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Late Cenomanian.Turonian (Cretaceous) ammonites from Wadi Qena, central Eastern Desert, Egypt : taxonomy, biostratigraphy and palaeobiogeographic implications

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Nagm E. , Wilmsen M.

Acta Geologica Polonica

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2012

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tom Vol. 62, no. 1

63-89

EN

In Egypt, marine Upper Cenomanian.Turonian strata are well exposed in the Eastern Desert. The southernmost outcrops are located in the central part of Wadi Qena, where the lower Upper Cretaceous is represented by the fossiliferous Galala and Umm Omeiyid formations. From these strata, numerous ammonites have been collected bed-by-bed and 13 taxa have been identified, which are systematically described herein. Four of them (Euomphaloceras costatum, Vascoceras globosum globosum, Thomasites gongilensis and Pseudotissotia nigeriensis) are recorded from Egypt for the first time. The ammonite ranges are used for a biostratigraphic zonation of the lower Upper Cretaceous succession in the northern and central part of Wadi Qena: the Upper Cenomanian.Lower Turonian has been subdivided into five biozones (including a new upper Lower Turonian biozone based on the occurrence of Pseudotissotia nigeriensis), and one biozone has been recognized in the Upper Turonian. Palaeobiogeographically, the ammonite assemblage has a Tethyan character. During the Early Turonian, influences of the Vascoceratid Province were predominant with strong affinities to typical Nigerian faunas. This shows the significance of faunal exchange between Egypt and Central and West Africa via the Trans-Saharan Seaway. Compared to contemporaneous ammonoid faunas from the northern part of the Eastern Desert, Boreal influences are much less obvious in Wadi Qena. Thus, the present study greatly enhances the knowledge of the Late Cretaceous palaeobiogeography and biostratigraphy of Egypt and adjacent areas.

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Palaeobiology and evolutionary context of Angulithes mermeti, a streamlined early Late Cretaceous shallow - water nautiloid

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Wilmsen M. , Nagm E.

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tom 64

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nr 4

EN

Cretaceous nautiloids are commonly characterized by inflated shells and prolonged stratigraphic ranges. In the Albian, the species of Angulithes appeared and compressed, short-lived forms with narrow venters emerged during the Cenomanian age. Based on a new description, the late Cenomanian nautiloid Angulithes mermeti is discussed with its palaeobiological background and placed in an evolutionary context of the Cenomanian lineage of Angulithes, considering contemporaneous palaeoenvironmental changes and inferred functional traits. A. mermeti is characterized by a nearly oxycone shell with sharp venter and narrow umbilicus, a fairly sinuous suture, low inter-septal distances, and an almost dorsal siphuncle. Its palaeobiogeographical occurrence was latitudinally restricted to shallow tropical–subtropical shelf seas with a preferred habitat depth between 5–50 m. Several morphological trends reflected by the Cenomanian species of the genus culminated in the late Cenomanian species A. mermeti, i.e., (i) increasing shell compression and sharpening of the venter, (ii) increasing folding of the septa, (iii) reduction of inter-septal distances, and (iv) dorsally directed migration of the siphuncle. The hydrodynamically efficient form was favorable to successfully populate the wide and shallow epicontinental seas that formed during the Cenomanian age. The increasing sutural sinuosity and the dense septal spacing aimed to buttress the shells against shell-breaking predators while the functional reason for the dorsal- directed migration of siphuncle is more elusive; it may have improved the efficiency of the hydrostatic apparatus and its internal position is beneficial in the case of predation, too. The gradual morphological change in the Cenomanian lineage of the genus Angulithes provides a well-constrained case study of rapid evolutionary response to major environmental pressure, i.e., the opening of newly available niches in the course of the great early Late Cretaceous transgression, in an otherwise rather bradytelic biotic group.

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Cenomanian Acanthoceratoidea (Cretaceous Ammonoidea) from the Koppeh Dagh, NE Iran : taxonomy and stratigraphic implications

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Mosavinia A. , Wilmsen M.

