PHYSICAL VOLCANOLOGY OF FRAGMENTED VOLCANIC ROCKS IN THE MIOCENE JACUMBA/ALVERSON VOLCANICS,DEVIL’S CANYON, CALIFORNIA

The surface geology at Devil’s Canyon, 9 km west of Ocotillo, California, contains fragmented Miocene volcanic and epiclastic deposits overlaying Eocene fluvial conglomerate and sandstone, which in turn sits above the Cretaceous La Posta Pluton. The Miocene volcanic and volcaniclastic rocks form one of several scattered outcrops in the NW corner of the Sonoran Desert mapped previously as the Jacumba volcanics and/or Alverson Formation. The lowest unit is composed of brecciated rocks of polylithologic character dominated by quartz-rich granitic clasts but also containing clasts of Miocene lava. The unit is poorly stratified but in places shows normal grading to fine sand and contains few scattered and discontinuous lenses of cross-bedded sandstone with angular clasts consistent with debris-flow deposition in an alluvial-fan or channel-fill environment with contemporaneous or later reworking to produce sandy lenses of fluvial transport. The granitic clasts decrease in abundance up section. The upper units are devoid of granitic clasts and contain a polymictic variety of volcanic clasts supported by a muddy matrix, consistent with lahar deposits that are best exposed in valleys cutting through the SE part of the Devil’s Canyon exposure. The intriguing, uppermost fragmented unit differs from all others. It is widespread, poorly sorted, monolithologic and well-indurated due to incipient welding of ashy matrix. It has all the characteristics of a block-and-ash flow derived from collapse of a growing lava dome, a type of deposit that has not been previously described in the Jacumba/Alverson volcanic succession. Fundamental features that support this conclusion are: (a) hot rocks with radial cooling joints typically found in volcanic domes, (b) flow banding delineated by aligned vesicular zones that develop along shear surfaces during dome growth, (c) elevated temperatures of the ashy matrix reflected in weak welding of the deposit, (d) a monolithologic character consistent with a provenance of dome collapse, and (e) reverse grading due to mechanical agitation during fluidized flow.