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Mount Sanford hyaloclastite (Holocene and Pleistocene). Unconsolidated, crudely bedded, coarse-sand- to silt-sized volcanic fragments that consist mostly of black to dark-gray, locally palagonitized, angular volcanic glass. Hyaloclastite contains fragments and blocks of andesite that range from a few centimeters to as much as 100 m in diameter. Smaller fragments mostly of aphyric andesite exhibit glassy rinds, radiating fractures, and breadcrust textures typical of pillows and bombs. Larger blocks (b), which invariably display very intricate columnar joints, consist of coarsely porphyritic andesite that contains phenocrysts of plagioclase (10-15%, as long as 2 cm), hypersthene (2-4%, as large as 2 mm), and olivine (trace) that exhibits hypersthene reaction rims, in intersertal groundmass of plagioclase, mafic minerals, glass, and opaque minerals. * * * Deposits apparently result from almost complete fragmentation of at least two physically distinct flows upon contact with water or ice. Field evidence, however, is equivocal in deducing relative age of deposit. As shown on map sheet, hyaloclastite partly fills present Sanford Glacier valley and, therefore, may be as young as Holocene; alternatively, hyaloclastite is possibly much older and underlies or is interbedded with Sanford lava flows (Qsl). Maximum thickness about 220 m. |