Rapid subsidence as a driver of phosphate deposition in the later Ordovician: Case study from the Alabama Appalachians

Abstract

An increase in accumulation of phosphate-enriched sediment occurred in several areas of Earth's oceans during the later Ordovician, suggesting that some fundamental change (s) in seawater and/or pore water chemistry were taking place, either syndepositionally or during early diagenesis. One hypothesis is that widespread cooling of the water column led to this increase, but another is that the increase could have been forced primarily by changes in seawater chemistry that accompanied an influx of siliciclastic sediments associated with tectonically driven subsidence. Here, we report on findings from study of the upper ~30 m of the >200 m of Middle and Late Ordovician strata exposed near Tidwell Hollow, Blount County, Alabama, and what the results suggest about the competing hypotheses. This Ordovician sequence, deposited along the southeastern margin of Laurentia during the initial (Blountian) stage of the Taconic Orogeny, records the transition from a restricted peritidal carbonate shelf environment (the “Black River lithofacies”) into a more normal marine carbonate environment (the “Trenton lithofacies”). In particular, the younger carbonate strata are notable for their measurably higher amounts of secondary phosphate minerals. This stratigraphic interval is also well-constrained chronostratigraphically by the presence of the Deicke and Millbrig K-bentonite beds (altered volcanic tephra layers), thus it is ideal for a focused geochemical, petrographic, and stratigraphic investigation of the increased phosphate content of a specific Upper Ordovician sequence. Analyses of bulk rock samples and of extracted collophane grains (via x-ray fluorescence, XRF, and in-situ laser ablation inductively coupled plasma mass spectrometer, ICPMS) suggest that phosphogenesis in these strata was episodic rather than slow and steady, with a depositional pattern of abrupt increases followed by abrupt declines. Observed trace element changes in the 12 m to 18 m sample interval include the occurrence of elevated Th/U ratios (maximum of 5.7) accompanied by Y/Ho ratios that range from 27 (base) to 51 (top). We attribute these changes to increasing silicate influx into the basin and an accompanying increase in available Fe, which would preferentially scavenge MREEs and then release them to the pore waters leading to MREE enrichment, which we also observe in this interval. In a rapidly subsiding basin, MREE enrichment could have resulted from initial restriction of bottom water circulation followed by gradually more open marine conditions. Some allochems and textures that are more characteristic of the older Black River lithofacies than the younger Trenton lithofacies (e.g. framework grains of calcareous green algae and Type 1 and Type 2 oncoids, fenestral lime mudstones, and associated small framestones of Tetradium sp. buildups) persist in relative abundance upsection into the ~10 m interval of strata above the Millbrig K-bentonite Bed, and this persistence supports the hypothesis that the Trenton transgression was not initially accompanied by widespread cooling of the Laurentian epicontinental sea. Instead, we propose that variable weathering rates, turbidity, and nutrient supply accompanied by relatively rapid subsidence could have governed the observed variability in phosphate deposition and elemental abundances in these sediments, and that this subsidence of the Laurentian margin could have been the driver of an uptick in phosphate deposition during the later Ordovician here, and perhaps at other locations as well where subsidence was occurring, given that increased phosphate deposition is a globally recognized and globally significant change in many Upper Ordovician sequences beyond eastern Laurentia.