Sedimentology, Ichnofacies, and Early Diagenesis, the Three Forks Formation, North Dakota and Montana- A Schizohaline, Storm Dominated, Intrashelf Basin

Abstract

The time-equivalent, landward deposits of the Famennian Palliser carbonate platform preserved within the Williston basin have been inaccessible until recent economic interest in the Three Forks Formation. A regional database of polished drill core slabs and petrographic thin sections allowed for detailed analysis of texture, sedimentary structures, ichnological and diagenetic features. XRD and Qemscan mineralogy data were compiled from a variety of cores around the basin. The Three Forks mixed carbonate and siliciclastic intrashelf mudstones facies associations (FA) include: 1) schizohaline storm-dominated shallow shelf, 2) arid shallow shelf and 3) mudflats. FA1 consists of the following lithofacies: (1) distorted claystones that contain moderate abundance of mobile feeding traces and syneresis cracks; (2) laminated mudstones; (3) dolomudstones that include the previous features plus escape burrows, and opportunistic suspension feeding colonies. These facies demonstrate scouring, high depositional rate, storm, and wave features. Additionally, (4) syndepositional deformation through dewatering, evaporite precipitation and dissolution and bioturbation is interpreted to produce brecciated to distorted mudstones; and (5) thin bedded quartz sandstones represent preserved and unmixed fluvial floods into the shallow basin. Arid shallow shelf (FA2) deposits consist of mosaic anhydrites which imply periods of restriction and evaporation at a basinal scale. Mudflat (FA 3) facies include (1) rarely preserved meteorological tide deposited laminated mudstones, and (2) abundantly preserved matrix-supported breccias to (3) massive or intraclastic mudstones. These deposits represent a spectrum of strength and influence of mudflat storm surge that locally transports siltstone clasts derived from the previous facies association. Massive mudstones also commonly have syndepositional anhydrite nodules when associated with the arid shallow shelf system. Storm-generated deposits are preferentially preserved over fairweather deposits. Variable storm strength and resultant geomorphology can result in high vertical facies variability. Regional changes in the stratigraphic section allude to a climatic shift from arid to seasonal in addition to influences from tectonics and sea level.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015