosive (able to generate sequence boundaries) nor aggrading (able to accumulate fluvial deposits). The stratigraphic record is therefore the result of deposition and erosion by streams that in the longer term do not maintain graded or equilibrium conditions. Another problem arises because stratigraphic sequences are commonly defined by their genetic link with eustatic or relative sea-level changes (e.g., Posamentier and Vail 1988; Posamentier et al. 1988; Van Wagoner et al. 1990; Posamentier and James 1993). Alluvial deposition, however, responds to change in the local accommodation regardless of whether this change is caused by upstream or downstream controls or indeed intrabasinal subsidence shifts. We have therefore used the concept of unconformity-bounded sequences as suggested by Mitchum (1977), and sequence boundaries have been placed along extensive erosion surfaces (tens of square kilometers) even though it is unknown whether all of these surfaces correspond to relative sea-level falls at downdip shorelines. It should be noted, however, that when these surfaces could be followed basinward (in the lower part of the succession) they are associated with marked falls in relative sea level (see similar examples in Shanley and McCabe 1993). CONTROLSON TIlEARCHITECTUREOF CLASTICWEDGES

A complex interaction of hinterland and basinal controls governs the interlayering of marine and nonmarine strata in clastic wedges. Sourcearea tectonics, in the form of uplift and tilting, can cause changes in the ' ~ SIUDV A a E A erosional, storage, drainage, and sediment-supply patterns in hinterland areas. Once sediment is available, it will eventually be released and deposited in the basin, given sufficient accommodation space. Basinal tectonics, mainly in the form ofbasinal subsidence rates, together with other base-level controls such as eustasy and nontectonic subsidence changes, cause the creation or destruction of accommodation in the basinal area. It is also noteworthy that changes in the size and shape of the drainage system and depositional basin may have an important effect on the total IFFs volume of available accommodation even though the point-by-point changes in stratigraphic base level along the basin axis remain unchanged. Climate exerts an important control on the amount of water and sediment o ii 40 supplied to a basin but may also affect the fluvial system more directly, 0 KM O0 e.g., by allowing resistant soil horizons to form or by causing increased FIG.2.-Map showingthe main physiographicfeatures of east-centralUtah, evaporation. In attempting to explain the detailed internal architecture of such clastic The study area north of Price is outlined.The line markingthe edge of the Book Cliffsand the WasatchPlateaucorrespondsto the cliff-forminglowerunit of the wedges, where coarse alluvial tongues derived from the hinterland interCastlegate Formation.(Modifiedfrom Pfaff 1985.) fingerwith shaly basinal tongues, the temptation to explain changes in the

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coarse proximal end of the wedge purely by changes in the upstream variables, and vice versa, should be avoided. Two lines of argument warn against such oversimple reasoning. Firstly, there is ample evidence that changes in the hinterland variables (e.g., overthrusting) can not only cause changes in the character of the wedge (e.g., petrologic shifts), but also can influence downstream characteristics such as subsidence-induced thickness shifts (e.g., Yingling and Heller 1992; DeCelles and Burden 1992; Bentham el al. 1992). Likewise, variations in downstream variables (e.g., relative sea-level changes, whether due mainly to eustasy or subsidence) can cause significant changes not only in the downstream character of the wedge (e.g., shoreline migration) but also in the extent and character of upstream erosion and sedimentary stacking patterns (e.g., Shanley and McCabe 1991, 1993; Bentham et al. 1992; Blum, in press). There is therefore likely to be a complicated linkage between hinterland structuring and relative sea-level change, i.e., between upstream and downstream controls and effects. A second line of argument against linking individual hinterland tectonic events too closely to individual coarse-grained tongues within the elastic wedge is the growing consensus that the most extensive and marked elastic tongues form during time intervals of relative tectonic stability and minimal creation of accommodation. This has been documented for clasticwedge development related to extensionally created accommodation space (Steel 1988), as well as for infilling of compressionally created basins (Blair and Bilodeau 1987; Heller and Paola 1989). In contrast, during active thrusting or extension, high subsidence rates in front of the tectonic lineaments cause the coarsest facies to be trapped in a narrow belt adjacent to the basin margin (Gloppen and Steel 1981; Yingling and Heller 1992; Paola et al. 1992). Irrespective of the complexity of the controlling factors, however, the important practical step to be taken after description of facies and interpretations of depositional processes is documentation of key surfaces of wide extent as well as trends indicative of rising or falling base level.

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STUDY AREA AND METHODS

The Price Canyon area consists of two main canyons, each c. 10 km long: Price Canyon itself and, at an approximately right angle to the northeast, Willow Creek (see insert map in Figure 4). A number of adjacent smaller tributary canyons were also studied. In this relatively high area of Utah (1800..-2400 m a.s.l.) vegetation is common and good exposures are confined to south-facing canyon sides. The study area has had a long history of geological research, the early part of which is summarized by Fisher et al. (1960). A paper by Young (1955) is significant for its recognition of the eastward-prograding system of elastic tongues. The regional depositional pattern of the Castlegate

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