Stratigraphy of the Cretaceous/Paleogene (K/Pg) boundary at the Global Stratotype Section and Point (GSSP) in El Kef, Tunisia: New insights from the El Kef Coring Project
Heather JonesThomas WesterholdHeather BirchPincelli M. HullMohamed Hédi NegraUrsula RöhlJulio SepúlvedaJohan VellekoopJessica H. WhitesideLaia AlegretMichael J. HenehanL. RobinsonJoep van DijkTimothy J. Bralower
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Abstract:
The Cretaceous/Paleogene (K/Pg) boundary is marked by one of the largest mass extinctions in Earth’s history, with geological evidence for this event being expressed in hundreds of locations worldwide. An extensively studied section located near El Kef, northwestern Tunisia, is characterized by the classic iridium-rich K/Pg boundary layer, abundant and well-preserved microfossils, and apparently continuous sedimentation throughout the early Danian with no previously described structural complication. These features led to its designation in 1991 as the Global Stratigraphic Section and Point (GSSP) for the base of the Danian (i.e., the K/Pg boundary). However, the outcrop section has become weathered, and the “golden spike” marking the GSSP is difficult to locate. Therefore, the El Kef Coring Project aimed to provide a continuous record of unweathered sediments across the K/Pg transition in cores recovered from five rotary-drilled holes located close to the El Kef GSSP. Here, we present new, high-resolution lithologic, biostratigraphic, and geochemical data from these cores. The recovered stratigraphic successions of each hole (all drilled within ∼75 m of one another) are unexpectedly different, and we identified a formerly unknown unconformity within planktic foraminiferal biozone P1b. Our results provide evidence that sedimentation at El Kef was not as continuous or free from structural complication as previously thought. Despite these challenges, we present a new composite section from the five El Kef holes and an age model correlated to the orbitally tuned record at Walvis Ridge, South Atlantic Ocean, which is critical in placing the paleoenvironmental and paleoecological records from El Kef in a global context.Keywords:
Paleogene
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Biozone
Lithology
Outcrop
Coring
Chronostratigraphy
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Geochronology
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Hiatus
Chronostratigraphy
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Calcareous nannofossil biostratigraphy reveals a continuous depositional record through the Upper Cretaceous‒Paleocene succession in the Zagros Basin; southwest Iran (southeastern part of the Neo-Tethys), especially through the Cretaceous‒Paleogene and the Danian‒Selandian boundaries. The upper part of the Gurpi Formation and the lower part of the Pabdeh Formation were investigated. Calcareous nannofossil assemblages from these formations are characterized by abundant, diverse and moderate- to well-preserved forms with Tethyan affinities. This study concentrates on two sections (the Gurpi and Kabir-Kuh sections) in the west and southwest of Iran in the Zagros Basin, which span biozones CC25c/UC20bTP to NP6/NTp10B in the Kabir-Kuh and CC25b/UC20aTP to NP5/NTp9 in the Gurpi section. Late Maastrichtian nannofossil abundance and diversity in the studied intervals is high with a sharp decrease slightly above the K/Pg boundary. The K/Pg boundary interval is marked by the FO of Biantholithus sparsus, an acme of the dinoflagellate cyst Thoracosphaera operculata, a notable decrease in Cretaceous calcareous nannofossil abundance and diversity, and the appearance of new Paleocene taxa. An increased abundance of Thoracosphaera is recorded similar to that in the El Kef (Global Stratotype Section and Point, GSSP, for the K/Pg boundary), Ellés (auxiliary section), Egypt, Atlantic Ocean and sections elsewhere in the world. Reworked Cretaceous calcareous nannofossils are recorded from the base of the Danian along with the new Paleocene species. The Danian‒Selandian boundary is marked by the appearance of Lithoptychius ulii, Lithoptychius pileatus and Lithoptychius janii at the base of NTp8C; this level is also marked by the second radiation of Fasciculithus.Azam Mahanipour [a_mahanipour@uk.ac.ir], Department of Geology, Faculty of Science, Shahid Bahonar University of Kerman, Kerman, Iran; Mohammad Parandavar [parandavar.m@gmail.com], Paleontological Research Department, Exploration Directorate of Oil and Gas, NIOC, Tehran; Mohamed Youssef [myousefgeology@gmail.com], Department of Geology, Faculty of Science, South Valley University, 83523 Qena, Egypt.
