Origin and Weathering of Late Pleistocene Ash Deposits on St. Vincent, B.W.I

Abstract

The island of St. Vincent consists of an active volcano and the remains of two or more that are extinct. Late Pleistocene subaerial ash deposits forming the uppermost geologic unit over much of the island are the object of the present study. Most of the deposits are between 20 and 40 feet thick, but they reach a maximum thickness of at least 120 feet. They consist of andesitic lithic, crystal, and vitric material or their weathered equivalents. Plagioclase ($$An_{50-100}$$), hypersthene, augite, and olivine are the most abundant minerals. The ash was erupted from the Soufrière. The pattern of grain size and thickness suggests that it was distributed by northwesterly Pleistocene winds quite different from those of the present. Four lithologic fades have been separated on the basis of sorting, grain size, and degree of weathering-namely, the Soufrièreslope, eastern-coastal, western-coastal, and upland facies. Differences between them can be explained by primary distribution of the ash and subsequent removal of the fine material on steeper slopes by sheet wash. The facies of ash therefore reflects the topography of the underlying land surface. Soils developed and were buried while the ash accumulated to form a sequence of fossil soils. The vitric material and much of the plagioclase in the ash have been weathered to halloysite$$\cdot 4H_{2}O$$, and much olivine has been weathered to iddingsite. All the pyroxene and many of the olivine and plagioclase crystals have been etched by solution. Fine vitric ash was altered first, then plagioclase of composition ($$An_{80-100}$$). Olivine of composition $$(Fo_{74})$$ was the first ferromagnesian mineral to alter and was followed by more magnesian olivine ($$Fo_{78-85}$$), hypersthene, and augite, all of which were destroyed at about the same rate. Hornblende weathered more slowly. Plagioclase phenocrysts in pumiceous fragments altered more rapidly than did discrete crystals of the same composition. Minerals are more etched and otherwise altered in poorly drained clayey beds than in permeable beds, probably because the poorly drained beds provide a moist soil environment. Initial permeability differences in newly deposited ash seem adequate to account for differences in the degree to which the vitric ash has been altered to clay.