MODERN CONTINENTAL MICROBIAL MAT STRUCTURES: ANALOGS FOR PRECAMBRIAN LANDSCAPES

The continental fossil record of microbial mats in Precambrian strata is sparse and may be related to the development of braided systems and hence the absence of preserved overbank deposits. Documentation of modern continental microbial mat systems aids in the recognition of the possible spectrum of microbial features that could be present in the rock record. Two types of deposits are documented - biological soil crusts (BSC) and small ephemeral pond mat features.

The Colorado Plateau BSC’s are composed of a poorly sorted mixture of silts and sands. These BSC’s display distinctive sedimentary features associated with stages of development that can be easily recognized in ancient strata. These include: 1) growth, smooth, pustule, pinnacled, and curved pinnacles, 2) metabolism, mineral precipitation and gas features, 3) destruction, mat erosion and shrinkage, and 4) decay and diagenesis, enhancement of sorting by cementation. Typically only the growth and destruction phase have been documented from the rock record.

Small, modern ephemeral mud ponds in eastern Pennsylvania display a suite of microbial mat related features that develop in hours to days, that include: shrinkage cracks, gas-generation related features related to metabolism, crinkled mats, folds, flip overs, and roll-up features. Subaqueous shrinkage cracks can develop. Roll-up features are caused by the microbial autotrophic gas generation that causes the mat to separate from the sediment surface then float. As the water level drops, the elastic microbial mat splits and recoils causing folds that parallel the pond margin, flip overs, and in some instances, mat roll up features. These ephemeral pond features are similar to marginal-marine MISS structures and caution should be used in interpreting the depositional setting based solely on the microbial mat features.