Premium
Environmental changes in the Hirnantian (upper Ordovician) of the Prague Basin, Czechoslovakia
Author(s) -
Brenchley P. J.,
Štorch P.
Publication year - 1989
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350240302
Subject(s) - flysch , geology , ordovician , marine transgression , paleontology , siltstone , sedimentary rock , geomorphology , structural basin , facies
Sedimentary sequences through the uppermost Ordovician (Hirnantian), Kosov Formation, are described. The Kosov Formation is a shale‐dominated formation with two thin horizons of sandy diamictites near the base and two ‘flysch’ units separated by shales forming the main part of the formation, which is overlain abruptly by Silurian graptolitic shales. The diamictites are interpreted as of glaciomarine origin, probably deposited from coastal winter ice. The flysch units have many beds with hummocky cross‐stratification and wave‐ripples indicating deposition from storm generated currents above storm wave‐base. The shales between the two flysch units have thin interbedded sandstones and siltstones with mainly unidirectional current structures, but also some wave‐ripples, suggesting deposition around storm wave‐base. The sequence shows clear evidence of two regressive phases, the second of which is the more pronounced, followed by rapid transgression near the Ordovician/Silurian boundary. The regressions and subsequent transgression are interpreted as being caused by Hirnantian glacioeustatic changes rather than being tectonic in origin. The importance of the glaciomarine diamictites is that they record the onset of cold climatic conditions in the region as being early Hirnantian. The disappearance of most of the fauna at about the same time might reflect the influence of low temperatures on faunal diversity, and could have significance for the end Ordovician extinction.