HENDRY, JAMES P., Department of Geology and Petroleum Geology, University of Aberdeen, UK; CONCHITA TABERNER, Institute of Earth Sciences, CSIC, Barcelona, Spain; and JAMES D. MARSHALL, Department of Earth Sciences, University of Liverpool, UK

ABSTRACT: Regional Diagenesis of the Upper Eocene Pre-Evaporite Reef Complex, South Pyrenean Foreland Basin, Northeast Spain

Progressive restriction of the South Pyrenean foreland basin during the Late Eocene induced a transition from normal marine to euxinic, evaporitic, and finally continental sedimentation. Final normal marine deposits on the basin margins comprise a laterally-extensive barrier reef complex. Uppermost fore-reef talus wedges are onlapped by basinal euxinic marls followed by evaporites, and the reef platform is overlain by a transitional hypersaline to continental facies association. We have investigated how regional environmental and sedimentological changes affected the diagenetic evolution of the reef complex by studying three well-exposed outcrops spaced at 25-35 km around the basin margin. Rather than finding consistent diagenetic histories related to the changing conditions in the ba in, our results show that local factors strongly influenced post-depositional alteration in the reef complex.

The reef outcrops display significantly different parageneses and cement compositions. All three contain evidence of meteoric diagenesis including dissolution of aragonite and precipitation of zoned, {18}O-depleted calcite cements. However, calcite (isotope){18}O values differ widely between locations (-57 o/oo vs. -9 o/oo PDB), and cementation varies from minor coral-replacement fabrics to extensive pore-filling mosaics. We infer that meteoric alteration took place at different times and under different paleoenvironmental conditions around the basin margins. Late diagenetic fluids from compactional dewatering in the basin precipitated Ba/Sr sulfates and non-ferroan to ferroan calcite spars in fractures and cavities throughout the reef complex. However, in one location subsequent diss lution of sulfate and calcite cements was followed by saddle dolomite and non-ferroan calcite precipitation. Low negative (isotope){18}O values of these localized latest cements suggests derivation from hydrothermal fluids.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.