Paleogeology

Paleogeology, Paleoclimate, in relation to Evolution of Life on Earth

Regional Geologic History

CanadaAcasta Gneiss ComplexSlave craton
GreenlandGreenland
USUS westUS west - alternative interpretations

maps: Canada : Alberta : British Columbia : Manitoba : New Brunswick : Newfoundland and Labrador : Nova Scotia : Nunavut : Northwest Territories : Ontario : Quebec : Saskatchewan :
Yukon Territory :
maps: Greenland: main periods of crust formation and orogeny :

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Acasta Gneiss Complex

Cleaned exposures of the Acasta gneisses at their discovery site. Ancient tonalites (4.03 Ga) occur on left side of the picture, and are intruded by highly deformed younger granite sheets and mafic dykes. Courtesy of Natural Resources Canada The Acasta Gneiss Complex is located in Canada's northwestern Slave Province, NWT. (more images at Geomorphology, lithology, geological history of the Slave craton.)

Dating from the Hadean at least 4.03-4.055 Ga, is Earth's oldest known crustal rock outcrop. The complex comprises mostly Gray gneiss (granodioritic gneiss), White gneiss (tonalitic to granitic gneiss), foliated granite (3.6 Ga), and many aplite and basaltic intrusions.


The gneisses are embedded in a large Mesoarchean-Hadean basement complex, which lies beneath the west-central Slave craton. This craton is a complex containing ancient collisional orogenic structures and accreted fragments. Its rock formations comprise ancient crust, juvenile arcs, mature arcs, and intervening accretionary prism material. All tectonostratigraphic units are cut by strike-slip faults and later-emplaced granitic plutons, and many strata show evidence of late extensional collapse structures. Most rock types are derived from basaltic protoliths. The basement complex is overlain by Neoarchean supracrustal sequences has plutonic intrusions dating from 2.72-2.58 Ga. (Synvolcanic plutons date from 2.72-2.67 Ga and granitic batholiths date from 2.59-2.58 Ga (late-orogenic).)

Zircon geochronology has demonstrated that at least four magmatic or metamorphic events occurred in the Acasta Gneiss Complex: 4.0~3.95 Ga, 3.75 Ga, 3.6 Ga and 3.4 Ga. Zircon from a White gneiss has revealed an age of 4,203 +/- 28Myr, implying that granitic continental crust was more widespread than previously thought, and that it was reworked into Early Archean continental crust.[r]

Weathering of the Acasta Gneisses indicates that they have been exposed by erosion for a considerable time. In other areas, quartzite, banded iron formations, and volcanic rocks sit unconformably atop the gneisses.

The Archean gneisses were united at the core of the Slave protocontinent, at least 2.9 Ga. In the nucleus of the old continent, juxtaposed rocks sometimes differ by a billion years in age, probably indicating episodic volcanic eruptions, fed by broad plumes of rock ascending periodically from the deep mantle, rather than having resulted from gradual tectonic accretion of crust at plate boundaries. Such lava flows presumably gradually built up the continental nuclei as a part of mantle mafic-ultramafic and crustal acid magmatism.

The Archean rocks subsequently experienced uplift, possibly because of a hot mantle plume. Elevation of the continent caused erosion, creating the unconformity. Two volcanic layers above the unconformity date to a little over 2.8 Ga. Subsequently the plume dissipated, and the region sank beneath an ancient ocean, accumulating sediments: quartz-rich sandstone, and then the banded iron formations (Yellowknife Supergroup). The volcanics accumulated as the Slave protocontinent was rifted apart about 2.8-2.7 Ga.

links: formation: Acasta Gneisses, 2; Acasta Gneisses; AG complex; hand-specimen: Acasta Gneiss, 1, 2, 3; Tonalite gneiss; Acasta Gneiss, 2; close-up: Acasta Gneiss; sem: Acasta Gneiss - sem; article (1999); Acasta gneiss and another old zircon; abstract, 2 pdf; article; news; Microstructure of Neoarchean zircon from the Acasta gneiss complex ... (pdf):

image of Acasta Gneisses courtesy of Natural Resources Canada (source website, Figures)

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Greenland

Much of Greenland is composed of a Precambrian shield that affected by major tectonic episodes: ~2.6 Ga (Archean), ~1.9 Ga (lower Proterozoic, Ketilidian orogeny), and ~1 Ga (middle Proterozoic); ~400 Ma (Caledonian orogeny).

