Pre-Silurian Stratigraphic Succession in Central Western Maine
Three major pre-Silurian stratigraphic divisions are recognized in the rocks of west-central Maine: Division I, Precambrian crystalline rocks of the Chain Lakes massif; Division II, Cambrian(?) to Lower Ordovician(?) eugeosynclinal rocks; and Division III, Middle Ordovician to Upper Ordovician(?) eugeosynclinal rocks. The rocks are sparsely fossiliferous and are cut by four plutonic series, the oldest of which predates the Highlandcroft Plutonic Series. Division I consists of massive-bedded granofels and thin- to massive-bedded gneiss, metavolcanic rocks, schist, metasandstone, and amphibolite (~3,100 m). The granofels and gneiss are characterized by an unusual fragmental texture w h o se origin is obscure. Division II has four lithologic units that represent a transition from the sedimentary and volcanic components of an ophiolite sequence in the lower part to a flysch sequence in the upper part. The four units, in ascending stratigraphic order, are (1) greenstone displaying relict pillow structure, amphibolite, keratophyric metavolcanic rocks, and iron formation (~1,500 m); (2) quartz-rich metaquartzwacke and metagrawacke, magnesian metapelite, and greenstone displaying relict pillow structure (~1,100 m); (3) dark sulfidic interbedded metapelite, metagraywacke, feldspathic metasand-stone, and quartzite (~600 m); and (4) green and red magnesian metapelite, metagraywacke, and quartzite, all with characteristic pinstripe lamination (> 800m).
Rocks in New Hampshire equivalent to those of Division II are intruded by the Highlandcroft Plutonic Series, which is similar to the Attean Quartz Monzonite, provisionally dated at about 470 m.y. The constancy of facing directions at contacts between units has made possible the definition of major fold axes and faults, and the definition of these structures has enabled us to make reasonable estimates of regional stratigraphic thicknesses. The stratigraphic succession established in this paper is believed to be reasonably comparable to ophiolite-flysch successions indicated from geophysical data and drilling in modern oceanic environments. If the comparison is valid, then the process of generating oceanic crust can be projected back to early Paleozoic time in this region.
Units 3 and 4 of Division II have been traced directly to the Maine-New Hampshire border, where they are continuous with the Dixville and Albee Formations. This correlation involves either inversion of part of the section or overthrusting from either the southeast or northwest of the section where units 3, 2, and part of 1 would be sturcturally superposed on 4. Division III, which has been described elsewhere, probably conformably overlies unit 4 in the southwest part of the region.