--> ABSTRACT: Controls and Timing of Quartz Cementation in the Rotliegend Sandstone, U.K., Southern North Sea, by Morgan D. Sullivan, R. Stuart Haszeldine, Anthony E. Fallick; #91020 (1995).

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Controls and Timing of Quartz Cementation in the Rotliegend Sandstone, U.K., Southern North Sea

Morgan D. Sullivan, R. Stuart Haszeldine, Anthony E. Fallick

Authigenic quartz is a pervasive pore-filling cement that occupies up to 30% of total rock volume in the Lower Permian Rotliegend Sandstone of the Leman field. Two generations of early diagenetic quartz are observed. The earlier generation is represented by small prismatic outgrowths (30 to 70 microns) that are commonly enclosed within later syntaxial over-growths. Both phases of quartz are best developed along the well-sorted, medium- to coarse-grained beds of cross-stratified dune foresets. Oxygen-isotope compositions of overgrowths range from +19.7^pmil to +22.3^pmil (SMOW). Fluid-inclusion data indicate that the greatest volume of quartz precipitated during the Late Triassic between 72° and 97°C (1.5 to 2 km burial) from an evolved meteoric brine with d 8O composition of-2^pmil (SMOW). Low measured amounts of intergranular pressure solution and late timing of feldspar dissolution (post-dating quartz) precludes derivation of the authigenic silica from within the Rotliegend. We suggest that the quartz cement was derived from silica released by diagenetic reactions occurring in underlying Carboniferous shales. Petrographic observations, including textural selectivity of authigenic quartz distribution, indicate that the silica was imported by migrating fluids rather than diffusion. This transport mechanism requires larger volumes of fluid movement than could have been supplied by compaction of underlying shales. Alternatively, silica may have been imported by deep circulation of highly evolved meteoric waters, possibly in association ith Triassic rifting.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995