The Itsaq Gneiss Complex in southwest Greenland comprises the most extensive and best-preserved fragment of early Archean continental crust.

Southwestern Greenland's Isua Greenstone Belt is an Archean greenstone belt that dates from 3.8-3.7 Ga. The belt's five tectonic domains contain the best preserved, metavolcanic, metasedimentary and sedimentary rocks on Earth.

Most of the Isua Greenstone Belt comprises fault-bounded rock assemblies derived from basaltic, high-Mg basaltic pillow lavas, and pillow lava breccias, intruded by numerous sheets of tonalite, chert-banded iron formations, and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks.

The recrystallized ultramafic bodies within the belt are thought to be intrusions or komatiitic flows, and these komatiites are very similar to the 3.5 Ga Barberton basaltic komatiites of South Africa. Both are Archean equivalents of modern boninites that are produced by hydrous melting in subduction zones. The boninitic geochemical signatures provide evidence that plate tectonic processes were responsible for the creation of the belt, while the pillow breccias and basaltic debris indicate that liquid water existed on the surface at the time of their formation.

Chert banded iron formations comprise the commonest sedimentary rocks, and the 3.5 Ga Isua basalt-komatiite-chert was parental to the enclosing 2.8 Ga Amitsoq Tonalite-Trondhjemite-Granodiorite (TTG) gneisses. It is considered that direct mantle melting produced the diorites and high magnesian granodiorites found in these Archaean cratons.

Quartz globules in undeformed pillow breccias are associated with a complex system of quartz veins that probably represent remnants of a sea-floor hydrothermal system that was contemporaneous with lava eruption and pillow basalt formation (3.75 Ga).

In the Southwestern Isua Greenstone Belt, kyanite in muscovite-rich schists developed when the region was subjected to deformation in high strain zones during the late Archaean. In the 2.7 Ga Manjeri Formation in the Belingwe Greenstone Belt, oxide and sulphide facies ironstones indicate a complex bacteria/archaea eclogical community.

On the opposite coast of Greenland from the Isua Greenstone Belt, the Eocene Skaergaard intrusion lies in contact with Archean gneisses. Geological features of the 55 Ma layered intrusion are the subject of excellent photographs on Kurt Hollocher's webpage and a Vanishing Ice, and view Greenland's Thinning Icecap.)

Maps : Greenland: 1:2 500 000 geological map; 1: 500 000 geological maps : geological maps of Greenland; main periods of crust formation and orogeny;

links: images: formations: Isua: aerial of Isua, satellite image of Isua (large), spectacular 20 000 nT anomaly of the Isua banded iron formation (magnetic total field); thrust-nappes in tonalite mylonite in the western footwall to the Isua Greenstone Belt; cross-cutting felsic dikes in the "Central gneisses", Isua Greenstone Belt, contact between a mafic Tarssartoq dike and tonalitic gneiss, Isua, Isua, oldest conglomerate, ancient sedimentary, oldest sedimentary, oldest pillow basalt, flattened pillow basalt, pillow lava, oldest ophiolite, oldest BIF, den båndende jernmalm ved Isua (BIF), BIF contact with ultramafic, felsic gneisses, rock types of the Isua, finely banded and intensely folded calc-silicate rocks (thought to have formed by carbonate metasomatism of predominantly basaltic protoliths), magnetite-tremolite, hand-specimen BIF, thin-section of graphite-bearing garnet in the ~3.8 Ga metasediment from the Isua Greenstone Belt; non-Isua: strongly metamorphosed Precambrian rocks, near Kangerlussuaq (West Greenland); deformed granites, gneisses and migmatites, Nordvestfjord; granitic vein, Liverpool Land Grundvigskirken - a needle-like mountain peak of hard, Caledonian cristalline rocks (granite, gneiss) in the Øfjord, shaped by ice-age glaciers; close-up: strongly deformed and metamorphosed rocks, Øfjord, inner Scoresbysund; gneiss, Nanortalik; webpages: The Precambrian shield; The Caledonian fold belt; Old Red and New Red; Basalts of the Blosseville Küste; World's 'oldest' volcanic rocks (Canada); Oldest evidence of photosynthesis

