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Production and preservation of organic matter during deposition of the Bakken Formation (Late Devonian and Early Mississippian), Williston Basin
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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. V6T 1Z4, 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 deposition. The recurrence of thin, organic-rich deposits Ł Corresponding
author. Tel.: C1 604 822 6179; Fax: C1 604 822 6088; E-mail: [email protected]
that accumulated synchronously, in many areas on a continent or wider scale over relatively short time intervals, 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 S 0 0 3 1 - 0 1 8 2 ( 9 8 ) 0 0 0 6 3 - 7
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Calvert and Pedersen (1992), Arthur and Sageman (1994) and Wignall (1994) demonstrate how the majority of studies consider either production or preservation of organic matter as the most important factor affecting black shale deposition while regulating the other to a non-essential role. Arguments for and against productivity and preservation models are a source of continuing controversy that has polarized much of the debate over black shale deposition. Wignall (1994, p. 56), an advocate of preservation, states that “preservation and productivity models are essentially incompatible because high productivity and enhanced preservation conditions do not coincide (Tyson, 1987) despite the claims of many”. What’s more Wignall’s (1994, p. 79) response to the idea of concurrent production and preservation of organic material during the deposition of Cretaceous black shales is that it constitutes “a rather unsatisfactory and strangely coincidental combination of causes”. Perhaps Arthur and Sageman (1994, p. 541) best illustrate the current state of uncertainty in black shale studies and the views of those who instead would advocate a productivity model when they state that “we do not fully understand the relative rates of organic carbon flux, dissolved oxygen (anoxia) and rate of sediment accumulation in producing relatively OC-rich sediments in modern environments, nor can we satisfactorily explain why preservation should be enhanced under anaerobic conditions, if it indeed is.” This paper focuses on palaeoenvironmental conditions in the Williston Basin during deposition of the black mudstone members of the Bakken Formation. Its purpose is to: (1) discuss regional conditions that control productivity and bottom water anoxia; and (2) test the hypothesis that elevated rates of productivity and enhanced preservation under anoxic bottom water conditions occurred simultaneously during organic-rich mud deposition. Despite the apparent “dichotomy of mechanisms for black shale formation” (Wignall, 1994, p. 60), this paper will argue the importance of increased organic matter production and enhanced preservation during the deposition of Late Devonian and Early Mississippian black mudstones in the Williston Basin.
2. Bakken Formation lithostratigraphy, mineralogy and geochemistry The Bakken Formation is part of a vast interval of Late Devonian and Early Mississippian (latest Famennian to earliest Tournaisian) black shale formations, including the Exshaw Formation (Foreland Basin), Chattanooga Shale (Appalachian Basin), Antrim Shale (Michigan Basin) and New Albany Shale (Illinois Basin), found throughout the interior of North America (Blatt et al., 1991 and many others). The Bakken Formation is composed of two hemipelagic black mudstone members (upper and lower) separated by a shallow marine, grey mudstone=sandstone middle member (Fig. 1; Smith, 1996; Smith and Bustin, 1996). The middle Bakken member is composed of: (1) a basal layer of burrow mottled and horizontal bedded offshore mudstone; (2) a middle layer of lenticular-bedded, wavy-bedded and flaser-bedded mudstone and sandstone, containing abundant wave and current ripple structures, overlain by trough cross-bedded and tabular cross-bedded fine-grained quartz sandstones deposited in a shoreface setting; and (3) a second upper layer of burrow mottled and horizontal bedded offshore mudstone. The Bakken Formation is restricted to the sub-surface in the Williston Basin in southern Saskatchewan, southwestern Manitoba, western North Dakota and northeastern Montana (Smith, 1996; Smith and Bustin, 1997). During deposition, the distribution of Bakken sediments was largely controlled by two regional sub-basins (Elbow and North Dakota) separated by broad platforms (Regina–Melville and Swift Current). The lower member averages 4 m in thickness and reaches a maximum of 20 m near the centre of the North Dakota Sub-basin, the deepest part of the Williston Basin during Late Devonian and Early Mississippian time (Fig. 2a). The upper black mudstone averages 2 m in thickness and it has maximum thicknesses of 7 m in the North Dakota and Elbow sub-basins and in isolated locations in southwestern Manitoba (Fig. 3a). The middle Bakken member has an average
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Fig. 1. Typical lithology of the Bakken Formation from Christie Quintana Melaval, 02-14-08-04W3, in south-central Saskatchewan. Cored interval: 1684 m to 1702 m below present land surface.
thickness of 10 m and maximum thicknesses of 30 m in the North Dakota Sub-basin, Elbow Sub-basin, and Hummingbird Synclinorium and Rocanville– Torquay Trend in south-central Saskatchewan. The reader is referred to Smith and Bustin (1997) for more detailed maps and cross-sections of Bakken members and sub-units. The Bakken Formation reflects two successive episodes of widespread, moderately deep-water (>200 m), organic-rich mud deposition in the Williston Basin, separated by a period of shallow water (10 to 200 m) offshore mud and shoreface sand deposition during accumulation of the North American
black shale ‘complex’ in Late Devonian and Early Mississippian time (Blatt et al., 1991; Smith, 1996). Changes in depositional conditions between members were caused by relative sea-level rise and fall (Smith et al., 1995). The lower Bakken member contains an average of 8% total organic carbon by weight (TOC) and a maximum concentration of 20% in the interior of the North Dakota Sub-basin (Fig. 2b). The upper member has an average of 10% TOC with high concentrations, including a maximum of 35% TOC, on the Regina– Melville Platform, a shelf setting in southeastern Saskatchewan adjacent to the deeper North Dakota
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Fig. 2. (a) Generalized regional isopach map of the lower Bakken member. (b) Regional distribution of total organic carbon content (TOC), in percent weight, for the lower Bakken black mudstone member.
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Fig. 3. (a) Generalized regional isopach map of the upper Bakken member. (b) Regional distribution of total organic carbon content (TOC), in percent weight, for the upper Bakken black mudstone member
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Fig. 4. Stratigraphic distribution of: (a) total organic carbon (TOC); (b) carbon isotopes; and (c) nitrogen isotopes in well C.P.O.G. Cedoux, 15-20-11-12W2 (Sask). Lower member thickness is 9.12 m.
Sub-basin (Fig. 3b). The middle Bakken member contains on average less than 1% TOC with rare concentrations of up to 7% TOC in offshore mudstone layers. The predominantly quartz, illite, calcite and pyritic mudstone of the lower and upper Bakken members contain high concentrations of mainly amorphous organic material. They contain an abundance of Type I and Type II organic matter (Fig. 4a,
Fig. 5. Distribution of: (a) total organic carbon (TOC); (b) carbon isotopes; and (c) nitrogen isotopes in well Triton Rocanville, 05-04-16-31W1 (Sask). Upper member thickness is 2.82 m.
Fig. 5a), which, along with the narrow ranges of Ž13 C values in organic material (Fig. 4b: lower member is 29.0‰ to 27.7‰; Fig. 5b: upper member is 29.6‰ to 28.3‰), and petrographically recognizable algal remains indicate that marine algae were the primary source of organic matter. Terrestrial organic material flushed into the basin from marginal source areas is responsible for only a small portion of their total organic matter content.
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Earliest Bakken studies recognized that deposition of black mudstone members took place in an offshore, deep-water marine setting (MacDonald, 1956) and that fine-grain, organic-rich mud may have been preserved on an “inhospitable basin floor” in a quiescent setting where “high levels of marine productivity” contributed abundant organic material to the bottom (Kents, 1959). However, more recent Bakken interpretations have often adopted a more singular approach which emphasizes fine-grained sedimentation and organic matter preservation in a stratified water column under anoxic bottom-water conditions (Lineback and Davidson, 1982; Holland et al., 1987; Webster, 1987; Karma, 1991; LeFever et al., 1991). The recent palaeoenvironmental interpretations of Smith (1996) and Smith and Bustin (1996) describe stagnant conditions across the floor of the Williston Basin characterized by sluggish bottom-water circulation, a low rate of clastic sedimentation and anoxic bottom water (>200 m deep) containing elevated concentrations of dissolved hydrogen sulphide, that were probably toxic to most benthic organisms. Such an interpretation is based on the well preserved micro-laminated structure of Bakken black mudstones, the presence of very rare trace fossils and body fossil remains and an abundance of in-situ pyrite nodules. Laminae bend around pyrite nodules indicating insitu and syndepositional formation. Less common pyrite nodules and pyritic shell remains occur in thin (<2 cm) discontinuous lag deposits of reworked inclusions that are believed to have formed by sluggish bottom-water circulation that winnowed clay and silt-size particles. Baird and Brett (1991) have attributed similar physical reworking of organic-rich muds in large Devonian depositional basins to deep-water cyclone-like gyres or geostrophic boundary currents. Bottom-water currents postulated for the Williston Basin were apparently incapable of deep, widespread erosion, significant redistribution of coarser-grained sediment or alleviation of bottom-water anoxia. While Smith and Bustin (1996) assert that stagnant conditions, particularly anoxia, were important because they enhance organic matter concentration by preventing significant physical redistribution and biological degradation of sediments, they were not a prerequisite for the deposition of organic-rich mud.
