HR: 1340h
AN: V53B-1558    [Abstracts]
TI: Stratigraphic and Granulometric Studies of three Pyroclastic Sequences from Soufriere Volcano, St. Vincent, Lesser Antilles.
AU: * Estrella, L J
EM: alsmith@csusb.edu
AF: Department of Geological Sciences, California State University, San Bernardino, CA 92407 United States
AU: Smith, A L
EM: alsmith@csusb.edu
AF: Department of Geological Sciences, California State University, San Bernardino, CA 92407 United States
AU: Roobol, M J
EM: alsmith@csusb.edu
AF: Saudi Geological Survey, P.O Box 54141, Jeddah, 21514 Saudi Arabia
AB: Soufriere volcano on the island of St. Vincent is a complex strato-volcano that has been active since at least the Late Pleistocene. Stratigraphic and granulometric studies have been carried out on three pyroclastic sequences from this volcano: 1) Deposits from Late Pleistocene eruptions. These deposits, which range in thickness from 50m in the north east to 2 m in the south and cover about 55% of the island, form a complex stratigraphy produced by a variety of eruptive styles including Plinian, Vulcanian and Strombolian; 2) Deposits from the 1902 eruption. These were studied in a series of newly excavated pits near the Wallibou river on the west coast. The stratigraphy of these deposits indicate an initial sequence of surges and flows overlying a well developed paleosol in the deposits from the 1812 eruption, followed by many thin fall beds. Both fall and flow deposits are composed of a combination of vesicular juvenile clasts, lithic fragments (altered and unaltered), juvenile crystals , accretionary lapilli and accreted grains. Although the abundance of accretionary lapilli, and accreted grains in most deposits indicates a significant hydrologic component throughout the eruption, the variations in their amounts between the different beds suggest that the water-magma contact fluctuated significantly during the eruption; 3) Deposits from the 1979 eruption. These are dominantly fall deposits with abundant accretionary lapilli, plus minor pyroclastic flows and surges. The fall deposits, which can be subdivided into a lower blue-grey (explosive events of April 13 and 14) and an upper yellowish brown unit (explosive events April 17-26), generally show a bimodal grain-size distribution with the secondary coarse mode usually produced by the abundant accretionary lapilli. The earlier eruptions (April 13-22) all had a general E-W distribution, in contrast the maximum distribution for the April 26 eruption was NW-SE. The presence of abundant uncharred wood and cauliflower-surfaced bombs in the pyroclastic flows, and the abundance of accretionary lapilli together with the fine-grained nature of the fall deposits all suggest that the eruptions were phreatomagmatic in origin. The apparent absence of juvenile fragments in the lower pyroclastic flows suggests that the initial eruptions represented the explosive shattering of the 1971 dome that existed in the crater prior to the 1979 eruption.
DE: 8455 Tephrochronology (1145)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005