Characteristics and Formation of Late Quaternary Incised-Valley-Fill Sequences in Sediment-Rich Deltas and Estuaries: Case Studies from China

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Valleys, Geology, Stratigraphic, Sedimentation and deposition, Incised valleys, SEPM, Sedimentary geology

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Four sediment-rich incised-valley systems in China, which underlie the Luanhe fan delta, the Changjiang delta, the Qiantangjiang estuary, and the Zhujiang delta, are examined based on over 800 drill cores. These four systems are of different shapes and sizes, and are located in different tectonic zones with different tide regimes ranging from microtidal to macrotidal. Because of the abundant fluvial sediment supply and relative dominance of river forcing, sediments in the modern Qiantangjiang and paleo-Changjiang estuaries display a fining-seaward trend. This is different from the classical estuarine facies model of coarse bay-head delta, fine central basin, and coarse bay-mouth deposits. The abundant sediment supply also results in the presence of relatively thick transgressive successions in the overall incised-valley fill. The transgressive succession constitutes more than 50% of the total strata thickness and approximately 60–70% of the total sediment volume within the valley. The river-channel facies in the transgressive succession was formed by retrogressive aggradation during postglacial sea-level rise. Retrogressive aggradation extends far inland beyond the reach of flood-tidal currents, and, therefore, no marine signatures were found at the lower portion of the incised-valley fill. The regressive succession in the incised-valley systems consists of fluvial facies or tidal facies and deltaic facies, and was developed as the estuary filled and evolved into a progradational delta. The tidedominated facies tends to be developed in the apical areas of funnel-shaped estuaries, such as the modern Qiantangjiang and paleo- Changjiang estuaries.

Four generalized facies successions (FS-I, FS-II, FS-III, and FS-IV) are recognized within the valley fill. An idealized schematic incisedvalley fill contains FS-I at the coastline region, FS-II and FS-III in the middle, and FS-IV at the apex area of the delta/estuary, reflecting a decreasing marine influence and increasing terrestrial contributions. The preservation of multiple incised-valley-fill sequences is controlled by the different regional tectonic characteristics. Vertically superimposed valleys are preserved beneath the Changjiang delta, whereas the Luanhe fan delta is characterized by lateral juxtaposition of valley as a result of channel switching.

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SEPM Special Publication, issue 85, p. 141-160

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