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2.1. Nitrogen isotopes and organic productivity During lower and upper member deposition, increased productivity was essential for the accumulation of organic-rich mud. In this study nitrogen isotope ratios from lower and upper Bakken member organic material are used, in the manner of Francois and Altabet (1992), Altabet and Francois (1994) and Farrell et al. (1995), as proxies for relative nutrient utilization that indicate the relative intensity of biological productivity in Williston Basin surface waters. The basic premise is that nitrate, a primary nutrient containing 15 N and 14 N isotopes, undergoes fractionation by marine algae during photosynthesis; marine algae preferentially utilize the 14 N isotope while discriminating against 15 N. Organic matter that has light Ž15 N values (i.e. low positive) reflects conditions of high nutrient supply to surface waters relative to biological consumption, thereby reflecting eutrophic conditions. Heavy Ž15 N values (i.e. high positive) reflect a more limited supply of nitrate relative to consumption which is indicative of oligotrophic conditions. Isotopically light nitrogen isotope signatures are thus interpreted to reflect an increase in availability of nutrients and thus provide an indication of high primary productivity at the time of deposition. In the lower Bakken member, consistently low Ž15 N values (Fig. 4c: Ž15 N D 1.9‰ to 5.2‰) imply an abundance of nutrients in the Williston Basin that could have supported a relatively high rate of marine productivity resulting in increased organic matter sedimentation to the basin floor. High concentrations of isotopically light organic material, similar to that in the lower member, have been identified in Holocene and Late Pleistocene sapropels in the eastern Mediterranean and the Devonian New Albany Shale, deposits associated with intense organic matter production in eutrophic conditions (Calvert et al., 1992, 1996). While a consistently high rate of productivity is inferred for the lower member, the temporal increase in organic matter concentration (Fig. 4a) is considered a function of improvements in preservation facilitated by higher rates of siliciclastic sedimentation. Smith (1996) interprets increases in the ratios of quartz to illite and alkali feldspar to illite (which in turn correlates with an increase in coarser grain size)
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to reflect progressively higher rates of siliciclastic sedimentation. Very similar temporal increases in siliciclastic sedimentation rate and organic matter concentration suggests that, while anoxia persisted in bottom water and primary productivity was consistently high, improved burial efficiency enhanced organic matter preservation during lower member deposition. The argument outlined above implies that siliciclastic sedimentation rates were balanced such that preservation was enhanced but sedimentation rates were not so high as to cause significant dilution of organic matter. Quantitatively, however, the role of enhanced siliciclastic sedimentation rate in leading to preservation of organic matter in the Bakken Formation is impossible to assess because of a lack of detailed biostratigraphy. In lower member black mudstone regionally high concentrations of organic matter were deposited in the North Dakota Sub-basin probably in response to the focusing of fine-grained sediment by slow moving deep-water currents. Currents preferentially concentrated very fine organic and mineral particulates in the deepest part of the Williston Basin while smaller concentrations accumulated nearer the basin margins. The middle Bakken member records a significant interruption of conditions affecting organic-rich mud accumulation. During a rapid drop in sea-level in latest Devonian time, offshore muds and shoreface sands accumulated across the interior of the Williston Basin in a physically more dynamic setting in which most of the organic matter was either consumed by an abundant, diverse macro-fauna or decomposed in the more oxygenated waters of the basin. In the upper Bakken member, the progressive stratigraphic increase in Ž15 N values (Fig. 5c: Ž15 N D 3.1‰ to 10.2‰) suggest, initially, greater utilization of 14 N isotope-bearing nitrate by marine algae during photosynthesis. This invokes a higher concentration of nutrients and an increased rate of organic productivity in basin surface water at the onset of deposition. The relative increase in Ž15 N values implies a progressive increase in the utilization of 15 N isotope-bearing nitrate which was probably caused by a reduction in nutrient supply. This inferred reduction in surface water nutrient supply is reflected in the temporal decrease in organic matter concentration
in the upper member (Fig. 5a) which is attributed to slower rates of primary productivity and organic matter sedimentation. In the upper member, temporal changes in organic matter concentration apparently took place independent of variations in the rate of siliciclastic sedimentation. There is no apparent correlation between organic matter distribution and variations in quartz to illite and alkali feldspar to illite ratios, as they indicate clay, silt and sand deposition (Smith, 1996). Furthermore, the occurrence of abundant organic remains on the Regina–Melville Platform records regional changes in sediment distribution independent of the deep basin centre and the effects of sediment focusing. Regional changes, like temporal variations, apparently reflect changes in productivity.
3. Production of organic matter in the Williston Basin Increased rates of organic productivity during lower and upper member deposition required an abundant supply of nutrients to the surface photic layer of the Williston Basin. The relative importance of nutrients transported from terrestrial source areas by fluvial runoff and=or supplied by upwelling water masses near the site of surface productivity is dependent upon the prevailing geographic, oceanographic and climatic conditions. 3.1. Terrestrial sediment sources During Late Devonian and Early Mississippian time the present-day North American continent was part of the much larger Laurasia land mass (Fig. 6; Ettensohn and Barron, 1981). The western interior of the North American craton, upon which the Bakken and other black mudstones were deposited, was covered by a large, shallow epicontinental sea described by Ettensohn et al. (1988) as the ‘Black-Shale Sea’. Marine black shales=mudstones, including lower and upper Bakken members, accumulated across the western North American craton in a series of interconnected or semi-isolated intra-cratonic, craton margin and foreland basins (Ettensohn, 1985; Blatt et al., 1991). The Acadian Mountains to the east and cratonic highlands to the north and northeast were
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Fig. 6. Palaeogeography of the North American craton, as part of the Laurasian land mass, in Late Devonian time (after Scotese and McKerrow, 1990).
