Publikacje

Abstract:

Numerous volcanic ash beds (K-bentonite) are recognized globally in Ordovician to Early Devonian sedimentary
successions, and are important for regional stratigraphic correlations. Relatively little, however, is understood of the origin and
transport of these ash deposits. Here, we examine ash beds preserved in the Upper Ordovician and Silurian dark shales and
mudstones of the Holy Cross Mountains, Poland, and discuss their origin in the context of contemporaneous tectonic
configurations. The Ordovician ash beds occur in the upper Sandbian and Katian mudrock succession, whereas the Silurian Kbentonites
form prominent beds in the upperWenlock and lower Ludlow graptolite shales. Their various thickness and irregular
distribution reflect fluctuation of volcanic activity and contribution of physical and biogenic factors in their preservation.
Deposition under dysoxic or anoxic conditions and relatively rapid burial by muddy sediment seem to be dominant factors
preventing physical and biological reworking and mixing of the volcanic ash. The Upper Ordovician ash beds in the Holy Cross
Mountains, as well as the Baltoscandian K-bentonites, appear to be accumulated from pyroclastic material delivered by
westerlies of the Southern Hemisphere from the Avalonian volcanoes. In the Silurian, the distribution of pyroclastic material
was controlled by SE trade winds, and therefore the source area for the Wenlock and Ludlow K-bentonites in the Holy Cross
Mountains appears to have been an active subduction zone developed close to the southeastern side of Baltica. However, the
delivery of volcanic ash to the central part of Baltica might have been strongly affected by climatically driven latitudinal
migration of the Intertropical Convergence Zone

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Upper Ordovician and Silurian ash beds in the Holy Cross Mountains, Poland: preservation in mudrock facies and relation to atmospheric circulation in the Southern Hemisphere