LAMINA STRUCTURE IN THE UNDERSTANDING OF STROMATOLITE MORPHOLOGY

Lamination has long been considered a fundamental property of stromatolites, yet the analysis of stromatolites typically focuses on description of macro-scale morphology (macrostructure) and the diversity and distribution of morphologies through geologic time, with a secondary emphasis on micro-scale elements (microstructure) such as the composition and behavior of microbial constituents or the relative contribution of chemically precipitated versus detrital elements. Here we argue that lamina geometry (mesostructure) is an underutilized link between these different scales and is a critical element to deciphering stromatolite genesis.

Stromatolite mesostructure provides a critical link between the basic description of morphology and the fundamental interpretation of how sediment-microbe interactions manifest these morphologies. This linkage is based on the understanding that a lamina represents the physical manifestation of the interface between the water column and the sedimentary substrate, and that each lamina marks a discrete time horizon during stromatolite growth. The interfacial aspect of stromatolite lamination is defined by the shape and synoptic relief of a given lamina and provides a record of both microbial growth and its interaction with the environment at the time of lamina formation. By contrast, the temporal dimension of stromatolite lamination is defined by inheritance, or the degree to which successive laminae inherit pre-existing morphology, which provides a record of the stability of the sediment-microbe interface over time.

Here we analyze both synoptic relief and lamina inheritance to reconstruct and interpret the growth of both simple and complex stromatolite morphologies. We provide evidence that complex macrostructure occurs in stromatolites with low synoptic relief and low inheritance. We argue that low inheritance reflects environmental disruption that fundamentally affects the communication between and continuity of successive microbial laminae. We therefore suggest that the canonical pattern of high stromatolite diversity in the Mesoproterozoic records specific environmental conditions (e.g. epeiric sedimentation and spatially variable DIC and redox conditions) during this time.