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The Ordovician-Silurian boundary section at Shanwangmiao, Honghuayuan, southeast of Tongzi County Town was thought to be one of the best sections through the Ordovician-Silurian transition in China where the boundary rocks are well-exposed, continuous, and abundant with shelly fossils and the biostratigraphically impartant graptolites. On the basis of detailed investigation on the Shanwangmiao section together with several other sections in southern Tongzi, including the Nanbazi section of Honghuayuan, Sancha section of Maoba, and Daijiagou section of Liaoyuan, new interpretations of the boundary rocks at the Shanwangmiao section are proposed. The Kuanyinchiao Bed, i.e. the Hirnantia Bed (AFA295-305) and the Shelly Bed (AFA306-311c) should be combined as a single lithostratigraphical unit of Ordovician age as they have similar lithology and yield the same brachiopod fauna. Comparison with the basal boundary stratotype of the Hirnantian Stage at Wangjiawan, northern Yichang, shows that the Kuanyinchiao Bed at Shanwangmiao can be correlated with the upper Normalograptus extraordinarius Biozone and the lower N. persculptus Biozone. There is a hiatus between the Kuanyinchiao Bed and its overlying Lungmachi Formation (AFA312), missing the upper N. persculptus Biozone, the Akidograptus ascensus Biozone, and most part of the Parakidograptus acuminatus Biozone. Thus, the recognition that the typical Hirnantia Fauna could extend to the earliest Silurian A. ascensus Biozone or even higher horizon in South China(Rong Jia-yu et al., 2002) is modified. The Kuanyinchiao Bed at many localities in Southwest China may be regarded as coeval rock units although its thickness is variable. The Hirnantia Fauna after the first episode of the end Ordovician mass extinction contains a series of unique benthic communities with special affinities, geologic age, and environmental background. It is not only age indicative, but also reflects sea water cooling, shallowing, and bottom oxygenation.
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The Cretaceous/Paleogene (K/Pg) boundary is marked by one of the largest mass extinctions in Earth’s history, with geological evidence for this event being expressed in hundreds of locations worldwide. An extensively studied section located near El Kef, northwestern Tunisia, is characterized by the classic iridium-rich K/Pg boundary layer, abundant and well-preserved microfossils, and apparently continuous sedimentation throughout the early Danian with no previously described structural complication. These features led to its designation in 1991 as the Global Stratigraphic Section and Point (GSSP) for the base of the Danian (i.e., the K/Pg boundary). However, the outcrop section has become weathered, and the “golden spike” marking the GSSP is difficult to locate. Therefore, the El Kef Coring Project aimed to provide a continuous record of unweathered sediments across the K/Pg transition in cores recovered from five rotary-drilled holes located close to the El Kef GSSP. Here, we present new, high-resolution lithologic, biostratigraphic, and geochemical data from these cores. The recovered stratigraphic successions of each hole (all drilled within ∼75 m of one another) are unexpectedly different, and we identified a formerly unknown unconformity within planktic foraminiferal biozone P1b. Our results provide evidence that sedimentation at El Kef was not as continuous or free from structural complication as previously thought. Despite these challenges, we present a new composite section from the five El Kef holes and an age model correlated to the orbitally tuned record at Walvis Ridge, South Atlantic Ocean, which is critical in placing the paleoenvironmental and paleoecological records from El Kef in a global context.
