Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30s.591v&link_type=abstract
Meteoritics, vol. 30, no. 5, page 591
Mathematics
Logic
Astroblemes, Craters, Impact, Paleogeography, Popigai
Scientific paper
Due to the lack of Upper Paleozoic, Mesozoic and Cenozoic sediments the Anabar Shield (Siberian Platform) was considered for a long time to be a positive area of denudation and planation during major part of Its Phanerozoic history. This interpretation is traditionally shown on all regional paleo- geographic maps, where seashore lines for variuos geological epochs are limited usually by the margin contours of corresponding sediments successfully preserved from erosion within the Khatanga Depression. The large Popigai crater of ~35 Ma age, with Its well-preserved impact formations may provide a new sight into the open sheets of Regional Phanerozoic history. Popigai impact formations should contain a full collection of target sedimentary rocks, in scope of large, ~100 km in diameter, area of cratering. The explosion cloud deposits (suevites), and the suevite megabreccia among them in the first turn, should be the most interesting in this aspect, especially if one takes into account a new model [1,2] of their transport during cratering (subvertical exavation for cape target formations). Next data are present in order to support the thesis mentioned above. (1) The fragments of K1 rocks collected by one of us (S.V.) from the suevite megabreccia, contain well-preserved in situ Early Valanginian fauna of Buchia keyserlingi (Lah.) (layered leptochlorite sandstone, NE part of crater, specimen 2315/2) as well as Early Valanginian fauna of Costacolpus septentrionales Beisel, Buchia sp. ind., Praeexogyra cf. anabarensis Bodyl. and others (phosphate sandstone, N part of crater, specimen 2374/6). This fauna is the evidence of open (100 km to seashore, at least) and shallow (20-40 m) sea. Another marine fauna of Valanginian age (bivalves and cephalopods) had been found in suevites of the crater by L.P.Smirnov (1957) and others. So, there was a sea regime in Valanginian time on the Northern part of the Anabar Shield. (2) By M.T.Kirjushina report [3], Late Aalean-Bajocian fauna of Pseudomonotis (Eumorphotis)? /Arctotis, in modern terminology/ cf. lenaensis (Lah.) was collected in the central part of the crater (sandy-carbonaceous concretion). Again, it is the evidence of shallow (not deeper then 100 m) sea on the Northern part of the Shield. (3) A special quest for Paleogenic rocks incorporated in Popigai impactites, which was made by one of us (S.V.), was unsuccessful, but reworked Paleogenic tree pollen (Tsuga, Abus) as well as marine Diatomea were reported by A.O.Yefomov (1966) and others to be present in postimpact N2-Q2 crater fill. So, continental as marine regimes were also possible for the Area in Paleogenian time. The bulk composition of Popigai suevites can allow an approximate thickness estimations for Mz + Pg(?) cape of target. Really, subvertical excavation of this cape during cratering should provide relatively uniform deposition of mobilized material. The top of the suevitic column (suevite sands, suevite megabreccia and Daldyn breccia) is broadly widespread in the crater and is made up of Mz + Pg(?) material mainly (up to 60-75% vol). This top's minimum thickness is up to 200 m (drilling data). So, the top should correspond to Mz +Pg(?) target cape of up to 120-150 m in thickness, at least, and the Northern part of the Anabar Shield was not a positive area only, but depositional (including marine regimes) basin also during the time considered. The examples described are the first steps only in using of such a unique storage of regional paleogeographic data as the Popigai crater. References: [1] Vishnevsky S. A. (1994) Novosibirsk: UIGGM Preprint, 1-66, in Russian. [2] Vishnevsky S. A. (1994) 2nd Intl. Workshop, ESF Sci.Network, Ostersund, May 31-June 5, 50. [3] Kirjushina M. T. (1959) Geology, 1, 50-55.
Beisel A. L.
Vishnevsky S. A.
Zakharov Alexander V.
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