Unconformities and Facies Relationships of Muddy Sandstone, Northern Powder River Basin, Wyoming and Montana: ABSTRACT

Abstract

The Muddy Sandstone, although commonly less than 100 ft thick, is one of the most complicated formations in the Powder River basin. Utilizing an integrated sedimentologic and stratigraphic analysis of outcrop, core, and well logs, the Muddy can be divided into six isochronous members. The recognition of two widespread, subaerial unconformities is critical to understanding the complex facies relationships and member-level stratigraphy of the Muddy. Over much of the Powder River basin, the Muddy contains an upward coarsening sequence at its base. At first glance, the sequence appears to be conformable with the underlying Skull Creek Shale. East-west cross sections, however, reveal westward truncation of siltstone and dahllite marker beds in the upper part of the Skull Creek. Rapid transgression of the sea planed off most of the erosional topography on the Skull Creek unconformity, reworking the dahllite concretions and concentrating them as a lag. A regressive, sand-rich strand plain, the Rozet member, was deposited during a subsequent sea level drop. As regression continued, a second unconformity, the Rozet, developed as streams eroded through it and into the underlying Skull Creek Shale, forming deep valleys. The next sea level rise resulted in the deposition of five regionally correlatable members withinmore » the previously eroded valleys and onlapping Rozet hillsides. Layers of volcanic ash frequently fell on the surface, preserved in the valleys as beds of bentonite. Ash that fell on the hillsides was dissolved by rainwater and contributed to growth of interstitial kaolinite within the upper part of the Rozet sand.« less