An ice sheet is a permanent layer of ice covering an extensive tract of land or sea.
An ice field is more than one glacier with a common source. Several ice fields can become an ice cap. When the ice cap becomes large enough it is an ice sheet.
Shown in the northern hemisphere image on the right is the rock distribution of sea ice and land ice.
Antarctic ice sheetsEdit
"The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of North America, the Weichselian ice sheet covered northern Europe and the Patagonian Ice Sheet covered southern South America."
At the south pole, Antactica, there is also an extensive ice sheet shown in the second image on the right. Seasonally, when the North polar sea ice and ice sheet has been contracting, the South polar sea ice and ice sheet has been expanding.
Apparent global warming that was progressively melting more and more of the north polar ice sheet each year has been countered by progressive expansion of the south polar ice sheet.
Greenland ice sheetsEdit
At the right is a satellite composite image of the ice sheet over Greenland.
"Active and passive microwave satellite data are used to map snowmelt extent and duration on the Greenland ice sheet. The passive microwave (PM) data reveal the extreme melt extent of 690,000 km2 in 2002 as compared with an average extent of 455,000 km2 from 1979–2003."
"Several PM-based melt assessment algorithms [Mote and Anderson, 1995; Abdalati and Steffen, 1995] are applicable to Scanning Multi-channel, Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I) instruments providing near-continuous coverage since 1979. The PM data as gridded brightness temperatures on polar stereographic grids (25 km resolution) [used] are from the National Snow and Ice Data Center [Maslanik and Stroeve, 2003], containing daily data spanning 25 melt seasons from 1979 to 2003."
In the second image on the right, (a) "shows the probabilities of the observed melt behavior on the Greenland ice sheet for several large melt years and indicates the extreme melt anomaly observed in northeastern Greenland in 2002."
"Prior to 2002, both 1995 and 1998 were extreme melt years in terms of maximum areal extent and total melt. During 1995 melt was dominated by a high frequency of melt along the western margin of the ice sheet. During 1998 melt was spatially diverse with slightly more melt than usual in the northeast and southwest. However, the high frequency melt in 2002 in the northeast and along the western margin is unprecedented in the PM record with a log likelihood of occurrence that is 35% lower than the previous record melt anomaly in 1991."
(c) "depicts the magnitude of the increasing trends in melt extent on a daily basis over the last 25 years. Although there is a large amount of inter-annual variability in melt extent on a given day, 56 days show statistically significant (alpha = 0.1) increasing trends in melt area."
"Melt along the west coast was extensive during 2002 but not atypical for large melt years. However melt in the north and northeast was highly irregular both in terms of extent and frequency. Nearly 3,000 km2[(b)] were classified as melting during 2002 that had not previously melted during any other year between 1979 and 2003."
The figure at the left "presents QSCAT backscatter and diurnal signatures, and ETH/CU AWS air temperature." Half-decade records for ETH/CU Camp station: (a) Top panel is for QSCAT backscatter, (b) middle panel for QSCAT diurnal signature, and (c) bottom panel for air temperature measured at the AWS site.
At the lower right QSCAT melt maps are shown on the climatological peak-melt day (1 August). Red color represents current active melt areas, light blue is for areas that have melted but currently refreeze, white is for areas that will melt later, and magenta is for areas that do not experience any melt throughout the melt season. The dark blue color surrounding Greenland is the ocean mask.
"QSCAT mapping can reveal details of the spatial pattern of surface melt evolution in time. There are large variabilities in melt extent and melt timing over different regions. [The figure at tje lower right] confirms that 2002 has the most extensive areal melt. In 2002, the northeast quadrant of the Greenland ice sheet, extending well into the dry snow zone, experienced at least some melt where melt never happened before (from satellite data records to date). Since the beginning of the QSCAT data record (July 1999), the smallest spatial extent of melt occurred in 2001, and melt extent was similar for years 2000 and 2003."
