CYCLIC FORMATION AND DESTRUCTION OF MICROBIALITES AT KELLY LAKE, INTERIOR BRITISH COLUMBIA, CANADA

Calcified stromatolites, thrombolites, and oncoids are forming on shallow (< 1 m deep) submerged platforms around the margins of Kelly Lake, a small freshwater lake in a steep-sided paleomeltwater valley in Interior British Columbia. The platforms, which are rooted upon inactive postglacial alluvial fans, are composed of carbonates variably mixed with siliciclastic detritus. The shallower parts of the platform are covered with small (1-5 cm high), laterally-linked domal stromatolites, thrombolites, oncoids, and their detritus, whereas charophytes are found along the outer margins and bench slopes. The stromatolite laminae are built by Calothrix and Fischerella, the denser thrombolites by Gloeocapsa and Synechococcus. Small (1-30 micron) calcite crystals precipitated in the microbialites are attached to filaments or cells.

Shallow (0.5–1 m) cores from the NW platform show abundant calcite sand and marl, charophyte and stromatolite debris, small oncoids, and reworked thrombolites, but no well-preserved mats or laminites. This implies that either mat growth began recently or that older mats have been destroyed. Maximum stromatolite growth occurs in summer when waters attain their highest level of calcite saturation. Physical processes, mainly strong wave action in the spring and autumn, fragment and rework shallow mats, especially where poorly calcified. During winter, ice impinging on the shallowest mats leads to further fragmentation. Early diagenetic corrosion is also implied by etched calcite grains.

The stromatolite-charophyte debris breaks down to carbonate sands and muds leaving little obvious record of the microbialites. Much of the debris is reworked by waves and transported to the platform margin, where some of the sediment is baffled and trapped by rooted charophytes. Other sediment is lost by mass flows down the platform slope. The platforms are equilibrium surfaces that are prograding, but show little aggradation. This is supported by the discovery of Bridge River tephra in one core, which implies only 30 cm of net aggradation during the past 2400 years.