PALEOECOLOGY OF A MID-CRETACEOUS PATCH REEF: FOSSIL ASSEMBLAGES YIELD EVIDENCE OF ENVIRONMENTAL CHANGE THROUGH TIME

Fossilized reefs can preserve a wealth of information about changes in marine environments over a relatively short period of time. However, the interpretation of these changes can be difficult because of the innate complexity of reef growth (e.g. vertical and lateral zonation, differing species growth habits, and chaotic architecture). In reef outcrops, rigorous mapping and data collection incorporating both paleoecological and sequence stratigraphic principles is needed to tease apart natural heterogeneity from biotic shifts linked to changing ocean conditions over time. At Paul Spur, a pristine reef outcrop southeast of Bisbee, AZ, a robust dataset consisting of UAV-derived 3D outcrop models, measured sections, point-counted quadrats, and numerous samples reveals five distinct stages of reef development: (1) initial shoal deposition; (2) pioneer reef growth; (3) reef diversification; (4) reef hiatus; and (5) rudist shoal development. These stages represent periods of deposition and reef growth within high-frequency transgressive-regressive sequences, but also reflect changes in local environmental conditions through time. Prevailing wind and current direction acted as higher order controls on overall reef architecture, producing a windward-leeward asymmetry; fluctuations in water depth and varying amounts of allochthonous sediment influx were important influences on reef community composition as well as growth habit. Although corals and rudists cohabited during much of the reef’s history, corals dominated when water depth was greater and sediment influx lesser, whereas rudists proliferated in shallower water during periods of high sediment influx. In addition to adding important insight into environmental controls that impacted Cretaceous reef communities, this work demonstrates that detailed evaluation of paleoecology and stratigraphy, with careful consideration of timelines and inherent heterogeneity, is essential for building a more thorough understanding of processes driving reef growth and evolution through time.