A Fossilized Opal A To Opal C-T Transformation On The Northeast Atlantic Margin- Support For A Significantly Elevated Palaeogeothermal Gradient During The Neogene?

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Journal Article: A Fossilized Opal A To Opal C-T Transformation On The Northeast Atlantic Margin- Support For A Significantly Elevated Palaeogeothermal Gradient During The Neogene?

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 crosscuts 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 24degreesC. 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. 60degreesC 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.

Authors 
R. J. Davies and J. Cartwright








Published Journal 
Basin Research, 2002





DOI 
Not Provided
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Citation

R. J. Davies,J. Cartwright. 2002. A Fossilized Opal A To Opal C-T Transformation On The Northeast Atlantic Margin- Support For A Significantly Elevated Palaeogeothermal Gradient During The Neogene?. Basin Research. (!) .