The E arly Aptian (C retaceous) stratigraphy of M ount Pagasarri (N Spain): Oceanic anoxic event‐1a
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Abstract:
Sedimentological, stratigraphical, palaeontological, and geochemical evidence from outcrops of the Early Aptian (Cretaceous) succession in Mount Pagasarri (northern Spain) dated by ammonites is described to elucidate the origin of organic carbon‐rich sediments and carbonate platforms in the Early Aptian of the northern margin of the Tethyan Ocean. A 1,600‐m‐thick section spans the latest Barremian to the end of the Early Aptian and is divided into 3 units: a lower marine nearshore, a middle rudist‐coral shallow‐water carbonate platform, and a deeper‐water outer shelf. Deshayesites oglanlensis , D. forbesi , D. deshayesi and Dufrenoyia furcata ammonite zones are represented. Three organic carbon‐rich intervals are recognised. The oldest (top Barremian) is correlated with the Taxy Level of France/Germany. A second organic layer in the lower part of the Deshayesites deshayesi Zone sediments correlates with the Selli Event of Italy. A third organic‐carbon enriched layer in the D. furcata Zone sediments post‐dates the Aparein Level of the Aralar area in Spain. Carbon isotope curves reveal a brief negative excursion followed by a more prolonged positive trend during the Early Aptian interpreted as oceanic anoxic event 1a (OAE1a). Three carbonate platform growth phases are recognised in the Basque‐Cantabrian Basin with OAE1a being developed between carbonate platform Phases 1 and 2. The Peñascal Limestones (late Early Aptian) represent carbonate platform Phase 3 of the Basque‐Cantabrian Basin succeeded by ammonite‐bearing marls of the latest Early Aptian, reflecting platform demise and subsequent marine flooding. The Mount Pagasarri area is a key locality for study of the oceanic anoxic events in the Greenhouse Earth of the Early Aptian.Keywords:
Aptian
Carbonate platform
Ammonite
Marl
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Aptian
Carbonate platform
Progradation
Marl
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Ammonoidea
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Detailed data on twenty-seven species of ammonites identified in three stratigraphic sections of northern Mexico, allow further refinement of the Aptian ammonite biozonations of previous authors. Four formal ammonite biozones are proposed. Dufrenoyia justinae Taxon-Range-Zone for the uppermost part of the lower Aptian (Bedoulian), Epicheloniceras cf. subnodosocostatum/Acanthohoplites acutecosta Interval Zone representative of the middle Aptian (Gargasian), and Acanthohoplites aschiltaensis and Hypacanthoplites cf. leanzae Taxon-Range-Zones, spanning the upper Aptian (Clansayesian). The ammonite zonation proposed represents the advancements on the development of a standard scheme towards a reliable correlation with the biozonal scheme of England and the ammonite zonal standard of the Mediterranean area.
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Mediterranean area
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The Lower Cretaceous sediments in the Monts de Tiaret (Oranian High Plains) (Tlemcenian Palaeogeographic Domain) and the Western Ouarsenis (Tellian Palaeogeographic Domain) are subdivided in lithostratigraphic units which differ for the Tlemcenian and the Tellian Domains. In the first one, a specific feature is the development of shalow, and in the second – of deep water sediments. The following formal lithostratigraphic units are distinguished in the Tlemcenian Domain: (1) Remaïlia Carbonate Formation – limestones, marls and dolomites (Berriasian – Early Hauterivian (p.p.); (2) Berthelot Sandstone Formation (Early Hauterivien (p.p.) – Aptian); (3) Djebel Tenfeld Carbonate Formation – alternation of marls and limestones, rich in shallow water molluscs. Within the Tellian Atlas (Tell ian Domain) are distinguished: (l) Oued Fodda Carbonate Formation (Late Tithonian – Early Aptian); (2) El Melaab Turbiditic Formation (new unit of Aptian (p.p.) – Early Albian age) subdivided in 2 new Members: (a) Oued Sidi Mohamed Member (Aptian) – marls with turbiditic sandstones – distal flysch; (b) Oued Mezdar Member (Early Albian) – sandstones with intercalations of marls – proximal flysch; (3) Rhedadoua Marls Formation (Middle – Late (p.p.) Albian; (4) Hassi ben Ziane Carbonate Formation – marls and limestones (Late Albian (p.p.) – Early Cenomanian). In the Chouala region crop out the allochtonous sediments of the Sidi Seba Achir Limestones-Marls Formation (new unit of Berriasian-Middle Aptian Age) subdivided in 3 new members: (a) Oued el Ouakal Member – alternation of marls and limestones (Berriasian – Valanginian); (b) Oued Malah Member - marls (Hauterivian); (c) Sidi Ounes Member – marls and limestones (Barremian – Middle Aptian). The region of the High Plains (Monts de Tiaret) was uplifted at the end of the Jurassic, and the Berriasian sea transgressed over it. The subsidence of the Tellian Domain continued since the Late Jurassic and became an area of turbiditic sedimentation during the Aptian-Early Albian. The sedimentation in the Tlemcenian and Tellian Domains continued without interruption into the Cenomanian.
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The present paper reports for the first time the presence of representatives of the ammonite subfamily Venezuellinae Kvantaliani in the Mediterranean area (in particular Bulgaria). Until now they were considered as endemic and were known only from Aptian sediments in South America (Venezuela). Species of g. g. Venezuella and Renziella were found in three sections on the territory of North Bulgaria. The lithology and the ammonite sequences of the sections are described. The representatives of g. Venezuella and Renziella associate with characteristic, including also zonal Aptian ammonites from A. nolani and H. jacobi zones. The concept of the Aptian Age of the ammonites of this group is advocated. The phyllogenetic concept for the origin and evolution of the subfamily is rejected. Five species of g. Venezuella and 1 species of g. Renziella are described and figured. Three of them – V. bulgarensis sp. n., V. sumerensis sp. n., V. gracilis sp. n., are new for the science.
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Ammonite
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Albian in Northwestern Bulgaria is rich in ammonite fauna. The representatives of suborder Lytoceratina are comparatively scarce in the Albian ammonite associations. They derive from the marls of the Sumer Formation and from the glauconitic marls and sandstones of the Malo Peštene Formation. A total of 18 species and subspecies are described, which are distributed as follows: Ammonoceratites (Ammonoceratites) - 1: A. (Argonauticeras) – 1; Pictetia – 2: Tetragonites – 1; Protetragonites – 1; Eogaudryceras (Eogaudryceras) – 2; E. (Eotetragonites) – 2; Anagaudriceras – 3; Kosmalella – 5. Most of the latter are established and described for the first time in Bulgarian paleontological literature. The specimens described are kept in the collection of the Paleontological Museum of Sofia University.
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