No abstract

The Devonian Chattanooga Shale contains an uppermost black shale interval with 7 dispersed phosphate nodules. This interval extends from Tennessee to correlative strata in 8 Kentucky, Indiana, and Ohio and represents a significant period of marine phosphate fixation 9 during the Late Devonian of North America. It overlies black shales that lack phosphate nodules 10 but otherwise look very similar in outcrop. The purpose of this study is to examine what sets 11 these two shales apart and what this difference tells us about the sedimentary history of the 12 uppermost Chattanooga Shale. 13In thin section, the lower black shales (PBS) show pyrite enriched laminae and compositional 14 banding. The overlying phosphatic black shales (PhBS) are characterized by phosbioclasts, have 15 a general banded to homogenized texture with reworked layers, and show well defined horizons 16 of phosphate nodules that are reworked and transported. In the PhBS, up to 8000 particles of P-17 debris per cm 2 occur in reworked beds, whereas the background black shale shows between 37-18 88 particles per cm 2 . In the PBS, the shale matrix contains between 8-16 phosphatic particles per 19 cm 2 . The shale matrix in the PhBS contains 5.6% inertinite, whereas just 1% inertinite occurs in 20 formation of secondary marcasite and phosphate. Repeated, episodic reworking caused repetitive 36 cycles of phosphatic dissolution and reprecipitation, enriching MREEs in reprecipitated apatite. 37 A generally "deeper" seated redox boundary favored P-remineralization within the sediment 38 matrix, and multiple repeats of this process in combination with wave and current reworking at 39 the seabed led to the formation of larger phosphatic aggregates and concentration of phosphate 40 nodules in discrete horizons. 41