Abstract

The writer interprets the Boston Bay group as partially intertongued lithofacies of coarse conglomerate and associated volcanic rocks (Roxbury conglomerate) and finely laminated siltstone and mudstone (Cambridge “slate”). He regards the Squantum “tillite” as multiple discontinuous lenses of unsorted pebbly to bouldery mudstone of various ages interstratified within this complex. Although fossil evidence is inconclusive, stratigraphic relationships and clastic compositions suggest that these deposits were formed during middle Paleozoic diastrophism. The writer does not consider the Squantum as a temporal equivalent to southern hemisphere Permo-Carboniferous glaciation, as advocates of a glacial origin had supposed. Lack of glacial pavement and comparison of composition, graded bedding, gross stratigraphy, and tectonic setting with known submarine slide and mass-flow deposits render the glacial interpretation suspect. Origin by gravity movement of rapidly deposited, volcanic-rich sediments and periodic resedimentation by turbidity currents in a Paleozoic eugeosynclinal belt is a more plausible alternative.

Very poor sorting, striated clasts, faceting, and even some rafted erratic fragments in fine, laminated mudstones can arise from mechanisms other than glaciation. A preserved, extensive, grooved and polished pavement overlain by poorly sorted, till-like material—particularly if nonmarine—is the most compelling glacial evidence. Very large erratic boulders are suggestive of ice movement, as are abundant rafted erratic fragments in fine muds, as in the Gowganda formation of Ontario. Independent biologic or isotopic cold-temperature indicators are sorely needed to strengthen glacial interpretations. General stratigraphic relationships and tectonic setting serve as important factors in judging probability of alternate interpretations, particularly in geosynclinal sequences. From these criteria, the writer judges that only the Permo-Carboniferous glaciation in certain parts of the southern hemisphere is firmly established. The Gowganda and some other Precambrian deposits are very likely glacial, but most postulated examples must be re-evaluated.

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