Leslie Kanat, Professor of Geology Johnson College, Johnson, VT.
Sunday, August 1, 2010. 9AM, 9:30AM & 10:00AM
Starting and Ending Location: Convention campground (Champlain Valley Fairgrounds).
Cost: $35 (includes lunch)
General Geologic Setting
Vermont’s landscape records more than one billion years of geologic history that is represented by rocks formed in a variety of depositional, metamorphic,and tectonic environments. Glacial ice and meltwater sculpted, eroded and modified the underlying bedrock. Water, wind and human actions continue to alter the VT landscape. Vermont’s geologic resources include glacial sand and gravel deposits, rock and mineral resources, and the spectacular landscape and natural systems used by the tourism industry.
Vermont is situated along the western half of the Northeast trending Appalachian Mountain system and east of the Adirondack Highlands of New York. The rocks record a highly varied depositional sequence of rift clastic and volcanic rocks, a composite continental platform which marks the ancient margin of eastern North America in the Cambrian and Ordovician, and a variety of depositional units associated with the collapse and destruction of the ancient margin during the Ordovician Taconic Orogeny. Subsequent deformations in the Devonian and later events, add complexity to the geologic history. During the Quaternary, VT was covered by glacial ice and the landscape was scoured. The period of deglaciation associated ice‐dammed lakes, variations in lake levels, and variations in flow of meltwater is recorded by deposits of glacial till, sands, lake‐bed clays, depositional features such as moraines and kame terraces, and erosional features such as broad river valleys and narrow gorges.
Vermont has a variety of rocks that are valued from both a scientific and economic standpoint. We have some outstanding fossil locations in the Champlain Valley and a beautiful natural setting in which to observe and work with our natural resources. Rock and mineral‐based industries of both historic and economic importance include abundant quality talc, slate, marble and granite, as well as a variety of ores and precious metals extracted during the 19th and early 20th centuries. Sand and gravel deposits, primarily a result of the Pleistocene glacial history of the state, are also marketable resources. The VT State Rocks, granite, marble, and slate, plus the State Mineral, talc, are activelyquarried in VT. Marble, a metamorphic rock composed principally of calcite, is found in western VT. The marble in VT formed by metamorphism of Cambrian to Ordovician age limestones. Marble from Vermont’s Danby quarry was used for the Thomas Jefferson Memorial in Washington, D.C., the United Nations Building in New York, and the Chiang Kai‐Shek Memorial in Taiwan. Most marble currently quarried in the state is crushed and used as filler in paint, paper, and plastic. Slate, also a metamorphic rock, is quarried insouthwestern VT. Slate is a very fine‐grained rock composed mainly of quartzand mica. VT slates formed about 475million years ago during the Taconic Orogeny. Vermont slates are generally black, green, purple ormottled, depending on the amount of chlorite (green) and iron (red to purple). Slate is on the roof of the VT State House and used for flooring and as a crushed product. Granite, an igneous rock, occurs as small to large plutons in eastern VT. Most granite in VT is partof the New Hampshire Plutonic Series and is Devonian age, making it quite a bit younger than the slates and marbles of western VT. The granite from Barre is world famous for its use as monumentstone, building stone, and stable bases for the high‐tech industry. Glacial deposits have provided Vermonters with raw material for stone walls and foundations. Sand and gravel deposits, found along valley floors and walls,are used for road and building construction. The flat valley bottoms, sites of rich agricultural land, are the traditional town population centers in VT.
The field trip will begin on the upper plate of the Champlain Thrust in Essex Junction, VT. From Essex the trip will cross the Champlain Thrust in Colchester slightly east of Sand Bar Bridge then traverse lower plate Ordovician shales and carbonates on the way to Isle LaMotte. The Champlain Thrust extends from Canada south to the Catskill Plateau in New York, a distance of approximately 320 kilometers. The thrust is a Middle Ordovician, east‐dipping fault along which older Cambrian rocks were placed on top of younger Ordovician rocks. Stanley (1987) estimated a maximum throw of 2700 meters and a displacement of 55 to 80 kilometers along the fault.
