Syndepositional Deformation Features In High-Relief Carbonate Platforms: Long-Lived Conduits for Diagenetic Fluids

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 clumpedisotope 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 d 18 O values of this calcite (28.8 to 214.0% VPDB) and clumped-isotope temperatures (16u to 32uC) indicate precipitation from meteoric fluids (d 18 O SMOW of 26.2 to 210.5%) 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, inclusionrich calcites that formed from warm (59u to 96uC) fluids. Calculated fluid isotopic compositions (d 18 O SMOW of 20.5 to 24.7%) imply mixing of meteoric and oilfield brines. Carbon isotope values (+2 to 217%) 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 (59u to 65uC) than outer-shelf fractures (70u to 96uC) 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 limestone...

Extensive, uplift-related and non-fault-controlled spar precipitation in the Permian Capitan Formation

“…Our analyses extend the data set to include fore-reef slope facies and therefore provide a more complete picture of potential spar precipitation events. While the fluids responsible for precipitating these spars may well have been sourced along faults as indicated by the Budd et al (2013) study, they also penetrated unfaulted formation in the fore-reef slope. The clumped isotope data from this study and Budd et al (2013) are evaluated together, along with previously reported outcrop and core relationship data (Scholle et al, 1992).…”

“…While the fluids responsible for precipitating these spars may well have been sourced along faults as indicated by the Budd et al (2013) study, they also penetrated unfaulted formation in the fore-reef slope. The clumped isotope data from this study and Budd et al (2013) are evaluated together, along with previously reported outcrop and core relationship data (Scholle et al, 1992). These results indicate that late stage spar precipitation was extensive, occurred long after deposition and included a significant meteoric fluid component, with cementation likely occurring during uplift and not during burial.…”

“…6; Mruk, 1985Mruk, , 1989Lohmann, 1985, 1986;Scholle et al, 1992;Budd et al, 2013). Spar calcites from McKittrick Canyon exhibit δ 13 C carb and δ 18 O carb values that range from − 5.6 to + 1.3‰ and − 16.7 to − 8.9‰ (VPDB), respectively.…”

“…The conditions associated with the formation of these spars has been discussed in several papers which present different models for their formation based on isotope and elemental data (Mruk, 1985;Meyers and Lohmann, 1985;Scholle et al, 1992). Recent clumped isotope analyses by Budd et al (2013) has demonstrated that spars of the outer-shelf and reef facies of the Capitan Formation formed in the presence of relatively high temperature fluids (up to~100°C). Some of these spars exhibit data reflective of very low fluid oxygen isotope values (δ 18 O fluid ), indicating the influence of meteoric fluids.…”

“…Some of these spars exhibit data reflective of very low fluid oxygen isotope values (δ 18 O fluid ), indicating the influence of meteoric fluids. The spars analyzed in Budd et al (2013) were sampled from Dark Canyon and most were directly associated with faults, providing a local delivery mechanism for the spar-yielding fluid at that locality.…”