Coarsening upward sequences - the products of laterally established river subsystems, M. Devonian, Hornelen Basin, Norway

Abstract

Investigations of sedimentary sequences in a marginal part of Homelen Basin (M. Devonian, W Norway) have resulted in the interpretation of gradually established fluvial and fluviodeltaic subsystems adjacent to a major river system. A succession of 8 coarsening upward (CU) sequences (4.5---19 m thick) wad studied. Three sequence types are recognized: sequence type A, B and C. The lower part of all sequence types is composed of silty lacustrine flood basin deposits. The upper part of all sequence types is composed of sandy low sinuosity fluvial channel deposits. The middle part of the three sequence types is composed of sandy sheet splay deposits (sequence type A), crevasse splay deposits (sequence type B) and deltaic mouth bar deposits (sequence type C). The genesis og the different types of CU sequences is explained by repeated lateral establishments of fluvial and fluvio-deltaic subsystems adjacent to a major humid fan-like river system. The establishment of the subsystems occurred in four phases: I) Initial fine-grained overbank flows into flood badin lakes. II) Sheet splay flows and/or crevasse splay flows. III) Establishment of distributary channels and associated deltaic mouth bars. IV) Progradation of channel/mouth bar couplets and filling up of the flood basin lakes.