Abstract
AbstractNumerical tidal modelling, when integrated with other geological datasets, can significantly inform the analysis of physical sedimentation processes and the depositional and preservational record of ancient tide‐influenced shoreline–shelf systems. This is illustrated in the Oligo–Miocene of the South China Sea, which experienced significant changes in basin physiography and where tide‐influenced, shoreline–shelf deposition is preserved in ca 10 sub‐basins. Palaeogeographic reconstructions, palaeotidal modelling and regional sedimentary facies analysis have been integrated in order to evaluate the spatial–temporal evolution and physiographic controls on tidal sedimentation and preservation during the ca 25 Myr Oligo–Miocene record in the South China Sea. Palaeotidal modelling, using an astronomically‐forced and global tidal model (Fluidity) at a maximum 10 km resolution, indicates that spring tides along Late Oligocene to Middle Miocene coastlines were predominantly mesotidal–macrotidal and capable of transporting sand, which reflects two main conditions: (i) increased tidal inflow through wider ocean connections to the Pacific Ocean; and (ii) tidal amplification resulting from constriction of the tidal wave in a ‘blind gulf’ type of basin morphology. Since the Middle to Late Miocene, a reduction in the amplitude and strength of tides in the South China Sea was mainly due to diminishing tidal inflow from the Pacific Ocean caused by the northward movement of the Philippines and Izu–Bonin–Mariana arc. Sensitivity tests to palaeogeographic and palaeobathymetric uncertainty indicate that regional‐scale (hundreds to thousands of kilometres) palaeogeographic changes influencing tidal inflow versus outflow can override local‐scale (one to hundreds of kilometres) changes to tidal resonance and convergence effects (funnelling and shoaling), such as shelf width and shoreline geometry. Palaeotidal model results compare favourably to the distribution and sedimentary fabric of Oligo–Miocene, tide‐influenced, shoreline–shelf successions in peripheral South China Sea basins. However, the preservation potential of tidal deposits is lower in open coastline environments, probably due to enhanced reworking during storms and river floods.
Highlights
The stratigraphic record is a consequence of the integration between deposition and preservation
The slight overprediction of sediment grain-size mobility by Fluidity compared to FES 2014 is most likely to be due to the coarser mesh and bathymetry resolution and lack of internal drag, which causes insufficient frictional damping of the tidal energy. These results suggest that Fluidity will be capable of modelling bed shear stress and potential sediment mobility in ancient domains provided that the palaeobathymetric uncertainties are adequately defined (Mitchell et al, 2010)
Palaeotidal modelling using an astronomically-forced and global tidal model (Fluidity) at a maximum 10 km resolution, suggests that in the Late Oligocene–early Middle Miocene South China Sea (SCS), >80% of coastline spring tides were mesotidal– macrotidal (>2 m), >30% were macrotidal (>4 m) and sand transport was possible along 80% of the coastline
Summary
The stratigraphic record is a consequence of the integration between deposition and preservation. Coleman and Wright, 1975; Galloway, 1975; Ainsworth et al, 2011) These processes have different preservation potential because they operate on varying timescales and are associated with different sedimentation rates and deposit thicknesses (Fig. 1) Higher magnitude-lower frequency processes (for example, infrequent fluvial floods and storms) have a higher preservation potential on a thousand-year timescale (Fig. 1) because these events: (i) erode the deposits of lower magnitude–high frequency processes; and (ii) form thicker deposits that minimize physical and biological reworking, including complete bioturbation, before the succeeding event Barrell, 1917; Dott, 1983; Plotnick, 1986; Ager, 1993) and records repeated episodes of deposition and repeated accidents of preservation Stratigraphy is inherently fragmentary (e.g. Barrell, 1917; Dott, 1983; Plotnick, 1986; Ager, 1993) and records repeated episodes of deposition and repeated accidents of preservation (e.g. Sadler, 1981; Miall, 2015)
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