Bedrock Geology of Ripogenus Dam Maine Geological Survey Maine Geologic Facts and Localities June, 2018 Ocean crust, fossils, and an unconformity: Bedrock Geology of Ripogenus Dam 45o 52’ 54” N, 69o 10’ 35” W Text by Ian Hillenbrand Maine Geological Survey, Department of Agriculture, Conservation & Forestry 1 Bedrock Geology of Ripogenus Dam Maine Geological Survey Introduction Did you know you can visit a mid-ocean ridge, put your finger on the gap of time related to a mountain building event, and observe coral fossils at one locality in northern Maine? At Ripogenus Dam, the head of Ripogenus Gorge, ancient bedrock recording Maine’s geologic past is exposed and freed from the glacial till which typically covers it. The 92-foot high by 702-foot long dam was constructed along the West Branch of the Penobscot in 1916, damming Chesuncook Lake to provide hydroelectric power for a paper mill in Millinocket. When the dam is opened, as in 2017, it releases incredibly large amounts of water. Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 1. Chesuncook Lake viewed from Ripogenus Dam. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 2 Bedrock Geology of Ripogenus Dam Maine Geological Survey Logging History: Sluiceway The paper industry played a large role in the recent history of this region. The spruce forests along the West Branch of the Penobscot have been harvested since at least the 19th century, with logs rafted through the gorge to sawmills as far south as Bangor. Before the construction of the dam, Ripogenus Gorge was altered by log driving rivermen with dynamite and stone- filled timber cribs to prevent jamming. The hydroelectric dam was constructed in 1916 to power paper mills in Millinocket. McKay Power Station was constructed as a retrofit in 1953. The dam impounds the largest storage reservoir ever built with private funding. 2,400 cubic feet per second of the river’s flow is diverted through a mile-long penstock to the McKay Power Station. Some photographs from Maine Geological Survey construction of the dam are available online. To Hillenbrand Ian by Photo allow for the transport of pulpwood after the Figure 2. Sluiceway (in foreground) with Ripogenus dam in dam’s construction a sluiceway was installed background. and used until 1971. After this time, Great Northern Paper Company began trucking the lumber to the mill via the Golden Road. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 3 Bedrock Geology of Ripogenus Dam Maine Geological Survey Geologic Setting Ripogenus gorge lies on the east limb of the Caribou Lake Anticline, part of the Boundary Mountains-Bronson Hill Anticlinorium (Kusky, 1994). The region was first mapped by Griscom (1976), and later mapped at a more detailed scale by Schoonmaker and Kidd (2013). Ripogenus Gorge exposes Silurian to Devonian sedimentary rocks of the Ripogenus Formation which unconformably overlie the Ordovician Dry Way volcanics. These rocks are part of a sequence as old as Cambrian and as young as Devonian. Older rocks in the region include the Cambrian Hurricane Mountain and Dead River Formations (Boom House Group on the geologic map). These units are intruded by the Devonian Katahdin Granite and overlain by deep water sediments of the Figure 3. Geologic map of the Ripogenus Dam area, modified from Seboomook group. Schoonmaker and Kidd (2013). Maine Geological Survey, Department of Agriculture, Conservation & Forestry 4 Bedrock Geology of Ripogenus Dam Maine Geological Survey Geologic Setting Figure 4. The stratigraphy of the Harrington Lake Quadrangle, modified from Griscom (1976) by Kusky et al. (1994). Box indicates the units exposed here. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 5 Bedrock Geology of Ripogenus Dam Maine Geological Survey Dry Way Volcanics - Pillow lavas The greenish to dark brown massive rocks on the south side of the river are basalt, a fine-grained igneous rock largely made of the minerals feldspar and pyroxene. These particular rocks have been named the Dry Way Volcanics. Looking closely at the outcrops, you may be able to find rounded, teardrop-like shapes within the basalt. Geologists refer to these structures as pillows, and they form when hot basaltic lava is erupted underwater. For more on pillow lavas in Maine, check out the GFL Pillow Lavas I have Known. Research by Schoonmaker and Kidd (2006) shows that these rocks are geochemically similar to basalts found at a mid- ocean ridge. This means that rocks that once were at the middle of an ocean (like the mid-Atlantic ridge) are now in the middle of Maine! Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 5. Large outcrop of pillow basalt at the base of the dam. