Bedrock Geology of the Katahdin-Traveler Area, Maine

Rankin, Douglas

Cambrian or Ordovician, Ordovician, Silurian, and Devonian volcanic and elastic sedimentary rocks, with an aggregate thickness of more than 23,000 feet, underlie about 65 percent of the Katahdin-Traveler area, i-1alne. The stratified rocks are in the chlorite zone of regional metamorphism a.'1.d were deformed during the Middle or Late Devonian Acadian orogeny into a r:iajor southwest plunging synclinorium, the Matagamon synclinorium. The deformed stratified rocks have been intruded by the post-tectonic Katahdin granite, probably of Late Devonian age. The oldest rocks in the area are tightly folded varicolored slates, quartzites, and grits of Cambrian or Ordovician age. These are overlain with probable unconformity by Middle Ordovician olivine basalt and the basalt is, in turn, unconformably overlain by coarse elastic rocks of Early Silurian age. Practically the only carbonate rocks in the area are thin (300+- feet) carbonate-rich siltstones of Middle Silurian age. This study is focused upon the Lower Devonian rocks, particularly upon the Traveler felsite. The oldest Devonian rocks are a monotonous series of graded beds of dark-colored slate and minor sandstone of the Seboomook formation. These are overlain by about 4500 feet of rusty weathering, cross-laminated sandstone and minor slate of the Matagamon formation. The Traveler felsite covers about 70 square miles and has an estimated volume of about 70 cubic miles. It is the largest of a chain of Lower Devonian felsite bodies in central Maine, and one of the largest bodies of extrusive felsites in the world. The felsite has been divided into a lower Pogy member, consisting largely of welded tuff, an upper Black Cat member, consisting largely of lava. Typically the Pogy member contains quartz, calcic andesine, and altered mafic silicate phenocrysts; the andesine has structure characteristics of the low thermal modification. Typically the Black Cat member contains calcic andesine, ferraugite, and biotite phenocrysts; the andesine has structure characteristics of an intermediate thermal modification. To a first approximation, the Traveler felsite is isochemical and is believed to have been erupted in a short interval of time. These relationships could be explained by a decrease in the volatile content of the magma during the course of the erupt ions. Because of the presence of welded tuff, it is believed that the bulk of the felsite was erupted subaerially. It is also believed that a large caldera was formed during and after the eruption of the felsite. Clastic rocks of the Trout Valley formation, the youngest stratified unit in the area, unconformably overlie the Traveler felsite. This formation includes at its base, a coarse felsite conglomerate lentil, the Gifford conglomerate lentil. Early terrestrial plant fossils occur abundantly in the formation. These, other fossils in the Trout Valley formation, and character of the rocks suggest that much of the formation was deposited in shallow brackish water, perhaps adjacent to an eroding volcanic is land. The Katahdin granite, a perthite, plagioclase, biotite granite, covers the entire southern third of the area of study. The granite exposed here is part of a large batholith, which extends to the southwest nearly 30 miles beyond the area of study. On the upper part of Mount Katahdin, the granite becomes pink, vuggy, and has a granophyric texture. This probably indicates proximity to the roof of the magma chamber. If so, the roof of the magma chamber appears to have been nearly flat in the vicinity of Mount Katahdin.