CEREBROID OOIDS IN THE MIDDLE JURASSIC ROCKS OF SOUTHERN MONTENEGRO

We report very interesting facies of cerebroid oolites discovered on the north slopes of Mt. Rumija in southern Montenegro. In the early stages of the Bajocian transgression basal parts of Mt. Rumija II Oolite were deposited over the exposed or very shallow carbonates of Adriatic Platform. Bluish gray oolitic grainstone is thinly bedded and made almost exclusively of cerebroid ooids which make up over 75% of all grains. The cerebroid ooid diameter ranges from 0.4 to 1 mm but most of the ooids are from 0.6 to 0.8 mm diameter with an average size of 0.68 mm. Spherical cerebroid ooids are most common but oval, ellipsoidal, square, dumbbell, and irregular ooids are present as well. The cerebroid cortex is 0.07-0.2 mm thick and is made of convex, yellowish calcite or dolomite alternating with concave, cryptocrystalline dark brown micrite. Dark concentric lines within the cortex mimic the cerebroid curvature of the convex and the concave areas. Nuclei of the cerebroid ooids are made of preexisting radial concentric and tangential/micritic concentric ooids, skeletal grains, and micrite/peloid cryptocrystalline calcite. Ostracodes, crinoids and gastropods are often fully preserved in the nucleus and in many instances enveloped in dark mud before the outer cerebroid cortex was added. Five types of cerebroid ooids were recognized: 1) Cerebroid ooids with a radial concentric inner cortex; 2) Cerebroid ooids with a tangential/micritic concentric inner cortex 3) Cerebroid ooids with a large palisade cortex growing from the skeletal nucleus 4) Superficial cerebroid ooids with a composite nucleus; and 5) Globular cerebroid ooids with no recognizable nucleus. Fractures are common within the cerebroid ooids and in some instances partial micritization or partial neomorphism and growth of granular blocky spar occur. We propose that the cerebroid oolite facies of Mt. Rumija II Oolite formed in shallow, quiet water, hypersaline lagoon environment. In an extremely seasonal climate the cerebroid ooids grew during the hot and dry summers when evaporation increased carbonate concentration in shallow and restricted lagoons. During the humid season the carbonate concentration was lower but the water agitation greater when concentric bands in cerebroid cortices developed.