Abstract

Black carbonate lithoclasts, along with blackened peloids, ooids, skeletal fragments, and entire limestone beds are found in the shallow subtidal, intertidal, and supratidal zones of modern and ancient carbonate environments. They are often associated with pedogenic features such as root-traces or caliche. The blackening is due to impregnation of the sediment by dissolved, colloidal, or finely particulate organic substances in an anoxic and alkaline environment or microenvironment. The organic matter is derived from decayed algae and/or decayed or burnt higher terrestrial plants. Iron sulfides contribute to the blackening, especially in samples containing algal matter. A complex interplay of adsorption of organic matter on carbonate-crystal surfaces, neomorphism, and microcrystalline cementation in the vadose or freshwater phreatic zones is thought to fix the organic matter and make the black coloration relatively resistant to oxidation. Black pebbles form through reworking of the preferentially cemented and blackened sediment by coastal erosion. They are mostly relics because the less-consolidated host sediment is washed away. Black pebbles may thus be valuable indicators of ancient coastal and terrestrial environments.

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