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The Oppelzone remains a controversial topic in stratigraphy, despite the attempts to systematize biozonal units in the International Stratigraphic Guide (ISG). In the first edition of the ISG, the Oppelzone was regarded as a particular kind of biozone, although its boundaries were recognized as 'difficult to define empirically'. This is probably the main reason why the Oppelzone was removed from the second edition of the ISG. Here, we review briefly the history and significance of the Oppelzone, starting from Jurassic zonal biostratigraphy as introduced by Oppel himself, and based mainly on ammonites, to its present usage, distinguishing the multiple meanings of the unit – biostratigraphical, chronostratigraphical, or even as a time interval. We review the Oppelzone as integral part of the current biostratigraphy of Palaeogene–Miocene larger foraminifera (the Shallow Benthic Zones, SBZ). Here, different species concepts in individual systematic groups result either in Oppelian (e.g. nummulitids, alveolinids) or non-Oppelian (e.g. lepidocyclinids, miogypsinids and in part orthophragmines) biozones. In addition, various regional larger foraminiferal zonations have been established. These different kinds of biozones are subsumed under the biochronostratigraphical SBZ system in a similar way as regional ammonite zonations are integrated in the standard ammonite zonation. To overcome issues of fuzzy-defined boundaries, a novel research programme is needed to: (1) establish the most suitable markers for biozonal boundaries; (2) enhance correlation with different systematic groups (especially planktonic foraminifera and calcareous nannofossils) and with other stratigraphical tools (magnetostratigraphy, radiometric dating, isotopic stratigraphy, etc.); and (3) extend morphometric criteria wherever possible to recognize the markers themselves.
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Incertae sedis
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Neogene
Geochronology
Magnetostratigraphy
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In the north of Iran in the Galanderud area, similar to those known from the eastern Tethys realm, experienced unusually adverse environmental conditions for planktic foraminifera during the last two million years of the terminal Cretaceous to early Danian. This section is studied to determine the foraminiferal biozones of the upper Cretaceous to lower Paleocene and to detect patterns of foraminiferal changes across the Cretaceous-Tertiary (K-T) boundary. All late Maastrichtian planktic foraminiferal biozones CF1 to CF4, and Danian biozones P0 (Parvularugoglobogerina extensa) P1a (Parvularugoglobogerina eugubina) and Parasubbotina pseudobulloides are present. Faunal studies show that all but four of the Cretaceous species identified disappeared at or below the K-T boundary in zone CF1 (P. hantkeninoides). Another four species (Heterohelix globulosa, H. dentata, H. monmouthensis, G. cretacea) appear to have survived in to the early Danian. Early disappearances appear to be environmentally controlled. Coarse ornamented species with small populations disappeared first, where as small species will little or no ornamented and generally large populations tended to survive after the environment changing. This indicates a pattern of gradual and selective faunal turnover in planktonic foraminifera during the latest Maastrichtian and in to the earliest Danian that is similar to that observed at the El kef stratotype of Tunisia, as well as K-T sequences in west of Iran, Egypt, Italy, Spain and Mexico.
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In order to use the foraminiferal contents for biostratigraphy and age dating, chronostratigraphy and explanation of some evolutionary trends the well-known foraminifera rich Abderaz Formation of the Kopeh-Dagh basin in northeastern Iran was sampled at the type section. The 300 meters thick section is mainly made up of grey shales and marls with two units of chalky limestone in the upper part. The lower contact of the formation with Aitamir Formation is disconformable while the upper contact with Abtalkh Formation is continuous. Fifty nine species belonging to 18 genera were identified and four biozones identical to those established more recently for the Tethyan were erected and compared with the existing biozonation. The biozones are quite correlatable through with minor changes in the boundaries. Based on these, an age of middle Turonian-early Campanian is quoted to the formation. The Neoflabellinids can be used to mark the Coniacian-Santonian boundary. Neoflabellinids has world-widely been distributed during the Coniacian-Santonian time. Four species of Neoflabellina including N.suturalis, N.praerugosa, N.gibbera, N.praecursor were identified. The N.gibbera and N.praecursor appear at the very base of the Santonian stage differentiating therefore, the Coniacian-Santonian boundary. The findings of this study is quite correlatable with those from the Global Standard Stratotype Section and point (GSSP).
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