PALEOCURRENT STRUCTURES AND PALEOGEOGRAPHY OF THE ULTRAHELVETIC FLYSCH BASINS, SWITZERLAND

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The Ultrahelvetic Flysch of the Swiss Prealps ranges from Late Cretaceous to Eocene in age. It occurs as isolated outcrops which are known by various local names—Waggital Flysch in the Glarus Alps, Schlieren Flysch in central Switzerland, Leissigen Flysch in the Internal Prealps southwest of Lake Thun, and Gurnigel Flysch in the Bernese External Prealps. These Flysch formations are characterized by thick sequences of interbedded sandstone, conglomerate, shale, and silty limestone. The sandstone of the Flysch exhibits sedimentary structures typical of turbidity-current deposition— e.g., graded bedding, flute casts, groove casts. About 400 measurements of the directional sedimentary structures were made from the various Flysch formations. Paleocurrent information thus obtained was interpreted within the framework provided by the stratigraphic and tectonic studies of alpine geologists. The major conclusions of this paper are: (1) The Ultrahelvetic Flysch basins of the central Swiss Alps were created as a result of isostatic adjustment subsequent to an episode of pre-Maestrichtian folding. During the Maestrichtian to middle Eocene time, the Ultrahelvetic region was characterized by a topography typical of block-faulted terranes. Rugged land mass adjoining deep-sea basins resulted in conditions favorable for periodic turbidity-current deposition in those basins. (2) The Schlieren Flysch, the Leissigen Flysch, and the Sandstone Flysch near Adelboden were deposited in a row of near-shore deep-sea basins just south of a shallow shelf sea where carbonates of the Helvetic facies accumulated. These Flysch formations derived their sediments from the south or southwest. They were subsequently overthrust as the nappes of the Internal Prealps. (3) The Waggital Flysch and the Gurnigel Flysch were deposited in a row of deep-sea basins farther offshore. These were separated from the near-shore basins by an island chain which furnished sediments to both units. Subsequently, the Gurnigel Flysch was overthrust as a nappe of the External Prealps.