Coastal-Plain Paleovalleys in the Lower Blackhawk Formation, Wasatch Plateau, Utah

Abstract

Compared to its well documentation in the downstream marine and coastal-plain strata of the Book Cliffs of Utah and Colorado, high-resolution sequence stratigraphy of the Upper Cretaceous Blackhawk Formation is poorly understood in the upstream non-marine and coastal-plain succession in the Wasatch Plateau, Utah. An integrated outcrop, core, and LIDAR dataset of a ~50 km outcrop belt allowed an improved characterization of two coastal-plain paleovalleys of the lower Blackhawk Formation in the Wasatch Plateau that is aligned in the North-South regional strike-orientation. These two valleys are distinctly wider with high aspect-ratio, indicating pronounced valley-widening that is interpreted to record lateral migration of channel-belt deposits even during sea-level fall. Well-developed paleosols have been found in the interfluves of these two valleys. Whilst paleosol is thicker (~10 m thick) and homogeneous for the lower valley, it is thinner (~3 m thick) and markedly heterogeneous for the upper valley. This upper valley paleosol is interspersed with layers of thin channelized deposits (each up to ~80 cm thick) with dune cross-stratification (~20 cm set thickness), suggesting subtle climate perturbations during the paleosol development during sea-level fall. Field documentation reveals an apparent organization in spatial-distribution of paleosol vs. valley-fill amalgamated channel sandbodies. While the paleosols are much developed towards the Northern part of the study area (likely flank-side of the two valleys), multistoried channel sandbodies are pronounced towards southern part (likely depocenter of the two valleys). At the bottom of each valley, multistoried sandbodies (~15–20 m thick) have been identified with highly-erosional base. Our integrated dataset allowed de-convolution of autogenic from allogenic signals preserved in valley-fill fluvial architecture. Key observation illustrates a positive correlation between thicknesses of underlying coal and overlying amalgamated sandbodies within these two paleovalleys. We interpret that differential subsidence linked to underlying coal-precursor-peat thickness was the major autogenic control that dominated over allogenic forcing to guide deposition of valley-fills.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015