Palaeozoic Bentonites

Research project P 14124 Palaeozoic Bentonites Hans Peter SCHÖNLAUB 24.01.2000 The search for evidence of explosive volcanism. in the form of volcanic ash beds, now referred to as K-bentonites, has become increasingly important in recent years as their potential as stratigraphic markers has been recognised. "Events" such as these ash eruptions are thought to be short lived, lasting only a few weeks at most and having wide lateral extension (BERGSTROM et al, 1995). In recent years research has been concentrated on Ordovician and Silurian occurrences as the K-bentonite levels have proved useful in the correlation of the palaeogeographical position of various Lower Palaeozoic terranes and consequently the timing of events such as the opening and closure of the Iapetus and Rheic oceans. This fact is of considerable importance with regard to the drift rate of the continental plates which has been shown to be up to 25 cm/year and will contribute to the confirrnation of the reliable palaeomagnetic data for this interval. This is also true in the case of the protoAlps during the Lower Palaeozoic. As much controversy surrounds the palaeogeographic position of the Southern Alps during the Late Ordovician to lower Devonian interval it is proposed to locate and identify Kbentonite levels within the well-dated graptolitic shale and carbonate sequences of this interval from various sections in the Carnic Alps and the Graz Palaeozoic in order to correlate them with those established in other areas of Europe and possibly North America. It may be possible to show that the bentonite levels are associated with known volcanic activity during the Ordovician to Lower Devonian in the Southern and Central Alps. The correlation of the bentonite levels across separate terranes and microcontinents and consideration of their spacial extension will help to document individual magmatic events at precise stratigraphic intervals. The distance from the source area may be implied as well as the width of intervening areas between individual terranes and thus help to clarify the rate of movement of the northern Gondwana terranes in relation to Baltica, Avalonia and Laurentia. The hypothesis that continued rifting was a "motor" for the movement of individual microcontinents and terranes separated from the Northern Gondwana margin (such as Avalonia, Perunica, Proto-Alps, Iberia etc.) from higher to lower latitudes during the Lower Palaeozoic may be proven. The possibility that K-bentonite levels may be found in the Southern and Central Alps area is quite high. Recent detailed studies have been done on the sections in these areas at both a centimetre and even millimetre scale which have highlighted the possible presence of bentonite levels. The exact stratigraphic position of the sedimentary sequences is well documented which will allow precise correlation with other areas during certain intervals. Recent data from palaeobiological studies show that the Carnic Alps occupied a more northern palaeogeographical position during the Silurian (Schonlaub, 1997) than previously supposed and the proposed bentonite investigations may sustain these findings.

In the Southern Alps the Upper Ordovician - Lower Silurian classic sections of (1) Seewarte, (2) Cellon, (3) Oberbuchach, (4) Dr Steinwender Hutte, (5) Nölbling Graben (6) Uggwa, (7) Valbertad and (8) Hoher Trieb which are stratigraphically well-dated by graptolites and conodonts were sampled in detail. A total of 159 possible K- bentonite levels were recognized and sampled from the sections mentioned which range in age from Upper Ordovican to Lower Devonian.. Immobile trace elements and REE have been used to provide information on the magmatic composition of K- Bentonite parent ashes and tectonic setting of the source volcanoes. The geochemical analysis of the samples taken indicate that the Cellon and Seewarte sections plot within the ocean ridge field, the Oberbuchach, Nölbling Graben Dr Steinwender Hütte, Uggwa, Valbertad and Hoher Trieb sections mainly plot within the volcanic arc and syn- collisional fields. The samples vary in composition and plot mainly within the andesite, rhyodacite/dacite fields. The abundant presence of these K-bentonite horizons in the Llandovery - Middle Ludlow sequences of the Carnic Alps is similar in the British Isles, Sweden, Canada and N. America and documents widespread volcanism related to the closing of the Iapetus Ocean. Silurian K-Bentonites which are Pridoli in age may be comparable with those described from Podolia for which a source area in the Rheic Ocean has been indicated.