Architectural Element Analysis of Regressive vs. Transgressive Tidal Sandstone Outcrops: Upper Cretaceous Sego Sandstone, Book Cliffs, Utah

Shuji Yoshida, Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254, phone: 512-471-1772, fax: (512) 471-9425, shuji@mail.utexas.edu, Ron Steel, Dept. of Geological Sciences, The University of Texas at Austin, 1 University Station, C-1100, Austin, TX 78712, Robert Dalrymple, Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L 3N6, Canada, and James MacEachern, Dept. of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada.

The Sego Sandstone in Book Cliffs, Utah, contains abundant, unequivocal tidal sedimentary structures, and has been used as exemplary tidal sandstone outcrops for both academics and the oil industry. However, its origin remains controversial. Traditionally, the Sego Sandstone was considered to reflect transgressive estuary valleys. Recent studies, however, suggest that it can possibly be interpreted as forced-regressive, tide-dominated deltas. We have quantitatively examined the Lower Sego Sandstone to evaluate these models, utilizing architectural element analysis methodology on several 3-D outcrops.

In Sego Canyon, the Lower Sego Sandstone has a gradational contact with the underlying open marine shale, with an upward increasing abundance of tidal sedimentary structures, coarsening of grain size, and concomitant decline in wave-generated structures. This is overlain by amalgamated tidal channels, in turn truncated by marine shales and HCS sandstones. In San Arroyo Canyon, the Lower Sego Sandstone contains two regionally extensive unconformities overlain by stacked bars that exhibit an overall upward increase in salinity toward open marine conditions, based on ichnological suites.

In both localities, paleourrents measured from the tidal bars and channels indicate the NW-directed flood-domination. Accretion surfaces dip either NE or SW, perpendicular to the paleoflow direction, suggesting lateral bar accretion. We postulate that only the basal part of the Lower Sego Sandstone clearly represents a forced-regressed delta, which is best preserved in Sego Canyon. The overlying amalgamated tidal bars/channels comprise the majority of the Lower Sego Sandstone, and represent either distributary channels or transgressive fills of incised valleys.

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