Bottom-Simulating Seismic Reflectors (BSRs)
KeywordsInstantaneous Frequency Impedance Contrast Methane Hydrate Blake Ridge Polygonal Fault
A seismic reflection occurring in the upper few hundred meters of marine sediments mimicking the seafloor, crosscutting sediment layers, and showing a phase reversal is known as a “bottom-simulating reflector.” Such a gas hydrate-related BSR originates from a large impedance contrast between a layer of gas-hydrated sediment above and a free gas layer below. A diagenetic-related BSR occurs at the opal-A/opal-CT transition zone, lies often deep and outside the base of the gas hydrate stability zone, shows no phase reversal, and does not always mimic the seafloor.
The intent of this article is to describe the two most commonly observed bottom-simulating reflectors (BSRs). The term BSR stems from their principal characteristic that these reflectors mimic the seafloor topography in marine seismic reflection data thereby crosscutting sedimentary strata. BSRs are known to occur in continental margin sediments in regions of gas hydrate and free gas (Shipley et al., 1979) and/or in siliceous ooze (diatoms, radiolaria, silica sponges, silicoflagellates) bearing sedimentary formations (Hein et al., 1978). The largest silica contribution comes from diatoms (Holland and Turekian, 2003), while the largest methane contribution derives from methane-producing Archaea in sub-seafloor sediments (e.g., Kotelnikova, 2002).
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) (Fig. 1). Often, polygonal faults occur in conjunction with diagenetic BSRs as they preferably form in similar types of sediments (Fig. 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
Assumptions on the possible presence of gas hydrate-/free gas-related BSRs along continental margins are based largely on modeling the GHSZ (Fig. 2) (Dickens and Quinby-Hunt, 1997; Zatsepina and Buffett, 1998). The modeling and thus the theoretical potential for the existence of a BSR are mainly based on water depth (pressure), seafloor temperature, and the geothermal gradient (Fig. 2). As these parameters are largely controlled by water depth from the shelf to the deep sea, the methane hydrate BSR shows two pinch-out zones, one at the shallow water depth where pressure becomes too low for hydrates to be stable and one toward the mid-ocean ridge where heat flow becomes too high. Thus, if one describes these two end members, one may still use the terminology BSR but should be clear that the BSR does not parallel the seafloor.
Seismic Character of BSRs
BSRs related to hydrate/free gas phase transitions can be distinguished from opal-A/opal-CT phase transitions based on their phase polarity. While diagenetic BSRs have the same positive phase as the seafloor reflection (Fig. 1), the hydrate/free gas BSRs show a phase reversal (Fig. 2). This reversed polarity is due to the negative impedance contrast between hydrated sediments above (higher density and velocity) and gas-saturated sediments below (Hyndman and Spence, 1992). However, caution is necessary if the phase of the BSR is the only criterion used. Gas trapped beneath a diagenetic BSR may also result in a phase reversal. As a consequence one should use several criteria such as instantaneous frequency, phase reversal, and GHSZ modeling to predict the BSR depth. Frequencies are useful as additional criteria for gas hydrate BSRs. A shift from higher frequencies in the gas hydrate zone to lower frequencies in the free gas zone (Berndt et al., 2004) beneath the BSR may be used.
Dynamics of BSRs
Both the diagenetic- and the hydrate-related BSR may be used to evaluate the thermal state or reconstruct the thermal development of a sedimentary basin (Grevemeyer and Villinger, 2001; Nouzé et al., 2009). Particularly, the hydrate-related BSR is widely used as a proxy for heat flow in marine sediments. On a small scale, BSR-derived heat flow changes may often be associated with structures that focus warm fluids from deep sediments like mud volcanoes (Depreiter et al., 2005) or chimneys (Rajan et al., 2013). In such instance, the BSR may not mimic the seafloor if lateral variations in heat flow or changes in gas compositions exist. Generally, an increase in heat flow causes a shoaling of a BSR, whereas an increase of higher-order hydrocarbons causes a deepening. Hence, the depth of the BSR may vary greatly as, for example, in the Barents Sea depending on the contribution and thus the amount of thermogenic gases migrating into the GHSZ (Chand et al., 2008; Rajan et al., 2013).
More recently, BSR observations and hydrate stability zone modeling of the upper pinch-out zone on continental margins have been used to assess past and contemporary changes in hydrate stability through warming of ocean bottom water (Vogt and Jung, 2002; Mienert et al., 2005; Biastoch et al., 2011; Ferré et al., 2012; Phrampus and Hornbach, 2012).
Inferred former positions of a BSR are often referred to as paleo-BSR. One of the best examples for a paleo-BSR can be found on the Blake Ridge, approx. 450 km offshore Georgia on the East Coast of the United States (Hornbach et al., 2003). This BSR formed when erosion by strong contour currents on the eastern flank of the Blake Ridge removed the top sediments of a hydrated formation causing an adjustment of the hydrate/free gas boundary by moving it deeper. The free gas layer beneath the former BSR crystallized into a newly formed concentrated layer of hydrates causing both a density and velocity increase. Beneath this paleo-BSR, the new BSR formed with free gas underneath the base of the gas hydrate stability zone (BGHSZ). Though the timing of the readjustment is unknown, it presents one good example for the dynamic behavior (in this case deepening) of a BSR due to erosion of sediments and a drop in seafloor temperature.
Summary and Conclusions
Bottom-simulating reflectors (BSRs) occur in a wide range of sediments in the world oceans. Such creation of BSRs involves the existence of free gas and water in the pore space of sediments under low temperature and high pressure, forming hydrates beneath the ocean floor. The depth of the BSR defines the base of the gas hydrate stability zone (BGHSZ) under which free gas accumulates. Free gas becomes trapped beneath the hydrate-charged layer causing a distinct impedance contrast and a seismic reflection of reversed (negative) polarity if compared to the seafloor. The GHSZ depends on temperature and pressure and to a lesser degree on salinity and gas composition (thermogenic, biogenic). The thickness of the GHSZ decreases toward the upper continental margins (lower pressure) and sedimented ocean ridges (higher temperature). The second type of BSRs concentrates in regions of siliceous ocean sediments. Here, increases in temperature with burial depth result in dissolution of siliceous skeletons, which in turn creates an interface with higher porosity and permeability above and lower values beneath the interface. If the contrast becomes large enough, a seismic reflector occurs but with positive polarity (no phase reversal). As a consequence, diagenetic BSRs occur commonly deeper, show no phase reversal, and exist over large areas at the opal-A/opal-CT interface in ocean sediments.
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