In two KIT sections of the French Basque Country, Hendaye and Bidart, ammonites have been found in the last few tens of centimetres of the Maastrichtian. In the Bidart section, one specimen was discovered less than five centimetres below the boundary clay layer. It contains a small, but significant amount of iridium and a few crystals of Ni-rich spinel, two markers of the K/T cosmic event. These findings support the idea that ammonites existed up to the very top of the Cretaceous and disappeared suddenly right at the K/T boundary.

Abstract The Cretaceous-Tertiary (K-T) boundary at Blake Nose, in the NW Atlantic, is recorded by a coarse, poorly graded and poorly cemented layer mostly consisting of green spherules that are mainly composed of smectite. Geochemical patterns across the boundary are governed by the source material of the spherule bed and postdepositional processes. The chemical composition and the nature of this bed indicate that it derived from melted target rocks from the Chicxulub impact structure. Ir and other typical extraterrestrial elements do not present significant enrichments, which suggests that the spherule bed material derived from crustal rocks. Ir instead reaches its highest concentration in the burrow-mottled calcareous ooze above the spherule bed, suggesting that it is associated to the finest fraction deposited after the target-rock-derived material. Only the Ni and Co content show slight enrichments within the upper part of the spherule layer, although most of the trace element profiles resulted from diagenetic alteration. During the alteration of glass to smectite, the concentrations of certain trace elements, such as the rare earth elements, were severely changed. In addition, oxygen-poor conditions also led to the remobilization of redox-sensitive elements, which show enhanced concentration at the top or above the spherule bed. Diagenetic remobilization may have also affected the Ir concentration.

The review of the Cretaceous-Tertiary interval deposits of the Elles section based on a closer sampling than done before allows to well characterise the K/T boundary and to reconstitute the geological events underlining this boundary. Than, the discovery of a thin Ir rich layer, with Ni and Cr rich spinel testify to the cosmic event. This sudden event added to some longer term other events (e.g. climatic, eustatic) generated a mass extinction, at the K/T boundary, of specialists among the globotruncanids and heterohelicids planktic Foraminifera species. This biological crisis is followed by a slow biotic turnover into the Danian which contains also ecological generalist species inherited from the Cretaceous lot. The underlining of the all Cretaceous-Tertiary interval biozones (i.e. A. mayaroensis zone, G. conusa zone, PV. eugubina subzone, P. pseudobulloides subzone, T. trinidadensis subzone and a T. uncinata zone) certifies that the Elles section is complete. This Elles section, compared to the El Fief section (considered up today as the K/T stratotype) exposes a better K/T interval outcrops. Besides, at the Elles section the interval deposits of the all indicated zones and subzones is slightly more expanded than elsewhere. All these characteristics prevail the Elles section to be proposed as a new stratotype or at least a parastratotype for the K/T boundary. Editions scientifiques et medicales Elsevier SAS.

Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424] reported the discovery of iridium-bearing sediments sandwiched between basalt flows in the Anjar area (Kutch province, India). They concluded that the signature of the K/T impact had been recorded and that onset of volcanism in the Deccan traps preceded the K/T boundary, excluding the possibility of a causal connection. This paper reports complementary analyses of Anjar outcrops by a joint Indo–French team, where we focused on cosmic markers (iridium and spinels) in the intertrappean sediments and 40Ar/39Ar dating and paleomagnetism of the lava flows. Anomalous Ir concentrations (up to 0.4 ng/g) are confirmed, with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections. Despite careful search, no Ni-rich spinels were found. Eight basalt samples provided 40Ar/39Ar results, four on plagioclase bulk samples, four on whole rocks. Spectra for whole rocks all indicate some amount of disturbance, and ages based on plagioclase bulk samples seem to be consistently more reliable [Hofmann et al., Earth Planet. Sci. Lett. 180 (2000) 13–28]. The three flows underlying the Ir-bearing sediments are dated at ∼66.5 Ma, and two overlying flows at ∼65 Ma. Magnetic analyses (both thermal and by alternating fields) uncovered clear reversed primary components in the upper flows, and more disturbed normal components in the lower flows, with evidence for an additional reversed component. There are reports [Bajpai, Geol. Soc. India Mem. 37 (1996) 313–319; Bajpai, J. Geol. Soc. London 157 (2000) 257–260] that the intertrappean sediments contain uppermost Maastrichtian dinosaur and ostracod remains above the uppermost Ir-bearing level, and may not be mechanically disturbed. We propose the following scenario to interpret these multiple field and analytical observations. Deccan trap volcanism started within uppermost Maastrichtian normal chron C30N at ∼66.5–67 Ma in the Anjar area. Volcanism then stopped at least locally, and lacustrine sediments were deposited over a period that could be in the order of 1–2 Ma. The K/T bolide impact was recorded as a deposit of Ir, and possibly (though not necessarily) spinels. Volcanism resumed shortly after the K/T boundary, within reversed chron C29R, as witnessed by the three reversely magnetised overlying basalt flows dated ∼65 Ma. This was responsible for erosion and destruction of part of the uppermost sediments (including spinels if there were any) and heterogeneous and non-uniform redeposition of Ir at a number of underlying sedimentary levels. This was also responsible for the partial remagnetisation of the underlying flows. These findings generally confirm and complement those of Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424], and are compatible with the occurrence of the K/T impact at the paleontological K/T boundary, and of Deccan trap volcanism straddling the boundary and starting before the impact. Anjar provides evidence for minor volcanism somewhat earlier than suggested by some authors, though still within normal chron C30N. There is no indication contradicting the view that the bulk of Deccan trap volcanism occurred over only three chrons (C30N, C29R, C29N) [Courtillot, Evolutionary Catastrophes: the Science of Mass Extinctions, Cambridge University Press, 1999; Courtillot et al., Earth Planet. Sci. Lett. 80 (1986) 361–374; Vandamme et al., Rev. Geophys. 29 (1991) 159–190].

