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Journal of the Geological Society; 1992; v. 149; issue.3; p. 431-441;
DOI: 10.1144/gsjgs.149.3.0431
© 1992 Geological Society of London

Article

Nature and origin of a laterally extensive alluvial sandstone body in the Middle Jurassic Scalby Formation

Jan ALEXANDER

Department of Geology, University of Wales College of Cardiff, PO Box 914, Cardiff CF1 3YE UK

Complex internal structure is characteristic of many high-sinuosity channel deposits. This is particularly true of meander-belts formed in lower delta plain or coastal plain settings, as demonstrated by an 'exhumed meander-belt' exposed for 3.5 km along the coast north of Scarborough, Yorkshire. Point bar, counterpoint bar, channel plug, floodplain lake and sheet flood deposits all contribute significantly to the internal architecture of the sandstone body. Units that are recognizably of point bar lateral-accretion origin, make up a little over 24% of the area of the 'exhumed meander-belt', counterpoint bar deposits over 1%. The bulk of the sandstone body is composed of channel plug and sheet flood sandstones and units whose origin is not easily recognizable due to partial preservation. The structure of the point bar and counterpoint bar deposits indicate that channels of a variety of types were responsible for the deposit. Although, in general the channels were small, mixed load and highly sinuous. The counterpoint bar deposits observed in the wave-cut platform vary in curvature, composition and structure. Counterpoint bars are sedimentary packages deposited on or near the outer bank of an alluvial channel meander. Four theoretical mechanisms for producing these deposits are suggested: (a) flow separation in a tight bend of a suspension load dominated river; (b) changing radius of curvature of a river bend; (c) local cut-bank embayment development during high stage flow filling at lower flow conditions and (d) complex meander loop development. Of these only two have been recognized in the 'exhumed meander-belt'.




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