GEOLOGY OF VERMOI'IT

Brewster Baldwin Middlebury College Middlebury, VT 05753

EARTH SCIENCE

Fall 1982 V. 35, fl. 3

copyright ® 1982

American Geological Institute geology of Vermont

ermont is a very small state, but its the present level of Lake Champlain. V geology is remarkable in variety However, as meltwater from the conti- and character. The surficial geology is nents flowed back into the oceans, sea the story of deglaciation, and the bed- level rose and water flooded back to rock evolution is the result of the open- form the 'Champlain Sea'. A whale skel- ing and closing of an ancient protoAt- eton in the University of Vermont geolo- lantic Ocean. gy museum testifies to that, as do mi- Pleistocene ice sheets covered Ver- crofossils in the clays. mont more than once. The last ice sheet The sea lasted until about 10,300 years moved south to Long Island about ago, after which it was cut off by glacial 75,000 years ago, and by 17,000 years rebound. That tilted the shoreline fea- ago it was melting northward faster tures (water planes), which rise north- than the ice itself was oozing south. By ward. Relicts of Lake Vermont's shore- 13,000 years ago the ice had largely line are about 160 m above sea level at melted from the mountains of Vermont, the same latitude as Middlebury, and but one lobe remained in the Champlain about 210 m at the Canadian border. Valley. Weight of the ice had depressed The water plane of the Champlain Sea the crust so that ancient Lake Vermont also rises northward. At the latitude of formed against the south end of the Middlebury, the features are about 30 m lobe. The lake drained south into the above the present level of Lake Cham- Hudson Valley and extended across the plain, or about 60 m above present sea Champlain Valley from the Green level. The sea water has been flushed Mountains on the east to the Adiron- out and a rock-ledge barrier has risen to dacks on the west. Now many active form the outlet at the north end of Lake sand and gravel pits are being worked Champlain, just north of Vermont. in the discontinuous bench that was Bedrock geology is a different—and formed along the Green Mountain front older—story. The proto-Atlantic ocean by meltwater flowing into Lake Ver- began opening about 700 million years mont. Farther west, away froin the ago, and sediments and volcanic materi- shore, clay deposits covered the lake als accumulated on the continental mar- floor. gin and ocean floor. Plate motion re- About 12,000 years ago, the ice lobe versed and the ocean began closing, melted north to the St Lawrence low- perhaps in Cambrian time (570-500 mil- lands, and the lake drained. For a short lion years ago). Late in the Ordovician, time, the level may have dropped below about 440 million years ago, the conti- nent collided with the volcanic arc that lay to the east; that was the Taconic orogeny. (Orogeny is mountain forma- tion from structural disturbance of Earth's crust, especially by folding and faulting. The Taconic orogeny is named for the Taconic Range of eastern New York.) By mid-Devonian time, about 360 million years ago, the ocean beyond that arc had closed, so that the continents collided, in what we call the Acadian orogeny.

10fall 1982 Earth science -- VTINH

550 m.y.a.

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The cross-sections show the opening and closing of the proto-Atlantic Ocean.

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This cross-section illustrates the subsurface geolo- gy between Burlington and St Johnsbury.

Earth science fall 1982 11 When publication of the Centennial ments underlie deep thrusts and contain geologic map of Vermont was celebrated at reserves of natural gas. a meeting in 1961, one geologist com- As for other mineral resources, one mented that the map indeed was accu- formation consists of marble that is be- rate in showing the kinds of rock and ing quarried for building stone and for where they occur, but he was not so calcite powder. The limestone, dolos- sure about the stratigraphic interpreta- tone, and quartzite have all been used tion—the age relations of one unit to an- in building, well displayed on the cam- other. We have only recently come to puses of Middlebury College and the suspect that the sequence of successive- University of Vermont, Burlington. ly 'younger' formations mapped east of the Green Mountains is not depositional South of Middlebury, the folded strata but, rather, a series of fault slices of of the continental shelf are buried be- rocks that are about the same age. neath a thrust mass of green and black Precambrian basement rocks are ex- slate and schist about 1 km thick. The posed in the Green Mountains, and also rocks now form the Taconic Mountains in domes south of Chester, just east of of southwest Vermont and supply green the Green Mountain axis. The Precam- and purple slate from quarries near Fair brian gneiss, schist, and marble were Haven, where outcrops show turbidites formed in an earlier stage of geologic with quartz and carbonate grains de- evolution. They are more than 1 billion rived from the shelf. The rocks are the years old and are related to rocks found same geologic age as those of the conti- in the Adirondacks. nental shelf and must have been deep- About 2 km of limestones, dolostones, water sediments that accumulated very and quartzites are seen in the Cham- slowly at the base of the continental plain Valley, and they accumulated on a slope. continental shelf in the Cambrian and North-trending belts of schist make Ordovician periods. The setting was like up central and eastern Vermont. The today's warm shallow water of the Ba- schists are greenish (with chlorite and hama platform, and most of the beds amphibole) or dark grey (from graphite). are carbonate rocks. However, erosion Schists west of the transition belt are of an area to the west provided quartz late Precambrian to mid-Ordovician sand that forms several formations. (640?-440 million years ago) in age; they Each one spread across the shelf when a are exposed in deep roadcuts of Inter- temporary drop in sea level caused the state 89 from Burlington southeast to shoreline to move eastward. The east Montpelier. Most of the rocks were edge of the shelf was just east of the probably sands and muds from a conti- Green Mountain front. Late in the Ordo- nental source, but some were basaltic vician, a thick (1,400 m) section of shale flows. From Montpelier east, almost to was deposited on the shallow-water the Connecticut River, the schists are Si- shelf sediments, signalling a change in lurian to Early Devonian (435-380 mu- tectonism. The continental shelf sediments are now folded, sheared, and cut by several These 5 main groups of rock record Vermont's geo- thrust faults. That includes the Cham- logic history. Interpretations here reflect the grow- plain thrust. The 'Eastern Overthrust ing understanding of environments of sediment de- Belt' is being explored by deep seismic position, geochemistry of the rocks, and deep structure, all in the context of plate tectonics. profiling. In 1980, the Cocorp research Symbols: pE = Precambrian basement rocks. project, out of Cornell University, made Ch=Chester. G=granite. S=shelf sediments. profiles across Vermont, and some pro- Mi = Middlebury College. UVM = University of files are being run this year by compa- Vermont. Bu =Burlington. T=Taconic al- nies searching for gas. Tentative results lochthon. FH= Fair Haven. M=schists. suggest greater thrusting than previous- Mp=Montpelier. Ch.t. = Champlain ly thought. Perhaps less-deformed sedi- Thrust. fit: •r4flf.fifl

