Rudist Assemblages Research Articles

The Turonian–Campanian rudist bivalve succession in the Central Iberian Basin

Four rudist assemblages: lower Turonian, upper Turonian, upper Coniacian, and Santonian–?Campanian, are distinguished in the shallow water carbonate platform successions of the Iberian Basin, nowadays Iberian Range, a Mesozoic intra-continental basin in the western margin of the Mediterranean Tethys. Because of the depositional evolution of the Iberian Basin, the occurrence, abundance, taxonomic diversity, diagenetic processes, and shell preservation, for each assemblage, is linked, both, to the shallow character of these carbonate platform successions, and the successive high and low frequency sea level falls. Twenty identified rudist taxa are described and figured: Hippuritidae, six species of two genera; Radiolitidae, ten species of six genera, one new, Hoyosites tozoi gen. et sp. nov; Requieniidae, one genus. The knowledge of the shell characters of some taxa has been improved and the taxonomic, biostratigraphic, and palaeobiogeographic significance of most of them increased. The precise positioning of the first three rudist assemblages in high-frequency depositional stacking pattern (parasequence sets) and their correlation and calibration with ammonite biozones provide biostratigraphic datums of great importance. This fact notably improves the chronostratigraphic framework of the Cretaceous sedimentary successions of the Iberian Basin, especially towards the coastal margins, and allows the accurate quantification of the hiatuses associated with the parasequence sets boundaries, so enabling their precise hierarchization.

Micro-biofacies of the Maastrichtian Tarbur Formation, Neo-Tethys Zagros Basin, Iran, clue to penecontemporaneous faults and environmental hydrodynamics

The main purpose of this research main is to study the role of local faults in the formation of rudist assemblage composition and thickness in the Maastrichtian deposits of the Tarbur Formation in the High Zagros sub-basin. Based on the taphonomic studies and micro-biofacies analysis, three micro-biofacies were identified in the study areas. These micro-biofacies were formed in the shallow marine environments in the study areas. The Tarbur Formation in the Ardal and Murak areas was deposited in a high-energy environment based on the distribution of micro-biofacies. Other areas were formed in low-energy environments. The activities of local faults were an important and effective factor in the formation of rudist association with high thickness and the distribution of hydrodynamic energy in different areas of the carbonate platforms. The thickness of these rudist colonies increased from northwest to southeast in the High Zagros zone.

The Lower Cretaceous Carpatho-Cimmerian bioprovince: The contribution of rudist bivalves (Hippuritida)

The Carpatho-Cimmerian bioprovince is a Barremian–lower Aptian entity. Its definition based on the coexistence of two calcareous algae and a foraminifera, is well supported by a distinctive rudist assemblage. Thirteen species: Requienia zlatarskii, Lovetchenia lovetchensis, Lovetchenia drinovi, Lovetchenia stratechi, Toucasia petersi, Parapachytraga gigantea, Paragyropleura paquieri, Debrunia ? dibrariana, Debrunia ? kafanensis, Homopleura balkanica, Homopleura ? minima, Homopleura? gigantea and Bicornucopina petersi are characteristic, endemic members of this bioprovince. The Carpatho-Cimmerian bioprovince shared some taxa with Western Europe and Apulian biomes. The Carpatho-Cimmerian rudist assemblage extends from Hungary, Romania, Bulgaria, Slovakia, Ukraine, Southern Russia, Northern Turkey, Armenia, Azerbaidjan and probably Turkmenistan. The South Bohemian shelfal discontinuity between the Western European and the Balkanic regions, accounts well for the dissimilarity of the corresponding faunas.