Goodsell Reef Preserve of the Chazy Reef, Isle La Motte, VT: The exposed limestones of the Goodsell Reef contain stromatoporoids, gastropods, and cephalopods and record several million years of evolutionary history. The significance of recognizing reefs or mounds of Ordovician age in VT is great. First, the occurence of reefs in modern environments at 20-30 degrees parallel to the equator provides a strong paleo-latitudinal control for this portion of VT 480 million years ago. Second, the narrow bathymetric control on modern reefs provides us with a constraint on water depth for these rocks. Third, the detailed orientation of the mounds (roughly aligned in a north‐south belt usingcurrent geographic coordinates) constrain the local paleogeography; the Iapetus Ocean was to the east, Laurentia lay to the west, and this region sat on the platform margin bordering deep water. In 2009 the Chazy Fossil Reef, as it occurs on Isle La Motte and on Valcour Island in Lake Champlain, was awarded the designation of National Natural Landmark by the National Park Service of the US Department of the Interior. The quarried limestone was used in Radio City Music Hall. Removing any rocks or fossils from the Quarry is strictly prohibited. The Goodsell Reef Preserve is an 81 acre site managed by the Isle La Motte Preservation Trust. (Contact information: http://www.anr.state.vt.us/DEC/GEO/chazytxt.htm)
The Beam, South Hero Island, VT: Stanley (1990) describes “The Beam” as an outcrop of a thin (30 cm) bed of micrite (finely crystalline limestone) surrounded by thicker sequences of well‐cleaved, calcareous shale in the Ordovician flysch of western VT The outcrop is part of the Cumberland Head Formaton (middle Ordovician age). This sequence records a complex history of imbricate faulting and associated folding. The micrite was shortened by low angle, imbricate thrust faulting andthe calcareous shale has been shortened by pressure solution. Crosscutting relations among the floor, ramp, and roof faults indicate that faulting progressed from west to east . All fault surfaces are covered by sparry calcite; most fractures are also filled with sparry calcite. Layer parallel shortening in both rock types is between 11 and 16%. Five imbricate thrust faults and associated ramps are exposed in the 30 cm thick bed of micrite that extends approximately 14 meters. The dominant cleavage dips steeply to the east. The outcrop scale foreland deformation observed at this location may provide insight into the processes and mechanics involved in larger foreland settings around the world.
Mt. Philo State Park, Charlotte, VT: Mt. Philo, Vermont’s first state park, provides a spectacular view to the west ofthe Lake Champlain valley and Adirondack Mountains of New York; the views from the peak (295 meters) are exceptional. The Adirondacks represent a 1300 m.a. massif of metamorphic and igneous rocks that are overlain by younger (Cambro‐Ordovician) sediments that dip gently east toward Lake Champlain. The isolated mountains (north to Pease Mt. and Jones Hill; south to ShellhouseMt., Buck Mt. and Snake Mt.) that are visible from this location help identify the location of the Champlain Thrust fault. These mountains are erosional remnants of the Monkton Quartzite. Much of the landscape has developed due to glaciation. The heavily glaciated terrain has left behind rounded, striated, and grooved bedrock. Ice dams formed from the melting glaciers about 12,000 years ago. As the glaciers retreated from VT, Mt. Philo was an island in an inland sea--marine sediments are common at the base of Mt. Philo. The VT State Fossil, the Charlotte Whale, is believed to have died in a shallow marsh of the Champlain Sea and covered by fine clay sediment. In 1849, while constructing the first railroad between Rutland and Burlington, a workman uncovered the bones of a whale (Howe 1993). The skeleton that was eventuallypieced together canbe seen at the Perkins Museum of Geology at the University of Vermont in Burlington, VT. The White Whale, Delphinapterus leucas, is one of 17 found in the Pleistocene deposits of Ontario, Quebec, and VT. The Cambrian (550 m.a.) sedimentary rocks of the Champlain Valley were deposited in shallow water along the continental margin of the Iapetus Ocean (a precursor to the Atlantic Ocean). The rock at the summit of Mt. Philo is the mid‐Cambrian aged Monkton Quartzite. Most of the rounded grains in the reddish‐brown Monkton Quartzite are terrigeneous, fine to medium‐granted, quartz and feldspar grains, ripple marks,cross‐bedding, mud cracks, and laminated beds provide additional evidence for ancient tidal flat deposits. The units are commonly interbedded with a pale yellow weathering dolostone (Gale 2007). Redstone Campus and other buildings in Burlington were built using the Monkton Quartzite. The rocks at the summit of Mt. Philo are older than the rocks in the valley below. The rocks were lifted up along the upper plate of the Champlain Thrust Fault that runs in a north‐south direction for at least 120 km along the western edge of VT. This thrust placed Cambrian rocks on top of Ordovician rocks.
Middlebury Quarry Geology
The marble layers quarried at the Middlebury Quarry are composed of the Columbian member of the Shelburne Formation. These north-south striking layers are located on the eastern limb of the Middlebury Synclinorium and locally are near vertical to slightly overturned. The Columbian member is bounded to the west by the blue-grey Cutting Formation which is exposed in the northwest (West Blue Contact) and southwest corners of the quarry. Exposed along the east side of the quarry is the grey to buff Intermediate Dolomite member of the Shelburne Formation. The Columbian member can be further separated into two distinct submembers. The Upper Columbian west layers are composed of white marble with interbedded greenish grey schistose layers. The Lower Columbian east layers are composed of light grey to white marble with relatively few schistose layers. A dolomite bed also snakes discontinuously through the lower (eastern) portion of the Lower Columbian layers.
The Shelburne Formation is part of a broader sequence of carbonate sediments deposited in the vicinity during the early Ordovician. The Taconic Orogeny metamorphosed, deformed and uplifted these rocks during the late Ordovician and subsequent deformations such as the Acadian Orogeny further metamorphosed these rocks. Finally, several hundred million years of erosion capped by Pleistocene glaciation ground these rocks down perhaps a couple miles to their present level. At the Middlebury Quarry, glaciation is evidenced by the widespread glacial polish on the marble surface and glacial till mantling the bedrock. Overlying the till, thin and discontinuous varved silts and clays with imbedded drop stones provide evidence for glacial Lake Vermont inundating this location at the end of the Ice Age.
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