CARBONATE INTERNAL BRECCIAS: A SOURCE OF MASS FLOWS AT EARLY GEOSYNCLINAL PLATFORM MARGINS IN GREECE

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Abstract
Internal breccias caused by dilation of slightly lithified limestones have been investigated in the Triassic and Jurassic of the island of Hydra (Greece). They occur between shelf platform and rift basin of the early Tethys and are in general composed of monomictic and closely fitted angular clasts of platform carbonates. The matrix consists frequently of reddish deep-water limestones with thin-shelled molluscs (filaments) and radiolarians; it was sucked-in from above as a result of dilation during brecciation. The vertical succession includes about 20–30 m beginning with (a) fractured and fissured shallow-marine limestones grading upward into (b) internal breccias and (c) mass flows (characterized by a low degree of fitting and considerable roundness). Internal breccias imply a very small lateral displacement; they provide an important source for mass flows near shelf-to-slope breaks. Repeated brecciation is typical. The breccias are most frequently composed of shallow-water limestones, but are overlain by basin sediments. This indicates that the brecciation was connected with tectonic downwarping. We suggest that these breccias were produced by large migrating flexures, and that such flexures are a tectonic alternative or substitute for faults. In the Triassic and lower Jurassic limestones of the island of Hydra, five major breccia horizons are recognized. They correlate well with major tectonic phases in the early geosynclinal history of the northern and eastern Alps, in which internal breccias were found as well. This coincidence emphasizes the significance of such breccias in the evolution of geosynclines.

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