Sometime around 600,000,000 years ago the huge paleocontinent of Rodinia started to break up. Eventually, an ocean called Iapetus Ocean opened up between paleo-North America (Laurentia) and paleo-Africa (Gondwanaland). Sometime around 500,000,000 years ago, the Iapetus started to close.
Still working from Northeast to Southwest across North America, the next set of terranes we encounter after those of the Iapetus seafloor are those that were originally deposited along the margin of ancient North America -- the Paleocontinent of Laurentia. These rocks were pushed into their present positions during the Ordovician as volcanic island arcs in the closing Iapetus Sea plowed into Laurentia pushing portions of the sea bottom in front of them.
The distinction between these rocks and those of Laurentia proper is often somewhat hazy. The rocks of these regions tend to be fault delimited slices and sometimes overturned folds -- often extending many tens of kilometers along the axis parallel to the Laurentian boundary, but much narrower perpendicular to that axis. The problem is that the rocks along the margin of Laurentia were sometimes crumpled and or moved a bit as the sea bottom to their East was pushed up into the basin to their East. In many cases, it's very hard to tell how far blocks were moved. Pragmatics require a boundary somewhere. I have chosen mine roughly along the line where most rocks to the East and South have been transported and most rocks to the North and West are probably more or less in place.
Somewhat more precisely, I have defined the Laurentian margin region for my purposes as follows: The Southern/Eastern boundary follows the Baia Verte Fault line in Newfoundland, then a line about 50km South of the South bank of the Saint Lawrence River in New Brunswick and Quebec. In Western New England, it is the first major (unnamed) thrust East of the Oak Hill-Hinesburg thrust structure which runs down the West side of the Green Mountains. South of New York City, it is the Western margin of the Piedmont region in the Mid Atlantic and Southern States. On the North and West sides I consider the zone to end somewhere in the strait between Labrador and Newfoundland, and then to swing West including the Saint Lawrence South shore exposures of New Brunswick and Quebec, but not the large Islands in the St Lawrence like Anticosti Island. In the vicinity of Quebec City, the Margin rocks overlap onto the North Shore of the St Lawrence. West of Quebec City, the limit then follows Logan's/Emmons' line West to the Richileu River. It then swings South down the Richileu River, the East Shore of Lake Champlain, the East side of the Hudson River and then swings Southwest generally including the first (Blue Mountain) ridge of the Appalachians along the line where Early Paleozoic marine sediments from the East have been thrust over generally younger Paleozoic rocks. In the Carolinas, Alabama and Georgia the western boundary moves further West into the Appalachians.
Largely my dividing line coincides closely with classical divisions like Logan's Line in Canada and Emmons' Line in the US.
Almost all of the rocks I am discussing have been transported from where they were originally deposited. That doesn't mean that the structures are always small. Indeed many are county sized or larger. A few terms that I use and/or will inevitably be found while researching sites:
I have somewhat arbitrarily divided the Laurentian Margin rocks into six terranes
These are shallow water Precambrian, Cambrian, Ordovician, and possibly a few Silurian rocks pushed into place prior to the formation of the Appalachian Mountains. The Western boundary is somewhat obscure. Unlike regions further North there seems to be extensive faulting and slicing in the Laurentian platform deposits West of the rocks pushed into place during the early part of Iapetus closure. The slicing results in repetition of Early Paleozoic formations as one moves from East and West. My inclination is to put the Appalachian Margin boundary at the most westerly rock sequences containing deep water/continental slope sediments and/or Iapetus seafloor volcanic rocks in the basement rocks. I believe that boundary is on the East side of the first major Appalachian Ridge (Blue Mountain) in Pennsylvania and moves westward over the mountain as one moves further South through Maryland, Virginia, the Carolinas and into Georgia.
I could well have parts of this all wrong.
