262 / 2022-03-31 09:23:54

Sea-level changes and strait topography control the distribution and facies development of the Changjiang coastal mud wedge

Changjiang coastal mud wedge,shallow-marine mud,coastal mud,fluid mud,Taiwan Strait,East China Sea,shallow-water contourite

The coastal mud wedge is one of the most important settings where shallow-marine muddy deposits accumulate. However, until recently, there are very few detailed sedimentological studies on it. To investigate the sedimentary processes of one of the largest coastal mud wedges, the Changjiang Coastal Mud Wedge (CCMW), we used chirp shallow seismic stratigraphy and ¹⁴C dating of a sediment core from the northern end of Taiwan Strait to provide a regional stratigraphic framework. Sedimentological observations and organic geochemical analysis of the sediment core, combined with detailed comparison with the eustatic sea-level curve, permit estimation of the changing paleowater depths and sedimentary processes responsible for the coastal mud wedge. The clay mineralogy and U–Pb detrital zircon geochronology were analyzed to illustrate the provenance evolution.

Three regional unconformities were identified that probably correspond to the last three glacial maxima, MIS2, MIS 6 and MIS 8. The MIS 6 and MIS 8 unconformities display pronounced incised valleys that were likely carved by the Minjiang River. The strata between the MIS2 and MIS6 unconformities (seismic unit 2) have ¹⁴C ages that indicate that the valley filling occurred during the long-term sea-level fall of MIS 3. The lower part of this valley fill is composed of bay-head delta plain to delta-front deposits, from base up. A provenance study of the lower fill reveals the predominance of the Minjiang sediments, with some Changjiang sediments, which confirms a deposit that accumulated in the Minjiang estuary, with some southward transport of Changjiang mud by the Zhe-Min Coastal Current. The middle valley fill contains shallow-marine mud that comprises significant amounts of Changjiang sediments. While the shoreline lay seaward of the core site (i.e., during initial filling of the valley), the main sediment supply was by the Minjiang, but after the shoreline moved landward of the core location (middle valley fill), the Minjiang contribution decreased (presumably because it was trapped in the contemporaneous estuary), and the contribution from the distant Changjiang became important/dominant. During the transgression of the shoreline, the Minjiang estuary was inundated and became a partially filled incised valley. The clay mineralogy of the upper valley fill (coastal mud) reveals the predominance of the Changjiang sediment, which suggests that the remainder of the valley was filled by the CCMW. This is also for the first time that the existence of the CCMW during MIS 3 is reported.

Integrated water-depth estimates, provenance and core observations reveal the presence of distinct shallow-water and deep-water facies within the CCMW. The shallow-water (<20 m water depth) facies contains fluid-mud layers, highlighting the role of waves and tides in resuspending previously deposited muds. The deep-water facies consist of muds that were influenced by ocean currents and are more intensely bioturbated and local rich in shells. The succession here is dominated by muddy deposits that initially look “simple”, but upon closer examination have a very complex sediment provenance, and with many unconformities, due to sea-level fluctuations during MIS3, in what might initially look like a continuous sedimentary succession.