"To provide a direct comparison of PM and QSCAT results, we overlay results for PM melt extent and QSCAT number of melt days in [the figure at the lower left] for years 2000–2003. PM XPGR melt extent is approximately confined to QSCAT melt areas experiencing 2 weeks or more of melting time [the figure at the lower left]. QSCAT melt areas outside of the PM melt extent represent the surface that has less melt corresponding to about 15 melt days or less. This is consistent with the relationship of relative melt strength measured by active and passive data as discussed above. Note that such areas can total up to a large region in year 2002. Surface albedo can reduce considerably once the snow melts for a period of 2 weeks. The albedo reduction may significantly impact the surface heat balance and thus change the mass balance. The large number of melt days around the northern perimeter of the ice sheet, which is shown as the narrow dark-red band in north Greenland in the 2003 map was an anomalous feature [the figure at the lower left]. This band was wider as defined by the PM melt extent in 2002 than in 2003. However, there were more QSCAT melt days in the 2003 northern melt band."
"The comparison reveals that the PM cross-polarized gradient algorithm classifies melt more conservatively than the scatterometer algorithm. The active microwave identifies melt approximately up to two weeks more than the PM at higher elevation in the percolation zone toward the dry snow zone [the figure at the lower left]. Both methods (active and passive microwave) consistently identify melt areas that have a melt duration of at least 10–14 days. The longer snowmelt duration can be sufficient to decrease surface albedo and affect surface heat and mass balance."
Himalayas ice sheetsEdit
Often called the third pole, the image on the right shows the rocky ice sheet over the top of the Himalayas.
Def. "a current of ice in an ice sheet or ice cap that flows faster than the surrounding ice" is called an ice stream.
Def. "a dome-shaped mass of glacier ice that spreads out in all directions" is called an ice cap.
Theoretical ice sheetsEdit
Def. "a dome-shaped mass of glacier ice that covers surrounding terrain and is greater than 50,000 square kilometers (12 million acres)" is called an ice sheet.
Earth has ice sheets, ice caps, ice fields, and glaciers.
The paleolithic period dates from around 2.6 x 106 b2k to the end of the Pleistocene around 12,000 b2k.
The Paleolithic or Palaeolithic is a period in human prehistory distinguished by the original development of stone tools that covers c. 95% of human technological prehistory. It extends from the earliest known use of stone tools by hominins c. 3.3 million years ago, to the end of the Pleistocene c. 11,650 cal BP.
|North America||England (Atlantic Europe)||Maghreb||Italy||Central Europe|
|10,000 years||Flandrian interglacial||Flandriense||Mellahiense||Versiliense||Flandrian interglacial|
|80,000 years||Wisconsin||Devensiense||Regresión||Regresión||Wisconsin Stage|
|140,000 years||Sangamoniense||Ipswichiense||Ouljiense||Tirreniense II y III||Eemian Stage|
|200,000 years||Illinois||Wolstoniense||Regresión||Regresión||Wolstonian Stage|
|450,000 years||Yarmouthiense||Hoxniense||Anfatiense||Tirreniense I||Hoxnian Stage|
|580,000 years||Kansas||Angliense||Regresión||Regresión||Kansan Stage|
|750,000 years||Aftoniense||Cromeriense||Maarifiense||Siciliense||Cromerian Complex|
|1,100,000 years||Nebraska||Beestoniense||Regresión||Regresión||Beestonian stage|
The Permian lasted from 299.0 ± 0.8 to 251.0 ± 0.4 Mb2k.
Late Paleozoic icehouseEdit
The late Paleozoic icehouse, formerly known as the Karoo ice age, was the climate state 360–260 million years ago (Mya) in which large land-based ice-sheets were present on Earth's surface.
"The late Paleozoic icehouse was the longest-lived ice age of the Phanerozoic, and its demise constitutes the only recorded turnover to a greenhouse state."
The Mississippian lasted from 359.2 ± 2.5 to 318.1 ± 1.3 Mb2k.
Prolecanites gurleyi is an index fossil of the Mississippian.
The Silurian spanned 443.7 ± 1.5 to 416.0 ± 2.8 Mb2k.
Hexamoceras hertzeri is an index fossil for the Silurian.
"Rolfe made the important observation that 'Other genera are pre-Devonian and hence cannot be ammonoid aptychi, but Ruedemann's suggestion that aptychi "would naturally also have existed in the Ordovician and Silurian cephalopods" has been largely overlooked'."
