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Abstract

Upper Cambrian microbial reefs established themselves on an extensive shallow shelf portion of Laurentia, also referred to as the Great American Carbonate Bank. Four dominant regressions and three transgressions synchronously occurred across Laurentia, based on measured sections, regional correlation, and biostratigraphy. The youngest transgression in Central Texas, corresponding to the Point Peak Member of the Wilberns Formation, is evident in several outcrops within a 2500 Km2 area that has been the focus of study by the Rice/Trinity Industry Microbial Research Consortium.

The Point Peak Member is divided into lower and upper portions by a regional time marker bed—the Plectrotrophia zone (Plectrotrophia bridgei and species of Billingsella). Microbial accumulations of the Lower Point Peak crop out along the Llano River and Mill Creek and consist of a series of 50cm-thick biostromes and some individual buildups one meter or less in height, intercalated with heterolithic facies, glauconitic siltstones, and oolitic, skeletal, and interclastic carbonate grainstone. These interrelated facies are interpreted to represent shallow subtidal to intertidal depositional environments. Farther offshore, equivalent thicker microbial buildups (up to 30 m thick) have been recorded in the literature, indicating the wide extent of subtidal microbial facies across the up to 50 km wide shelf.

Above the Plectrotrophia zone, spectacular outcrops of Upper Point Peak reveal large microbial reefs (10-14 m high and tens of meters in diameter). These reefs are exposed in 2D and 3D, along the James and Llano rivers, and Mill Creek, providing unique opportunities to quantify their distribution and heterogeneity and to better place them into a sequence stratigraphy framework. Meter-thick skeletal and oolitic grainstone inter-reef beds, contemporaneous to the buildup growth evolution, are intercalated with a series of siliciclasticrich silty beds onlapping the different buildup growth phases. These large reefs are equivalent in depositional setting to the offshore large buildups below the Plectrotrophia zone.

The microbial buildups both below and above the Plectrotrophia zone are interpreted as a response to sea level rises, whereas siliciclastic-rich beds, in particular the thick bed onlapping the final phase of buildup growth, are most likely a result of sea level falls. The belt of thicker buildups in the Upper Point Peak is located farther landward relative to that of the Lower Point Peak, indicating that these “higher-frequency” sea-level changes were occurring within an overall transgression.

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