The fluid mud flow deposits represent mud caps of Holocene hybrid event beds from the widest and gentlest shelf

Abstract

The internal fabric of mud caps, which are on top of sediment gravity flow deposits, remains unexplored so far. We identified four hybrid event beds on one of the widest and gentlest shelf (East China Sea Shelf). Three types of lithofacies, including massive sand, chaotic mud and homogeneous mud, were distinguished. Homogeneous mud represents mud caps of these hybrid event beds. Scanning electron microscopy performed in both homogeneous mud and chaotic mud has revealed the presence of mud aggregates (mud clasts) conforming sand-size clasts. Chemical analyses of the aggregates show the presence of abundant gypsum/anhydrite and clay mineral components as well as relicts of quartz, muscovite and feldspar, suggesting an origin related to erosional events and an intraclast origin. The vertical shortening, deformation together with the pore spaces of mud clasts indicate a shallow burial depth of erosion, where mud sediments were still unconsolidated and water-saturated. No background sedimentation is present in the mud caps, because we did not observe any bioturbated interval in core images and X-radiographs. The lack of laminated muds and synsedimentary trace fossils in the homogeneous muds suggest an origin related to fluid mud flows. As proved on previous experiments, the mud caps could be related to head erosion and shearing of the muddy debris flow/mud flow's upper surface which generated a slower moving fluid mud flow. Our study further confirms that no laminated sand/silt (H4 division) would be present in hybrid event beds with mud-dominated H3 division. Head erosion of muddy debris flow/mud flow would not allow laminated H4 division to form, because the fluid mud flow derived from the parent mud-dominated debris flow was mainly composed of grains smaller than 30 μm which can effectively damp turbulence effectively. Grain size analysis performed on samples from fluid mud flow deposit fall into pelagites/hemipelagites area (T area), suggesting that fluid mud flow deposits are also a candidate for T area, and the area does not necessarily reflect a deposition under un-stressed environment.