October 16, 2009 -   Following are the abstracts for the papers that have been accepted from UT GeoFluids researchers for the upcoming 4th International Symposium of Submarine Mass Movements and Their Consequences in Austin, Texas, November 7 – 12, 2009.

For more information on this conference, visit http://www.beg.utexas.edu/indassoc/dm2/Conference2009/home.htm

Subaqueous Landslides in Clay-Rich Systems
Derek E. Sawyer, Peter B. Flemings, David Mohrig

We simulated subaqueous landsliding within sedimented beds of clay-rich material. We deposited beds inside a flume and gradually increased the bed angle until failures developed. In a preliminary experiment, localized debris flows developed at bed angles of ~25° in a thin (2 cm thick) bed of kaolinite clay (60% by weight) and silica silt (40% by weight). Failure surfaces were confined to the upper 0.5cm. In one flow we observed outrunner blocks that accelerated away from the main flow and created linear grooves along the upper bed surface. In future experiments we aim to understand: 1) how landslide rates and styles vary as a function of material properties (clay mineralogy, grain size, and presence of thin interbeds of sand); and 2) the process of retrogression by measuring pore pressure in several locations behind retrograding headscarps. This work will illuminate the processes that drive subaqueous landsliding on continental slopes.

Exploring the Origin and Characteristics of Mass Transport Deposits
Ursa Basin, Gulf of Mexico
Hilary E Strong, Peter B Flemings, Ruarri J. Day-Stirrat, Derek E. Sawyer, Julia Schneider

Seismic, core, and logging data from Integrated Ocean Drilling Program (IODP) Expedition 308 record multiple Mass Transport Deposits (MTDs) within the upper 600 meters below seafloor (mbsf) of the Ursa Basin, northern Gulf of Mexico. The most prominent, MTD-2, is 35 to 100m thick, spans all three drill sites – U1324, U1323 and U1322 – and is located approximately 100 mbsf. MTD-2 is seismically imaged with a positive, low-amplitude top reflection, and negative, high-amplitude basal reflection. MTD-2 is identified in core and logging data as a low porosity, high bulk density, zone. At U1324, the basal sediments of MTD-2 are 7 porosity units less than those immediately below, while at U1322, the basal sediments are 10 porosity units less. This decline in porosity corresponds to a 5% increase in bulk density from overlying bounding sediments to MTD-2. We hypothesize that MTD densification is a result of sediment remolding during debris flow. Remolding is defined as shearing sediment at unaltered water content, thus removing the original fabric and resulting in weaker, more compressible material. We also suggest that the arrested flow is buried and consolidated under uniaxial strain. Initial high resolution X-ray texture goniometry (HRXTG) fabric analysis shows greater basal plane alignment of smectite and chlorite in a MTD specimen, compared to a non-MTD specimen. Mercury Injection Capillary Pressure (MICP) tests confirm a reduction in pore throat size within the MTD-2; however this is a function of decline in porosity, not position within the MTD. Consolidation curves from initial uniaxial experiments show that at a given porosity, a synthetically remolded specimen has on average a 2MPa lower vertical effective stress than the original intact specimen. Pre-consolidation effective stresses measured for Constant Rate of Strain (CRS) consolidation tests on 11 MTD and 13 non-MTD Ursa specimens suggest that the pre-consolidation stresses are approximately the same whether within or outside of an MTD. We further explored these hypotheses through CRS tests on 3 intact Ursa core specimens above MTD-2 (75mbsf), below MTD-2 (125mbsf) and within MTD-2 (115mbsf). Our results show (1) Weaker consolidation curves for MTD vs. non-MTD specimens; (2) Decreased sediment compressibility correlating with decline in porosity, irrespective of location within MTD vs non MTD; (3) Slightly higher permeability within the MTD, but still within the range of Ursa mudstones; and (4) No distinction in linear pre-consolidation stress trend between MTD and non-MTD specimens. Ultimately, MTDs pose a hazard because it takes longer for suction anchor piles and jetted conductors – installed for production platforms – to penetrate MTDs relative to bounding sediment.

