Sedimentology of Late Jurassic Barrier-Island Complex--Lower Sundance Formation of Black Hills

Abstract

The lower Sundance Formation (Upper Jurassic, Callovian) in the Black Hills region of South Dakota and Wyoming contains a sequence of sediments deposited during progradation of a regressive, northeast-trending barrier-island complex. The regressive complex, which comprises the Lak, Hulett, and upper part of the Stockade Beaver Members, overlies the transgressive Canyon Springs sandstone that was deposited as large, submarine sand waves. The regressive sequence begins with the Stockade Beaver shale, an offshore, marine-shale facies that grades vertically into the Hulett sandstone through a transition facies of flaser- and lenticular-bedded sandstone and shale. In the northern Black Hills, the transition unit and a bioturbated, small-scale cross-bedded, and low-angle laminated facies of the Hulett sandstone comprise an upward-coarsening sequence that formed in the inferred beach environments of lower, middle, and upper shoreface and foreshore. An ichnofauna of vertical, tubelike Monocraterion and Diplocraterion implies very shallow water depths. A channel facies of cross-bedded sandstones locally replaces the barrier-island profile and represents probable tidal-channel deposits. The barrier sequence (up to 30 m thick) s overlain by a greenish silty and sandy lagoonal facies containing sandstones interpreted as washover fans. The Hulett sequence in the southern Black Hills is thin (5 to 6 m), and consists of a flaser- and lenticular-bedded transition facies overlain by the silty and sandy lagoonal facies. In this region, the transition facies is characterized by indicators of intermittent emergence such as flattened ripples and wrinkle marks, and is interpreted as having formed low on an intertidal flat behind the barrier. The barrier sequence passes on the west into a sequence of silty cross-bedded sandstones and shales interpreted as an offshore-bar deposit. The Lak Member overlies the Hulett in most of the region, and consists of well-sorted silty red beds. Well-developed stratification is absent, and the rock has a peculiar, faint wispy fabric. The red color, the apparent gradational position above a barrier complex in an overall regressive sequence, and the total lack of fauna suggest a nonmarine origin for the Lak Member. An eolian origin seems plausible for this enigmatic unit.