TONIAN MICROBIAL BUILDUPS AND SEA-LEVEL CHANGE: NEOPROTEROZOIC LITTLE DAL GROUP, NORTHWEST TERRITORIES, CANADA

The Little Dal Group is renowned for its enormous basinal calcimicrobial reefs, but the paleoenvironmental extent of these structures remains unclear. This study demonstrates that calcimicrobial buildups also grew in basin-margin ramp facies, where they occurred with stromatolites, but in different facies belts. The rocks were deposited on a broad, intermediate energy, homoclinal ramp and the adjoining epicratonic basin. The ramp was influenced by two scales of sea-level fluctuation resulting in six symmetrical 3rd-order sequences (100-250 m thick) superimposed on an asymmetric 2nd-order sequence (>900 m thick). Distribution of the two major biogenic lithofacies is directly linked to this cyclicity. Stromatolitic units form thick (up to150 m), regionally extensive, multi-storied biostrome/bioherm complexes composed of columnar and branching stromatolites. They occur in the high energy inner ramp and mid-ramp, as evidenced by interbedding of shallow-water facies such as oolite and intraclast grainstone. Thick units are limited to highstand systems tracts of 3rd-order sequences. Calcimicrobial buildups, in contrast, are confined to outer ramp and deeper basinal facies. These ~15 m-high mounds are built by a suite of calcimicrobes and contain growth cavities, synsedimentary cement, and geopetal sediment. Mounds are surrounded by dark, parted lime mudstone and formed only during the transgressive systems tract of the deepest 3rd-order cycle, at maximum accommodation. Such facies-specific mound growth implies that a major oceanographic interface separated stromatolitic buildups and calcimicrobial mounds. Possible environmental controls on this depth segregation include energy, turbidity, temperature, and light availability. This calcimicrobial consortium persisted into the early Phanerozoic when metazoan-calcimicrobial reefs dominated high-energy platforms. This study suggests initial establishment of calcimicrobial buildups in deep water, and subsequent gradual adaptation to occupy higher energy environments.