THE PALEOECOLOGY AND DEPOSITIONAL SETTING OF A SILURIAN MIXED SILICICLASTIC SYSTEM: RIPOGENUS DAM, MAINE

The Silurian Ripogenus Formation consists of shallow marine siliciclastics and fossiliferous carbonates. It is located on the Caribou Lake Anticline in north-central Maine, and is truncated to the east by the Katahdin Pluton. It is in contact with Dry Way Volcanics. The best exposure of the Ripogenus Formation lies at the southeastern end of Ripogenus Lake, in a gorge at the base of Ripogenus Dam, 28 miles northwest of Millinocket. Although the formation has been noted previously to contain abundant shallow marine invertebrates, few paleontological studies have been conducted. This project evaluated the fossil assemblage and unique lithology of the Ripogenus Formation.

The most distinctive lithostratigraphic interval is a ~15 m section of alternating sandstone and limestone beds, typically 10 to 40 cm and 5 to 20 cm in thickness, respectively. The limestones have been weathered heavily, and exhibit a conspicuous pitted appearance. The fauna is restricted to these limestones and consists of benthic invertebrates associated with reef environments. These include framework builders such as stromotoporoids and Halysites, as well as, reef dwellers such as bryozoans, pentamerid brachiopods, and Archaegastropods. The fossils are found both in situ and organically bound as allochthonous elements within the carbonate unit.

The relative abundance and specific composition of the assemblage strongly resembles a Silurian reef community. A paleoecological analysis of the assemblage confirms the presence of a protected reef-flank or proximal-reef community subject to cyclical sea-level fluctuations with changes in sedimentation processes.