ABSTRACT
The Portoro Limestone is jet black with white veinlets and gold stylolites, and is much used as ornamental stone. Vague, pale-gray nodules represent precocious lithification at shallow depths below the sediment surface, probably the result of bacterial colonies consuming organic matter and mobilizing carbonate. Gnocchi (now turned upside down) are partly filled with geopetal, fine dolomite sediment; original tops are filled with sparry calcite and baroque dolomite. These probably represent dissolved evaporite nodules. Despite the black color, the rock contains very little organic matter. The rock is made of 10-20 µm metamicrite as the result of thermal metamorphism; crystals are so clear that the minute amount of organic matter is readily visible. There are many generations of dolomite; most abundant is postmetamorphic, post-fracturing dolomite that contains inclusions of metamicrite and has ragged crystal form. Earlier dolomite behaved as a mechanical sediment, accumulating as geopetal filling within cavities, on top of clasts, and within tectonic fractures.
