MICROSTROMATOLITE: SOLE COMPONENT OF STROMATOLITE, DENDROLITE AND THROMBOLITE FABRICS IN MICROBIAL CONES AND RIMMED COLUMNS, LATE CAMBRIAN OF TEXAS

Shallow marine biostromes in the ~490 Ma upper Cambrian Point Peak Member of the Wilberns Formation, Texas, are dominated by stromatolitic inverted cones, up to 30 cm high and 10 cm wide. These locally develop into steep-sided rimmed columns, up to 60 cm high and wide. Each column consists of a laminated stromatolite rim and a grainy dendrolite interior. The dendritic fabric in the column interior has a clotted (thrombolite) appearance in plan view. The well-preserved fabrics show that the stromatolite and dendrolite fabrics in these cones and columns are both formed solely by microstromatolite. The individual microstromatolites are layered, fine-grained, millimetric columns with parallel or upward-widening sides, and width to height ratios ranging 3:1 to 1:3. Microstromatolites have been recognized in normal marine carbonates in the late Cambrian of Nevada and the middle Cambrian to early Ordovician of North China. Microstromatolites resemble Proterozoic microdigitate stromatolites in size, but are often relatively small, less well laminated and more irregular in form. They typically occur as short columns with disjunct branches, and consist of peloidal to microclotted irregularly laminated fabrics. Their microfabrics are consistent with a biogenic origin. Recognition of the ability of microstromatolite to create a variety of fabric types in close proximity is unexpected. We infer that rapid microstromatolite accretion formed slightly elevated column rims, and that as a result the column interior accumulated current- and wave-borne allochthonous sediment by a ‘bucket effect’. It is likely that examples of rimmed columns with clotted interiors, reported elsewhere in Cambrian–Early Ordovician shallow carbonate seas, could have a similar – potentially time-limited – origin.