CHRONOSTRATIGRAPHIC INTERPRETATIONS OF SOME STRATIGRAPHIC BREAKS IN LOWER CRETACEOUS CARBONATE FACIES IN THE TERRITORY OF GEORGIA (CAUCASUS)

During Early Cretaceous, the region occupied by the present territory of Georgia was an active oceanic margin of the Tethys region. Specifically during late Barremian-Aptian, in some places in Georgia, carbonate subplatform facies sequences developed. The general order of facies from bottom to top includes brecciated limestones, limestones, sandy limestones, and marly limestones, containing glauconite at some levels. Also, there are some sequence-bounding stratigraphic breaks within the Lower Cretaceous section. At Godogani, these stratigraphic breaks occur at four levels: mid-Barremian, astride the Barremian-Aptian boundary, in lower-upper Aptian (D. furcata Biozone is absent), and in uppermost Aptian (at the C. tobleri-A. nolani Biozone boundary). At Tvishi (30 km to the north), stratigraphic breaks occur at mid-Barremian, lower-upper Aptian, and in uppermost Aptian (at the same biozone boundary as in Godogani). Both the lower-upper Aptian and the uppermost Aptian stratigraphic breaks are likely coeval at Godogani and Tvishi and some other places, but at Alpana and Bethlevi (corresponding to more deep areas of the shelf) the upper/uppermost Aptian transition is conformable. The lower-upper Aptian stratigraphic break may be related to a short-term global eustatic sea-level drop, whereas formation of the uppermost Aptian break, besides a short-term global eustatic event, apparently was related to regional tectonic events as well. Unlike the upper Barremian-lower Aptian break at Godogani, there is no break at all on the corresponding level at Tvishi. Moreover, due to lack of sufficient data on biostratigraphy, it is not clear whether the mid-Barremian stratigraphic breaks at Godogani and Tvishi are the same age. Originally the chronostratigraphic relations of these breaks were correlated using mainly ammonite biostratigraphy, but in this study we are also using Sr87/Sr86 isotopic analysis to refine the ages of these stratigraphic breaks.