2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 11
Presentation Time: 1:30 PM-5:30 PM


LEE, Keumsuk1, MCMECHAN, George2, GANI, Royhan2, BHATTACHARYA, Janok3 and XIAOXIAN, Zeng2, (1)Geosciences Department, The University of Texas at Dallas, Center for Lithospheric Studies, P.O. Box 830688, Richardson, TX 75080, (2)Geosciences, The University of Texas at Dallas, Center for Lithospheric Studies, P.O. Box 830688, Richardson, TX 75080, (3)Geosciences Dept, University of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083, kxl015500@utdallas.edu

Ground-penetrating radar (GPR) is used to study the 3-D facies architectural elements of a top-truncated, mixed-influenced, delta-front reservoir analog at Raptor Ridge in the Wall Creek member of the Frontier Formation, Wyoming, and to provide a km-scale deltaic depositional model. The GPR study was integrated with outcrops, photomosaics, cores, and GIS data. Two orthogonal grids of 2-D GPR profiles provide information on the 3-D facies architecture and stratigraphy of the deltaic sequence. The delta front is composed of three main GPR-derived facies elements. Distal bars are characterized by seaward dipping (foreset beds). Proximal bars are characterized by multi-directional, low-angle, thicker foreset beds. The distal barsets are interrupted by a sub-horizontal radar facies that is interpreted as tidally modulated bars, and shallow delta-slope channels characterized by landward (northwest) dipping accretionary reflections.

The GPR interpretation implies two distinct phases of depositional development in the Upper Cretaceous of the Raptor Ridge site. During the first phase, river-borne sediments were deposited in a deltaic setting, forming a fan-shaped delta lobe composed of coalesced mouth bars; river-flood discharge enhanced progradation of the deltaic lobe before the the lobe was abandoned. During the second phase, younger sediments were deposited on the old lobe, forming a new lobe which prograded into the basin in the same direction as the old bar deposits. The younger bedsets prograded further seaward until a regional transgressive ravinement.

An architectural comparison between the barsets in two separate delta lobes (Murphy Creek Reservoir and Raptor Ridge) ~30 km apart in the Wall Creek Member indicates that the bar complexes were deposited on the delta fronts as detached forced regressive deposits during a relative sea-level fall. The older deltaic sequence at Murphy Creek Reservoir prograded south-eastward (124°), whereas the younger bars at Raptor Ridge expanded southward (171°).