ON THE ORIGIN OF PHOSPHATE NODULES IN THE CHATTANOOGA SHALE (UPPER DEVONIAN) OF TENNESSEE – A COMBINED SEDIMENTOLOGIC AND PETROGRAPHIC STUDY

Although the formation of phosphate nodules generally implies remobilization of phosphate from biogenic apatite and decaying organic matter, diagenetic phosphate formation takes many forms and likely has multiple pathways. The Devonian Chattanooga Shale is part of a widespread black shale interval, and contains an uppermost interval where phosphate nodules are dispersed in a black shale matrix. This interval overlies another black shale unit that lacks phosphate nodules but otherwise looks very similar in outcrop. Our objective for this study was to see what, in addition to phosphate, sets these two shales apart and what it tells about sedimentary history.

In thin section, the lower black shales (PBS) show pyrite enriched laminae and compositional banding, and are distinct from the upper phosphatic black shales (PhBS). The latter are characterized by phosbioclasts, have a generally banded to homogenized texture with intermittent reworked layers, and show well defined horizons of phosphate nodules (1-15 cm size). Nodules show evidence of reworking and transport (rotation, imbrication). In PhBS, up to 8000 particles (silt to sand size) of P-debris per cm² occur in reworked beds, whereas the background black shale shows between 40-90 particles per cm². In PBS, the shale matrix looks similar to the background shale in PhBS, but contains only between 8-35 phosphatic particles per cm².

The presence of a regional erosion surface between the two shale units places the PhBS unit at the base of a transgressive systems tract, probably deposited in somewhat shallower water. Thus the higher phosphate content in PhBS appears to be a combination of smaller overall sedimentation rates, coupled with reworking/winnowing episodes that concentrated phosphate even further. A higher degree of homogenization and secondary marcasite formation in the PhBS suggests more effective meiofaunal sediments reprocessing and intermittent downwards migration of the redox boundary. The associated oxidation of pyrite forced a drop of pH and dissolution of phosphate, and was followed by formation of marcasite and reprecipitation of phosphate. A generally “deeper” redox boundary favored P-remineralization within the sediment matrix, and multiple repeats of this process led to the formation of larger phosphatic aggregates (nodules).