A fossilized Opal A to Opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated Palaeogeothermal gradient during the Neogene?

• Published in: Basin Research
• Main Authors: Davies, R. J., Cartwright, J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2002
• Subjects: Northeast Atlantic
• Online Access: http://dx.doi.org/10.1046/j.1365-2117.2002.00184.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1365-2117.2002.00184.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2117.2002.00184.x

ABSTRACT Rock samples – collected from a recent deep‐water exploration well drilled in the Faeroe‐Shetland Channel, northwest of the UK – confirm that a distinctive high‐amplitude seismic reflector that cross‐cuts the Upper Palaeogene and Neogene succession and covers an area of 10 000 km 2 is an example of a fossilized Opal A to Opal C/T (Cristobalite/Tridymite) transition. Analysis of these rock fragments tied to an extensive two‐dimensional and three‐dimensional seismic database constrains the time at which the boundary was fossilized and in addition reveals the unusual geometrical characteristics of a relict bottom‐simulating reflector. The diagenetic transformation of biogenic silica (Opal A) to Opal C/T is predominantly temperature‐controlled and requires sediments that contain biogenic silica. The reflector (termed as Horizon E) probably initially represented a biosiliceous ooze or a siltstone that contained a component of biogenic silica that underwent transformation as the diagenetic front migrated upsection during burial. The parallelism it shows with a shallower early Pliocene reflector and its apparent upsection migration during a compressional episode in the basin indicate that it was active during the middle and late Miocene and ceased activity during the early Pliocene when there was between 200 and 400 m of overburden. The present‐day burial depth of the boundary is ca. 700 m and the temperature at the inactive diagenetic front at the well location is 24 °C. Given these temperature and depth constraints, we hypothesize that even if this is an example of a relatively low‐temperature Opal A to Opal C/T transformation, a temporarily elevated geothermal gradient of ca. 60 °C km −1 would have been required to initiate and arrest upsection migration of the boundary during the middle and late Miocene. Factors such as climatic deterioration and the onset of cold deep‐water circulation are likely to only have had a contributory role in arresting the upward migration of the boundary.