distal sources of detrital sediment to the interior sea. Towards the west and north the Antler, Caribou and Ellesmerian Mountains remained marginal sediment source areas (Richards, 1989). The Williston Basin was located in the interior of the epicontinental sea, near the north–south-trending western shoreline of the Laurasia land mass, approximately 5 to 10ºN from the Equator (Fig. 6; Ettensohn and Barron, 1981; Parrish, 1982; Ettensohn, 1985; Van der Voo, 1988; Brand, 1989; Scotese and McKerrow, 1990). During Bakken deposition it was a partially enclosed, saucer-shaped intra-cratonic basin that maintained a seaway connection to deeper-water, open-ocean conditions at the western margin of the North American craton through the Montana Trough, a relatively narrow passage bounded by the Alberta and Wyoming platforms and partially silled by the Sweetgrass Arch (Kume, 1960, 1963; Gerhard and Anderson, 1988; Richards, 1989). The average rate of sediment accumulation dur-
ing Bakken black mudstone deposition is estimated by Huber (1986) at 1 to 3 m per million years or 0.1 to 0.3 cm=1000 years, a rate comparable to present-day, deep-water, open-ocean conditions (Libes, 1992). In comparison, sediment accumulation rates on modern continental shelves and in the Black Sea Basin average 30 cm=1000 years (Libes, 1992). Sediment starvation in the Williston Basin was a product of two factors: (1) the geographic isolation of the basin, which caused much of the detrital material debauched into the interior seaway from marginal orogenic belts to accumulate in the intervening intra-cratonic Michigan and Illinois basins and Appalachian and Antler foreland basins (Lineback and Davidson, 1982; Richards, 1989); and (2) rising sealevel in latest Devonian and Early Mississippian times (Sandberg et al., 1982; Johnson et al., 1985; Ross and Ross, 1985; Smith et al., 1995) which contributed to low rates of sedimentation offshore by increasing water depth and sediment accommo-
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dation space at basin margins. During Bakken black mudstone deposition, fine-grained siliciclastic sediments (i.e. clay, silt- and fine sand-size quartz) were probably derived from terrestrial sources on the Laurasian land mass to the east and northeast. They were transported offshore by prevailing east to west winds and settled through the water column as a fine-grained rain of hemipelagic sediment. The relatively small amount of terrestrial sediment contributed to the Williston Basin by fluvial runoff and aeolian transport is thought to have made only a minor contribution to nutrients supplied to basin waters. 3.2. Upwelling in the Williston Basin as a source of nutrients Increased production of organic matter is probably the result of upwelling marine circulation. During Bakken deposition the prevailing climate across much of the emergent Laurasian land mass was tropical to savannah-like with seasonally wet–dry conditions and generally low rainfall levels (Woodrow et al., 1973). Ettensohn and Barron (1981) and Ettensohn et al. (1988) surmise that much of the interior of North America, including the epicontinental sea where black shales accumulated, lay within a rain shadow imposed by the Acadian Mountains to the east and that prevailing trade winds blowing from east to west contributed little in the way of precipitation. However, Ettensohn and Barron (1981) and Ettensohn et al. (1988) also state that close to the Equator, in the vicinity of the Williston Basin, moisture-laden, warm air masses were largely unaffected by orographic barriers and passed freely to the west. The result is that a warm, tropical climate that included ‘seasonal’ precipitation characterized the conditions over the Williston Basin (Ettensohn and Barron, 1981; Ettensohn et al., 1988). Climatic conditions, in particular prevailing east– west winds, and the semi-enclosed, isolated geography of the Williston Basin may have produced a two-layer pattern of estuarine-like water circulation, between basin water and the open-ocean conditions at the western edge of the North American craton, that included upwelling in the basin (Fig. 7). This implied pattern of wind-driven water circulation, which includes opposing surface and deeper water lay-
ers, mimics the dynamics of estuarine circulation in modern coastal settings. During Bakken deposition, east–west winds would have forced surface water in the Williston Basin to flow towards the north-northwest (because of divergence to the right associated with Ekman transport) where it would have exited the Williston Basin through the northwest-trending, partially silled Montana Trough. To maintain a balance of flow, surface water outflow would have been replaced with an influx of water into the basin at depth through the Montana Trough. This inflow of water, once inside the basin, would have upwelled to the surface replacing water lost from the basin. This pattern of estuarine-like circulation is a possible mechanism by which nutrients were transported from deep-water in the basin to the surface where they contributed to organic matter production during black mudstone deposition. Estuarine-like circulation described for the Williston Basin is consistent, for example, with the productivity models of Strakhov (1971); Calvert et al. (1987) and Calvert (1990) for the Holocene Black Sea Basin and with the origin of recent organicrich muds in Kau Bay, Indonesia (Middelburg et al., 1991). These studies postulate a similar circulation pattern, which includes the inflow of relatively dense water into a partially enclosed basin, which enters at or near the bottom and displaces nutrient-enriched deep-water already in the basin upwards towards the surface. The result is increased biological productivity and a higher rate of organic sedimentation. Unlike the models for the Black Sea Basin and Kau Bay, implied estuarine-like circulation in the Williston Basin does not include density stratification of the water column caused by the presence of a surface freshwater layer. Indeed, during Late Devonian and Early Mississippian time, the Williston Basin was: (1) geographically isolated from sources of fluvial discharge; (2) affected by a rising sea-level, which led to a marine transgression across the interior craton; and (3) influenced by a humid climate with seasonal rainfall. All these factors would have mitigated the effects of evaporation and freshwater input to significantly change basin water salinity or density from normal marine conditions and create a stratified water column. Bakken black mudstone deposition occurred in relatively long-term episodes characterized by in-
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Fig. 7. Wind-driven surface water circulation in the semi-isolated Williston Basin produces a pattern of estuarine-like water circulation between the basin and the western margin of the North American craton.
creased productivity. As such, upwelling of in-situ deep-water in the Williston Basin was probably insufficient to sustain prolonged, intense productivity. Another source of nutrients is needed for the Williston Basin. A possible source of nutrients to the Williston Basin, with the potential to sustain productivity during lower and upper member deposition, as well as potentially other North American black shales, is the inflow of deeper water to the basin originating from the equatorial undercurrent in the adjacent Pacific Ocean Basin (Fig. 7). According to Jewell (1995) the nutrient-enriched, oxygen-depleted water of the equatorial undercurrent was well established in the Pacific Ocean Basin during Late Devonian time where it impinged upon the western margin of North America. Rising sea-level during Bakken black mudstone deposition would have caused the undercurrent, which in the present Pacific Basin occurs at depths of 100 to 200 m (Jewell, 1995), to transgress onto the craton margin. The geography of
the Williston Basin, with its opening to the western craton margin, could have allowed this undercurrent to pass through the Montana Trough and enter the Williston Basin. Once in the basin, as part of the system of estuarine-like circulation, it would have upwelled towards the surface photic zone layer providing an abundant supply of nutrients to sustain increased productivity (Fig. 7). Regional distribution of organic material in the upper black mudstone is consistent with the possible effects of increased productivity caused by the estuarine-like circulation and upwelling of nutrientenriched equatorial undercurrent water in the Williston Basin. High concentrations of organic matter (up to 35% TOC) occur on the Regina–Melville Platform, which was a shelf setting at the eastern margin of the Williston Basin next to the deeper water of the North Dakota Sub-basin (Fig. 3b). If undercurrent flow entered the basin, it would have moved across at depth and encountered the shelf margin between the North Dakota Sub-basin and the Regina–
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Fig. 8. Estuarine-like circulation in the Williston Basin during the time of upper member deposition when return flow was deflected upwards at the margin of the North Dakota Sub-basin, leading to increases in upwelling, nutrient supplies and productivity over the adjacent Regina–Melville Platform.
Melville Platform (Fig. 8). Here it would have been deflected up into the surface photic zone potentially increasing productivity and the rate of organic matter sedimentation on the Regina–Melville Platform. Similar, albeit less intense, upwelling could have occurred during lower Bakken deposition while bottom water circulation was such that organic remains were concentrated in the basin interior. Upwelling in this manner is potentially important as it invokes an extra-basinal source of nutrients supplied by a pattern of marine circulation that transcends the boundaries of the Williston Basin to include conditions in the adjacent open-ocean basin. Higher rates of productivity in the Williston Basin would have contributed to widespread, persistent anoxia in bottom water during black mudstone deposition. However, of potentially equal importance to sustained anoxia are prevailing geographic and oceanographic conditions. During Late Devonian and Early Mississippian time, the apparent lack of significant differences in
density or salinity between water in the Williston Basin and water entering from the adjacent Pacific Ocean Basin suggests that inflow from the equatorial undercurrent probably did not sink to the bottom, but instead remained at a shallow depth, possibly 100 to 200 m, below the surface. If inflow remained this close to the surface, estuarine-like circulation would have been restricted to the upper part of the water column and the vertical exchange of deep-water (below 200 m depth) and shallow water within the basin minimized. A lack of significant vertical water mixing would have established a closed system of circulation in deep basin water, isolated from a more vigorous open system in basin surface water, that promoted the slow accumulation of fine-grained, micro-laminated organic-rich sediments on the basin floor. These conditions are a possible means by which a puddle of poorly or non-circulating, stagnant, anoxic bottom water formed, and was allowed to persist, across the basin floor. Rising sea-level during black mudstone deposition would have allowed
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this stagnant puddle to expand across the basin floor. While the puddle described here invokes similar studies by Wignall (1991, 1994) and Wignall and Hallam (1991) which suggest an ‘expanding puddle’ as the cause of enhanced preservation and high concentration of organic matter, in this paper it is more a symptom of a high rate of organic sedimentation, perpetuated by sustained biological productivity, and isolation of basin bottom water.
