Facies and sequence stratigraphy of two Cambrian grand cycles: implications for Cambrian sea level and origin of grand cycles

摘要

The Pika, Arctomys and Waterfowl formations comprise two "grand cycles" as originally defined by Aitken (1966, 1978, 1981). A detailed across-strike fades stratigraphic reconstruction based on measured outcrop sections and subsurface data reveal that the Pika and Waterfowl Formations consist of five transgressive―regressive sequences of subtidal facies and tidal flat facies each up to 100 m thick and some 10~5 to 10~6 years duration. The bulk of the Arctomys Formation consists of terrestrial to marginal marine playa deposits, correlates with an unconformity in the adjacent subsurface, and was deposited during a sea level low stand. The upper third of the Arctomys Formation records a single minor marine transgressive―regressive cycle. Transgressive facies can extend eastward onto the craton up to 1000 km from the platform to slope transition and record local relative increases in sea level of various magnitudes. Maximum transgressive or regressive phases do not necessarily correspond with the bottoms or tops of grand cycles. A sea level curve derived from our facies stratigraphy is different than the curve proposed by Bond et al. (1989) from R2 analysis, and also different to the sealevel curve of Montanez and Osleger (1993) for the southern Great Basin.