engleski

The architecture of incised valley-fill as a record of the relative rates of sea-level rise and sediment supply

The deposits of transgressive and highstand systems tracts have the highest preservation potential in incised valleys, which suffer little erosion from storms and are virtually free of sediment removal by alongshore drift. Incised valley-fill successions, therefore, provide a valuable record of the relative rates of base-level rise (accommodation-space creation) and terrestrial sediment supply (accommodation-space infilling). In terms of these two parameters, four scenarios for an incised valley-fill are suggested by previous studies (Fig. 1): (A) a rapid sea level rise combined with relatively low coarse-sediment supply, resulting in a Gilbert-type bayhead delta prograding slowly in a rapidly aggrading and shallowing prodelta environment (e.g., many postglacial fjord-head deltas) ; (B) a rapid sea-level rise combined with relatively high coarse-sediment supply, resulting in a quickly prograding Gilbert-type delta ; (C) a gradual sea-level rise combined with relatively low coarse-sediment supply, resulting in the classical estuary with a bayhead delta, muddy central basin and an outer barrier/tidal-inlet complex ; and (D) a gradual sea level rise combined with relatively high coarse-sediment supply, resulting in a fluvial dominated valley-fill. Considering the additional controlling role of climate, syndepositional tectonics, and marine regime, it is rather obvious that this range of basic scenarios may be far from exhausting the natural spectrum of incised valley-fill cases. It is suggested that, in particular case studies, instead of pigeonholing the case into one of the four categories (Fig. 1) – an attempt should rather be made to distinguish it from these end-members and recognize its own specific formative conditions. A case study from the southern margin of the Corinth Rift, Greece, is presented to show an ‘intermediate’ type of valley-fill, additionally influenced by syndepositional tectonics. The present study of the Pleistocene Akrata palaeovalley documents a gravelly valley-fill that lacks evidence of tidal activity, shows considerable wave influence and is dominated by bayhead shoal-water deltas succeeded by a Gilbert-type delta. The Akrata palaeovalley was ~3 km long, up to ~2 km wide and at least 120 m deep, incised in similar older syn-rift deposits probably due to the rift margin uplift combined with the OIS 10 eustatic sea-level fall. A stepwise rapid marine invasion drowned the valley, resulting in an infilling in ~30 ka in association with the OIS 7e and 7c eustatic sea-level rises. The age of the valley-fill deposits is estimated from the available chronostratigraphical data (McNeill & Collier, 2004 ; Rohais et al., 2007) integrated with a new field survey. The filling of the Akrata valley commenced with a gravelly basal alluvium deposited during the relative sea-level lowstand. The first step of the subsequent marine transgression resulted in a shoal-water bayhead delta comprising mouth bars stacked in a compensational manner, indicating a still limited sediment supply and an autogenic lateral switching of delta distributaries under a rising relative sea level. The second step of rapid marine flooding gave rise to a valley-wide, single-lobe shoal- water delta with a wave-worked front and steeper slope, indicating significant increase in sediment supply. The vertical stacking of the bayhead shoal-water deltas increased the bathymetric relief of the valley. The next step of marine transgression drowned the relief, resulting in a Gilbert-type delta that overstepped the previous ones and reached the valley mouth, while forming the bulk of the valley-fill succession. The advancing delta recorded shorter-term sea level fluctuations, as evidenced by changes in the delta's brink- zone trajectory and the geometry of its foreset-topset contact. Since the magnitude of the eustatic sea- level rises OIS 7e and 7c was insufficient to create a 120-m accommodation space (Westaway, 2002), a strong impact of tectonic subsidence is inferred. The formation of a valley-parallel relay ramp is thought to have pinned down the nucleation point of the consecutive deltas and apparently acted as a topographic barrier for marine invasions. As a result, the gravelly alluvium in the upstream segment of the valley was piled up into a monotonous succession combining TST and HST. All three marine-flooding events were rapid, with a negligible thickness of transgressive deposits, but the stepwise transgression caused a substantial increase in valley-floor relief and prepared the stage for a highstand growth of the Gilbert-type delta. This field case (model E, Fig.1) would thus represent a variety of the wave-worked, shallow-water fluvio- deltaic system of model D with a major flooding, turned at sea-level highstand into the deep-water, Gilbert-type fluvio-deltaic system of model B.

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