EVIDENCE FOR CHANNELIZED FLOW PATTERNS OF THE MIOCENE LOVEJOY BASALT, NORTHERN CALIFORNIA

Variations from the generally assumed southwestward flow direction of the Miocene Lovejoy Basalt in the western Sierra Nevada foothills of northern California are indicated by orientations of stretched vesicles and pipe vesicles measured along the basal surface of the basalt at Oroville Table Mountain. These indicators suggest localized westward to northwestward flow direction and are consistently perpendicular to less diagnostic linear undulations measured on the upper surface of the basalt. Field observations of subcropping units combined with cross sectional profiles indicate pre-existing channels and paleotopographic basement obstructions, such as the Bald Rock pluton, which further channelized Miocene lava flows in the vicinity of the present northern Sierra Nevada-Sacramento Valley.

The distribution of the original channels is difficult to establish due to erosional removal of the channelized material, beginning with the margins. This often resulted in an inverted topographic relief and consequential destruction of channel morphology. Also, matching of discrete lava flows across areas of non-exposure is difficult due to the homogeneous appearance and chemical uniformity of the Lovejoy Basalt. Although no consistent patterns have yet emerged, our preliminary relative magnetic susceptibility measurements and trace element data from a regional selection of outcrops, in conjunction with detailed field observation, suggest promising means for identifying the extent and distribution of individual lava flows.

The surface distribution of Lovejoy Basalt remnants is highly modified by post-eruptive uplift and subsequent erosional patterns which may provide a misleading impression of the original channel distribution. A higher degree and more complex pattern of channelization than generally assumed could help explain how the Lovejoy Basalt flows were able to travel great distances and may lead to a new estimate of the total volume of lava erupted.