Expedition 308 synthesis: overpressure, consolidation, and slope stability on the continental slope of the Gulf of Mexico.

Flemings, P. B., John, C. and Behrmann, Jan and IODP Expedition 308 Scientists (2012) Expedition 308 synthesis: overpressure, consolidation, and slope stability on the continental slope of the Gulf of Mexico. Open Access Proceedings of the Integrated Ocean Drilling Program, 308 . pp. 1-19. DOI 10.2204/iodp.proc.308.215.2012.

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Abstract

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.

Document Type: Article
Keywords: Gulf of Mexico, mudstone, overpressure, slope stability, IODP, Expedition 308, hydrogeology
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.2204/iodp.proc.308.215.2012
ISSN: 1930-1014
Projects: Future Ocean
Expeditions/Models/Experiments:
Date Deposited: 04 Feb 2013 08:18
Last Modified: 23 Jan 2019 13:38
URI: http://oceanrep.geomar.de/id/eprint/20241

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