ABSTRACT: A Spectrum of Transgressive Valley-Fill Sequences: Implications to Sequence Stratigraphy and Petroleum Exploration

David M. Wheeler, Alan J. Scott, Paul E. Devine, John C. Horne

Valley incisement and transgressive valley fill deposition has been well documented from the Texas coast. These successions are in response to glacially induced eustatic sea level fluctuations. This generalized valley fill model has an erosional base representing a sequence boundary and a succession of fluvial sands overlain by estuarine and bay muds. A spectrum of valley fill models is discussed based upon examples from the stratigraphic record including the Morrowan (Pennsylvanian) of southeastern Colorado, the Minnelusa-Opeche (Permian) of Wyoming, the Muddy Sandstone (Cretaceous) of Wyoming, and the lower Wilcox (Paleocene) of the Texas Gulf Coast. Differences within the spectrum can be attributed to variations in the rate of sea level rise or subsidence, sediment sup ly, and tectonic setting.

Rapid rates of subsidence and/or sea level rise relative to sediment supply results in an abrupt landward shift in facies and a valley filled primarily with estuarine muds. Examples include the Minnelusa-Opeche formations and the lower Wilcox Formation. The valley fill mudstones in these units trap petroleum in laterally adjacent facies.

Slower rates of sea level rise or increased sediment supply result in valley fill sequences with thicker basal fluvial deposits abruptly overlain by estuarine to marine sands and muds. Petroleum is produced from Morrowan fluvial reservoirs within this type of valley fill in southeastern Colorado and southwestern Kansas.

Complex valley fill sequences consisting of multiple backstepping progradational parasequences occur in association with varying rates of sea level rise or subsidence relative to sediment supply. Well-documented examples from the Muddy Sandstone illustrate such complexities. Petroleum has been produced from fluvial, bayhead-delta and barrier island facies of the Muddy Sandstone valley fills.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990