LAURENTIAN MARGIN DEPOSITS

Donald Kenney (donaldkenney@gmail.com)
Last Update: Thu Oct 8 01:18:34 2020



Introduction

Still working from Northeast to Southwest accross 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 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. Pragmatics require a boundary somewhere. I have chosen mine roughly along the line where most rocks to the East have been transported and most rocks to the West are more or less in place. Largely my dividing line coincides closely with classical divisions like Logan's Line in Canada and Emmons' Line in the US.

Background

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 some are county sized or larger. A few terms that I use and/or will inevitably be found while researching sites: . *allocthonous* -- Rocks that have been transported. Often a long way. ' *autocthonous* -- Rocks that remain where they were originally deposited. . *para-autocthonous* -- Rocks that have been moved a small distance, but 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 characteristics. 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 the larger "Laurentian Margin Terrane"

Sometime before 600 million years ago almost all the continental rock on the planet collected itself into a single "supercontinent" that geologists call Rodinia. Rodinia is not to be confused with the much younger supercontinent of Pangea which assembled itself about 300 million years ago and broke up roughly 200 million years ago. About 600 million years ago, rifts developed in Rodinia. It fractured into as many as eight fragments. One of these fragments consists of the rocks underlying most of North America. It is called Laurentia. A second fragment included much of what are now Africa, South America, Australia, and Antarctica. It was called Gondwana.

Laurentia and Gondwana were adjacent parts of Rodinia. The rift between them opened and eventually became a full fledged ocean -- the Iapetus Sea. Shortly after the Iapetus started to open, a rift developed in Gondwona that calved a long narrow strip of continental rock called Avalonia into the Iapetus. As time passed, the ocean between Laurentia and Avalonia ceased to widen. One of more subduction zones appeared where surface rocks were sucked into the Earth, and the Iapetus started to close. Eventually, it closed completely folding many hundreds of kilometers of sea bottom into a confused jumble of rocks that can easily be driven across in a few hours. On the western margin of that jumble are rocks that started off on the Laurentian continental shelf and the continental slope leading down toward the floor of the Iapetus Sea.

These rocks lying along the Eastern margin of ancient Laurentia were pushed into their current positions by a long series of collisions between Laurentia and landmasses arriving from the "East" as the Iapetus Sea disappeared -- first, volcanic island arcs, then the microcontinent of Avalonia, and finally the continent of Gondwana. The collisions stressed and sometimes altered the margin rocks. Blocks of countryside were pushed into sometimes unlikely positions. But overall, the rocks tend for the most part to be somewhat in sequence with continental slope rocks to the East of continental shelf rocks and rocks in the Western part of the margin generally transported shorter distances than those on the East. Fossils in the margin rocks tend to be more numerous and better preserved than those of the Iapetus Seafloor with abundance and preservation improving as one approaches ancient Laurentia.

I have defined the Laurentian margin region somewhat arbitrarily. In general, it consists of Newfoundland North and West of the Baia Verte Fault line (the West Coast of Newfoundland), New Brunswick and Quebec along a line about 50km South of the South bank of the Saint Lawrence River. The first major (unnamed) thrust East of the Oak Hill-Hinesburg thrust structure which runs down the West side of the Green Mountains and then the Western margin of the Piedmont region in the Mid Atlantic and Southern States. On the West side 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. The limit then follows Logan's/Emmons' line 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.

Rocks

The rocks of the Laurentian margin tend to be extremely diverse with rapid facies changes and extensive faulting -- both thrust and normal -- juxtaposing unrelated rocks in unexpected ways. Throw in the often extensive verneer of glacial debris and the humid climates that will grow vegetation on just about anything and the result is a jumble of random exposures that is extremely difficult to interpret.