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Slave craton

northwestern Canadian Shield courtesy of USGS. Click to go to larger original. In this new map, shield rocks are subdivided into dozens of units distinguished by age, lithology and originThe Slave craton is a complex containing ancient collisional orogenic structures and accreted fragments, which now sits at the northeast of the North America craton. (map, geological map, very simplified, maximum protolith age, cross-section, northern Slave mafic intrusions).

(The Slave Province is "A" in the image at left - magnify - legend)

(more images at  Geomorphology, lithology, geological history of the Slave craton.)

Rock formations of the Slave Craton comprise ancient crust including the Acasta Gneiss Complex (A), which contains the oldest known intact Earth rocks. The craton also contains juvenile arcs, mature arcs, and intervening accretionary prism material. All tectonostratigraphic units are cut by strike-slip faults and later-emplaced granitic plutons, and many strata show evidence of late extensional collapse structures.

The Slave craton is a mere fragment of ancient crust, surrounded by Paleoproterozoic rifted margins. Cratons such as the Slave are remnants that preserve parts of the much larger, ancient pre-tectonic and tectonic systems in which they were generated. The Slave craton originated from the break-up of a much larger late Archean landmass and preserves a complex and spatially heterogeneous record of crustal growth that spans nearly 1.5 billion years. (garnets in subcontinental lithospheric mantle (SCLM))

large late Archean supercontinent Kenorland (~2.7 Ga)
_____________________________
late Archean parental supercraton Sclavia Slave _ + Rae (~2 Ga) Laurentia
__________________________________________________________+
_________________________________________Baltica, Ukraine, Amazonia, Australia
_____________________________________________ and possibly Siberia, North China and Kalahari
________________________________________________

______________________Columbia (Nuna, Hudsonland, Hudsonia) 1.8-1.5 Ga

The original landmass could have been the speculative late Archean supercontinent Kenorland, followed by, or perhaps more likely instead, a smaller landmass referred to as the supercraton Sclavia (a late Archean supercraton of unknown size and configuration, which is considered to be parental to the Slave craton).

The Archean gneisses were united at the core of the Slave protocontinent, at least 2.9 Ga. In the nucleus of the old continent, juxtaposed rocks sometimes differ by a billion years in age, probably indicating episodic volcanic eruptions, fed by broad plumes of rock ascending periodically from the deep mantle, rather than having resulted from gradual tectonic accretion of crust at plate boundaries. Such lava flows presumably gradually built up the continental nuclei as a part of mantle mafic-ultramafic and crustal acid magmatism.

Within the later Laurentian/Nuna supercontinent, the Archean rocks subsequently experienced uplift, possibly because of a hot mantle plume. (time chart, Yellowknife domain time chart) Elevation of the continent caused erosion, creating the unconformity. Two volcanic layers above the unconformity date to a little over 2.8 Ga. Subsequently the plume dissipated, and the region sank beneath an ancient ocean, accumulating sediments: quartz-rich sandstone, and then the banded iron formations (Yellowknife Supergroup). The volcanics accumulated as the Slave protocontinent was rifted apart about 2.8-2.7 Ga. (Hadean to Mesoarchean basement of CSBC)

The Slave craton has thick sequences of tholeiitic greenstone sequence date from ~2.7 Ga, younger arc-like sequences from 2.69-2.61 Ga, and extensive turbidite sequences from about 2.68 to 2.62 Ga. Syn-orogenic conglomerates were deposited about 2.6 Ga. Some of Canada's largest volcanogenic massive sulfide (E) deposits lie in arc-like sequences formed above the attenuated basement and in progressively widening, juvenile, back-arc-like basins. (evolution of Slave craton)

Archean cratons of similar age (Zimbabwe, Wyoming) also have quartzite layers and banded iron formations lying over gneisses. Geologist, Wouter Bleeker of the Geological Survey of Canada (GSC) says, "These may be several pieces of a larger continent of which the Slave nucleus is just one remnant."

