Fossilisation of a silica diagenesis reaction front on the mid-Norwegian margin

The diagenetic transformation of biogenic silica from opal-A to opal-CT was recognised on seismic reflection data over an area of 78 × 10³ km² on the mid-Norwegian margin. The opal-A/CT diagenetic boundary appears as a positive, high amplitude reflection that generally cross-cuts the hosting stratigraphy. We demonstrate that it is not a sea bottom simulating reflection (BSR) and also that is not in thermal equilibrium with the present day isotherms. We present arguments that three styles of deformation associated with the opal-A/CT reflection – polygonal faulting, regional anticlines and synclines and differential compaction folding – indicate that the silica diagenesis reaction front is fossilised at a regional scale. Isochore maps demonstrate the degree of conformity between the opal-A/CT reflection and three seismic horizons of Late Miocene to Early Pliocene age that potentially represent the paleo-seabed when ‘fossilisation’ of the reaction front took place. The seismic interpretational criteria for recognition of a fossilised diagenetic front are evaluated and the results of our study are integrated with previous studies from other basins of the NE Atlantic in order to determine if the arrest of silica diagenesis was diachronous along this continental margin.     

Estimation of biogenic silica contents in marine sediments using seismic and well log data: Sediment Drift 7, Antarctica

Petrophysical properties (wet bulk density, porosity, P-wave velocity) are used to predict biogenic silica contents along a seismic reflection profile that ties two well sites, 1095 and 1096, drilled by Ocean Drilling Program (ODP) Leg 178 on sediment drifts on the Pacific continental margin of the Antarctic Peninsula. The biogenic silica contents along the seismic reflection profile were estimated on the basis of three hypotheses about petrophysical properties distributions in the two boreholes and statistical relationships between biogenic silica and other petrophysical properties, which were established on various sediment layers within the boreholes. Our study demonstrates the possibility to reliably predict the distribution of biogenic silica in the sub-seabed sediments if seismic data processed with amplitude preservation are used and statistical relations are considered. We conclude that the statistical extrapolation of biogenic silica content along seismic reflection profiles tied to borehole data is an efficient tool to quantify the amounts of silica undergoing crystalline transformation, which may have strong implications for submarine slope destabilisation.     

The limited suitability of silica diagenetic boundaries as isothermal markers: Insights from seismic reflection imaging,Offshore Sakhalin,Russian Far East

In many sedimentary basins the seismic reflections that mark the conversion of opal-A to opal-CT and the subsequent conversion of opal-CT to quartz, are parallel to the present-day seabed. As the reactions are in part thermally activated these boundaries have been proposed as potential isothermal markers and could have utility for hydrocarbon exploration. We describe opal-A to opal-CT and opal-CT to quartz diagenetic boundaries using 2D seismic data from the North Sakhalin Basin (NSB). These are not parallel to the present-day seabed, but for 80% of the area of the basin are parallel to an unconformity of Late Miocene age and may represent palaeo-isotherms that were parallel to the Late Miocene seabed. This characteristic has been identified in other basins and may indicate silica diagenetic boundaries do not make reliable present-day isothermal boundaries. We propose diagenetic boundaries, which are not parallel to the seabed, are not present-day isothermal boundaries. This characteristic could result from: (1) temperature decrease, such as through a declining geothermal gradient or erosion of the overburden, which will cause the rate of conversion to slow; (2) variations in the burial rate that would cause changes to the rate of conversion; and (3) a change in the rate of conversion as a result of variation in the physico-chemical factors influencing the silica diagenetic reactions.