Internal Breccias near Early Geosynclinal Platform Margins: ABSTRACT

Abstract

The Triassic and lower Jurassic limestones of the island of Hydra (Greece) were deposited on the Pelagonian platform, near its western edge which strikes north-northwest-south-southeast. In this region, five main breccia horizons are recognized. Internal breccias are characterized by mutual fitting of clasts, indicating relatively small displacement. Transitions downward into fissures and almost undisturbed rock sections, and upward into mass flows, provide important clues as to their origin. The clasts are generally monomictic and consist of shallow-water, slope or basin ridge limestones. The matrix is derived from above and consists of basin sediments which are commonly red. Each of the five breccia horizons represents a sequence of: (a) platform buildup; (b) tilting caused by unequal subsidence; (c) deposition of basin sediments on top of the platform carbonates; and (d) brecciation of the platform limestones and absorbing of the overlying basin sediments. In many places, early lithification and repeated brecciation also occur. These main breccia horizons correlate very well with major tectonic phases in the early geosynclinal history of the northern and eastern Alps. Although submarine breccias are commonly related to faults, there is no evidence for this in the Triassic and Jurassic sequences of Hydra. We suggest that the breccias were produced by large migrating flexures, and that such flexures are a tectonic alternative or substitute for faults in the early stages of Tethys formation. The study of brecciation of the type discussed may provide more precise information on the configuration and evolution of early geosynclinal platform margins and shelf-to-slope breaks. End_of_Article - Last_Page 928------------