Parts of the Central Slave Basement Complex contain quartzite gneiss similar to the quartzite found in the Southern Cross Province of the Yilgarn craton.

Many geologists dispute any possibility that plate tectonics could have operate on the early Earth. The planet was much hotter 3-4 billion Ga – too hot, many believe, for formation of rigid continental plates. Further, the hot surface layer would have been too buoyant to sink in subduction zones, preventing development of plate-tectonic cycles. Eventually Earth cooled sufficiently for the crust to form rigid, less buoyant plates, so that plate tectonics evolved as the dominant geophysical force. Bleeker speculates that the breakup of the Slave nucleus some 2.8-2.7 Ga could mark the beginning of plate tectonics.

The oldest parts of the Slave province lack the long, narrow belts of accreted and deformed rocks that are key signatures of plate tectonics. Mapping of the Slave craton has even disconfirmed some supposed relics of ancient plate tectonics.

Key elements of the geology of the Slave craton are illustrated in field photographs on the Slave Craton website, refer to cross-section of the craton (legend):
A. The Acasta gneisses at their discovery site, with ancient tonalites (4.03 Ga) on left.
B. Basal quartzites of the Central Slave Cover Group overlying basement of the Central Slave Basement Complex. To right, in low foreground are low-weathering basement gneisses. The dark ridge in background has ca. 2.7 Ga basalts overlying the quartzites.
C. Syn-Kam Group quartz-porphyritic tonalite intrusion (ca. 2713 Ma), silling into the northern part of the Yellowknife greenstone belt. The large sill-like body is cut by somewhat younger mafic dykes that likely fed the upper part of the greenstone belt. Inset altered quartz-porphyritic tonalite.
D. Quartz porphyritic rhyolite breccia with carbonate matrix, typical for the uppermost part of 2690-2660 Ma felsic volcanic edifices.
E. Massive sulphide mineralization of the Sunrise deposit, associated with ca. 2670 Ma felsic volcanic rocks just below the interface with the Burwash Formation turbidites.
F. Thickly bedded sandy turbidites typical of the Burwash Formation in its type area east of Yellowknife. Oblique areal photo shows F1 syncline refolded by north-northwest trending F2 folds.
G. Silicate facies iron formation interlayered with turbiditic greywackes, George Lake, northeastern Slave. This banded iron formation hosts significant epigenetic gold mineralization.
H. Passive margin strata of the Coronation Supergroup (Epworth Group) overlying the western margin of the rifted Slave craton, structurally at the base of Wopmay orogen.
I. Dense Proterozoic mafic dyke swarms cutting extended Slave crust and its cover

Legend for Field photos accompanying cross-section:
B. Typical 2.95 Ga foliated tonalites of the Central Slave Basement Complex with transposed 2734 Ma mafic dykes.
C, D & E. Basal quartz pebble conglomerate, fuchsitic quartzite, and banded iron formation of the Central Slave Cover Group that overlies the basement complex.
F. Variolitic pillow basalts of the Kam Group, Yellowknife.
G. Syn-Kam Group K-feldspar porphyritic granodiorite pluton in basement below greenstone belts.
H. Polymict conglomerate, including 10-30 cm granitoid cobbles, which occurs locally at the base of the younger, 2.69-2.66 Ga, volcanic cycle.
I. Carbonate-cemented rhyolite breccia typical for the younger volcanic cycle.
Well-preserved sub-biotite grade turbidites in the core of the Yellowknife structural basin, showing graded bedding and load casts.
J. Aerial photo of large scale, upright, fold structures in turbidites of the Yellowknife structural basin.
K. Late-tectonic conglomerates, formation: Acasta Gneisses, 2; Acasta Gneisses; AG complex; hand-specimen: Acasta Gneiss, 2; Tonalite gneiss; Acasta Gneiss, 2; close-up: Acasta Gneiss; sem: Acasta Gneiss - sem; article (1999); Acasta gneiss and another old zircon; abstract, 2 pdf; article; news; Microstructure of Neoarchean zircon from the Acasta gneiss complex ... (pdf); websites: The Slave Craton: Geological and Metallogenic Evolution : Abstract
Introduction : Ancient Basement Complex : The Cover Sequence : Ca. 2.73-2.70 Ga Tholeiitic Volcanism : Post-2.70 Ga Volcanism : Ca. 2.68-2.66 Ga Sedimentation : Ca. 2.65-2.63 Ga Closure Of The Burwash Basin : Post-2.63 Ga Turbidites : 2.60-2.58 Ma, Final Orogenesis : Cratonization And Beyond : Summary : References : Table : Figures : Appendix

