Authigenic siliceous-carbonate concretions in Cenomanian-Turonian carbonates of southern Croatia; geochemical and biotic record

摘要

Numerous regular and irregularly shaped concretions were discovered in southern Croatia within the Upper Cretaceous carbonate deposits, which consist mostly of bioclastic wackestones and packstones. The size of the concretions varies from 2 to 25 cm in diameter for the regular, and up to 40 cm in length for the irregular ones. The concretions have different internal structures and compositions, which vary from those of the host rocks. Some concretions are entirely siliceous, while others exhibit alternating opal silica and carbonate-rich zones. Silica is derived from opal-A secreting marine biota, mostly sponges, whose spicules can be found within the concretions and in the surrounding limestones. The regular shape and alterations of zones are the result of the diffusive supply and fast "consummation" of silica at the growth site. Determined fossil community and bitumen found in the surrounding limestones indicate deposition in the oxygen-depleted deep-marine environment of the Adriatic Carbonate Platform, which belongs to the Sveti Duh Formation (latest Cenomanian-Early Turonian). The carbon isotope composition of the concretions corresponds to the globally known Cenomanian-Turonian 'Oceanic Anoxic Event' (OAE 2).