ARCHITECTURE OF A MID-CRETACEOUS PATCH REEF: HIGH RESOLUTION MAPPING PROVIDES NEW INSIGHT INTO FACIES STRUCTURES AND ECOLOGICAL RELATIONSHIPS AT PAUL SPUR (BISBEE, ARIZONA)

At the beginning of the Cretaceous Period, corals dominated reef ecosystems as primary framework builders. As time progressed, a new group of heterodontid bivalves called rudists appeared and rose to prominence in the reef-building role. The rise of rudists as dominant reef-building organisms at the expense of corals has been attributed to either competition or environmental adaptation. The present study analyzes reef structure at Paul Spur, an exposure of Aptian-Albian Mural Limestone southeast of Bisbee, Arizona, in order to better understand rudist-coral relationships.

Paul Spur preserves an open marine shelf interior patch reef that provides exceptional three-dimensional exposures of coral-algal-rudist reef buildups, which are ideal for assessments of spatial and temporal ecological variability. Differential GPS mapping at sub-meter spacing and point clouds constructed from unmanned aerial vehicle (UAV) photogrammetric imaging allowed comprehensive mapping of the facies structure of the exposed top of the patch reef over a 0.15 km² area. Previous work at Paul Spur hypothesized that rudists occupied a different niche than corals, occurring in reef climax and back reef communities while corals occupied pioneer and framework communities. More recent work has shown that the exposure more likely reflects the evolution of a reef through time with increasing abundance of rudists. Our analysis of lateral facies distribution on the top of the reef exposure agrees with the latter work; however, it is important to note that the reef biotic composition is not uniformly continuous laterally. Our data suggest that rudists and corals cohabited more than previously thought, and that rudist colonization was not sudden and invasive. Solitary upright conical rudists commonly occur in facies dominated by platy corals and algal buildups. Furthermore, rudists were only observed monopolizing the ecosystem in small localized mounds. Rudist-coral competition was therefore not likely the cause of rudist domination in Cretaceous reef communities; adaptations for fluctuating environments were more likely what allowed rudists to flourish.