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US

US geological tapestry

US terrain or topographical relief









Click here for geologic history of the western US.



US state boundaries

















links: animation: click and drag panorama; webpages: USGS links to boundaries

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US west

Dr. Ron Blakey's website provides a very good overview, complete with maps, of the geologic history of the southwestern US.

Geologists continue to debate the merits of alternative interpretations of the geological history, from 270 to 150 Ma, of the western US. In one interpretation, most elements of the western US are assumed to have been derived tectonically from North America. In the second tectonic interpretation ("or"), in which some exotic elements are considered to have originated far from North America (accreted terranes).

Roll-over the dated links for preview images of maps of the time periods, or visit the website for links to major periods. Click here to follow continental and offshore alternative sequences.

Playlist: Geology of the Grand Canyon (not my videos): link.



[links: all maps in sequence: 1.7 Ga, 1.1 Ga, 650 Ma, 510 Ma, 470 Ma, 470 Ma w, 430 Ma, 430 Ma w, 370 Ma, 370 Ma w, 340 Ma, 340 Ma w, 310 Ma, 310 Ma w, 280-270 Ma, 280-270 Ma w, or 270 Ma w, 250 Ma, 250 Ma w, or 250 Ma w, 240 Ma, 240 Ma w, 215 Ma, 215 Ma w, 200 Ma, or 200 Ma w, 170 Ma, or 170 Ma, 150 Ma, or 158 Ma, or 155 Ma, or 150 Ma, 145 Ma, 130 Ma, 90 Ma, 75 Ma, 65 Ma, 50 Ma, 35 Ma, 20 Ma, 10 Ma ; geological age of rocks by State: Arizonia, California, Colorado, New Mexico; Nevada; Utah (adjacent Idaho, Kansas, Nebraska, Oregon, Texas, Wyoming), boundaries; webpages: Geological History of the western US: Paleogeology of the southwestern US; Precambrian, Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Tertiary]

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US w alternatives

Dr. Ron Blakey's website provides a very good overview, complete with maps, of the geologic history of the western US.

Geologists continue to debate the merits of alternative interpretations of the geological history, from 270 to 150 Ma, of the western US. In one interpretation, most elements of the western US are assumed to have been derived tectonically from North America. In the second tectonic interpretation ("or"), in which some exotic elements are considered to have originated far from North America (accreted terranes).

Roll-over the 'dated' links for preview images of maps of the time periods, or visit the website for links to major periods. Click here for all geological maps in sequence.

roll-over links for maps: continental sequences: 1.7 Ga, 1.1 Ga, 650 Ma, 510 Ma, 470 Ma, 430 Ma, 370 Ma, 340 Ma, 310 Ma, 280-270 Ma, 250 Ma, 240 Ma, 215 Ma, 200 Ma, 170 Ma, 150 Ma, 145 Ma, 130 Ma, 90 Ma, 75 Ma, 65 Ma, 50 Ma, 35 Ma, 20 Ma, 10 Ma ; offshore, to west of continent: contiguous interpretation: 470 Ma w, 430 Ma w, 370 Ma w, 340 Ma w, 310 Ma w, 280-270 Ma w, 250 Ma w, 240 Ma w, 215 Ma w, 200 Ma w, 170 Ma w, exotic interpretation: or 270 Ma w, or 250 Ma w, or 170 Ma, or 158 Ma, or 155 Ma.

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. . . stratifying since 10/06/06