Tectonically-induced strontium isotope changes in ancient restricted seas : the case of the Ediacaran-Cambrian Bambuí foreland basin system, east Brazil.

Abstract

The Bambuí Group is a marine sedimentary record of an intracratonic foreland basin developed at the terminal

Ediacaran and early Cambrian during the assembly of West Gondwana. Here we present a basin-scale high- resolution Sr isotope stratigraphy for the basal Bambuí Group, aiming to understand the spatial and temporal var- iations of the 87Sr/86Sr ratios and to explore the controls over the Sr isotope system in intracontinental marine

environments. Assessment of the stratigraphic evolution of both Sr concentrations and Sr isotopes shows a major increase in Sr/Ca ratios (up to 0.004) and a decrease in the 87Sr/86Sr ratios from 0.7086 to 0.7076 in the high stand system tract of the basal 2nd-order sequence. These changes precede a large positive δ13C excursion

typically found across the basin in the middle Bambuí Group. The high variability of both 87Sr/86Sr and Sr/Ca ra- tios was not caused by globally uniform changes in isotopic compositions of seawater, but rather likely reflect

marine restriction and paleogeographic changes of the depositional environments at basin scale. This would re- sult from the tectonic uplift of Neoproterozoic orogenic belts around the São Francisco craton, which generated

an isolated foreland marine basin. Compared to the global ocean, such a smaller intracontinental reservoir would be more sensitive to the Sr isotope composition from the different rock sources. We suggest that changes on the

balance between carbonate production and accommodation associated with tectonically-related flexural subsi- dence progressively modified the continental drainage patterns, sedimentary sources and the chemical

weathering regimes, altering the strontium influxes and isotopic compositions of the seawater in the early Bambuí basin cycle. Similar anomalies in the strontium isotope record are also recorded in coeval marine basins across West Gondwana and suggest that tectonics might have played an important role on seawater chemistry at the Neoproterozoic-Paleozoic transition.