Two diagenetic paths of Callovian oolites from SE Adriatic Carbonate Platform

Four exposures of the Middle Jurassic (Callovian) oolites from coastal Montenegro were studied to determine their origin and diagenesis. The distal exposures (Livari and Orjen) appear in banks and thin to medium beds. Their basal contact with supratidal black pebble rudstone (Bathonian) is sharp, while their upper contact with coral framestone (Oxfordian) is gradational. Large (~ 1mm) ooids with micritic cortices make up most (~80 %) of grainstones in these two exposures. The central exposures (Vrsuta and Han) have the same basal contact, but their upper contact is unconformable with the Clypeina jurassica packstones (Kimmeridgian). Micritic ooids in Vrsuta grainstones are larger (~1mm) and more abundant (~ 97% of allochems) than ooids in Han grainstones (~0.65mm and 70% respectively).

In the distal exposures grain contacts are floating and pore space is high. Grains are coated with three generations of cement and the remaining pore space is filled with drusy spar. Late diagenetic neomorphism affects some peloids and ultimately leads to creation of secondary porosity. Central exposures show far less pore space and much greater grain compaction. Pore space in these samples is filled with meniscus, blocky, and granular spar, with patches of late diagenetic poikilotopic spar. Near the top of central exposures microstylolitic grain contacts predominate and the remaining pore space is filled with micrite.

The oolites present in central exposures, indicate high sedimentation rates along the shoals that kept pace with transgression. Filling of the accommodation space lead to exposure and meteoric diagenesis of these oolites. Distal exposures reflect drowning shoals with low sedimentation rates and early marine phreatic diagenesis. This portion of the Adriatic Carbonate Platform, at the end of the Middle Jurassic, consisted of shallow ooid shoals (Vrsuta and Han) and deepening bathymetry with subsequent reef development at the distal locations (Livari and Orjen).