Diversity dynamics, trophic, community and environmental changes in early cretaceous rudist bivalves (Hippuritida): New insights on the late Barremian “Agriopleura extinction event” and subsequent recovery

The “Agriopleura event”, which expresses the regional extinction, in northwestern Europe, of the rudist genus Agriopleura, is associated with environmental and biological changes. This event marks the boundary between two distinctive late Barremian regional rudist assemblages, the pre-event Brouzet-les-Alès and post-event Orgon faunas. The succeeding Palorbitolina episode is coeval with a rudist eclipse. The ensuing Rustrel fauna identifies the phase of post-extinction recovery. The last occurrence of Agriopleura, the stem genus of the Radiolitidae, coincides with the extinction of 41% of the pre-event Requieniidae and Monopleuridae. The recovery phase is characterized by new species and a low extinction pattern. This phase is marked by a burst of speciation, in part indigenous and in part due to immigrants i.e. Caprinidae. The key morphotypes, clingers, elevators and recumbents record contrasting changes in specific diversity, clingers being dominant due to the substantial diversity of Requieniidae and Monopleuridae. During the extinction event elevators decrease and suffered a reduction in size, large size species being selectively eliminated. The entry of the Caprinidae identifies the recovery phase and accounts for size increase and an important contribution of recumbent. The main agent of the extinction, combines cooling, anoxia, platform exposure and a trophic factor assumed to account for the selective removal of the elevators. A dual partition in feeding behavior of clingers and elevators is suggested: elevators exploiting the phytoplankton flux and the re-suspended bottom biodeposits; and clingers feeding on the benthic boundary layer. Significant community changes are associated with Agriopleura extinction and its aftermath, which clearly respond to changes in taxonomic composition. Elevator dominated communities with the Agriopleura are replaced by polytaxic requieniid rich communities then by caprinids. The Palorbitolina episode, records a trophic peak, cooling and a deepening trend. The recovery phase mainly records an aragonitic pulse coupled with temperature increase.

Taxonomy and paleobiogeographic implication of Glabrobournonia Morris and Skelton (Hippuritida, Radiolitidae) from the Late Cretaceous Yigeziya Formation, southwestern Tarim Basin

A re-examination of the specimens that were identified as Biradiolites minor Pojarkova from the late Campanian to early Maastrichtian middle member of the Yigeziya Formation of southwestern Tarim Basin reveals that they should be assigned to the genus Glabrobournonia Morris and Skelton. Glabrobournonia is a genus of radiolitids characterized by indented radial bands, salient ridges on the shell margins and absence of fine ribs on the surface of the right valve. Apart from southwestern Tarim Basin, Glabrobournonia minor (Pojarkova) has also been recorded from the late Campanian of Fergana and Alai basins. The central Asian, late Campanian to early Maastrichtian G. minor differs from the late Campanian to Maastrichtian, eastern Arabian type species Glabrobournonia arabica Morris and Skelton in the flat left valve and an additional fourth ridge on the junction of the dorsal and posterior sides of the right valve. Biradiolites ingens (Des Moulins) could be the direct ancestor of Glabrobournonia. The paleogeographic distribution of Glabrobournonia suggests that this genus dispersed to central Asia from the late Campanian time, becoming widely distributed in the eastern Tethyan region rather than endemic to eastern Arabia. Correspondingly, specimens belonging to Gyropleura yielded from the same bed as G. minor in southwestern Tarim Basin, are similar to the specimens which were attributed to the eastern Arabian Gyropleura sp.; Campanian to early Maastrichtian Osculigera specimens described from the Yigeziya Formation are comparable with those known from the Campanian–Maastrichtian of Iran, Afghanistan and eastern Arabia. The similarity of the rudist assemblages between central Asia and eastern Arabia suggests a faunal connection and affinity between the north and south margins of the eastern Tethyan realm during Campanian to early Maastrichtian times.

Ecological successions of rudist communities: A sedimentological and palaeoecological analysis of upper Cenomanian rudist assemblages from the South-Provence Carbonate Platform (SE France)

