Low Accommodation Depositional Systems in the Lower Cretaceous Dakota Formation, Northwestern Nebraska and Southern Utah

Jonathan Antia¹, Christopher R. Fielding¹, and Matthew Joeckel^2
1Geosciences Department, University of Nebraska, Lincoln, NE
²School of Natural Resources, University of Nebraska, Lincoln, NE

The “J” sand and Dakota Formation represent up to 30m of the Lower Cretaceous stratigraphy in northwestern Nebraska and in southern Utah, respectively. The units lie within mudrock-dominated sections and are hydrocarbon prospective in the Denver-Julesburg basin. Understanding of their external geometry and internal architecture is limited. An improved understanding of depositional history was achieved by describing and correlating cores and wireline logs from wells within Sioux, Dawes and Box Butte Counties, Nebraska and outcrops in the Henry Mountains, Utah. The “J” sand is composed of fine grained, amalgamated, sharp based sandstones, alternating with siltstone and shale, with abundant carbonaceous material. Layers of wavy, hummocky, and flat laminated sandstones occur through the unit along with soft sediment deformation. Ripple laminations, lenticular bedding and millimeter scale rhythmic, pinstripe laminations of fine grained sandstone and siltstone also occur. Mudstone rip-up clasts, siderite, pyrite, and few mollusk shells occur throughout. Low diversity, size-reduced variants of the Skolithos and Cruziana ichnofacies are common in most of the unit. Bioturbation indices are variable. Multiple rooted horizons and coals are present. The near absence of marine fossils along with ichnofossil characteristics suggests stressed depositional environments. Physical structures indicate significant sediment flux to the area. Rhythmic, pinstripe laminations and abundant carbonaceous material imply near shore environments influenced by tidal activity. Together, this dataset suggests that the “J” sand in northwestern Nebraska was formed in estuarine depositional environments. In southern Utah, Hanksville to Ticaboo, outcrops of the Dakota Formation expose the variability associated with estuarine and marginal marine depositional systems comparable to the “J” sandstone.

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