scholarly journals Ice-rafted dropstones in “postglacial” Cryogenian cap carbonates

Geology ◽  
2020 ◽  
Author(s):  
D.P. Le Heron ◽  
M.E. Busfield ◽  
C. Kettler
Keyword(s):  
Thin Section ◽  
Mass Flow ◽  
Cold Climate ◽  
Conventional Wisdom ◽  
Warm Water ◽  
Cap Carbonates ◽  
Cap Carbonate ◽  
Carbonate Deposition ◽  
Mass Flow Deposits ◽  
Paleoclimatic Significance

Dropstones of ice-rafted origin are typically cited as key cold-climate evidence in Cryogenian strata and, according to conventional wisdom, should not occur in postglacial, warm-water carbonates. In Namibia, the Chuos Formation (early Cryogenian) contains abundant dropstone-bearing intervals and striated clasts. It is capped by the Rasthof Formation, composed of laminites in its lower portion and microbial carbonates above. These laminites are locally found to contain pebble- and granule-sized lonestones in abundance. At the Omutirapo outcrop, meter-thick floatstone beds occur at the flanks of a Chuos paleovalley and are readily interpreted as mass-flow deposits. At Rasthof Farm, however, the clasts warp, deflect, and penetrate hundreds of carbonate laminations at both the outcrop and thin-section scale. We propose that these are dropstones, and we infer an ice-rafting mechanism. Evidence for vestigial glaciation concomitant with cap carbonate deposition thus merits a reappraisal of the depositional conditions of cap carbonates and their paleoclimatic significance.

scholarly journals Episode of intense chemical weathering during the termination of the 635 Ma Marinoan glaciation

2016 ◽  
Vol 113 (52) ◽  
pp. 14904-14909 ◽  
Author(s):  
Kang-Jun Huang ◽  
Fang-Zhen Teng ◽  
Bing Shen ◽  
Shuhai Xiao ◽  
Xianguo Lang ◽  
...  
Keyword(s):  
South China ◽  
Chemical Weathering ◽  
Snowball Earth ◽  
Carbonate Precipitation ◽  
Isotopic Data ◽  
Low Intensity ◽  
Cap Carbonates ◽  
Cap Carbonate ◽  
Carbonate Deposition ◽  
Global Precipitation

Cryogenian (∼720–635 Ma) global glaciations (the snowball Earth) represent the most extreme ice ages in Earth’s history. The termination of these snowball Earth glaciations is marked by the global precipitation of cap carbonates, which are interpreted to have been driven by intense chemical weathering on continents. However, direct geochemical evidence for the intense chemical weathering in the aftermath of snowball glaciations is lacking. Here, we report Mg isotopic data from the terminal Cryogenian or Marinoan-age Nantuo Formation and the overlying cap carbonate of the basal Doushantuo Formation in South China. A positive excursion of extremely high δ26Mg values (+0.56 to +0.95)—indicative of an episode of intense chemical weathering—occurs in the top Nantuo Formation, whereas the siliciclastic component of the overlying Doushantuo cap carbonate has significantly lower δ26Mg values (<+0.40), suggesting moderate to low intensity of chemical weathering during cap carbonate deposition. These observations suggest that cap carbonate deposition postdates the climax of chemical weathering, probably because of the suppression of carbonate precipitation in an acidified ocean when atmospheric CO2 concentration was high. Cap carbonate deposition did not occur until chemical weathering had consumed substantial amounts of atmospheric CO2 and accumulated high levels of oceanic alkalinity. Our finding confirms intense chemical weathering at the onset of deglaciation but indicates that the maximum weathering predated cap carbonate deposition.

Pleistocene mass-flow deposits of basaltic hyaloclastite on a shallow submarine shelf, South Iceland

1991 ◽  
Vol 53 (8) ◽  
pp. 597-611 ◽  
Author(s):  
Steffen G Bergh ◽  
Gudmundur E Sigvaldason
Keyword(s):  
Mass Flow ◽  
Mass Flow Deposits

Depositional Environments of Plant Bearing Sediments

1988 ◽  
Vol 3 ◽  
pp. 1-13 ◽  
Author(s):  
Scott L. Wing
Keyword(s):  
Mass Flow ◽  
Depositional Environments ◽  
Fluvial Systems ◽  
Volcanic Origin ◽  
Sedimentary Environments ◽  
Plant Remains ◽  
Mass Flow Deposits ◽  
Fluvial Environments

Plants can become incorporated into the sediments of virtually any environment, from the oozes of abyssal plains to the silts and sands of delta fronts to brecciated mudflows of volcanic origin. However there is a much narrower range of sedimentary environments in which identifiable plant remains are found in abundance. Generally speaking these are the very shallow or subaerial portions of deltas and estuaries, the channels and floodplains of fluvial systems, lakes of all sizes, ash-falls, and mass-flow deposits such as mudflows. For the purposes of this paper peat swamps are considered as unusual subtypes of deltaic and fluvial environments in which clastic input is low relative to organic accumulation.

How tillite weathering during the snowball Earth aftermath induced cap carbonate deposition

Geology ◽  
2012 ◽  
Vol 40 (11) ◽  
pp. 1027-1030 ◽  
Author(s):  
Sébastien Fabre ◽  
Gilles Berger
Keyword(s):  
Snowball Earth ◽  
Cap Carbonate ◽  
Carbonate Deposition

scholarly journals Middle and Late Jurassic radiolarians from the Neotethys suture in the Eastern Alps

2016 ◽  
Vol 91 (1) ◽  
pp. 25-72 ◽  
Author(s):  
Luis O’Dogherty ◽  
Špela Goričan ◽  
Hans-Jürgen Gawlick
Keyword(s):  
Mass Flow ◽  
Late Jurassic ◽  
Eastern Alps ◽  
Accretionary Wedge ◽  
Taxonomic Study ◽  
New Genera ◽  
Two New Species ◽  
Mass Flow Deposits ◽  
Thermal Overprint ◽  
Radiolarian Assemblages

AbstractCherty limestones, marls and radiolarites are widespread in the Hallstatt Mélange of the Northern Calcareous Alps. The mélange was formed during the Neotethyan orogeny in a series of deep-water basins that progressively developed in front of the advancing nappe front. The low thermal overprint of these rocks favors the good preservation of radiolarians (polycystines) that have been used for dating and reconstructing the Jurassic tectonostratigraphy of the area. This paper describes rich Middle–Late Jurassic radiolarian faunas from four localities in the Hallstatt Mélange near Bad Mitterndorf in Austria. Two different successions, both spanning from the Bathonian to the Oxfordian, are dated. In the first succession, the radiolarites are intercalated between or occur as matrix in mass-flow deposits originating from the accretionary wedge. The second succession is nearly 100 m thick but is devoid of mass-flow deposits and documents a continuous radiolarite deposition in greater distance from the nappe stack. Both successions are ascribed to the Sandlingalm Basin, which evolved on a relatively distal continental margin during early stages of the orogeny.The highly diverse and well-preserved radiolarian assemblages have been used for a detailed taxonomic study. Two new families are described: Minocapsidaen. fam. andXitomitridaen. fam.; six new genera are describedDoliocapsan. gen.,Crococapsan. gen.,Parvimitrellan. gen.,Xitomitran. gen.,Campanomitran. gen., andMizukidellan. gen. In addition, one new replacement name,Takemuraella(proTriversus), is introduced, the diagnoses of 6 genera are emended, and two new species are described:Hemicryptocapsa nonagintan. sp. andMizukidella mokaensisn. sp.

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