4. Modern analogues? There is clearly no modern analogue for the unique conditions that led to widespread organicrich black shale deposition throughout North America, Europe and parts of Asia during the latest Devonian and Early Mississippian interval. However, studies such as Demaison and Moore (1980), Parrish (1982) and Wignall (1994) and others illustrate the importance of modern depositional settings, similar to the palaeo-depositional conditions envisaged here for the Williston Basin, to elaborate on organic matter deposition. Two areas to consider are the modern Peruvian margin and Namibian shelf. Both occur under a vigorous coastal upwelling system responsible for high primary productivity in surface waters with persistent bottom water anoxia (Devries and Pearcy, 1982; Calvert and Price, 1983; Suess et al., 1987; Wefer et al., 1990; Oberha¨nsli et al., 1990; Dingle, 1995). On the Peruvian margin organic-rich marine mud is accumulating in a series of structurally isolated marine basins (Salavarry and Lima Basins) less than 300 m deep, on the continental shelf of the eastern Pacific Ocean Basin. The average sedimentation rate on the Peruvian margin is 19 cm=1000 years (Oberha¨nsli et al., 1990) which contributes to a maximum organic carbon content in recent sediments of about 12% by weight (Wefer et al., 1990). On the Namibian shelf off Walvis Bay, in water depths of less than 140 m, diatom oozes are accumulating with as much as 22% total organic carbon by weight with concentrations commonly in excess of 5%. On the Namibian continental margin terrigenous input is minimal and uncompacted sedimentation rates are between 30 and 120 cm=1000 years (refer to summary by Calvert and Price, 1983). According to Pedersen and Calvert (1990) the
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rate of organic productivity in settings such as the Peruvian margin and Namibian shelf exceeds 200 g m 2 yr 1 . In comparison, the rate of organic matter productivity in the surface waters of modern deepsea ocean basins is <50 g m 2 yr 1 (Pedersen and Calvert, 1990). Based on a water depth of 300 m and organic matter flux rates by Suess (1980) and Pedersen and Calvert (1990), approximately 30% of organic matter produced in the photic zone will reach the ocean floor. However, the amount of organic material ultimately preserved in buried sediment is also dependent upon its exposure time on the bottom and post-deposition alteration. According to Bralower and Thierstein (1984) the preservation factor for organic carbon in laminated black shales, deposited in euxinic conditions, is approximately 2%. Sediment accumulation rates on the Peruvian margin (19 cm=1000 yr) and the Namibian shelf at Walvis Bay (30 to 120 cm=1000 yr) are much faster than those estimated during Bakken deposition (0.1 to 0.3 cm=1000 yr), even taking into consideration compaction of Bakken mudstones. The implied slow average rate of Bakken black mudstone sedimentation is more comparable to present-day, deep-water, open-ocean conditions where sediment is supplied primarily by aeolian transport from distal source areas. Because neither the net sedimentation rate nor preservation potential is confidently known, organic productivity rates during Bakken deposition are poorly constrained. It is nevertheless instructive to calculate potential productivity levels based on some simplistic assumptions. Assuming the preservation factor of 2% proposed by Bralower and Thierstein (1984) for laminated Cretaceous shales and an average estimated sedimentation rate for Bakken lower and upper members of 0.2 cm=1000 yr and a mudstone density of 2.65 g=cm3 , a TOC content of 20 wt.%, yields organic carbon accumulation rates 1 of about 50 g m 2 yr 1 . Assuming 30% of what is produced accumulates (i.e. Pedersen and Calvert, 1990), the most organic-rich Bakken shales have 1
Net carbon accumulation D 2 ð 10 4 cm=yr ð 2:65 gm=cm3 ð 0:20 g C=grock D 1:06 g C=m2 =yr; organic accumulation rate D .100=2/ ð 1:06 g C m 2 yr 1 D 53 g C m 2 yr 1 (assuming 2% preservation); productivity D .100=30/ ð 50 g C m 2 yr 1 D 166 g C m 2 yr 1 (assuming that 30% of productivity reaches the sea floor).
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an organic productivity of about 160 g m 2 yr 1 which is comparable to the 200 g m 2 yr 1 cited for the modern Peruvian margin and Namibian shelf. It must be noted, however, that few Bakken source rocks contain 20% organic matter and a preservation potential of 2% is probably highly optimistic considering the slow rate of sedimentation of the Bakken shales. Based on the range of TOC concentrations in the lower and upper members, during much of Bakken black mudstone deposition organic carbon accumulation rates may have been even less than the 50 g cm 2 yr 1 found in modern open-ocean settings. It must be emphasized that approximating rates of productivity and organic matter sedimentation for palaeoenvironments using modern depo-settings, while instructive, is fraught with potential errors due to: (1) the lack of accurate modern analogues; (2) difficulties in estimating sedimentation rates where accumulation has been extremely slow and=or varied over time and where lithified deposits are compared with uncompacted sediments; and (3) differences between recent, relatively fresh organic remains and ancient, diagenetically altered material. While evidence here suggests that productivity was the primary factor in organic-rich mud accumulation, more rigorous estimation of organic productivity and preservation rates and how they may change during deposition await more detailed chronostratigraphic analyses of Bakken black mudstone members and better estimates of sedimentation rates than is currently possible.
5. Conclusions The geography, climate and oceanography of the Williston Basin during Late Devonian and Early Mississippian time produced a pattern of estuarinelike circulation between the semi-enclosed intra-cratonic Williston Basin and the adjacent open-ocean conditions in the Pacific Ocean Basin. Flow, into the Williston Basin from the equatorial undercurrent, supplied nutrients from an extra-basinal source as part of a pattern of circulation that transcended the boundaries of the Williston Basin. Entrainment and upwelling of inflow provided a source of nutrients which could have sustained increased biological pro-
ductivity in the surface photic zone. The implied absence of a water density gradient between the Williston Basin and open-ocean conditions suggests that estuarine-like circulation was confined to the upper part of the water column. While intense production of organic matter occurred in surface waters, anoxia was established in bottom water because of the high rate of organic sedimentation and the lack of vertical mixing in the water column of this semi-isolated basin. Continued productivity and rising sea-levels allowed a puddle of anoxic bottom water to expand across the basin floor. During Late Devonian and Early Mississippian time, interaction of prevailing geographic, climatic and oceanographic conditions in the Williston Basin resulted in a depositional setting in which abundant organic matter is produced near the surface and exported to a basin floor where persistent anoxic bottom water enhanced its preservation. The Bakken Formation reflects unique conditions in the Williston Basin, that include productivity and preservation as essential parts of the same depositional model. These conditions may provide new insight into the factors affecting the widespread deposition of other contemporaneous black shale formations (Exshaw, Chattanooga, Antrim, New Albany) on the North American interior.
Acknowledgements Funding and logistic support for this study was provided by Wascana Energy Inc., PanCanadian Petroleum, Shell Petroleum, UNOCAL, Gulf Resources, Amoco Ltd. and an NSERC grant to Bustin. We thank all the journal reviewers but particularly Paul Jewell for his careful critique of the paper and many positive suggestions.
