Paleoenvironmental and diagenetic implications of δO and δC isotope ratios from the Upper Jurassic Plassen limestone (Northern Calcareous Alps, Austria)

The first δ¹⁸O and δ¹³C data from the Upper Jurassic of the Northern Calcareous Alps are presented. The interpretation of stable isotope ratios serves as an approach for paleoenvironmental and diagenetic studies of the Plassen carbonate platform, which cannot be obtained by paleontological methods and microfacies analyses alone. The studied part of the Plassen limestone is characterized by (1) lithoclast facies, also called ‘intraformational breccia’; the origin of lithoclasts was formerly unknown; (2) peloid facies; (3) bioclastic facies, composed of peloids, porostromate algae, green algae and red algae; and (4) oncoid facies. Two types of fracturing and four cement generations can be distinguished. Isotope ratios of the matrix, oncoids, three cement generations and whole rock samples revealed that (1) the studied section represents an open marine carbonate platform with high water circulation and high input of cool oceanic waters; (2) the platform was not affected by emersion and fresh water influence; normal marine conditions prevailed; (3) carbonate cements were precipitated in a closed diagenetic system, but burial diagenesis was absent; (4) both fabric-selective and non-fabric-selective fracturing occurred in a normal marine environment, affecting the formation of ‘intraformational breccias’.