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Controls on the Formation of a Phosphatic Interval in the Chattanooga Shale (Upper Devonian) of Tennessee – A Combined Sedimentologic and Geochemical Study

Abstract

The Devonian Chattanooga Shale 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 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. The upper phosphatic black shales (PhBS) are characterized by phosbioclasts, have a generally banded to homogenized texture with intermittent reworked layers, and show well defined horizons of phosphate nodules that are reworked and transported. In PhBS, up to 8000 particles 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 contains between 8–35 phosphatic particles per cm2. The shale matrix in PhBS contains 5.6% Inertrinite, whereas just 1% Inertrinite is found in PBS. The shale matrix in both units displaying flat REEs pattern, whereas phosbioclasts in PhBS is featured by high concentrations of REEs, with particular MREEs enriched. All the samples show negative Ce anomalies, with maximum negative Ce anomalies in Phosbioclasts. Redox-indicative elements (Co, U, Mo) are more strongly in PBS than those in PhBS. Trace elements (Cu, Zn, Cd, Ni) associated with organic matter show reverse trend of enrichments. Deposited atop a sequence boundary, the PhBS unit represents a transgressive systems tract and was probably deposited in shallower water than the underlying PBS interval. The higher phosphate content in PhBS appears to be a combination of lower sedimentation rates, coupled with reworking/winnowing episodes. Strong negative Ce anomalies, high degree of homogenization and secondary marcasite formation in the PhBS suggest more effective meiofaunal sediments reprocessing, improved aeration of the water column, and intermittent downwards migration of the redox boundary. A lowering of pore water pH caused by pyrite oxidation forced dissolution of detrital biogenic phosphate, which was followed by formation of marcasite, and reprecipitation of phosphate with reincorporation of dissolved MREEs. 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) in discrete horizons.