PRIMORDIAL METAZOAN-CALCIMICROBIAL REEFS - TOMMOTIAN (EARLY CAMBRIAN)OF THE SIBERIAN PLATFORM

With the possible exception of some older aggregations of tubicolous s keletal forms, bioherms in. the Pestrotsvet Formation (Tommotian) of t he southeastern Siberian Platform represent Earth's earliest metazoan- mediated reefs. They are certainly the earliest known bioconstructions of metazoan-calcimicrobial type. The bioherms are geographically limi ted to the transitional facies tract of the Siberian Platform, separat ing restricted lagoonal from deep ramp and slope sediments. Bioherms a re intergrowths of archaeocyaths, calcimicrobes (Renalcis) and rare co ralomorphs (Cysticyathus) in lime mudstone which was cemented early on the sea poor. They consist of meter-scale mounds occurring either sin gly or stacked together. Mounds cart be categorized into component dom ains occupied by associations of lime mud, archaeocyaths, calcimicrobe s and cement. Some mounds are simply a conglomeration of such domains, with no discernible pattern of internal distribution, while others po ssibly show incipient ecological succession. The calcimicrobe Renalcis was the major bioconstructor; it and the synsedimentary cement were t he principal contributors to framework construction. Archaeocyaths of themselves rarely produced a framework, except where stereoplasm was a bundant. Nevertheless, the archaeocyaths provided substrates for the R enalcis and cement, and facilitated the development of internal caviti es. Archaeocyaths appeared, as bioconstructors, in the basal Tommotian sunnaginicus Zone. In the succeeding regularis Zone and thereafter, t hey flourished throughout the mounds, in some cases forming thickets o f large sticklike cups atop mature mounds. The bioconstructional regim e established in these Tommotian bioherms prevailed throughout the Ear ly Cambrian reef-building phase, particularly in argillaceous limeston e facies. Likewise, the trophic web of this very early Cambrian commun ity remained a blueprint for shallow wafer, open, marine habitats thro ughout the epoch.