"A major glacial episode at c. 440 Ma, is recorded in Late Ordovician strata (predominantly Ashgillian) in West Africa (Tamadjert Formation of the Sahara), in Morocco (Tindouf Basin) and in west-central Saudi Arabia, all areas at polar latitudes at the time. From the Late Ordovician to the Early Silurian the centre of glaciation moved from northern Africa to southwestern South America."
The maximum extent of glaciation developed in Africa and eastern Brazil.
The Andean-Saharan was preceded by the Cryogenian ice ages (720–630 Ma, the Sturtian and Marinoan glaciations), often referred to as Snowball Earth, and followed by the Karoo Ice Age (350–260 Ma).
The Ordovician lasted from 488.3 ± 1.7 to 443.7 ± 1.5 Mb2k.
Def. "a geologic era within the Proterozoic eon; comprises the Tonian, Cryogenian and Ediacaran periods from about 1000 to 544 million years ago, when algae and sponges flourished" is called the Neoproterozoic.
The Baykonurian occurs about 547 Ma.
The Gaskiers glaciation is a period of widespread glacial deposits (e.g. diamictites) that lasted under 340 thousand years, between 579.63 ± 0.15 and 579.88 ± 0.44 million years ago – i.e. late in the Ediacaran Period – making it the last major glacial event of the Precambrian.
Deposits attributed to the Gaskiers - assuming that they were all deposited at the same time - have been found on eight separate palaeocontinents, in some cases occurring close to the equator (at a latitude of 10-30°), where the 300 m-thick name-bearing section at Gaskiers-Point La Haye (Newfoundland) is packed full of striated dropstones. Its δ13
C values are really low (pushing 8 ‰), consistent with a period of environmental abnormality. The bed lies just below some of the oldest fossils of the Ediacaran biota, where there is in fact a 9 million year gap between the diamictites and the 570 Ma macrofossils.
The Varangian apparently spans 610 to 575 Ma.
"The Elatina glaciation has not been dated directly, and only maximum and minimum age limits of c. 640 and 580 Ma, respectively, are indicated."
"The Elatina glaciation is of global importance for several reasons:
- its diverse and excellently preserved glacial and periglacial facies represent a de facto type region for late Cryogenian glaciation in general;
- the Elatina Fm. has yielded the most robust palaeomagnetic data for any Cryogenian glaciogenic succession; and
- the recently established Ediacaran System and Period (Knoll et al. 2004, 2006; Preiss 2005) has its Global Stratotype Section and Point (GSSP) placed near the base of the Nuccaleena Fm. overlying the Elatina Fm. in the central Flinders Ranges [...]."
"Feeder dykes for volcanic rocks near the base of the [Adelaide Geosyncline] sedimentary succession have been dated at 867 ± 47 and 802 ± 35 Ma (Zhao & McCulloch 1993; Zhao et al. 1994) and 827 ± 6 Ma (Wingate et al. 1998)."
"No volcanism is known in the region during the Elatina glaciation."
"The Neoproterozoic–early Palaeozoic succession in the Adelaide Geosyncline was deformed by the Delamerian Orogeny at 514 – 490 Ma (Drexel & Preiss 1995; Foden et al. 2006)."
"The Yerelina Subgroup at the top of the Cryogenian Umberatana Group embraces all the glaciogenic formations of the Elatina glaciation (Preiss et al. 1998)."
"The Yerelina Subgroup is unconformably to disconformably overlain by the Ediacaran Wilpena Group."
"Deposition in the North Flinders Zone commenced, possibly following an erosional break, with the 1070-m-thick Fortress Hill Fm., which comprises laminated siltstone with gritty lenses and scattered dropstones, some faceted, marking the onset of glacial deposition (Coats & Preiss 1987; Preiss et al. 1998). Clast lithologies include granite, quartzite, limestone, oolitic limestone and dolostone. The Fortress Hill Fm. is typical of the dominantly fine-grained units of the Yerelina Subgroup that are interpreted by Preiss (1992) as outer marine-shelf deposits."
"The Fortress Hill Fm. is sharply overlain by sandstone and conglomerate at the base of the Mount Curtis Tillite (90 m) that may record a lowering of relative sea level and mark a sequence boundary (Preiss et al. 1998)."