Newfoundland

Newfoundland geology was described by Sir Charles Murray head of the Newfoundland Geologic Survey in the 19th Century. He defined a number of zones of rocks deposited in strips running Northeast to Southwest parallel to the West coast of Newfoundland. Some of these zones have survived into modern usage sometimes a bit mangled. The Avalon zone on the East is described here as the Avalon Terrane. The Humber Zone on the West will be described along with the other Taconic-Laurentian Margin rocks. The original Dashwoods, Notre Dame, Dunnage, Gander, Exploits, Zones are usually combined into two zones. The Dunnage zone found East of the Humber zone consists of obvious deep sea bottom sediments -- volcanics and deep water sediments. It is split by the Red Indian Line which is thought to represent a major divide between surviving rocks from the Eastern and Western Iapetus seabottoms. The Gander zone further East consists of "clastics" -- sandstones and shales -- intruded by igneous rocks such as granite and of metamorphosed rocks of various sorts. It is separated from the Dunnage Zone by the Reach Fault. Since granite is a rock typical of continental volcanic activity rather than deep sea vulcanism it is thought that Avalonian continental rock underlies the Gander rocks at depth. Other nomenclature is used by various authors. Being clairvoyance deprived, I will not attempt to describe the nomenclature and geology further.

The Dunnage-Gander rocks contain a few fossils. Enough to demonstrate that they are early Paleozoic and are of marine origin. Some Silurian fossils are found near Glenwood.

Newfoundland

Age Dunnage (Notre Dame) Dunnage (Exploits) Gander
Carboniferous Maritime Cover Terrane
Devonian (intrusives)
Silurian Mimac lake, Cape St John Group Indian Islands Group, Botwood Group
Ordovician< (intrusives),Snooks Arm,Flat Water Pond Wild Bight Group*, Davidsonville Group, Badger Group
(Ophiolites) Victoria Lake Supergroup Indian Bay-Big Pond
Cambrian Fleur de Lys Supergroup Victoria Lake Supergroup Jonathans Pond

*The Wild Bight Group consists of five formations -- oldest to youngest -- Seal Bay Brook, Side Harbour, Omega Point, Sparrow Cove, Pennys Brook

New England, Quebec and New Brunswick

In New England, the rocks not belonging to younger "cover terranes" such as the Triassic Basin rocks resemble the Dunnage Zone of Newfoundland to some extent. Rock types are more diverse as they include sediments apparently laid down in shallow waters adjacent to volcanic island arcs as well as some Devonian deposits laid down in shallow basins or in lowlands remaining after the basins filled with sediment. I have quite arbitrarily placed the "Dunnage-Humber" boundary at roughly the Easternmost occurrences of well preserved, shallow water, Laurentian fossils even though in some cases, these manage to be somewhat East of regions of what are clearly continental slope deposits and in other cases regions of fairly strongly metamorphosed rocks exist to the West of my boundary.

A few outcrops of Ordovician and Silurian rocks are found in Maine. Devonian marine fossils and plants are found in basins in the central part of the state.

A few Silurian and Devonian fossils are found in a strip of mildly altered sandstones and limestones along the Connecticut River from Bernardston, MA to and beyond Littleton, NH. Graptolites are found at a number of localities in Northern and Central Vermont.

In Quebec, Dunnage rocks are confined to a narrow, largely unfossiliferous, strip along the borders of New Brunswick, Maine, New Hampshire, and Vermont. At least one Ordovician and one Devonian outcrop have recognizable fossils

In New Brunswick the Northwestern third of the province is covered with Paleozoic rocks similar those of Maine. A relatively narrow strip on the Southeast side of this belt is mapped as Dunnage and Gander rocks with the beds further Northwest being Silurian and Devonian sediments. The Southeast portion of the province is covered with Maritime Cover Terrane and Avalonian rocks.

For the present, I am assuming that Ordovician and older rocks in New Brunswick are Dunnage-Gander or Avalonian and that Silurian or later rocks are Avalonian or questionably Dunnage. Many authors would disagree and possibly allocate Silurian/Devonian rocks in Northwest New Brunswick to the Humber zone of Laurentian Margin rocks.

