Foraminiferal Paleoecology and Paleoceanography of the Greenhorn Cycle Along the Southwestern Margin of the Western Interior Sea
Oona L.O. West, R. Mark Leckie, Maxine Schmidt, 1998. "Foraminiferal Paleoecology and Paleoceanography of the Greenhorn Cycle Along the Southwestern Margin of the Western Interior Sea", Stratigraphy and Paleoenvironments of the Cretaceous Western Interior Seaway, USA, Walter E. Dean, Michael A. Arthur
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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.
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This volume presents the results of a coordinated, multidisciplinary study of Cretaceous carbonate and clastic rocks in cores collected along a transect across the old Cretaceous seaway that extended from the Gulf Coast to the Arctic by a team of academic, industry and U.S. Geological Survey scientists. The overall goal was to construct a subsurface transect of mid-Cretaceous strata that were deposited in the U.S. Western Interior Seaway. In particular, the papers in this volume focus on the Graneros Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation and equivalents in cores from six drillholes from western Kansas, southeastern Colorado and eastern Utah.