RELATING SEA LEVEL CHANGE TO THE STRUCTURE OF STROMATOLITE REEFS: CRYPTIC SEQUENCE BOUNDARIES IN THE 1.3 GA DISMAL LAKES GROUP, ARCTIC CANADA

The ~1.3 Ga Dismal Lakes Group, Arctic Canada, contains an extensive stromatolite reef system that shows complex internal structure, consisting of several distinct morphologic forms. Large conical stromatolites (up to 15 m synoptic relief) occur at a single stratigraphic level. Individual cones are widely spaced and lack inter-stromatolitic debris. Inter-stromatolitic space is filled with stacked packages of low-relief (<1 m), prograding domes. These stacked, prograding "arms" extend outward from a single Conophyton lamina, thereby postdating cone growth. A transition upward to low-relief, laterally-linked domal stromatolites capped by a pisoid horizon marks the termination of reef growth.

Nucleation and initial growth of the Dismal Lakes reef occurred during a major transgression above a type I sequence boundary with >2 m of erosional relief. Overall shoaling of the reef is indicated by the presence of pisoids at the top of the succession. However, distinctive relationships between reef growth phases indicate a more complex sea level history. The extraordinary synoptic relief of conical stromatolites demands significant rise in sea level. Aggradational growth apparently did not exceed the rate of transgression and, in fact, the absence of inter-stromatolitic debris suggests that this phase of reef growth occurred entirely below storm wave base. In contrast, the progradational character of the intra-reef arms suggests that upward growth was limited by sea level and the arms developed on an intra-reef sequence boundary. The presence of at least three stacked sets of prograding arms therefore requires multiple fluctuations of sea level. Having recognized the presence of these cryptic sequence boundaries within the reef, we are currently examining the detailed sequence stratigraphy of correlative facies, both landward and basinward of the reef. This analysis will allow us to better define the magnitude and frequency of sea level change and its relationship to stromatolite reef structure.