Significance of Isolated Shelf Turbidites and Coarse-Grained Sandstone Bodies in Shallow Marine Correlations

Simon A. Pattison
Department of Geology, Brandon University, Brandon, MB, Canada

Mancos Shale-encased sandstone bodies are concentrated along a variety of intervals in the Book Cliffs region of eastern Utah, including a 70 to 90m thick zone that straddles the upper Aberdeen and lower Kenilworth members (Blackhawk Formation, Campanian). Two broad-categories of isolated bodies are observed: (1) fine-grained turbidite-rich channels and lobes deposited in a shelf setting, and (2) transgressively-modified pods of coarse-grained sandstones that were originally deposited during falling stage and lowstand. The former are observed in the distal parts of three Aberdeen (APS3 to APS5) and two Kenilworth (KPS1 to KPS2) highstand parasequences, while the latter are concentrated along the top Aberdeen sequence boundary. The turbidite-rich layers "sandwich" the coarse-grained sandstones. The unique sedimentology (i.e. turbidite-rich shelf channels and lobes) and sequence stratigraphy (i.e. coarse-grained sandstones at the top of the Aberdeen) is attributed to a short-term pulse of tectonic uplift and subsidence that was centered northwest of the Book Cliffs region. Tectonic activity is linked to the generation of shelf turbidite bodies, rotation of the paleoshoreline trend and introduction of coarse-grained sediments into the basin. Early results highlight two important implications for correlating in a shoreface-to-shelf systems: (1) regional outcrop and subsurface data demonstrates a gradual basinward thinning of the Aberdeen and Kenilworth parasequences (i.e. 30 % reduction in thickness over 40+ km) and therefore it would be incorrect to apply an abrupt parasequence-scale clinoform correlation-style over these distances, and (2) parasequences that contain shelf turbidite bodies are characterized by anomalous basinward patterns of increased sandstone and thickness.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas