First Common Occurrence Research Articles

Sequence stratigraphy and reservoir quality of the Gulf of Suez syn-rift deposits of the Nukhul formation: Implications of rift initiation and the impact of eustacy and tectonic on deposition

The Nukhul Formation represents the sediments deposited during the earliest phase of the syn-rift period in the Gulf of Suez rift. It is the primary hydrocarbon reservoir for fifteen fields in the region. However, the timing of rifting needs to be better constrained, and the influence of local tectonics and global eustatic sea-level fluctuations on reservoir quality and deposition is still being determined. This research aims to establish a stratigraphic sequence framework, determine the age of the Suez Rift commencement, use changes in paleobathymetry, and assess reservoir quality. The methods employed involve the analysis of four boreholes in the Abu Rudeis-Sidri field, incorporating biostratigraphy, well-logs, and 3D seismic data. The integrated paleo-structure and biostratigraphic analyses indicate that the beginning of rifting in the Gulf of Suez occurred at 23.53 Ma at the first common occurrence (FCO) of Trilobatus primordius. The paleobathymetric analysis of the Nukhul Formation reveals deeper paleo-water depths during certain intervals, indicating deposition during a phase of high subsidence and rift-climax. The Nukhul stratigraphic sequence reflects sea-level changes leading to shifts in accommodation, and facies changes, erosion, and sequence formation during the late Chattian and early Burdigalian stages. However, the reservoir characterization of the Nukhul Formation is significantly affected by various factors, including cementation, pore system properties, sediment sorting, rift system geometry, and relative sea-level fluctuations. These factors must be carefully considered in any regional exploration or development efforts. The study also reveals that tectonic activity played a role in forming fault-bounded basins, and eustatic sea-level changes influenced the rate and timing of sediment accumulation. These findings can aid in predicting the distribution of the Nukhul sedimentary deposits and improve the ability to explore and develop their resources.

Integrated Quantitative Calcareous Plankton Bio-Magnetostratigraphy of the Early Miocene from IODP Leg 342, Hole U1406A, Newfoundland Ridge, NW Atlantic Ocean

High-resolution quantitative analyses of the planktonic foraminifera and calcareous nannofossil content have been carried out on IODP Leg 342, Hole U1406A (Northwest Atlantic Ocean) in the core interval 10H2–2H4, plotting the abundance distributions of the biostratigraphically and quantitatively most significant species. Qualitative analyses have been used to identify the precise depth of marker bioevents. For almost the whole succession, relevant magnetostratigraphic data were provided by previous studies. All chrons and subchrons in the interval C6Cn–C6AAr were recognised in core interval 10H2–4H1 and calibrated to the GPTS 2012, but no magnetostratigraphic data are available for the highest portion (above 35 m). The investigated succession falls between Sphenolithus delphix FO (First Occurrence)—uppermost Chattian bioevent—in core 10H2 and the Sphenolithus belemnos LO (Last Occurrence)–Burdigalian bioevent – in core 2H5, embracing the Zone O7 (uppermost part)–M3 foraminiferal zone interval and the Zone CNO6–CNM5 calcareous nannofossil zone interval. A calcareous plankton biochronological framework has been produced integrating calcareous plankton and magnetostratigraphic data. Calculated ages for the most significant recognised bioevents are S. delphix FO 23.35 Ma, S. delphix LO 23.02 Ma, Paragloborotalia kugleri FO 23.00 Ma, Globoquadrina dehiscens FO 22.59 Ma, Sphenolitus disbelemnos FO 22.56 Ma, Helicosphaera carteri FCO (First Common Occurrence) 22.18 Ma, P. kugleri LO 21.23 Ma. Two hiatuses have been recognised within core 4H, whereas only one was recognised in previous studies. The first one in core 4H5 spans the interval 21.17–19.01 Ma and coincides with a phosphate-rich layer; the second in core 4H2 spans the interval from 18.87 to 18.21 Ma and corresponds to a glauconite-rich layer. The results obtained from Hole U1406A allow some considerations about potential reference bioevents concerning the still open issue of Burdigalian GSSP (Global Stratotype Section and Point).

New insights in late Miocene lower Chelif basin biostratigraphy based on planktonic foraminifera (Algeria)

Quantitative and qualitative analyses based on planktonic foraminifera of two sections (Oued Derdoussa and Djebel Meni) from lower Chelif basin (northern Algeria) enable us to identify for the first time a sequence of bioevents calibrated with the geomagnetic polarity time scale. The identified bioevents of late Miocene formations are useful for a high-resolution correlation in the whole western Mediterranean at local and regional scales. In particular, this work reveals that the sequence covers an interval of time that extends from the upper Tortonian up to the pre-evaporite Messinian period. Indeed, the base of the succession starts with a change in the coiling direction (from dextral to sinistral) of Neogloboquadrina acostaensis as the first bioevent. This later corresponds to the bioevent (1–8) assigned to the Tortonian sediments. This period is marked by the presence of Globorotalia menardii in sinistral coiled form that was substituted gradually with dextral coiled. The Tortonian/Messinian (T/M) boundary coincides to some extent with a sharp replacement of G. menardii group (I and II) with the first common occurrence (FCO) of Globorotalia miotumida plexus few meters below the development of the Tripoli diatomite formation (bioevent 9). This later is dominated mainly by highly convex species (G. miotumida plexus) marked by the presence of Globorotalia mediterranea in contrast with the marly formation at the base of the section. In this part, the recorded bioevents (10 to 17) are assigned to the Messinian sediments. Thus, the Tripoli formation shows the influx of Globorotalia nicolae and a small incursion of the dextrally coiled of Globorotalia scitula at the top of Oued Derdoussa section as a local bioevent that may be related to the local palaeoenvironment. We notice the absence of the change in the coiling from sinistral to dextral of N. acostaensis during the upper Messinian, this may be due to the coincidence of this bioevent with the barren levels.

The barren Messinian Tripoli in Sicily and its palaeoenvironmental evolution: suggestions on the exploration potential

New observations on the Sicily Messinian Tripoli have yielded a variably thick diatomitic, calcareous and shaly rock interval marked by an upwards disappearance of calcareous and siliceous plankton (barren lithosome), coexisting with a variability in vegetal remains and significant amounts of amorphous organic matter (AOM). Facies analysis associated with biostratigraphy and palaeoecology of the several field and borehole sections has been framed in a well-accepted chronological scheme that points to this barren interval coinciding with the stratigraphic upper and younger part of some Tripoli sections (bituminous Tripoli) located in the northern part of the study area. Biostratigraphically, the barren lithosome falls within the Non-Distinctive Zone (NDZ) and is generally younger than the First Common Occurrence (FCO) of T . multiloba . The age of its diachronous base appears older than the Messinian salinity crisis (MSC) deposits, because is well constrained by the calcareous plankton biostratigraphic markers along the underlying portions of the Tripoli. The top boundary of the barren rock interval, that is not always identifiable, could be age dated, following the proposed stratigraphic criteria. We infer that the Tripoli initial open-marine environment became, in more inner areas, progressively confined, with freshwater floodings alternating with short marine-water incursions in a shallow-water to continental domain. The collected information appears useful in addressing further exploration.

The Pliocene Mediterranean infilling of the Messinian Erosional Surface: New biostratigraphic data based on calcareous nannofossils (Bajo Segura Basin, SE Spain)

The Bajo Segura Basin (eastern Betic Cordillera) is a Mediterranean marginal basin where the Messinian Erosional Surface (MES), formed during the Messinian Salinity Crisis sea-level fall, is well developed. Overlying this major discontinuity the lower Pliocene transgressive sediments record the reflooding of the Mediterranean and the return to an open marine environment, the continental shelf being rebuilt after the Messinian erosion. The stratigraphic and biostratigraphic study of seven sections allow two transgressive-regressive sequences filling the MES to be distinguished, correlated with the previously distinguished Mediterranean offshore seismic units. Ten calcareous nannofossil bioevents have been identified. The lower sequence can be dated according to nannofossil biozones NN12 to NN14 and the upper sequence by NN15 to NN16. The boundary between both lower Pliocene sedimentary sequences is dated by the First Common Occurrence (FCO) of Discoaster asymmetricus in the uppermost sediment of the lower sequence and the First Occurrence (FO) of Discoaster tamalis in the lowermost part of the upper sequence. This discontinuity can be calibrated at between 4.1 and 4.0 Ma ago.

Integrated stratigraphy of the St. Thomas section (Malta Island): A reference section for the lower Burdigalian of the Mediterranean Region

Abstract The 91.15 m thick St. Thomas section belongs to the Middle Globigerina Limestone, which is the intermediate member of the Globigerina Limestone Formation, and crops out along the eastern cliff of the Delimara Peninsula (the south-eastern part of Malta Island). The sedimentary record is composed of alternating calcareous marls and marly limestones with subordinated prominent bioturbated indurated limestones that are deposited in a pelagic environment. For the first time paleomagnetic analyses of a Burdigalian succession have provided reliable data that allow the identification of five magnetozones that have been correlated to the Astronomically Tuned Neogene Time Scale, spanning the Early Miocene C6n–C5Dn interval. Calcareous plankton quantitative analyses integrated with paleomagnetic analyses, allowed us to identify and date several bioevents that have great potential for Mediterranean and extra-Mediterranean correlations in the interval between ~ 19.7 and ~ 17.2 Ma. In particular, the complete distributional range of the calcareous nannofossil Sphenolithus belemnos has been recorded as follows: the First Occurrence (FO) at 19.12 Ma, the First Common Occurrence (FCO) at 19.05 Ma, the Last Common Occurrence (LCO) at 18.44 Ma, and the Last Occurrence (LO) at 18.02 Ma. In addition, the FO and the FCO of Sphenolithus heteromorphus have been documented at 18.29 Ma and at 17.99 Ma, respectively. A new paracme interval in the lower part of the range of this species is described between 17.56 and 17.31 Ma. Concerning the planktonic Foraminifera, the main bioevents are the Common Interval top of Paragloborotalia siakensis at 19.55 Ma, a Common Interval of Globoquadrina dehiscens between 19.34 and 18.48 Ma, the Globigerinoides subquadratus FO at 18.43 Ma, and the onset of an acme interval of Paragloborotalia acrostoma that changes its coiling from random to prevalently sinistral at the same time, at 18.40 Ma. These new biostratigraphic data allowed us to place the succession in the standard Mediterranean calcareous plankton zonal schemes and to make some amendments to these schemes. The FCO of S. belemnos was revealed a more reliable marker than its FO for the base of the MNN3a Zone. With regard to the planktonic Foraminifera, the P. acrostoma AB-r/s has been used as a subzonal marker of the G. dehiscens / Catapsydrax dissimilis Zone, which now comprises three subzones instead of two. The deep marine paleoenvironmental setting, the excellent outcrops and the recognition of a continuous succession of bio-magnetostratigraphic events suggest that the St. Thomas section should be considered as a reference section for the lower Burdigalian of the Mediterranean area and for future studies in the definition of the Burdigalian GSSP.

Biostratigraphic and palaeoenvironmental analyses of the Messinian/Zanclean boundary and Zanclean succession in the Moncucco quarry (Piedmont, northwestern Italy)

Abstract Integrated biostratigraphic and palaeoenvironmental analyses of foraminiferal, ostracod and calcareous nannofossil assemblages were carried out on the Messinian/Zanclean succession of the Moncucco quarry (Piedmont, northwestern Italy). The uppermost Messinian deposits may be correlated to the “Lago-Mare” deposits of the Mediterranean area. They yield a brackish ostracod assemblage, which belongs to the Loxocorniculina djafarovi zone of the Messinian post-evaporitic interval, indicating their deposition in oligo-mesohaline shallow waters and the influx of Paratethyan faunas. Only reworked marine microfossils are present. The boundary between the “Lago-Mare” deposits and the overlying Zanclean Argille Azzurre Fm. is marked by an irregular erosional surface, followed by a barren, 10–50 cm thick, black arenitic layer. The Zanclean succession consists of grey to whitish clayey marls (Argille Azzurre Fm.), rich in microfossils, which document the MPl1 (Sphaeroidinellopsis acme) zone up to the MPl4a foraminiferal zone, and the MNN12 to MNN14-15 calcareous nannofossil zones. Micropalaeontological data confirm that the Pliocene flooding event was synchronous at the Mediterranean scale. Some foraminiferal bioevents recognized at the Mediterranean scale and related to precessional cycles are recorded in the studied succession : the Globorotalia scitula common occurrence (CO) (cycle 3), the occurrence of Siphonina reticulata (cycle 6), and the Globorotalia margaritae first common occurrence (FCO) (cycle 10). A short hiatus could be inferred in the lowermost Zanclean sediments for the absence of Triquetrorhabdulus rugosus and of the Reticulofenestra pseudoumbilicus paracme. Hiatuses or reduced sedimentation could also have affected the bottom during the MPl2-MPl3 zones. Foraminiferal and ostracod assemblages document a rather deep basin just from the lower Zanclean, warm surficial waters (common Globigerinoides spp.), rather high seasonal primary productivity (common phytoplankton grazer Neogloboquadrina acostaensis) and mesotrophic bottom (common Eponides pusillus, Hoeglundina elegans, Argilloecia acuminata, Krithe compressa, Paijenborchella iocosa). Bottom depths in the lower Zanclean of northwestern Italy appear to be shallower than depths suggested by coeval southernmost assemblages. A deepening of the basin during the MPl1 and MPl2 zones is suggested by the increasing diversity of microfossils and by the spreading of deep bathyal taxa (Cibicidoides robertsonianus, Paijenborchella malaiensis cymbula). Warm, oligotrophic conditions in the water column become more constant. Palaeoenvironmental conditions change progressively during the upper MPl2, the MPl3 and MPl4a zones. Cold, eutrophic planktonic foraminifers (Globigerina bulloides, N. acostaensis) become dominant, as well as benthic taxa tolerant of low oxygen content and high organic matter (Uvigerina peregrina, Brizalina spp.), suggesting an increase in eutrophic conditions. Deep benthics such as Cibicidoides kullenbergi, C. robertsonianus disappear, and many outer neritic to bathyal taxa decrease in abundance, whereas inner shelf taxa increase, suggesting a shallowing of the basin and episodes of strong winnowing, probably tectonically controlled, in the uppermost Zanclean succession.

Morphologic variability of the Late Oligocene and the Early Miocene elliptical reticulofenestrids from the Central Paratethys: biostratigraphical, paleoecological and paleogeographical implications

The analysis of size changes (length of placoliths, their width, length of central opening and its width) in elliptical reticulofenestrids from the NP25-NN5 zonal interval of the Central Paratethys allowed to dicriminate two size categories of placoliths: (i) small Reticulofenestra minuta ( 8.0 iðehas been established in the Zone NN2 while its FCO (first common occurrence) in the Zone NN5. This study shows that the FOs of sizedefined morphotypes of the R. haqii-pseuduumbilicus group differ in the Central Paratethys and oceanic realm. Blooms of R. minuta at the Oligocene-Miocene boundary and in the Early Middle Miocene may be correlated with the incoming of warm water into the higher latitude Central Paratethys basins during connection wit...

Micropaleontological study on the Miocene calcareous turbidite deposits of Faeto and Tufillo formations (eastern sector of the Southern Apennine chain)

A biostratigraphic study was carried out on four sections of the Miocene Tufillo and Faeto Formations (Southern Apennines). The sediments analyzed were referred to the late Burdigalian-early Tortonian on the basis of planktonic foraminifers (MMi2b Subzone through the MMi8 Zone) and calcareous nannofossils (MNN3b Zone through the MNN7 Zone). Almost the majority of the bioevents of the recently published Miocene biozonal schemes of Mediterranean area were identified. Particularly, the marked changes in the abundance pattern of Paragloborotalia siakensis resulted very useful to correlate the sediments. A new acme abundance of the letter species has been recorded in the uppermost part of the Burdigalian planktonic foraminiferal MMi2b Subzone. Moreover, the integrated calcareous plankton biostratigraphy revealed that the First Common Occurrence (FCO) of Sphenolitus heteromorphus occurs before the Last Occurrence (LO) of Catapsidrax dissimilis , in the upper part of Burdigalian record. This study indicates that the lower Langhian portion of the Tufillo Formation is characterized by arkose sandstones which pass upwards into calcarenites and/or calcirudites and marly deposits. Field data suggest that these sandstones directly overly the Numidian Sandstones. The Faeto Formation starts with calcarenites and calcareous marls, in the calcareous nannofossil MNN3b Zone (Burdigalian), and lies on the Numidian Sandstones as well. High-resolution sampling and biostratigraphy reveal that the deposition of the Numidian Sandstones ends in the late Burdigalian stage, just below the FCO of S. heteromorphus (MNN3b Zone) and the LO of C. dissimilis (MMi2b Subzone).

EARLY TO LATE OLIGOCENE CALCAREOUS NANNOFOSSIL BIOEVENTS IN THE MEDITERRANEAN (UMBRIA-MARCHE BASIN, CENTRAL ITALY)

