Evidence of soft sediment deformation, fluid escape, sediment failure and regional weak layers within the late Quaternary mud deposits of the Adriatic Sea

摘要

The late Quaternary sediments of the Adriatic Sea show clear evidence for fluid flow and shallow subsurface sediment mobilization. Amplitude blanking in high-resolution seismic data and limited sediment core information document that free gas is diffused at very shallow stratigraphic levels; pockmarks and blow-out pipes indicate expulsion of overpressured fluid; possible polygonal fault systems denote areas of sediment contraction and dewatering. Core data and VHR seismic profiles suggest that very high accumulation rates of homogeneous and fine-grained deposits result in fluid accumulation (high pore water pressure) within low-permeability sediments. The fluid flow processes that are active under such regime are still poorly understood. Enhanced accumulation rates during particular intervals can potentially result in trapping of fluids leading to increased pore pressure and decreased shear strength. Seismicity is high in the study area, and recurrent earthquakes or tsunamis provide an effective mechanism leading to the cyclic loading of sediment, favoring mobilization of gas-charged sediments. This paper provides an assessment of the shallow deformation styles, possible fluid flow processes and subsurface sediment mobilization features in the Quaternary muddy succession of the Adriatic shelf and slope. This succession includes forced regression deposits originated during repeated eustatic falls, lowstand prograding deposits formed during the Last Glacial Maximum (LGM), backstepping late Quaternary transgressive deposits and the late Holocene highstand progradational mud wedge. In each of these units, a variety of sediment deformation patterns are confined to well-defined stratigraphic intervals, typically above regionally extensive downlap surfaces that likely acted as detachment surfaces and left unaffected the succession beneath them. It is suggested that intervals of rapid progradation (resulting in rates of deposition in the order of 30 m in less than 10 kyr) generate downlapping deposits that are more prone to failure if perturbed soon after a critical thickness has accumulated and internal overpressures, likely related to the rapid rate of deposition, have not yet dissipated. Among the broad variety of sediment deformations identified, the late Holocene mud wedge shows a high degree of complexity in internal geometry just above a regional downlap surface (the maximum flooding surface [mfs]). According to geochronological determinations and tephra correlation, the mfs marks also an interval of condensed deposition between ca. 6 and 3.5 cal. kyr BP. The progradational wedge deposited above the mfs during the last ca. 3.5 cal. kyr, with sediment accumulation rates up to as much as 10 cm yr, likely hindering a rapid dissipation of interstitial fluids. A suite of deformation features occurs above the mfs regional downlap surface affecting this fast-depositing progradational unit. The picture is further complicated by variable deposition on preferred sides of seafloor irregularities caused by sediment deformation, indicating that bottom currents interact with the complex topography created by sediment deformation. These complex features (inferred to be genetically linked) nowhere affect the deposits underlying their basal downlap surface, a character shared also by the other reported cases of sediment deformations within older rapidly deposited units in the Adriatic.