The structural and metamorphic history of the Taconic unconformity in western Massachusetts: Discussion

Abstract

Uzuakpunwa and Brownlow (1978) have presented an interpretation of a pre-Taconic angular unconformity in western Massachusetts that disagrees significantly with our published conclusions that we derived from abundant detailed mapping in same area. Their statements and interpretations appear to disregard data that we published, and we cannot agree with their arguments for a preTaconic unconformit y. Uzuakpunwa and Brownlow include in their paper a regional map of western Massachusetts at a scale of 1:500 000 (their Fig. 1) and a more detailed map of parts of Blandford and Woronoco quadrangles at a scale of 1:48 000 (their Fig. 2). Nowhere do they indicate how much mapping they have done, in which areas, nor at what scale. Our mapping was done at a scale of 1:24 000 for all area of their Fig. 1 that pertains to their discussion and was available as quadrangle maps (Hatch and Hartshorn 1968; Chidester et al. 1967; Osberg et al. 1971; Hatch 1969; Hatch et al. 1970; Hatch and Stanley 1976) or as maps in other papers (Stanley 1967,1975; Hatch and Stanley 1973). Uzuakpunwa and Brownlow's maps differ in some important details from ours. They indicate (their Fig. 1) that occurs at a unique stratigraphic horizon and is nearly continuous across Massachusetts from Connecticut to Vermont. Our mapping shows that Chester is a lens which can be traced for only 10 km along strike, and that no amphibolitelgreenstone extends continuously across breadth of state. The amphibolites/greenstones that Uzuakpunwa and Brownlow included in their Chester Amphibolite vary in composition and bedding characteristics, and have very different stratigraphic positions with respect to other mapped lithologic members of Rowe Schist. Uzuakpunwa and Brownlow stated (pp. 1941, 1945) that Chester Amphibolite forms a basal unit to Moretown Formation. Our mapping shows that rocks above and below type Chester body are lithically identical and that distinctively thin bedded, more granulose rocks characteristic of Moretown Formation are not present below a horizon 200-300 m stratigraphically above Chester. Where Chester thins to a feather edge, schists that are no longer separated by it are indistinguishable. Uzuakpunwa and Brownlow asserted (p. 1943) that the Rowe Schist is generally exposed west of western Massachusetts ultramafic belt and infer that this relationship has stratigraphic importance. Our published maps show that ultramafic bodies are found at all stratigraphic positions between base of Rowe Schist and lower part of Moretown Formation (see maps and reports listed above). The stratigraphically lowest body of ultramafic rock within lower Paleozoic section is 200 m above Hoosac-Rowe contact in Rowe quadrangle (Chidester et al. 1967). Uzuakpunwa and Brownlow have ignored stratigraphic redefinitions proposed by Hatch et al. (1966), and have instead adopted a stratigraphy similar to that of Emerson (1898) without any discussion or documentation of their reasons for so doing. Although Emerson (1898) originally mapped Chester Amphibolite as a single horizon across Massachusetts, on his final (Emerson 1917) presentation of this geology, he showed Chester as being discontinuous, but showed separate formations above and below Chester (horizon). We