WRITTEN IN THE MAARS: PRELIMINARY MAPPING AND STRATIGRAPHY OF HYDROMAGMATIC ERUPTION DEPOSITS IN THE CLEAR LAKE VOLCANIC FIELD, CALIFORNIA

With its most recent eruptions likely occurring as recently as the Holocene, the Clear Lake Volcanic Field is the northernmost and youngest field in a chain of four volcanic provinces in the California Coast Range mountains. Effusive and explosive activity has spanned at least 2.1 Ma, with the youngest eruptions comprising a series of maar craters at the edges of, and within, Clear Lake itself. Pyroclastic deposits associated with this activity drape older lava flows and domes around the lake’s southeastern end, but previous work did not differentiate or document their stratigraphy; in addition, the only age constraints on these eruptions are relative (100 to 10 kya). Indigenous (Pomo) oral histories contain descriptions of potential volcanic phenomena, suggesting that explosive activity occurred within the last ~24,000 years. Therefore, thoroughly documenting the eruptive history of this hydromagmatism is crucial to hazard assessment at the CLVF.

This presentation explores hydromagmatic deposit mapping in 11 localities on southwestern side of the lake. While all pyroclastic material was lumped into a single unit on the original geologic map, for this study maar deposits were distinguished on the basis of lithology and volcaniclastic structures from pyroclastic breccias associated with older lava flows. Grain size and lithic assemblage analyses were combined with detailed stratigraphic mapping to attempt to identify local packages within the overall maar field. However, given the close geographic spacing of the maars and the possibility of multiple eruptions within individual craters, establishing a complete correlative stratigraphy is likely to be difficult. Even if this proves true, ¹⁴C dates on organic material found within surge deposits in several outcrops will provide age constraints on the maars and help calculate recurrence intervals for eruptions in the CLVF. In the future, a high-resolution bathymetric survey will also reveal any as-yet unmapped maar craters and, in combination with ongoing geophysical investigations, help relate the activity to the tectonic and magmatic evolution of the field.