Carbonate platforms developed in the southwestern Provence area during the late Cretaceous on the rims of a narrow deep E-W oriented basin. The upper Cenomanian lower Member Fontblanche 1 of the Fontblanche Limestones Formation is exceptionally rich in rudists. This member is well exposed in two quarries with flat outcrop surfaces, thanks to sawn surfaces, that allow 3D investigation of its litho- and biofacies and sedimentary structures. The paper describes the sedimentary sequences of each quarry from a quantitative study of both sedimentary and palaeontological data, both macroscopic and microscopic, derived from quadra counts in the field and thin section analyses. The sequences are one to several metres thick and each of them shows a basal drowning and a final emersion surface with evidence of reworking and dissolution. The microfacies are dominated by floatstone texture, most common grains being bioclasts derived from biocorrosion of rudist shells and benthic foraminifers. The matrix is made mostly of microbioclasts and micrite, resulting from a strong microborers activity. Fossil organisms are regarded as autochthonous or parautochthonous. The overall depositional environment was shallow marine with a maximum depth of a few metres in an inner, peritidal, sheltered and quiet platform. We found a link between the sedimentary facies and rudist ecotypes. The statistical analysis of the fossil counts, using multivariate statistics and clustering, differentiates four fossil assemblages, namely the Chondrodonta assemblage, the Apricardia-Sauvagesiinae assemblage, the Caprotina assemblage, and the Nerinea assemblage, which are considered to represent biotic communities. The replacement of communities through Fontblanche 1 and within the elementary sequences illustrates a relationship between the benthic communities, the degree of biocorrosion, the sedimentary texture, and the sedimentation rate so that ecological processes affect the sedimentary dynamics which in turn affects communities through feedback mechanism. Such an intimate link between the sedimentary facies and ecosystems provides evidence that biotic parameters played an important role in the rudist-rich facies succession.

Stratigraphic and taxonomic considerations on the Late Cretaceous rudist fauna of Aksai Chin (Western Tibet, China) from the De Filippi Collection

The rudist fauna collected in western Tibet in the Aksai Chin area by the De Filippi expedition in 1914 has been reprised and redescribed. This fauna is composed of Radiolites cf. lusitanicus, Radiolites sp., Gorjanovicia cf. endrissi, ? Sauvagesia sp., Sphaerulites sp., Durania sp., and Gyropleura sp. The rudist-bearing beds belong to the Tielongtan Group of the Tianshuihai terrane. The Turonian-? Coniacian Xiloqzung Formation (Fm.) bear the older rudists (Radiolites cf. lusitanicus, Radiolites sp., Sphaerulites sp., Durania sp.), while younger ages have been determined through microfossil analysis which, compared with the western Neotethys records, suggests an early-mid Campanian age. This allowed to ascribe the younger rudists of the collection (Gorjanovicia cf. endrissi, ? Sauvagesia sp., Gyropleura sp., Radiolites sp.) to the Dongloqzung Formation. Our data confirm that rudist-bearing facies in the Tielongtan Group extend at least up to the middle Campanian. The Aksai Chin rudist assemblage should belong to the Southwestern Asian assemblage of the Eastern Mediterranean Subprovince.

Facies and sequence stratigraphic architecture of the Mural Limestone (Albian), Arizona: Carbonate response to global and local factors and implications for reservoir characterization

AbstractThis study documents the detailed facies and sequence stratigraphic architecture of a multi‐cyclic patch‐reef and its associated ramp interior facies that formed during Oceanic Anoxic Event 1b in the Mural Limestone, Arizona, USA. Ramp interior facies are comprised of bedded wackestone/packstone, rudist build‐up and coral–algal patch‐reef facies located north of Bisbee, Arizona, at the Grassy Hill locality. The larger multi‐cyclic patch‐reef that developed coevallyca5 km to the south of Grassy Hill consists of a high‐angle windward margin with a narrowca70 m long reef frame containing vertically zonatedMicrosolena, Actinastrea, diverse branching coral and rudist assemblages, and an 870 m long low‐angle leeward margin comprised of reef debris rudstone and grainstone shoal facies. Similar reef geomorphology and orientation is documented across the Gulf of Mexico and reflects the shelf‐wide north to north‐east‐trending prevailing wind and current energies. Controls affecting reef formation and growth patterns include changes in accommodation space associated with low‐amplitude global sea‐level rise and regional thermotectonic subsidence, local accommodation space and nutrient fluctuations associated with the inner shelf depositional setting within a humid and siliciclastic‐rich environment. Four aggradational to retrogradational high‐frequency sequences are documented in Arizona: High‐frequency sequences 1 and 2 represent the first pulse of patch‐reef development in an overall second‐order marine transgression over the Sonora/Bisbee Shelf. These sequences correlate to δ13C signatures associated with Oceanic Anoxic Event 1b across the Gulf of Mexico and suggest that carbonate reefs persisted on the ramp interior during this time. High‐frequency sequences 3 and 4 record a second brief transgression and backstepping of reef facies followed by the final regression of shallow shelf carbonates that correlates to more robust patch‐reef development in Sonora, Mexico. The patch‐reef at Paul Spur is an excellent outcrop analogue for productive patch‐reefs in the Maverick Basin (Comanche Shelf) of Texas. Detailed facies mapping of this outcrop analogue shows that the greatest reservoir potential is contained within the backreef grainstone shoals where primary porosity of up to 15% is observed.

Shajia, a new genus of polyconitid rudist from the Langshan Formation of the Lhasa block, Tibet, and its palaeogeographical implications

A new polyconitid rudist Shajia tibetica gen. et sp. nov., of late Aptian to Albian age, is described from the Langshan Formation of Nyima County, northern Lhasa Block, Tibet. Though comparable in size and external morphology with Horiopleura haydeni Douvillé, which is a common endemic species in southwestern Asia, Shajia differs from the latter species in its possession of an inwardly inclined, instead of outwardly facing, posterior myophore in the right valve. In addition, a single specimen from Ladakh, which was previously assigned to Polyconites? sp., on account of a similar myophoral distinction from H. haydeni, is transferred to the new genus. Shajia is considered most likely to have been derived from one of a group of Horiopleura species that lived on the southern margin of the Mediterranean Tethys.The so-called ‘Yasin fauna’ represented by the late Aptian to Albian Horiopleura haydeni/Auroradiolites gilgitensis rudist association, is considered to be restricted to southwestern Asia, including Afghanistan, Kohistan in northern Pakistan and Ladakh in northern India, though those two species in particular have not so far been recorded from the Lhasa Block of Tibet. Nevertheless, S. tibetica co-occurs with Auroradiolites biconvexus (Yang et al.), which probably evolved directly from A. gilgitensis (Douvillé), and the age of the latter association is in accordance with the generally accepted age of the Yasin fauna as late Aptian to Albian. Hence the S. tibetica and A. biconvexus association can be considered a regional variant of the Yasin fauna, which had evidently already dispersed to the Lhasa Block by the late Aptian. So the Langshan Formation can be considered palaeogeographically linked with other mid-Cretaceous shallow-marine carbonate deposits in adjacent southwestern Asian regions. These findings also provide new evidence that the age of the rudist assemblage of the Lhasa Block is late Aptian to Albian, although a slightly younger age cannot be excluded.

The Madenli (Central Taurides) Upper Cretaceous platform carbonate succession: Benthic foraminiferal biostratigraphy and platform evolution

The Upper Cretaceous succession in the Madenli area (western Central Taurides, Southern Turkey) consists of platform carbonate rocks deposited in entirely peritidal environments, which are sensitive to sea level changes driven by global eustasy, but also strongly affected by local and regional tectonics. It includes economically important bauxite deposits. Previous works suggest different ages for bauxite formation ranging from the Albian to the Santonian. Benthic foraminiferal biostratigraphy and facies analysis of the Madenli and Doğankuzu outcrop sections allow for a more precise dating of the platform emersion periods. The footwall limestones of the bauxite deposits consist of well-bedded limestones (Unit-1), which contain a benthic foraminiferal assemblage (BFA) including mainly Biconcava bentori and Pastrikella biplana, Chrysalidina gradata (BFA I), assigned to the middle-upper Cenomanian. In the Madenli section, the first bauxite deposit occurs in the upper part of Unit-1 as a layer interbedded with pinkish sparitic and dolomitic beds (subunit-1a) deposited in supratidal environment. Subunit-1a is stratigraphically equivalent to the Doğankuzu and Mortaş bauxite deposits considered as karst-related, unconformity-type deposits. The hanging-wall limestones of the bauxite are represented by the massive limestones (Unit-2) starting locally with either the upper Cenomanian characterized mainly by the presence of Pseudolituonella reicheli or upper Campanian comprising mainly Murciella cuvillieri and Moncharmontia apenninica (BFA II). There is no field evidence of a discontinuity surface at the contact between the lower part of Unit-2, including BFA I, and the upper part of Unit-2, including BFA II. This contact is defined as a paraconformity indicating a stratigraphic gap from the Turonian to the early Campanian. The top of Unit-2 is truncated by another discontinuity surface associated with a minor bauxite deposit. The overlying Unit-3 is characterized by well-bedded, rudist-bearing limestones topped by laminated and dolomitized limestones organized in shallowing upward cycles. It is assigned to the upper Maastrichtian based on the presence of Rhapydionina liburnica (BFA III) and rudist assemblage. A third emersion period of the platform corresponds to the early Maastrichtian.

