Organic Carbon Burial and Phosphogenesis in the Antler Foreland Basin: An Out-of-Phase Relationship During the Lower Mississippian

Abstract

ABSTRACT Stratigraphic analysis of Lower Mississippian successions at four localities in Utah and Wyoming establish the relative timing of a large carbon isotope excursion ( +7‰) and episode of regional phosphogenesis in the Antler foreland basin and adjacent shelf (Delle Phosphatic Member). The 13C excursion, which appears global in scope, is marked by peak values of +7.3‰ in western Wyoming, which are among the heaviest known from Phanerozoic successions. The isotope excursion is well dated to the late Kinderhookian S. isosticha and early Osagean lower G. typicus zones and precedes the Delle phosphorites of upper G. typicus Zone age. The Delle phosphatic event, which resulted in the demise of the shallow-water carbonate factory over a wide area, is marked by a heterogeneous succession of pelletal phosphatic crusts, phosphatic pisolites, and winnowed and reworked grain aggregates. The observed stratigraphic relationship between the positive 13C excursion and Delle Phosphatic Member is consistent with a scenario in which high organic-carbon burial and seawater phosphate enrichment under anoxic conditions took place in deeper water masses of the Antler foreland system. This period of high 13C in surface waters above the pycnocline was followed by an episode of persistent upwelling into a salinity-stratified water column and phosphogenesis that was triggered by regional uplift within the Antler foreland basin system in response to tectonic loading. Despite the high productivity associated with upwelling and the accumulation of organic matter on the seafloor, 13C values remained low during phosphogenesis. This is attributed to the counterbalancing effects of organic-matter oxidation during lowstands of sea level, which ventilated the water mass and resulted in sediment winnowing and condensation of phosphatic particles and the incorporation of light carbon from the upwelling water mass. A low in 13C coincident with phosphorite deposition suggests that upwelling, carbon oxidation efficiency, and phosphogenesis reached maxima at approximately the same time, a situation that may be analogous to several Precambrian-Cambrian intervals and the Middle Ordovician but is opposite to that observed for the Early Cretaceous.