Mega-pockmarks surrounding IODP Site U1414; insights from the CRISP 3D seismic survey

Author(s): Nale, S. M.; Kluesner, J. W.; Silver, E. A.; Bangs, N. L.; McIntosh, K. D.; Ranero, C. R.
Author Affiliation(s): Primary:
University of California, Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, CA, United States
University of Texas at Austin, United States
Institució Catalana de Recerca i Estudis Avançats, Spain
Volume Title: AGU 2013 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2013; American Geophysical Union 2013 fall meeting, San Francisco, CA, Dec. 9-13, 2013. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Visualization of neural network meta-attribute analyses reveals fluid migration pathways associated with mega-pockmarks within the CRISP 3D seismic volume offshore southern Costa Rica, near site U1414 of IODP Expedition 344. A 245 km2 field of mega-pockmarks was imaged on the Cocos Ridge using EM122 multibeam bathymetry, backscatter and 3D seismic reflection aboard R/V Marcus G. Langseth during the 2011 CRISP seismic survey. We utilize the OpendTect software package to calculate supervised neural network meta-attributes within the 3D seismic volume, in order to detect and visualize probable faults and fluid-migration pathways within the sedimentary section of the incoming Cocos plate [see Kluesner et al., this meeting]. Pockmarks imaged within the 3D volume near the trench commonly show a two-tier structure with upper pockmarks located above the steep walls of deeper, older pockmarks. The latter appear to truncate surrounding strata, including widespread high-amplitude reverse polarity reflectors (RPRs), interpreted as trapping horizons. In addition, RPRs are also truncated by positive polarity crosscutting reflections (CCRs), most of which form the base and sides of lens-like structures below the RPRs that are frequently located next to imaged pockmarks. Site U1414 intersects one of these lens-like structures and this appears to correlate to a sharp density and porosity swing observed at ∼255 mbsf. In addition, preliminary geochemical analyses from site U1414 show evidence of lateral fluid flow through sediments below the RPR [Expedition 344 Scientists, 2013]. Thus, we interpret the 3D lens-like structures to be pockets of trapped gas and/or over-pressured fluid. Based on 3D imaging we propose a 3-stage pockmark evolution: (1) Overpressure and blowout along RPRs, resulting in pockmark formation, (2) sustained seepage along pockmark walls, resulting in preferential deposition near the center of the pockmark, and (3) rapid burial as pockmarks near the trench axis. On the seafloor, small high-backscatter mounds are found near the walls of a subset of pockmarks, suggesting recent or active seafloor seepage. Further geochemical analyses are needed to determine the source of fluid/gas migration associated with the pockmark structures.
Year of Publication: 2013
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Costa Rica Seismogenesis Project; East Pacific; Expedition 344; Geophysical methods; Geophysical surveys; IODP Site U1414; Integrated Ocean Drilling Program; Marine sediments; North Pacific; Northeast Pacific; Pacific Ocean; Sediments; Seismic methods; Surveys; Three-dimensional models
Coordinates: N083014 N083014 W0841332 W0841332
Record ID: 2015031154
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