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 6 Bedrock Geology of Ripogenus Dam Maine Geological Survey Thin chert bed representing ocean deposits Within the Dry Way Volcanics, and between basalt pillows, you may find thin sections of yellow-green to purple, fine grained rock often one to two cm thick and occasionally up to 10 cm. This is a metamorphosed sedimentary rock primarily composed of silica called chert. It was deposited as sediment onto the pillow lavas while they were exposed on the ocean floor. The best exposure of this type of rock occurs right next to the concrete base of the dam and best viewed when the water discharge is low. The yellow-green and, in places, purple chert bed is up to 10 cm thick. These layers of chert are parallel with each other and close to vertical in orientation, showing the sediments were originally deposited horizontally in an ancient ocean. Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 6. Green to purple bed of chert between green basalt of the Dry Way Volcanics. Field notebook for scale. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 7 Bedrock Geology of Ripogenus Dam Maine Geological Survey Ripogenus Formation - basal conglomerate Rocks on the northern side of the river differ greatly from those on the south; these are metamorphosed sedimentary rocks, and are significantly younger (by as much as 10 million years) than the Dry Way Volcanics. These rocks are difficult to reach, and best accessed during periods of low flow. At a few locations you may observe a sharp line between the green massive basalt and a reddish rock made of pebbles and sandstone, which is the contact between the two units. Within this 1 to 3 meter thick unit are a few red pebbles of jasper, an opaque form of quartz sometimes used in jewelry. Geologists refer to the rule of inclusions, which states that the pebbles must be older than the rock they are within. A rock made of many large clasts is called a conglomerate. Geologists attribute the gap in the geologic record to uplift and erosion of the older unit during a mountain building event. Such a gap is referred to as an unconformity, and this particular unconformity is attributed to the Taconic Orogeny. Maine Geological Survey Hillenbrand Ian by Photo Figure 7. A yellow field notebook lies on top of a thin horizon of pebble conglomerate in the base of the Ripogenus Formation. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 8 Bedrock Geology of Ripogenus Dam Maine Geological Survey Quartzose and Calcerous Sandstones of the Ripogenus Formation A few meters of yellow rock sit just above the basal conglomerate. This is sandstone, mostly made up of the mineral quartz. Above these rocks, just below and above the spillway are rocks containing conspicuous rows of 10 to 20 cm thick pits (Kusky et al., 1994). These pits are the result of weathering of calcareous material within the sandstone, which erodes at a much faster rate than the rest of the sandstone. These sedimentary rock types are typically deposited in a shallow ocean (Kusky et al., 1994). Similar rocks were deposited at the same time along the Bronson Hill-Boundary Mountains Anticlinorium as far south as Connecticut and Massachusetts (Kusky et al., 1994). Maine Geological Survey Photos by Ian Hillenbrand Ian by Photos Figure 8b. Closeup of a pit in calcareous sandstone of the Ripogenus Formation. Note the white highlights on the elliptical pit where calcite, Maine Geological Survey dissolved from the calcareous rock, has been Figure 8a. Pitted sandstone (top) overlies deposited. more massive quartzose sandstone. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 9 Bedrock Geology of Ripogenus Dam Maine Geological Survey Fossils in the limestone In some calcareous pits of the Ripogenus Formation you may find fossils of corals and stromatoporoids. Stromatoporoids are a class of aquatic invertebrates that were common in the Ordovician through the Devonian. Bradley et al. (2000) recovered fossilized teeth from a conodont, a sea creature that resembled eels, in the cliffs above the dam. Conodonts are used by geologists as an important tool for dating and correlating Palaeozoic rocks around the world because of their abundance, their highly diversified and rapidly evolved morphology and their wide distribution. The conodonts recovered from the Ripogenus formation lived between 420 and 415 million years ago (Bradley et al., 2000). Figure 9b. Scanning electron microscope images of fossilized conodont teeth from Bradley et al. (2000). Samples 1 through 5 were recovered from the Ripogenus Formation. Maine Geological Survey Photo by Ian Hillenbrand Ian by Photo Figure 9a. Coral fossils in the limestone, field notebook for scale. Maine Geological Survey, Department of Agriculture, Conservation & Forestry 10 Bedrock Geology of Ripogenus Dam Maine Geological Survey Faults The bedrock around Ripogenus Dam is cut by several faults, none of which are currently active. At least three faults are in the area just below the dam. One fault, which is shown on the geologic map, is in the West Branch of the Penobscot River.