Abstract Changing sea level is a major factor in the pattern of enrichment of organic carbon in marginal and epicontinental seas. Organic-carbon-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 mid-Holocene 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 salinity to 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 water-column 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.

We have analyzed the concentration and sulfur isotope composition of trace sulfate in carbonate from three Proterozoic formations in Death Valley, California. Trace sulfate concentrations for the Crystal Spring Formation and Beck Spring Dolomite, which were deposited in the late Mesoproterozoic and mid-Neoproterozoic and are not associated with glacial sediments, range from 0 to 144 ppm with δ 34 S sulfate values spanning 11.0‰–27.4‰. Within these formations, stratigraphic shifts in δ 34 S sulfate of up to ∼9‰ occur over <50 m. Trace sulfate concentrations for the Noonday Dolomite, which was deposited in the late Neoproterozoic and directly overlies glacial sediments associated with the “snowball Earth” events, range from 2 to 272 ppm with δ 34 S sulfate values varying between 15‰ and 35‰. The ∼17‰ δ 34 S sulfate increase at the base of the Noonday Dolomite is similar in magnitude and rate to the >20‰ positive δ 34 S shifts recorded in Neoproterozoic postglacial carbonates from Namibia. The results indicate that the sulfur cycle behaved differently in the late versus early Neoproterozoic as a possible consequence of severe late Neoproterozoic glacial events. Furthermore, based on δ 34 S sulfate patterns and carbonate-associated sulfate concentrations recorded in the Crystal Spring and Beck Spring formations, we speculate that late Mesoproterozoic to mid-Neoproterozoic oceanic sulfate concentrations were ∼10% of modern values (e.g., ∼3 mM).

Abstract The Cretaceous Western Interior Seaway Drilling Project was begun in 1991 under the auspices of the U.S. Continental Scientific Drilling Program. It was intended to be a multidisciplinary study of Cretaceous carbonate and siliciclastic rocks in cores from bore holes along a transect across the Cretaceous Western Interior Seaway. The study focuses on middle Cretaceous (Cenomanian to Campanian) strata that include, in ascending order, Graneros Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation. The transect includes cores from western Kansas, eastern Colorado, and eastern Utah. The rocks grade from pelagic carbonates containing organic-carbon-rich source rocks at the eastern end of the transect to nearshore coal-bearing units at the western end. These cores provide unweathered samples and the continuous depositional record required for geochemical, mineralogical, and biostratigraphic studies. The project combines biostratigraphic, paleoecological, geochemical, mineralogical, and high-resolution geophysical logging studies conducted by scientists from the U.S. Geological Survey, Amoco Production Company, and six universities.

Abstract The Upper Albian-Coniacian section cored in the Amoco No. 1 Rebecca K. Bounds well in Greeley County western Kansas, serves as a reference section for the timing of depositional events in the Western Interior Seaway. Chronostratigraphy of this section was calibrated by a multidisciplinary study of nannofossils, dinoflagellates, spores, pollen, foraminifers, and mollusks. Range data of the biota in the Bounds core were compared by graphic correlation to a global composite standard that includes key reference sections in Europe and North Africa. The basal Upper Albian sequence boundary is overlain by transgressive facies of the Purgatoire Formation dated as 102.8 Ma. The upper Upper Albian sequence boundary between the Purgatoire and Dakota Formations marks a hiatus in deposition from 99.4 to 98.2 Ma. The Albian-Cenomanian intra-Dakota sequence boundary spans from 96.0 to 94.1 Ma. The Turonian-Coniacian sequence boundary between the Carlile and Niobrara Formations spans from 89.9 to 88.3 Ma. Maximum flooding is documented within the Purgatoire at 101.4 Ma and in the Graneros Shale at 93.7-92.8 Ma. The Albian-Cenomanian boundary defined by European ammonites and correlated by dinoflagellates is placed at the intra-Dakota unconformity. Graphic correlation is an independent method of measuring the durations of Milankovitch-scale depositional cycles and can separate climatic cycles from longer tectono-eustatic cycles. Four orders of depositional cycles are recorded by lithological changes, and their durations are constrained by graphic correlation. The longest cycles range from 2.0 to 3.4 My and are found in the sequences defined by the Purgatoire Formation, the lower part of the Dakota Formation, the upper Dakota and Graneros Formations, and the Greenhorn and Carlile Formations. The next lower order comprises transgressive-regressive subcycles of about 0.5 My long in the Purgatoire. The third-scale cycles include sandstone-mudrock cycles in the Dakota, limestone-marl cycles in the lower part of the Greenhorn, and cyclical strata in the Fort Hays Limestone Member of the Niobrara Formation that are about 100 ka long. The shortest cycles are limestone-marl couplets in the upper Greenhorn that are about 41 ka long.

