Conodont biostratigraphy and stable isotope chemostratigraphy of the lower Henryhouse Formation (Gorstian-early Ludfordian, Ludlow, Silurian), southern Oklahoma, USA

The lower member of the Henryhouse Formation (Ludlow-Přídolí) is a thin (<14 m) succession of extremely argillaceous carbonate mudstones and shales that are restricted to the central Arbuckle Mountain region in southern Oklahoma. Conodont faunas of the Kockelella crassa, Ancoradella ploeckensis and Polygnathoides siluricus zones indicate that the lower member ranges in age from the early Gorstian into the middle Ludfordian (Ludlow). Mudstone lithofacies, Dapsilodus conodont biofacies and presence of graptolites indicate relatively deep water, offshore deposition of the lower member. The variable thicknesses of the lower member, of units within it, and of conodont zones over short distances suggest that deposition was discontinuous and occurred on an irregular, shifting surface, such as may have existed along the northern edge of the failed-rift margin of the Southern Oklahoma Aulacogen. Values of δ ¹³C and δ ¹³O are relatively uniform through the lower member and resemble Ludlow isotopic curves from Lithuania and Podolia. Short-lived negative δ13C excursions at unit boundaries are attributed to diagenetic effects at discontinuity surfaces. The Baltic pattern of Gorstian-early Ludfordian oceanic episodes are difficult to recognise and no lithofacies, conodont faunal or carbon isotope evidence of the late Gorstian Linde Event was recovered. The thicker upper member of less argillaceous carbonate strata (up to 100 m) rests unconformably on the lower member and the unconformity coincides with the mid-Ludfordian Lau Event and carbon isotope excursion (CIE). The diverse shelly fauna of the Henryhouse Formation does not occur in the lower member, but is restricted to the upper member, which is late Ludfordian and Přídolí in age.