GROWTH DYNAMICS OF STROMATOLITE REEFS IN THE PROTEROZOIC ATAR GROUP, MAURITANIA

Laterally extensive stromatolite reefs in the Proterozoic Atar Group, Mauritania show a complex internal structure consisting of a variety of stromatolite morphologies (conical, branching conical, and branching columnar forms) and several distinct matrix components (detrital micrite, intraclastic breccia, and early marine cement). In all cases, initial reef nucleation occurred with the growth of vertically-walled, coniform stromatolites during marine transgression over a gently dipping marine platform. Shoaling of reefs is represented in offshore environments by a transition to peritidal carbonate facies and, in nearshore environments, by the appearance of brecciated stromatolite pavements. Detailed examination of individual components within the reef, however, reveal a much more complex sea level history.

A complex relationship between reef growth and sea level change produces a cyclic pattern of superposition among reef elements. At the beginning of each cycle, seafloor cement coated all preexisting surfaces. This cementation event was followed by upward growth of coniform stromatolites. High synoptic relief (typically 50-70 cm) and an absence of inter-stromatolite debris suggest initial reef growth at or beneath storm wavebase. Upon sea-level fall, increased wave activity disrupted initial stromatolite growth and eroded stromatolite walls, producing platy intraclasts. Stromatolite growth in wave-dominated environments is characterized by branching forms with low synoptic relief (<2 cm) and considerable inter-stromatolitic debris. In higher-energy environments, inter-stromatolitic sediment is dominated by intraclastic breccia, whereas micritic carbonate dominates low-energy, nearshore environments. Each new cycle is marked by a coating of marine cement across the substrate and nucleation of new conical stromatolites above detrital surfaces and exposed dormant cones or branching stromatolites.

Recognition of parasequence-scale cyclicity in the Atar Group reefs sheds new light on the growth dynamics of Proterozoic stromatolite reefs in which both the character of individual parasequences and parasequence stacking patterns are critically dependent on the position of wave base relative to the evolving substrate.