CAP CARBONATE PLATFORM FACIES MODEL, NEOPROTEROZOIC NOONDAY FORMATION, SE CALIFORNIA

The Neoproterozoic outcrop belt of the Death Valley region, CA, preserves an oblique cross-section of the Noonday Formation, a mixed carbonate–siliciclastic platform that hosts distinctive basal Ediacaran (Marinoan) cap carbonate–affiliated sedimentary structures, stromatolite textures, and δ¹³C_carb values. We address the relative contributions of syndepositional tectonism and recovery from extreme deglaciation in dictating Noonday platform architecture and an intra-Noonday sequence boundary. Noonday facies record the transition from a carbonate ramp to a stromatolitic reef-rimmed shelf. Subsequent base-level fall exposed the reef crest to karstic dissolution and cut incised valleys into the seaward margin of the reef; overlying strata record the backfilling of incised valleys and re-establishment of a carbonate-dominated backstepping ramp. The Noonday encompasses two depositional sequences that reveal two cycles of relative sea-level change within strata conventionally considered to record a single, rapid glacioeustatic sea-level rise.

Noonday deposition coincided with active extension of the Laurentian margin during Rodinian supercontinent disaggregation. Within this framework, previous work suggests the intra-Noonday sequence boundary records growth faulting that reinforced differential topography, uplifting reef-rimmed horsts—exposing the reef crest to karstic dissolution—and down-dropping footwall half-grabens. However, we trace the intra-Noonday sequence boundary seaward of the reef crest and demonstrate that, for a time, wave-base was situated downdip of the reef escarpment. Thus, if the Noonday margin were undergoing extension, then the intra-Noonday sequence boundary records a decrease in accommodation due, perhaps, to post-glacial isostatic uplift at a rate more rapid than tectonism. We speculate that the Noonday sequence architecture records immediate deglacial flooding, isostatic rebound induced by a hiatus in melt-water flux or rapid ice sheet collapse against a background of global deglaciation, and resumed flooding following complete deglaciation. Rift-related tectonism could amplify or counter isostasy, thus local estimates of the amplitude of post-glacial sea-level change require robust estimates of syndepositional tectonism on the margin.