Lucia Simone

Chlorozoan versus foramol lithofacies in Upper Cretaceous rudist limestones

Abstract The study of Upper Cretaceous limestones of the Nurra region (northwestern Sardinia, Italy) that crop out with open shelf, rudist-rich deposits has shown that during the early Senonian (Coniacian-Santonian interval) sediments characterized by assemblages dominated by molluscs, with variable amounts of red algae, bryozoa and echinoids, replaced sediments including hermatypic corals, green algae and variable amounts of non-skeletal grains. These latter components were largely dominant in the previous carbonate-shelf contexts of the Nurra during Jurassic-early Cretaceous times, becoming less numerous until they disappeared in the transgressive deposits that followed the tectonically induced middle Cretaceous emersion phase. A rhodalgal association (typical of anomalous tropical areas or transitional ones) has been recognized in the replacing limestones that may be more generically considered foramol-type deposits. This seems to document conditions of stress in the waters impinging on the Nurra carbonate platform during the Coniacian-Santonian transition. The observation that, in the late Cretaceous, a similar evolutionary trend seems to have characterized many shallow neritic successions, from many different areas, leads us to hypothesize more general causes of stress (e.g. induced upwellings on the marginal sectors of the shelves, increased oceanic overturns and hyperproductivity due to terrestrial runoff on post-emersive transgressed substrata). Peculiar oceanic conditions were able to create depositional contexts that were considerably changed with respect to the previous ones, such that the more opportunistic forms of the foramol assemblages flourished. The change in dominant assemblages resulted in a different organization of the sediment as a response to the different producing communities. The resulting temperate-type carbonate shelves were characterized by a low growth potentiality essentially due to a higher dispersion rate and this could have facilitated their drowning.

The fate of foramol (“temperate-type”) carbonate platforms

Abstract On rimmed shelves of Bahamian-type, characterized by chlorozoan associations and typical of tropical seas, carbonate production keeps pace with normal sea-level rise except when rapid rise or drastic environmental changes occurs. On the other hand, open temperate carbonate shelves are characterized by low carbonate production of the foramol association (molluscs, benthic foraminifera, bryozoans, coralline algae, etc.) and generally show seaward relict sediments, because carbonate production cannot keep pace with normal rate of sea-level change. Several examples of recent drowning foramol carbonate platforms (e.g., large areas of the Mediterranean Sea, eastern-northeastern Yucatan Shelf) as well as analogous ancient drowned foramol-type carbonate platforms (e.g., early to middle Miocene of the Southern Apennines; Miami Terrace) may support the idea that the drowning of many ancient carbonate platforms has been favoured by their biogenic (foramol sensu lato) constitution. Because of their typically low rate of growth, foramol carbonate platforms are fated to be drowned even if the sea-level rise is one with which the normal growth of chlorozoan platforms can keep pace. Similar conditions may also occur in tropical areas where variations in environmental conditions, such as the presence of cold waters, changes in salinity and increased nutrients, preclude the development of chlorozoan associations.

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

SummaryIn 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.

Development of rudist lithosomes in the Coniacian–Lower Campanian carbonate shelves of central-southern Italy: high-energy vs low-energy settings

Abstract The Late Cretaceous shallow-water depositional areas of southern Tethys were complexes of unprotected shelves occupied by foramol assemblages that produced loose, diagenetically stable bioclastic debris not involved in significant in situ cementation processes. Both storm- and wind-induced currents and waves exercised a strong control on the distribution of the shifting biogenic sediments which covered the open sea-floor, constituting large coalescing sheets of winnowed fine to coarse skeletal sands. Rudists spread over all shelf sectors, from more open and external areas to more internal ones, occupying different substrata and furnishing the bulk of the skeletal component by means of bioerosion processes. They colonised mobile sediments giving rise to complex bodies with peculiar characteristics related to environmental parameters of the different sectors of the shelf. On the basis of detailed sedimentological, taphonomic and palaeontological data, we recognised two main rudist-rich depositional settings (‘end members’) in the southern Italy Senonian rudist-bearing successions. In successions pertaining to hypothesised marginal shelf sectors, characterised by high-energy regime deposits, rudist lithosomes are metric in thickness and lateral extent and lens-like in morphology, rich in bioerosion-derived skeletal sand and silt. Rudists are highly diversified. Large elongated cylindro-conical hippuritids (mostly pertaining to the genera Hippurites and Vaccinites), thick-shelled radiolitids and plagioptychids largely dominate. Rudists clustered in life position are subordinate; they often form small bouquets. More commonly these organisms appear fallen but only barely reworked. The rudist-rich bodies laterally pass into clean bioclastic grainstone in which sedimentary structures, related to current and/or storm erosional action, are common. No evidence of significant original relief of the rudist bodies in respect of the neighbouring sediment can be recognised. The submarine erosion and/or the high-energy processes operating presumably inhibited the aggradation of the tidal sediments above the marginal ones. As a consequence the vertical facies organisation shows widespread subtidal cycles, as commonly recognised in open shelves with ramp-like morphologies. In successions pertaining to more internal and/or low-energy sectors, rudist-rich beds rhythmically alternate with finer-grained foraminiferal limestones. Small elevator radiolitids with oligospecific diversity are dominant, mostly concentrated in clumps. Rudists in growth position are abundant, although a large quantity of shells appear toppled with little reworking. They may form laterally continuous biostromal shell beds. Sedimentary structures such as cross-lamination and gradation are only occasionally present. The resulting facies are commonly arranged in peritidal/shallow subtidal cycles in which evidence of subaerial (up to pedogenic modifications on a large and small scale) and, less frequently, submarine exposure is common. Intermediate successions have been recognised, characterised by deposits of silty-sand plains, which present intercalations of graded, bioclastic, storm-related beds. Sedimentological characteristics seem to document more open conditions in which submarine erosion was intermittently prevalent. In these successions rudist species that are commonly found both in high-energy and low-energy assemblages coexist.

