High-resolution event stratigraphy, carbon geochemistry, and paleobiology of the Upper Cenomanian Hartland Shale Member (Cretaceous), Greenhorn Formation, Western Interior, U.S

The Hartland Shale Member is an interval of well-laminated, organic-rich shale deposited in the Western Interior Basin during a sea level rise event in the Late Cenomanian (Greenhorn Cyclothem). It has been traditionally viewed as a regional "anoxic event" in the Western Interior sea (i.e., stagnant basin). Because the interval is well-exposed at outcrop localities throughout the Western Interior, it provided the opportunity to test concepts and models for: (a) depositional processes of fine-grained, organic-rich facies developed during sea level rise (stratigraphy/ sedimentology); (b) the nature and distribution of organic carbon and carbonate in epicontinental black shales (geochemistry); and (c) the adaptation of organisms and benthic communities to low-oxygen environments (paleobiology). Integrating these three types of analysis allowed detailed paleoenvironmental reconstruction of the Hartland Shale Member.;The methods of High-Resolution Event Stratigraphy were used to investigate the study interval at 7 sections representing a wide range of Western Interior depositional environments. Ten other measured sections were used to correlate between the principal sites. The main conclusions of the study are: (a) Development of genetic surfaces and sequences (including Milankovitch cycles, transgressive pulses cycles, and 3rd-order sequences) in the Hartland Shale predominantly resulted from variations in sediment supply to the basin. These variations were controlled by changes in base level related to eustatic fluctuations and/or changes in climate; (b) the quality and quantity of preserved organic matter was mainly controlled by burrowing activity, which varied in response to bottom water oxygen, substrate firmness, and the presence or absence of benthic boundaries; (c) associations of low-oxygen tolerant taxa comprise distinct biofacies in the Hartland Shale. Three biofacies types, including predominantly infaunal, predominantly epifaunal and mixed infaunal/epifaunal, and 6 biofacies levels, reflecting gradations in paleoenvironment from anoxic/soupy substrate to fully oxygenated/firm substrate, were recognized; (d) integrated sedimentologic, geochemical and paleobiologic data indicate that the Hartland Shale was not a stagnant basin, but in fact was highly dynamic. Although a positive water balance maintained low oxygen levels in the bottom water, frequent storm events mixed the water column enough to prevent long-term anoxia.