ANCIENT FAULT-FLUID INTERACTIONS IN NORTHERN OKLAHOMA

In order to better understand controls on induced seismicity in the region, previous studies of the uppermost igneous basement in northeastern Oklahoma have documented abundant fracturing and fluid alteration features. These reports have largely focused on near-vertical mineralized fractures. Continuing investigation of drill core material from these rocks has also shown evidence of ancient faulting, with faults and slickensides present in most inspected cores. Much like the steeply dipping fractures previously studied, these shear features commonly display evidence of fluid alteration such as oxidation, carbonate ± clay mineralization, and/or hydrothermal leaching. More well-developed shear structures, which likewise show evidence of complex slip and fluid histories, are also present in several cores.

Core from the Sinclair Louisa M. Jones 46 (Pawnee Co.) contains a sheared carbonate interval with slickenside striations, rhyolitic clasts and clay-rich slip surfaces. Petrographic inspection has shown evidence for multiple episodes of carbonate remineralization and slip. This fault is also associated with hydrothermal leaching of the host rock.

The Amax T-1 (Cherokee Co.) contains a distinctive interval of fractures filled with very fine-grained dark material which is variably replaced by carbonate, contains rhyolite breccia and is associated with shear fractures. This interval strongly resembles a spheroidal carbonate-chlorite texture associated with hydrothermal replacement of vesicular glass and may represent an altered pseudotachylyte. Similar material has been variably altered to clay-dominant or carbonate-dominant fracture fill in other intervals, suggesting interaction with multiple fluids. Some of the calcite-dominant intervals show leaching similar to that seen in the Pawnee Co. core.

Carbon and oxygen isotopic data for carbonates from fractures and faults in the basement rock of these cores show significant overlap with those from the overlying Arbuckle dolomites, indicating the possibility of fluid communication between them. The alteration textures and chemistry show that the ubiquitous fractures and faults in this region have a long and complex fluid history which likely influences the behavior of fluids and the mechanical behavior of faults today.