Sea-level Control on Source-rock Development: Perspectives from the Holocene Black Sea, the Mid-cretaceous Western Interior Basin of North America, and the Late Devonian Appalachian Basin

Changing sea level is a major factor in the pattern of enrichment of organic carbon in marginal and epicontinental seas. Organiccarbon-rich facies accumulate preferentially during major transgressive episodes. Rising sea level promotes retention of nutrients in marginal seas through several possible mechanisms, leading to higher organic production and/or eutrophic conditions. Transgressive seas also create circumstances that lead to seasonally or longer-term enhanced water-column stratification and development of anoxia in combination with eutrophism. Finally, rising sea level promotes nearshore trapping of terrigenous clastic material, creating condensed intervals that are characterized by enrichments in organic carbon. The interplay of these mechanisms is illustrated by integrated studies of the Holocene Black Sea, the Cenomanian–Turonian of the U.S. Western Interior Basin, and strata of the Middle to Late Devonian Appalachian Basin. Anoxia and high productivity developed in the Holocene Black Sea around 7.6 ka, leading to deposition of a sapropel with up to 20 wt. % organic carbon. The development of eutrophic conditions coincided with rising sea level and overflow of saline waters from the Mediterranean Sea. Trapping of river-derived nutrients in the Black Sea behind the shallow sill to the Mediterranean and the high freshwater flux to the Black Sea Basin during climatic warming are additional causes of eutrophication associated with the global midHolocene transgression. Organic-carbon contents increase towards the basin center because of lower clastic dilution and focusing of organic-carbon transport from margins to center. Repeated transgressive–regressive episodes in the Cenomanian–Turonian led to progressive flooding of the Western Interior Basin of North America, culminating in deposition of maximum highstand intervals in the early Turonian. High organic-carbon contents characterize transgressive episodes at multiple temporal scales as indicated by sequence stratigraphic analysis. The major enrichments of organic carbon in the Cenomanian Graneros, Lincoln, and Hartland shales during transgression are interpreted as reflecting enhanced stratification under salinityto thermally stratified conditions in a silled basin characterized by high fluvial input. River-derived nutrients were responsible for higher production of organic matter preserved under anoxic conditions that resulted, in part, from enhanced watercolumn stratification. Episodes of organic-carbon enrichment in the Bridge Creek Limestone and Fairport Chalk occur during rising sea level and highstands and are interpreted as representing eutrophication caused by the entrainment of nutrient-rich, oxygen-poor waters of an oxygen-minimum zone that impinged on the southern basin sill. These waters were tapped only during transgressive episodes that exceeded about 75–100 m water depth over the sill. The combination of eutrophication and decreased clastic dilution associated with the transgressive episodes led to maxima of organic-carbon contents (to 8 wt. %) that were highest in distal portions of the basin. Rising sea level, in combination with tectonic subsidence, also profoundly influenced the pattern of organic enrichment in Middle to Late Devonian strata of the Appalachian Basin. Sediment starvation during transgressions led to organic enrichment in shales, which, together with seasonal benthic anoxia beneath thermally stratified waters, enhanced remineralization of nutrients from sedimented organic matter. These nutrients fueled a “eutrophication pump” that may have augmented an already rising nutrient inventory resulting from the evolution of vascular land plants and concomitant increases in the flux of land-derived nutrients. Comparison of data sets among these three intervals of organic enrichment, widely separated in time, clearly illustrates the linked roles of sedimentation, nutrient supply, primary production, and microbial metabolism, with change in relative sea level acting as a master variable influencing each set of processes. The Deposition of Organic-Carbon-Rich Sediments: Models, Mechanisms, and Consequences SEPM Special Publication No. 82, Copyright © 2004 SEPM (Society for Sedimentary Geology), ISBN 1-56576-110-3, p. 35–59. INTRODUCTION The general correlation between black-shale deposition and transgressive stratigraphic sequences has long been observed (see reviews by Arthur and Sageman, 1994; Wignall, 1994). Various mechanistic hypotheses have been proposed to account for this association, including: (1) insufficient ventilation due to deepening combined with water column stratification, (Demaison and Moore, 1980); (2) transgressive condensation (Hallam and Bradshaw, 1979; Loutit et al., 1988; Middleburg et al., 1991); and (3) an expanding “puddle” of anoxic water due to interplay of thickening water column and decreased accumulation rate of bulk sediment (Wignall, 