Depositional controls on sediment body architecture in the Eagle Ford/Boquillas system: Insights from outcrops in west Texas, United States

Abstract

ABSTRACT A detailed sedimentological study of the middle Eagle Ford/Boquillas (outcrop analog of subsurface producing strata) units was conducted on numerous roads cut along US Highway 90 in Val Verde County and on outcrops in Big Bend National Park, Brewster County, Texas, using field-based petrographical and high-resolution image capture methods (light detection and ranging and GigaPans). This study demonstrates that vertical and lateral facies distribution is controlled by the interaction of sediment productivity under the influence of bottom currents below storm wave base. Vertical cyclicity is the result of alternating periods of lower primary productivity and relatively low sediment accumulation rates (globigerinid argillaceous wackestones) and shorter periods of high primary productivity and higher accumulation rates (pelagic grainstones), driven by the absence or presence of iron from volcanic ash deposition. Lateral variations are controlled by the deposition and reworking of pelagic sediment under the influence of below–storm wave base bottom currents. Pelagic grainstones accumulated in the form of isolated barchanoid hydraulic dunes, sand ridges, coalesced sand ridges, and sand sheets and less commonly as continuous beds. Detailed measurements show that pelagic grainstones have no more than 50% continuity, and ash beds have 72% continuity. Application of sequence stratigraphic principles needs to be done with caution because the deeper-water depositional setting is affected not by sediment input from a shallow-water benthic carbonate factory but by pelagic sediment from the open-marine environment subject to bottom current reworking. Packages of strata may be more reliable for longer-distance correlations.