"The Mount Curtis Tillite is a sparse diamictite with erratics of pebble to boulder size, some faceted and striated, in massive and laminated, grey-green dolomitic siltstone. Clast lithologies are mostly quartzite, limestone and dolostone, but also include granite and porphyry (Coats & Preiss 1987). Granite boulders attain 3 x 8 m."
"The Mount Curtis Tillite is overlain by the medium-grained, feldspathic Balparana Sandstone (130 m), which contains interbeds and lenses of calcareous siltstone and pebble conglomerate."
"The Balparana Sandstone is disconformably overlain by the Wilpena Group. The main source for the glaciogenic deposits may have been the Curnamona Province to the present east [...] and possibly the now-buried Muloorina Ridge immediately north of the North Flinders Zone (Preiss 1987)."
"The lower-most, laminated siltstone facies of the Fortress Hill Fm. shows progressively greater amounts of scattered, ice-rafted granules and pebbles. The shallow-water Gumbowie Arkose (45 – 90 m) disconformably overlies these early deposits at a possible sequence boundary and is conformably succeeded by the Pepuarta Tillite (120 – 197 m), which is a sparse diamictite with scattered clasts up to boulder size in massive and laminated, grey calcareous siltstone. Faceted and striated boulders reach 2.5 m in diameter. Clast lithologies include pink granite, granite gneiss, grey porphyry, quartz-granule conglomerate, various quartzites, and vein quartz. The siltstone facies with scattered large clasts of extrabasinal provenance implies deposition from floating ice."
"The widespread Grampus Quartzite (60 m) disconformably overlies the Pepuarta Tillite, possibly at a sequence boundary defining a third genetic sequence of the Yerelina Subgroup (Preiss et al. 1998)."
"It is conformably overlain by the laminated to cross-laminated, calcareous, pale grey Ketchowla Siltstone (271 m) (Preiss 1992). The Ketchowla Siltstone contains scattered ice-rafted granules, pebbles and boulders up to 1 m across, and is ascribed by Preiss (1992) to outer marine-shelf deposition under generally waning glacial conditions. It is overlain disconformably by the Nuccaleena Fm., with any Ketchowla Siltstone deposited in the North Flinders Zone having been completely removed by erosion at this sequence boundary (Preiss 2000)."
"The outer marine-shelf successions of the Fortress Hill Fm. and Ketchowla Siltstone record the waxing and waning of glacial conditions, respectively. The Pepuarta Tillite and the correlative Mount Curtis Tillite mark the glacial maximum of the Elatina glaciation (Preiss et al. 1998)."
"A U–Pb age of 657 ± 17 Ma was obtained for a zircon grain of uncertain provenance from the Marino Arkose Member of the underlying Upalinna Subgroup (Preiss 2000). Re – Os dating gave an age of 643.0 ± 2.4 Ma for black shale from the Tindelpina Shale Member at the base of the Tapley Hill Fm., which overlies glacial deposits of Sturtian age in the Adelaide Geosyncline (Kendall et al. 2006). Zoned igneous zircon from a tuffaceous layer near the top of the Sturtian-age glaciogenic succession gave a SHRIMP U – Pb age of c. 658 Ma (Fanning & Link 2006). Mahan et al. (2007) reported a Th–U–total Pb age of 680 ± 23 Ma for euhedral laths of monazite, interpreted as authigenic, from the Enorama Shale of the Upalinna Subgroup."
The Nantuo glaciation apparently occurred 654 ± 3.8 Ma.
Ice Brook glaciationEdit
The Ice Brook glaciation apparently spans 651 to 659 Ma.
"Dropstone-bearing glaciomarine sedimentary rocks of the Ghaub Formation within metamorphosed Neoproterozoic basinal strata (Swakop Group) in central Namibia contain interbedded mafic lava flows and thin felsic ash beds. U-Pb zircon geochronology of an ash layer constrains the deposition of the glaciomarine sediments to 635.5 ± 1.2 Ma, providing an age for what has been described as a “Marinoan-type” glaciation. In addition, this age provides a maximum limit for the proposed lower boundary of the terminal Proterozoic (Ediacaran) system and period. Combined with reliable age constraints from other Neoproterozoic glacial units—the ca. 713 Ma Gubrah Member (Oman) and the 580 Ma Gaskiers Formation (Newfoundland)—these data provide unequivocal evidence for at least three, temporally discrete, glacial episodes during Neoproterozoic time with interglacial periods, characterized by prolonged positive δ13C excursions, lasting at most ∼50–80 m.y."
