engleski

Comparative facies analysis of the sigmoidal and oblique foreset deposits of Gilbert-type deltas: implications for the recognition of short-term relative sea-level changes

The geometrical relationship of fluvial topset to subaqueous foreset of a Gilbert-type delta may be 'sigmoidal' (transitional) or 'oblique' (erosional) and is attributed to a rise or fall of the delta's shoreline time-distance trajectory, thus reflecting relative sea-level change. However, every episode of a relative sea-level fall will force the fluvial system to cut down, which may remove the delta-brink sigmoidal signature of a preceding episode of relative sea-level rise. The geometrical record of short-term relative sea-level changes in a Gilbert-type delta thus tends to be obliterated by fluvial erosion. The component facies of a Gilbert-type delta foreset are deposits of turbidity currents (whether slope collapse- generated brief surges or longer-duration hyperpycnal flows), dense debris flows and loose coarse- debris falls. The present study of the Plio-Pleistocene Gilbert-type deltas in the Gulf of Corinth, Greece, focuses on the relative proportion of these facies in sigmoidal and oblique foreset deposits – revealing significant differences. The foreset beds corresponding to a sigmoidal delta-brink geometry are mainly debris-flow deposits (~50 %), with subordinate turbidites (~27 %) and debris-fall deposits (~23 %). A relative sea- level rise apparently promotes excessive sediment deposition in the delta’s brink zone during low fluvial discharges, resulting in gravitational collapses (debris flows) ; forces sediment bypass during higher discharges (turbidity currents) ; and causes an excessive accumulation of failure-prone fluvial lag gravel during the highest discharges (debris falls). The foreset beds corresponding to oblique delta-brink geometry comprise debris- flow deposits (~45%), more turbidites compared to sigmoidal intervals (~40%) and minor debris- fall deposits (~15%). A relative fall or stability of sea- level apparently promotes an increased sediment bypass (turbidity currents) accompanied by denser sediment failures (debris flows) and only sporadic gravel-lag collapses (debris falls). The study suggests that a detailed quantitative analysis of the delta foreset facies may help to reveal the record of short-term relative sea-level changes whose brink-zone geometrical signatures may have been erased by delta-top fluvial erosion.

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