ABSTRACT: Parasequence Architecture in the Wall Creek Member of the Frontier Formation (Upper Cretaceous), Powder River Basin, Wyoming

Marilyn C. Huff, Dag Nummedal

Depositional sequences have a predictable arrangement of systems tracts, each of which has an internal architecture composed of stacked parasequences. Each parasequence also has a predictable architecture. We differentiated two such patterns, one simple and the other complex. Existing literature states that siliciclastic parasequences are entirely progradational, that they shoal upward, and that no sedimentation takes place during the flooding, or transgressive, phase of their development. In contrast, this study documents complex parasequences that have a transgressive component at their base and record sedimentation during both the increase as well as decrease in water depth.

Complex parasequences consist of a flooding surface overlain by transgressive shelf sediments that are then overlain by progradational sediments. Simple parasequences consist of a flooding surface overlain only by progradational strata. In both simple and complex parasequences, transgressive lags may be formed at the flooding surfaces and condensed sections or omission surfaces may be present at the base of the progradational package. A ravinement surface resulting from the erosional retreat of the shoreface during water deepening merges with the marine or basinal flooding surface. Landward of the shoreface, the ravinement surface may be found above the back barrier flooding surface.

Examples of both simple and complex parasequences are found in outcrops of the Late Turonian Wall Creek Member of the Frontier Formation. In this area, the Wall Creek (or First Frontier) consists of interbedded sandstones, siltstones, shales, and rare bentonites deposited in nearshore and inner shelf environments of the Western Interior Seaway. Fossil zones encompassed are Scaphites whitfieldi to Prionocyclus quadratus. Wall Creek parasequences are typically 5-15 m thick. Inner shelf muds and shelf sand sheets and ridges comprise the transgressive facies. Condensed sections are marked by nodule horizons and hardgrounds. The regressive facies are prograding shorefaces in many places. In general, the transgressive portions of complex parasequences are much thinner than the regressive po tions. Up to five stacked backstepping parasequences comprise this transgressive systems tract.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990