2014 Rocky Mountain Section AAPG Annual Meeting

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Sedimentary Processes and Trace Fossil Communities in the Upper Shale Member of the Upper Devonian-Lower Mississippian Bakken Formation, North Dakota, USA

Abstract

Black, organic-rich rocks of the upper shale member of the Upper Devonian-Lower Mississippian Bakken Formation, a world-class petroleum source rock in the Williston Basin of the United States and Canada, contain a diverse suite of mudstone lithofacies. The succession consists of 3 facies associations (FAs). These are: 1) siliceous mudstones, 2) quartz and carbonate-bearing laminated mudstones, and 3) macrofossil debris-bearing massive mudstones. These FAs were deposited in three facies belts that reflect proximal to distal relationships. The macrofossil debris-bearing massive mudstones (FA 3) occur in the proximal facies belt and contain erosion surfaces, some with overlying conodont and phosphate-lithoclast lag deposits, mudstones with abundant millimeter-scale siltstone laminae showing irregular lateral thickness changes, and shell debris. In the medial facies belt, quartz and carbonate-bearing laminated mudstones dominate, exhibiting sub-millimeter siltstone layers with variable lateral thicknesses and local mudstone ripples. In the distal siliceous mudstone facies belt, radiolarites, radiolarian-bearing mudstones, quartz and carbonate-bearing laminated mudstones dominate. Total organic carbon (TOC) contents range between about 3 and 10 wt %, with a general proximal to distal decrease in TOC content. Abundant evidence of bioturbation exists in all FAs, and the lithological and TOC variations are paralleled by changes in burrowing style and abundance: two horizontal burrows and two types of fecal strings are recognized in the proximal facies belt, and only one horizontal burrow and one type of fecal strings characterize mudstones in the distal facies belt with none present in some millimeter-thick radiolarites. Bed-load transport processes, likely caused by storm-induced turbidity currents were active across all facies belts. Suspension settling from near the ocean surface, however, most likely played a role for depositing some of the mudstones, and was probably responsible for deposition of the radiolarites. The distribution pattern of high-TOC sediments in proximal and lower TOC deposits in some distal facies is likely a function of higher accumulation rates during radiolarian depositional events leading to a decrease of suspension-derived organic carbon within radiolarite laminae. The presence of burrows in all facies associations and nearly all facies in the upper Bakken shale member indicates that dysoxic to oxic conditions prevailed during deposition throughout this mudstone sedimentary system. This study shows that in intracratonic high-TOC mudstone successions such as the upper Bakken shale member, bed-load processes most likely dominated sedimentation, and conditions promoted a thriving infaunal benthic community. As such, deposition of the upper Bakken shale member through dynamic processes in an overall dysoxic to oxic environment represents an alternative to conventional depositional models for world-class source rocks.