Miocene point-bar deposits in the Ebro Basin, Northern Spain*


  • *

    Presented at the VIIIth International Sedimentological Congress in Heidelberg (1971).


Pyrenean debris dispersed in relation to the Miocene tectonic phase filled the northern part of the Ebro basin. In the studied area (50 km south of Pamplona) the Miocene sediments are represented by fluviatile sequences consisting of flood basin and point-bar deposits forming fining-upward cycles.

The point-bar character of part of these sediments is not only evidenced by the internal sedimentary structures, but also by the fact that the corresponding meander bends, about 200 m in radius, are clearly visible on aerial photographs. In this way a unique opportunity is given to compare directly the vertical profile and the surface pattern of ancient small size point bars.

The internal structure consists of many imbricated and concentrically arranged bundles corresponding to the accretional topography of the point bar. The separation planes between the bundles are erosional and dip toward the channel axis. The internal structure of each bundle is festoon mega-cross-stratification whose direction points toward the convex bank, away from the channel axis. The resulting structure resembles the Epsilon cross-stratification of Allen (1965a) and its origin is the same as postulated by Allen for this kind of cross bedding.