Environmental controls on photosynthetic microbial mat morphogenesis on a 3.42 Ga clastic-starved platform

Three morphotypes of microbial mats are preserved in rocks deposited in shallow-water facies of the 3.42 Ga Buck Reef Chert (BRC). Morphotype α consists of fine anastomosing and bifurcating carbonaceous laminations which loosely drape underlying detrital grains or form silica-filled lenses. Morphotype β consists of meshes of fine carbonaceous strands intergrown with detrital grains and dark laminations which loosely drape coarse detrital grains. Morphotype γ consists of fine, even carbonaceous laminations that tightly drape underlying detrital grains.

All BRC mats are preserved in the shallowest-water interval of those rocks deposited below normal wave base and above storm wave base. This interval is bounded below by a transgressive lag developed during regional flooding and above by a small condensed section marking a local relative sea level maximum. Morphotypes α and β dominate the lower, coarse-grained half of this interval, while morphotype γ dominates the fine-grained upper half, suggesting that either current energy or light intensity acted as a first-order control on mat morphotype distribution.

Morphotypes α and β grew continuously up through detrital layers less than ~1-mm-thick, possibly trapping and binding the finer fraction of detrital sediment otherwise bypassing sites of growth. Thicker layers were draped by new mats. The thickness of these mats was therefore controlled largely by the intensity of individual detrital sedimentation events. In contrast, while thick stacks of γ-type laminations developed in finer-grained intervals, this mat morphotype never grew down into or up through detrital layers.

The restriction of all mat morphotypes to the shallowest interval of the storm-active layer in the BRC ocean reinforces previous interpretations that these mats were constructed primarily by photosynthetic organisms. The strong environmental controls on distributions of individual morphotypes suggest an interplay between detrital sedimentation and current activity/light intensity in modulating biological processes to produce distinctive mat morphologies.