The Anna Shale is an organic-rich, marine black shale that commonly overlies the Herrin (No. 6) Coal of the Carbondale Formation, Illinois Basin. Disk-shaped iron sulfide concretions commonly up to 10 cm or more across, called pyrite suns, are found in mines in the Sparta, IL region. This area is the only known location where pyrite suns of this size have been found, suggesting that unusual geochemical and/or depositional conditions led to their formation. The primary objective of this study was to evaluate the geochemical conditions at the time of Anna Shale deposition in the area where the pyrite suns formed. Four cores of the interval were selected from Washington Co., IL, along with several in-mine samples of the Anna roof shale and pyrite suns collected from the Prairie Eagle underground coal mine near Pinckneyville, IL. Samples were analyzed using a Bruker hand-held XRF-unit (for major, minor, and trace elements) and by XRD (for mineralogy). Total carbon and total sulfur were also determined. Rock-Eval pyrolysis was used to determine the quantity and quality of organic matter (OM), and organic petrology was used to evaluate OM sources to better define characteristics of the depositional setting.
Trace element redox proxies (V/Cr, Ni/Co, V/(V+Ni)) indicate paleo-redox conditions fluctuated between dysoxic and anoxic throughout deposition of the Anna Shale. C-S-Fe plots indicate conditions were at least dysoxic throughout deposition. Total organic carbon values range from 37.0% in strongly anoxic intervals to 2.2% in dysoxic intervals, with an average of 15.9%. The Anna Shale is composed of Type II to Type III kerogen with hydrogen indices (HI) ranging from 54 to 438 mg HC/g TOC (averaging 225 mg HC/g TOC), is immature (with Tmax <430°C), and has excellent oil potential (typical S2 values ~10-150 mg HC/g).
A better understanding of the conditions under which the Anna Shale was deposited provides insight into the formation of pyrite suns while also providing new information to supplement interpretations of the depositional environment of black shales in Illinois Basin cyclothems.