Miocene carbonate factories in the syn-rift Sardinia Graben Subbasins (Italy)

SummaryMiocene syn-rift carbonate production areas are exposed in south-eastern Sardinia (Italy) along the eastern side of the Plio-Pleistocene Campidano Graben. Here the downfaulted margins of the Oligo-Miocene Sardinia rift-basin crop out with a complex alignment of smaller subbasins. Carbonate factories developed during the Aquitanian times in these tectonically controlled small depositional subbasins, where terrigenous input might be high due to the erosion of the uplifted pre-Neogene substrata. Terrigenous deposits generally acted as passive substratum for pioneer communities of rhodalgal-type (red algae, ostreids, bryozoans) and locally evolved into coral-dominated assemblages.Carbonate production areas occur on tilted, uplifted blocks as well as along basement margins. These have varying organic communities and facies characteristics strongly dependent on different environmental conditions. In footwall areas, devoid of (or with a very reduced) terrigenous supply, open foramol carbonate factories occur with aggradational-progradational stratigraphic geometries. The resulting pure calcareous successions are organized in sequences, bounded by tectonically driven discontinuity surfaces.In sectors close to the hinterland area, with a higher freshwater input and a consequent significant clastic input, coral-dominated assemblages grew during long periods of quiescence between flash floods. Repeated coral-rich communities developed on thick wedges of terrigenous debris.Carbonate facies composition and distribution were mainly controlled by local syn-sedimentary tectonics and eustasy as well as by climate. However, although the inception of the carbonate factories was presumably not synchronous, their growth represented the response to a relative regional sea level rise subsequent to episodes of significant clastic supply: the expression of an early syn-rift stage.During the Burdigalian (N6 zone) a hemipelagic cover uniformly sealed the neritic successions. This basin wide drowning event appears to correspond to the transgressive system tract of the TB2.1 cycle ofHaqet al. (1987) but it can also be interpreted as the expression of an evolved syn-rift stage passing to late syn-rift and quiescence stages.Beneath the hemipelagic cover in some areas, ?late Aquitanian-early Burdigalian deep erosive surfaces, terrigenous clastics and paleosoils have been recognized. These suggest a relative sea level fall and may relate to the global sea level drop at the TB1-TB2 boundary (seeHaqet al., 1987) or to diacronous uplift of different fault blocks.

Rhodalgal/bryomol assemblages in temperate-type carbonate, channelized depositional systems: the Early Miocene of the Sarcidano area (Sardinia, Italy)

Abstract During Aquitanian-Burdigalian times, temperate-type carbonate-siliciclastic successions were deposited in basins located on the graben and half-graben along the Oligo-Miocene Sardinia Rift Basin (Sardinia, Italy). In one of these basins, the Isili Basin, rhodalgal/bryomol limestone was deposited in a temperate-type carbonate depositional system in which a shallow carbonate factory, a marginal tributary belt and a main channel were identified. In this paper, the Isili Limestone is described in terms of biogenic components and taphonomic characterization. To understand the palaeoecology of the depositional settings and the related palaeoceanographic controls, the coralline algal assemblages were differentiated according to coralline growth forms, rhodolith shapes, rhodolith inner structure and taxonomic composition. In addition, a first analysis was carried out in order to discriminate among the different bryozoan and mollusc assemblages present in different localities corresponding to different palaeoecological settings. The results obtained provide fundamental support for palaeoenvironmental reconstruction that combines palaeobiological analyses (i.e. rhodolith characteristics, bryozoan growth forms, taphonomic features) with previous physical (sedimentological and geometrical) observations. This allows a more detailed reconstruction of the complex relationships between the different sectors of the sedimentary system: the carbonate factory, the smaller erosive tributary channels and the deeper main channel.