Calcareous nannofossil assemblages have been investigated by means of quantitative analyses in three Oligocene pelagic sections located in the Umbria-Marche Apennines (Central Italy). The studied sections mainly consist of marly limestones and marls belonging to the Scaglia Cinerea Formation, and include the interval between NP23 and NP25 representing a time interval of about 3.5 Ma. Biostratigraphic resolution is extremely low and only two standard bioevents are known, which are the FO of Sphenolithus ciperoensis and the LO of Sphenolithus distentus . The distribution patterns of poorly known or recently described calcareous nannofossils provided a valuable tool for improving the current biostratigraphic framework. The studied interval is characterized by significant changes in the calcareous nannofossil assemblages and by several extinction events. The last occurrence (LO) and/or the last common occurrence (LCO) here proposed are: the LO of Sphenolithus akropodus , the LO of Reticulofenestra circus , the LCO of Helicosphaera ethologa , the LCO of Helicosphaera compacta the LO of Discoaster tanii nodifer . The reversal in abundance between Sphenolithus predistentus and S. distentus provided an additional biostratigraphic constraint at the NP23-NP24 transition. In addition biometric criteria enabled the recognition of the first common occurrence (FCO) of Cyclicargolithus abisectus > 12 mm as potential bioevent within NP24. The quantitative distribution of Sphenolithus distentus suggests to rely on the LCO of the species rather than on the LO, for the identification of NP24-NP25 boundary. The identified bioevents is a first step towards the improvement of the present Mediterranean biostratigraphic framework of the Oligocene geological record. A preliminary correlation of the bioevents to the Geomagnetic Polarity Time Scale is presented.

Calcareous nannofossil bioevents and environmental control on temporal and spatial patterns at the early–middle Pleistocene

Quantitative distributions of calcareous nannofossils are analysed in the early–middle Pleistocene at the small Gephyrocapsa and Pseudoemiliania lacunosa zone transition in deep-sea cores from the Mediterranean Sea and North Atlantic Ocean (Ocean Drilling Program [ODP] Sites 977, 964 and 967, Deep Sea Drilling Project [DSDP] Site 607). The temporal and spatial mode of occurrence of medium-sized gephyrocapsids and reticulofenestrids has been examined to refine biostratigraphic constraints and evaluate possible relationships of stratigraphic patterns to environmental changes during a period of global climatic deterioration. The timing of bioevents has been calibrated using high-resolution sampling and correlation to the δ 18O record in chronologically well-constrained sections. Newly identified events and ecostratigraphical signals enhance the stratigraphic resolution at the early–middle Pleistocene. The first occurrence (FO) of intermediate morphotypes between Pseudoemiliania and Reticulofenestra ( Reticulofenestra sp.) is proposed as a reliable event within marine isotope stage (MIS) 35 or at the MIS 35/34 transition. The distribution of Reticulofenestra asanoi is characterized by rare and scattered occurrences in its lowest range, but the first common occurrence (FCO) is consistently identified at MIS 32 or 32/31; the last common occurrence (LCO) of the species is a distinctive event at MIS 23. In the studied interval, Gephyrocapsa omega dominates among medium-sized Gephyrocapsa. The FO of G. omega and contemporaneous re-entry of medium-sized gephyrocapsids at the lower–middle Pleistocene transition are diachronous between the Atlantic Ocean and Mediterranean Sea and from the western to eastern Mediterranean. In the Mediterranean, the LO of G. omega falls at MIS 15, insolation cycle 54 and is isochronous among the sites. Abundance fluctuations of G. omega show notable relations to early–middle Pleistocene climate changes; they considerably increase in abundance at the interglacial stages, suggesting warm water preferences. Gephyrocapsa omega temporarily disappears during the glacial MIS 22 and MIS 20. Above MIS 20, an impoverishment in G. omega and in the total abundance of medium-sized gephyrocapsids occurs. A decrease in abundance of G. omega is observed between the western Site 977 and the easternmost Site 967 in the Mediterranean Sea, as a possible response to high salinity and/or low nutrient content. Possible environmental influences on the distribution of R. asanoi and of Reticulofenestra sp. are discussed.

CALCAREOUS NANNOFOSSIL EVENTS IN THE LOWER-MIDDLE PLEISTOCENE TRANSITION AT THE MONTALBANO JONICO SECTION AND ODP SITE 964: CALIBRATION WITH ISOTOPE AND SAPROPEL STRATIGRAPHY

We present quantitative data on calcareous nannofossil biostratigraphy in the section of Montalbano Jonico (Southern Italy). This is one of the candidate Global Statotype Section and Point (GSSP) of the Middle Pleistocene. The first common occurrence (FCO) and last common occurrence (LCO) of Reticulofenestra asanoi are well detectable in the section and are here proposed as additional events for improving biostratigraphic resolution in the interval corresponding to the transition between the small Gephyrocapsa Zone and Pseudoemiliania lacunosa Zone. The potential value of the bioevents is tested in a Mediterranean deep-sea core of ODP Site 964, located in the Ionian Sea. At Site 964 the FCO of R. asanoi occurs in the marine oxygen isotope stage (MIS) 30, and the LCO of the species correlates with MIS 23. The ages of the FCO and LCO of R. asanoi have been estimated at 1.06 Ma and 0.93 Ma respectively. The first occurrence (FO) of Gephyrocapsa sp. 3 has been correlated to MIS 25, both at Site 964 and at the Montalbano Jonico section. The age of the FO of Gephyrocapsa sp. 3 is estimated at 0.95 Ma, based on the astronomical datings of sapropels occurring at the Montalbano Jonico section. In addition, an interval of temporary disappearance of Gephyrocapsa sp. 3 has been identified within the P. lacunosa Zone. The beginning of this interval is correlated with MIS 20/19 and can be considered a reliable event in the Mediterranean area, occurring close to the Brunhes/Matuyama magnetic boundary. The recognized events provide a basis for interpreting the Montalbano Jonico section within a standard chronostratigraphic framework.

CALCAREOUS PLANKTON HIGH RESOLUTION BIOSTRATIGRAPHY (FORAMINIFERA AND NANNOFOSSILS) OF THE UPPERMOST LANGHIAN – LOWER SERRAVALLIANRAS IL-PELLEGRIN SECTION (MALTA)

Results of an integrated biostratigraphic (calcareous nannofossils and planktonic foraminifers) study of the uppermost Langhian/lower Serravallian Ras il-Pellegrin section (Fomm ir-Rih Bay – Malta) are presented. This high resolution study allowed us to recognise several useful lower Serravallian bio-events in the Mediterranean and to provide a detailed distribution pattern of the recognised taxa. The astrochronological calibration, reported in a different paper of this volume, provided absolute ages of the bio-events of the studied section. The LO (Last Occurrence) of Globorotalia peripheroronda at 13.39 Ma, the Paragloborotalia partimlabiata FO (First Occurrence) at 12.62 Ma and the Paragloborotalia mayeri FCO (First Common Occurrence) at 12.34 Ma among the planktonic foraminifera, and the Sphenolithus heteromorphus LO at 13.59 Ma, the Cyclicargolithus floridanus LCO (Last Common Occurrence) at 13.39 Ma, the Reticulofenestra pseudoumbilicus FCO at 13.32 Ma, the Calcidiscus macintyrei FO at 12.57 Ma and the Calcidiscus premacintyrei LCO at 12.51 Ma among the calcareous nannofossils, were recorded. Moreover, our results confirm the LO of S. heteromorphus as a fairly synchronous event in the Mediterranean and in the Atlantic and Pacific Oceans. Therefore, we suggest a lithological level near or coincident with this event for the definition of the GSSP of the Serravallian. The Ras il-Pellegrin section may be considered a good candidate for this definition.

Calibration of Pliensbachian calcareous nannofossil events in two ammonite-controlled sections from northern Spain (Basque-Cantabrian area)

Common and diverse calcareous nannofossil assemblages were found in two continuous and expanded successions, sedimented on hemipelagic carbonate ramp. Based on refined ammonite biostratigraphy, closely spaced sampling and semiquantitative analysis, the main purpose of this paper is to calibrate against ammonite zones and subzones, the nannofossil composition changes and the identified biohorizons. In the study area, the first notable composition change is in the lower Carixian, with the appearance and abundance increase of the genus Similiscutum, that represents a large amount of the assemblages up to the lower Domerian. Although, the appearance of the genera Biscutum and Bussonius is easily detectable around the Carixian/Domerian boundary, the other relevant Pliensbachian composition changes took place in the lower Domerian. It includes the sharp increase in abundance of the genus Calcivascularis and the appearance of the genus Lotharingius , that dominate the upper Domerian assemblages, also characterized by the significant presence of the genus Biscutum . The set of easily recognizable and reliable events includes the First Occurrences (FOs) of Similiscutum avitum and Similiscutum cruciulus (Jamesoni Zone), the FOs of Lotharingius barozii and Lotharingius hauffii (Stokesi/Margaritatus Zone boundary), the First Common Occurrence (FCO) of Lotharingius hauffii (Margaritatus/Spinatum Zone boundary) and, probably, the FCO of Calcivascularis jansae (Stokesi/Margaritatus Zone boundary). Instead, the FOs of Biscutum dubium, Bussonius prinsii (Davoei Zone), Biscutum novum, Biscutum grande and Biscutum finchii (Stokesi Zone) are slightly difficult to calibrate against ammonite zones.