Tectonically controlled autocyclicity in the Furongian (late Cambrian) carbonate platform, central Nevada and western Utah, USA.

摘要

A cyclo- and sequence stratigraphic study of the Furongian carbonate platform in the western United States was undertaken to investigate the temporal and spatial variations of carbonate cycles and their stacking patterns that developed under a supergreenhouse climate. The well-exposed succession (ca. 502 to 496 Ma) which crops out in central Nevada and western Utah consists of 16 carbonate and siliciclastic facies, which are grouped into seven facies associations, including (1) deep subtidal, (2) shallow subtidal, (3) sand shoal, (4) subtidal shelf, (5) lagoonal, (6) peritidal complex, and (7) subaerial exposure. The architecture of these facies and facies associations show local- and regional-scale progradation, aggradation and retrogradation in response to the interplay between post-rift thermal subsidence, local faulting and changes in carbonate productivity. Several types of meter- and decameter-scale cycles can be recognized, including peritidal, shallow subtidal and deep subtidal cycles. Tracing the cycles and cycle boundaries in continuous outcrops reveals significant lateral variability of cycles. Cycle boundaries disappear within tens to hundreds of meters and component facies within individual cycles pinch out or interfinger with other component facies. Within a cycle set (i.e cycle stacking patterns bounded by key surfaces), cycle numbers and thickness vary locally and regionally, suggesting dominant autogenic formation of cycles and stacking patterns under the warm climate during the late Cambrian. Sequence boundaries also show spatial and temporal complexities that are best interpreted by forced regression and are not correlatable with those from other continents. These observations are inconsistent with the existing interpretations in which carbonate cycles and sequences of the Furongian carbonate platform in western Laurentia were formed by glacio-eustatic sea-level fluctuations under Milankovitch astronomical forcing. Instead, the temporal and spatial variations of cycles and cycle stacking patterns favor autogenic cyclicity controlled by differential tectonic subsidence and carbonate productivity across the Furongian carbonate platform.