--> Uppermost Pleistocene Coralgal Reefs and Upper Cambrian Microbial Bioherms: Morphologies and Sea Level-Induced Evolution

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Uppermost Pleistocene Coralgal Reefs and Upper Cambrian Microbial Bioherms: Morphologies and Sea Level-Induced Evolution

Abstract

Many of the world's oil and gas discoveries are in carbonate reef reservoirs, and understanding the morphological evolution of reefs from select key examples, when forcing mechanisms such as sea level fluctuations are well established, provides insight into ancient reef growth. This study focuses on a comparison between Uppermost Pleistocene drowned coralgal reefs along the south Texas shelf edge and Upper Cambrian microbial bioherms in Mason County (Central Texas). Similarities and differences between the two geological settings make an interesting and insightful comparison. Major similarities include, low-latitude, shelf edge, mixed systems; differences reside in the nature of the reef builders from coralgal to microbial. State of the art technologies were applied to quantitatively establish detailed morphologies of these reefs. High-resolution multibeam surveys (<1m/pixel) of the south Texas shelf reefs clearly established true reef morphologies such as spurs and grooves along their margins, clear atoll development, and a series of distinct terrace levels. These findings, when combined with 3.5 kHz seismic profiles, clearly demonstrate that these distinct reef morphologies can be explained through shrinking and backstepping of the reefal production areas, responding to a well established step wise sea level rise during last deglaciation. 3D digital outcrops (1 cm/pixel) were built using drone-based photogrammetry for the Upper Cambrian microbial bioherms. Based on qualitative and quantitative morphological analyses, it was determined that their overall evolution occurred in three distinct growth phases. During the latest deglacial transgression, a switch from shallow coastal siliciclastics to neritic carbonates is observed in the south Texas shelf edge reefs. A similar switch is also observed at the base of the Upper Cambrian microbial bioherms where shallow subtidal to intertidal mixed carbonate-siliclastic depostional environments suddenly evolved into a pure carbonate system by the initial growth of microbial bioherms on top of thin sheets of flat rip-up clasts. These observations could be used to hypothesize that a transgression during the Upper Cambrian led to the establishment of microbial reefs providing enough accommodation to grow vertically in three distinct phases triggered by step-wise sea level rise. Once drowned, both systems ended up being buried by siliciclastics that most likely did not contributed to their demise.