The Heterogeneity of Paleocavern Systems Developed Along Syndepositional Fault Zones

Abstract

The results are presented from an integrated sedimentologic, structural, stratigraphic, and diagenetic study of syndepositional faults and fractures that cut the Permian Capitan reef and equivalent platform strata within three outcrop windows in Slaughter Canyon, Guadalupe Mountains, New Mexico. The studied faults are dip slip, have displacement of up to 30 m parallel the shelf margin, and are vertically and laterally segmented. The faults dip steeply shelfward and basinward, and grew incrementally during deposition of the Yates and Tansill Formations. Steep polycyclic paleocavern systems developed along faults and fractures. These can have cumulative vertical relief of more than 270 m, extend at least 1.2 km along strike, and are typically less than 10 m wide but can attain a width of 90 m. The paleocaverns can extend more than 110 m below the top of the Capitan reef. The paleocaverns are filled mainly with sediments deposited during Capitan progradation and aggradation. Seven Permian lithologies are distinguished: (1) limestones and limestone brecdas, (2) microspar-lithified breccias, (3) carbonate-rich breccias, (4) reworked and remnant brecdas, (5) beige dolomitic siltstone-sandstone and associated breccias, (6) pink dolomitic siltstone-sandstone and associated breccias, and (7) spar-cemented breccias. The fills vary vertically and along strike within the paleocaverns but have a clear organization and stratigraphy. The integration of the stratigraphy of the paleocaverns with the structural and sequence stratigraphie framework developed in back-reef strata provides evidence for incremental fault growth and multiple episodes of dissolution, brecciation, collapse, deposition, cementation, and dolomitization within the paleocaverns during Capitan times. Solution-modified syndepositional faults extend at least 33 km along strike from Slaughter Canyon and are considered to be an integral component of the Capitan platform. The fault-zone paleocaverns contain a unique internal record of events and processes that have no counterpart in the shelf succession. Their study provides new insights into the internal heterogeneity and diagenesis of the Capitan platform. The results have important implications for the Capitan platform, and more generally for the heterogeneity of syndepositional fracture-controlled karst systems formed in carbonate platforms with steep unstable margins, subject to compaction-induced tilting and/ or developed in active tectonic settings.