SEDIMENTOLOGICAL FRAMEWORK OF THE TROUT VALLEY FORMATION, MIDDLE DEVONIAN, MAINE

The Trout Valley Formation of Middle Devonian age is located in Baxter State Park of North-Central Maine. These rocks were examined first in the late 1950s and early 1960s, and the fossil plants preserved within this sequence have provided insights into the early evolution of terrestrial ecosystems. However, to date, there has been no detailed stratigraphic and sedimentological study of the outcrop belt from which a sedimentological model for deposition could be established. In the summer of 2001, a detailed evaluation of the Trout Valley Formation was conducted. Photomosaics were taken of all exposed outcrops, samples were collected for thin-section analysis, and the sequence was described in detail.

The basal unit of the formation is a cobble-to-boulder conglomerate overlain by fining upward sequences composed of granule conglomerate fining to medium sandstone, with primary structures including channel form geometries and cross-bed stratification. Overlying lenticular fine-grained orthoquartzites are organized into hummocky (?) cross-stratified features and found beneath heavily bioturbated siltstones that display ladder and starved ripple structures, as well as vertical and horizontal trace fossils. Overlying these rocks are several cycles comprised of fine sandstone with herringbone cross-stratification overlain by rippled siltstones bearing fossil plant material. The formation previously has been interpretated as a terrestrial/brackish marsh. Based upon the present study, the section is interpreted to represent a basal braidplain, overlain by transitional to marine deposits. The fossil-plant bearing lithologies at the top are interpreted as having been deposited in an estuarine setting. This reevaluation of the Trout Valley Formation has provided new insights into the depositional systems originating from erosion of Acadian orogenic sources.