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 δ13Ccarb 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.

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