Plio-Pleistocene alluvial architecture in marginal extensional narrow sub-basins: examples from southwest Greece

Abstract

At the eastern margin of the southern part of the northwest-trending Kalamata Basin, four small extensional sub-basins were formed during the Plio-Pleistocene period. These sub-basins developed on the footwall of a major north-northwest-trending marginal fault, which separates the Taygetos mountains from the Kalamata Basin. Two different basin configurations, mostly resulting from different subsidence/sedimentation ratio (R), produced two different depositional types. In the first type, where R>1, gravelly alluvial fans were deposited in narrow and restricted sub-basins. In the second type, where R<1, braid-deltas were deposited in open, low-gradient sub-basins with marine influence. Braid deltas consist mostly of sandstone and mudstone, and only in the northern part of one sub-basin did gravel accumulate. The paucity of gravel is associated with rapid basin subsidence. Since factors such as climate and source-rock lithologies were essentially similar for the different basins, these different alluvial stratigraphies result principally from tectonic processes. Maximum sediment thickness formed on the hangingwall of faults parallel to the major basin-margin fault. Transfer faults at right angles to this major fault have strongly influenced the pathways of braided low-sinuosity streams. The initial open-channel width-to-depth ratio of the channels in this study (range 5–20) is comparable to suspended load channels and channel-fill deposits.