OOIDS AND VADOIDS IN THE LOWER JURASSIC OOLITE FROM THE SOUTHERN ADRIATIC CARBONATE PLATFORM, MONTENEGRO

Two distinct assemblages of the Lower Jurassic Oolite were recognized along the margin of the Adriatic Carbonate Platform exposed in the coastal mountains of Montenegro. Medium bedded grainstones in the southeastern exposures (Mt. Rumija) reveal shoal development, partial exposure, and then drowning during the Toarcian transgression. Small (< 0.3 mm) superficial ooids in the basal part of the oolite pass into polyphase ooids (0.4-1.2 mm) with radial, radial-concentric, tangential and micritic fabrics, which indicates frequent small scale sea level fluctuations. Early chertification affected the basal parts of the oolite by completely replacing the cement and partially replacing the ooids. Grain compaction, spalling off the ooid cortices and bivalve rupturing, and the presence of meniscus cement and vadose fill indicate a partial exposure and a meteoric vadose diagenesis in the top 5 m of the oosparite. Brachiopod and crinoid rich grainstones separate the Lower Jurassic Oolite from the Middle Jurassic Oolite.

Thin to medium bedded Lower Jurassic grainstones and packstones in the central and northwestern exposures (Mt. Lovćen and Mt. Orjen) are intercalated with massive wackestones, and transition into Middle Jurassic Oolite beds, reflecting high sedimentation rates during the Toarcian transgression. Vadoids (0.4-0.8 mm) are the only allochems in the packstones. The polygonal fitting of vadoids, their pendant grain protrusions and micrite nuclei, pervasive micritization of the grain surfaces, the total absence of skeletal grains, and micrite matrix with patchy microspar in this facies indicate either a coastal caliche or a hypersaline pond environment. The grainstones, however, contain well sorted ooids (0.3-0.6 mm) with micrite nuclei and radial or radial concentric fabric sometimes enveloping encrusted forams, which indicates a lagoon environment. Massive wackestones contain a few ooids scattered in a muddy matrix. Partial to complete dolomitization of radial ooids and matrix occurred early in a supratidal environment.