CONODONTS ACROSS THE SILURIAN-DEVONIAN BOUNDARY AND THE KLONK EVENT IN THE FRAME FORMATION, SUBSURFACE WEST TEXAS

In the subsurface of West Texas, the Frame Formation comprises a unit of shale, argillaceous carbonate mudstone, wackestone and rare skeletal packstone that crosses the Silurian-Devonian boundary. These strata accumulated in a slope to basin setting south of the Fasken shelf of Ruppel and Holtz (1994). A 150-foot core of the upper Frame Formation from the Amoco Three-Bar 74 well in southern Andrews County, Texas, was processed for conodonts. The Three-Bar 74 well lies just south of the transition from Fasken shelf facies to Frame slope/basin facies, the latter which locally reaches nearly 500 feet in thickness; the top of the core begins 60 feet below the base of the overlying Thirtyone Formation. Conodonts were obtained from nearly all samples, but abundance was low, usually 10 to 50 conodonts/kg. Conodont faunas from the West Texas core are remarkably like those described by Barrick and Klapper (1992) from the Henryhouse-Haragan transition in southern Oklahoma. The lowest beds in the core yielded a Pridoli fauna dominated by coniforms (Belodella, Dapsilodus, Panderodus) and containing Oulodus elegans elegans and rare Ozarkodina. The overlying detorta Zone differs by the presence of the zonal index and the appearance of abundant B. coarctata near the top of the zone. As in Oklahoma, the upper detorta Zone fauna disappears abruptly, and is replaced by an undiagnostic fauna dominated by surviving coniforms and more common Oz. remscheidensis. This may represent the Klonk Event of Jeppsson (1995), which occurs near or at the base of the Devonian and may serve as a reliable means of identifying the boundary. Icriodus postwoschmidti appears slightly higher (eurekaensisZone) and is moderately abundant in intervals of skeletal packstone that may represent shallow-water skeletal sands transported to the slope by gravity flows. Forms transitional to Ancyrodelloides and Lanea species occur in upper part of the core, which suggest a mid-Lochkovian (omoalphaZone?) or younger age for the top of the Frame Formation.