CORRELATION OF CARBON ISOTOPE (δ13C) STRATIGRAPHY WITH SHIFTS IN LITHOFACIES AND TRILOBITE BIOFACIES, LATE ORDOVICIAN VIOLA GROUP, SOUTHERN OKLAHOMA

The Ordovician Viola Group of south-central Oklahoma is a thick, shallowing-upward succession of carbonate rocks that records infilling of the Southern Oklahoma aulacogen. Five sedimentary facies occupy a depth gradient along a carbonate ramp sloping into the aulacogen. The facies of the deep ramp consists of laminated to weakly bioturbated carbonate mudstone that was deposited below wave base under anaerobic to dysaerobic conditions. Increased bioturbation and the appearance of thin, storm-winnowed horizons characterizes the transition to middle ramp. In the shallow ramp, interlayed wackestone and bioclastic packstone to rudstone record periodic storm winnowing between storm and fair-weather wave base. Bryozoan-rich bioclastic intervals represent shoals that formed near fair-weather wave base. The Viola Group contains at least 52 trilobite species and 36 genera. Cluster analysis identifies four discrete trilobite biofacies, each of which occupied a distinct habitat along the carbonate ramp. Species diversity declines from shallow to deep ramp settings. On the margins of the aulacogen, near Fittstown, an initial abrupt deepening is recorded by the appearance of a thin interval of deep ramp litho- and biofacies above peritidal carbonates of the Bromide Formation. The change is expressed by a shift towards negative carbon isotope (δ¹³C) values. Upward shallowing is marked by progradation of shallow ramp litho- and biofacies, and a return to increasingly positive δ¹³C values. At the top of the local succession, deepening and replacement of a diverse trilobite fauna with graptolite-rich assemblage is marked by a return to more negative values. Thus, variation in isotopic values within the Viola correspond well to sedimentological and faunal trends that were driven by relative sea level changes.