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Modern and Ancient Alluvial Systems
A Unique Mass Flow Marker Bed in a Miocene Streamflow Molasse Sequence, Switzerland
A unique sedimentary marker bed outcrops within a Middle Miocene sequence of conglomerates and finer clastics in the North Alpine molasse basin. The sequence is interpreted as the deposits of large, gently-sloping (≤ 1°) alluvial fans building out from the foot of the Alps. Up to 6 m thick, the marker bed has a volume of the order of 109 m3 and occurs across the entire width (65 km) of one of these fans.
Using information from 216 profiles, the marker bed is subdivided into four facies, all characterized by a more restricted, carbonate-rich composition than the enclosing fluvial deposits. One of these facies, confined to a 18 km wide mid-fan region, consists of several superimposed depositional units. These are either 0.5 - 3 m thick and consist of clast-supported rudites which are ungraded or inversely-graded at the base, or dm thick, matrix-supported rudites. Common features include relatively small, angular carbonate clasts and a carbonate mud matrix. Even in the thicker beds, maximum clast size (< 10 cm) is finer than the surrounding streamflow conglomerates. These beds have a measurable width of several 100 m, and are interpreted as having been deposited from a series of small subaerial debris flows which originated from a uniquely large event. This was possibly an exceptional rockslide from the front range of the Alps. The other facies of the marker bed comprise fluvial conglomerates and detrital lacustrine limestones, both of which represent reworked material from the large event.
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