Abstract

Isolated lenses of breccia within portions of the Potsdam Sandstone in northwestern New York State provide evidence for syndepositional silica cementation in a probable subaerial environment. The breccias consist of a chaotic arrangement of angular to subrounded pebble- to cobble-size clasts of highly indurated, silica-cemented quartz arenite lying in a matrix of less well-cemented, clay and iron oxide-rich quartz sandstone. The lenses of breccia are small (0.6-1.2 m lateral extent, 0.3-0.6 m in thickness) and lie on top of, and are in turn overlain by, "normal" horizontally-bedded or crossbedded sandstones. The breccias described indicate the local breaking up of a well-cemented surface layer of quartz sand, and subsequent foundering of the clasts into underlying layers of uncemented, clay- and ferric oxide-rich quartz sand. The surficial silica cementation probably took place in a subaerial environment similar to that described by Smale (1974) and Williamson (1957) for the silcrete soil horizons of semi-arid areas in South Africa and Australia. Recognition of silcrete brecciation within the Potsdam provides evidence for deposition of at least some of these sands in a very shallow water environment, where subaerial exposure was common.

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