Cretaceous rudist-bearing platform carbonates from the Lycian Nappes (SW Turkey): Rudist associations and depositional setting

Lycian Nappes (in SW Turkey) lie between the Menderes Massif and Bey Dağları carbonates and comprise thrust sheets (nappes piles) of Paleozoic-Cenozoic rocks, ophiolitic and tectonic mélanges and serpentinized peridodites. This study focuses on identification of rudists and their palaeoenvironmental features observed within the Cretaceous low grade metamorphic successions (dominated by recrystallized limestones) from the Tavas and Bodrum nappes. The study is based on fifteen stratigraphic sections measured from Tavas, Fethiye, Köyceğiz, Bodrum, Ören and Bozburun areas. The Lower Cretaceous successions with rudists are very sparse in the Lycian Nappes and a unique locality including a Berriasian epidiceratid-requieniid assemblage is reported so far. A new requieniid-radiolitid assemblage was found within the pre-Turonian (?Albian-?Cenomanian) limestones. Four different Late Cretaceous rudist assemblages were firstly identified as well: 1) Caprinid-Ichthyosarcolitid assemblage (middle-late Cenomanian); 2) Distefanellid assemblage (late Turonian); 3) Hippuritid-Radiolitid assemblage (late Coniacian-Santonian-Campanian); 4) Radiolitid-Hippuritid assemblage (‘middle’-late Maastrichtian). Microfacies data and field observations indicate that the rudists lived in the inner and outer shelves of the Cretaceous carbonate platform(s) in this critical part of the Neotethys Ocean. Rudists formed isolated patchy aggregations in very shallow palaeoenvironments and deposited as shell fragments particularly on the outer shelf environment, which is characterized by higher energy and platform slope characteristics.

The Campanian-Maastrichtian rudist bivalves succession in the Chiapas Central Depression, Mexico

The restudy of the Late Cretaceous rudist fauna of the Chiapas Central Depression, considered Campanian-Maastrichtian with no more precisions in literature, reveals that, in fact, three successive rudist assemblages occur: (1) a lower one, early and middle Campanian, in the uppermost part of the Sierra Madre Formation (distinguished as Suchiapa Formation); (2) a middle one, early Maastrichtian, in the upper part of the Ocozocoautla Formation; and (3) an upper one, late Maastrichtian, in several horizons within the Angostura Formation. Published data on planktic and benthic foraminifers and inoceramid bivalves, as well as new findings of ammonites, helped bracketing the age of the three rudist assemblages. Their stratigraphic position fits with a depositional model assuming a basal carbonate platform fragmented into blocks each following a different tecto-sedimentary evolution: (1) a drowned block constituting the Tuxtla Gutiérrez Basin, deepening towards the neighbor raised block and receiving terrigenous material from the emerged Sierra Madre de Chiapas, that, after a long hiatus, was filled up and, subsequently, a marginal shallow carbonate platform, surrounded by a detritic belt and prograding towards the basin, installed on top; (2) a raised block constituting a high where, after a long hiatus, an insular shallow carbonate platform installed.

Aptian–Albian rudist bivalves (Hippuritida) from the Chilean Central Andes: Their palaeoceanographic significance

The study of Aptian–Albian rudist faunas from the Chilean Central Andes documents the presence of two forms, ascribed to the Monopleuridae: Douvillelia skeltoni an early Aptian species known hitherto from Mexico, and Jerjesia chilensis, an endemic Albian species from Chile and Peru, with a complex taxonomic history. The regional stratigraphy of the Central Andes, combining ammonites and rudists, is consistent with the Caribbean stratigraphic distribution of Jerjesia and Douvillelia. Andean occurrences of the two genera broaden their distributional area on the Pacific side of Americas, and testify their biostratigraphic value. Oceanographic conditions of the Chilean Pacific margin during the Aptian–Albian, including moderate but effective cold oceanic current, upwellings, high productivity and thermal instability, may acknowledge the low taxonomic diversity of rudist assemblages, which look impoverished when compared to their low latitude homologues from the Caribbean regions which were functioning as a dispersal centre.

Response of proto-North Atlantic carbonate-platform ecosystems to OAE1a-related stressors

Integrated biostratigraphic–chemostratigraphic studies provide evidence that the proto-North Atlantic realm witnessed major changes in carbonate platform production in the run-up of the Early Aptian oceanic anoxic event (OAE) 1a. Whereas pervasive growth of Lithocodium microencrusters represents an early harbinger of OAE1a-related environmental perturbation, the subsequent replacement of oligotrophic rudist-coral-nerineid by mesotrophic orbitolinid-oyster communities was clearly associated with the event itself. In order to test the supra-regional relevance of this major community replacement, two shallow-water sections in the southern Lusitanian Basin (Portugal) are investigated by means of geochemistry (carbon and oxygen isotopes), cement petrography and detailed sedimentological analysis. The focus is on a regional, prominent discontinuity surface (S4) at the transition between oligotrophic and mesotrophic carbonate platform production, which might indicate that the major biotic change could have been associated with a phase of non-sedimentation and possibly erosion. The studied sections (São Julião, Crismina) provide evidence that the major Early Aptian biotic turnover was preceded by numerous subordinate but significant changes in platform ecology, which mirrored a series of progressive short-term environmental changes in the course of OAE1. Several transient mass occurrences of orbitolinids indicate repeated phases of ecological stress arguably due to enhanced nutrient input and deepening. Small-scale sea-level changes at parasequence level below the major discontinuity surface are revealed by alternations of rudist assemblages dominated by clinger or recumbent forms as well as intercalated hardground and subaerial exposure stages. Expanded phases of subaerial exposure, however, can be largely ruled out following the geochemical and cement-petrographic data presented here. Enhanced continent-derived siliciclastic input characterising the lower orbitolinid-oyster dominated limestones is in support of a shift to more humid conditions during the middle Early Aptian. This is in line with palaeoclimatic data, which propose a southward movement of the mid-latitude arid climate belt during this time. The documented rapid replacement of oligotrophic assemblages by various environmental-stress adapted carbonate platform communities might be seen as explanation for ongoing Early Aptian proto-North Atlantic carbonate production during a time of widespread platform demise and drowning in the northern Tethyan realm.

Strontium isotope (87Sr/86Sr) stratigraphy from the Coalcomana–Caprinuloidea rudist assemblage in the Greater Antilles (Puerto Rico, Dominican Republic and Jamaica)