Rudist lithosomes related to current pathways in Upper Cretaceous, temperate-type, inner shelves: a case study from the Cilento area, southern Italy

Abstract Shallow-water foramol limestones have been studied from a locality in the southern Apennines in which outcrop conditions show an excellent overview of the lateral and vertical evolution of rudist bodies and allow their geometry and the dynamic aspects to be reconstructed. The lithofacies suggest open depositional settings characterized by peloidal silty-muddy sediments. Rudists inhabited well-defined sectors of these shelves, giving rise to wide biostromal bodies, and supplied most of the skeletal debris via bioerosion and minor physical breakdown. In particular, the characteristics of rudist lithosomes document the existence of a complex network of channel-like depressions. In such a depositional context, the evolution of rudist lithosomes was controlled by the environmental hydrodynamic conditions. The resulting composite rudist assemblages are characterized by often-toppled individuals, suggesting continuous sediment removal between the organisms. The good preservation of the shells and the common articulation of the valves, however, point to an absence of sustained transport but rather a slight sediment destabilization. The gross lenticular geometry of the shell beds could be related to the above-mentioned patterns of weak, perhaps channelized, pathways. In such a depositional context, rudist colonization on channel margins assumes particular importance as it documents the rudist ability to exploit a wide array of environments, comparable to that of oysters in Recent seas, and reflecting the probable opportunistic nature of rudists.

Rudist-bearing rhodalgal facies in the post-Turonian recovery of peri-Tethyan carbonate systems: a case history from the Nurra region (northwestern Sardinia, Italy)

ABSTRACT The presence of non-geniculate coralline red algae and bryozoans (rhodalgal lithofacies) in association with rudists has been mentioned only rarely in the literature. Nevertheless, because of the significance of rhodalgal facies in the characterization of shallow-water carbonate factories, a correct interpretation of the related ecological factors may improve the palaeo-environmental reconstruction of some rudist-bearing carbonate depositional systems. Uppermost Coniacian-Santonian rhodolith-rich rudist-bearing carbonate successions in the Nurra region (northwestern Sardinia, Italy) record several discrete depositional settings, from autochthonous shallow-water mobile skeletal deposits including coralline algal fragments and rhodoliths, to re-mobilized deposits rich in skeletal components with rhodoliths. The rudist-bearing rhodalgal limestone studied is part of an uppermost Turonian-Campanian transgressive sequence that covers a tectonically-modelled Lower Cretaceous substrate. The recovery of the Upper Cretaceous carbonate factories followed an interval of time during which the Lower Cretaceous carbonate systems, dominated by chlorozoan assemblages and non-skeletal grains, had experienced “Mid-Cretaceous” worldwide crises presumed to have been caused by global climatic/oceanographic perturbations. In particular, Early-Middle Turonian times, characterized by the hyper-greenhouse conditions then prevailing, witnessed a significant reduction or even complete demise of highly productive carbonate factories. In carbonate settings, biotic assemblages grew in mesotrophic/eutrophic environments. Cyanobacterial consortia, with variable contributions from rudists, largely prevailed in shallow-water domains. Thus far, the latest Turonian-Coniacian recovery of Sardinian carbonate factories with “impoverished chlorozoan assemblages” might be considered as an indication of ameliorated environmental conditions. However, ecological constraints did not allow the tropical “chlorozoan assemblages” to thrive in the Late Cretaceous low-latitudinal carbonate shelves of Sardinia. A Santonian shift toward foramol/rhodalgal depositional systems occurred with sciaphile- (shadow preferring), and mesotrophic-adapted (“rudist-bearing rhodalgal”) assemblages flourishing in the new shallow-water domains. In Santonian times relatively cool and mesotrophic to eutrophic conditions are presumed to have become dominant in the water mass impinging on the marginal sectors of the shelf or distal ramp of the Nurra carbonate system. Deterioration in the quality of water presumably caused the demise of large sectors of the Nurra carbonate factory, which underwent local drowning episodes controlled by tectonic activity.

Re-sedimented Rhodoliths in Channelized Depositional Systems

The knowledge of re-sedimented rhodolith deposits has always lagged behind that of in situ deposits, which can be formed in shallow and deeper water carbonate and mixed siliciclastic-carbonate depositional settings. A combination of detailed outcrop analyses from three published case studies reveals a series of palaeobiological and taphonomic signals that are used to identify fossil re-sedimented rhodoliths. The re-sedimented rhodolith deposits of the middle Eocene carbonates in the Venetian area (northeast Italy), the lower Miocene carbonates from southern Sardinia (Italy), and the lower–middle Miocene carbonates from Southern Apennines (southern Italy) are described in terms of rhodolith morphology, coralline algal assemblages, inner arrangement, outer growth-forms, and taphonomic signatures. In all the cases, shallow water rhodolith beds were redeposited to feed offshore deposits through submarine channel systems. The sedimentological features, rhodolith characteristics and taphonomic signatures of the rhodolith deposits are compared from the carbonate factory, through the shelf-margin to the proximal and distal parts of the tributary belt. Within submarine channelized carbonate settings, complex relationship patterns of autochthonous/parautochthonous and allochthonous rhodolith deposits were governed by the interplay of changes in environmental factors such as water energy, light irradiance, substrate characteristics, and residence time on the sediment-water interface.