Sedimentary architecture of the distal lower freshwater molasse and its relation to the diagenesis and petrophysical properties demonstrated by the example of the Langenthal boreholes

Sedimentary study at outcrop and quarry exposures of distal alluvial deposits in the upper part of the Lower Freshwater Molasse ('Aquitan') has revealed four principal facies. The same facies may also be recognised in boreholes (Langenthal, Weiach, Schafisheim) penetrating the continental succession. These facies make up distinct sediment bodies or 'architectural elements'. Each architectural element may be assigned to a particular depositional setting: Meander Belt RG (MB); Crevasse Splays and Crevasse Channels DFR (CS), Levees and Distal Splays UW (LS), Overbank, Paleosols and Swamps UPS (OPS) and Lacustrine LAK (LAC). Meander belt sandstones in the distal Lower Freshwater Molasse reflect the occurrence of two types of mixed load channels. Architectural modelling permits consideration of the distal Lower Freshwater Molasse in terms of a sandy framework, comprising the channel sandstone bodies, each in contact with their associated crevasse splay and crevasse channel sandstones, and a muddy-silty matrix consisting of interbedded floodplain sediments. Comparison with the morphologies of modern alluvial systems suggests that channel sandstone bodies in the Lower Freshwater Molasse are likely to show greater longitudinal continuity, greater width and a higher degree of interconnectedness in an upstream direction. Conversely, primary depositional porosities and permeabilities of the sandstone units are likely to decrease downstream towards more distal areas in the east and at the northern margin of the basin, although regional differences in tectonic style and diagenesis will also exert a major control on the petrophysical and hydrological properties in any given area. (author) figs., tabs., 54 refs