Interaction of an antecedent fluvial system with early normal fault growth: Implications for syn-rift stratigraphy, western Corinth rift (Greece)
Résumé
Continental ‘overfilled’ conditions during rift initiation are conventionally explained as due to low creation of accommodation compared with sediment supply. Alternatively, sediment supply can be relatively high from the onset of rifting due to an antecedent drainage system. The alluvial Lower Group of the western Plio–Pleistocene Corinth rift is used to investigate the interac- tion of fluvial sedimentation with early rifting. This rift was obliquely super- imposed on the Hellenide mountain belt from which it inherited a significant palaeorelief. Detailed sedimentary logging and mapping of the well-exposed syn-rift succession document the facies distributions, palaeocurrents and stratigraphic architecture. Magnetostratigraphy and bios- tratigraphy are used to date and correlate the alluvial succession across and between fault blocks. From 3!2 to 1!8 Ma, a transverse low sinuosity braided river system flowed north/north-east to east across east–west-striking active fault blocks (4 to 7 km in width). Deposits evolved downstream from coarse alluvial conglomerates to fine-grained lacustrine deposits over 15 to 30 km. The length scale of facies belts is much greater than, and thus not directly controlled by, the width of the fault blocks. At its termination, the distribu- tive river system built small, stacked deltas into a shallow lake margin. The presence of a major antecedent drainage system is supported by: (i) a single major sediment entry point; (ii) persistence of a main channel belt axis; (iii) downstream fining at the scale of the rift basin. The zones of maximum sub- sidence on individual faults are aligned with the persistent fluvial axis, sug- gesting that sediment supply influenced normal fault growth. Instead of low accommodation rate during the early rift phase, this study proposes that facies progradation can be controlled by continuous and high sediment sup- ply from antecedent rivers.