Abstract Calcareous nannofossil assemblages were investigated in two cyclic stratigraphic intervals from the Late Cretaceous Western Interior Seaway to determine the causes of lithologic variations. Relative abundance data were collected from the Bridge Creek Limestone Member (Cenomanian-Turonian) of the Greenhorn Formation and the transition between the Fort Hays Limestone and Smoky Hill Chalk Members (Coniacian-Santonian) of the Niobrara Formation from two cores, the No. 1 Amoco Rebecca Bounds Core, western Kansas, and the USGS No. 1 Portland Core, central Colorado. Stable-carbon and -oxygen isotopic analyses of fine (<38pm) fractions were carried out on samples from the interval with the best preserved nannofossils, the Fort Hays Limestone-Smoky Hill Chalk transition in the Bounds core. Preservation of nannofossil assemblages varies in the two cores. Correlations between CaCO 3 and the abundance of nannofossil species, which are used as proxies for organic productivity, are rarely significant, which indicates an inconsistent relationship between fertility and lithology. Fine-fraction, oxygen-isotopic values from the Fort Hays Limestone-Smoky Hill Chalk transition of the Bounds core correlate highly with nannofossil fertility markers, indicating a close relationship between organic productivity and the amount of run-off into the basin. However, the lack of consistent correlation between organic productivity (as seen in nannofossil assemblages) and lithology suggests that lithology was influenced by a variety of complex processes including variations in carbonate productivity and dilution with clastic material. Carbonate productivity was likely influenced by multiple water masses, including run-off from mountainous regions to the west, warm waters from the Tethys, and cooler waters from the Arctic. The interplay of the water masses and the additional dilution signal render the lithologic cycles out of phase with the periodicities that control organic productivity.

Abstract The Cenomanian-Santonian calcareous nannofossil biostratigraphy of the Rebecca Bounds, Portland, and Escalante cores was investigated. Zonal markers were used to correlate the Cenomanian-Turonian boundary interval in the cores with outcrop sections in the Western Interior basin. However, it is difficult to apply existing zonations in the remainder of the section. We determined several new biohorizons that are useful for drawing correlations between the cores. These biohorizons are combined with some of the standard zonal markers in defining informal zonal units. Environmental factors appear to have reduced nannofossil diversity along the western margin of the basin and led to the premature extinction of several nannofossil markers in the Cenomanian-Turonian boundary interval.

Abstract Foraminifera in shales and mudrocks of the Greenhorn Cycle (late Cenomanian-middle Turonian age) in the Cretaceous Western Interior Basin were strongly influenced by sea level change. This long-term record of third-order sea level rise and fall is superposed by fourth-order relative sea level cycles as delimited by carbonate and sedimentological data. The study interval includes the Cenomanian-Turonian boundary and the early Turonian record of the highest stand of sea level in the western interior. We document stratigraphic variations in foraminiferal assemblages and their response to changing sea level for one drill core through the Tropic Shale (Escalante, Utah) and two outcrop sections of the Mancos Shale (Lohali Point, Arizona; Mesa Verde, Colorado) from the Colorado Plateau. The three sections record deposition along the southwestern margin of the Greenhorn Sea and provide a temporal and spatial framework for interpretations of paleoecology and paleoceanography. Earlier studies demonstrate that fluctuations in planktic foraminifera and calcareous and agglutinated benthic foraminifera track the transgression and regression of the Greenhorn Cycle. Results of assemblage analyses presented here show that benthic taxon dominance also correlates to fourth-order sea level changes, and to the type of systems tract. Assemblages of calcareous benthic foraminifera are dominated by two species, Gavelinella dakotensis and Neobulimina albertensis. Neobulimina, an infaunal taxon, dominated during the late transgression and highstand of the Greenhorn Sea (early Turonian) when warm, normal salinity, oxygen-poor Tethyan waters advanced northwards into the seaway. In contrast, the epifaunal/shallow infaunal taxon Gavelinella proliferated briefly during times of water mass renewal and when deposition of organic matter increased at the transition between fourth-order cycles. Peaks in abundance of other calcareous benthic species delimit transgressive pulses prior to the spread of oxygen-poor Tethyan water masses. These broad-based correlations may result from an intricate relationship among changing water masses, flux of terrestrial and marine organic matter, sedimentation rates, and benthic oxygenation. Regression of the Greenhorn Sea resulted in a greater restriction of oceanic circulation and in the withdrawal of Tethyan waters that were replaced by cooler, lower salinity water masses of Boreal affinity. An abrupt change to dominance by agglutinated benthic foraminifera and loss of nearly all planktic foraminifera marks this paleoceanographic event. Enhanced biological productivity accompanied regression in south-central Utah. Depauperate benthic foraminiferal assemblages reflect the stress of low-oxygen conditions despite an abundance of food. Enhanced salinity stratification during later stages of regression may have reduced ventilation on the seafloor and led to dysoxic bottom waters. Sea level change helped produce distinctive assemblages of benthic foraminifera that can be used to delimit successive systems tracts. Foraminiferal assemblages also provide insight into their evolutionary responses to rapidly changing paleoenvironments. Our results indicate no evolution in the foraminiferal biota of the study sections, which we think points to evolutionary stasis.