Catégorie : Geology

The Kerinitis is one of some eight conglomeratic Gilbert-type fan deltas of exceptional dimensions that are preserved within the syn-rift stratigraphy of the Corinth rift. It is probably the best known and most frequently described example of a footwall-derived, coarse-grained, Gilbert-type fan delta deposited in the hangingwall of an active normal fault. Several studies have used the exceptional exposures of this delta to examine the interplay between tectonics and eustasy in controlling delta stratigraphic architecture (e.g., Gawthorpe & Colella 1990; Ori et al. 1991; Poulimenos et al. 1993; Dart et al. 1994; Zelilidis & Kontopoulos 1996). Recent biostratigraphic dating indicates that these deltas were deposited from Early to Middle Pleistocene over a period of between 500 and 800 ka (Ford et al. 2007; Rohais et al. 2007).

Figure.1. Geological map of the central part of the Pirgaki-Mamoussia fault block.

With a basal radius of 3.8 km and an arc of 125°, the Early-Middle Pleistocene Kerinitis Gilbert-type delta, covers a surface area of 15 km2 and contains approximately 9 km3 of (preserved) coarse grained sediment. It is very well exposed along a SW-NE 3.8 km long, 600 m high, natural cliff section cut by the Kerinitis river. This seismic-scale cliff represents a true section through the SE side of the delta, which prograded toward the north. Foresets on this river section dip 25° (average) toward N042° (Backert et al, submitted). The point source for the delta was probably inherited from an antecedent drainage system that coincided with the present day Kerinitis river, where it incises into footwall carbonates (Figs 1, 2).

Figure 2. Simplified map of the Lower – Middle Pleistocene Kerinitis and Vouraikos deltas and their geometrical relationship with controlling faults. Point sources are shown by blue arrows.

As the base of the delta is not exposed, we assign a minimum delta thickness of 600 m in the Kerinitis river section. The delta base is erosive lying below 200 m altitude in the Kerinitis valley but at 500 m altitude in the Selinous valley to the west. Clasts are dominantly carbonates with minor radiolarites (Upper Triassic to Upper Cretaceous) and some ’flysch’ sandstones (Upper Cretaceous to Eocene), all principally derived from the underlying Pindos thrust sheet (Hellenide orogen, Oligocene-Miocene). The Kerinitis delta is separated from the Vouraikos delta by the Keranitis fault (Fig. 2). To the west, its uppermost west-dipping foresets underlie the east-dipping foresets of the Selinous delta. The red Kolokotronis delta (Upper Group) overlies and masks the Kerinitis delta, with a significant erosional boundary.

The Kerinitis river profile is divided into three zones. To the SW, the 1.5 km long Zone 1, comprises up to 440 m of stacked topsets representing the most proximal part of the delta edifice. Zone 2 is 900 m long and includes the most complex stratal architecture of the delta. In the middle of this zone, a NW-SE trending valley about 1600 m deep, cuts at right angles to the main cliff, providing 3D information. To the NE, Zone 3 (1.4 km long) exposes 600 m of stacked foresets, which pass rapidly north-eastward into bottomset and prodelta facies.

Figure 4. View south along the Kerinitis river valley. the Lower to Middle Pleistocene Keranitis Gilbert type delta lies in the hangingwall of the Kerintis and Pirgaki normal faults.

Figure 5. View SSW from Mamoussia of the Northern part of the Kerinitis delta showing the stacked foreset packages.

Symmetrical hangingwall delta with fixed point sediment supply.

Figure 1. Geological map of the Vouraikos delta and surrounding area showing stratigraphy and main controlling faults (Ford et al. 2OO7)

Figure 2. The E-W Mamoussia cliff at the SW edge of the Vouraikos Gilbert delta, showing the gradual infill of a palaeo-valley.

The delta shows a near perfect radial fan of foreset dips (Fig.3a) recording progradation toward the NNW (345°). This symmetry suggest that the Vouraikos delta had a fixed point sediment supply from the footwall throughout its history and that this was the same river that is now incising into it (Type A feeder system of Postma, 1990, 1995).

 

The delta built into a marine basin.

An algal limestone bed (Fig 3b) forms a step at the gate to the ancient city of Keryneia (located in Figure 2b). Along with another bioclastic limestone found on the delta front, this limestone facies indicates that the delta built into a marine basin (Ford et al., 2007).

Figure 3 (a) Rose diagram and polar plot of dip and dip directions of 104 foresets of the Vouraikos delta showing average direction of 345°N. (b) Photomicrograph (5 mm across) of Mamoussia limestone bed (located in Figure 33b), showing algal mat texture (Ford et al. 2007).

In a 600km2 area south of Aegion to Xylocastro (Fig.1) the onshore syn-rift stratigraphy is divided into three informal groups (Rohais et al. 2007; Ford et al. 2007; Backert et al. in revision).

Figure 1. General geology of the central south coast, showing syn rift sediments and major Lower to Middle Pleistocene Gilbert deltas (adapted from Ghisetti et Vezzani 2006; Rohais et al. 2007; Ford et al. 2007; Backert et al. in revision). The areas covered by various authors are indicated.

Figure 2. Synthetic map (unpublished) of structure and Plio-Pleistocene syn-rift stratigraphy of the Corinth rift adapted from Rohais et al. (2007), Ford et al. (2007), Ghisetti & Vezzani (2005), Backert et al. (in rev) and unpublished field data (Ford 2009). (b) EW section of lateral correlations.(c) Legend. 

Lower Group: 1.3 km alluvial-fluvial sediments in W, 400 m fluvio-lacustrine sediments in NW (south of Aegion), 1.8 km fluvio-lacustrine sediments E of Akrata. All sourced from W –WSW. estimated age :Pliocene-early Pleistocene

Middle Group: Gilbert-type fan deltas, Sourced from S, prograde mainly N, locally NE (Prioni), and NW (Ilias). Up to 1000 m thick. Rift narrows in W (deltas restricted to north of the PM fault), but remains wide in E. Estimated age: early-Middle Pleistocene.

Upper Group: Local onshore deposits of small Gilbert deltas, fluvial terraces, red soil terraces, slope deposits and marine terraces on the range front. All were deposited during uplift of the south coast. Deposition offshore (Bell et al. 2008). Estimated age: Middle to Late Pleistocene

The borehole was drilled in 2002 and instrumented with a permanent monitoring system installed from September 2003 to February 2005. Only a small portion of the scheduled system has been installed: the fault could not be sealed, and only pore pressure data could be recorded under a surface plug, together with a surface hydrophone.

The whole system has been installed in 2007, but above the fault.

The data available are:
 electrical image log (UBI) from 2002
 pore pressure from the first phase of the instrumentation, from 2003 to 2005.

contact: F. Cornet, M.L. Doan