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Hampson, Gary1
(1) Imperial College London, London, United Kingdom

ABSTRACT: Facies Architecture and Stratigraphy of “Stray” Shelf Sandstones, Late Cretaceous Mancos Shale, Northern Utah and Colorado

Isolated, “stray” sandstones are a key component of shelf stratigraphy, but their origin remains enigmatic and their distribution is challenging to predict. This ongoing study analyses sandbody character, distribution and stratigraphic architecture in two continuously exposed, 200-km dip-transects through Campanian (late Cretaceous) shelf strata of the US Western Interior Seaway in northern Utah and Colorado. An extensive subsurface dataset comprising c. 3000 well-logs allows 3D sandbody distribution to be mapped in between these two transects and linked into a high-resolution sequence stratigraphic framework derived from coeval shoreline strata in the Book Cliffs, Utah.
“Stray” sandstones along the northern shelf transect, through north-western Colorado, occur at four discrete stratigraphic levels and form widespread, discontinuous sheets. Each of these sandstones comprises one or more thin (5-15 m), gradationally based, upward-coarsening successions, which are of two types: (1) shales grading into moderately bioturbated, cross-bedded sandstones with abundant silt drapes and reactivation surfaces, and (2) shales grading into “clean”, very well-sorted, cross-bedded sandstones that are moderately to intensely bioturbated by deep-tier burrows (Thalassinoides, Ophiomorpha). Hummocky cross-stratification is rare. Palaeocurrents are predominantly oriented towards the south, perpendicular to the highstand shoreline trend, although localised populations of opposed, northward-directed palaeocurrents are observed. Both types of succession are interpreted as the products of tidal currents. The gradational bases of the “stray” sandstones, which lack transgressive lags, their widespread extents and their relatively coarse grain size (medium-grained sandstone) imply that the “stray” sandstones are tide-dominated falling stage and/or lowstand shorelines with complex, non-linear palaeogeographies.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.