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Rep. 96-1. Smith, M.G., Bustin, R.M., Caplan, M.L., 1995. Sequence stratigraphy of the Bakken and Exshaw formations: a continuum of black shale formations in the Western Canada Sedimentary Basin. In: Hunter, L.D.V., Schalla, R.A. (Eds.), 7th International Williston Basin Symposium. Montana Geol. Soc. Spec. Publ. pp. 399–409. Strakhov, N.M., 1971. Geochemical evolution of the Black Sea in the Holocene. Lithol. Miner. Resour. 3, 263–274. Suess, E., 1980. Particulate organic carbon flux in the oceans — surface productivity and oxygen utilization. Nature 288, 260–263. Suess, E., Kulm, L.D., Killingley, J.S., 1987. Coastal upwelling and a history of organic-rich mudstone deposition off Peru. In: Brooks, J., Fleet, A.J. (Eds.), Marine Petroleum Source Rocks. Geol. Soc. Publ. 26, 181–198. Tyson, R.V., 1987. The genesis and palynofacies characteristics of marine petroleum source rocks. In: Brooks, J., Fleet, A.J. (Eds.), Marine Petroleum Source Rocks. Geol. Soc. Spec. Publ. 26, 47–68. Van der Voo, R., 1988. Paleozoic paleogeography of North America, Gondwana and intervening displaced terranes; comparisons of paleomagnetism with paleoclimatology and paleogeographical patterns. Geol. Soc. Am. Bull. 100, 311–324. Webster, R.W., 1987. Petroleum source rocks and stratigraphy of the Bakken Formation in North Dakota. In: Peterson, J.A., Kent, D.M., Anderson, S.B., Pilatzke, R.H., Longman, M.W. (Eds.), Williston Basin: Anatomy of a Cratonic Oil Province. Rocky Mountain Association of Geologists, Denver, Colo., pp. 269–285. Wefer, G., Heinze, P., Suess, E., 1990. 21. Stratigraphy and sedimentation rates from oxygen isotope composition, organic carbon content, and grain-size distribution at the Peru upwelling region: holes 680B and 686B. Proc. ODP, Sci. Results 112, 355–362. Wignall, P.B., 1991. Model for transgressive black shales? Geology 19, 167–170. Wignall, P.B., 1994. Black Shales. Oxford Monographs on Geology and Geophysics 30, Oxford University Press, Oxford, 127 pp. Wignall, P.B., Hallam, A., 1991. Biofacies, stratigraphic distribution and depositional models of British onshore Jurassic black shales. In: Tyson, R.V., Pearson, T.H. (Eds.), Modern and Ancient Continental Shelf Anoxia. Geol. Soc. Spec. Publ. 58, 291–309. Woodrow, D.L., Fletcher, F.W., Ahrnsbrak, W.F., 1973. Paleogeography and paleoclimate at the depositional sites of the Devonian Catskill and the Old Red Facies. Geol. Soc. Am. Bull. 84, 3051–3064.
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. V6T 1Z4, 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 deposition. The recurrence of thin, organic-rich deposits Ł Corresponding
author. Tel.: C1 604 822 6179; Fax: C1 604 822 6088; E-mail: [email protected]
that accumulated synchronously, in many areas on a continent or wider scale over relatively short time intervals, 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 S 0 0 3 1 - 0 1 8 2 ( 9 8 ) 0 0 0 6 3 - 7
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Calvert and Pedersen (1992), Arthur and Sageman (1994) and Wignall (1994) demonstrate how the majority of studies consider either production or preservation of organic matter as the most important factor affecting black shale deposition while regulating the other to a non-essential role. Arguments for and against productivity and preservation models are a source of continuing controversy that has polarized much of the debate over black shale deposition. Wignall (1994, p. 56), an advocate of preservation, states that “preservation and productivity models are essentially incompatible because high productivity and enhanced preservation conditions do not coincide (Tyson, 1987) despite the claims of many”. What’s more Wignall’s (1994, p. 79) response to the idea of concurrent production and preservation of organic material during the deposition of Cretaceous black shales is that it constitutes “a rather unsatisfactory and strangely coincidental combination of causes”. Perhaps Arthur and Sageman (1994, p. 541) best illustrate the current state of uncertainty in black shale studies and the views of those who instead would advocate a productivity model when they state that “we do not fully understand the relative rates of organic carbon flux, dissolved oxygen (anoxia) and rate of sediment accumulation in producing relatively OC-rich sediments in modern environments, nor can we satisfactorily explain why preservation should be enhanced under anaerobic conditions, if it indeed is.” This paper focuses on palaeoenvironmental conditions in the Williston Basin during deposition of the black mudstone members of the Bakken Formation. Its purpose is to: (1) discuss regional conditions that control productivity and bottom water anoxia; and (2) test the hypothesis that elevated rates of productivity and enhanced preservation under anoxic bottom water conditions occurred simultaneously during organic-rich mud deposition. Despite the apparent “dichotomy of mechanisms for black shale formation” (Wignall, 1994, p. 60), this paper will argue the importance of increased organic matter production and enhanced preservation during the deposition of Late Devonian and Early Mississippian black mudstones in the Williston Basin.
2. Bakken Formation lithostratigraphy, mineralogy and geochemistry The Bakken Formation is part of a vast interval of Late Devonian and Early Mississippian (latest Famennian to earliest Tournaisian) black shale formations, including the Exshaw Formation (Foreland Basin), Chattanooga Shale (Appalachian Basin), Antrim Shale (Michigan Basin) and New Albany Shale (Illinois Basin), found throughout the interior of North America (Blatt et al., 1991 and many others). The Bakken Formation is composed of two hemipelagic black mudstone members (upper and lower) separated by a shallow marine, grey mudstone=sandstone middle member (Fig. 1; Smith, 1996; Smith and Bustin, 1996). The middle Bakken member is composed of: (1) a basal layer of burrow mottled and horizontal bedded offshore mudstone; (2) a middle layer of lenticular-bedded, wavy-bedded and flaser-bedded mudstone and sandstone, containing abundant wave and current ripple structures, overlain by trough cross-bedded and tabular cross-bedded fine-grained quartz sandstones deposited in a shoreface setting; and (3) a second upper layer of burrow mottled and horizontal bedded offshore mudstone. The Bakken Formation is restricted to the sub-surface in the Williston Basin in southern Saskatchewan, southwestern Manitoba, western North Dakota and northeastern Montana (Smith, 1996; Smith and Bustin, 1997). During deposition, the distribution of Bakken sediments was largely controlled by two regional sub-basins (Elbow and North Dakota) separated by broad platforms (Regina–Melville and Swift Current). The lower member averages 4 m in thickness and reaches a maximum of 20 m near the centre of the North Dakota Sub-basin, the deepest part of the Williston Basin during Late Devonian and Early Mississippian time (Fig. 2a). The upper black mudstone averages 2 m in thickness and it has maximum thicknesses of 7 m in the North Dakota and Elbow sub-basins and in isolated locations in southwestern Manitoba (Fig. 3a). The middle Bakken member has an average
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Fig. 1. Typical lithology of the Bakken Formation from Christie Quintana Melaval, 02-14-08-04W3, in south-central Saskatchewan. Cored interval: 1684 m to 1702 m below present land surface.
thickness of 10 m and maximum thicknesses of 30 m in the North Dakota Sub-basin, Elbow Sub-basin, and Hummingbird Synclinorium and Rocanville– Torquay Trend in south-central Saskatchewan. The reader is referred to Smith and Bustin (1997) for more detailed maps and cross-sections of Bakken members and sub-units. The Bakken Formation reflects two successive episodes of widespread, moderately deep-water (>200 m), organic-rich mud deposition in the Williston Basin, separated by a period of shallow water (10 to 200 m) offshore mud and shoreface sand deposition during accumulation of the North American
black shale ‘complex’ in Late Devonian and Early Mississippian time (Blatt et al., 1991; Smith, 1996). Changes in depositional conditions between members were caused by relative sea-level rise and fall (Smith et al., 1995). The lower Bakken member contains an average of 8% total organic carbon by weight (TOC) and a maximum concentration of 20% in the interior of the North Dakota Sub-basin (Fig. 2b). The upper member has an average of 10% TOC with high concentrations, including a maximum of 35% TOC, on the Regina– Melville Platform, a shelf setting in southeastern Saskatchewan adjacent to the deeper North Dakota
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Fig. 2. (a) Generalized regional isopach map of the lower Bakken member. (b) Regional distribution of total organic carbon content (TOC), in percent weight, for the lower Bakken black mudstone member.
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Fig. 3. (a) Generalized regional isopach map of the upper Bakken member. (b) Regional distribution of total organic carbon content (TOC), in percent weight, for the upper Bakken black mudstone member
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Fig. 4. Stratigraphic distribution of: (a) total organic carbon (TOC); (b) carbon isotopes; and (c) nitrogen isotopes in well C.P.O.G. Cedoux, 15-20-11-12W2 (Sask). Lower member thickness is 9.12 m.
Sub-basin (Fig. 3b). The middle Bakken member contains on average less than 1% TOC with rare concentrations of up to 7% TOC in offshore mudstone layers. The predominantly quartz, illite, calcite and pyritic mudstone of the lower and upper Bakken members contain high concentrations of mainly amorphous organic material. They contain an abundance of Type I and Type II organic matter (Fig. 4a,
Fig. 5. Distribution of: (a) total organic carbon (TOC); (b) carbon isotopes; and (c) nitrogen isotopes in well Triton Rocanville, 05-04-16-31W1 (Sask). Upper member thickness is 2.82 m.
