Highest Phanerozoic strontium isotopic ratios of pre-rift Late Cambrian passive margin in New York State, USA: products of continental weathering and orogenesis

Abstract

The Late Cambrian Sauk Sequence, a passive margin in New York State, is composed predominantly of carbonate rocks that were deposited in discrete provinces: on the shelf, the Beekmantown Group includes the Galway Formation, and the Wappinger Group includes the Pine Plains Formation. On the paleoslope, the Taconic Sequence is a deep-water deposit and includes the Hatch Hill Formation. The shallow-water carbonates of the Galway and Pine Plains Formation are dolostones, whereas those of the Hatch Hill Formation are deep-water limestones. Strontium isotopic values of the carbonates are much higher than others determined for the Late Cambrian and are the highest values for the Phanerozoic and probably for all geologic times. They show the dominance of continental sources of strontium. Continental and groundwater runoff supplied radiogenic strontium to the Late Cambrian ocean. An apparent increase in seawater 87 Sr/ 86 Sr values may record rapid unroofing and attendant increased erosion as well as collapse of mountains that had been constructed during the continental collisions that assembled Gondwana during the Pan-African and Braziliano mountain-building episodes. As a result of extensive chemical weathering, erosion and high uplift rates, seawater 87 Sr/ 86 Sr recorded peak values. Diagenetic alteration may affect radiogenic strontium. In this study, petrography, mineralogical analysis, and isotopic analysis were used to interpret diagenesis. These data provided evidence that diagenesis was not involved.