Description: Integrated Ocean Drilling Program Expedition 308 quantified the coupling between sedimentation, consolidation, overpressure, fluid flow, and slope instability in continental margin settings. We summarize and synthesize peer-reviewed hydrogeologic studies published since the end of Expedition 308 that focus on Expedition 308 sites drilled in Ursa Basin: Sites U1322, U1323, and U1324. There is a rich stratigraphic complexity in the Ursa Basin, deepwater Gulf of Mexico. The sandstone-prone Blue Unit is a permeable aquifer overlain by mudstone-prone leveed-channel deposition. Multiple mass transport deposits, each densified relative to surrounding material, are present. Eight coeval surfaces mapped across the drilling transect span ~70 ka to present. Sedimentation rates were 〉10 mm/y at Site U1324. In situ penetrometer measurements document severe overpressures that begin at or near the seafloor and extend to the base of the drilled holes. Uniaxial consolidation experiments quantified the compression behavior of the sediments and provided an approach to predict pressure from porosity. Laboratory analysis of permeability and compressibility document that the coefficient of consolidation (hydraulic diffusivity) of Ursa Basin mudstones is 10–8 m2/s over the effective stresses encountered during drilling at Ursa Basin (0.1 to 5 MPa). This low and constant value for the coefficient of consolidation is responsible for the presence of high overpressure very near the seafloor. Although permeability is very high, the sediments are also highly compressible. Forward modeling of Ursa Basin sediments shows that overpressure near the seafloor is reasonable given the rock properties and sedimentation rates that are observed. Modeling also illustrates that flow is driven laterally along an underlying aquifer, the Blue Unit. Soon after Blue Unit deposition, lateral flow induced submarine landslides. Later in the evolution of this system, overpressure may have preconditioned the slope to failure by earthquakes of magnitude 5.0.

Description: We report the preliminary results of a triaxial and ring shear study on clay-rich, fine-grained Pleistocene sediments cored in Ursa Basin, Gulf of Mexico continental slope. Specimens from Integrated Ocean Drilling Program (IODP) Expedition 308 Sites U1322 and U1324 document friction coefficients in the range of 0.13–0.31, with internal angles of friction of ~7.4°–17.2° in ring shear experiments. At intermediate (7.624 MPa) to high (15.237 MPa) overburden pressure, the majority of the samples tested show velocity weakening, whereas lower overburden pressures do not give a clear trend regarding velocity weakening or strengthening of the samples. In consolidated-undrained triaxial tests, peak shear stresses observed are between 27 and 140 kPa, with the strongest sample by far coming from a core catcher section. We suspect that this is an effect of fabric changes induced during hydraulic piston coring. One sample coming from the base of a mass transport deposit at Site U1322 is the weakest one tested. Young’s moduli calculated range from 2 to 17.4 kPa. Stress paths indicate slight overconsolidation of the samples, which is in line with the information gained from preconsolidation stresses in other studies. Permeability determined from consolidation data is in the range of 10–16 to 10–17 m2, and hydraulic conductivity is around 10–9 to 10–10 ms–1. Grain density of the tested samples is around 2.7 g/cm3, and water content ranges from 18.3% to 30.7%.

Description: Turbidites and debris-flow deposits are exposed in the Upper Devonian to late Lower Carboniferous Markstein Group of the Southern Vosges Mountains (NE-France). Medium to coarse-grained sandstones are the dominating lithology alternating with fine-grained units. Sedimentary facies and their stacking patterns indicate a prograding evolution of sandy submarine fans from an outer-fan facies to channel-lobe-transional deposits and coarse conglomerates. Sedimentological and tectonical data confirm the strong geological relationship between the sedimentary units of the Southern Vosges and the Badenweiler-Lenzkirch Zone in the southern Schwarzwald.The grain composition of the turbidite sandstones is of metagranodioritic and volcanic origin. It reflects the erosion of an uplifted plutonic basement, episodically mixed with detritus from basaltic to dacitic active volcanoes. Trace element patterns and element ratios in sandstones from mafic-volcanic as well as from non-volcanic stratigraphic levels point to a subduction related setting. Trace element patterns in sandstones from dacite-volcanic active stages have normalized values, which can be compared with those from overthickened active margins or collisional belts. However, large-scale frontal collisional processes between the Protoalpine and the Moldanubian Zone cannot be evidenced by tectonic or provenance data. Instead, the favoured plate tectonic scenarios for the depositional setting are an arc-hinterland collision followed by dextral transpressive deformation through largescale wrench faults or a transpressively deformed active margin basin. In the first case a prograding retro-arc foreland is regarded as the tectonic setting of the submarine fans, in the second case an extensional or strike-slip setting in the fore-arc or back-arc area can also be imagined. Some turbidite complexes share petrographic and sedimentological similarities which in combination with the occurrences of overturned units point to an accretionary tectonic environment. Therefore the first model is favoured here. Our results are of interest for the paleogeographic reconstruction of the southern border of the Variscan Moldanubian Zone.