Ocean crust, fossils, and an unconformity: Bedrock Geology of Ripogenus Dam

Transcription

1 Maine Geologic Facts and Localities June, 2018 Ocean crust, fossils, and an unconformity: 45 o N, 69 o W Text by Ian Hillenbrand, Department of Agriculture, Conservation & Forestry 1

2 Photo by Ian Hillenbrand 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. Figure 1. Chesuncook Lake viewed from Ripogenus Dam., Department of Agriculture, Conservation & Forestry 2

3 Photo by Ian Hillenbrand 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 stonefilled 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 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 construction of the dam are available online. To allow for the transport of pulpwood after the dam s construction a sluiceway was installed and used until After this time, Great Northern Paper Company began trucking the lumber to the mill via the Golden Road. Figure 2. Sluiceway (in foreground) with Ripogenus dam in background., Department of Agriculture, Conservation & Forestry 3

4 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 Seboomook group. Geologic Setting Figure 3. Geologic map of the Ripogenus Dam area, modified from Schoonmaker and Kidd (2013)., Department of Agriculture, Conservation & Forestry 4

5 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., Department of Agriculture, Conservation & Forestry 5

6 Photo by Ian Hillenbrand 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 midocean 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! Figure 5. Large outcrop of pillow basalt at the base of the dam., Department of Agriculture, Conservation & Forestry 6

7 Photo by Ian Hillenbrand 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. Figure 6. Green to purple bed of chert between green basalt of the Dry Way Volcanics. Field notebook for scale., Department of Agriculture, Conservation & Forestry 7

8 Photo by Ian Hillenbrand 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. Figure 7. A yellow field notebook lies on top of a thin horizon of pebble conglomerate in the base of the Ripogenus Formation., Department of Agriculture, Conservation & Forestry 8

9 Photos by Ian Hillenbrand 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). Figure 8a. Pitted sandstone (top) overlies more massive quartzose sandstone. Figure 8b. Closeup of a pit in calcareous sandstone of the Ripogenus Formation. Note the white highlights on the elliptical pit where calcite, dissolved from the calcareous rock, has been deposited., Department of Agriculture, Conservation & Forestry 9

10 Photo by Ian Hillenbrand 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 9a. Coral fossils in the limestone, field notebook for scale. 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., Department of Agriculture, Conservation & Forestry 10

11 Photo by Ian Hillenbrand 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. Weakness in the rock created by motion along fault increased erosion, leading the course of the river to follow the line of the fault. Schoonmaker and Kidd indicate that this fault continues under the dam into Chesuncook Lake. Two other faults cut the Dry Way Volcanics, creating a V- shape. Figure 10. Annotated image showing several faults, as seen from the top of Ripogenus Dam., Department of Agriculture, Conservation & Forestry 11

12 Geologic History: Tectonic Setting of Dry Way Volcanics Schoonmaker and Kidd (2006) suggest that the Dry Way Volcanics were erupted at a mid-ocean ridge that in an active tectonic setting. They suggest that the direction of subduction was to the northwest (modern day coordinates), and the plate on which the Dry Way Volcanics was erupted onto was being subducted underneath the Chain Lakes microcontinent. Figure 11. Schematic cartoons of the Early to Middle Ordovician Taconic ocean illustrating: (A) Ridge subduction beneath Chain Lakes Massif (s.l. sea level), and (B) relationship of Chain Lakes microcontinent to pre-taconic Laurentian margin and Taconic arc. From Schoonmaker and Kidd (2006)., Department of Agriculture, Conservation & Forestry 12

13 Geologic History: The Angular Unconformity The gap in time during the Ordovician or Silurian between the eruption of Dry Way volcanics and start of the deposition of the Ripogenus Formation is seen in rocks of similar age in northern Maine. Schoonmaker et al. (2011) propose that the Dry Way volcanics and older rocks had amalgamated to the Chain Lakes microcontinent onto the continental margin of North America in the late Ordovician or Silurian. The collision resulted in the uplift of the Dry Way Volcanics and associated rocks causing erosion and therefore a break in the rock record. Figure 12. Schematic geologic model of the Taconic Orogeny in the Ordovician as the Chain Lakes microcontinent (Dashwoods) collides with the margin of North America. From van Staal et al. (2009)., Department of Agriculture, Conservation & Forestry 13

14 Geologic History: Deposition of the Ripogenus Formation The Silurian Ripogenus Formation indicates shallow water deposition in an initially near-shore, overall shallow marine environment during late Silurian to early Devonian time (Kusky et al., 1994; Schoonmaker et al., 2011). Following the deposition of the sequence exposed at Ripogenus Dam the water depth increased significantly and rapidly (Schoonmaker et al., 2011). Some of the faults that cut the Ripogenus Formation and Dry Way volcanics may have developed at this time (Schoonmaker et al., 2011). Figure 13. Schematic paleogeographic map of Maine and neighboring regions showing the deposition of carbonate rocks of the Ripogenus Formation in a far foreland basin, distant from the front of the Acadian Orogeny. From Bradley et al. (2000)., Department of Agriculture, Conservation & Forestry 14

