Opal‐A/opal‐CT phase boundary inferred from bottom‐simulating reflectors in the southern South Korea Plateau, East Sea (Sea of Japan)

Abstract

Multi‐channel seismic reflection profiles from the trough areas of the southern South Korea Plateau, East Sea (Sea of Japan), reveal high‐amplitude, positive‐polarity bottom‐simulating reflectors (BSRs) at about 500 ms two‐way travel time (ca. 450 m) below the seafloor. The positive polarity and no noticeable drop of frequency (low‐frequency shadow) below the BSR strongly suggest that the BSR is due to an opal‐A/opal‐CT phase boundary. The BSR often marks a sharp interface between low‐amplitude reflections above and high‐amplitude reflections below. The weak, but distinct reverse‐polarity event, seen 70–150 ms two‐way travel time below the BSR, may represent the base of the silica diagenetic zone. The geothermal gradients (78–111°C/km) estimated from the sub‐bottom depth of the BSR are comparable to that (98°C/km) measured at the nearby Kita‐Yamato Trough, where the opal‐A/opal‐CT boundary has been penetrated by the Ocean Drilling Program (ODP) drilling.

Geological controls on BSR occurrences in the incipient arc-continent collision zone off southwest Taiwan

“…The positive acoustic impedance contrast between the transition from opal A above to opal CT below yields the same BSR polarity as the seafloor reflection. The above diagenetic transformation is likely to occur in siliceous sediments of biogenic origin (e.g., oozes, Kastner et al, 1977) and therefore this type of BSR is reported to exist in passive continental margins with slow rate of sedimentation (e.g., Berndt et al, 2004;Lee et al, 2003). The study area off SW Taiwan is characterized by rapid sedimentation with large amounts of orogenic detritus (Yen and Lundberg, 2006) derived from the rapidly denudating Taiwan island (Dadson et al, 2003) and systematic analyses of seafloor sediments found no siliceous ooze (Jiang et al, 2006) in both regions of the accretionary wedge and adjacent South China Sea continental slope.…”

The limited suitability of silica diagenetic boundaries as isothermal markers: Insights from seismic reflection imaging, Offshore Sakhalin, Russian Far East

“…Hein et al, 1978;Nobes et al, 1992a;Davies and Cartwright, 2002;Lee et al, 2003). Hein et al (1978) suggested the opal-A to opal-CT boundary, which is parallel to the present-day seabed in the Bering Sea, is a time transgressive boundary and not hosted by a lithology of a single age.…”

“…Because temperature is one of the dominant controls on the silica reactions the opal-A to opal-CT diagenetic boundary may be isothermal marking a specific temperature at which the reaction occurred over extensive areas of basins (Hein et al, 1978;Kuramoto et al, 1992;Lee et al, 2003). Therefore, opal-A to opal-CT boundaries have tentatively been used as low-temperature present-day isothermal markers to help reconstruct the thermal history of the basin in order to determine source rock maturity and the timing of the maturation (e.g.…”

The character and amplitude of ‘discontinuous’ bottom-simulating reflections in marine seismic data

“…A fourth type of BSR, known as diagenetic BSRs, form as the result of diagenetic boundaries in siliceous 78 sediments where sediment density increases with depth, most commonly due to the transformation of 79 biogenic silica (opal-A) to opal-CT (Berndt et al, 2004;Davies and Cartwright, 2002;Goldberg et al, 80 1987). The acoustic impedance associated with the relative change in density results in a prominent 81 reflection with the same polarity as the seafloor; this reflection follows an isothermal gradient, and 82 therefore parallels seafloor topography (Davies and Cartwright, 2002;Lee et al, 2003). This is unlike 83…”