Sequence Stratigraphic and Tectonic Controls on Shannon Incised-Valley Distribution, Hartzog Draw, Wyoming: ABSTRACT

Abstract

The Upper Cretaceous Shannon Sandstone in the Powder River Basin, Wyoming is interpreted as stacked tidal bars infilling northwest-southeast trending incised valleys. The Shannon Sandstone can be subdivided into 3 sequences: Copenhagen Blue, Crimson Red, and Canary Yellow. The Copenhagen Blue sequence boundary is a regional unconformity at the base of the Shannon Sandstone marked by distal tidal-bar deposits resting on offshore mudstones of the Cody. The overlying Crimson Red sequence is the main reservoir interval at Hartzog Draw and is composed of proximal tidal-bar deposits. The highly erosional Canary Yellow sequence boundary forms the trap at Hartzog Draw by juxtaposing the reservoir sandstones of the Crimson Red sequence with the overlying offshore mudstones of the Canary Yellow sequence. The Shannon Sandstone is therefore an erosional remnant of originally more extensive tide-dominated deltas deposited within northwest-southeast trending incised valleys. The orientation and distribution of the Shannon incised valleys is the result of the interplay between tectonics and eustacy. During lowstand incised valley development the orientation of fluvial systems incising the shelf was strongly controlled by pre-existing northwest-southeast trending structural elements in the basin. During the subsequent sea-level rise the shoreline backstepped in a landward direction. The shoreline also gradually rotatedmore » from an east-west orientation during the lowstand to a north-south orientation during the highstand systems tract. Ultimately the highstand shorelines were located over 160 kms landward of the lowstand shorelines and oriented subparallel to the lowstand incised valleys.« less