Abstract: Pliocene Basaltic Dikes Along the Clayton Fault on the Northeastern Flanks of Mount Diablo

Raymond Sullivan, Carl F. Schaefer, M. C. Erskine Jr.

Basaltic dikes of Pliocene age are associated with major north-south faults on the northeast side of Mount Diablo. They outcrop in small dome-like structures along the Clayton fault zone in the Concord Naval Weapon Station. A magnetometer survey across the basalt outcrops indicated a dike system feeding lateral surface flows that often occur along former stream channels. The main volcanic field is located at an elevation of 375 feet on the valley floor but other basalt flows occur on the fault ridges to an elevation of 1150 feet above sea level. Basalt has also been reported in the subsurface further north along the Clayton fault zone in wells in the Los Medanos Gas Field. Basaltic flows are again present a few miles to the northeast along the Kirker Pass fault in the Kel er Canyon Landfill Site. The basalts along the Clayton fault were dated at 4.8 Ma by the USGS where they intrude the mid-Eocene Markley Formation. A younger age is suggested, however, since the basaltic lava along the Kirker Pass fault overly the Lawlor tuff of 4.5 Ma age. The dike material does not appear to be gas charged, and scorious flows are absent. From this evidence, we can assume the magma to have a minimum density of 2.8g/cc and it did not rise diapirically in a sedimentary section of average density of 2.3 to 2.5g/cc. The magma, therefore, must have been tectonically pumped; probably by the compressive regime that produced the topography and stacked thrusts of the present Mount Diablo.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California