1991, 1994; Wignall and Maynard, 1993). However, it is likely that the forcing is much more complex than any single factor. For example, sedimentary condensation alone probably does not produce a widespread black shale unit, but in concert with changes in circulation, ventilation and/or higher primary production, condensation produces a unit more enriched in organic carbon (OC) than otherwise. The purpose of this study is to reexamine the relationship between black-shale deposition and marine transgressions on the basis of recent developments in the study of organic-matter burial, and to elucidate the combinations of forcings that accompany marine transgressions to produce widespread black shales. In doing so, we focus on three examples from our own studies of modern and ancient OC-rich strata—the Holocene Black Sea, MICHAEL A. ARTHUR AND BRADLEY B. SAGEMAN 36 Cenomanian–Turonian strata of the Western Interior Seaway of North America, and Upper Devonian black shales of the Appalachian Basin. In our view, these regionally extensive “black shales” exhibit a strong imprint of the effects of rising sea level on OC deposition and preservation and allow us to illustrate the complexities of these effects. In the examples to follow, enhanced burial of Type II marine (amorphous, high hydrogen index) organic matter (OM) is controlled by a number of factors, either individually or in combination. Most important is OM supply and quality resulting from relatively high primary production. Under favorable export and burial conditions, high surface-water primary productivity, due primarily to enhanced nutrient delivery, results in OM supply that exceeds the capacity of the system for organic-matter decomposition. In the case of enhanced OM preservation, favorable export conditions, such as a shallow water column (e.g., Müller and Suess, 1979) and reduced O2 exposure time (Hartnett et al., 1998) are critical. High primary productivity results in a greater OC flux to the sediment–water interface (SWI), where other factors come into play that can increase the efficiency of burial and preservation of the OC. The latter results in a shift to less efficient microbial metabolism and lower levels of OM degradation during early burial, i.e., a dominance of anaerobic metabolism (e.g., Canfield, 1989). A favorable sediment accumulation regime modulates both O2 exposure time and sulfide generation rate; sulfide plays a key role in limiting heterotrophic sediment aerators. (Bioturbation also increases residence time of organic matter in the shallow oxygenated zone of sediments.) However, sulfate reduction in anaerobic water columns does represent a significant source of organic-matter degradation in some basins; for example, in the Cariaco Basin water-column sulfate reduction is estimated to consume about two-thirds of the export flux of organic C (Thunell et al., 2000). We will demonstrate, using several different examples from the geologic record, that rising sea level in epeiric seas exerts a master control on OM burial (Fig. 1). This control is exerted primarily through the effects of deepening of the water column and landward retreat of the shoreface, which results in trapping of sediment inshore. In many marginal basins and epicontinental seas, deepening decreases the frequency and magnitude of bottom-water ventilation, with consequent development of oxygen deficiency, thereby enhancing OC preservation. This relationship between transgression and anoxia is illustrated by many Holocene examples, particularly where fresh-water fluxes to the basin are high, including the Baltic Sea (e.g., Sohlenius et al., 2001), the Cariaco Basin (e.g., Richards, 1975; Peterson et al., 1991; Piper and Dean, 2002), and the Black Sea (Degens and Stoffers, 1976; Arthur and Dean, 1998). In addition, deeper connections to adjacent oceanic basins can allow nutrient-rich, O2-depleted water masses of the oxygen-minimum zone (OMZ) to be advected into a basin, especially if there is a quasi-estuarine circulation pattern; this may enhance primary productivity while adding oxygendeficient water masses to basinal deep waters (Fig. 1). The effects of deepening on water-mass stratification occur because most epeiric seas were probably too shallow to maintain permanently stratified water columns (e.g., Parrish, 1982; Heckel, 1991). In flooded but shallow epicontinental seas stratification was probably dominated by development of seasonal thermoclines, which were mixed to a greater or lesser degree by winter storms, depending on long-term variance in storm intensity (e.g., Tyson and Pearson, 1991). This was especially the case at maximum flooding stages when only the very largest storms would mix to the bottom. Such an oceanographic regime is optimal for the remineralization of biolimiting nutrients from decomposing OM, nutrient buildup in bottom waters, and intermittent recirculation to surface waters, thus providing a further mechanism of enhancing primary production levels. Another sea-level effect on OC concentrations involves condensation. When deepening is accompanied by shoreface transgression (i.e., rate of sea-level rise exceeds rate of siliciclastic supply, and/or complements subsidence of a foredeep), finegrained sediments may be sequestered in drowned river valleys (estuaries), causing a shift to lower average bulk accumulation rates in the distal facies tracts. In this way, relative siliciclastic condensation leads to increased concentration of biogenic sediments and OC concentrations are generally higher if oxidation is not too extensive. In this environment, the development of black shales probably reflects enhanced OC preservation because of a combination of faster relative burial of OM into the SO4 = reduction zone, H2S buildup, decreased burrowing, and less dilution by carbonate during development of mudrock facies (calcareous shales, marly shales, marlstones). Reduced delivery of reactive iron, which characterizes maximum flooding stages, when terrigenous input to the deeper basin is at a minimum, may aid in this process by lowering H2S buffering capability (Meyers et al., 2004). Deposition of organic-carbon-poor limestone facies occurs when OM burial is slow, H2S is rare, burrowers are dominant, and OM remineralization by aerobes is very effective. Either black shales or limestones can be deposited during transgressive intervals, but limestones form when clastic-sediment supply and nutrient input from rivers to a basin is relatively low and overall oligotrophic conditions prevail. Thus, rising sea level plays a very important role in the genesis of black shales. We will examine the intricate interplay of the effects of transgressive systems that produce black shales in epicontinental and marginal seas. Although we do not provide detailed sequence stratigraphic frameworks for each of the examples, we do discuss “black shales” related to both basal transgressive episodes and and maximum flooding episodes, as distinguished, for example, by Wignall and Maynard (1993). THE HOLOCENE BLACK SEA Background and Depositional Setting The Black Sea is a tectonically isolated basin with only a shallow connection with the saline waters of the Aegean and Mediterranean seas and a maximum depth of nearly 2250 meters. A permanent halocline at a depth of about 80 m in the center of the basin coincides with the redoxcline or “chemocline”, which separates the fresher, oxic surface waters from more saline, anoxic and sulfidic deep waters. This redox interface deepens as much as 210 m along the basin margins (e.g., Murray et al., 1989). The hydrochemical structure of the Black Sea depends critically on freshwater inputs from rivers, atmospheric forcing, topography, and seawater inputs through the Bosporus (e.g., Özsoy and Ünlüata, 1997). During the last glacial maximum, seawater inputs from the Mediterranean were cut off, and the Black Sea was apparently a fully enclosed freshwater lake (Degens and Ross, 1972); as sea level rose, seawater entered the Black Sea system, either as a trickle (Degens and Ross, 1972), a catastrophic event (Ryan et al., 1997), or something between these two scenarios (Arthur and Dean, 1998). At some point after the initial influx of seawater to the basin, water-column anoxia developed. Ryan et al. (1997) have suggested that development of anoxia accompanied the abrupt spillover event, which, in their hypothesis, occurred at about 8 ka. Arthur and Dean (1998) adopt about the same age for the development of anoxia on the basis of radiocarbon data from Jones and Gagnon (1994) but suggest that the anoxia required at least 2 kyr to develop after the initiation of 37 SEA-LEVEL CONTROL ON SOURCE-ROCK DEVELOPMENT: PERSPECTIVES FROM THREE SEPARATE AREAS spillover of saline waters from the Mediterranean Sea. From 8 ka to the present, the Black Sea has been typified by a large volume of deep, anoxic and sulfidic water with resulting fine-grained sediments that are thinly laminated and relatively rich in organic carbon. Despite long historical usage of the Black Sea as a type example of an anoxic basin and analog for black shale-forming basins of the past (e.g., Degens and Stoffers, 1976), the details of the temporal and chemical evolution of the deep anoxic water remain somewhat controversial. For example, some workers favor high biologic productivity coupled with an oxygenated water column (e.g., Calvert, 1990; Pedersen and Calvert, 1990) while others argue for long-term water-column anoxia, perhaps with high productivity, as the primary control on deposition of laminated, organic-carbon-rich sediments in the Black Sea (e.g., Degens and Ross, 1972; Arthur and Dean, 1998). FIG. 1.—A) Flow chart illustrating important factors involved in the development of organic-carbon rich facies in an epicontinental setting during transgressions. B) Diagrammatic cross section of an epicontinental basin bounded by an orogen, showing spatial distribution of factors involved in the production and preservation of organic carbon in sediments during sea-level-rise events. Large arrows represent water-column mixing; small arrows reflect basinward sediment transport (“focusing”). Note: large vertical exaggeration and diagram not to scale. A