The six original specific scientific objectives of Expedition 308 and their rationale were as follows.
1. Document how pressure, stress, and geology couple to control fluid migration on passive margins.We hypothesized that flow-focusing is present in Ursa Basin (Fig. F3). This should result in a characteristic spatial distribution of fluid pressure and rock properties (e.g., consolidation, permeability, and shear strength) in the mudstone overlying the Blue Unit. Our objective was to measure fluid pressure and rock properties through downhole tools, core, and logs to establish the vertical and lateral variation in pressure and rock properties above the Blue Unit. These data would test and refine the flow-focusing model.
2. Establish reference properties at Brazos-Trinity Basin #4.We wished to establish reference logging and core properties where overpressure is hypothesized to be zero and thus to measure properties at a range of effective stresses. These data would serve as a baseline against which the properties measured in Ursa Basin could be compared, allowing us to establish the deviation in sediment and fluid properties caused by fluid overpressure and low effective stress.
3. Illuminate the controls on slope stability.Massive paleolandslides are present in Ursa Basin. We wished to determine pore pressure, rock properties, and overburden stress to predict the potential for slope failure in the present and to estimate the conditions that drove previous slope failures.
4. Understand timing of sedimentation and slumping.Sedimentation rate drives the generation of overpressure. However, the age of strata in Ursa Basin is poorly understood. We wished to establish an age model to predict the sedimentation rate and the timing of paleolandslides.
5. Establish geotechnical and petrophysical properties of shallow sediments.We wished to understand the state and evolution of geotechnical and petrophysical properties of mudstone at effective stresses encountered between the seafloor and 500 mbsf. A complete logging suite, in situ measurements of permeability and pressure, and core samples will allow us to understand compaction and flow processes near the seafloor.
6. Provide an extraordinary data set to observe ponded and channelized turbidite systems.Brazos-Trinity Basin #4 and Ursa Basin are foci of study for turbidite depositional systems. Logging and core data will provide a unique opportunity to study turbidity currents and related mass deposits.