New Brunswick

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
Rocks above here are probably Humber Zone?
Arsenault
(Opiolitic melanges)
Cambrian Murphy Creek

bold=fossiliferous italic=calcareous ()=largely volcanic

Northern Maine

Age |Formations (West) Formations (East)
Mississippian (Miscellaneous intrusives) (Miscellaneous intrusives)
Devonian Upper Mapleton
Devonian Middle (Miscellaneous intrusives) (Miscellaneous intrusives)
Devonian-Lower Tomhegan Trout Valley
(unnamed)
Tarratine Mantagammon
Seboomook
Silurian-Upper Dagett Ridge Allsbury,Perlham, Kellyland,Smyrna Mills
Hardwood Mountain
Lobster Lake
Silurian-Middle Chesuncook
Maple Mountain Frenchville
Ordovician Pyle Mountain
Rockland
(Dunn Brook, Nine Lake,Hove)
Battie
(Kennebec) (Shin Brook)
Ellsworth
Cambrian Grand Pitch

bold=fossiliferous italic=calcareous ()=largely volcanic

New Hampshire-Western Maine

Age Formations (West) Formations (East)
Jurassic?/Cretaceous? (unnamed) (unnamed )
Devonian Waits River,(Putney),Littleton Sharpleigh,Towow,Littleton
Gile Mt,(Putney),Littleton Sharpleigh,Rindgemere,Littleton,Hildreth
Erving,(Putney), Littleton Sharpleigh,Gonic,Littleton,Seboomook
Merrimack,Berwick,Madrid,Vassalboro
Silurian Berwick,Merrimack Merrimack,Berwick,Casco Bay, Small Falls
Fitch,Eliot,Merrimack Merrimack,Eliot,Perry Mt,Waterville
Mayflower Hill
Clough,Kittery,Merrimack Merrimack,Kittery,Rangeley
Ordovician (intrusives) (intrusives)
Partridge,Dixville Dixville
(Ammonoosuc) (Rye)
Albee Albee
Rye
Aziscomos Aziscomos

*bold=fossiliferous* ~calcareous~ ()=largely volcanic

Central and Eastern Vermont

Age Formations
Jurassic?/Cretaceous? (unnamed)
Devonian Upper (unnamed)
Devonian Waits River,(Putney),Littleton
Gile Mt,(Standing Pond),Littleton
Northfield
Shaw Mt
Silurian ???
Clough
Ordovician (intrusives)
Missisquoi
Cambrian Ottauquechee

bold=fossiliferous calcareous ()=largely volcanic

Western Massachusetts and Western Connecticut

Age Formations
Triassic *Newark Supergroup*
Devonian (unnamed)
Erving,Littleton,Mt Pisgah,Brimfield?
~Waits River~,Littleton,Brimfield?
Silurian Goshen,Littleton,Brimfield?,Hamilton Reservior
Russell Mt, Brimfield?
Clough,Bigelow Brook
Hebron, Southbridge
Wepawaug
Ordovician (intrusives)
Wallomsac,Cobble Mt,Partridge,Paxton,Hartland
Wallomsac,(Hawley),(Ammonoosuc), Straits
Wallomsac,Moretown,Collinsville
(Middletown), Tatnic Hill
Stockbridge,Rowe/Waterbury/Hartland,(Monson),(Mamcoke),(Quinebaugh)
Stockbridge,Rowe/Waterbury/Hartland,New London
Stockbridge,Rowe/Waterbury/Hartland,Savin,Glastonbury
Cambrian Hoosac,Waramaug
(Unnamed)
Everett,Cavendish,Plainfield

