Sequence and Chemostratigraphy of the Middle Cambrian Succession in Nevada and Utah

摘要

The House Range Embayment of western Utah and eastern Nevada was a prominent topographic feature on the passive margin of western North America during the middle and late Cambrian. In this study, detailed documentation of sequence boundaries, and their intervening sequences across a platform-to-basin transect of the House Range Embayment was used to establish a sequence-stratigraphic framework from which the depositional history of the embayment was deciphered. This framework was then used to test the hypothesis that the House Range Embayment formed by tectonic subsidence. In addition, the chemostratigraphic (δ13Ccarb) record across the embayment was analyzed within this framework to (1) document the relationship between the FAD of P. atavus and the negative δ13C Drumian Carbon Isotope Excursion (DICE), (2) clarify the relationship between sea-level change and the DICE, (3) refine the definition of the DICE event in a sequence stratigraphic framework, (4) test the hypothesis that the embayment was a restricted anoxic basin, and (5) explore the possibility of using carbon isotope excursions for regional/global stratigraphic correlation.Five sequence boundaries were identified in the platform and correlated with condensed intervals in the basin within the known biostratigraphic framework. These sequence boundaries document the initial tectonic formation of the embayment and its subsequent infilling during tectonic quiescence. Five carbon isotope profiles indicate that the DICE is located stratigraphically above the FAD of Ptychagnostus atavus in the embayment, associated with transgressive deposits on the southern platform, and with stratigraphic condensation in the basin. Two longer carbon isotope profiles do not reveal significant isotopic variability that can be attributed solely to local carbon cycling in an anoxic basin but they do reveal the effect of diagenetic alteration of platform values. The results also indicate that chemostratigraphic correlation in poorly dated successions deposited from significantly different paleoenvironments should be approached with caution.