Early Diagenesis of New Albany Shale (Upper Devonian) Flooding Surfaces—an Intertwined Record of Fe-Sulfide, Fe-Carbonate, and Silica Deposition in Dysoxic Surface Sediments

Juergen Schieber
Indiana University, Bloomington, IN

Deposited under conditions of extreme sediment starvation, flooding surface deposits in the Upper Devonian New Albany Shale of Indiana and adjacent areas may consist to almost 100% of authigenic minerals, such as pyrite, marcasite, quartz, ankerite, siderite, dolomite, and calcite. Due to the highly carbonaceous nature of the New Albany Shale, pore waters of surface sediments were reducing, fostering formation of framboidal pyrite as the first mineral precipitate. Pyrite is followed by chalcedony and chert precipitation in pore spaces (due to dissolution of radiolaria) that extends through the remainder of early diagenetic history and may dominate the rock fabric.

Adjacent pore spaces with strongly differing mineral associations (e.g. chalcedony and coarse euhedral pyrite vs. successions of siderite to manganiferous ankerite) point to closely spaced but distinct chemical microenvironments. Patches/micro-concretions of dolomite and calcite are prominent in beds with lower levels of silicification, possibly deposited when conditions of sediment starvation were less extreme. Marcasite, in the form of coarse or radial-fibrous aggregates and overgrowths on pyrite can make repeated appearances in the course of early diagenetic history.

The following themes emerge from high resolution textural studies of these deposits: (1) biofilms that coated decaying organic matter played a key role in mineral localization; (2) framboid sizes and manganiferous carbonates suggest that the redox boundary was located within the sediment (dysoxic rather than anoxic bottom waters); and (3) marcasite formation is suggestive of multiple episodes of downward oxidation (“burndown”). The latter may reflect intervals of comparatively small surface productivity.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005