“BREAD LOAF” MOUNDS: MICROBIAL REEFS ON A MIXED CARBONATE SILICICLASTIC INBOARD SHELF, UPPER CAMBRIAN WILBERNS FORMATION (MASON COUNTY, CENTRAL TEXAS)

Large 10-14 m-high microbial reefs, well known in the upper Point Peak Member, are uniquely exposed in 3D along the Llano/James Rivers and Mill Creek (Mason County, TX) and are interpreted to have grown on an outboard shelf setting. In contrast, <0.90 cm in height, recently discovered microbial mounds in the Lower Point Peak, referred to as “Bread Loafs” and interpreted as growing on an inboard shelf setting, crop out on a pavement along the Llano River. The main objective of this study is to explain the growth phases of these microbial mounds, from their initial establishment, to their growth, and their final demise.

The “Bread Loafs” mounds established themselves on top of a bed made of reworked coarse sediments such as ooids, intraclastic pebbles, and skeletal fragments, organized in a series of poorly defined fining upwards layers, with carbonate content values increasing towards its top. High energy conditions, with individual storm events reworking coarse grains from the outboard shelf and overall decreased turbidity, became ideal conditions for microbial mats to be generated and preserved.

Up to 80 cm high, round and flat top mounds were organized by increasing heights towards one direction and aligned in sub-parallel curved bands. The mounds were apparently influenced by topography with the flat top mounds growing in shallower water depth than the round ones. Generally, both types of mounds display oblong shapes, with their long axis orientated in a similar direction, though within 30 degrees of the band orientation; their shape was possibly influenced by a preferential wave direction.

The heart of the mounds consists of precipitated microbial micrite associated with trapping and binding of large skeletal bioclasts, mostly trilobites fragments, often oriented tangentially to their growth direction. High energy had to occur to transport those from the outboard to the inboard shelf. The mound outskirt consists of mostly precipitated microbial micrite lacking trapping and binding processes. Their final growth is characterized by a defined crust referred to as a rind, made of clotted micrite microbialites. The carbonate content values of the mounds are high, oscillating between 88-92%. The mounds were buried in unlithified low carbonate siltstone, Increased flux of siliciclastics most likely triggered their demise.