DNG 2001 Abstract

Multiple fluid flow events and diagenetic evolution in the Palaeozoic clastic sediments of the Cantabrian Zone, Spain

C. Reinl & T. Bechstädt, Geologisch-Paläontologisches Institut, Universität Heidelberg

PUBLIKATIONEN

Palaeozoic sediments of the Cantabrian Zone, which represents the foreland of the Iberian Variscides, have been complexly folded and thrusted, resulting in several thrust units with diagenetic to very low grade metamorphic overprint. Major clastic sediments are quartzitic sandstones of Cambrian to Ordovician and Upper Devonian age, which have been sampled in two thrust units (Somiedo-Correcillas and Sobia-Bodon) of the southern part of the Cantabrian Zone. The samples consist of almost pure sandstones, which exhibit a more or less pronounced quartz cementation by quartz overgrowth. The remaining pore space is partly filled with authigenic clay minerals, mostly illite. Rare feldspars are completely altered.
The earliest generation of fluid inclusions (FI) is developed along the boundary of detrital quartz grains and quartz overgrowth and within the overgrowth. These aqueous FI have moderate salinities which correlate positively with the homogenisation temperatures (Th) of 100 to 150°C reflecting an increasing salinity with increasing burial. A second generation of aqueous FI occurs as trails, which crosscut several detrital grains and overgrowth, and in quartz veins. This fluid has a very low salinity and Th around 110°C and is probably of meteoric origin. A third generation of aqueous FI with high salinity and Th of about 90°C is also developed in quartz overgrowths and in quartz veins, where an alternation of zones with FI of low and high salinity is found.
In general, the clastic sediments are marked by a quartz cementation from moderately saline fluids during burial and subordinate from high salinity fluids during subsequent uplift. These high salinity fluids are likely to originate from deeper crustal levels and where episodically transported to shallower levels where they alternate with low salinity fluids, which penetrate from higher levels during extensional periods. Th in the Sobia-Bodon unit are generally a little lower than in the Somiedo-Correcillas Unit reflecting the shallower burial of the former unit. Quartz cementation is followed by the formation of authigenic illites.