Geochemistry and occurrence of authigenetic phosphate nodules from the Desmoinesian cyclic Excello epeiric sea of the Midcontinent, USA

Abstract

Abstract Pennsylvanian cyclothemic black shales of the Midcontinent, which were deposited in glacial-related epeiric seas containing high marine algal productivity due to the occurrence of swamps prior to maximum transgression, coastal upwelling and terrigenous runoff, are very rich in phosphate nodules. The high nutrient content of the epeiric sea supported phytoplankton populations that created favourable environments for the accumulation of phosphorites in anoxic bottom-waters. This close association of organic matter and phosphate-controlled nodule morphology (spherical, elongated, bladed, platy) appear to be related to progressive decreases in the nutrient supply of the sea water from the ancient Pacific Ocean (Panthalassa) in the south to the palaeoshorelines in the north. The major and trace element contents of ten phosphorite nodules from representative exposures along the 400-mile long outcrop belt from the northern Quachita to central Missouri were determined. Major elements of the phosphorites are P 2 O 5 = 22.33–34.19, SiO 2 = 3.56–25.15, Fe 2 O 3 = 0.73–2.94, CaO = 30.77–43.22, Na 2 O = 0.30–4.51, K 2 O = 0.35–2.27, Al 2 O 3 = 4.31–8.02 and CO 2 = 1.2–2.0 wt%. The distribution of Mn, Zn, Cd and Cu shows a sharp increase at locations 40 and 50, where terrigenous runoff waters were dominant. The distribution of Ni, U, Co, Li, Cr, V and Mo does not show any significant change across the outcrop belt. Programmed pyrolysis indicates that the nodules are thermally immature with respect to hydrocarbon generation and kerogen types belong to Type II and Type III. Pyrolysis results are T max = 418–432° C , Ol = 17–168, Hl = 11–481, Pl = 0.04–0.4 and TOC = 0.65–7.07 wt%. Direct precipitation in micropores (1–4 μm in size) is shown by petrographic and SEM studies. However, the occurrence of nodules is explained by a calcite replacement model. The ubiquitous association of organic matter and phosphorus is vitally important for growing of phytoplankton, which extract phosphorus from sea water. When phytoplankton die, organic phosphorus oxidizes in the micropores to inorganic phosphorus species such as HPO 4 2− , which replaces calcite to form apatite as a function of alkalinity, pH, Eh and available ions. Concentric layers in nodules are due to progressive growth reaction with interstitial water. The model proposed for the occurrence of phosphate nodules in the Excello epeiric sea with bottom-water anoxia is similar to that for the highest productive regions of the world, such as Walvis Bay and off the coast of Peru and Chile.