Quaternary Alluvial Fans in Southwestern Crete: Sedimentation Processes and Geomorphic Evolution
- M. Marzo and
- C. Puigdefábregas
Published Online: 14 APR 2009
Copyright © 1993 The International Association of Sedimentologists
How to Cite
Nemec, W. and Postma, G. (1993) Quaternary Alluvial Fans in Southwestern Crete: Sedimentation Processes and Geomorphic Evolution, in Alluvial Sedimentation (eds M. Marzo and C. Puigdefábregas), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444303995.ch18
- Published Online: 14 APR 2009
- Published Print: 16 SEP 1993
Print ISBN: 9780632035458
Online ISBN: 9781444303995
- Quaternary alluvial fans;
- reconstruction of the depositional alluvial-fan complexes;
- onshore geomorphology;
- tectonics and sea level changes;
- The oldest fan- relatively small;
- The younger (Pleistocene) fans are relatively large
This is a study of the Quaternary alluvial sedimentation and its controls in the piedmont zone of the Lefka Ori limestone massif in southwestern Crete, Greece. The piedmont was sculptured by wave erosion during a major marine transgression in Pliocene time. The sea-level highstand controlled also the development (erosion base level) and early alluvial infill of large intramontane valleys, whose outlets at the mountain front are ‘hanging’ high above the piedmont plain. These palaeovalleys were once filled with thick gravelly alluvium, and periodically conveyed large sediment and water discharges during Quaternary times, when the highest mountain range was subject to inferred glaciations and deglaciations. The palaeovalleys have later been degraded by deep incision of narrow axial trenches. The Quaternary piedmont deposits occur as an array of coalescent alluvial-fan complexes, each comprising several generations of superimposed fans built of limestone gravel. Three main generations of fans have been distinguished as broadly isochronous depositional ‘stages’.
1 The older fans (earliest Pleistocene), which have apices at altitudes of 300–400 m, are relatively small (radii of c. 1 km), very steep (20–22° near apices) and composed of reddish debris-flow deposits. These fans were built of highly immature debris derived locally through headward erosion of mountain-front ravines, and received little or no sediment from the pre-existing intramontane valleys. Climate was probably arid to semiarid, such that the valleys stayed inactive. The fan toes were modified and cliffed by a marine incursion into the piedmont zone.
2 The younger (Pleistocene) fans, which are several tens of metres thick and constitute the bulk of the piedmont alluvium. They have apices at altitudes of 320–370 m and comprise at least five generations of variously stacked fan lobes. These fans are relatively large (radii in excess of 2 km) and steep (up to 13°), but consist of streamflow deposits. Moreover, their longitudinal morphometric profiles tend to be convex, broadly sigmoidal in shape. The deposits are relatively well-sorted, grey gravels that occur as alternating channel-lag and channel-bar sheets. This stage involved large water discharges, attributed to mountain ice-cap melting, whereby the pre-existing intramontane valleys had been activated as principal drainage and sediment-yield areas. The hanging valley outlets made the systems prograde on to the piedmont plain as relatively steep, ‘cascading’ fans. The preserved toes of smaller, lower-lying fans in the east show the record of deposition in a protected marine embayment.
3 The Holocene fans and trench-confined lobes, which are of minor volumetric importance and consist of grey, unconsolidated, openwork ‘sieve’ gravels. Their origin is attributed to ephemeral stream-flood surges issued from the degraded intramontane valleys in semiarid, possibly monsoonal, climatic conditions.