Stratigraphy and paleoenvironments of rudist biostromes in the Upper Cretaceous (Turonian-upper Santonian) limestones of southern Istria, Croatia

Abstract

Abstract Rudists are well preserved in the Upper Cretaceous limestones of southern Istria, where they flourished within different shallow-water environments during the Turonian to upper Santonian. In order to determine paleoenvironments, a vertical succession of the rich rudist assemblages has been investigated and described within sequence stratigraphy framework. The Turonian to upper Santonian sediments of southern Istria, in the sense of sequence stratigraphy, comprise one complete third-order sequence and the beginning of the transgressive systems track (TST) of the next sequence. Lower Turonian pelagic limestones were deposited on a foundered carbonate platform, and represent the beginning of the TST. Limestones of the late TST-lowstand systems track (LST) were deposited on the inner shelf, and the early TST of the next sequence was deposited on the outer shelf. All the rudists which appeared after the early Turonian disappearance and partial extinction were elevators. The genera Durania and Praeradiolites thrived in bouquets, which may coalesce to form thickets in inner shelf environments of the late TST-LST. Their growth depended completely ( Durania ) or partly ( Praeradiolites ) on net sediment accumulation. The environment in which Durania and Praeradiolites thrived was subtidal and deep enough to be out of reach of emersion. Within the same depositional environment, these genera were later replaced vertically by Radiolites . Individuals with slim, cylindrical shells, and wide growth lamellae, have full contact within the thickets, in which their growth depended little, if at all, on net sedimentation rate. Also, Radiolites occupied the shallower parts of the subtidal zone with a commissure rim rising above the sediment surface. Renewed deepening in the late Santonian resulted in the outer shelf environment of the next TST. Gorjanovicia , together with rare Vaccinites , flourished after the Radiolites . Gorjanovicia differed from Radiolites only in possessing more pronounced vertical ribs. In their slightly deeper subtidal environment with pelagic influxes, Gorjanovicia thrived in bouquets. Loosely packed Gorjanovicia gave Vaccinites an opportunity to grow among them. The environmental setting of the southern Istria rudist biostromes and their correlation with other shallow-water deposits in the Mediterranean region suggests that a sequence stratigraphy approach can be applied to correlate rudist biostratigraphy on a regional, or even inter-regional, scale.