PHYSICAL VOLCANOLOGY OF CAMBRIAN BASALTS AND RHYOLITES IN THE SOUTHERN OKLAHOMA AULACOGEN FROM SURFACE EXPOSURES AND DEEP DRILLING PENETRATIONS

The Cambrian Southern Oklahoma Aulacogen (SOA) is a failed-arm rift zone that strikes inland from the Laurentian continental margin and contains >250,000 km³ of bimodal volcanic and plutonic rocks. During Pennsylvanian inversion of the SOA, rift-related igneous rocks were thrust over younger, oil-bearing Paleozoic strata, exposing parts of the rift fill in the Arbuckle and Wichita Mountains in southern Oklahoma. Examination of cuttings from 28 wells drilled into the overthrust igneous rocks provides access to 37.8 km of uncorrelated igneous section and a more complete view of the volcanic succession than available in surface outcrops.

A-type rhyolite lavas are abundant within the rift fill. Most of the exposed rhyolites show a standard vertical zonation of groundmass textures inferred to reflect emplacement of thick (up to 600 m), laterally extensive flow units; some flows are separated by bedded volcaniclastic rocks £100 m thick. Unweathered subsurface rhyolites show a similar variety of textures including formerly glassy flow margins, as well as abrupt color/textural changes between adjacent flows, with generally thin interbeds of tuffaceous mudstone and rhyolitic sandstone. One well closer to the rift axis, however, penetrated a cumulative thickness of 1.2 km of texturally immature rhyolitic sediments, suggesting that thick clastic aprons are present around eruptive centers in parts of the SOA not exposed at the surface.

Subsurface work has also revealed the previously unrecognized intercalation of rhyolite with significant volumes of dominantly subalkaline, tholeiitic basaltic lava. Groundmass textures in basaltic cuttings range from intergranular (developed in slowly cooled flow interiors) to tachylitic glass present along inferred flow margins, consistent with cooling patterns in subaerial basaltic flows. Basaltic pyroclastic deposits up to 58 m thick are present in six wells in the Arbuckle Mountains area and contain variably vesicular ash and lapilli consisting of altered sideromelane glass, pointing to rapid quenching of magma during explosive phreatomagmatic interactions with external water. Similar pyroclasts occur in diatremes up to 1.4 km across that cut the volcanic sequence exposed in the Arbuckles and provide examples of the types of vents that produced the basaltic tephra.