The origin of massive sandstone facies in an ancient braided river deposits

Abstract

Lateral profiling techniques have been utilised to define the three-dimensional fluvial architecture of the Fell Sandstone Group (Arundian-Holkerian) of the Northumberland Basin, UK; the Lee-type sandstones (Morrowan-Atokan) of the central Appalachian Basin, USA; the Mansfield and Brazil Formations (Morrowan-Atokan) of the Illinois Basin, USA; and the Anisian Hawkesbury Sandstone of the Sydney Basin, Australia. These strata are characterised by sandstones of braided fluvial origin. Individual fluvial channels are dominated by downstream accreting mesoforms and macroforms, interpreted to represent mid-channel and bank attached bars and dunes. Palaeocurrents are unimodal and of low variance. Evidence of low stage reworking is rare, indicating that the fluvial systems were perennial. Cross-stratified sandstones are interbedded with structureless sand bodies, which display three distinct geometric forms: Sms, Smc and Sme. The texture and composition of facies Sms, Smc and Sme are distinct from associated structured facies. Facies Sms forms erosively based sandsheets <8 m thick and >250 m parallel and transverse to the flow. The upper surface is planar. Facies Smc forms elongate channels trending at high angles to the palaeoflow of fluvial channels. The sandbodies preserve a symmetrical cross-section with margins dipping <50º . Concentric laminae are preserved parallel to these margins, and grade into a structureless sandstone fill. Individual units of facies Sme are >6 m thick, and may be traced >200 m parallel and transverse to flow direction. Amalgamation of the facies results in sandsheets >20m thick. Scours, elongate both parallel and oblique to fluvial flow are preserved along the basal surface. A classification scheme of massive sandstone facies has been developed. The facies are interpreted in terms of deposition from highly concentrated, laminar sediment/water flows. Sediment-laden currents were generated through primary and secondary mechanisms related to flooding and mass flow.