TRILOBITE BIOSTRATIGRAPHY OF THE LATE ORDOVICIAN VIOLA GROUP IN SOUTH-CENTRAL OKLAHOMA

Trilobite faunas have been used for high-resolution biostratigraphy in the Cambrian and Early Ordovician but have been underutilized in younger strata. The diverse and abundant Late Ordovician (Mohawkian-Cincinnatian) trilobite fauna in the Viola Group of Oklahoma is used to construct high-resolution biozones that can be employed to evaluate and refine previous studies using graptolites and conodonts. Biostratigraphic data from this study are particularly important in lithofacies that do not preserve graptolites, such as those deposited in high-energy, shallow-water environments. Bioclastic sediment of the Viola Group was deposited along a depth gradient within and on the margins of the Southern Oklahoma aulacogen in south-central Oklahoma. Four lithofacies define a shallowing-upward sequence in the Viola Springs Formation as sediment filled the basin and these grade upward into shallow platform deposits of the Welling Formation. Sections along a transect from the depocenter of the aulacogen out onto the margins record varying thicknesses of each lithofacies. The base of the Viola Group is marked by deep-water deposits that formed following rapid subsidence of the aulacogen and preserves a low-diversity fauna limited to graptolites and such trilobites as Cryptolithus, Isotelus and Anataphrus. Near the margins of the aulacogen, sections shallow rapidly upward into coarse-grained facies interpreted to represent deposition in shallow subtidal and shoal environments. Graptolites are rare to absent, limiting their use as biostratigraphic tools, while the trilobite faunas, which include isotelines (Isotelus, Anataphrus), illaenines (Thaleops, Bumastoides), cheirurines (Ceraurus, Ceraurinella), pterygometopids (Achatella, Calyptaulax) and Flexicalymene, are abundant and diverse. The base of the Welling Formation is gradational at each section and is not isochronous across the basin. The Welling Formation consists of well-washed, coarse bioclastic pack-, grain- and rudstone that is rich in such trilobites as Isotelus, Anataphrus, Ectenaspis and Calyptaulax but nearly devoid of graptolites. This study demonstrates that trilobite biostratigraphy can be used to increase resolution in Late Ordovician deposits for which biostratigraphic data using graptolites and conodonts already exists.