History of pore pressure build up and slope instability in mud-dominated
sediments of Ursa Basin, Gulf of Mexico continental slope
Roger Urgeles, Jacques Locat, Derek E. Sawyer, Peter B. Flemings, Brandon Dugan4 and Nguyen Thi Thanh Binh

The Ursa Basin, at ~1000 m depth on the Gulf of Mexico continental slope, contains numerous Mass Transport Deposits (MTDs) of Pleistocene to Holocene age. IODP Expedition 308 drilled three sites through several of these MTDs and encompassing sediments. Logs, sedimentological and geotechnical data were collected at these sites and are used in this study for input to basin numerical models. The objective of this investigation was to understand how sedimentation history, margin architecture and sediment properties couple to control pore pressure build-up and slope instability at Ursa. Measurements of porosity and stress state indicate that fluid overpressure is similar at the different sites (in the range of 0.5 to 0.7) despite elevated differences in sedimentation rates. Modeling results indicate that this results from pore pressure being transferred from regions of higher to lower overburden along an underlying more permeable unit: the Blue Unit. Overpressure started to develop at ~53 ka, which induced a significant decrease in FoS from 45ka, especially where overburden is lower.

Failure caused by breaching in subaqueous sand
Yao You, Peter B. Flemings, David Mohrig

Submarine failures can be divided into two categories: liquefaction failure and breaching failure. During liquefaction failure the sediment matrix contracts while during breaching failure the sediment matrix dilates. Dilation causes the pore pressure in the sediment deposit to decrease, thereby temporarily increasing its shear strength. The tangent of failure angle increases in proportion with the ratio of effective stress to total stress: . Degree of dilation is greatest near the failure front and decays quadratically with distance away from failure. Finer sand or poorly sorted sand creates a higher failure angle and slower erosion rate during breaching because of stronger dilative response and lower diffusivity. In preliminary experiments we have documented pressure drawdown and identified the fraction due to dilation and the part due to change in stresses. We will present recent results from laboratory experiments and a numerical model that characterize how the behaviors of the dilative failures vary with different sand compositions.

June 10, 2009 -     Hilary Strong, an M.S. student with the UT GeoFluids Consortium,  received third place in the AAPG student poster competition  for her Poster entitled, Consolidation Characteristics of Mass Transport Deposits in Ursa Basin, Northern Gulf of Mexico

ABSTRACT: Seismic, core, and logging data from Integrated Ocean Drilling Program (IODP) Expedition 308 record multiple Mass Transport Deposits (MTDs) within the upper 600 meters below seafloor (mbsf) of the Ursa Basin, northern Gulf of Mexico. The most prominent, MTD-2, is 35 to 100m thick, spans all three drill sites – U1324 U1323 and U1322 – and is located approximately 100mbsf. MTD-2 is seismically imaged with a positive, low-amplitude top reflection, and negative, high-amplitude basal reflection. MTD-2 is identified in core and logging data as a low porosity, high bulk density, zone. At U1324, the basal sediments of MTD-2 are 7 porosity units less than those immediately below, while at U1322, the basal sediments are 10 porosity units less. This decline in porosity corresponds to a 5% increase in bulk density from overlying bounding sediments to MTD-2. We hypothesize that MTD densification is a result of sediment remolding during debris flow. Remolding destroys the original chaotic fabric, resulting in shear-aligned grains that have lower porosity. We also suggest that the arrested flow is buried and consolidated under uniaxial strain. Initial high resolution x-ray texture goniometry (HRXTG) fabric analysis shows greater basal plane alignment of smectite and chlorite in a MTD specimen, compared to a non-MTD specimen. Consolidation curves from initial uniaxial experiments show that at a given porosity, a synthetically remolded specimen has on average a 2MPa lower vertical effective stress than the original intact specimen. Pre-consolidation effective stresses measured for Constant Rate of Strain (CRS) consolidation tests on 11 MTD and 13 non-MTD Ursa specimens suggest that the pre-consolidation stresses are approximately the same whether within or outside of an MTD. We will further explore these hypotheses through HRXTG fabric analysis and CRS tests on natural and synthetically remolded Ursa core specimens. If our hypotheses are correct, we expect (1) weaker consolidation curves for MTD vs. non-MTD specimens; (2) Similar consolidation curves for MTD and remolded specimens; (3) No distinction in linear pre-consolidation stress trend between MTD and non-MTD specimens; and (4) Chaotic fabric in non-MTDs, and shear-aligned fabric in remolded and MTD specimens. Ultimately, MTDs pose a hazard because it takes longer for suction anchor piles and jetted conductors – installed for production platforms – to penetrate MTDs relative to bounding sediment.