The stratigraphic range of the Coalcomana–Caprinuloidea rudist assemblage from Puerto Rico, the Dominican Republic and Jamaica has been revised based on numerical ages derived from strontium isotope (87Sr/86Sr) analyses of the calcitic outer shell of requieniid rudists. The extent of diagenetic alteration was assessed by cathodoluminescence microscopy and trace element concentration analyses (Sr, Mg, Fe and Mn), and only the least altered shells were selected for the 87Sr/86Sr analyses. The 87Sr/86Sr analyses from all the analyzed Antillean rudists resulted in average ages close to the Aptian/Albian boundary (112.56 Ma). These results show that the Antillean Coalcomana–Caprinuloidea rudist assemblage is correlative to the latest Aptian to early Albian Coalcomana interval zone from Texas and Mexico. There are, however, subtle differences in the orbitolinid foraminifer and rudist bivalve records of the Antillean Coalcomana–Caprinuloidea rudist assemblage that still need further revisions. In the Dominican Republic, for example, the orbitolinid fauna in the Central Cordillera include Coskinolinoides texanus (Keijzer), which is reported from middle Albian successions in Texas and Mexico. Moreover, the presence of primitive and advanced morphotypes of Caprinuloidea also suggests a possible bipartite division of the Coalcomana–Caprinuloidea rudist assemblage. Although the collected material from the localities with the foraminifer C. texanus and the advanced form of Caprinuloidea failed the preservation screening for 87Sr/86Sr analyses, we speculate about the possible extension of the Coalcomana–Caprinuloidea rudist assemblage from the Aptian/Albian boundary through the middle Albian.

Stratigraphy of the mid-Cretaceous shallow-water limestones of the Apulia Carbonate Platform (Murge, Apulia, southern Italy)

The geological mapping of the 438 Sheet «Bari» (scale 1:50.000), located in the northern part of the Murge area (Apulia, southern Italy), offered the opportunity to revise the local stratigraphy of the mid-Cretaceous carbonate succession belonging to the Calcare di Bari Fm. Despite the flat topography, the strong urbanization and a widespread cover of «terra rossa» soils prevented the study of a continuous and undisturbed succession, several fragmentary lithostratigraphic and biostratigraphic data have been collected and correlated along many stratigraphic sections. This correlation allowed us to reconstruct a 470 m-thick composite section mostly made up of mud-supported shallow-water limestones and dolomites. This thickness is considerably lower than about 1000 m estimated before in the same area for the same time interval. In agreement with the previous edition of the Geologic Map of Italy few stratigraphic intervals showing a rich content in rudist assemblages and an interval of dolomitic breccias have been used as reference layers and successfully used for lithostratigraphic correlations. New biostratigraphic data show the first record of «primitive» orbitolinid assemblages and other important index taxa which refer the lower and the middle part of the studied succession to the Albian in disagreement with the previous attribution to the Cenomanian.

Maastrichtian Rudist Fauna from Tarbur Formation (Zagros Region, SW Iran): Preliminary Observations

The uppermost Cretaceous Tarbur Formation of the Zagros region (SW Iran) is mainly siliciclastic in composition, though it also incorporates some carbonate units including several rudist lithosomes. Two sections through this formation, in the Semirom and Gerdbisheh areas, have been chosen for study of the lithosomes and their rudist fauna. These lithosomes vary in faunal content, geometry and internal organization (density and diversity). Preliminary investigation of the specimens collected from the studied sections reveals a diverse rudist fauna. Eleven genera and 23 species have been determined, belonging to the rudist families Hippuritidae, Radiolitidae and Dictyoptychidae. These rudist assemblages indicate a Maastrichtian age for the Tarbur Formation in these areas. With regard to their growth geometries, most of the specimens are of elevator rudist morphotype, forming many different associations (e.g., bouquets and clusters). Comparison between the present rudist fauna, particularly taxa considered endemic to this part of the Mediterranean province, with the Late Cretaceous fauna recorded from other parts of the Zagros, Turkey and South of the Persian Gulf (Oman and UAE) show similarities that confirm the faunal connection between them.

