Tsunami killed and backwashed accumulated crinoids in Middle Triassic (Anisian) intracratonic Germanic Basin carbonates of central Europe

• Autores: Cajus G. Diedrich
• Localización: Carbonates and Evaporites, ISSN 0891-2556, Vol. 32, Nº. 4, 2017, págs. 435-458
• Idioma: inglés
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• Resumen

  • In Germany tens to hundreds of kilometres-laterally distributed sedimentological heterogene crinoid accumulation bed with partly articulated Encrinus liliiformis specimens is dated within a complete trans-regression cycle of higher order into the upper Illyrian, Anisian (Middle Triassic, pulcher biozone). The crinoid bearing layer differs much from under- and overlaying tempestite parasequence sets of the Meißner Formation. The tsunamite layer consists of different laterally grain size sorted facies types ranging from unsorted bioclastic floatstones of large-scaled antidune megaripples, which continue in oscillation-ripple-marked oolithic grainstone surfaces that end into crinoid filled scour troughs in wacke/mudstones (=horizontal grain sorting in backwash direction). The tsunamite bed of Lamerden that consists of mixed reworked oolithic bar to crinoid ramp facies areas (sediments/biota to the outer ramp facies) can be correlated to a crinoid bed of Bissendorf within northwestern Germany at minimum (about 50 km) and appears within the tectonical deepening period of the Central European intracratonic Germanic Basin. The heterotope crinoid taphocoenosis was built in the outer ramp Tonplatten facies, in the deepest part of the Germanic Basin (Hesse-Depression) due to backwash. The mixed macro fauna originate from the crinoid bioherms and terebratulid hardgrounds of the inner/middle ramp facies (such as the sediments). From 350 analysed crinoid individuals (of 1500), only 1 % are complete with 120–160 cm short-stemmed ecotypes of the inner ramp crinoid meadows which settled along the sand bars basinwards. The Lamerden crinoids are scattered or are concentrated differently condensed in scour trough-like structures on wackestones/mustone areas and more rare or scattered on osciallation-ripple marked grainstones and allow only local bottom current estimations. In interfingering floatstones, more rare crowns are found unsorted within dominat chaotic oriented terebratulids. Most crinoid specimens (83 %) were found with relic stems and the others (16 %), only as crowns. The echinoderms survived a tsunami event, for at least a few hours/days under 10–30 cm of mud coverage. Only 1 % of the crowns have opened arms (=postmortal embedding); all others (99 % are closed crowns) are “buried alive” specimens. The crinoids closed the nutrition canal immediately and started regeneration of the distal stem for new attachment purposes; therefore, those have all rounded distal stems. The northern German Lamerden crinoid layer is most similar to the southern German Neckarwestheim site, but is different in its origin to other German sites, where crinoids have been found around bioherms or with long-stemmed specimens not far from their patch-reef like habitats in channels of the upper middle ramp facies. The tsunamite demonstrate the problem of the “Triassic time scale” which is tempestite and Milankovitch cycle based on an intracratonic basin, because there seem to be several further tsunamite bioclastic layers, which were misidentified as “tempestites”.