Abstract: Lacustrine deposits of Upper Eocene that crop out near Kmik village, NW Ankara, consist of algal bioherms and various geometric forms of stromatolites- Small algal bioherms contain two types of carbonates-, tufa andcarbonate cones. Tufa is an organic carbonate mass made up of green algae which were rapidly encrusted by micritewhile alive. Rapid micritization is probably in response to photosynthetic release of GO2- The other bioherm groups(digitate-columnar carbonate cones) are also represented by radial growth of blue-green algae.The bioherms are covered and partly surrounded by bedded pellet-intraclasts and bioclastic grainstones-packestonesdeposited during the temporal fall and rise in lake level. A thin laminated crust formed under subaerial conditions,cover the grainstone-packestone layers and the bioherms.In the studied area, stromatolites generally overlie the algal bioherms. These are different kinds of organo-sedimentarystructures formed by trapping and binding actions of the blue-green algae in lacustrine environment. Despite the factthat the stromatolites were built only by blue-green algae genus «Schizotrix», increasing water energy and availablesediment led to different geometric forms. These forms can be grouped as LLH-lateral linked hemispheroids, SH-stacked hemispheroids and SS-oncoidal stromatolites.Organic and inorganic CaCO3 precipitation caused rapid calcification and cementation in bioherms. Mat actions ofthe blue-green algae and other biochemical events are the main effects on the cementation and configuration ofthe porosity of stromatolites.