"Dropstones are ubiquitous within the finer-grained (Ghaub) lithofacies, and their presence, along with the facies context for subglacial and near grounding-line deposition, indicates a glacigenic origin for the Ghaub Formation, despite its subtropical paleolatitude and distal foreslope setting."
Apparently the major glacial period the Marinoan occurred during the Cryogenian.
A similar period of rifting, to the break up along the margins of Laurentia, at about 650 Ma occurred with the deposition of the Ice Brook Formation in North America, contemporaneously with the Marinoan in Australia.
The Marinoan glaciation ended approximately 635 Ma, at the end of the Cryogenian.
The Marinoan glaciation was a period of worldwide glaciation that lasted from approximately 650 to 635 Ma, where the end of the glaciation may have been sped by the release of methane from equatorial permafrost.
The name is derived from the stratigraphic terminology of the Adelaide Geosyncline (Adelaide Rift Complex) in South Australia and taken from the Adelaide suburb of Marino to subdivide the Neoproterozoic rocks of the Adelaide area and encompass all strata from the top of the Brighton Limestone to the base of the Cambrian. The corresponding time period, referred to as the Marinoan Epoch, spanned from the middle Cryogenian to the top of the Ediacaran and included a glacial episode within the Marinoan Epoch, the Elatina glaciation, after the 'Elatina Tillite' (now Elatina Formation). The term Marinoan glaciation came into common usage because it was the glaciation that occurred during the Marinoan Epoch.
The term Marinoan glaciation was applied globally to any glaciogenic formations assumed to correlate with the Elatina glaciation in South Australia. The Elatina glaciation in South Australia and the Gaskiers also occurs within the wide ranging Marinoan Epoch.
The Earth may have underwent a number of glaciations during the Neoproterozoic era.
There were three (or possibly four) significant ice ages during the late Neoproterozoic, periods of nearly complete glaciation of Earth are often referred to as "Snowball Earth", where it is hypothesized that at times the planet was covered by ice 1–2 km (0.62–1.24 mi) thick.
During the Marinoan glaciation, characteristic glacial deposits indicate that Earth suffered one of the most severe ice ages in its history, where glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.
The melting of the Snowball Earth is associated with greenhouse warming due to the accumulation of high levels of carbon dioxide in the atmosphere.
Glacial deposits in South Australia are approximately the same age (about 630 Ma), confirmed by similar stable carbon isotopes, mineral deposits (including sedimentary barite), and other unusual sedimentary structures.
Two diamictite-rich layers in the top 1 km (0.62 mi) of the 7 km (4.3 mi) Neoproterozoic strata of the northeastern Svalbard archipelago represent the first and final phases of the Marinoan glaciation.
The Marinoan "is separated from the Sturtian by a thick succession of sedimentary rocks containing no evidence of glaciation. This glacial phase could correspond to the recently described Ice Brooke formation in the northern Canadian Cordillera."
The Gucheng is apparently comparable to the Marinoan.
The Jiangkou spans the Chang'an through the Gucheng.
The Chang'an occurred about 715.9 ± 2.8 Ma.
Port Askaig glaciationEdit
The Port Askaig glaciation is above the Elbobreen-Wilsonbreen glaciation.
The Elbobreen-Wilsonbreen glaciation in Svalbard occurred c. 720 Ma.
Cryogenian ice ageEdit
"Late Proterozoic glaciogenic deposits are known from all the continents. They provide evidence of the most widespread and long-ranging glaciation on Earth."
Def. "a geologic period within the Neoproterozoic era from about  to 600 million years ago" is called the Cryogenian.
The end of the period also saw the origin of heterotrophic plankton, which would feed on unicellular algae and prokaryotes, ending the bacterial dominance of the oceans.
Apparently two major glacial periods occurred during the Cryogenian: the Marinoan and the Sturtian, formerly considered together as the Varanger glaciations, from their first detection in Norway's Varanger Peninsula.
The Cryogenian is a geologic period that lasted from 720-635 Mya.
The Cryogenian period was ratified in 1990 by the International Commission on Stratigraphy.
