COLUMNAR JOINTING INFERRED FROM BASALTIC SILLS IN THE GUAYMAS BASIN DRILLED ON IODP EXPEDITION 385

Columnar jointed volcanic rocks are well known from geologic exposures on land, where the joints formed by tensile cracking during rapid cooling, with the joints orthogonal to the isothermal surfaces. There are only rare examples of columnar jointing from shallow submarine intrusions. IODP Expedition 385 recovered over 4 km of core from 25 drill holes at 8 sites in the Guaymas Basin in the central Gulf of California, Mexico. This basin straddles the Pacific North America plate boundary, extending by faulting and subsidence along two spreading axes as well as off-axis intrusion and volcanism. It is characterized by a high deposition rate of terrigenous and biogenic sediments. Young sills in the sedimentary section were imaged in multichannel seismic reflection profiles and targeted for drilling. Hypabyssal mafic sill material was recovered from various locations and depths: 350-420 m subseafloor at Site U1546 and 65-210 m subseafloor at or near the Ringvent (Sites U1547 and U1548) (50 and 30 km off-axis, respectively.). Much of the basaltic and doleritic rock was massive, retrieved as cylindrical cores either broken along old mineralized fractures or with curved surfaces shaped by drilling. However, some core fragments had planar surfaces intersecting at the classic 120-degree angle that is found in columnar jointed basalts on land. Such fragments were found over a range of depths at the Ringvent site but were not encountered at Site U1546. We infer that the samples indicate columnar joints, with dimensions ranging from a few cm to larger than the 6 cm core diameter can indicate. The columnar jointing is not ubiquitous but rather restricted to very shallow intrusions. Studies of columnar jointing in young lava flows in Iceland indicate that the columnar joints form at high temperature but sub solidus and can increase the permeability by many orders of magnitude compared to unfractured basalt. We characterize the likely locations of the columnar jointing within the sills sampled by IODP Exp 385 because in these locations, the sills did not present an impermeable barrier to fluid flow. Rather, increased fracture-controlled permeability would have locally enhanced the rate of cooling, sediment injection, and fluid circulation after sill emplacement.