Syndepositional fault control on dolomitization of a steep-walled carbonate platform margin, Yates Formation, Rattlesnake Canyon, New Mexico
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Syndepositional deformation features are fundamental components of carbonate platforms both in the subsurface and in seismic-scale field analogs. These deformation features are commonly opening-mode, solution-widened fractures that can evolve into extensional faults, and reactivate frequently through the evolution of the platform. They also have potential to behave as fluid flow conduits from the earliest phases of platform growth through burial and uplift, and can be active during hydrocarbon generation. As such, diagenetic alteration in the margins of these carbonate platforms is often intense, may demonstrate a preferential spatial relationship to the deformation features rather than the depositional fabrics of the strata, and may impact the permeability development of reservoir strata near deformation features. This study focuses on a syndepositional graben known as the Cave Graben fault system in the Yates Formation of Rattlesnake Canyon in the Guadalupe Mountains, and investigates the distribution of dolomite around the faults and associated opening-mode fractures, in an effort to understand the control the Cave Graben faults exert on fluid flow through the platform margins. Two generations of dolomite are identified on the outcrop: a fabric retentive dolomite located in the uppermost facies of the platform, and a fabric destructive dolomite that forms white, chalky haloes around syndepositional deformation features. The first generation of dolomite is dully luminescent and has very small crystal sizes, as well as a low trace element concentration and an ¹⁸O-enriched stable isotopic signature compared to Permian marine carbonate ratios. This dolomite is interpreted to have formed from the penecontemporaneous refluxing of concentrated lagoonal brine, and shows little fault control on its distribution. The second generation of dolomite is brightly luminescent and has much larger crystal sizes, as well as a higher trace element concentration and a slightly ¹⁸O-depleted isotopic signature compared to the first generation of dolomite, though it is still enriched in ¹⁸O compared to Permian marine carbonate. This dolomite is interpreted to have formed in a burial environment due to the transport of concentrated brines from the overlying evaporites through syndepositional deformation features. Overall, this study suggests that, once open, syndepositional deformation features may become the primary fluid conduit through otherwise impermeable strata, and may control the distribution of diagenetic products over a long period of geologic time. It provides valuable insight into the interaction of syndepositional faults and fractures and fluid flow, and may improve understanding of diagenesis in analogous subsurface carbonates reservoir intervals.