REVISIONS TO THE AREAL DISTRIBUTION OF THE GINKGO AND SAND HOLLOW BASALTS, FRENCHMAN SPRINGS MEMBER, COLUMBIA RIVER BASALT GROUP: AN ILLUSTRATION OF THE INTERACTION BETWEEN LARGE LAVA FLOWS

The Ginkgo and Sand Hollow basalts are the most voluminous units in the Frenchman Springs Member. Both units were erupted from dikes and vents exposed in south-central Washington and north-central Oregon, inundated much of the Columbia Basin, flowed into western Oregon (OR) and Washington (WA), and flows from both reached the Pacific coast.

New mapping suggests that Ginkgo flows occur in and east of the Grand Coulee and along the Wallula Fault zone between the Tri-Cities, WA, and Milton-Freewater, OR, significantly beyond the previously recognized flow margins. In addition, the vent system for the Ginkgo basalt forms a prominent aeromagnetic lineation that extends northwest from the dike exposures along the Snake River. Boreholes in the Othello, WA, region preserve an unusually thick Ginkgo section with multiple thin shelly pahoehoe flows and vent material. This region lies at the northern end of the aeromagnetic lineation suggesting that the Ginkgo vent system may be at least 140 km in length (comparable to the well-documented Roza Member vent system).

While new mapping does not alter the previously known margins of the Sand Hollow basalt, examination of the boreholes in the Othello region indicates that the Sand Hollow is not present in this area. Given the unusual thickness of the Ginkgo basalt in these boreholes, we suggest that flows erupted from the Ginkgo vent system formed a topographic high that prevented the younger Sand Hollow lava flows from covering this region. The Sand Hollow flows reduced this constructional topographic high such that lava flows of the next Frenchman Springs unit, the Sentinel Gap basalt, covered the Ginkgo vent system.