Geometry and Lateral Accretion Patterns in Meander Loops: Examples from the Upper Oligocene–Lower Miocene, Loranca Basin, Spain

Abstract

The Loranca Basin was infilled by two coalescing fluvial systems from Late Oligocene to Early Miocene times. High sinuosity rivers left meander loop deposits on the fan surfaces. Today these are preserved as exhumed point bar deposits which provide excellent three-dimensional exposures, enabling detailed examination of the internal organization and architecture of the sandstone bodies. It is possible to reconstruct the geometry of individual depositional components and thus chronicle the development of individual meander loops. Laterally stacked point bars are delimited by discordant surfaces marking changes in the displacement direction of meander bends. Two types of point bars are distinguished on the basis of grain size distributions: type A bars display a wide range, whereas in type B bars some of the grain sizes are lacking. The vertical organization of point bar deposits was described from longitudinal or oblique sections through lateral accretion units. Type A point bars display three types of vertical bedding profiles: thickening-thinning, thickening and thinning. In contrast type B point bars grew from recurrent thickening-thinning bedding cycles. The bed thickness of type A point bars has a positive correlation with sedimentation and channel migration rates, which in turn are controlled by channel curvature. In contrast the recurrent bedding cycles which characterize type B point bars are ascribed to lateral accretion episodes accompanied by scroll bar formation. The discordances between adjacent point bars resulted from episodic channel adjustment in response to the development of critical curvatures.