15 Geologic History: Acadian Orogeny The Acadian Orogeny was a major mountain building event that affected the Ripogenus area, caused by the collision of the Avalon microcontinent with North America. The orogeny had three major effects: 1) depositing deep water sediments of the Seboomook Group in much of the surrounding area, 2) deforming and metamorphosing the Ripogenus Formation, and 3) leading to the extrusion of the Traveler Rhyolite and intrusion of the Katahdin Granite. Bradley et al. (2000) constrained the Acadian Orogeny to have affected the region between 406 and 408 million years ago, but the exact tectonics of the Acadian are still debated by geologists because of the complex geology and general lack of bedrock exposure. Four possible tectonic scenarios of the Acadian Orogeny, presented by Schoonmaker et al. (2001), are (A) east-dipping subduction, (B) west-dipping, Laramide-style shallow subduction, (C) west-dipping subduction with embryonic back-arc basin, and (D) two opposing subduction zones similar to current subduction in the Molluca Sea. Figure 14. Tectonic models presented by Schoonmaker et al. (2011) showing for the onset of the Acadian orogeny in north-central Maine., Department of Agriculture, Conservation & Forestry 15

16 Google Maps (5/1/2018) Directions Take exit 244 from I-95 towards Medway/Millinocket. Follow ME-157, Millinocket Rd and Golden Rd for approximately 41 miles. Take a right turn at the fork, continue past the cabins and park on the spaces on the south side of the dam. The gorge can be viewed from walking out on the dam and looking down onto the outcrops. There is also a trail back along the gravel road to the east about 100 meters to the first bend. From there turn left and walk down the track going down the wooded slope to the north. You can venture onto the outcrops below the dam. If you do this, be aware that it is possible for a remotely controlled opening of some of the dam sluice gates to occur. In the case of this event, a warning is supposed to be given through the sirens mounted at places on and near the dam and the powerhouse downstream. If these sound, depart for higher ground immediately! For most locations in the gorge, this will mean moving to the south where the access road will be encountered. Also, the traverse of the gorge can be treacherous and physically demanding. Care should be exercised while in the gorge, especially when the rocks are wet. N 5 miles Figure 15. Location of Ripogenus dam in north-central Maine., Department of Agriculture, Conservation & Forestry 16

17 References and Additional Information Bradley, D. C., Tucker, R. D., Lux, D. R., Harris, A. G., and McGregor, D. C., 2000, Migration of the Acadian Orogen and foreland basin across the northern Appalachians of Maine and adjacent areas: United States Geological Survey Professional Paper 1624, 49 p. Griscom, A., 1976, Bedrock geology of the Harrington Lake area, Maine: [Ph.D. thesis], Harvard, Cambridge, MA, 373 p. Kusky, T., Bradley, D., Winsky, P., Caldwell, D., and Hanson, L., 1994, Paleozoic stratigraphy and tectonics, Ripogenus Gorge and nearby areas, Maine, in L. Hanson (ed.), Guidebook to field trips in North-Central Maine: New England Intercollegiate Geological Conference 86th Annual Meeting, trip C1, p Schoonmaker, A. and Kidd, W.S.F., 2006, Evidence for a ridge subduction event in the Ordovician rocks of north-central Maine: Geological Society of America Bulletin, v. 118, p Schoonmaker, Adam, Kidd, William S. F., Reusch, Douglas N., Dorais, Michael J., Gregg, Thomas, and Spencer, Christopher, 2011, Stratigraphic context, geochemical, and isotopic properties of magmatism in the Siluro- Devonian inliers of northern Maine: implications for the Acadian orogeny: American Journal of Science, v. 311, p Schoonmaker, A., and Kidd, W.S.F., 2013, Tectonic significance of Cambro Ordovician and Siluro Devonian stratigraphy and magmatism in the Chesuncook Lake and Ripogenus Gorge area, north-central Maine, Field trip A-4, in Hanson, L.S., ed., Guidebook for field trips in Central Maine: New England Intercollegiate Geological Conference 105th Annual meeting, Millinocket Lake,Maine, October 2013, Salem State University, Salem, MA, p van Staal, C. R., Whalen, J. B., Valverde-Vaquero, P., Zagorevski, A., and Rogers, N., 2009, Pre-Carboniferous, episodic accretion-related, orogenesis along the Laurentian margin of the northern Appalachians, in Murphy, J. B., Keppie, J. D., and Hynes, A. J., editors. Ancient orogens and modern analogues: Geological Society, London, Special Publications, v. 327, p , Department of Agriculture, Conservation & Forestry 17