INTERNAL GRAPTOLITE CASTS FROM THE SILURIAN (LUDLOW, PRIDOLI) HENRYHOUSE FORMATION IN SOUTH-CENTRAL OKLAHOMA

In the Arbuckle Mountains and Lawrence Uplift of south-central Oklahoma, strata of the Hunton Group are exposed, including the Late Silurian (Ludlow-Pridoli) Henryhouse Formation. The Henryhouse Formation consists mainly of argillaceous wackestone and mudstone with some beds of skeletal wackestone and packstone. The lower member extends from the Gorstian (Kockelella crassa Zone) into the middle Ludfordian (Polygnathoides siluricus Zone). The top of the lower member is a disconformity that coincides with Lau Event and Isotope Excursion. The lower member of the Henryhouse is particularly argillaceous and contains several graptolite-bearing shale beds. Compressed, carbonized fragments of Ludlovian graptolites have previously been recovered from the lower member of the Henryhouse Formation, but internal casts of graptolites have not previously been identified from this region. Indeed, internal graptolite casts are rare worldwide.

At sections Highway 77 and Dougherty West, internal graptolite casts of the monograptolid genera Pristiograptus, Saetograptus, and Bohemograptus are found in samples from the upper part of Unit 2 and Unit 3 in the middle and upper parts of the lower member of the Henryhouse Formation. The casts appear to be composed of grains of silt and fine sand cemented together with an acid-insoluble binder. The surface and interior portions of the casts became coated in pyrite and limonite during diagenesis. A reflective black carbonaceous film remains attached to a small number of specimens.

The internal casts are a mixture of siculae and rhabdosome fragments composed of two to five conjoined thecae. The casts represent the interior shape of the tubarium, and often show banding of fusellar growth rings in thecae and internal constrictions of siculae. However, the external details of the original chitinous structures such as the thecal apertures are not preserved.