HR: 0830h
AN: OS51B-0854    [PDF]
TI: Recently Discovered Near-Shore Gascharged Sediments and Pockmarks, Northern Iceland
AU: * Richter, B
EM: br@isor.is
AF: Iceland Geosurvey, Grensasvegi 9, Reykjavik, 108 Iceland
AU: Driscol, N
EM: ndriscoll@ucsd.edu
AF: Scripps Institution of Oceanography, Univ. of California, San Diego, CA 92037 United States
AU: Detrick, R S
EM: rdetrick@whoi.edu
AF: Woods Hole Oceanographic Institution, Woods Hole, Boston, MA 02543 United States
AU: Fornari, D
EM: dfornari@whoi.edu
AF: Woods Hole Oceanographic Institution, Woods Hole, Boston, MA 02543 United States
AU: Brandsdottir, B
EM: bryndis@raunvis.hi.is
AF: Science Institute, Univ. of Iceland, Hagi, Hofsvallagata 53, Reykjavik, 107 Iceland
AB: The approx. 150 km long and 50 km wide Tjornes Fracture Zone (TFZ), formed in response to the eastward jump of the spreading axis in N-Iceland that began in late Miocene time (7-9 Ma). The fracture zone is made up of three N-S trending extensional half graben, Eyjafjordur, Skjalfandi and Oxarfjordur, and bordered to the south by a WNW-trending transtensional fault, the Husavik-Flatey fault (HFF). A 0.5-4 km thick sedimentary sequence has accumulated within the TFZ graben. Near-shore marine sediments are exposed on the Tjornes peninsula, at the eastern margin of the Skjalfandi graben, where approx. 500 m thick Miocene-Pliocene sediments contain several lignite layers. Natural gas emitting from geothermal areas at the shore of Oxarfjordur contain high concentrations of evolved hydrocarbons. The gas probably originates from marine sediments and lignites similar to those observed on the Tjornes peninsula. In 2001, a sidescan sonar imaging and CHIRP subbottom profiling were conducted along the HFF, within Skjalfandi Bay. CHIRP data show amplitude anomalies and acoustic wipe-out zones, indicative of gas accumulation within the sediments. In addition, the sidescan data reveal a number of shallow, circular and elongated depressions up to 30 m in diameter along the northern edge of the HFF. Areas of large, elongated and circular depressions (pockmarks) within Skjalfandi Bay were mapped during a multibeam bathymetric survey in 2002. A second Chirp subbottom and sidescan-sonar survey was conducted this summer along with gravity coring and digital bottom photography in order to assess the origin of these pockmarks and define the distribution of the gas charged sediments. Where present, acoustic wipe-out zones associated with the gas obscured all underlying reflectors as shallow as 5 meters below the ocean floor, in areas as large as 2 square km. The gas charged sediments are more widespread than previously observed within the bay. In northern Skjalfandi the gas seems connected to rather large, elongated pockmarks, which appear active, but in the southernmost Skjalfandi no pockmarks seem associated with the gas. These pockmarks reach 400-500 meters in length and 100-200 m in width and are commonly 2-5 meters deep. The pockmarks in northeastern Skjalfandi bay are all elongated NE-SW, with deeper NE ends. The pockmarks in the northwestern part of the bay are elongated WNW-ESE, with deeper WNW ends. The northeastern pockmarks seem to follow N-S lineaments and are possibly linked to sediment covered N-S aligned marginal faults of the Skjalfandi graben whereas the northeastern pockmark field seems to be linked to two WNW-ESE trending transform faults with little or no vertical displacement. Sidescan data show the larger pockmarks to be made up of a cluster of smaller pocks, none which exceeds 25-30 meters in length. Pockmarks along the northern edge of the HFF seem to be of similar dimensions, on the EM300 maps, but are more rounded. Gas was also detected there, and possible seafloor gas-emissions photographed. Sediment samples are being analysed. The newly discovered pockmarks and gas charged sediments within the TFZ seem to be associated with deep transform and normal faulting. The unusually high geothermal gradient (50-170$\deg$C) locally, and findings of thermogenic gas nearby, suggest that the pockmarks are formed by active gas seeps associated with oil and/or gas forming processes.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3045 Seafloor morphology and bottom photography
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL
DE: 4820 Gases
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting