ESTIMATING PALEOBATHYMETRY IN CARBONATE SHELF STRATA: AN EXAMPLE FROM THE SEVEN RIVERS FORMATION, NM

Stratal geometries in shelf-top carbonates constrain key depositional variables, especially the relationship between water depth and facies. However, primary depositional dips are typically modified by early faulting and fracturing. If the relative magnitudes of structural and depositional dips are unknown, stratal geometries yield a non-unique interpretation of the bathymetric profile. For example, stratal geometries adjacent to the Capitan Reef in the Delaware Basin, USA, have been interpreted in support of both a shallow reef near sea level (<10 m) or a deep subtidal reef below fair-weather wave base (up to 70 m). Reducing the uncertainty in water-depth estimates would improve our understanding of shelf-top processes and environmental influences on the Capitan Reef.

This study examines seaward-dipping (up to 18°) shelf-top strata of the Permian Seven Rivers Formation (equivalent to the early Capitan Reef) in McKittrick Canyon, New Mexico. Traditional field mapping and a digital outcrop model permit quantitative assessments of the relationships among facies, structures, and stratal geometries. Our results show that there are at least two origins for variations in stratal architecture: (1) primary dips associated with steepening of the bathymetric profile towards the shelf edge and (2) both landward and seaward rotation of fault blocks during deposition of successively younger strata. We employ stepwise reconstruction of shelf-top geometries to remove structural rotation and recover the original depositional profile. The resulting water depth estimates for the Capitan shelf-break are 40-60% shallower than previous work. Our result has implications for future studies of other shelf-margin reefs where water depth can be estimates from shelf-top strata.