Abstract

Strata containing abundant authigenic feldspar, which heretofore have been considered rarities in the geologic column, now appear to be characteristic of the Cambrian-Ordovician rocks that formed on the shelf of the proto-Atlantic Ocean. They crop out from the mid-continent of North America to the present Appalachian Mountain chain and along the paleo-shelf edge through Newfoundland, Scotland, and Greenland. Abundant authigenic K-feldspar deposits in Cenozoic alkaline lake beds of the western United States are known to have formed from tephra by replacement of an intermediate zeolite stage. The presence of some volcanic rocks of probable Cambrian and Early Ordovician age in New England suggested the possibility that the authigenic K-feldspar of the shelf rocks is the only remaining evidence of older tephra. Chemical analyses of over 1,300 carbonate rock samples of Cambrian to Devonian age from the shelf in New York State indicate that Cambrian and Lower Ordovician strata contain 7 times more normative K-feldspar than the younger strata. Petrographic study utilizing cathodoluminescence of the Tribes Hill Formation of Early Ordovician age shows that the K-feldspar is not entirely authigenic but rather, is composed of subequal amounts of detrital K-feldspar as grain cores and authigenic K-feldspars as overgrowths. Detrital quartz always is associated with the K-feldspar. A portion of the Canadian Shield near the present New York area was exposed during the Cambrian and Early Ordovician and probably served as the source for the K-feldspar and quartz detritus deposited on the shelf during that time. Tephra may have provided potassium, aluminum and silicon ions to the formation of the abundance of K-feldspar overgrowths, although no direct evidence was found. By Middle Ordovician time that part of the shield which served as the source of detritus was submerged, and the supply of detrital K-feldspar necessary for the growth of authigenic K-feldspar ended.

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