Productivity, preservation, and cyclic sedimentation within the Mowry Shale depositional sequence, Lower Cretaceous, western interior seaway

[摘要] The late Albian Mowry Shale of Wyoming and Montana, and the time equivalent Big River Formation of Saskatchewan, were studied to determine the cause of anoxia within the Mowry Seaway and the presence or absence of small scale (fourth order) cyclicity. The Mowry Shale is an important source rock, characterized by high total organic carbon content (TOC) (0.5-5.0%), abundant fish debris, numerous bentonites, and prolific radiolarians. Minimal bioturbation at distal sites from Wyoming to Canada indicates that the preservational quality of bottom water with respect to organic matter was high throughout the basinal area of the seaway. TOC values were highest at the southern end however, coincident with a high in radiolarian abundance. This correlation suggests an added input of planktonic organic carbon at the southern end of the seaway, due to higher levels of paleoproductivity. Therefore, anoxia within the Mowry Seaway may have resulted from two processes, stagnation in Canada, and elevated planktonic fertility in Wyoming and Montana. Seasonal upwelling has been predicted along the SE margin of the Albian seaway from independent climate models. Burial rates of Si and P in the Mowry Shale indicate paleoproductivity levels comparable to lower values reported from modern coastal upwelling zones. This fertility could have been supported entirely by nutrients derived from a (depleted) seawater source, even with limited renewal rates. Six locations in N. Wyoming were tied into an onshore - offshore resistivity well log transect. TOC, phosphorous content, and lithologic profiles from each section could then be correlated using numbered bentonite markers. Vertical sedimentation trends were divisible into three intervals. Within the context of sequence stratigraphy, these units may be interpreted as the Transgressive Systems Tract (decreasing sand, increasing TOC), the Condensed Interval (minimum sand, peak TOC), and the Highstand Systems Tract (increasing sand, decreasing TOC). Higher order cyclicity is present in the Mowry Shale, as 1-10m coarsening-up cycles near the western margin, and 5-30m fertility cycles in the basin. Correlation of marginal to basinal cycles indicate that during shallowing-up, decreases in TOC on the shelf coincide with minor increases in TOC in the basin.

[发布机构] Rice University