Several glacial periods are evident, interspersed with periods of relatively warm climate, with glaciers reaching sea level in low paleolatitudes.
Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.
The deposits of glacial tillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply frozen planetary oceans called "Snowball Earth".
"Most Neoproterozoic glacial deposits accumulated as glacially influenced marine strata along rifted continental margins or interiors."
Fossils of testate amoeba (or Arcellinida) first appear during the Cryogenian period.
New groups of life evolved during this period, including the red algae and green algae, stramenopiles, ciliates, dinoflagellates, and testate amoeba.
The break up along the margins of Laurentia at about 750 Ma occurs at about the same time as the deposition of the Rapitan Group in North America, contemporaneously with the Sturtian in Australia.
The Sturtian glaciation persisted from 720 to 660 million years ago.
A Sturtian age was assigned to the Numees diamictites.
"Glaciogenic rocks figure prominently in the Neoproterozoic stratigraphy of southeastern Australia and the northern Canadian Cordillera]. The Sturtian glaciogenic succession (c. 740 Ma) unconformably overlies rocks of the Burra Group."
The Sturtian succession includes two major diamictite-mudstone sequences, which represent glacial advance and retreat cycles, stratigraphically correlated with the Rapitan Group of North America.
The Sturtian is named after the Sturt River Gorge, near Bellevue Heights, South Australia.
Reusch's Moraine in northern Norway may have been deposited during this period.
The Numees has a Sturtian age.
The Tereeken occurred < 727 ± 8 Ma.
"The Rapitan Group (Cryogenian) of western Canada is similar to the Chuos Formation in both lithofacies and basin context, representing deposition in a paraglacial rift basin (Young, 1976; Eisbacher, 1985). An iron-rich, dropstone-bearing unit (the Sayunei Formation) is capped by a diamictite unit (the Shezal Formation) (Hoffman and Halverson, 2011). Measured sections (Fig. 3 of Eisbacher, 1985) illustrate that the most complete successions have a basal ferruginous shale sequence bearing occasional dropstones. These deposits pass gradationally upward, via 5–40 m jaspillite-hematite ironstone at the top of the Sayunei Formation, into diamictites. The ironstone is laterally persistent in depocentres (Eisbacher, 1985). Sea-ice removal may have triggered local grounding line advance, resulting in deposition of the Shezal Formation (Eisbacher, 1985): Hoffman and Halverson (2011) recognised this as a possible catalyst for ironstone precipitation. In addition to an abiotic “rusting of the seas” model, a biologically-mediated mechanism was also considered. Once “the ice cover thinned and finally disappeared, anoxic and oxygenic photosynthesis could have precipitated Fe2O3-precursor from anoxic Fe(II)-rich basin waters” (Hoffman et al., 2011). [...] Such a biogenic mechanism for ironstone precipitation, via for example photosynthetic stromatolites, would be in agreement with our observations in Namibia."
The Port Nolloth extends from the Kaigas formation upwards to the Murmees.
The Kaigas glaciation was a hypothesized snowball earth event in the Neoproterozoic Era, preceding the Sturtian glaciation inferred based on the interpretation of Kaigas Formation conglomerates in the stratigraphy overlying the Kalahari Craton as correlative with pre-Sturtian Numees formation glacial diamictites; however, the Kaigas formation was later determined to be non-glacial, and a Sturtian age was assigned to the Numees diamictites.
The Vendian occurred about 740 Ma.
"The "grainstone prism" was a major submarine drainage system localized in a paleovalley carved during the Chuos glaciation, which was occupied by a transverse ice-stream that cut the Duurwater trough during the Ghaub glaciation."
"Despite early indications of two distinct glaciations (Kröner and Rankama, 1972; Guj, 1974), the prevailing view of a single glaciogenic horizon that could serve as a basis for correlation throughout the Otavi Group (Hedberg, 1979; SACS, 1980; Miller, 1997) led to the former "Otavi Tillite" (le Roex, 1941) being assigned to the Chuos Formation of Gevers (1931), a glaciogenic diamictite with an intimately associated banded iron formation that is widely distributed within the orogens bounding the Otavi platform (Martin, 1965a, 1965b). More recently, two glaciations have been firmly established in the Otavi Group (Hoffmann and Prave, 1996; Hoffman et al., 1998a; Hoffman and Halverson, 2008), the older Chuos Formation and a younger glaciation represented by the "Otavi Tillite" (le Roex, 1941), and its correlative carbonate-clast breccia unit of the Fransfontein homocline (Frets, 1969; Guj, 1974). Hoffmann and Prave (1996) renamed this younger glaciogenic unit the Ghaub Formation, after a farm near the section originally described by le Roex (1941)."
