Distal alluvial deposits in a foreland basin setting—the Lower Freshwater Miocene), Switzerland: sedimentology, architecture and palaeosols

Abstract

The Lower Freshwater Molasse (Untere Susswasser Molasse) crops out over a wide area of the Swiss Molasse Basin. Coarse grained alluvial fan conglomerates dominate in proximal basin areas along the Alpine front. These conglomerates pass northwards into sandstones and mudstones of an extensive northeastward draining meandering river system which ran parallel to the basin axis. Sedimentological study of outcrops, quarry exposures and boreholes in the basal Miocene (‘Aquitanian‘) has permitted detailed facies analysis of this distal alluvial sequence. The distal Aquitanian is made up of distinct ‘architectural elements’characterized by their geometries and sedimentary structures. Each may be assigned to a particular depositional setting: meander belt, levees, crevasse channels and splays, overbank fines and palaeosols, and lacustrine. Meander belt sandstones were deposited in mixed load channels with a dominant bedload component. Sandstones commonly comprise amalgamated and locally stacked ribbon bodies 2–15 m thick and 150–1500 m wide. Interbedded rippled, laminated and mottled fine grained levee sandstones and siltstones form lenticular packages up to 3 m thick and 30–100 m across. Small scale crevasse channel sandstones 2–4 m thick and 5–10 m across pass laterally into metre scale, medium to fine grained crevasse sandstone sheets. Rare laminated lacustrine siltstones occur only in the north‐east part of the basin. Floodplain mudstones and marls make up the remainder of the succession. These display a variety of pedogenic features recording cyclical palaeosol development. Palaeosols show strong variations in morphology and maturity both laterally across the floodplain and downstream along the basin axis, reflecting local variation in aggradation rate associated with proximity to alluvial channel courses as well as regional variation in subsidence and floodplain drainage. Analysis of the organization and distribution of the various sediment bodies permits reconstruction of the fluvial system and allows development of a model for the sedimentary architecture of the Lower Freshwater Molasse in the study area. Integration of palaeosol studies into a well defined architectural framework assists recognition of areal facies belts and may aid location of sand‐prone sequences in the subsurface.

The Pennsylvanian Springhill Mines Formation: sedimentological framework for a portion of the Joggins Fossil Cliffs UNESCO World Heritage Site

“…Although not present in the Springhill Mines Formation, calcretes are present in basin-margin redbeds of the broadly coeval Tynemouth Creek and New Glasgow formations (Plint and van de Poll 1982;Chandler 1998), suggesting that, at specific times and locations, conditions were suitable for carbonate accumulation if soils were given sufficient time to develop. Such conditions would be more likely in these proximal units, which likely experienced better drainage and slower sedimentation rates (Platt and Keller 1992). The development of crude cyclicity in the Springhill Mines Formation is consistent with astronomically forced cycles in the Milankovitch band (Calder 1994).…”

Sedimentological and palynological constraints on the basal Triassic sequence in Central Switzerland

“…The massive texture, observed in the coarse-grained sandstone beds, suggests episodic transport of sediment presum- ably as granular flows (Platt & Keller 1992). The presence of cross-stratified beds with a flat base indicates occasional transport of sand grains as bedload by unconfined subcritical flows (Southhard 1991).…”

Possible environmental effects on the evolution of the Alps-Molasse Basin system

“…Detailed sedimentological analysis of well-exposed sections reveal that the deposits of the axial drainage are made up of distinct architectural elements. Each are assigned to a particular depositional setting including riverbelts, levees, crevasse channels and splays, floodplain fines and palaeosoils, and lacustrine (Platt & Keller, 1992). The riverbelt sandstones are between 150 and 1500 m wide and arranged as amalgamated and locally stacked ribbon bodies 2-15 m thick.…”