Stratigraphy, Sea Level Fluctuations, and Depositional History of Lower Cretaceous Muddy Sandtone, Northern Powder River Basin, Wyoming: ABSTRACT

Abstract

The Muddy Sandstone may be the most complicated Cretaceous formation in the Powder River basin. Although generally less than 100 ft thick, it contains a wide variety of facies. Two unconformities, one of which is largely intraformational, are associated with the Muddy. It has long been known that Muddy deposition was preceded by a lowering of sea level in the Western Interior. Their study indicates that there were, in fact, two cycles of lowering/rising of sea level. The first lowering and the subsequent rise were abrupt; they were accompanied by only minor deposition of sandy sediments. The second lowering of sea level was slower. Sand provided by longshore drift from a southwestern source accumulated as an extensive strand-plain system. As sea level continued to lower, rivers cut downward locally through the old strand plain and into the Skull Creek Shale. The deepest Muddy valleys formed at this time. The following transgression was not continuous but comprised at least five pulses of abrupt rise separated by periods of stability. During periods of abrupt rise, valleys flooded to form estuaries; during still-stands, valleys were largely filled with estuarine, bayhead-deltaic, and fluvial sediments. Barrier islands and associated tidal inlets formed near the mouthsmore » of the estuaries. All of the sandy facies of the Muddy - fluvial, strand plain, barrier island, and bayhead delta, in decreasing order of importance - serve locally as reservoirs for oil and gas.« less