FROM SIMPLE MOUNDS TO COMPLEX REEFS: ASSESSING THE ROLES OF SIZE, RELIEF, AND BUILDER MORPHOLOGY IN THE DEVELOPMENT OF HABITAT HETEROGENEITY IN ORDOVICIAN THROUGH DEVONIAN REEFS OF LAURENTIA

Across the Ordovician, reefs transition from primarily microbial buildups to the complex sponge- and coral-dominated constructions that characterize 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 are: 1) reef size and relief; and 2) the morphology of individual constituent framework builders. With respect to reef size, larger buildups are more likely to exhibit strong hydrodynamic influence on local sedimentary environments, providing opportunity for greater differentiation of core and flanking facies. As for framework builders, delicate morphologies promote higher self-sedimentation and better trapping of sediment than encrusting or massive counterparts. The morphological diversity of framework builders, and their distribution across a reef, also directly influence physical habitat complexity, and can further promote differentiation between reef core facies.

Through field observations and an extensive review of the literature, I find that the transition from low facies-diversity within Ordovician reefs to high facies diversity in the Devonian can be largely characterized by three changes: 1) a dramatic increase in reef size and relief; 2) a concurrent 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 water depth (where available), I use identified reef dimensions and framework-builder morphologies to constrain relative timing of these changes, 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 provide insight into the evolution of habitat heterogeneity and the promotion of high biological diversity within the reef fauna.