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Siliceous nodular concretions from Upper Cretaceous limestone near Vrgorac (Croatia)

Siliceous-carbonate nodular concretions – perfectly spheroid, in the form of oblate spheres and some of irregular shape – were found within Upper Cretaceous limestone near Vrgorac. Nodule is a mineral body that differs in composition from rock in which it is formed, commonly imestone and dolomite. Sediment surrounding the concretions is wackestone-packstone with abundant pelagic particles (pithonellas and calcispheres), echinoderm fragments and primitive planktic foraminifera. These deposits belong to the hemipelagic Cenomanian–Turonian Sveti Duh Fm (GUŠIĆ & JELASKA, 1990). Concretion outer surface and surrounding host sediment is often impregnated with bituminous material. The source of bituminous material was organic remains possibly giving opal (silica) material for concretion growth. Preservation of bituminous material in carbonates is connected with stagnant pore waters and rapid sea-level rise due to the Oceanic Anoxic Event (OAE) at the Cenomanian– Turonian transition (GU[I] & JELASKA, 1990, and references therein).

silica; nodular concretion; diffusion; Upper Cretaceous; Vrgorac; Croatia

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