COMPLEX INTERPLAY OF SHALLOW MARINE TO COASTAL PLAIN ENVIRONMENTS IN MARGINAL-MARINE DEPOSITS: STAR POINT AND BLACKHAWK FORMATIONS, WASATCH PLATEAU, UTAH

Late Cretaceous Star Point Blackhawk Formations superbly crops out in the north-eastern Wasatch Plateau, Utah. Using cliff-face photomosaics, measured sections, and core data, this outcrop study characterizes facies and stratigraphic complexity of these deposits at the transition of marine to non-marine depositional environments. The transition of shallow-marine Star Point strata to coastal-plain Blackhawk strata was documented in detail with one well core and eight outcrop “windows” along the Wasatch Plateau cliff. 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 part of the Blackhawk Formation constitutes an overall coastal-fluvial environment, which contains a number of facies representing various sub-environments.

Transitional complexity at the boundary of the two formations is greatest in the northern part of study area (Wattis Road location), where several intertonguing of marine and coastal-plain strata were observed. At Wattis Road, facies architecture of the outcrop shows up-dip pinch-outs of two shallow-marine parasequences into coastal-plain Blackhawk deposits. One prominent incised-valley fill deposit (~10 m thick) was also observed that eroded the upper part of the shallow-marine parasequence. More incised valleys were documented in other southern outcrop “windows” that erosionally overly the uppermost marine sandbody, indicating the existence of a possible 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. Overall, our results illustrate a complex land-ocean interaction in the marginal-marine setting along the Wasatch Plateau outcrops.