Fig. 5a), which, along with the narrow ranges of Ž13 C values in organic material (Fig. 4b: lower member is 29.0‰ to 27.7‰; Fig. 5b: upper member is 29.6‰ to 28.3‰), and petrographically recognizable algal remains indicate that marine algae were the primary source of organic matter. Terrestrial organic material flushed into the basin from marginal source areas is responsible for only a small portion of their total organic matter content.
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Earliest Bakken studies recognized that deposition of black mudstone members took place in an offshore, deep-water marine setting (MacDonald, 1956) and that fine-grain, organic-rich mud may have been preserved on an “inhospitable basin floor” in a quiescent setting where “high levels of marine productivity” contributed abundant organic material to the bottom (Kents, 1959). However, more recent Bakken interpretations have often adopted a more singular approach which emphasizes fine-grained sedimentation and organic matter preservation in a stratified water column under anoxic bottom-water conditions (Lineback and Davidson, 1982; Holland et al., 1987; Webster, 1987; Karma, 1991; LeFever et al., 1991). The recent palaeoenvironmental interpretations of Smith (1996) and Smith and Bustin (1996) describe stagnant conditions across the floor of the Williston Basin characterized by sluggish bottom-water circulation, a low rate of clastic sedimentation and anoxic bottom water (>200 m deep) containing elevated concentrations of dissolved hydrogen sulphide, that were probably toxic to most benthic organisms. Such an interpretation is based on the well preserved micro-laminated structure of Bakken black mudstones, the presence of very rare trace fossils and body fossil remains and an abundance of in-situ pyrite nodules. Laminae bend around pyrite nodules indicating insitu and syndepositional formation. Less common pyrite nodules and pyritic shell remains occur in thin (<2 cm) discontinuous lag deposits of reworked inclusions that are believed to have formed by sluggish bottom-water circulation that winnowed clay and silt-size particles. Baird and Brett (1991) have attributed similar physical reworking of organic-rich muds in large Devonian depositional basins to deep-water cyclone-like gyres or geostrophic boundary currents. Bottom-water currents postulated for the Williston Basin were apparently incapable of deep, widespread erosion, significant redistribution of coarser-grained sediment or alleviation of bottom-water anoxia. While Smith and Bustin (1996) assert that stagnant conditions, particularly anoxia, were important because they enhance organic matter concentration by preventing significant physical redistribution and biological degradation of sediments, they were not a prerequisite for the deposition of organic-rich mud.
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2.1. Nitrogen isotopes and organic productivity During lower and upper member deposition, increased productivity was essential for the accumulation of organic-rich mud. In this study nitrogen isotope ratios from lower and upper Bakken member organic material are used, in the manner of Francois and Altabet (1992), Altabet and Francois (1994) and Farrell et al. (1995), as proxies for relative nutrient utilization that indicate the relative intensity of biological productivity in Williston Basin surface waters. The basic premise is that nitrate, a primary nutrient containing 15 N and 14 N isotopes, undergoes fractionation by marine algae during photosynthesis; marine algae preferentially utilize the 14 N isotope while discriminating against 15 N. Organic matter that has light Ž15 N values (i.e. low positive) reflects conditions of high nutrient supply to surface waters relative to biological consumption, thereby reflecting eutrophic conditions. Heavy Ž15 N values (i.e. high positive) reflect a more limited supply of nitrate relative to consumption which is indicative of oligotrophic conditions. Isotopically light nitrogen isotope signatures are thus interpreted to reflect an increase in availability of nutrients and thus provide an indication of high primary productivity at the time of deposition. In the lower Bakken member, consistently low Ž15 N values (Fig. 4c: Ž15 N D 1.9‰ to 5.2‰) imply an abundance of nutrients in the Williston Basin that could have supported a relatively high rate of marine productivity resulting in increased organic matter sedimentation to the basin floor. High concentrations of isotopically light organic material, similar to that in the lower member, have been identified in Holocene and Late Pleistocene sapropels in the eastern Mediterranean and the Devonian New Albany Shale, deposits associated with intense organic matter production in eutrophic conditions (Calvert et al., 1992, 1996). While a consistently high rate of productivity is inferred for the lower member, the temporal increase in organic matter concentration (Fig. 4a) is considered a function of improvements in preservation facilitated by higher rates of siliciclastic sedimentation. Smith (1996) interprets increases in the ratios of quartz to illite and alkali feldspar to illite (which in turn correlates with an increase in coarser grain size)
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to reflect progressively higher rates of siliciclastic sedimentation. Very similar temporal increases in siliciclastic sedimentation rate and organic matter concentration suggests that, while anoxia persisted in bottom water and primary productivity was consistently high, improved burial efficiency enhanced organic matter preservation during lower member deposition. The argument outlined above implies that siliciclastic sedimentation rates were balanced such that preservation was enhanced but sedimentation rates were not so high as to cause significant dilution of organic matter. Quantitatively, however, the role of enhanced siliciclastic sedimentation rate in leading to preservation of organic matter in the Bakken Formation is impossible to assess because of a lack of detailed biostratigraphy. In lower member black mudstone regionally high concentrations of organic matter were deposited in the North Dakota Sub-basin probably in response to the focusing of fine-grained sediment by slow moving deep-water currents. Currents preferentially concentrated very fine organic and mineral particulates in the deepest part of the Williston Basin while smaller concentrations accumulated nearer the basin margins. The middle Bakken member records a significant interruption of conditions affecting organic-rich mud accumulation. During a rapid drop in sea-level in latest Devonian time, offshore muds and shoreface sands accumulated across the interior of the Williston Basin in a physically more dynamic setting in which most of the organic matter was either consumed by an abundant, diverse macro-fauna or decomposed in the more oxygenated waters of the basin. In the upper Bakken member, the progressive stratigraphic increase in Ž15 N values (Fig. 5c: Ž15 N D 3.1‰ to 10.2‰) suggest, initially, greater utilization of 14 N isotope-bearing nitrate by marine algae during photosynthesis. This invokes a higher concentration of nutrients and an increased rate of organic productivity in basin surface water at the onset of deposition. The relative increase in Ž15 N values implies a progressive increase in the utilization of 15 N isotope-bearing nitrate which was probably caused by a reduction in nutrient supply. This inferred reduction in surface water nutrient supply is reflected in the temporal decrease in organic matter concentration
in the upper member (Fig. 5a) which is attributed to slower rates of primary productivity and organic matter sedimentation. In the upper member, temporal changes in organic matter concentration apparently took place independent of variations in the rate of siliciclastic sedimentation. There is no apparent correlation between organic matter distribution and variations in quartz to illite and alkali feldspar to illite ratios, as they indicate clay, silt and sand deposition (Smith, 1996). Furthermore, the occurrence of abundant organic remains on the Regina–Melville Platform records regional changes in sediment distribution independent of the deep basin centre and the effects of sediment focusing. Regional changes, like temporal variations, apparently reflect changes in productivity.
3. Production of organic matter in the Williston Basin Increased rates of organic productivity during lower and upper member deposition required an abundant supply of nutrients to the surface photic layer of the Williston Basin. The relative importance of nutrients transported from terrestrial source areas by fluvial runoff and=or supplied by upwelling water masses near the site of surface productivity is dependent upon the prevailing geographic, oceanographic and climatic conditions. 3.1. Terrestrial sediment sources During Late Devonian and Early Mississippian time the present-day North American continent was part of the much larger Laurasia land mass (Fig. 6; Ettensohn and Barron, 1981). The western interior of the North American craton, upon which the Bakken and other black mudstones were deposited, was covered by a large, shallow epicontinental sea described by Ettensohn et al. (1988) as the ‘Black-Shale Sea’. Marine black shales=mudstones, including lower and upper Bakken members, accumulated across the western North American craton in a series of interconnected or semi-isolated intra-cratonic, craton margin and foreland basins (Ettensohn, 1985; Blatt et al., 1991). The Acadian Mountains to the east and cratonic highlands to the north and northeast were
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Fig. 6. Palaeogeography of the North American craton, as part of the Laurasian land mass, in Late Devonian time (after Scotese and McKerrow, 1990).
