The Nastapoka arc is a distinctively arcuate segment of the coastline of the southeastern shore of Hudson Bay in

Quebec Quebec ( ; )According to the Canadian government, ''Québec'' (with the acute accent) is the official name in Canadian French and ''Quebec'' (without the accent) is the province's official name in Canadian English is one of the thirtee ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

, that extends from the most northerly of the Hopewell Islands to Long Island near the junction with

James Bay James Bay (french: Baie James; cr, ᐐᓂᐯᒄ, Wînipekw, dirty water) is a large body of water located on the southern end of Hudson Bay in Canada. Both bodies of water extend from the Arctic Ocean, of which James Bay is the southernmost pa ...

. It is a prominent, near-perfect circular arc, covering more than 160° of a 450-km-diameter circle.Beals, C.S., 1968. ''On the possibility of a catastrophic origin for the great arc of eastern Hudson Bay.'' In: Beals, C.S. (Ed.), pp. 985-999. ''Science, History and Hudson Bay'', Vol. 2, Department of Energy Mines and Resources, Ottawa. While the circular shape has led to suggestions that it represents an

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

, there is no evidence for this hypothesis, and it is thought to have been formed as a result of

lithospheric flexure Lithospheric flexure (also called regional isostasy) is the process by which the lithosphere (rigid, thin outer layer of the Earth) bends under the action of forces such as the weight of a growing orogeny or changes in ice thickness related to glac ...

during the

Trans-Hudson orogeny The Trans-Hudson orogeny or Trans-Hudsonian orogeny was the major mountain building event (orogeny) that formed the Precambrian Canadian Shield and the North American Craton (also called Laurentia), forging the initial North American contine ...

Geology

The bedrock that comprises the

shoreline A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...

and landscape that lies inland of the Nastapoka arc largely consists of

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Arc ...

age rocks of the Superior craton. In areal distribution, these rocks consist of about 60% Archean

granitic A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz- ...

plutons and granitic

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

. The granitic rocks include typically foliated

granodiorite Granodiorite () is a coarse-grained ( phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gr ...

quartz diorite Quartz diorite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less potassium feldspar. Quartz Quartz is a hard, ...

quartz monzonite Quartz monzonite is an intrusive, felsic, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic (coarse-grained) to porphyritic granitic rock. The plagiocl ...

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

s; related intrusive rocks; and their

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

equivalents. Less common in occurrence are layered gneisses,

migmatite Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock th ...

s and hybrid rocks that often form easterly trending linear belts.Martini, I.P., 1986. ''Canadian Inland Seas'', Elsevier oceanography series, Vol. 44, Elsevier Science, 494 pp. About 20% of the Superior craton consists of metamorphosed Archean

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

and

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s. They occur as tightly folded, lightly metamorphosed (

greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite ...

) preserved in easterly trending, elongate to irregular, structural basins known as greenstone belts. The most abundant volcanic rock is metamorphosed

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

s that exhibit primary structures, e.g.

pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of discont ...

s, indicative of underwater volcanic

eruption Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...

s. The metamorphosed sedimentary strata consist largely of interbedded mudstones and

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

s exhibiting

sedimentary structure Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particl ...

s indicative of

turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites wer ...

s. A large triangular area surrounding Richmond Gulf, which includes a short segment of the Nastapoka arc, is underlain by about of undeformed, pink and red, mainly fluvial,

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

-rich

that is interbedded with minor beds of conglomerate and basaltic, subaerial, lava flows. These strata, which are known as the ''Richmond Gulf Group'', rest

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

on Archean crystalline rocks of the Superior craton and are now preserved by downfaulting only within the Richmond Gulf Graben. The graben, its faults, and the Richmond Gulf Group are unconformably overlain by the strata of the Nastapoka Group.Chandler, F.W. and Schwarz, E.J., 1980. ''Tectonics of the Richmond Gulf area, northern Quebec-a hypothesis''. ''Current research, part C.'' Geological Survey of Canada, Paper, pp. 59-68.Chandler, F.W. and Parrish, R.R., 1989. ''Age of the Richmond Gulf Group and implications for rifting in the Trans-Hudson Orogen, Canada.'' ''Precambrian Research'', 44(3-4), pp. 277-288. The portion of Hudson Bay immediately offshore of and partially encircled by the Nastapoka arc is underlain by Early Proterozoic strata of the Nastapoka Group. Adjacent to the Nastapoka arc, these strata form a

homocline In structural geology, a homocline or homoclinal structure (from old el, homo = same, cline = inclination), is a geological structure in which the layers of a sequence of rock strata, either sedimentary or igneous, dip ...

that dips gently westward and consist of unmetamorphosed to slightly metamorphosed sandstone,

stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). T ...

-bearing

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness ...

, and basalt. Further west, exposed in the Ottawa and

Belcher Islands The Belcher Islands ( iu, script=latn, ᓴᓪᓚᔪᒐᐃᑦ, Sanikiluaq) are an archipelago in the southeast part of Hudson Bay near the centre of the Nastapoka arc. The Belcher Islands are spread out over almost . Administratively, they belo ...

