Stratigraphic Variations within the Sego Sandstone, Book Cliffs, Central Utah

Willis, Brian J.¹, Matthew Fey¹, Eric Robinson¹, Stanley Scott Birkhead¹ (1) Texas A&M University, College Station, TX

Interpretations of the Sego Sandstone, one of the classic outcrop analogs for tide-influenced reservoir deposits, remain controversial because: complex internal facies variations have been mapped in detail only along specific limited segments of this huge outcrop belt; sequence stratigraphic interpretations have evolved to recognize the importance of sea floor erosion during falling-stage regression; and numerous field trip parties have assessed stratigraphic relationships based on local field stops out of context of a clear regional stratigraphic framework. This poster presents detailed cross-sections of facies and stratigraphic surfaces across two additional segments of this outcrop belt. A cross section of the upper Sego Sandstone spanning the 20 km segment between Horse and Sego Canyons indicates deposits are dominantly a regressive deltaic succession very locally cut out by a fluvial-deposit-filled valley, in contrast to previous interpretations that this unit is a dominantly transgressive succession floored by a lowstand sequence boundary. A series of parallel outcrops near the much visited San Arroyo Canyon area were mapped in 3D using photogrammetry and surface modeling techniques to better document previous interpretations of a classic valley fill succession cut into the top of the lower Sego Sandstone. There fluvial channel deposits at the base of the valley fill fine upward to a major ravinement shell lag that onlaps the valley fill margin, and then deposits coarsen upward into a landward-reworked sand shoal deposit. These results are combined with those of several earlier mappings of different Sego outcrop segments along the Book Cliffs to assess current ideas about regional Sego Sandstone stratigraphy.