12 tall 1982 Earth science

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Earth science fall 1982 13 lion years ago) in age. The muds had probably caused movement on the evidently been deposited in the fairly Champlain thrust and faulting of the shallow ocean between the continent Precambrian basement. Sediments and and arc after the collision. volcanics of the transition zone probably In central Vermont, many tiny lenses formed during the collision. Events as- of serpentinized ultramafic rocks yield sociated with the continent-arc collision, talc, soapstone, asbestos, and verde an- in the last 15 million years of the Ordo- tique. These lenses form a zone that vician, constitute the Taconic orogeny. trends north to the large asbestos depos- We have found no record after that its of Thetford Mines in Quebec. Those until about mid-Silurian (about 415 mil- mines are in a huge mass of ultramafic lion years ago); probably the post-colli- rock. Along with associated pillow ba- sion uplift made it a source area for sed- salts and ocean-floor cherts and argil- iments shed westward into New York. lites, the rocks constitute a flake of an- From Montpelier east, the rocks are cient ocean crust, now emplaced on the mid-Silurian to mid-Devonian dark grey margin of an ancient continent. Almost schists with some limestones that accu- certainly the tiny ultramafic lenses in mulated between the continental margin Vermont also occur as fault slices; that is and arc. In New Hampshire, similar consistent with faults now being recog- rocks were evidently deposited east of nized and mapped in the schists. the arc, in the remaining ocean. Rocks The line of gneiss domes in New in southeast New Hampshire and the Hampshire, just east of the Connecticut Boston area are continental basement River, is evidently the root of a former that had been on the far side of the volcanic arc associated with subduction. proto-Atlantic ocean. The location of the arc, and the occur- The sediments were folded and meta- rence of ultramafics farther west, sug- morphosed in the Acadian orogeny of gest that the subduction zone dipped Middle and Late Devonian age, due to east (not west, as had been suggested in collision of the continents. In southern early reports of plate tectonics and the Vermont the closure was extreme, and Appalachians). the formations were stretched to thin As the ocean closed and the ocean belts. crust was subducted, the sediments and Grey granite bodies intruded the Silu- pieces of ocean crust were caught and nan and Devonian schists in northeast stacked into an accreting wedge. The Vermont; the deep quarries at Barre are several schist belts were probably about famous for the quality of stone. The the same age, but had been deposited granite magmas were probably from on different parts of the ocean floor. melting of deeply buried sediments dur- By mid-Ordovician time, about 450 ing collision of the continents. Erosion million years ago, the east edge of the of the resulting uplift supplied red allu- continent reached the trench and began vial and shore-zone sediments that are to subside rapidly, and thick shale was now exposed in the Catskill Mountains deposited. (The modern analog is the of New York. Australian platform, which is rapidly Vermont geology gives glimpses of '- subsiding as it enters the trench by Ti- post-Devonian history, but that record is, mor, so that shallow-water mid-Pliocene better seen in other parts of New Eng- limestone is now down 2.7 km!) At the land. Some structures and small igneous same time, the continental margin start- intrusions are related to the opening of ed to wedge beneath the 'pre-assem- the present Atlantic Ocean, and some bled' stack of schist belts, which there- are related to the White Mountain mag- fore became emplaced on and against ma series of New Hampshire. the continental margin. As noted above, the Taconic allocthon was emplaced on Brewster Baldwin the folded and sheared continental shelf Department of Geology, Middlebury College, sediments. The collision with the arc Middlebury, Vt., 05753

14 fall 1982 Earth science