Print this pageComplexity of Silica Diagenetic Reaction Zones from the Basin Scale
Silica diagenetic reaction zones hosted in Oligo-Pliocene strata on the north-east Atlantic margin are imaged using 3D seismic data, and commonly exhibit extraordinary patterns over areas ~104 km2. Understanding the characteristics of silica diagenetic transformations has implications on slope stability and subsurface fluid flow in silica-rich successions such as those in the Arctic. We have determined the structure of a silica diagenetic reaction zone on the Gjallar Ridge based upon key seismic characteristics, with correlation to a nearby well. We have interpreted an opal-A to opal-CT boundary, a zone of alteration, an opal-CT to quartz boundary, and regions that have undergone more complete conversion of opal-A to opal-CT.
We propose a new model to explain preferential advance of the main opal-A to opal-CT transition which is consistent with the previously documented patterned geometry at the upper boundary of the silica diagenetic reaction zone. When opal-A transforms to opal-CT and opal-CT transforms to quartz within a silica-bearing succession hosting a polygonal fault system, released pore water is expelled up the faults. This displaces water with lower magnesium and hydroxyl concentrations around the upper fault tips. Subsequently, the reaction zone advances more rapidly through those zones because enhanced magnesium and hydroxyl concentrations in pore water promote opal-A to opal-CT transformation at lower temperature.
These are the first seismic observations to confirm that, rather than simply being narrow boundaries, silica diagenetic reaction zones can be hundreds of metres in thickness, as has been observed at outcrop. Furthermore, it opens up the possibility that other diagenetic reactions may be controlled by catalysts at the basin scale.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil