Amalgamated Sandstone Conundrum: Sequence stratigraphic analysis of the Drip Tank Sandstone Member of the Straight Cliffs Fm., Grand Staircase-Escalante National Monument, Utah

William L. Schellenbach
New Mexico State University, Department of Geological Sciences Las Cruces, New Mexico USA
[email protected]

Existing sequence stratigraphic models for the Drip Tank Member of the Straight Cliffs Formation in the Kaiparowits Plateau have different interpretations of important sequence stratigraphic surfaces, primarily the sequence boundary. The three models that are being evaluated include a eustatic model, a fluvial gradient model, and a tectonic model. The eustatic model places the amalgamated facies tract of the Drip Tank Member entirely above the sequence boundary while the fluvial gradient and tectonic models place the amalgamated facies tract entirely below the sequence boundary.

This project integrates outcrop measured sections, paleocurrent orientations, detrital zircon provenance data, and petrography to address the placement of the sequence boundary and other important sequence stratigraphic surfaces. The following observations indicate a gradual change upsection from the underlying John Henry and an abrupt change at the upper contact with the Wahweap Formation: (1) The contact of the underlying John Henry and Drip Tank is gradational and even difficult to consistently identify; (2) truncation of the upper story or more of the Drip Tank is observed locally in several outcrop sections; (3) abrupt composition and paleocurrent changes take place at the truncation surface that marks the upper contact with a change from quartzose detritus in the Drip Tank to feldspathic detritus in the overlying valley-fill sandstone.

I infer that the Drip Tank Member underlies the sequence boundary and that the fluvial gradient model, whereby sediment bypass results from increased stream gradients from the progradation of the braided-fluvial system, best explains the deposition of the Drip Tank.

AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects