AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Complex Transition of Shallow-Marine Star Point Formation to Coastal-Plain Blackhawk Formation, Wasatch Plateau, Utah
(1) Earth and Environmental Sciences, University of New Orleans, New Orleans, LA.
(2) Department of Earth Science and Engineering, Imperial College, London, United Kingdom.
Using cliff-face photomosaics, measured sections, GPR (ground penetrating radar), and core data, this outcrop study characterizes facies and stratigraphic complexity at the transition of marine to non-marine depositional environments as preserved in the stratigraphic transition of the late Cretaceous Star Point Formation to Blackhawk Formation that superbly crops out in the north-eastern Wasatch Plateau, Utah. Data from seven “windows” of the outcrop, including GPR profiles through selected sandbodies, and one coal-mining core demonstrate this complexity at a range of scales. The youngest two sandbodies (i.e. parasequences) of the Star Point Formation constitute an overall wave-dominated shoreface environment with hummocky to swaley cross-stratification, dune cross-bedding, and marine trace fossils. The lower Blackhawk Formation constitutes an overall coastal-fluvial environment, which contains a number of facies representing various sub-environments, including: (1) thin to thick coal beds with Teredolites burrows, (2) fluvial channel deposits with dune cross-bedding and lateral-accretion surfaces, (3) tidal channel deposits with IHS (inclined heterolithic stratification), and (4) coastal to bay-fill mudstones.
In the northern part of study area (Wattis Road location), the complex transition from marine to continental strata is expressed by intertonguing of marine and coastal-plain strata. Facies architecture of the tidal channel strata reveals development of multiple IHS sets, as tidal channels migrated across the coastal plain. A depositional dip-oriented section shows up-dip pinch-outs of two shallow-marine parasequences into coastal-plain Blackhawk deposits. A prominent incised valley (~10 m thick) was also observed that eroded the upper part of this shallow-marine sandstone. At the same stratigraphic level, more incised valleys were documented in other outcrop “windows” that erosionally overly the uppermost marine sandstone, indicating the existence of a sequence boundary. Transitional complexity progressively decreases southward; the Cottonwood Canyon outcrop shows a rather simple upward transition from shallow-marine strata to coastal-plain strata.