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Cretaceous Chondrodonta (Bivalvia) accumulations from Istria (Croatia) and Gargano (Italy): a comparative study

Chondrodonta is an oyster-like bivalve which occurs commonly in worldwide Barremian – Turonian rudistbearing limestones and shows particularly high concentrations and predominance with respect to other biota in specific stratigraphic intervals. However, the causes of the flourishing of Chondrodonta are still unclearand it is not completely understood whether and how the Cretaceous palaeo-climate perturbations may have played a dominant role in the explosion of this bivalve in carbonate platforms. In this work we will highlight similarities and differences in the sedimentological and taphonomic signatures of the Chondrodonta accumulations during two specific stratigraphic intervals of the Cretaceous. As first case study we present a sedimentological and taphonomic analysis of the upper Cenomanian Chondrodonta joannae (Choffat) accumulations, which crop out in the Adriatic Carbonate Platform (Istria Peninsula, Croatia). These accumulations occur within moderate-energy inner carbonate platform facies, deposited in a tidal flat/lagoonal setting where intertidal alternates to subtidal foraminiferal wackestonepackstones and rudist floatstones. C. joannae is very abundant in the upper part of the succession, where floatstone-rudstones to boundstones beds with Chondrodonta prevail over rudists. The thickest concentration is recorded within a lens-shaped beds-set, with a maximum thickness of about 1.8 m and some tens of meters of lateral extension. The shells are densely packed in horizontal sheets with scattered autochthonous and small individuals, arranged in bouquet-like aggregates, in the lower part, and very elongated parautochthonous individuals in a toppled position in the upper part. Sedimentological and taphonomic characteristics of C. joannae concentrations are compared to lower Aptian Chondrodonta glabra (Stanton) accumulations from San Giovanni Rotondo Limestones of the Apulia Carbonate Platform (Gargano Promontory, Italy). These older accumulations occur below the peak of OAE1a, in a succession composed of stromatolites and shallow-subtidal mud-rich facies. C. glabra shows an analogue interval of predominance with respect to the other bivalves and crops out in almost monospecific para- to autochthonous accumulations, arranged in metrical tabular- to small mound-shaped boundstone beds-sets. The shells are closely packed and show a variable orientation changing from horizontal in dense sheets to chaotical and sub-vertical with scattered bouquet-like aggregates.

Chondrodonta, Croatia, Italy

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