THE PYROCLASTIC SURGE DEPOSIT OF THE ~680 YR BP PANUM CRATER ERUPTION, EASTERN CALIFORNIA

Panum Crater represents the youngest eruption of the Mono-Inyo Volcanic Chain in eastern California (Sieh and Bursik, 1986). Previous work by Sieh and Bursik (1986) determined a stratigraphy for this eruption characterized by a throat clearing breccia overlain by a pyroclastic flow deposit (“dune” flow), pyroclastic surge deposit, a block-and-ash-flow deposit found locally to the northwest of Panum Crater, capped by a tephra ring deposit and a rhyolite lava dome. This study investigates the “pyroclastic surge deposit” horizon of this stratigraphy along flow-parallel transects with increasing distance from vent in terms of thickness, internal stratigraphy, granulometry, and pyroclast morphology. Surge deposits within ~300 m of the crater rim are up to 4 meters thick and can be subdivided into 2 units, with a 3-meter-thick cross-bedded ash-rich unit at the base (P1) overlain by light beige ash-rich beds with cross-bedding and planar-bedding structures interbedded with 15 to 20-cm-thick reversely graded coarse-grained pumice lapilli and lithic fall deposits (P2). Surge deposits ~500 meters of the crater rim are 1 to 2 meters thick and can be subdivided into 3 units, with a dark beige fines-rich cross-bedded base unit (P1), a middle unit composed of beige fines-rich planar bedded layers interbedded with reversely-graded pumice fall (P2), overlain by a light tan fine-grained crystal-rich friable unit (P3). Surge deposits thin rapidly from 0.5 meters thick at distances of 700 meters from the crater rim to <4 cm thick at distances beyond 2 km from the crater rim. At these distances, only the upper friable and crystal-rich unit (P3) is observed. Granulometric analysis yields supportive data to the stratigraphic descriptions mentioned previously. For samples located ~500 meters from the Panum Crater rim, the percentage of fines decrease with stratigraphic height from 53% in P1 to 44% in P3, whereas median grain size increases slightly from 0.1 mm in P1 to 0.2 mm in P3. Sorting values range from 1.4 to 1.5 throughout the stratigraphy. We interpret this stratigraphy as the result of a dry, dilute, and turbulent pyroclastic density current (P1) followed by simultaneous deposition of fall and pyroclastic density current deposits (P2) capped by a co-ignimbrite ash fall (P3).