The sedimentary facies and setting of eocene point bar deposits, Monllobat formation, southern Pyrenees, Spain.

Abstract

Point bar deposits from a small, Eocene, molasse-type basin have been studied in detail in order to establish a spatial point bar model, which can be used to locate a vertical section in a threedimensional sence. A model is designed which shows the sedimentation processes in the palaeochannel and the evolution of point bars. However, some of the deposit characteristics can only be explained in terms of the wider, environmental setting. The development of facies and of larger sedimentary features can be recognised in the surface of a meander lobe, associated palaeochannel and fine fill, and in the vertical section of a comparable meander lobe. Most deposits comprise a lower, medium- to coarse-grained sandstone facies with trough-shaped crossbedding (0.15 m thick) and an upper facies with an inclined interbedding of sandstone and mudstone. Epsilon Cross Stratification has been developed extensively. Strong mottling has destroyed most of the sedimentary structures in the upper lithofacies interval. The exposed palaeochannel varies considerably in width and depth. Palaeochannel sediments are well graded in a vertical sense; laterally, however, there is only a minor gradation. During high discharge the lower lithofacies developed on a low platform at the point bar base. The upper facies originated on a steep upper point bar slope. Mud drapes were deposited during falling discharge. During periods of low discharge the upper part of the deposits was exposed to the air and strongly mottled and oxidized. Point bar deposits are arranged in meander lobes. A uniform dip direction often appears in transverse sections. Lateral grain-size variation in these sections can be large. In this respect the relatively coarse, initial part is important. Mud fills are found in the last stage channel. A fine member is present, but not all fine material was deposited by meandering rivers. The meander lobes developed in the distal parts of a small, highly energetic, alluvial system with marked fluctuations in discharge. In the sedimentary basin there was strong tectonic control on the sedimentation.