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

Abstract

Three-dimensional seismic data from the NE Atlantic margin have revealed opal-A to opal-CT reaction zones hosted in Oligo-Pliocene strata as bright reflection events that cross-cut stratigraphic reflections. In some areas, the topographic relief at the top of the reaction zone has a remarkable geometry comprising roughly circular flattish regions, spaced 1-3 km apart, separated by networks of steep-sided troughs or ridges with a relief of up to 200 m. Here we show that these topographic structures formed around the upper tips of polygonal faults, and that remnants exist at deeper stratigraphic levels below the opal-A to opal-CT crosscutting reflection. Furthermore, we deduce that the chemistry of formation water expelled transiently through the polygonal faults must have played a key role in their formation. Within silica-bearing successions, the transformation of opal-A to opal-CT is followed by the transformation of opal-CT to quartz at greater depth. The opal-CT to quartz transformation releases magnesium, which is known to promote opal-A to opal-CT transformation at lower temperatures. Consequently, we suggest that when the opal-A to opal-CT reaction zone nears the top of the polygonal fault system, it advances more rapidly through strata flushed by pore water that is richer in magnesium.

Diagenetically induced shear failure of fine-grained sediments and the development of polygonal fault systems

“…Several recent studies have identified a prominent seismic reflection as the boundary between Opal A bearing and Opal C/T bearing sediments within this interval (Brekke, 2000;Berndt et al, 2004;Ireland et al, 2010;Neagu et al, 2010). This seismic event corresponds to the reaction front for the diagenetic transformation of Opal A to Opal C/T (ACT reflection on Fig.…”

Structure of a silica diagenetic transformation zone: the Gjallar Ridge, offshore Norway

“…5) have previously been interpreted as regions in which transformation of opal‐A to opal‐CT has taken place earlier (Davies & Cartwright, 2007). The amplitude maps in Figs 6B and 7C show similar cell patterns in plan view, but on stratigraphic horizons below Horizon 1, as identified by Ireland et al. (2010).…”

“…5) have previously been interpreted as regions in which transformation of opal-A to opal-CT has taken place earlier (Davies & Cartwright, 2007). The amplitude maps in Figs 6B and 7C show similar cell patterns in plan view, but on stratigraphic horizons below Horizon 1, as identified by Ireland et al (2010). These amplitude variations in stratigraphic reflection events are attributed to the same cause: the circular zones of higher amplitude represent locations where more complete transformation of opal-A to opal-CT has taken place in the silica-rich beds.…”

“…, 1977). Ireland et al. (2010) have proposed that the cell structures identified at the opal‐A to opal‐CT transformation boundary owe their existence to variations in pore fluid composition due to transient expulsion of pore water up polygonal faults.…”

“…, 1977), the possibility that the cells and circular amplitude anomalies within the transformation zone are caused by advection of silica is discounted for two reasons. Firstly, the solubility of silica is so low that unrealistically large volumes of water would be needed to transport sufficiently large amounts of dissolved silica (Ireland et al. , 2010).…”