ABSTRACT

The Neogene borate deposits of Anatolia have thin sedimentary covers and have never been deeply buried. The major rock-forming, calcium-bearing borates are colemanite and ulexite. Under progressive evaporation, borate precipitation occurred in a number of settings, from stable playas to perennial lakes, the latter evolving to shallow lakes and playa lakes. In all these lakes, shallowing-upward cyclicity is common. Colemanite facies occupy the margins, and ulexite facies the centers of the basins; this mineral zonation is depositional and cannot be ascribed to diagenetic processes. Colemanite and ulexite formed mainly as interstitial growths under synsedimentary conditions, and as mineralogically primary, displacive and/or cementing lithofacies, which were affected by progressive compaction. Heliothermal conditions might have favored the precipitation of colemanite in some lakes; in others, subaerial (playa) conditions seem to have been appropriate. Locally, some colemanite is secondary and proceeds from the (macroscopic) replacement of precursor ulexite; this replacement occurred during either early diagenesis or (more probably) late diagenesis. Minor evidence exists for colemanite formation from replacement of precursor inyoite (?).