Depositional system and response to sea level oscillations of the senonian rudist-bearing carbonate shelves. Examples from Central Mediterranean areas

In the Late Cretaceous the carbonate platforms modified the organization of their depositional systems owing to vast and complex geologic events. In this view, detailed analyses have been made on Senonian shelf-to-slope rudist-bearing limestones resting on pre-Coniacian erosive surfaces or slope facies in the Nurra region (northwestern Sardinia, Italy), in the central-southern Apennines and in the Gargano area (central-southern Italy). The main characteristic of the analyzed deposits is the spreading of rudists in a context of foramol-type calcite-dominated benthonic sediment-producer communities. The reconstructed Senonian depositional environments match a large complex of unprotected shelves that produced loose, diagenetically stable mollusc-dominated bioclastic debris which were not involved in significantin situ cementation processes. High energy episodes led to repeated and more or less total remobilization of the sedimentary sheet. On the shelves, both storm- and wind-induced currents and waves exercised a strong driving control on the sedimentary arrangement of the shifting biogenic sediments. The latter constituted large coalescing sheets of winnowed, loose, fine-to-coarse skeletal sands. Sandy sediments were easily involved in remobilization processes across the shelves toward the redepositional sites. Transport modality largely depended on the granular composition of the sediments. The early and almost continuous sweeping of the finer fraction (bioeroded-derived silt) resulted in an effective pre-sorting of the skeletal debris stored in the Senonian open shelf settings.In situ preservation potentiality of the produced skeletal material was low and huge amounts of sands may have concurred in forming slope aprons. In the studied successions a two-stage evolution is documented during the Senonian. – - All over the latest Turonian-early Campanian interval the rudist-bearing shallow neritic platforms retreated, with seabed opening and deepening, and an underfeeding of the slope occurred. Probably, only where rudists strongly dominated the shelf assemblages (as in the case of the southern Tethyan carbonate platforms), their relatively high rate of bioclastic sediment production and supply might partially compensate for the increased accommodation space reducing the effects of the early Senonian transgressive phase. – - In the late Senonian a huge amount of foramol skeletal sands prograded over the upper slope by means of impressive gravitative flows suggesting that main depocenters moved down-slope. The persistence of healthy, producing foramol open-shelves may be inferred by the occurrence of compositionally coherent displaced skeletal sands even if reduced findings of late Campanian-Maastrichtian shallow water limestones are known characterized by a clear upward shallowing trend. A reduced accommodation space in shallow water settings may have enhanced the high off-bank sand dispersion via an increased winnowing action exerted on loose foramol-bioclastic sediments in periods in which the shelf tops were exposed to intense current winnowing. The generalized down-slope migration of the main depocenters occurred during the late Senonian regressive phase. Owing to the peculiar characteristics of the foramol-type open shelves (e.g., physiography, sediment production and composition), the sediment distribution patterns of the Senonian rudist-bearing carbonate factories and their response to sea level fluctuations were strongly modified with respect to the commonly accepted carbonate platform chlorozoan standard model. Major progradational episodes of marginal sands occurred during both relative lowstands and terminal highstands of sea level. During transgressive phases only where the sediment production was sustained (southern Tethyan carbonate platforms), the rudist-bearing depositional systems might have dampened the typical drowning tendency of the foramol open shelves. The reconstructed Senonian depositional environments match a large complex of unprotected shelves that produced loose, diagenetically stable mollusc-dominated bioclaslic debris which were not involved in significant in situ cemen-tation processes. High energy episodes led to repeated and more or less total remobilization of the sedimentary sheet. On the shelves, both storm- and wind-induced currents and waves exercised a strong driving control on the sedimentary arrangement of the shifting biogenic sediments. The latter constituted large coalescing sheets of winnowed, loose, fine-to-coarse skeletal sands. Sandy sediments were easily involved in remobilization processes across the shelves toward the redepositional sites. Transport modality largely depended on the granular composition of the sediments. The early and almost continuous sweeping of the finer fraction (bioeroded-derived silt) resulted in an effective pre-sorting of the skeletal debris stored in the Senonian open shelf settings. In situ preservation potentiality of the produced skeletal material was low and huge amounts of sands may have concurred in forming slope aprons. In the studied successions a two-stage evolution is docu-mented during the Senonian. - All over the latest Turonian-early Campanian interval the rudist-bearing shallow neritic platforms retreated, with sea¬bed opening and deepening, and an underfeeding of the slope occurred. Probably, only where rudists strongly domi¬nated the shelf assemblages (as in the case of the southern Tethyan carbonate platforms), their relatively high rate of bioclastic sediment production and supply might partially compensate for the increased accommodation space reduc¬ing the effects of the early Senonian transgressive phase. - In the late Senonian a huge amount of foramol skeletal sands prograded over the upper slope by means of impressive gravitative flows suggesting that main depocenters moved down-slope. The persistence of healthy, producing foramol open-shelves may be inferred by the occurrence of compo-sitionally coherent displaced skeletal sands even if reduced findings of late Campanian-Maastrichtian shallow water limestones are known characterized by a clear upward shallowing trend. A reduced accommodation space in shal¬low water settings may have enhanced the high off-bank sand dispersion via an increased winnowing action exerted on loose foramol-bioclastic sediments in periods in which the shelf tops were exposed to intense current winnowing. The generalized down-slope migration of the main depocenters occurred during the late Senonian regressive phase. Owing to the peculiar characteristics of the foramol-type open shelves (e.g., physiography, sediment production and composition), the sediment distribution patterns of the Senonian rudist-bearing carbonate factories and their re¬sponse to sea level fluctuations were strongly modified with respect to the commonly accepted carbonate platform chlorozoan standard model. Major progradational episodes of marginal sands occurred during both relative lowstands and terminal highstands of sea level. During transgressive phases only where the sediment production was sustained (southern Tethyan carbonate platforms), the rudist-bearing depositional systems might have dampened the typical drown¬ing tendency of the foramol open shelves.