Holocene debris flows recognized in a lacustrine sedimentary succession: sedimentology, chronostratigraphy and cause of triggering

This study focuses on the sedimentary characteristics and the chronostratigraphy of Holocene masse ow deposits recognized in a lake-e ll sedimentary succession. These deposits in lake Ulva dalsvatnet, western Norway, are discrete, sharp-bounded units of sand-sized sediment, running from gravelly and graded to silt-rich, and characterized by low total carbon and water contents. They are rich in terrestrial macroe ora detritus, dark brown in colour, and interpreted as high-density turbidity current deposits attributed to subaerial debris e ows that plunged into the lake. Thirty-three 14 C AMS dates were derived from three cores, and though the ages are somewhat inconsistent (macroe ora invariably younger than bulk sediment samples), they indicate a marked increase in debris-e ow processes after c. 2200 cal. yr BP, considered to ree ect increased occurrence of heavy rainstorms.