STABLE-ISOTOPE EVIDENCE FOR THE ORIGIN OF DIAGENETIC CARBONATE MINERALS FROM THE LOWER JURASSIC INMAR FORMATION, SOUTHERN ISRAEL

The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater comp osition can be related to major geological events within southern Isra el. In particular, saline brines played an important role in late (Pli ocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonate s included early siderite (delta(18)O(SMOW)= +24.4 to +26.5 parts per thousand; delta(13)C(PDB)= -1.1 to +0.8 parts per thousand), late dolo mite, ferroan dolomite and ankerite (delta(18)O(SMOW)= +18.4 to +25.8 parts per thousand; delta(13)C(PDB)= -2.1 to +0.2 parts per thousand) and calcite (delta(OSMOW)(18)= +21.3 to 32.6 parts per thousand; delta (13)C(PDB)= -4.2 to +3.2 parts per thousand). The petrographic and iso topic results suggest that siderite formed early in the diagenetic his tory at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early t o late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperatur e) and the oxygen isotopic compositions of the dolomitic cements. In a ddition, the delta(18)O values of the dolomitic phases in the northern Negev and Tudea Mountains are in isotopic equilibrium with present fo rmation waters. This behaviour suggests that formation of secondary do lomite post-dates the tectonic activity responsible for the present re lief of southern Israel (Upper Miocene to Pliocene) and that the dolom ite crystallized from present formation waters. Such is not the case i n the Central Negev. In that locality, present formation waters have m uch lower salinities and delta(18)O values, indicating invasion of fre shwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.