Detailed studies of the morphology, ontogeny, variation, and stratigraphic distribution of the ostracodes in the Henryhouse (Silurian) and superjacent Haragan (Devonian) Shales indicate the presence of an unconformity at the Siluro-Devonian boundary in the Arbuckle Mountains of Oklahoma. The ostracodes indicate a late Niagaran (early Ludlovian) age for the Henryhouse Shale and Helderbergian age for the Haragan Shale.

The ostracode faunas of both stratigraphic units are large and diversified. The Henryhouse Shale contains 46 species representing 28 genera and 17 families. The Haragan ostracode fauna consists of 53 species, 27 genera, and 16 families. These taxa are distinctive and readily identifiable.

Biostratigraphic evidence for the unconformity between the Henryhouse and Haragan follows. (1) At the specific level the two faunas are completely distinct; no species are common to both units. (2) Of the 28 genera in the Henryhouse, 13 are restricted to it. Twelve of the 27 Haragan genera are not present in the Henryhouse. (3) The two faunas are nowhere known to occur together or interfinger with one another. (4) The faunal change is abrupt. There is no evidence for an intermediate (gradational) fauna between the two distinct faunas recognized.

The stratigraphic distribution of ostracodes within the Henryhouse and Haragan Shales indicates no definite biostratigraphic zonation within either unit. Each species which is significantly abundant ranges throughout, or almost throughout, the unit in which it occurs. The Henryhouse and Haragan Shales are distinct and discrete stratigraphic units, each having its own ostracode fauna.

Detailed sampling of the Henryhouse and Haragan formations has been carried out in all the outcrop areas of the Hunton Group, which consists of four formations, in descending order: Bois d’Arc Limestone, Haragan Shale, Henryhouse Shale, and Chimneyhill Limestone. The ostracode faunas obtained are distinctive and readily identifiable. Eleven ostracode species previously described as Haragan species are now known to have come from, and to be restricted to, the Henryhouse Shale. The ostracode faunas are distinct and discrete. Therefore, the Siluro-Devonian contact in south-central Oklahoma is unconformable.

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