SUBSURFACE CAMBRIAN FELSIC IGNEOUS ROCKS OF THE SOUTHERN OKLAHOMA AULACOGEN

Igneous rocks of the Cambrian Southern Oklahoma Aulacogen (SOA) originated in a failed rift striking inland from the Laurentian continental margin. Pennsylvanian inversion thrust the northern edge of the aulacogen fill over younger oil-bearing strata, resulting in multiple wells drilled through the thrust. Microscopic examination of ~13,000 drill-cutting samples has documented ~41 km of uncorrelated igneous section in 30 wells across south-central and SW Oklahoma.

In the Arbuckle Mountains region in south-central Oklahoma, 25 boreholes penetrated 31.1 km of igneous rocks, including one 4.87 km igneous section with 2.83 km of basalt and andesite. Most igneous intervals are truncated by the Washita Valley Fault (WVF), an original south-dipping rift fault reversed by Pennsylvanian uplift. Felsic rocks constitute 54% of the cumulative drilled section. Rhyolites, consisting primarily of lavas, account for 49% of the total volcanic rocks. Pyroclastic deposits are limited to thin beds between flows. Trace element geochemistry indicates a highly fractionated within-plate signature similar to exposed felsic rocks in the SOA. Recognizing subsurface flow boundaries from cuttings is challenging, but textural variations such as formerly glassy margins and felsitic flow interiors provide useful markers. Eruptive sequences can be broadly divided into thicker, lithologically similar flow packages with minimal granite and diabase intrusions to the east, grading westward in the Arbuckle Mountains area into thinly intercalated rhyolite and basalt sequences with a maximum drilled thickness of 2.7 km. One continuous subsurface rhyolite sequence in the east is 1.16 km thick with up to 17 observed flows.

Granites make up 18% of the total igneous section and are present in all but one well. Subsurface granites are nearly identical to their exposed counterparts. Alkali-feldspar granite with granophyric porphyritic texture and biotite ± hastingsite or ferroedenite amphibole dominates cuttings. Groundmass textures and amphibole compositions suggest that many of the vertical penetrations sample the same plutonic body with depth. The maximum drilled granite thickness of 1.37 km, itself incomplete as it is truncated by the WVF, appears to be within a single pluton, unlike the thin (~500 m) sheet granites in surface outcrops.