Cretaceous Sussex Sandstone in House Creek Field (Wyoming, Usa): Transgressive Incised Shoreface Deposits

Abstract

Abstract The Upper Cretaceous Sussex Sandstone at House Creek field (Powder River Basin, Wyoming) has previously been interpreted as a prograding succession of sandstones that were transported southeastward by currents along a relatively shallow marine shelf. These sandstones were deposited many tens of kilometers seaward of the shoreline and a few tens of kilometers from the inferred shelf-slope break. Sand was believed to have been reworked by shelf processes into an offshore bar complex. This interpretation poses difficult problems and fails to account for several of the features observed in the Sussex including 1) pebbly sandstone, 2) stratigraphic variability of the sandstone, 3) erosional base and top of the Sussex and 4) regionally extensive depositional discontinuities marked by juxtapositions of facies, eroded mudstone clasts, glauconite and chert pebbles. These features suggest that fluctuations in rates of relative sea level change were important during Sussex deposition. In the House Creek area, the base of the Sussex is a shallow northwesterly dipping planar surface with respect to the lower silry marker and is characterized by the abrupt appearance of interbedded sandstone and bioturbated mudstone on top of black mudstone. Burrows of the Glossifungites ichnofacies commonly occur along this contact. The top of the Sussex is marked by an abrupt change from pebbly sandstone to conglomerate which passes upward through pebbly bioturbated sandstone and pervasively bioturbated pebbly mudstone into laminated black mudstone. The presence of erosional discontinuities in the Sussex Sandstone is suggested by 1) a sharp basal erosion surface that truncates underlying parasequences in the Cody Shale, 2) burrows of the Glossifungites ichnofacies at the basal contact, 3) lateral continuity of the individual facies contained between the discontinuity surfaces, 4) abrupt vertical facies changes and the association of glauconite, chert pebbles and mudstone rip-up clasts with these changes, 5) stratigraphic variability of the sandstones and 6) erosional termination of sandstone deposition and progradation. These discontinuities are characterized by an erosional escarpment with steeper landward margins that flatten basinward. This geometry and the linearity of the overlying sandstone contained in these erosional escarpments suggest incised shoreface deposits. These asymmetrical escarpments and their overlying sandstone form a rerrogradarional stacking pattern suggesting shoreface incision during an overall transgression. The Sussex Sandstone in the House Creek area is reinterpreted here as six back stepping incised shoreface deposits for med during a transgression punctuated by periods of decreased rates of relative rise and/or relative stillstand