AUTHIGENIC CHLORITE IN LATE TR IASS IC SANDSTONES FROM THE CENTRAL GRABEN,

ABSTRACT: Late Triassic sandstones from Quadrant 22 in the North Sea contain two generations of chlorite cement. Grain-coating chlorites consisting of curved, irregular crystal plates formed very early in diagenesis and are largely confined to marginal marine beds. Later diagenetic pore-filling chlorites, consisting of euhedral platy crystals, occur throughout the sequence in fluvial as well as marginal marine beds. Both generations of cement show similar XRD patterns; the 001 basal reflection of 14.0-14.06 A is broad and of low intensity, whereas the 002 reflection at 7.05 A is sharp and intense. Slight contraction of the 14 A basal spacing to 13.6 A occurs after heating to 600 ~ Electron microprobe analyses show the cements to be Fe-tich clinochlores with Fe/(Fe + Mg) ratios varying between 0.41 and 0-50. They are more Mg-and Si-enriched than most other reported North Sea authigenic chlorites. The grain-coating chlorites have an average composition of (Fe3.06A13.93Mg3.32) [(Si6.76All.24)O20] (on) l 6. The pore-filling chlorites tend to show more variable compositions. The high Si values probably reflect both minor interstratification with smectite or vermiculite and contamination f the analyses by a closely associated illite/smectite clay. An origin related to alteration ofa precursor swelling chlorite or corrensite isattractive on chemical nd morphological grounds for the earl