, the strata of the Nastapoka Group become highly faulted and tightly, often isoclinally, folded. These strata unconformably overlie Archean strata of the Superior craton and Proterozoic Richmond Group. The

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

between the Early Proterozoic Nastapoka Group and the underlying Archean Superior craton lies just inland of Nastapoka arc as defined by the edge of arcuate eastern coastline of Hudson Bay. The unconformity consists of undeformed stromatolite-bearing dolomites overlying either foliated Archean granodiorite or the tilted and eroded strata of the Richmond Gulf Group. A thin, conglomeratic quartz sandstone separates the dolomites from the underlying strata and forms the base of the Nastapoka Group.Ciesielski, A., 1983
''Géologie d'une portion sud de l'arc Nastapoka, Baie D'hudson, Quebec.''
Geological Survey of Canada, Open File no. 896. Scale 1:250,000

Origin

The origin of the Nastapoka arc has been a source of disagreement and discussion among

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althoug ...

s, other

Earth scientist Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spheres, ...

s, and planetary geologists. Noting the paucity of impact structures on

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

in relation to the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

and

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

and remarkable curvature of the shoreline of this part of Hudson Bay, Beals proposes that the Nastapoka arc is possibly part of a Precambrian extraterrestrial meteorite impact structure that is comparable in size to the

Mare Crisium Mare Crisium (Latin ''crisium'', the "Sea of Crises") is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. The basin is of the Pre-Imbrian period, 4.55 to 3.85 billion years ago. Characteristics Mare Cri ...

on the Moon. In the same volume, WilsonWilson, J.T., 1968. ''Comparison of the Hudson Bay arc with some other features.'' In: Beals, C.S. (Ed.), pp. 1015–1033. ''Science, History and Hudson Bay'', Vol. 2. Department of Energy Mines and Resources, Ottawa. evaluates Beals' interpretation and proposes an alternative hypothesis that the Nastapoka arc formed as the result of a continental-scale collision of pre-existing Archean continents and closure of an ancient ocean basin. In August 1972, Robert S. Dietz and J. Paul BarringerDietz, R.S., and Barringer, J.P., 1973. ''Hudson Bay Arc as an Astrobleme: a Negative Search''. Meteoritics. 8(1), pp. 28–29. conducted an extensive search of much of the Nastapoka arc with

First Nations First Nations or first peoples may refer to: * Indigenous peoples, for ethnic groups who are the earliest known inhabitants of an area. Indigenous groups *First Nations is commonly used to describe some Indigenous groups including: **First Natio ...

and

Inuit Inuit (; iu, ᐃᓄᐃᑦ 'the people', singular: Inuk, , dual: Inuuk, ) are a group of culturally similar indigenous peoples inhabiting the Arctic and subarctic regions of Greenland, Labrador, Quebec, Nunavut, the Northwest Territories ...

canoe A canoe is a lightweight narrow water vessel, typically pointed at both ends and open on top, propelled by one or more seated or kneeling paddlers facing the direction of travel and using a single-bladed paddle. In British English, the ter ...

s and fishing boats in an investigation of its impact origin. They examined the abundant and extensive rock exposures that occur within the region of the Nastapoka arc and found a complete lack of

shatter cones Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions. They are evidence that the rock has been subjected to a shock with pressures in the ra ...

suevite Suevite is a rock consisting partly of melted material, typically forming a breccia containing glass and crystal or lithic fragments, formed during an impact event. It forms part of a group of rock types and structures that are known as im ...

-type or other unusual melt rocks,

pseudotachylite Pseudotachylyte (sometimes written as pseudotachylite) is an extremely fine-grained to glassy, dark, cohesive rock occurring as veinsTrouw, R.A.J., C.W. Passchier, and D.J. Wiersma (2010) ''Atlas of Mylonites- and related microstructures.'' Sprin ...

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

, radial faults or fractures, unusual injection

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

s, or any other evidence of shock metamorphism. Based on numerical modelling, regional geology, and lack of evidence for a hypervelocity impact, the current, general consensus is that it is an arcuate boundary of tectonic origin between the Belcher Fold Belt and crystalline rocks of the Superior craton created during the

about 2.0–1.8 billion years ago.Hynes, A., 1991. ''The gravity field of eastern Hudson Bay: Evidence for a flextural origin for the Hudson Bay (Nastapoka) Arc?.'' ''Tectonics'', 10(4), pp. 722–728.Darbyshire, F.A., and Eaton, D.W., 2010. ''The lithospheric root beneath Hudson Bay, Canada from Rayleigh wave dispersion: No clear seismological distinction between Archean and Proterozoic mantle'', ''Lithos''. 120(1–2), 144–159, doi:10.1016/j.lithos.2010.04.010.Eaton, D.W., and Darbyshire, F., 2010. ''Lithospheric architecture and tectonic evolution of the Hudson Bay region'', ''Tectonophysics''. 480(1–4), 1–22, doi:10.1016/j.tecto.2009.09.006. However, other Earth scientistsGoodings, C.R. & Brookfield, M.E., 1992. ''Proterozoic transcurrent movements along the Kapuskasing lineament (Superior Province, Canada) and their relationship to surrounding structures.'' ''Earth-Science Reviews'', 32: 147–185.Bleeker, W., and Pilkington, M., 2004. ''The 450-km-diameter Nastapoka Arc: Earth's oldest and largest preserved impact scar?'' ''Program with Abstracts - Geological Association of Canada; Mineralogical Association of Canada: Joint Annual Meeting, 2004'', Vol. 29, pp. 344. have proposed that the preexisting structure of an older Archean impact structure might have been reactivated during and modified by the Trans-Hudson orogeny to form the Nastapoka arc. The deformation accompanying the Trans-Hudson orogeny could have masked evidence of such an Archean impact.

References

External links

Aerial Exploration of the Nastapoka Arc
Hudson Bay Geology of Quebec {{Quebec-geo-stub

Geology

Origin

See also

References

External links