Abstract:

Syndepositional faults and fractures are known to affect early fluid flow in carbonate platforms. Less clear is whether they are active fluid conduits throughout the entire history of the platform strata. Syndeformational fractures in Permian (late Guadalupian) carbonates exposed in Dark Canyon, Guadalupe Mountains, New Mexico, U.S.A., address this question. Transmitted-light and cathodoluminescent petrography, stable-isotope and fluid-inclusion analyses, and clumped-isotope thermometry show that there were multiple episodes of fracturing, dissolution, cementation, and replacement in the fractures. Dolomite cement or dolomitized marine cements line the walls of some fractures and indicate the syndepositional reflux of evaporated Permian seawaters through the fractures. Fine- to medium-crystalline, luminescently zoned calcite may overlie the dolomite and marine cements, line fracture walls where those phases are absent, or cement karst breccia on fracture walls. The δ18O values of this calcite (−8.8 to −14.0‰ VPDB) and clumped-isotope temperatures (16° to 32°C) indicate precipitation from meteoric fluids (δ18OSMOW of −6.2 to −10.5‰) associated with episodic sea-level lowstands during the development of high-frequency depositional sequences. The early calcites can themselves be fractured, rotated, and recemented, indicating recurrent deformation and meteoric influx.

Evaporite cements were once the dominant pore-filling phase in the fractures, forming both before and after the early meteoric cements. The earliest evaporites formed during deposition of Tansill limestone, probably from the same brines that formed dolomites. Evaporites that postdate the early calcite probably did not form until Permo-Triassic burial, when geomechanical analysis indicates that the syndepositional fractures were likely reactivated and brines could have been sourced from overlying bedded evaporite. All evaporite cements subsequently were calcitized, mainly by coarse-crystalline, inclusion-rich calcites that formed from warm (59° to 96°C) fluids. Calculated fluid isotopic compositions (δ18OSMOW of −0.5 to −4.7‰) imply mixing of meteoric and oil-field brines. Carbon isotope values (+2 to −17‰) indicate microbial degradation of hydrocarbons in some of those fluids, but not all. Geomechanical analysis indicates the potential for syndepositional fractures to have failed (reactivated) during Basin and Range extension, and the warm basinal fluids are interpreted to have migrated upward through the fractures during that event. Platform-margin fractures (unassociated with faults) witnessed cooler fluids (59° to 65°C) than outer-shelf fractures (70° to 96°C) because outer-shelf faults tapped waters from greater depths. The most recent fluid flow through the fractures generated dissolution features and laminated speleogenetic calcites, which are interpreted to result from intrastratal karsting associated with exhumation and weathering.

Diagenetic features in the syndepositional fractures are equivalent to those observed in the adjacent limestones, suggesting active fluid communication between matrix and fractures throughout the diagenetic history of the rocks. The complex fracture paragenesis also indicates that syndepositional fractures are not only conduits for early fluid-flow networks, but they can also impact a rock's entire diagenetic history if reactivated by changing stress fields.

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