Journal of Sedimentary Research, 2007, v. 77, 303–323 Research Article DOI: 10.2110/jsr.2007.031 3-D ARCHITECTURE AND SEQUENCE STRATIGRAPHIC EVOLUTION OF A FORCED REGRESSIVE TOP- TRUNCATED MIXED-INFLUENCED DELTA, CRETACEOUS WALL CREEK SANDSTONE, WYOMING, U.S.A. KEUMSUK LEE, 1,2 GEORGE A. MCMECHAN, 1 M. ROYHAN GANI, 1,3 JANOK P. BHATTACHARYA, 1,4 XIAOXIAN ZENG, 1 AND CHARLES D. HOWELL, JR. 1 1 Center for Lithospheric Studies, The University of Texas at Dallas, P.O. Box 830688, Richardson, Texas 75083-0688, U.S.A. , 2 Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713-8924, U.S.A. , 3 Energy & Geoscience Institute, The University of Utah, 423 Wakara Way 300, Salt Lake City, Utah 84108, U.S.A. 4 The Geosciences Department, University of Houston, 4800 Calhoun Road, Houston, Texas 77204-5007, U.S.A. e-mail: keumsuk.lee@beg.utexas.edu ABSTRACT: Ground-penetrating radar (GPR) is used to delineate the 3-D facies architectural elements and examine the evolution of a top-truncated, forced-regressive, mixed-influenced delta front in the Cretaceous Wall Creek Member of the Frontier Formation, Wyoming. The GPR data provide a bridge between outcrop facies architecture and recently published high-resolution 2-D seismic studies of Quaternary delta systems. The GPR data were integrated with outcrops, photomosaics, cores, and GPS data. Two orthogonal grids of 2-D GPR profiles provide information on the 3-D facies architecture and stratigraphy of the deltaic deposits. Three main GPR architectural elements are identified within the delta front: (1) mouth bars are characterized by seaward-dipping reflections (foreset beds) in dip view, and mounded bidirectionally downlapping reflections in strike view; (2) shallow delta-slope channels show truncation and onlap and also show low-angle landward (northwest) dipping reflections. The mouth bars are locally interrupted by (3) sub-horizontal radar reflections that are interpreted as tidally modulated bars. Within the mouth-bar radar facies, both top-preserved (proximal) and top-truncated (distal) examples are observed. Top-preserved proximal bars show evidence of mounded bar crests and landward accretion, as is observed in the upstream end of mouth bars on the modern Atchafalaya Delta in the Gulf of Mexico. Distal bar deposits primarily comprise the top-truncated, seaward-dipping foresets. The GPR interpretation shows two laterally overlapping delta lobes, interpreted to represent autocyclic delta switching. The older lower lobe forms a fan-shaped delta lobe composed of top-truncated coalesced mouth bars. The second phase of bars is largely top-preserved, building over the older bars. The presence of thicker bedsets and upstream accretion suggests that the younger lobe was quickly abandoned, as seen in the overlying transgressive ravinement. Although marine erosion during transgression was less effective on these later-stage bar deposits, ravinement increased landward. A comparison of the architectural elements at a more regional scale in two separate Wall Creek delta lobes (Murphy Creek and Raptor Ridge, , 30 km apart) indicates that the bar complexes were deposited on the delta fronts as accretionary forced-regressive deposits during a relative sea-level fall; the older river-dominated deltaic deposits at the Murphy Creek site prograded southeastward (, 124u), whereas the younger and more tidally influenced Raptor Ridge bar deposits expanded southward (, 171u) before final abandonment and regional transgression. The most distal lowstand lobe sediments within the offlapping parasequence at Raptor Ridge are more completely preserved and also show the greatest tidal influence, compared to the more severely top- truncated Murphy Creek, which is in an otherwise more landward location. This challenges the notion that tidal facies are predominantly associated with transgressive or highstand systems tracts. Greater preservation of deltaic facies in forced- regressive and lowstand deltas, as shown here, has also been documented in Quaternary shelf-edge deltas, such as the Lagniappe in the Gulf of Mexico and the Mahakham in Kalimantan. INTRODUCTION The large-scale stratigraphic architecture of deltaic clastic wedges is well known from many sequence stratigraphic studies (e.g., Duncan 1983; Suter and Berryhill 1985; Tesson et al. 1990; Bhattacharya 1993; Plint 2000; Barton et al. 2004; Garrison and van den Bergh 2004; Porebski and Steel 2005; and numerous other examples quoted in Bhattacharya 2006). However, the regional nature of these studies provides little information about internal bed-scale facies variability within individ- ual delta lobes. These older seismic and sequence stratigraphic examples also emphasize dip-oriented cross-sectional depictions of shelf depositional systems, versus along-strike variability. Strike-oriented variability reflects the timing and spacing of overlapping lenses or lobes, which may be dependant on the number, spacing, and avul- sion frequency of distributary channels but may also depend on the shape of the sea floor, especially if there is differential subsidence or uplift related to tectonics, since deltas commonly fill low areas on the sea floor. Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-303/$03.00