Lateral variability of facies and cycles in the Furongian (late Cambrian) carbonate platform: an example from the Big Horse Member of the Orr Formation in western Utah, U.S.A.

摘要

Carbonate depositional cycles and sequences have been proposed to be formed by glacioeustatic sea-level changes. This mechanism would be questionable during times of high atmospheric CO2 and negligible continental ice sheets such as the supergreenhouse time in the Furongian (Late Cambrian), during which limited glacioeustatic sea-level changes would be expected. A detailed sedimentological study of the Furongian Orr Formation in western Utah is aimed at testing the hypothesis that, under supergreenhouse climate conditions, most meter-scale carbonate cycles may have been formed through autocyclic processes and thus they should be laterally variable. The research was conducted in a small area ( 1.2 km2) where excellent exposure permits lateral tracing of key surfaces and facies.;The Big Horse Member of the Orr Formation in the study area mainly consists of shale, siltstone, cryptic microbialites, thrombolites, wackestone, and cross-laminated oolitic grainstone/packstone that were deposited from deep subtidal to supratidal environments. Meter-scale cycles are expressed by shallowing-upward trends with subtidal shale/siltstone at the base and supratidal microbialites with desiccation cracks, dissolution cavities, and karstic breccias at the top. Among the seven closely-spaced sections with traceable marker beds, the cycle numbers vary and the thickness of individual cycles change from 2 m to 36 m. Individual cycles are found to change within a few hundreds of meters to non-cyclic interval or, in some cases, several cycles merge into a single cycle within 200--1100-meter distance. However, two stratigraphic discontinuities marked by intensive subaerial exposure were traceable among sections. These features suggest that meter-scale cycles of the Big Horse Member were mainly formed by autocyclic process through interactions among tidal island aggradation, local carbonate production rates, and tectonic/thermal subsidence. Forced regression during times of high carbonate production formed the laterally persistent discontinuities, but their duration may have varied, with significant lag time recorded in some sections.