Upper Cretaceous ramp limestones from the Sorrento Peninsula (southern Apennines, Italy): micro- and macrofossil associations and their significance in the depositional sequences

Abstract

Abstract The Upper Cretaceous limestones of the Sorrento Peninsula are primarily characterized by wackestone/packstone with benthic foraminifers, thaumatoporellaceans and Aeolisaccus sp. and by rudist floatstones. Rudists in growth position are rare; most of them appear toppled and locally oriented but not reworked. In some cases they form small bouquets no more than 30 cm thick. The successions analyzed in the Sorrento Peninsula encompass a variety of facies representative of inner shelf environments in a ramp-like depositional setting. On the basis of lithologic and sedimentologic characteristics and on a qualitative analysis of the biofacies, three major facies associations have been recognized corresponding to: (a) intertidal silty-sand flats and shallow lagoons; (b) subtidal mobile foraminiferal sand sheets; (c) subtidal rudist dwelt sand plain. In the lower part of the successions periodic (more or less prolonged) emersions (subaqueous with fresh/brackish waters or subaerial exposure) are documented. The upper part of each succession lacks any emersion evidence; submarine exposure surfaces are testified by firmground-related features. The periodic influence of high-energy regimes (some storm-related events) is documented by wave- and cross-laminations, HCS and the lack of fine sediments. Lithofacies are arranged in depositional cycles that may correspond to individual beds. The boundaries of the depositional cycles show evidence of subaerial or submarine exposures. The nature and position of these discontinuity surfaces in the successions provide an important clue to interpretation in terms of both depositional environment and vertical evolution. The increase in thickness of the depositional cycles and the gradual change from peritidal/shallow-subtidal cycles to dominantly subtidal cycles, document an increase in the amount of accommodation space which resulted from a long-term rise in relative sea-level. A detailed study of benthic associations (micro- and macrofauna) has been carried out in order to identify accurately further signs of periodicity contained within the successions as a whole. These Upper Cretaceous limestones are abundantly fossiliferous with only a moderately diverse fauna. There are no significant differences in terms of species, the assemblages simply reflecting differences in abundance of the same species. Taphonomic studies of rudist-rich beds permit seven major shell-bed types to be distinguished. Six main foraminiferal assemblages have been detected, and the distribution and relative abundance of the species have been examined within the environments inferred through facies analysis. The taphonomic data and the foraminiferal assemblage abundance and diversity, have been compared with lithologic and sedimentologic data in order to elucidate their distribution within depositional cycles and throughout the successions. The successions show little up-section changes in lithology and in taxonomic composition of the foraminiferal assemblages. It is therefore difficult to establish a paleoenvironmental trend independent of the rudist shell concentrations, which do change up-section. We noticed an overall increase in thickness and abundance of the shell beds and a shift in types of shell concentration and in taxonomic composition. These characteristics, associated with the upward disappearance of emersion surfaces, the higher frequency of storm and/or wave intercalations, the increase in thickness of the depositional sequences and the gradual change from peritidal/shallow-subtidal cycles to dominantly subtidal cycles, document a general deepening-upward trend. More open water conditions with a depth between fairweather and storm wave-base became established as a result of relative sea-level rise and to a consequent increase in the available accommodation space.