• Introduction
  • Terminology
  • The Rocks
    • APPALACHIAN MARGIN TERRANE
      • Rocks
      • Formations
    • TACONIC TERRANE
      • Rocks
      • Formations
    • CHAMPLAIN THRUST TERRANE (NORTHERN BASIN)
      • Rocks
      • Formations
    • CHAMPLAIN THRUST PLATE NORTH OF LaMOILLE RIVER
    • CHAMPLAIN THRUST TERRANE (SOUTHERN PLATFORM)
      • Rocks
      • Formations
    • CHAMPLAIN THRUST PLATE SOUTH OF LaMOILLE RIVER
    • HINESBURG THRUST TERRANE
      • Rocks
      • Formations
    • ST LAWRENCE TERRANE
      • Rocks
      • Quebec Formations
      • New Brunswick Formations
      • New Brunswick
    • HUMBER TERRANE
      • Rocks
      • Formations
        • Western Slice
        • Eastern Slice
  • Localities
  • Maps and Figures
    • Geologic Maps
    • Formations
  • References

Introduction

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.

Terminology

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:

• allocthonous == alien -- Rocks that have been transported. Often a long way.
• autocthonous == native -- Rocks that remain where they were originally deposited.
• para-autocthonous == quasi-native Rocks that have been moved a small distance, but are not vastly displaced
• slice -- A fault delimited "outcrop" that has been moved from its original position, but is more or less in an ordered sequence with adjacent slices.
• terrane - A collection of rocks with related provenance. Terrane boundaries can be flexible. For example, I have identified a "Laurentian Margin Terrane" that we are discussing here. But the 200km long, 40km wide "Taconic Terrane" (mostly) in New York is a valid and useful subset of my larger "Laurentian Margin Terrane"

The Rocks

I have somewhat arbitrarily divided the Laurentian Margin rocks into six terranes

• Appalachian Margin Terrane - Shallow water sediments found from Pennsylvania to Georgia pushed up onto and in places over, the Easternmost ranges of the Appalachian Mountains.
• Taconic Terrane - Continental margin seafloor rocks pushed up onto the Laurentian Margin along a line from Fairhaven, Vermont to around Poughkeepsie, NY and bounded on the West by the line of the Hudson River (Note that modern geologists include some rocks found a bit West of the Hudson).
• Champlain Terrane - Shallow water and basinal deposits pushed up onto Laurentian margin rocks from the vicinity of Rutland Vermont to the St Lawrence River. Bounded on the West by Lake Champlain and the Richileu River.
• Hinesburg Thrust Terrane - Precambrian and Lower Cambrian shallow water rocks of obvious Laurentian affinity pushed up onto the Taconic and Champlain Terranes from the East.
• St Lawrence Terrane - Shallow water and continental margin rocks found along the South bank of the St Lawrence River from the mouth of the Richileu River (East of Montreal) to the Gaspe Peninsula.
• Humber Zone - Shallow water Lower Cambrian-Ordovician marine rocks exposed along the entire West coast of Newfoundland.

APPALACHIAN MARGIN TERRANE

Rocks

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.

Formations

TACONIC TERRANE

Rocks

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

Formations

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.

CHAMPLAIN THRUST TERRANE (NORTHERN BASIN)

Rocks

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

Formations

CHAMPLAIN THRUST PLATE NORTH OF LaMOILLE RIVER

CHAMPLAIN THRUST TERRANE (SOUTHERN PLATFORM)

Rocks

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.

Formations

CHAMPLAIN THRUST PLATE SOUTH OF LaMOILLE RIVER

HINESBURG THRUST TERRANE

Rocks

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.

Formations

ST LAWRENCE TERRANE

Rocks

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.

Quebec Formations

New Brunswick Formations

New Brunswick

HUMBER TERRANE

Rocks

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.

Formations

Western Slice

Eastern Slice

Formations in US largely based on American Association of Petroleum Geologists Geological Highway Maps

Localities

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.

Maps and Figures

Geologic Maps

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

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://dec.vermont.gov/geological-survey/publication-gis/VTrock

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

Overview of NY Geology

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

Virginia

North Carolina Generalized map

Access to detailed maps

South Carolina

Georgia High Resolution map

Alabama

Formations

See Rocks

References

• Nice Stratigraphy for Lower Humber Arm in Newfoundland http://www.nr.gov.nl.ca/mines&en/geosurvey/publications/cr2001/Burden.pdf
• https://web.archive.org/web/20130610024828/http://www.newfoundlandquarterly.ca/issue410/rock.php
• http://www.cspg.org/abstracts/2005Core/marcil_j_gaspe_basin.pdf
• http://www.atmos.albany.edu/facstaff/wkidd/whitehalltaconicmapartc.pdf
• http://web.archive.org/web/20060910144225/http://earth.geology.yale.edu/~ajs/1994/04.1994.01Schwab.pdf
• http://web.archive.org/web/20060908205015/http://earth.geology.yale.edu/~ajs/1998/02.1998.01Hibbard.pdf
• http://www.nr.gov.nl.ca/mines&en/geosurvey/publications/cr2001/Valverde.pdf
• American Association of Petroleum Geologists Geological Highway Maps
• The Acadian orogeny : recent studies in New England, maritime Canada, and the autochthonous foreland By David C. Roy, James William Skehan
• New Brunswick Bedrock Geology ]
• Newfoundland Geology
• http://www.nr.gov.nl.ca/mines&en/geosurvey/publications/cr2000/OBrien.pdf
• http://www.jstor.org/stable/10.1086/515976?journalCode=jg
• http://www.ramblermines.com/files/Rambler_Competent_Persons_Technical_Report_Sept2004.pdf
• New Hampshire Bedrock Geology
• Vermont Geology Info

Copyright 2006-2012 Donald Kenney (Donald.Kenney@GMail.com). Unless otherwise stated, permission is hereby granted to use any materials on these pages under the Creative Commons License V2.5.

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