Bedrock geology of the Grand Lake area, Aroostook, Hancock, Penobscot, and Washington counties, Maine

Abstract

The Grand Lake area in eastern Maine generally has low relief and contains widespread glacial drift and many lakes and swamps. It is underlain chiefly by metasedimentary rocks of Silurian and Silurian (?) age and to a lesser extent by rocks of Ordovician and possible Cambrian age. Intrusive rocks of Devonian age underlie more than a third of the area. Northeast-trending grabens in the metasedimentary and igneous rocks have preserved unmetamorphosed rocks of Late Devonian through Pennsylvania!! age. A diabase dike, possibly 100 miles long, trends southwest; it is younger than the Devonian granitic intrusives.The predominant metasedimentary rocks are metasiltstone, impure quartzite, slate, and quartzite metaconglomerate. Thin units of rhyolitic metatuff, thin black metachert, limestone metaconglomerate, phyllite, and schist are present. The metasedimentary rocks are isoclinally folded; dips are steep in places beds are overturned and folds trend northeast. Most major folds plunge to the northeast. The total thickness of the stratigraphic section is estimated to be at least 10,000 feet.The plutonic rocks are chiefly quartz monzonitic granite, but a large body of gabbro-diorite is present in the southeastern part of the area. The contactmetamorphic aureoles are three-fourths of a mile or less in width and, compared with adjacent wallrock, are characterized by retrograded cordierite, traces of chalcopyrite, increased biotite content, magnetite, and small amounts of iron sulfides. Ghiastolite and sillimanite also occur in the gabbro-diorite contact aureole.Faults of two ages, pre-Devonian and post-Pennsylvanian, are probably present. Pre-Devonian faults trend northeast, and post-Pennsylvanian faults trend northeast and northwest.