Abstract

High-resolution stratigraphic analyses shows that at least three orders of cyclicity produced the stratigraphy of the Yates Formation on the shelf as well as the time-equivalent basinal deposits of the Bell Canyon Formation. Evidence suggests that orbitally-forced, 400- and 100-k.y. (4th- and 5th-order, respectively) duration sea-level cycles (Milankovitch, long and short eccentricity cycles) were predominant events.

Stratigraphic computer modeling illustrates how a distinct Yates accommodation profile, hierarchical sea level history, and the interaction of carbonate and siliciclastic systems were fundamental controls on Yates stratigraphy. Simulations show how the Yates topography (flat platform with a steep margin), low subsidence, and “keep up” carbonate factory provided a distinct accommodation profile that resulted in rapid fluctuations between highstand- and lowstand-shoreline settings with only minor fluctuations in relative sea-level. Modeling suggests strong reciprocation between shelfal deposition during (decreasing rates of) relative sea-level rise and basinal deposition during (increasing and then decreasing rates of) relative sea-level fall occurred on a much shorter time scale (5th-order, 100-k.y.) than appreciated by previous workers.

Modeling and outcrop stratal geometries show how increased accommodation near the shelf edge resulted in a zone of greatest potential for cyclostratigraphic analysis and suggests that “fall-in” beds were the byproduct of a hierarchical sea-level operating across an outer shelf accommodation gradient. Periods of “fall in” occurred during 4th-order, sea-level lowstands when accommodation space shifted seaward and off a previously deposited carbonate bank. Modeling results also point out the fundamental importance of evaluating high-frequency cycles to understand shelf evolution and siliciclastic bypass. In the Yates model, sediment bypass to the basin is a high frequency phenomena that is varied by longer term cycles. Furthermore, 4th-order cycle (sequence) boundaries are defined by zones of closely spaced 5th-order bypass surfaces and 3rd-order sequence boundaries are, in turn, defined by zones of low-accommodation, 4th-order cycles.