Determing Detrital Sources and Depositional Dynamics of a Late Pennsylvanian Black Shale: Geochemistry of the Stark Shale, Mid-Continent, USA

Many Middle and Late Pennsylvanian high-frequency sequences of the northern mid-continent contain well-developed black shale facies in their maximum transgressive condensed intervals. These high TOC and trace metal enriched shales preserve a detailed record of fluctuating redox conditions (from dysoxic to euxinic) driven by a combination of upwelling and the development of a pycnocline produced by estuarine circulation. High precipitation in the Appalachians during these interglacial highstands, producing the freshwater plume driving the pycnocline, is a key component of depositional models for these shales. This study examined the lower Missourian Stark Shale in four cores from the northern mid-continent to document potential paleogeographic variations in the geochemistry of a typical black shale and to assess how these variations might impact depositional models of these shales.

Two of the cores are from the Forest City Basin (FCB) in northwest Missouri and two are from the Nemaha Uplift (NU) in northern Kansas, approximately 185km west. The two basinal cores are characterized by relatively high TOC (>25% max values), good correlation between TOC and trace metal variation, relatively low Si/Al ratios, and relatively high (CIA-CaO) values. The uplift cores are characterized by comparatively low TOC (<25%), poor correlation between TOC and trace metal variation, relatively high Si/Al ratios, and relatively low (CIA-CaO) values.

We interpret these patterns as follows. The FCB is dominated by subaqueous deposition of clay- and organic detritus–rich sediment from a mature source under protracted anoxic/euxinic conditions, which is a clear signature of the equatorial estuarine circulation system. However, the NU experienced a weaker, fluctuating redox system with a subtle but significant contribution of non-clay detritus from a less mature source area. We interpret this contribution as eolian dust derived from paleo-northeastern cratonic sources.