Age | North and West | South and East | Principle Rock Types |
Silurian, Middle | ??? | ||
Silurian, Lower | Tuscarora | Sandstones | |
Ordovician,Upper | Juniata | Sandstones and shales | |
Ordovician,Upper | Oswego | Sandstones | |
Ordovician,Middle | Martinsburg | Shales | |
Ordovician,Middle | Chambersburg | Limestones | |
Ordovician,Middle | New Market,Row Park | Limestones | |
Ordovician,Lower | Beekmantown | Pinesburg Station | Limestones |
Ordovician,Lower | Rockdale Run | Limestones | |
Ordovician,Lower | Stonehenge | Limestones | |
Cambrian,Upper | Conococheague | Limestones | |
Cambrian,Middle | Kinzers,Ledger,Elbrooke | Brooke | Shales and Limestones |
Cambrian,Lower-Middle | Kinzers | Waynesboro | Shales and Limestones |
Cambrian,Lower | Vintage | Tomstown | Shales and Limestones |
Cambrian,Lower | Antietam | quartizes | |
Cambrian,Lower | Harpers | quartizes | |
Cambrian,Lower | Weaverton-Loudun | quartizes | |
Ediacaran? | Catoctin | quartizes and conglomerates | |
Ediacaran? | Swift Run | quartizes and conglomerates | |
Ediacaran? | {Flattop} | Metamorphic rocks | |
Ediacaran? | {Montezuma} | Metamorphic rocks | |
Ediacaran? | {Linville} | Metamorphic rocks | |
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
Continental slope deposits thrust over Ordovician continental sediments from the vicinity of Fairhaven, Vermont South to Poughkeepsie, New York. The rocks consist mostly of slates with minor limestones, conglomerates, sandstones, etc. There are many commercial slate quarries. The Taconic Terrane has been thrust over native Laurentian rocks to the West and also over rocks of the Champlain Trust plate to its Northwest. On the East side of the terrane, rocks of the Hinesburg Thrust have been pushed over the Taconic rocks.
The Lower Cambrian beds contain a distinctive Elliptocephala asaphoides fauna typically in limestones or limestone conglomerates. Some younger beds contain graptolites
AGE | FORMATIONS | ROCK TYPES |
---|---|---|
Middle Ordovician | Austin Glen | greywacke (granitic sandstone) |
Middle Ordovician | Mt Merino | Chert and Shale |
Middle Ordovician | Pawlet | interbedded black slate and greywacke |
Middle Ordovician | Mount Hamilton (Indian River) | maroon,red,light green slates |
Lower Ordovician | Mount Hamilton (Poultney) | Green to grey slates |
Lower to Upper Cambrian | Mount Hamilton (Hatch Hill - White Creek) | Black slate with ribbon limestones |
Lower Cambrian | Mount Hamilton (Hatch Hill - Boomoseen) | Sooty black slate above limestone |
Lower Cambrian | Mount Hamilton (Middle Granville Slate) | maroon,grey,green slates |
Lower Cambrian | Mount Hamilton (Browns Pond) | Gray to Black slates, limestones, quartzites |
Lower Cambrian | Bull (Mettawee) | Purple and green slate, mudstone, and phyllite |
Lower Cambrian | Bull (North Brittain conglomerate) | intraformational limestone-pebble, slate-matrix conglomerate |
Lower Cambrian | Bull (Mudd Pond) | A thin but persistent orthoquartzite |
Lower Cambrian | Bull (Zion Hill) | A discontinuous graywacke or sub-graywacke unit ranging from a pebble conglomerate to a mudstone |
Lower Cambrian | Bull (Bomoseen) | massive, olive-drab, medium-grained graywacke |
Lower Cambrian | Bull (Nassau,Truthville) | |
Lower Cambrian | Biddie Knob | purple and green slate and phyllite with minor beds of limestone and quartzite |
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
The Taconic sequence has been studied extensively now that plate tectonics explains its nature which was quite baffling to older geologists. Analysis is difficult due to the many thrust faults and the large number of lithologically similar slate formations that are difficult or impossible to distinguish from each other where the intervening beds are missing. Formation nomenclature is undergoing frequent revision.
Rocks deposited in what appears to be a basin that developed in the Lower Cambrian. There is a substantial facies change along the line of US2 and the LaMoille River between shallow water sandstones/carbonates to the South and deeper water siltstones/sandstones with minor carbonates to the North The Champlain Thrust Block has clearly been pushed into place from the East. The thrust fault which places Lower Cambrian rocks of the thrust plate over Ordovician rocks is exposed at several locations along the East shore of Lake Champlain. On the East, the Champlain plate is bounded by the rocks of the Hinesbug-Oak Hill Thrust. To the South basin rocks appear as thin slates along the line of US2 and thicken rapidly to the North displacing the sandstones and dolomites of the platform to the south with finer grained and less calcareous sediments. In the North, a thrust fault within the plate reveals Upper Cambrian Gorge formation dolomites thrust over Ordovician Morses Line slates.
