ANDESITIC PEPERITE-BEARING DEBRIS FLOWS: PHYSICAL VOLCANOLOGY AND PALEOENVIRONMENT OF THE MIOCENE MEHRTEN FORMATION, CALIFORNIA

The Mehrten Formation is a voluminous Miocene suite of andesitic-dacitic primary volcanic and reworked equivalent rocks, emplaced along the crest and flanks of the Sierra Nevada mountains in northern California. At Kirkwood, California the formation comprises an upper sequence of andesitic debris flow deposits, debris avalanche deposits, block-and-ash flow deposits, near-surface andesitic and dacitic intrusions and fine-grained rhyolitic ignimbrite deposits, overlying a basal sequence of fluvially-reworked heterolithic andesitic debris flow, hyperconcentrated flood, streamflow and lacustrine deposits. At Kirkwood, California both sequences are intruded by numerous andesitic intrusions (often dike-like) that display fluidal and blocky peperitic margins. Debris flow deposits in the upper sequence also contain numerous very similar, discrete peperite domains (<1m to 300m across) of brittle or ductile-fragmented andesitic, poorly vesiculated clasts, which range from millimeters to meters in size. The matrix in these domains varies from laminated, well-sorted, pyroclastic (phreatomagmatic?) ash to poorly-sorted heterolithic volcaniclastic sandstone. Some peperite domains are clearly fed from dike-like intrusions, but most display no clear continuity. The debris flow deposits also contain large rip-up domains (to several meters across) derived from the underlying fluvial sequence. These observations suggest that peperite-bearing debris flows were generated when near-surface degassed andesitic magma intruded water-saturated, waterlain (?), unconsolidated pyroclastic deposits and fluvial volcaniclastic deposits in a valley or paleovalley setting. The absence of feeder intrusions to many of the peperite domains and the incorporation of large rip-ups of fluvial deposits, suggests that the discrete peperite domains also represent rip-ups entrained during flow. The voluminous peperite-bearing debris flow deposits of the Mehrten Formation suggest the possibility that near-surface intrusion into wet “sediment” can generate hazardous debris flows (lahars). It is not yet clear how applicable this mechanism may be to other volcanogenic debris flow deposits elsewhere, though similar associations are common at other Mehrten Formation localities.