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tom Vol. 61, no. 2

175-192

EN

Seven species of the acanthoceratoidean genera Forbesiceras Kossmat, 1897, Mantelliceras Hyatt, 1903, Acanthoceras Neumayr, 1875 and Cunningtoniceras Collignon, 1937 are described and illustrated from the upper (i.e., Cenomanian) part of the Aitamir Formation of the Koppeh Dagh, northeast Iran. The mantelliceratines were collected from Lower Cenomanian silty shales while the rest of the fauna stems from lower Middle Cenomanian glauconitic sandstones in the upper part of the formation. The ammonite association allows recognition of the lower Lower Cenomanian Mantelliceras mantelli and the lower Middle Cenomanian Acanthoceras rhotomagense zones. The upper Lower Cenomanian M. dixoni Zone is not proven by its index but is most likely represented by a unit of fossil-poor shales intercalated between the two above-mentioned zones. The lowermost Middle Cenomanian Cunningtoniceras inerme Zone, however, is potentially at least partly missing due to a major sea-level fall and lowstand in the latest Early to earliest Middle Cenomanian. A preliminary sequence stratigraphic interpretation of the successions suggests the presence of Lower Cenomanian sequence boundaries Sb Ce 1-3. The Aitamir Formation is truncated along a major regional unconformity at the base of the overlying Abderaz Formation (Turonian.Coniacian). The Upper Cenomanian and most likely also (parts of) the Lower Turonian are missing. This major unconformity has a tectonic origin as it deviates from the eustatic sea-level trend which was very high at this time. Furthermore, contemporaneous tectonic unconformities are also known from Central Iran and may have their origins in rotational movements of the Central-East Iranian Microcontinent.

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Occurrence and significance of Cenomanian belemnites in the lower Danubian Cretaceous Group (Bavaria, southern Germany)

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Wilmsen M. , Niebuhr B. , Chellouche P.

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tom Vol. 60, no. 2

231-241

EN

The belemnite records of the lower Danubian Cretaceous Group (DCG, northeastern Bavaria, southern Germany) are compiled, taxonomically described and placed within the new integrated stratigraphic framework of the group. Three specimens from the lower Regensburg Formation (Saal Member) south of Regensburg can be assigned to Neohibolites cf. ultimus (d'Orbigny) and are dated as late Early Cenomanian (Mantelliceras dixoni Zone). Eight specimens represent Praeactinocamax plenus (Blainville) and occur in an event (plenus Event) in the lower Eibrunn Formation (Regensburg area) or basal Regensburg Formation (Roding area in the Bodenwohrer Senke). Biostratigraphy and carbon stable isotopes suggest that the belemnite horizon with P. plenus in the DCG has strictly the same chronostratigraphic position (mid-Late Cenomanian, middle Metoicoceras geslinianum Zone) as elsewhere in Central and NW Europe. The lithostratigraphic units of the lower Danubian Cretaceous Group (i.e., the Regensburg and Ebirunn formations), however, are characterized by a pronounced diachronism based on their time-transgressive (i.e., onlapping) deposition during the Cenomanian.Early Turonian transgression. The distribution of P. plenus around the Mid-European Island can be easily explained by migration around the positive area without the necessity of a marine strait across the Bohemian Massif.

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Integrated stratigraphy of the Upper Hauterivian to Lower Barremian Agua de la Mula Member of the Agrio Formation, Neuquen Basin, Argentina

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Archuby F. , Wilmsen M. , Leanza H.

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tom Vol. 61, no. 1

1-26

EN

The Upper Hauterivian to Lower Barremian Agua de la Mula Member of the Agrio Formation (Neuquen Basin, Argentina) was studied applying an integrated stratigraphic approach and facies analysis. The ammonite biostratigraphy of the member has been improved based on bed-by-bed collecting. The already defined biozones (Spitidiscus riccardii, Crioceratites schlagintweiti, Crioceratites diamantensis and Paraspiticeras groeberi) were recognized, precisely related to the succession, and further refinement was proposed. Sequences of different order are built by stacked starvation/dilution (s/d) sequences, regarded here as sixth-order sequences with only two components that can be unequivocally distinguished: the lower starvation hemisequence and the upper dilution hemisequence. Pro- and retrogradational stacking pattern of s/d sequences define supra-ordinate sequences. The sequence-stratigraphic analysis resulted in the subdivision of the member into four main depositional sequences (dsAM-1 to -4) and several subordinate sequences. Previously published sequence stratigraphic charts of the Neuquen Basin did not relate sedimentary sequences to biozones, and are hence not comparable to the scheme presented here and other charts. Our study shows a good agreement with the sequence-chronostratigraphic scheme of european basins, thus arguing in favour of a predominantly eustatic control on sequence development during the Late Hauterivian to early Barremian. A latest early Barremian age is proposed for the almost ammonite-barren upper part of the Agrio Formation, based on correlations of sequence boundaries.