Diagenesis-related BSR
Siliceous ooze occurs in a wide range of sedimentary
basins indicating times of higher ocean productivity, for
example, along equatorial, polar, or coastal upwelling
regions (Holland and Turekian, 2003). With increasing
burial depth, the temperature and pressure increases,
resulting in the dissolution of the siliceous skeletons.
The dissolution process causes a collapse of the siliceous
framework and a geochemical reaction that converts
amorphous opal-A into opal-CT (e.g., Hesse, 1989;
Knauth, 1994). Deep-sea drilling project studies allowed
documenting the chemical, mineralogical, and structural
changes occurring during this process with increasing
depth (e.g., Hurd and Birdwhistell, 1983). As a consequence of the skeleton dissolution, an interface develops
where the opal-CT formation causes an increase in density
and compressional-wave velocity and a decrease in porosity and permeability (e.g., Tribble et al., 1992). If the seismic impedance contrast between opal-A (lower density
and velocity and higher porosity and permeability) and
opal-CT becomes large enough, a seismic reflector with
a positive polarity occurs (Figure 1) (Berndt et al.,
2004). It is believed that the temperature increase with
burial depth is the main parameter controlling the opalA/opal-CT transition aside from the time since burial, type
of surrounding sediment material, and interstitial waters
(e.g., Hein et al., 1978). The opal-A/opal-CT diagenesis
causes a volume reduction of as much as 30–40 %
(Davies and Cartwright, 2002). Since large areas of siliceous ooze exist in sedimentary formations, a diagenetic
BSR (Figure 1) often shows a very large lateral extent,
which is uncommon for gas hydrate-/free gas-related
BSRs. Moreover, diagenetic BSRs are believed to develop
at greater depth below the seafloor, at temperatures
(35–50
C) where hydrate is normally no longer stable
(Berndt et al., 2004).
Opal-A/opal-CT BSRs have been reported from many
areas containing siliceous sediments such as from the
mid-Norwegian margin (e.g., Brekke, 2000; Berndt
et al., 2004) (Figure 1). Often, polygonal faults occur in
conjunction with diagenetic BSRs as they preferably form
in similar types of sediments (Figure 1) (Cartwright and
Dewhurst, 1998; Davies and Cartwright, 2002). Polygonal
faults show an interruption and vertical offset of continuous reflections leading to short reflection segments. The
BSR may be difficult to identify if it runs parallel to the
strata, because normally it also does not show a reduced
instantaneous frequency (Berndt et al., 2004). The BSR
has a strong amplitude, has an apparent polarity that is
positive, lies deeper than a gas hydrate-related BSR, and
is often interrupted by polygonal faults.
Gas hydrate-related BSR
Gas hydrates occur as an icelike substance composed of
water molecules forming a rigid lattice of cages that trap
a guest molecule (Sloan, 2003). The predominant guest
Bottom Simulating Seismic Reflectors (BSR), Figure 1 Reflection seismic profile from the mid-Norwegian margin siliceous
sedimentary formation showing an example of an opal-A/opal-CT BSR. The BSR and amplitudes are offset by polygonal faults. The
origin of BSR 2 is speculative and may be related to transformation from smectite to illite at higher temperatures (Figure is from
Berndt et al., 2004, Figure 4).
BOTTOM SIMULATING SEISMIC REFLECTORS (BSR)
63
Siliceous ooze occurs in a wide range of sedimentary
basins indicating times of higher ocean productivity, for
example, along equatorial, polar, or coastal upwelling
regions (Holland and Turekian, 2003). With increasing
burial depth, the temperature and pressure increases,
resulting in the dissolution of the siliceous skeletons.
The dissolution process causes a collapse of the siliceous
framework and a geochemical reaction that converts
amorphous opal-A into opal-CT (e.g., Hesse, 1989;
Knauth, 1994). Deep-sea drilling project studies allowed
documenting the chemical, mineralogical, and structural
changes occurring during this process with increasing
depth (e.g., Hurd and Birdwhistell, 1983). As a consequence of the skeleton dissolution, an interface develops
where the opal-CT formation causes an increase in density
and compressional-wave velocity and a decrease in porosity and permeability (e.g., Tribble et al., 1992). If the seismic impedance contrast between opal-A (lower density
and velocity and higher porosity and permeability) and
opal-CT becomes large enough, a seismic reflector with
a positive polarity occurs (Figure 1) (Berndt et al.,
2004). It is believed that the temperature increase with
burial depth is the main parameter controlling the opalA/opal-CT transition aside from the time since burial, type
of surrounding sediment material, and interstitial waters
(e.g., Hein et al., 1978). The opal-A/opal-CT diagenesis
causes a volume reduction of as much as 30–40 %
(Davies and Cartwright, 2002). Since large areas of siliceous ooze exist in sedimentary formations, a diagenetic
BSR (Figure 1) often shows a very large lateral extent,
which is uncommon for gas hydrate-/free gas-related
BSRs. Moreover, diagenetic BSRs are believed to develop
at greater depth below the seafloor, at temperatures
(35–50
C) where hydrate is normally no longer stable
(Berndt et al., 2004).
Opal-A/opal-CT BSRs have been reported from many
areas containing siliceous sediments such as from the
mid-Norwegian margin (e.g., Brekke, 2000; Berndt
et al., 2004) (Figure 1). Often, polygonal faults occur in
conjunction with diagenetic BSRs as they preferably form
in similar types of sediments (Figure 1) (Cartwright and
Dewhurst, 1998; Davies and Cartwright, 2002). Polygonal
faults show an interruption and vertical offset of continuous reflections leading to short reflection segments. The
BSR may be difficult to identify if it runs parallel to the
strata, because normally it also does not show a reduced
instantaneous frequency (Berndt et al., 2004). The BSR
has a strong amplitude, has an apparent polarity that is
positive, lies deeper than a gas hydrate-related BSR, and
is often interrupted by polygonal faults.
Gas hydrate-related BSR
Gas hydrates occur as an icelike substance composed of
water molecules forming a rigid lattice of cages that trap
a guest molecule (Sloan, 2003). The predominant guest
Bottom Simulating Seismic Reflectors (BSR), Figure 1 Reflection seismic profile from the mid-Norwegian margin siliceous
sedimentary formation showing an example of an opal-A/opal-CT BSR. The BSR and amplitudes are offset by polygonal faults. The
origin of BSR 2 is speculative and may be related to transformation from smectite to illite at higher temperatures (Figure is from
Berndt et al., 2004, Figure 4).
BOTTOM SIMULATING SEISMIC REFLECTORS (BSR)
63