Fossils include occasional Ollenelus fauna fossils in the Cheshire and Dunham formations. A more diverse and abundant Lower and Middle Cambrian fauna in the Parker formation and Upper Cambrian/Lowest Ordovician fossils in the Gorge and Highgate formations
AGE | FORMATIONS | ROCK TYPES |
---|---|---|
Ordovician | Morses Line | Black slates |
Upper Cambrian | Gorge | Sandy Dolomite, black shales |
Middle Cambrian | Parker-Skeels Corner | slates, shales |
Lower Cambrian | Parker-Skeels Corner | slates, shales |
Lower Cambrian | Dunham | Dolomite, some limestones |
Lower Cambrian | Cheshire | Grey Quartzite |
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
Rocks deposited in what appears to be a shallow water environment South of a basin that developed in the Lower Cambrian. There is a substantial facies change along the line of US2 and the LaMoille River between shallow water sandstones/carbonates to the South and deeper water siltstones/sandstones with minor carbonates to the North. To the West, the platform rocks end at the eastern shore of Lake Champlain. There is a classic exposure on the bike trail North of Burlington of Lower Cambrian Dunham Dolomite thrust over Middle Ordovician black shales.
Fossils include occasional Ollenelus fauna fossils in the Monkton, Cheshire, and Dunham formations. A few fossils are found in the younger formations, which appear to be somewhat altered versions of the rock sequence found in place in Eastern Laurentia. The Middle Ordovician limestones (Black River,Chazy,etc are present here, but are altered to marble and have different names. For the most part the rocks have been altered extensively and most fossils have been destroyed. Most of the classic Vermont marble quarries are in these altered limestones between Shelburne and Rutland.
AGE | FORMATIONS | ROCK TYPES |
---|---|---|
Ordovician | several formations -- Chipman, Beldens, Weybridge, Burchards,etc | Marbles (altered versions of Bridport,Chazy, Black River, etc |
Upper Cambrian | Sandy Dolomites, Quartzites | |
Middle Cambrian | Winooski | Dolomite |
Lower Cambrian | Monkton| red and varicolored quartzites | |
Lower Cambrian | Dunham | Dolomite, some limestones |
Lower Cambrian | Cheshire | Grey Quartzite |
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
These rocks are part of a huge overturned fold that extends along the Westernmost edge of the Green Mountains from Quebec to Northern Connecticut. The oldest rocks appear to be billion year old "Grenville" sediments of the North American core. Most sediments deposited on floor of the Iapetus Sea as it opened. The youngest rocks appear to be shallow water and beachfront sediments deposited during the Lower Cambrian. Older, pre-plate tectonic, literature indicates that the Dunham Dolomite and other rocks of the Champlain Thrust Terrane are in place above the Cheshire, but that is very likely an inferred relationship rather than an observed relationship.
The relationship of the shallow water Hinesburg Thrust rocks to the continental slope rocks of the Taconic Terrane that they are thrust over on their West is unclear.
Because the structure is an overturned fold, the formations are upside down in places with younger rocks lying under older rocks.
Fossils -- tracks and a very few body fossils are found in the youngest formations -- Lower Cambrian Cheshire quartzite and underlying Fairfield Pond Phyllite. The condition of the rocks varies from place to place. Only slightly altered in some places, melted and quite distorted in others.
AGE | FORMATIONS | ROCK TYPES |
---|---|---|
Lower Cambrian | Cheshire - Gilman | quartzites |
Ediacaran?-Lower Cambrian? | Fairfield Pond - Gilman | silicified siltstones with quartz stringers |
Ediacaran? | several thin laterally variable formations | slates-limestone |
Ediacaran? | Pinnacle | Greywacke ((granitic sandstone)) |
Ediacaran? | (Tibbet Hill) | Underwater Lavas |
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
I have been unable to locate much material describing the sedimentary rocks exposed along the hundreds of kilometers on the South shore of the St Lawrence River East of the Richileu River other than those on the Gaspe Peninsula and those exposed opposite Quebec City. At a guess, they consist of two sequences. To the North are Cambrian and Ordovician rocks -- possibly continental shelf deposits similar to those of the Taconic Terrane. Thrust over them from the South are Ordovician, Silurian, and Devonian marine sediments that are (possibly?) an extension of the similar sequence in the Connecticut River Valley. Both of these sequences are only slightly altered, especially in the Gaspe region of Eastern Quebec.
