SEDIMENTOLOGY AND DIAGENESIS OF MIOCENE COLEMANITE-ULEXITE DEPOSITS (WESTERN ANATOLIA, TURKEY)

The Neogene berate deposits of Anatolia have thin sedimentary covers a nd have never been deeply buried. The major rock-forming, calcium-bear ing berates are colemanite and ulexite. Under progressive evaporation, berate precipitation occurred in a number of settings, from stable pl ayas to perennial lakes, the latter evolving to shallow lakes and play a lakes. In all these lakes, shallowing-upward cyclicity is common. Co lemanite facies occupy the margins, and ulexite facies the centers of the basins; this mineral zonation is depositional and cannot be ascrib ed to diagenetic processes. Colemanite and ulexite formed mainly as in terstitial growths under synsedimentary conditions, and as mineralogic ally primary, displacive and/or cementing lithofacies, which were affe cted by progressive compaction, Heliothermal conditions might have fav ored the precipitation of colemanite in some lakes; in others, subaeri al (playa) conditions seem to have been appropriate. Locally, some col emanite is secondary and proceeds from the (macroscopic) replacement o f precursor ulexite; this replacement occurred during: either early di agenesis or (more probably) late diagenesis. Minor evidence exists for colemanite formation from replacement of precursor inyoite (?).