Contrasting Expressions of Sequence Boundaries Across Upper Orodovician-Lower Silurian Carbonate Shelf and Shallow Ramp Facies of the Eastern Great Basin (Utah and Nevada)

Mark T. Harris, Lora A. Sexton, Peter M. Sheehan

Eight stratigraphic sequences (five Upper Ordovician and three Lower Silurian) accumulated on a westward-dipping carbonate ramp and attached shelf in the Eastern Great Basin prior to formation of a steep, rimmed margin in middle Llandovery time. The sequence stratigraphic framework depends upon the identification of sequence boundaries. Recognition criteria differ from boundary to boundary, and vary regionally for any single boundary.

Sequence boundaries in shallow shelf settings are delineated by vertical facies changes, stacking patterns of meter-scale cycles, and caliche horizons. In deeper shelf and shallow ramp settings, some boundaries are expressed by systematic changes in abundance of subtidal water depth indicators such as shoal deposits and storm beds. Lowstand oncolite shoals overlie some shallow ramp boundaries. Boundaries may be subtle to obscure in middle ramp settings, marked by localized low-angle scour surfaces and breccias with allochthonous carbonate clasts and a quartz sand matrix.

The most spectacular sequence boundary is an early Llandovery karst surface marked by dissolution features and caves filled with quartz sand and carbonate breccias. Unexpectedly, the best developed sand-filled karst features occur in the most seaward section (Pancake Range) where they extend at least 50 meters below the exposure surface.

We conclude that usable criteria for sequence boundary recognition are closely tied to the depositional setting. More specifically, middle ramp carbonates have few of the typical sequence boundary features such as lowstand wedges, facies shifts, and extensive erosional surfaces.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995