Fluid flow due to the advance of basin-scale silica reaction zones

Abstract

The conversion of biogenic silica (opal-A) to opal-CT (cristobalite and tridymite) in biosiliceous sediment causes increased rates of water expulsion because of the reduction in sediment porosity and dehydration of the amorphous opal-A phase. This release of water occurs over large tracts of sedimentary basins during sediment burial within discrete, diagenetic, reaction zones. Analysis of two-dimensional and three-dimensional seismic data sets from basins in the Northern Hemisphere provides geophysical evidence for a variety of fl uid conduits and roughly circular erosional depressions at the contemporaneous seabed. We interpret these features as indicative of water expulsion and focused fl uid fl ow emanating from opal-A to opal-CT reaction zones at burial depths within the range 200-800 m.The rate at which water is expelled depends upon the degree of porosity reduction and the weight fraction of bound water at the reaction zone as well as the rate of advance of the reaction zone. Where the reaction is actively taking place within homogeneous biosiliceous sediment, the rate of water expulsion is independent of the reaction rate. This is because water is released across the entire reaction zone; therefore, slow reaction rates are compensated for by expulsion of water across wider reaction zones. We calculate the rate and volume of water expulsion for the Faeroe-Shetland Basin, where the sediment immediately below the reaction zone contains, on average, ~30% opal-CT by weight. The estimated volumetric rate of water expulsion per unit surface area at the present day is ~6 m 3 My -1 per square meter, which is greater than the vertical fl ux of water at the same depth from compaction of the deeper basin fi ll. The average volumetric rate of water expulsion is ~120 km 3 My -1 across the whole basin.Biogenic silica is particularly rich in Neogene successions in high latitude and equatorial regions, and where silica reaction zones are identifi ed, they should be factored into sediment compaction and fl uid-fl ow histories.

Influence of pore water chemistry on silica diagenesis: evidence from the interaction of diagenetic reaction zones with polygonal fault systems

“…These pockmarks are also located above the upper tips of polygonal faults and constitute evidence of transient fluid expulsion at a palaeo-seabed. Similar evidence for fluid flow up fault planes has been identified from the Faeroe-Shetland Basin, where amplitude anomalies tracking faults indicate that fluid flow occurred up the faults from the level of the opal-A to opal-CT reaction zone to the contemporaneous seabed (Davies et al 2008). Other examples of seismic images of fluid flow pathways associated with faults in soft sediments have been published by Berndt et al (2003) and Gay & Berndt (2007).…”

“…Isaacs 1981;Tada 1991), releasing large volumes of pore water and resulting in a large acoustic impedance contrast, which can give rise to a seismic reflection event that cross-cuts stratigraphy (Hein et al 1978). Analysis of 2D and 3D seismic data has provided evidence for a variety of fluid flow conduits above reaction zones, indicative of water expulsion from the reaction zones and transient upward flow (Davies et al 2008).…”

Messinian evaporites and fluid flow

“…In the case of high sedimentation rates and tectonic loading fine-grained deep-water sediments are thus particularly prone to local retention of pore waters and overpressuring. Equally, any substantial biogenic fractions in the hemipelagites might render them susceptible to overpressure development through diagenesis (Davies et al, 2008). Smectite-Illite transformation could be another mechanism contributing to overpressure generation in the pre-Messinian succession (Powers, 1967;Swarbrick, 2002), possibly mitigated by the exceptionally high thermal conduction provided by the thick evaporites.…”

Systematic sequence-scale controls on carbonate cementation in a siliciclastic sedimentary basin: Examples from Upper Cretaceous shallow marine deposits of Utah and Coloradao, USA

“…Whilst these models give important insights into patterns of diagenetic alteration, they are invariably based on individual sequence-or parasequence-scale observations, often focused around only one or two stratal surfaces (e.g., Klein et al, 1999). The recognition of true basin-scale patterns of diagenesis within siliciclastic successions has generally been limited to mesogenetic (burial diagenetic) processes (e.g., Machel et al, 1996;Bjørlykke et al, 1988;Davies et al, 2008), although it is noted that within carbonatedominated successions meteoric fluid eogenetic alterations related to uncomformity development have been documented (e.g., Dorobek, 1987;Rheinhold and Kaufmann, 2010).…”