Assessing the impact of black shale processes on REE and the U–Pb isotope system in the southern Appalachian Basin

Abstract

Upper Devonian Java and Huron shales from the southern Appalachian Basin exhibit evidence of REE redistribution and resetting of the U–Pb isotopic system at the time of deposition (ca. 365 Ma). This alteration to the primary detrital signature of these shales is indicative of black shale diagenetic/depositional processes that in effect obscure paleoenvironmental and provenance information that may be recorded. The most direct evidence of these black shale processes is the rotated nature of the whole-rock REE patterns from this sequence of presumably source related shales. The neo-formation of REE-bearing diagenetic phases such as apatite and possibly carbonate has led to an unusually large range in the whole-rock 147Sm/ 144Nd and the Ce/Ce* for this sequence of shales. Changes to these whole-rock ratios will effectively alter the Nd model age and create a Ce/Ce* that records evidence of diagenetic processes rather than information about the paleooceanographic conditions of the basin. Simultaneously, the sequestration of U at the time of deposition and subsequent fixation in diagenetic phases such as apatite has effectively reset the 238U– 206Pb isotopic system at 340±50 Ma overprinting the Pb isotopic signature of the source area. The trace-element and isotopic signatures recorded by the Java and Huron formation shales from the study area reveal a syn-/postdepositional history dominated by what can be referred to as black shale processes. The diagenetic conditions that develop at and below the sediment water interface during the deposition of organic-rich shale are unique and consequently leave behind a unique geochemical fingerprint that can potentially overprint paleoenvironmental and provenance information.