SULFATE-BORATE RELATIONS IN AN EVAPORITIC LACUSTRINE ENVIRONMENT - THE SULTANCAYIR GYPSUM (MIOCENE, WESTERN ANATOLIA)

Calcium-berates, mainly pandermite (priceite) and howlite, but also ba kerite and colemanite, are intercalated within the Sultancayir Gypsum (Miocene, Sultancayir Basin, western Anatolia). This lacustrine unit, represented by secondary gypsum in outcrop, is characterized by: (1) a clear facies distribution of depocentral laminated lithofacies and de bris-flow deposits, a wide marginal zone of sabkha deposits, and at le ast one selenitic shoal located toward the basin margin; (2) evaporiti c cycles displaying a shallowing-upward trend; and (3) a diagenetic ev olution of primary gypsum to (burial) anhydrite followed by its final re-hydration. The calcium berates precipitated only in the depocentre of the lake and were partly affected by synsedimentary reworking, indi cating that they formed during very early diagenesis. The lithofacies, which are made up of a host gypsum (finely laminated) and berates (no dules, irregular masses and discontinuous bands; also fine laminations ), indicate that the berates grew interstitially because of the inflow and mixing of berate-rich solutions with basinal brines. Berate growt h displaced and replaced primary gypsum beneath a relatively deep depo sitional floor. Berate formation as free precipitates was much less co mmon. The anhydritization of primary gypsum took place during early to late diagenesis (burial <250 m deep). This process also resulted in p artial replacement of pandermite and accompanying berates (bakerite an d howlite) as well as other early diagenetic minerals (celestite) by a nhydrite. Final exhumation resulted in the replacement of anhydrite by secondary gypsum, and in the partial transformation of pandermite and howlite into secondary calcite.