The "Chuos glaciation occurred during a period of active faulting, which is reflected by the diversity of its debris and a low-angle (1.5°) structural unconformity [...] that cuts out ~2 km of strata (Hoffman et al., 1998a)."
The Rasthof Formation [is] the postglacial cap carbonate overlying the Chuos diamictite".
Below the Chuos glaciation is the Naauwpoort dated at 746 ± 2 Ma giving an upper age limit to the base of the Chuos.
"U–Pb ages from the Askevold Formation (Hoffman et al., 1996) [Nabis Formation 747 ± 2 Ma (Hoffman et al., 1996)] are from further west: this formation is not preserved beneath the Chuos Formation in [the Ghaub and Varianto farm areas of the Otavi Mountain Land]."
"Earlier analyses of the Chuos Formation concentrated on meta-sediments in the vicinity of its type section south of Windhoek and in the Damara Belt (Gevers, 1931; de Kock and Gevers, 1933; Martin et al., 1985; Henry et al., 1986; Badenhorst, 1988). More modern stratigraphic analyses several hundred kilometres to the west of the Otavi Mountain Land demonstrate that the Chuos Formation is cradled in a rift-related, fault bounded palaeotopography (Hoffman and Halverson, 2008), and hence its substrate also changes along strike, across the southern flank of the Owambo Basin. In the area of Ghaub and Varianto farms, the study interval comprises the Nabis Sandstone Formation of the Nosib Group, overlain by the Chuos Formation and succeeded by the Berg Aukas Formation [...]. This particular area has been mapped at the 1:250,000 scale (Geological Survey of Namibia, 2008). Age constraints include 747 ± 2 Ma from the Naauwport volcanics, locally beneath the Chuos Formation (Hoffman et al., 1996) and 635 ± 1 Ma from ash beds in the younger Ghaub Formation (Hoffmann et al., 2004)."
The Beiyixi is later than 755 Ma.
The Kundelungu is dated to 765 ± 5 Ma.
Def. "a geologic era within the Proterozoic eon; comprises the Calymmian, Ectasian and Stennian periods from about 1600 to 900 million years ago, when the Rodinia supercontinent was formed" is called the Mesoproterozoic.
Def. "a geologic era within the Proterozoic eon; comprises the Siderian, Rhyacian, Orosirian and Statherian periods from about 2500 to 1600 million years ago, when cyanobacteria increased the amount of oxygen in the atmosphere and changed life on Earth for ever" is called the Paleoproterozoic.
"In its eastern domain, the Transvaal Supergroup of South Africa contains two glacial diamictites, in the Duitschland and Boshoek Fms. The base of the Timeball Hill Fm., which underlies the Boshoek Fm., has a Re-Os date of 2,316 ± 7 My ago (13). The Boshoek Fm. correlates with the Makganyene diamictite in the western domain of the Transvaal Basin, the Griqualand West region. The Makganyene diamictite interfingers with the overlying Ongeluk flood basalts, which are correlative to the Hekpoort volcanics in the eastern domain and have a paleolatitude of 11° ± 5° (14). In its upper few meters, the Makganyene diamictite also contains basaltic andesite clasts, interpreted as being clasts of the Ongeluk volcanics. The low paleolatitude of the Ongeluk volcanics implies that the glaciation recorded in the Makganyene and Boshoek Fms. was planetary in extent: a snowball Earth event (15). Consistent with earlier whole-rock Pb–Pb measurements of the Ongeluk Fm. (16), the Hekpoort Fm. contains detrital zircons as young as 2,225 ± 3 My ago (17), an age nearly identical to that of the Nipissing diabase in the Huronian Supergroup."
The "Makganyene glaciation begins some time after 2.32 Ga and ends at 2.22 Ga, the three Huronian glaciations predate the Makganyene snowball."
