DIAGENESIS OF A MIXED SILICICLASTIC EVAPORITIC SEQUENCE OF THE MIDDLEMUSCHELKALK (MIDDLE TRIASSIC), THE CATALAN COASTAL RANGE, NE SPAIN/

The Middle Muschelkalk (Middle Triassic) of the Catalan Coastal Range (north-east Spain) comprises sandstone, mudstone, anhydrite and minor carbonate layers. Interbedded sandstones and mudstones which are domin ant in the north-eastern parts of the basin are terminal alluvial fan deposits. South-westward in the basin, the rocks become dominated by i nterbedded evaporites and mudstones deposited in sabkha/mudflat enviro nments. The diagenetic and pore water evolution patterns of the Middle Muschelkalk suggest a strong facies control. During eodiagenesis, for mation of microdolomite, anhydrite, baryte, magnesite, K-feldspar and mixed-layer chlorite/smectite was favoured within and adjacent to the sabkha/mudflat facies, whereas calcite, haematite, mixed-layer illite/ smectite and quartz formed mainly in the alluvial facies. Low delta(18 )O(SMOW) values for microdolomite (+23.7 to +28.4%) of and K-feldspar overgrowths (+17.3 to +17.7 parts per thousand) suggest either low-tem perature, isotopic disequilibrium or precipitation from low-O-18 porew aters. Low-O-18 waters might have developed, at least in part, during low-temperature alteration of volcanic rock fragments. During mesodiag enesis, precipitation of quartz overgrowths and coarse dolomite occurr ed in the alluvial sandstones, whereas recrystallization of microdolom ite was dominant in the sabkha/mudflat facies. The isotopic compositio ns of these mesogenetic phases reflect increasing temperature during b urial. Upon uplift and erosion, telogenetic calcite and trace haematit e precipitated in fractures and replaced dolomite. The isotopic compos ition of the calcite (delta(18)O(SMOW) = +21.5 to = +25.6 parts per th ousand; delta(13)C = -7.7 to -5.6 parts per thousand) and presence of haematite indicate infiltration of meteoric waters.