Architecture of a mid-Cretaceous patch reef: High resolution mapping provides new insight into facies geometries and ecological relationships at Paul Spur, Bisbee, Arizona.
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Patch reef complexes are commonly found in the shelf interior of carbonate platforms. These small scattered buildups are potential hydrocarbon targets in the Maverick Basin and more broadly within Cretaceous reservoirs in the Middle East. The three-dimensional facies architecture within patch reefs is difficult to determine using only subsurface data. Lateral facies distribution and overall patch reef architecture is better assessed in outcrop analogs. The Paul Spur patch reefs near Bisbee, Arizona are ideally suited for assessing three-dimensional spatial and temporal facies variability. Previous interpretations of this 1.5 km-long exposure of Mural Limestone disagree as to the overall history of the reef with regards to facies relationships and distribution. Early work at Paul Spur attributed spatial facies distribution to biotic zonation of a reef during one period of growth, while later work concluded that it preserves multiple stages of reef growth with facies succession and variation both spatially and temporally controlled. This work better resolves the depositional history and biotic composition of the Paul Spur patch reef complex with respect to stratal geometry and both spatial and temporal facies relationships. High-resolution lateral facies mapping of the exposed reef top is integrated with three-dimensional digital outcrop modeling techniques to facilitate improved understanding of the history of reef growth and patch reef architecture. The new reef architecture interpretations are integrated into the preexisting depositional model. At Paul Spur, multiple stages of reef growth are preserved and exhibit variable architectures controlled largely by local sea-level fluctuations and sediment influx. While coral diversity increases throughout the depositional history of the reef complex, overall abundance decreases as rudists become more common, reflecting an evolution of the reef community through time. Reef constituents are heterogeneously distributed within facies, highlighting the need for careful analysis and outcrop scale synthesis of the paleoecological data to avoid erroneous characterization of depositional environments based solely on the organisms found within a small area. The new depositional model developed in this study improves the utility of Paul Spur as an outcrop analog for patch reefs identified in the subsurface and furthers understanding of the relationship between environmental controls and reef development.