Carbonate submarine fan deposits of the Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico

Abstract

Deep-water carbonate deposition is relatively poorly understood but an area of vigorous research in academia and industry, where these deposits are a significant component of many unconventional petroleum reservoirs. Recent studies of modern deep-water carbonates have highlighted the wide variety of depositional processes, sediment types and resultant geomorphology; however, well-documented outcrops of ancient systems, their rock types and architecture are relatively sparse. The Mississippian Lake Valley Formation provides world-class exposures of slope-basinal carbonate deposits. The Tierra Blanca and Doña Ana members comprise submarine fans that are >14 to 20 km in length, >5 km wide, and exposed in strike and dip view, affording a unique opportunity to constrain the architecture, rock types and sedimentary processes. Tierra Blanca and Doña Ana sedimentation was dominated by crinoids shed from an up-dip platform and supplemented by sediments sourced locally from Waulsortian mounds. Depositional processes include turbidity flows, debris flows and hybrid sediment-gravity flows. The Tierra Blanca submarine fan thins towards its lateral flanks and distal fringe, where deposits become more mud-dominated, gravelly grain-supported flows are less common, and fewer beds have scoured bases. In proximal settings, bed tracing complemented by measured sections allow mapping of stratal surfaces and identification of stories, elements and complexes. The Tierra Blanca evolved from more unconfined to confined deposition. Point-sourced deposition of the Tierra Blanca fan required a funnelling mechanism, probably due to bathymetry created by Waulsortian mounds or possibly a platform margin re-entrant. Outcrop exposures illustrate that younger Doña Ana submarine fan deposits onlap onto, and compensationally stack with, the thickest portions of the antecedent Tierra Blanca fan. These outcrops illustrate both similarities and differences between carbonate and siliciclastic gravity flow deposits. Similarities include comparable deposit types, depositional processes and architecture; differences relate to hydrodynamics of carbonate grains, funnelling mechanisms for point-sourced deposits and sequence stratigraphic forcing.