| Abstrakt: |
The Serravallian Altınkaya fan-delta complex has been analysed in its stratigraphic context of the Köprü Basin and compared with coeval analogous complexes in the adjoining Manavgat Basin, southwestern Turkey. The sedimentary record appears to bear signal of broad inter-basinal events, both tectonic and eustatic, as well as the local signal of basin-margin tectonics. The study demonstrates the importance of fan-deltaic successions in recording the depositional and tectonic history of basin margins and outlines a conceptual approach to their stratigraphic analysis. An understanding of fan-deltaic basin-fill succession requires: (1) recognition of its basic regressive-transgressive wedges, their geometry, facies anatomy and origin (forced-regressive vs. normal-regressive delta advances); (2) careful analysis of the vertical stacking pattern of the wedges within a fan-delta complex (i.e., a lower-order, composite regressive-transgressive wedge), with the distinction of their forward-stepping, aggradational and backstepping sets; and (3) recognition of the stratigraphic stacking pattern of successive fan-deltaic complexes, which can similarly be of the forward-stepping, aggradational or backstepping type, but may involve tectonic unconformities and invariably includes an intervening record of apparent “stasis”, where the marine basin’s physico-chemical regime and ecological conditions are more completely revealed. The morphodynamics of wave-dominated, shoal-water fan-deltas vary markedly in response to relative sea-level change. Normal-regressive basic wedges are twofold sequences comprising a highstand systems tract composed of extensive alluvium overlying a coarsening-upward package of shoreface and delta-front beach deposits; and a transgressive systems tract composed of a landward-thickening alluvium interspersed with marine horizons and truncated by a ravinement surface covered with products of wave erosion. The normal-regressive wedges can be regarded as 2-D clastic wedges, because the essential aspects of their internal facies variation and heterogeneity are revealed by longitudinal cross-section. Forced-regressive basic wedges are fourfold sequences comprising a highstand systems tract (similar as above) overlain with deep scour by a falling-stage systems tract of incised alluvium and seaward offset delta-front deposits, covered further by a lowstand system tract of basinward-thickening alluvium and aggradational delta-front deposits, in turn overlain by a transgressive system tract (similar as above but generally thinner). The forced-regressive wedges have to be regarded as 3-D clastic wedges, because the main aspects of their internal variation that need to be revealed require both longitudinal and transverse cross-sections. The surrounding offshore-transition deposits are persistently tempestitic, but isolated turbiditic palaeochannels occur associated with the incised fluvial palaeovalleys of the falling-stage tracts of 3-D wedges. |
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