The geology of the Anson quadrangle, Maine

Thesis (Ph.D.)--Boston UniversityThe Anson quadrangle is located in west-central Maine in a region which offers complex problems in structural geology, petrology, and stratigraphy. The geomorphology is also considered in this study. The formations which make up the stratigraphy of the quadrangle include the following (from oldest to youngest): The Perry Mountain, Parmachenee, Madrid, Kennebec, and Anson formations. The Perry Mountain formation, which outcrops only in the extreme northwest corner of the quadrangle, is composed of well bedded quartzites intercalated with sandy biotite-muscovite schists and staurolite schists. The Parmachenee formation consists of thinly bedded gray phyllites, impure biotite quartzites and black limestones. The formation contains notable amounts of pyrite and pyrrhotite. The Madrid formation is composed predominantly of calcareous quartzite with lime-silicate minerals in the middle grade metamorphic zones. The formation contains only minor amounts of argillaceous rocks, and calcareous beds, and sulfides are absent or inconspicuous. The quartzites grade upward into the Dyer Hill member which is composed of chloritoid-bearing black slates. The Kennebec formation is composed of thinly bedded black sulfide-rich limestones, phyllites, and quartzites. The Anson formation consists of brown-mottled grits and arenaceous phyllites. The brown spots are caused by the oxidation of siderite. The position and formational status of the Kennebec and Anson formations are still in doubt. The Anson formation correlates on the basis of similar lithology with the Vassalboro formation in the Vassalboro quadrangle, which is Clinton in age. The edge of two intrusive bodies project into the quadrangle the Black Hill pluton and the Sandy River pluton. Only one phase is present, quartz monzonite, which varies in texture from medium to coarse grain. The rocks near the Black Hill pluton show distinct contact metamorphic affects, including the presence of andalusite. The low-grade regionally metamorphosed rocks are restricted to the south and southeast portions of the quadrangle, while the mille-grade metamorphic zone is found in the north and northwest area. There is no well-defined garnet isograd. The relationship of the siderite to the sulfides is decreased in detail. Siderite formed at an oxidation potential higher than that needed for pyrite. The sideritic rocks formed near the shore of the marine basin where more oxygen was available. The pyrrhotite in this region was considered to have been formed from pyrite under metamorphic conditions. An alternate explanation is suggested: namely that initial sulfide from which the pyrrhotite formed may have been FeS rather than FeS2. This would involve a subtraction of iron from the primitive FeS to form the pyrrhotite. The rocks containing the sulfides were formed in a strongly reducing environment, probably a barred basin with restricted circulation. The major fold structures trend northeast-southwest. The axial planes of the isoclinal folds are slightly overturned and dip to the northwest. The absence of noses of folds and the consistently steep dip of the beds may also be explained in terms of pleated folding. No large scale faulting has been observed in the quadrangle, although minor longitudinal faulting is not uncommon. Glaciation modified the pre-existing topography slightly by deposition and erosion. Glacial striations trend between S20°E and S30°E. Post-glacial geomorphic features, which include alluvial terraces, falls, ice push, discordant junctions, and sand dunes, are all described and their origins are discussed. The marine limit in the quadrangle is considered to be at least at the 400 foot elevation as evidenced by marine fossils found just south of the Anson quadrangle