Cap carbonate platform facies model, Noonday Formation, SE California

Abstract

The Neoproterozoic outcrop belt of the Death Valley region, California, preserves an oblique cross section of the Noonday Formation, a mixed carbonate-siliciclastic platform that hosts distinctive basal Ediacaran cap carbonate−affiliated sedimentary structures, stromatolite textures, and δ^(13)C_(carb) values. The Noonday platform encompasses two depositional sequences that reveal two cycles of relative sea-level change within strata conventionally considered to record a single, rapid, postglacial sea-level rise. In updip localities, facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite-bearing, “tubestone”-textured, reef-rimmed platform; downdip, localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge. Base-level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef. Overlying strata record the backfilling of a submarine incised valley and reestablishment of a back-stepping, carbonate-dominated ramp prior to a second subaerial exposure event that defines the contact between the Noonday and Johnnie formations. We address the relative contributions of syndepositional tectonism and recovery from low-latitude deglaciation in dictating Noonday platform architecture and the intra−Noonday Formation sequence boundary. Noonday Formation deposition coincided with extension of the Laurentian margin during disaggregation of the Rodinian supercontinent. Within this framework, previous work has suggested that the intra−Noonday Formation sequence boundary records growth faulting that reinforced differential topography, uplifting reef-rimmed horsts—exposing the reef crest to karstic dissolution—and downdropping grabens. However, we trace the intra−Noonday Formation sequence boundary seaward of the reef crest and demonstrate that, for a time, wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks. Thus, if the Noonday margin were undergoing extension, then the creation of the intra−Noonday Formation sequence boundary required a concomitant decrease in accommodation due, perhaps, to postglacial isostatic uplift attendant with low-latitude deglaciation. We speculate that Noonday Formation sequence architecture records (1) immediate deglacial flooding, (2) shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice-sheet collapse against a background of global deglaciation, and (3) resumed flooding following complete deglaciation. As rift-related tectonism could amplify or counter glacial isostasy, inferences of the amplitude of local postglacial sea-level change will require robust estimates of syndepositional extension across the Noonday margin.