ELSEVIER Palaeogeography, Palaeoclimatology, Palaeoecology 142 (1998) 185–200 Production and preservation of organic matter during deposition of the Bakken Formation (Late Devonian and Early Mississippian), Williston Basin Mark G. Smith, R. Marc Bustin * Department of Earth and Ocean Sciences, The University of British Columbia, Vancouver, B.C. V6T1Z4, Canada Received 16 October 1997; accepted 19 March 1998 Abstract The organic-rich (up to 35% TOC) lower and upper black mudstone members of the Bakken Formation (Late Devonian and Early Mississippian) accumulated in response to both increased productivity and enhanced preservation that resulted from unique palaeogeographic, palaeoceanographic and palaeoclimatic conditions. During Bakken deposition the semi-isolated Williston Basin was located close to the Equator (5º to 10ºN) in a zone of prevailing east to west winds. A pattern of estuarine-like marine circulation between the Williston Basin and open-ocean conditions at the western craton edge of North America is proposed. Surface water discharged from the basin was replaced by nutrient-rich deeper water sourced from the equatorial undercurrent in the Pacific Basin. Undercurrent flow into the Williston Basin upwelled providing a potentially sustainable source of nutrients to enhance productivity in the surface photic zone during black mudstone deposition. In response to increased productivity, that led to a high rate of organic sedimentation, and quiescent, deep-water conditions, due primarily to the geographic isolation of the basin and the restriction of estuarine-like circulation to the upper water column, anoxic bottom water developed on the basin floor. As sea-level rose during lower and upper black mudstone deposition, anoxic bottom water expanded across the basin enhancing organic matter preservation. Conditions in the Williston Basin demonstrate that both increased productivity and improved preservation can be essential and complementary factors affecting organic-rich mud deposition. These results have potentially important implications for the accumulation of similarly organic-rich deposits across the North American interior during Late Devonian and Early Mississippian time.  1998 Elsevier Science B.V. All rights reserved. Keywords: Bakken; preservation; organic matter; productivity; black shale; Williston Basin 1. Introduction Organic-rich marine black shale formations are the manifestation of unique environments of depo- sition. The recurrence of thin, organic-rich deposits Ł Corresponding author. Tel.: C1 604 822 6179; Fax: C1 604 822 6088; E-mail: bustin@unixg.ubc.ca that accumulated synchronously, in many areas on a continent or wider scale over relatively short time in- tervals, invokes depositional conditions which have no modern analogue (Demaison and Moore, 1980; Arthur and Sageman, 1994; Wignall, 1994). Considerable research effort has focused on the interpretation of black shale depositional settings. The summaries of Demaison and Moore (1980), 0031-0182/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S0031-0182(98)00063-7