A prominent BSR has been described by Rebesco et al. (1997) on the steeper side of the continental rise sediment drifts, where it is rendered more visible than on the gentler side by the unconformity between seafloor and deeper reflectors (Fig. 8).
The BSR is found consistently at a depth of 600-700 ms twt, approximately coincident with the lower boundary of the 'drift maintenance1 depositional stage. The reflector should be regarded as a surface of sharp change of sediment reflectivity, which becomes weaker below. The BSR has not been interpreted as a gas hydrate related reflection by Rebesco et al.(1997) because of the following arguments: 1) The reflector lies too deep (600-800 m below seafloor instead of expected 300-500 m) if heat flow is in the normal range for ocean floor of the age that underlies the sediment drifts where it occurs (about 33 Ma). Preliminary observations of the geothermal gradient on this site performed during ODP Leg 178 drilling operations (Barker, Camerlenghi, Acton et al., 1999), confirm this initial statement (Barker, personal communication). 2) The reflector does not show a reverse polarity with respect to the seafloor reflection, 3) The reflector is not associated with a blanking zone above.
During the site survey data analysis prior to drilling ODP Leg 178, a velocity analysis using the same method of acoustic tomography as in Tinivella et al. (1998), revealed that there is no velocity inversion immediately below the BSR, while the velocity profile above the BSR does not show any positive anomaly with respect to the velocity curve of a normally consolidated sediment (Tinivella & Camerlenghi, unpublished
Fig. 7 - Migrated seismic profile acquired along the South Shetland margin, where the BSR strength is particularly pronounced. It crosses a sedimentary anticline without loss in amplitude. Location in Fig. 4.
data). Therefore the seismic data do not suggest any presence of gas in either form above and below the BSR. Additionally, drilling to a few tens of meters above the expected depth of the BSR during ODP Leg 178 did not provide relevant shows of gas, while it confirmed the abundance of siliceous microfossils (diatoms and radiolarians in particular) in the form of opal-A. It is concluded that the BSR in the sediment drifts of the Pacific margin of the Antarctic Peninsula is produced by a diagenetic transition opal-A to -CT.
SW 195-130a 5km NE
Fig. 9 - Part of the multichannel seismic profile acquired on the South Orkney platform during the ODP site survey (Barker et al., 1990), where a supposed BSR was detected. Location in Fig. 5.
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