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

Paper No. 6
Presentation Time: 2:45 PM


CHRISTENSEN, Amy E., Institute of Tectonic Studies, New Mexico State University, Las Cruces, NM 88003 and LAWTON, Timothy F., Department of Geological Sciences, New Mexico State Univ, Las Cruces, NM 88003, a_e_christensen@yahoo.com

Sequence architecture, paleocurrent analysis and sandstone detrital modes of the upper Santonian Drip Tank Member of the Straight Cliffs Formation in southwestern Utah indicate that this amalgamated fluvial sandstone unit records progressive filling of the Late Cretaceous foredeep depozone. We infer that the Drip Tank Member underlies, rather than overlies, an important sequence boundary and thus does not conform well to popular sequence stratigraphic models for continental deposits. Originally described as a mappable lithostratigraphic unit, the Drip Tank Member does not represent a single genetic package of strata. The bulk of the Drip Tank Member consists of stacked, tabular multistory lithosomes deposited by braided rivers that occupied the foredeep depozone. At its uppermost extent, the Drip Tank Member is capped by laterally discontinuous dark orange quartzolithic valley fill sandstone. Dividing this dark orange sandstone from the underlying sandstone is an erosional surface that truncates a minimum of 8 meters of the underlying sandstone and represents a by-pass surface and sequence boundary. Sandstone composition and paleocurrent indicators record mixing of detritus in a foreland basin by a fluvial system that flowed primarily from the south and west and evolved to an eastward-flowing system in the upper part of the Drip Tank Member. Accompanying this shift was a decrease in feldspar content. Limited fossil and palynomorph data, combined with the presence of an unconformity represented by a sequence boundary near the top of the Drip Tank Member, suggest a probable latest Santonian age. Although previous models place the amalgamated fluvial facies tract entirely above the sequence boundary, in the Kaiparowits Plateau the Drip Tank Member includes the highstand systems tract, a sequence boundary, and the overlying lowstand or transgressive systems tract. Paleocurrent indicators record an initially underfilled basin that evolved to an overfilled basin in the Henrieville and Kaiparowits basins.