Chronicles of vadose zone diagenesis: cone-shaped iron oxide concretions, Triassic Trujillo Formation, Palo Duro Canyon, Texas

Abstract

Unusual cone-shaped iron oxide concretions occur in the Late Triassic, lower fluvial sandstone member of the Trujillo Formation at Palo Duro Canyon in the Texas panhandle. In situ concretions are significant because they record both historical information about past processes that occurred within the geologic unit and present-day information about the ability of the unit to conduct fluids. The dominant orientation of the concretions is cone-apex up, body radiating down and out, with long axis perpendicular to bedding. Concretion morphologies are associated with the sedimentary texture and primary bedding structure of the host rock and the corresponding hydrologic regime (i.e. advection versus dispersion for iron-transport behavior). Three lithofacies in the lower Trujillo member exhibit different cone forms. Field observations of cone orientation and morphology suggest vadose conditions for diagenetic precipitation of iron oxide cements, with timing potentially represented by the major pre-Miocene unconformity.