Huronian ice ageEdit
The Huronian Ice Age is known "mainly from Canada and the United States in North America, where dated rocks range from 2500 to 2100 million years old."
"The period from 2.45 Ga until some point before 2.22 Ga saw a series of three glaciations recorded in the Huronian Supergroup of Canada (11) [in the above centered image]. The final glaciation in the Huronian, the Gowganda, is overlain by several kilometers of sediments in the Lorrain, Gordon Lake, and Bar River formations (Fms.). The entire sequence is penetrated by the 2.22 Ga Nipissing diabase (12); the Gowganda Fm. is therefore significantly older than 2.22 Ga."
"The three Huronian glacial units, penetrated and capped by the Nipissing diabase, predate the Makganyene diamictite in the Transvaal. The uppermost Huronian glacial unit, the Gowganda Fm., is overlain by hematitic units, perhaps reflecting a rise in O2. The basal Timeball Hill Fm. contains pyrite with minimal [mass-independent fractionation] MIF (26), whereas the upper Timeball Hill Fm., which we suggest is correlative to the Lorrain or Bar River Fms., contains red beds. The Makganyene diamictite records a low-latitude, snowball glaciation (29), perhaps triggered by the destruction of a CH4 greenhouse. It is overlain by the Kalahari Mn Field in the Hotazel Fm., the deposition of which requires free O2."
"In its eastern domain, the Transvaal Supergroup of South Africa contains two glacial diamictites, in the Duitschland and Boshoek Fms. The base of the Timeball Hill Fm., which underlies the Boshoek Fm., has a Re-Os date of 2,316 ± 7 My ago (13). The Boshoek Fm. correlates with the Makganyene diamictite in the western domain of the Transvaal Basin, the Griqualand West region. The Makganyene diamictite interfingers with the overlying Ongeluk flood basalts, which are correlative to the Hekpoort volcanics in the eastern domain and have a paleolatitude of 11° ± 5° (14). In its upper few meters, the Makganyene diamictite also contains basaltic andesite clasts, interpreted as being clasts of the Ongeluk volcanics. The low paleolatitude of the Ongeluk volcanics implies that the glaciation recorded in the Makganyene and Boshoek Fms. was planetary in extent: a snowball Earth event (15). Consistent with earlier whole-rock Pb–Pb measurements of the Ongeluk Fm. (16), the Hekpoort Fm. contains detrital zircons as young as 2,225 ± 3 My ago (17), an age nearly identical to that of the Nipissing diabase in the Huronian Supergroup. As the Makganyene glaciation begins some time after 2.32 Ga and ends at 2.22 Ga, the three Huronian glaciations predate the Makganyene snowball."
"In contrast to the Makganyene Fm., the three Huronian diamictites are unconstrained in latitude. Poles from the Matachewan dyke swarm, at the base of the Huronian sequence, do indicate a latitude of ≈5.5° (18), but ≈2 km of sedimentary deposits separate the base of the Huronian from the first glacial unit (19), which makes it difficult to draw conclusions about the latitude of the glacial units based on these poles. Low latitude poles in the Lorrain Fm. (20, 21), which conformably overlies the Gowganda diamictite, are postdepositional overprints (22)."
"Some of the earliest continental red beds were deposited in the Firstbrook member of the Gowganda Fm. and in the Lorrain and Bar River Fms. in Canada, as well as in the upper Timeball Hill Fm. in South Africa. The basal Timeball Hill Fm. has recently been dated at 2,316 ± 7 My ago (13). In our proposed correlation, all of the red bed-bearing units were deposited after the last Huronian glaciation and before the Makganyene glaciation. The formation of the red beds could involve local O2, although it does not demand it (34). Syngenetic pyrite from the basal Timeball Hall Fm. shows only slight MIF of S (26), consistent with the initiation of planetary oxygenation or enhanced glacial activity."
The Transvaal is approximately 2700 Ma.
The Pongola glaciation is dated "at 2.9 Ga". It extends to 2780 Ma.
"The oldest known midlatitude glaciation, recorded in the Pongola Supergroup diamictite, occurred at 2.9 Ga (10)."
Def. "a geologic era within the Archaean eon from about 3200 to 2800 million years ago; stromatolites have existed from this time" or the "era from 3,200 Ma to 2,800 Ma" is called the Mesoarchean.
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