The origin of magnetization and geochemical alteration in a fault zone, Kilve, England

✍ Scribed by W. G. Bixler; R. D. Elmore; M. H. Engel

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 455 KB
• Volume: 33
• Category: Article
• ISSN: 0072-1050
• DOI: 10.1002/(sici)1099-1034(1998040)33:2<89::aid-gj773>3.0.co;2-s

✦ Synopsis

The origin of an apparently syndeformational chemical remanent magnetization (CRM) and geochemical alteration in a fault zone in the Bristol Channel Basin, southwest England, was investigated. Deformation in the fault zone occurs in Jurassic aged, organic-rich limestones and consists of numerous normal and oblique-slip faults and associated folds. Migration of basinal, radiogenic ¯uids is indicated by elevated 87 Sr/ 86 Sr values for calcite veins that occur throughout the fault zone. Some of the calcite veins contain hydrocarbons sourced from deeper strata. Elevated 87 Sr/ 86 Sr values in the host Jurassic limestones indicate that they were also extensively altered by radiogenic ¯uids that migrated through microfractures in addition to major fault and fracture planes. Folded and tilted host limestones contain a pervasive secondary CRM residing in magnetite that was acquired during deformation in the Tertiary. The association between this pervasive CRM and the pervasive geochemical alteration is consistent with a genetic connection between the orogenic ¯uids and the CRM although the timing of CRM acquisition (Tertiary) is not consistent with structural interpretations for the timing of most veining. An alternative remagnetization mechanism which is not triggered by externally derived ¯uids, such as diagenesis of hydrocarbons, might account for the CRM. Hydrocarbon-bearing veins also contain a CRM that resides in magnetite, although the time for remanence acquisition is not well constrained by ®eld tests.