FROM SIMPLE MOUNDS TO COMPLEX REEFS: REGIONAL TRENDS IN REEF FACIES DEVELOPMENT FROM THE ORDOVICIAN THROUGH DEVONIAN

Across the Ordovician, reefs transition from primarily microbial buildups, to the complex sponge and coral-dominated constructions that would come to define the Mid-Paleozoic reef system. However, in comparison to their later counterparts, Ordovician metazoan reefs are relatively small, with limited traditional reef facies development and low facies diversity. When, and in what fashion, does the transition to complex, facies-diverse reefs occur?

Although there are many factors that influence facies diversity, two primary elements can be identified as: 1) reef size/relief; and 2) the individual morphology of constituent framework builders. With respect to reef size, larger buildups are more likely to hydrodynamically influence local sedimentary environments, and may provide opportunity for greater differentiation of reef core facies. As for framework builders, more branching, tabular, or vertical morphologies can promote greater self-sedimentation, and better trapping of sediment. In addition, the morphological diversity of framework builders directly influences physical habitat complexity, and can further promote differentiation between reef core facies.

The transition between low facies diversity within Ordovician reefs, and the high facies diversity of their Devonian counterparts, can be largely characterized in three ways: 1) a dramatic increase in reef size/relief through the Silurian; 2) a shift from encrusting and massive framework builders to more vertical, tabular and branching morphologies; and 3) a transition towards reefs with high morphological diversity among framework builders. Controlling for reef type and depth (where available), I used reef dimensions and the morphology of framework-builders, recorded in the literature, to constrain relative timing of these changes between the Ordovician and Devonian, and identify regional variation across the Laurentian paleocontinent. With this approach, I hope to highlight the relative importance of reef size and reef builder morphology in the development of the Mid-Paleozoic reef system, and perhaps provide some insight into the evolution of habitat heterogeneity and the promotion of high biological diversity among the reef fauna.