ORIGIN OF LOW DELTA-O-18, PRE-COMPACTIONAL FERROAN CARBONATES IN THE MARINE STO FORMATION (MIDDLE JURASSIC), OFFSHORE NW NORWAY

Siderite (delta(18)O(PDB) = -9.3 to - 5.7 parts per thousand) and Fe-d olomite/ankerite (delta(18)O(PDB) = -13.7 to -7.7 parts per thousand) occur as texturally early diagenetic, pre-compactional cements in sand stones and mudstones of the nearshore to inner shelf St phi Formation (Middle Jurassic), Troms I area, Barents Sea. However, these carbonate s have anomalously low delta(18)O, probably clue to alteration of volc aniclastic materials to clay matrix, as well as to clay ooids and pelo ids th rough the interaction with marine porewaters. The lowermost del ta(18)O values could, however, be related to replacement during deep b urial and/or to upwelling of hot fluids related to the Jurassic riftin g in the Hammerfest Basin. The alteration of volcaniclastic materials also has most likely been an important source of Fe and Mg for the for mation of the carbonates.