distal sources of detrital sediment to the interior sea. Towards the west and north the Antler, Caribou and Ellesmerian Mountains remained marginal sediment source areas (Richards, 1989). The Williston Basin was located in the interior of the epicontinental sea, near the north–south-trending western shoreline of the Laurasia land mass, approximately 5 to 10ºN from the Equator (Fig. 6; Ettensohn and Barron, 1981; Parrish, 1982; Ettensohn, 1985; Van der Voo, 1988; Brand, 1989; Scotese and McKerrow, 1990). During Bakken deposition it was a partially enclosed, saucer-shaped intra-cratonic basin that maintained a seaway connection to deeper-water, open-ocean conditions at the western margin of the North American craton through the Montana Trough, a relatively narrow passage bounded by the Alberta and Wyoming platforms and partially silled by the Sweetgrass Arch (Kume, 1960, 1963; Gerhard and Anderson, 1988; Richards, 1989). The average rate of sediment accumulation dur-
ing Bakken black mudstone deposition is estimated by Huber (1986) at 1 to 3 m per million years or 0.1 to 0.3 cm=1000 years, a rate comparable to present-day, deep-water, open-ocean conditions (Libes, 1992). In comparison, sediment accumulation rates on modern continental shelves and in the Black Sea Basin average 30 cm=1000 years (Libes, 1992). Sediment starvation in the Williston Basin was a product of two factors: (1) the geographic isolation of the basin, which caused much of the detrital material debauched into the interior seaway from marginal orogenic belts to accumulate in the intervening intra-cratonic Michigan and Illinois basins and Appalachian and Antler foreland basins (Lineback and Davidson, 1982; Richards, 1989); and (2) rising sealevel in latest Devonian and Early Mississippian times (Sandberg et al., 1982; Johnson et al., 1985; Ross and Ross, 1985; Smith et al., 1995) which contributed to low rates of sedimentation offshore by increasing water depth and sediment accommo-
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dation space at basin margins. During Bakken black mudstone deposition, fine-grained siliciclastic sediments (i.e. clay, silt- and fine sand-size quartz) were probably derived from terrestrial sources on the Laurasian land mass to the east and northeast. They were transported offshore by prevailing east to west winds and settled through the water column as a fine-grained rain of hemipelagic sediment. The relatively small amount of terrestrial sediment contributed to the Williston Basin by fluvial runoff and aeolian transport is thought to have made only a minor contribution to nutrients supplied to basin waters. 3.2. Upwelling in the Williston Basin as a source of nutrients Increased production of organic matter is probably the result of upwelling marine circulation. During Bakken deposition the prevailing climate across much of the emergent Laurasian land mass was tropical to savannah-like with seasonally wet–dry conditions and generally low rainfall levels (Woodrow et al., 1973). Ettensohn and Barron (1981) and Ettensohn et al. (1988) surmise that much of the interior of North America, including the epicontinental sea where black shales accumulated, lay within a rain shadow imposed by the Acadian Mountains to the east and that prevailing trade winds blowing from east to west contributed little in the way of precipitation. However, Ettensohn and Barron (1981) and Ettensohn et al. (1988) also state that close to the Equator, in the vicinity of the Williston Basin, moisture-laden, warm air masses were largely unaffected by orographic barriers and passed freely to the west. The result is that a warm, tropical climate that included ‘seasonal’ precipitation characterized the conditions over the Williston Basin (Ettensohn and Barron, 1981; Ettensohn et al., 1988). Climatic conditions, in particular prevailing east– west winds, and the semi-enclosed, isolated geography of the Williston Basin may have produced a two-layer pattern of estuarine-like water circulation, between basin water and the open-ocean conditions at the western edge of the North American craton, that included upwelling in the basin (Fig. 7). This implied pattern of wind-driven water circulation, which includes opposing surface and deeper water lay-
ers, mimics the dynamics of estuarine circulation in modern coastal settings. During Bakken deposition, east–west winds would have forced surface water in the Williston Basin to flow towards the north-northwest (because of divergence to the right associated with Ekman transport) where it would have exited the Williston Basin through the northwest-trending, partially silled Montana Trough. To maintain a balance of flow, surface water outflow would have been replaced with an influx of water into the basin at depth through the Montana Trough. This inflow of water, once inside the basin, would have upwelled to the surface replacing water lost from the basin. This pattern of estuarine-like circulation is a possible mechanism by which nutrients were transported from deep-water in the basin to the surface where they contributed to organic matter production during black mudstone deposition. Estuarine-like circulation described for the Williston Basin is consistent, for example, with the productivity models of Strakhov (1971); Calvert et al. (1987) and Calvert (1990) for the Holocene Black Sea Basin and with the origin of recent organicrich muds in Kau Bay, Indonesia (Middelburg et al., 1991). These studies postulate a similar circulation pattern, which includes the inflow of relatively dense water into a partially enclosed basin, which enters at or near the bottom and displaces nutrient-enriched deep-water already in the basin upwards towards the surface. The result is increased biological productivity and a higher rate of organic sedimentation. Unlike the models for the Black Sea Basin and Kau Bay, implied estuarine-like circulation in the Williston Basin does not include density stratification of the water column caused by the presence of a surface freshwater layer. Indeed, during Late Devonian and Early Mississippian time, the Williston Basin was: (1) geographically isolated from sources of fluvial discharge; (2) affected by a rising sea-level, which led to a marine transgression across the interior craton; and (3) influenced by a humid climate with seasonal rainfall. All these factors would have mitigated the effects of evaporation and freshwater input to significantly change basin water salinity or density from normal marine conditions and create a stratified water column. Bakken black mudstone deposition occurred in relatively long-term episodes characterized by in-
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Fig. 7. Wind-driven surface water circulation in the semi-isolated Williston Basin produces a pattern of estuarine-like water circulation between the basin and the western margin of the North American craton.
creased productivity. As such, upwelling of in-situ deep-water in the Williston Basin was probably insufficient to sustain prolonged, intense productivity. Another source of nutrients is needed for the Williston Basin. A possible source of nutrients to the Williston Basin, with the potential to sustain productivity during lower and upper member deposition, as well as potentially other North American black shales, is the inflow of deeper water to the basin originating from the equatorial undercurrent in the adjacent Pacific Ocean Basin (Fig. 7). According to Jewell (1995) the nutrient-enriched, oxygen-depleted water of the equatorial undercurrent was well established in the Pacific Ocean Basin during Late Devonian time where it impinged upon the western margin of North America. Rising sea-level during Bakken black mudstone deposition would have caused the undercurrent, which in the present Pacific Basin occurs at depths of 100 to 200 m (Jewell, 1995), to transgress onto the craton margin. The geography of
the Williston Basin, with its opening to the western craton margin, could have allowed this undercurrent to pass through the Montana Trough and enter the Williston Basin. Once in the basin, as part of the system of estuarine-like circulation, it would have upwelled towards the surface photic zone layer providing an abundant supply of nutrients to sustain increased productivity (Fig. 7). Regional distribution of organic material in the upper black mudstone is consistent with the possible effects of increased productivity caused by the estuarine-like circulation and upwelling of nutrientenriched equatorial undercurrent water in the Williston Basin. High concentrations of organic matter (up to 35% TOC) occur on the Regina–Melville Platform, which was a shelf setting at the eastern margin of the Williston Basin next to the deeper water of the North Dakota Sub-basin (Fig. 3b). If undercurrent flow entered the basin, it would have moved across at depth and encountered the shelf margin between the North Dakota Sub-basin and the Regina–
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Fig. 8. Estuarine-like circulation in the Williston Basin during the time of upper member deposition when return flow was deflected upwards at the margin of the North Dakota Sub-basin, leading to increases in upwelling, nutrient supplies and productivity over the adjacent Regina–Melville Platform.
