Biostratigraphy of the Caballos Novaculite-Tesnus Formation boundary, Marathon Basin, Texas

Abstract

The Caballos Novaculite is a radiolarian/sponge spicule chert and siliceous mudstone which represents the final episode of a 200 Ma span of dominantly biogenous hemipelagic sedimentation in Marathon Basin. It is overlain by the Tesnus Formation, an upward-coarsening flysch sequence produced during the Ouachita Orogeny by the collision of Laurasia and Gondwana. Detailed radiolarian biostratigraphy of the boundary between these two units has provided greater age control as to the onset of flysch deposition and provides a greater understanding of the sedimentary patterns produced in Ouachita basinal facies when switching modes of sedimentation from a passive to an active regime. Radiolarians and conodonts recovered from this interval show a hiatus equivalent to much of the Early Mississippian occurring in the vicinity of the boundary between the Caballos and Tesnus formations. In three sections, Late Mississippian (Meramecian) radiolarians overlie earliest Mississippian (early Kinderhookian) radiolarians in an apparently conformable sequence of siliceous shale. In a fourth section, Meramecian radiolarians overlie Late Devonian (Famennian) radiolarians and conodonts, separated by a conglomerate layer. These data indicate that a hiatus equivalent to the late Kinderhookian to Osagean time period is present and that local erosion into the Famennian part of the section occurred. Lithostratigraphically, this hiatus is coincident with the Caballos Novaculite-Tesnus Formation boundary in the central part of the basin, while at the western margin, it occurs 8 m above the base of the Tesnus Formation, immediately above a clastic zone containing olistoliths. Results from this investigation show that localized deposition of olistoliths occurred in the western basin margin during the Famennian (Late Devonian), followed by an interval of non-deposition in the Early Mississippian. Finally, basin-wide flysch deposition in the Meramecian.