DID ENHANCED LITHIFICATION PLAY A ROLE IN THE PROLIFERATION OF EARLY TRIASSIC STROMATOLITES? A CASE STUDY FROM THE VIRGIN LIMESTONE MEMBER, MOENKOPI FORMATION, WESTERN NEVADA

Stromatolite morphogenesis can be described as the interplay between upward microbial mat growth, mat lithification, and sediment rain. Thus, stromatolite formation occurs in a “goldilocks” zone where the interplay between growth, lithification, and sedimentation is “just right”. Without the lithification term, stromatolites will not likely be preserved in the rock record. Recent advances have been made in the study of modern marine stromatolite lithification, but the role and details of stromatolite lithification remain elusive for ancient stromatolites. The microstructure of most Precambrian stromatolites is quite different from most modern marine stromatolites, such that modern lithification models may not directly apply to the ancient forms. The issue resides in our ability (or inability) to predict carbonate saturation and precipitation dynamics in ancient systems where direct observation and measurements cannot be made and where modern “analogues” may be not be appropriate.

The Lower Triassic (Spathian) Virgin Limestone Member of the Moenkopi Formation contains stromatolites that have been considered “disaster forms” by other workers due to their position in the aftermath of the Permo-Triassic extinction concomitant with (hypothesized) suppressed metazoan activity. An investigation of the basin architecture reveals that cm-scale aragonite seafloor fans grew in the slope facies (the Union Wash Formation) at the same time the stromatolites formed in the shallower carbonate ramp facies (Virgin Limestone Member). The (putative) abiotic seafloor fans provide suggestive evidence for enhanced carbonate saturation at the time of stromatolite growth. In this example, we can tentatively correlate enhanced carbonate saturation, and therefore lithification potential, to a period of stromatolite proliferation. Other factors (metazoan suppression, etc.) may or may not have played a role. The cause of the anomalous carbonate alkalinity is unclear and may be related to continued unusual conditions during Early Triassic time in the aftermath of the end-Permian extinctions.