Melville Platform (Fig. 8). Here it would have been deflected up into the surface photic zone potentially increasing productivity and the rate of organic matter sedimentation on the Regina–Melville Platform. Similar, albeit less intense, upwelling could have occurred during lower Bakken deposition while bottom water circulation was such that organic remains were concentrated in the basin interior. Upwelling in this manner is potentially important as it invokes an extra-basinal source of nutrients supplied by a pattern of marine circulation that transcends the boundaries of the Williston Basin to include conditions in the adjacent open-ocean basin. Higher rates of productivity in the Williston Basin would have contributed to widespread, persistent anoxia in bottom water during black mudstone deposition. However, of potentially equal importance to sustained anoxia are prevailing geographic and oceanographic conditions. During Late Devonian and Early Mississippian time, the apparent lack of significant differences in
density or salinity between water in the Williston Basin and water entering from the adjacent Pacific Ocean Basin suggests that inflow from the equatorial undercurrent probably did not sink to the bottom, but instead remained at a shallow depth, possibly 100 to 200 m, below the surface. If inflow remained this close to the surface, estuarine-like circulation would have been restricted to the upper part of the water column and the vertical exchange of deep-water (below 200 m depth) and shallow water within the basin minimized. A lack of significant vertical water mixing would have established a closed system of circulation in deep basin water, isolated from a more vigorous open system in basin surface water, that promoted the slow accumulation of fine-grained, micro-laminated organic-rich sediments on the basin floor. These conditions are a possible means by which a puddle of poorly or non-circulating, stagnant, anoxic bottom water formed, and was allowed to persist, across the basin floor. Rising sea-level during black mudstone deposition would have allowed
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this stagnant puddle to expand across the basin floor. While the puddle described here invokes similar studies by Wignall (1991, 1994) and Wignall and Hallam (1991) which suggest an ‘expanding puddle’ as the cause of enhanced preservation and high concentration of organic matter, in this paper it is more a symptom of a high rate of organic sedimentation, perpetuated by sustained biological productivity, and isolation of basin bottom water.
4. Modern analogues? There is clearly no modern analogue for the unique conditions that led to widespread organicrich black shale deposition throughout North America, Europe and parts of Asia during the latest Devonian and Early Mississippian interval. However, studies such as Demaison and Moore (1980), Parrish (1982) and Wignall (1994) and others illustrate the importance of modern depositional settings, similar to the palaeo-depositional conditions envisaged here for the Williston Basin, to elaborate on organic matter deposition. Two areas to consider are the modern Peruvian margin and Namibian shelf. Both occur under a vigorous coastal upwelling system responsible for high primary productivity in surface waters with persistent bottom water anoxia (Devries and Pearcy, 1982; Calvert and Price, 1983; Suess et al., 1987; Wefer et al., 1990; Oberha¨nsli et al., 1990; Dingle, 1995). On the Peruvian margin organic-rich marine mud is accumulating in a series of structurally isolated marine basins (Salavarry and Lima Basins) less than 300 m deep, on the continental shelf of the eastern Pacific Ocean Basin. The average sedimentation rate on the Peruvian margin is 19 cm=1000 years (Oberha¨nsli et al., 1990) which contributes to a maximum organic carbon content in recent sediments of about 12% by weight (Wefer et al., 1990). On the Namibian shelf off Walvis Bay, in water depths of less than 140 m, diatom oozes are accumulating with as much as 22% total organic carbon by weight with concentrations commonly in excess of 5%. On the Namibian continental margin terrigenous input is minimal and uncompacted sedimentation rates are between 30 and 120 cm=1000 years (refer to summary by Calvert and Price, 1983). According to Pedersen and Calvert (1990) the
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rate of organic productivity in settings such as the Peruvian margin and Namibian shelf exceeds 200 g m 2 yr 1 . In comparison, the rate of organic matter productivity in the surface waters of modern deepsea ocean basins is <50 g m 2 yr 1 (Pedersen and Calvert, 1990). Based on a water depth of 300 m and organic matter flux rates by Suess (1980) and Pedersen and Calvert (1990), approximately 30% of organic matter produced in the photic zone will reach the ocean floor. However, the amount of organic material ultimately preserved in buried sediment is also dependent upon its exposure time on the bottom and post-deposition alteration. According to Bralower and Thierstein (1984) the preservation factor for organic carbon in laminated black shales, deposited in euxinic conditions, is approximately 2%. Sediment accumulation rates on the Peruvian margin (19 cm=1000 yr) and the Namibian shelf at Walvis Bay (30 to 120 cm=1000 yr) are much faster than those estimated during Bakken deposition (0.1 to 0.3 cm=1000 yr), even taking into consideration compaction of Bakken mudstones. The implied slow average rate of Bakken black mudstone sedimentation is more comparable to present-day, deep-water, open-ocean conditions where sediment is supplied primarily by aeolian transport from distal source areas. Because neither the net sedimentation rate nor preservation potential is confidently known, organic productivity rates during Bakken deposition are poorly constrained. It is nevertheless instructive to calculate potential productivity levels based on some simplistic assumptions. Assuming the preservation factor of 2% proposed by Bralower and Thierstein (1984) for laminated Cretaceous shales and an average estimated sedimentation rate for Bakken lower and upper members of 0.2 cm=1000 yr and a mudstone density of 2.65 g=cm3 , a TOC content of 20 wt.%, yields organic carbon accumulation rates 1 of about 50 g m 2 yr 1 . Assuming 30% of what is produced accumulates (i.e. Pedersen and Calvert, 1990), the most organic-rich Bakken shales have 1
Net carbon accumulation D 2 ð 10 4 cm=yr ð 2:65 gm=cm3 ð 0:20 g C=grock D 1:06 g C=m2 =yr; organic accumulation rate D .100=2/ ð 1:06 g C m 2 yr 1 D 53 g C m 2 yr 1 (assuming 2% preservation); productivity D .100=30/ ð 50 g C m 2 yr 1 D 166 g C m 2 yr 1 (assuming that 30% of productivity reaches the sea floor).
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an organic productivity of about 160 g m 2 yr 1 which is comparable to the 200 g m 2 yr 1 cited for the modern Peruvian margin and Namibian shelf. It must be noted, however, that few Bakken source rocks contain 20% organic matter and a preservation potential of 2% is probably highly optimistic considering the slow rate of sedimentation of the Bakken shales. Based on the range of TOC concentrations in the lower and upper members, during much of Bakken black mudstone deposition organic carbon accumulation rates may have been even less than the 50 g cm 2 yr 1 found in modern open-ocean settings. It must be emphasized that approximating rates of productivity and organic matter sedimentation for palaeoenvironments using modern depo-settings, while instructive, is fraught with potential errors due to: (1) the lack of accurate modern analogues; (2) difficulties in estimating sedimentation rates where accumulation has been extremely slow and=or varied over time and where lithified deposits are compared with uncompacted sediments; and (3) differences between recent, relatively fresh organic remains and ancient, diagenetically altered material. While evidence here suggests that productivity was the primary factor in organic-rich mud accumulation, more rigorous estimation of organic productivity and preservation rates and how they may change during deposition await more detailed chronostratigraphic analyses of Bakken black mudstone members and better estimates of sedimentation rates than is currently possible.
5. Conclusions The geography, climate and oceanography of the Williston Basin during Late Devonian and Early Mississippian time produced a pattern of estuarinelike circulation between the semi-enclosed intra-cratonic Williston Basin and the adjacent open-ocean conditions in the Pacific Ocean Basin. Flow, into the Williston Basin from the equatorial undercurrent, supplied nutrients from an extra-basinal source as part of a pattern of circulation that transcended the boundaries of the Williston Basin. Entrainment and upwelling of inflow provided a source of nutrients which could have sustained increased biological pro-
ductivity in the surface photic zone. The implied absence of a water density gradient between the Williston Basin and open-ocean conditions suggests that estuarine-like circulation was confined to the upper part of the water column. While intense production of organic matter occurred in surface waters, anoxia was established in bottom water because of the high rate of organic sedimentation and the lack of vertical mixing in the water column of this semi-isolated basin. Continued productivity and rising sea-levels allowed a puddle of anoxic bottom water to expand across the basin floor. During Late Devonian and Early Mississippian time, interaction of prevailing geographic, climatic and oceanographic conditions in the Williston Basin resulted in a depositional setting in which abundant organic matter is produced near the surface and exported to a basin floor where persistent anoxic bottom water enhanced its preservation. The Bakken Formation reflects unique conditions in the Williston Basin, that include productivity and preservation as essential parts of the same depositional model. These conditions may provide new insight into the factors affecting the widespread deposition of other contemporaneous black shale formations (Exshaw, Chattanooga, Antrim, New Albany) on the North American interior.
Acknowledgements Funding and logistic support for this study was provided by Wascana Energy Inc., PanCanadian Petroleum, Shell Petroleum, UNOCAL, Gulf Resources, Amoco Ltd. and an NSERC grant to Bustin. We thank all the journal reviewers but particularly Paul Jewell for his careful critique of the paper and many positive suggestions.
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