Carboniferous | Maritime Cover Terrane | |
Devonian, Upper | Escuminac | sandstone,shale |
Devonian, Upper | Fleurant | Conglomerate |
Devonian, Middle | Touladi | limestone |
Devonian, Middle | Mountain Wharf,Famine | Conglomerate,limestone,shale |
Devonian, Middle | Malbaie | Conglomerate,sandstone |
Devonian, Lower | Battery Point | |
Devonian, Lower | York River | Sandstone |
Silurian | Cabano | |
Point aux Trembles | ||
Silurian, Upper | West Point | |
Sayabec/La Vieille | ||
Ordovician-Silurian | White Head | carbonates |
Ordovician? | Arsenault, Garin, Pabos | |
Ordovician | Chloridrome,Magog | Black Shale |
Ordovician,Lower | Riviere Ouelle | |
Cambrian,Upper | Grosses-Roches Formation | |
Cambrian,Lower | ||
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
Age | Humber(?) and Dunnage Formations | |
---|---|---|
Carboniferous | Maritime Cover Terrane | |
Devonian | Gaspe Sandstone | |
Gaspe | ||
Escuminac | ||
Fleurant | ||
Pirate Cove | ||
Devonian | La garde | |
Silurian | Gascons,IndianPoint,Roncelles | |
Sayabant | ||
Val-Brillant | ||
Awantjish,Burnt Jam Brook,Sources | ||
White Head | ||
Ordovician | Pabos | |
Garin | ||
older | [Iapetus Sea Floor Terranes IAPETUS.HTM ] | |
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
The rocks of Western Newfoundland seem to be broken into two slices -- a Western slice consisting of local (autocthonous) rocks and an Eastern slice of alien (allocthonous) rocks that were pushed over the local rocks during the Ordovician Taconic Orogeny.
Devonian-Carboniferous | Maritime Cover Terrane | |
Silurian-Devonian | Clam Bank | |
Ordovician,Upper | Long Point (Lourdes) | |
Inthrust granitic rocks | ||
Ordovician,Middle | Goose Tickle | Shales |
Ordovician,Middle | Table Point | Limestones |
Ordovician,Lower | Aguathuna | Limestones |
Ordovician,Lower | Catouche | Limestones |
Ordovician,Lower | Boat Harbour,Barbace Cove | Limestones |
Ordovician,Lower | Watts Bight | Limestones |
Cambrian,Upper | Berry Head | Dolostones,Limestones |
Cambrian,Middle-Upper | Point Jardin | Dolostones,Limestones,Shales |
Cambrian, Lower | Hawkes Bay | Sandstones |
Cambrian, Lower | Forteau | Shales |
Cambrian, Lower | Brandore | Sandstones |
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
Devonian-Carboniferous | Maritime Cover Terrane | |
Silurian-Devonian | Clam Bank | |
Ordovician,Upper | Long Point (Lourdes) | |
Ordovician, Lower | Cow Head | shales |
Lower Head | sandstones | |
Ordovician, Lower | Maiden Point | sandstones |
Cambrian, Upper | Maiden Point - Cooks Brook/Middle Arm Point/Eagle Island | conglomerate,shale,limestone |
Cambrian, Lower | Maiden Point - Blow Me Down Brook/Summerside/Irishtown | sandstones,shales |
bold=fossiliferous italic=calcareous ()=largely volcanic {} = intrusives,deep sea metamorphics |
Formations in US largely based on American Association of Petroleum Geologists Geological Highway Maps
No recommended localities. Although fossils are quite diverse and are much more common than in the collapsed Iapetus Seafloor region to the East, nowhere other than possibly the Perce region in Quebec's Gaspe peninsula fossils really abundant. Mostly, the outcrops of the Atlantic margin seem to be best suited to local collectors who can make multiple visits in order to scope out the sites and specific outcrops.
Devonian outcrops at Miguasha National Park on the Restigouche River across from Dalhousie, NB. Have yielded a wide variety of Devonian fishes. There is an interpretive center. Collecting is forbidden.
Newfoundland
Island Map http://www.nr.gov.nl.ca/mines&en/geosurvey/maps/nf.pdf
Detailed maps http://www.nr.gov.nl.ca/mines&en/geosurvey/maps/nl/
New Brunswick
Province map http://www2.gnb.ca/content/gnb/biling/en.html
Quebec -- So far, I have not located a province scale bedrock geology map of Quebec
Gaspe Peninsula Geology
http://ebookbrowse.com/marcil-j-gaspe-basin-pdf-d17372744
Vermont
State map http://www.anr.state.vt.us/DEC/geo/centmap.htm
Massachusetts
State map (simplified) http://geology.about.com/library/bl/maps/blmassachusettsmap.htm
State map http://www.geo.umass.edu/stategeologist/frame_maps.htm
Connecticut
State maps http://web.archive.org/web/20120306154657/http://www.wesleyan.edu/ctgeology/images/CtGeoMap_big.jpg
State maps https://web.archive.org/web/20050911162123/http://tmsc.org/geology/bedrock/
New York
Generalized Bedrock Geology of NY
New Jersey
Simplified map of NJ
Index to online geology maps of New Jersey
Pennsylvania
Simplified map of PA
Digital Geologic map of Pennsylvania
Maryland
Geologic Map
North Carolina
Generalized map
Georgia
High Resolution map
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