ASSESSING THE IMPACT OF BLACK SHALE PROCESSES ON REE AND U-PB PROVENANCE SIGNATURES FROM THE SOUTHERN APPALACHIAN BASIN

Black shales are susceptible to syn- and post-depositional changes in whole rock geochemistry that may obscure provenance information retained by the shale prior to deposition. Resultantly, it is critical to characterize the nature and extent of these potential disturbances prior to interpreting the provenance signature of a black shale. It has been demonstrated that the primary lanthanide and actinide provenance signatures of a black shale are particularly vulnerable to alteration. Here we present evidence of alteration of the REE and U-Pb provenance signature from a sequence of black shales deposited in the southern Appalachian basin during the Acadian orogeny. These Upper Devonian distal shales were deposited during the latest Frasnian and earliest Famennian stages near Big Stone Gap in southwestern Virginia.

REE patterns for some of these shales have been altered and are relatively enriched in the MREE and HREE. Shale normalized patterns and mass balance considerations suggest the addition of as much as 5% diagenetic apatite to the most enriched shales. This neo-formation of apatite is common during early diagenetic conditions associated with the deposition of organic-rich shale and requires the redistribution of REE. The same reducing conditions responsible for the precipitation of diagenetic apatite will enhance the preservation of U, hence enriching the shale relative to upper crustal values for U (2.8 ppm). The same shales that are enriched in the MREE and HREE are also enriched in U by up to a factor of 20 over upper crustal and average shale values (PAAS=3.1 ppm). The timing of this enrichment (ca. 365 Ma) is apparently recorded by the Pb isotopic system of the shales. Owing to the U enrichment, the Pb isotopic systematics of these shales have been disturbed and no longer faithfully record the Pb isotopic signature of the source area.

While the complications associated with the alteration of primary REE and U-Pb provenance signatures are significant in black shales, a careful characterization of the nature and extent of the alteration will allow for a more complete interpretation of basin history. In fact, given adequate constraints, the primary provenance signature may be recovered given a relatively simple post-depositional history.