bold=fossiliferous calcareous ()=largely volcanic

Mid Atlantic states

South and West of the Hudson River, there is probably a wide strip of Dunnage zone rocks arcing down toward Alabama. But the Eastern part of it is covered with Cretaceous and Tertiary sediments of the Atlantic Cover Terrane. The rocks exposed West of these relatively young sediments and East of the occasionally fossiliferous Taconic-Laurentian Margin Terrane are generally fairly deformed, non-fossiliferous and in large part buried under Triassic basin rocks. There are possibly fossil exposures in the Southern part of this region where it is broader. The Dunnage zone roughly corresponds to the Piedmont region between the Appalachians and the coastal lowlands. The boundary between Humber rocks and Laurentian rocks appears to swing inland in the Southern Appalachians and it would possibly not be surprising to see the Humber-Dunnage boundary swing West as well. In the Carolinas, the Carolina Terrane exposed on the East side of the Piedmont is thought -- on the basis of a few fossils -- to be Avalonian

Some of the formations listed below may belong in the Taconic-Laurentian margins Terrane and vice versa -- especially in Southeast Pennsylvania and Georgia where the sources I have examined so far do not differentiate the two.

Mid Atlantic States-Southeast Coast

Age Formations
Cretaceous-Tertiary Atlantic Margin Cover
Triassic Newark Supergroup
Undifferentiated Lower Paleozoic (Devonian-Cambrian) Peach Bottom
Arvonia
Cardiff
Irvington Grove
Virgilina Tomstown
Virgilina Urbana
Undifferentiated Late Proterozoic Sediments Virgilina Sugarloaf Mt
Libertytown
Sams Creek
Swift Run Wakefield
Silver Run
Liamsville
Lynchburg
Marburg
Wilmington-Wissahickon
Cockeysville
Setters
Baltimore Gneiss Baltimore Gneiss

bold=fossiliferous calcareous ()=largely volcanic

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

Localities

No recommended localities. Examine the fossil list for Nova Scotia. All Devonian and older localities in the Southern half of the province will be Meguman.

Maps and Figures

Regional Setting

Geologic Maps

Formations

Geology Background (Shelf,Slope and Fan Deposits)

References

.https://en.wikipedia.org/wiki/Meguma_terrane .https://gsa.confex.com/gsa/2008NE/finalprogram/abstract_134961.htm .https://gsa.confex.com/gsa/2007NE/finalprogram/abstract_117716.htm .http://museum.novascotia.ca/mnh/nature/nhns/t2/t2-2.pdf may have moved to https://ojs.library.dal.ca/NSM/article/view/3774/3457

.http://www.nrcan.gc.ca/earth-sciences/%20/7538

.http://www.nrcresearchpress.com/action/cookieAbsent .https://journals.lib.unb.ca/index.php/ag/article/viewFile/1814/2178

.good gelogic map of Nova Scotia http://earthsciences.dal.ca/people/schenk/Meguma/Meguma_Field_Guide.html .https://web.archive.org/web/http://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 http://bookstore.aapg.org/

.[The Acadian orogeny By David C. Roy, James William Skehan p106ff Google Books http://books.google.com/books?id=PU_RpfZ_2YgC&amp;pg=RA1-PA107&amp;lpg=RA1-PA106&amp;ots=Ny8-xswbKN&amp;dq=new+brunswick+stratigraphy+dunnage&amp;output=html]

.New Brunswick Bedrock Geology http://web.archive.org/web/20160501222531/http://www2.gnb.ca/content/gnb/en/departments/energy.html

.Newfoundland Geology http://www.nr.gov.nl.ca/mines&en/geosurvey/publications/cr2005/squires.pdf

.http://www.nr.gov.nl.ca/mines&en/geosurvey/publications/cr2000/OBrien.pdf

.http://www.journals.uchicago.edu/doi/abs/10.1086/515976?journalCode=jg .http://www.ramblermines.com/files/Rambler_Competent_Persons_Technical_Report_Sept2004.pdf

.New Hampshire Bedrock Geology http://geology.about.com/library/bl/maps/blnewhampshiremap.htm

.Vermont Geology Info http://dec.vermont.gov/geological-survey/


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.

This page will have been validated as Valid HTML 4.01 Transitional prior to posting on the web site. W3C Logo Image omitted.