Late Campanian Rudist Assemblages and Biometrical Analysis of Pseudopolyconites from Bačevica (Eastern Serbia)

The lithological and faunal succession cropping out close to the famous palaeontological locality of Bacevica (Eastern Serbia) is described along a very discontinuous and ill-exposed section. Since the section is dominated by clastic sediments, no-vestige of the so-called 'Vrbovac reef' has been observed. Rhythms consisting of a limestone breccia lower unit and a fossil-rich upper part characterize the base of the section while rare silt and sand-rich limestone breccias and prevailing silty/sandy soils are the lithologies observed in the upper part of the same. The faunal succession shows an alternation of monospecific-paucispecific assemblages and much more diverse assemblages of rudists. The stratigraphic distribution of the different examples of rudist bivalves recovered at Bacevica may represent a significant tool for biostratigraphic correlations as far as the late Campanian deposits in the Central Tethys area are concerned. In particular, the Pseudopolyconites-bearing strata seem to be included within a few fossiliferous lithosomes. Therein the Serbian rudistologists instituted twenty species of Pseudopolyconites on the base of the characteristics of their ligamental ridge. Biometrical analyses have been made on the transverse sections of the right valves of the Pseudopolyconites holotypes illustrated in the scientific literature. It is suggested that the wide variability of the ligamental ridge shape alone does not warrant the institution of so many species of the genus in question but the existing ones should probably be re-considered as simple eco-morphotypes.

Cenomanian - Turonian rudist assemblages and sequence stratigraphy on the North Sinai carbonate shelf, Egypt

ABSTRACT The Middle to Upper Cenomanian Halal and Turonian Wata formations crop out at Gabal Minsherah, Gabal Yelleg and Gabal Maaza in the northern Sinai Peninsula, Egypt. This paper describes the rudist assemblages of the two formations in a sequence stratigraphic framework. Tabular thickets of conical to cylindrical elevator radiolitid rudists were the most common benthic type on the northern Cenomanian – Turonian Sinai shelf. The thickets were either single beds or multiple beds intercalated with marl or packstone. Most biostromes were laterally restricted and do not extend to nearby outcrops. The Cenomanian rudist biostromes are thin and composed mainly of several species of Radiolitidae and less common Caprinidae together with coralline sponges and benthic foraminifers. The Turonian rudist biostromes are composed of several species of Radiolitidae. The Cenomanian Halal Formation grades upward from basal quartz arenite into bioclastic-oolitic grainstone overlain by bioclastic packstone and wackestone. Dolomite beds are part of the highstand facies. The Formation is composed of transgressive-regressive Halal Sequences 1 and 2. The base of the overlying Wata Formation is Sequence Boundary 3 at the base of Wata Sequence 3, which is Lower to Middle Turonian. The sequence boundary between the Halal and the Wata is marl-on-marl with Upper Cenomanian ammonites overlain by a condensed interval with ammonites of several Lower Turonian zones that are particularly well exposed at Gabal Minsherah. The boundary is interpreted by graphic correlation as a million-year-long hiatus after which carbonate deposition resumed and low-diversity rudist assemblages recovered. The hiatus is correlated to the global anoxic event OAE 2. Following OAE 2 carbonate deposition resumed and produced microfacies similar to those of the Cenomanian. The Upper Turonian Wata Formation is composed of Wata Sequences 4 and 5.

Systematic paleontology, biostratigraphy, paleobiogeography of Loftusia (foraminifera) and Rudist assemblages in a regressive sequence in the Hekimhan-Malatya area (Eastern Anatolia) Turkey

The Maastrichtian regressive sequence of the Hekimhan-Malatya area in Eastern Turkey consists of dolomitic limestones, limestones and calcareous mudstones which are dominated by rudists and Loftusia (foraminifera) assemblages. Several Loftusia species indicating middle to late Maastrichtian age such as Loftusia anatolica Meric, L. baykali Meric, L. coxi Henson, L. harrisoni Cox, L. minor Cox and L. morgani Douville have been recorded. Other benthic foraminifers present include Orbitoides medius d’ Archiac, Omphalocyclus macroporus (Lamarck), Laffitteina conica Drooger, Laffitteina mengaudi (Astre) and Laffitteina oeztuerki Inan. The rudists are abundant in the dolomitic limestones in the study area. The genus Miseia Patrulius is dominant and represented by Miseia bilacunosa Ozer and Miseia hekimhanensis Karacabey-Oztemur. The Loftusia species and the Rudists assemblage indicates middle to late Maastrichtian age for the formation. The paleobiogeographic distribution of the assemblage has been discussed taking this find into account.