The review of the Cretaceous-Paleogene interval deposits of the Elles section based on a detailed sampling gives a good characterization of the WT boundary and reconstitution of the geological events underlining this boundary. Thus, the discovery of a thin Ir-rich layer, with Ni and Cr-rich spinel and shocked quartz is a well preserved record of the known cosmic event that occurred at that time. This sudden event is also corroborated by the quantitative and qualitative organic components distribution along the WT interval. Added to some other long-term events (e.g. climatic, eustatic), it generated a mass extinction, at the WT boundary, of specialists among the Globotruncanids and Heterohelicids planktic foraminifera species. Following this biological crisis, the biotic turnover into the Danian is slow. The underlining of all the Cretaceous-Tertiary interval biozones attests that the Ellis section is complete. If compared to the WT stratotype and Fl Kef II sections. the Elles section is found to display a better exposure of the WT interval and shows more expanded zones and subzones. Such characteristics promote the Elles section to be considered as a parastratotype. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.

The clay mineral assemblages of the Cretaceous-Tertiary (K/T) transition recorded in southern Peten sedimentary rocks have been studied in sections dated by calcareous nannofossils and foraminifera. The deposits contain cosmic and impact remains (iridium, Ni-rich magnetite, tektite) and typical Cheto smectites resulting from weathered tektites (glass spherules) produced by the Yucatan impact. The clay mineral assemblages are used for describing the palaeoenvironmental and tectonic evolution of southern Peten near the K/T boundary.

The clay mineral assemblages of the Cretaceous-Tertiary (K/T) transition recorded in southern Peten sedimentary rocks have been studied in sections dated by calcareous nannofossils and foraminifera. The deposits contain cosmic and impact remains (iridium,Ni-rich magnetite, tektite) and typical Cheto smectites resulting from weathered tektites (glass spherules) produced by the Yucatan impact. The clay mineral assemblages are used for describing the palaeoenvironmental and tectonic evolution of southern Peten near the K/T boundary.

The Cretaceous-Tertiary (K/T) boundary is marked by the collision of the Earth with an asteroid or a comet. This event produced high concentrations of iridium and other PGEs in K/T boundary sediments. Although delivered by a very brief event, Ir is observed over a sizeable thickness of sediments simulating a long duration phenomenon (several tens of thousands of years). In order to understand this point we have analysed rocks from the K/T section of El Kef (Tunisia), a site which offers ideal conditions (high deposition rate and low bioturbation) for a detailed stratigraphic study. The Ir distribution shows a millimetre thick pulse-like feature occurring in perfect coincidence with the beginning of the biological crisis. This feature is superimposed to a diffuse component extending over about two metres of sediment from the upper Maastrichtian to the top of the Globigerina eugubina zone in the Danian. The pulse-like feature is associated with crystals of Ni-rich spinel, a mineral derived from meteoritic material. This pulse, reminiscent of the brevity of the K/T event, results from the prompt deposition of a large amount of debris of the projectile dispersed world-wide as a result of the impact event. As far as the diffuse component is concerned, its duration agrees with the estimated residence time of Ir in sea water. This component is explained by the transit through the oceanic reservoir of the iridium associated with the finely divided fraction of the projectile.

The upper part of the near K/T boundary limestone breccia deposits which crop out in Guatemala near 16 degrees N contains impact remains: altered glass spherules (tektites) and elevated concentrations of iridium. This suggests that these deposits could result from the impact of the K/T boundary asteroid on the Yucatan. However, their matrix contains foraminifers of the Eugubina Zone and calcareous nannofossils of the CP1a Zone, an association characterizing Early (but not the earliest) Danian. This unusual assemblage of K/T and Danian components suggests that the upper part of the breccia was reworked several thousands of years after the K/T event. ((C) Academie des sciences / Elsevier, Paris.)

The discovery of Paleocene planktonic Foraminifera (subzones P1c to P3b) in the western sub-Pyrenean zone (Haute-Garonne) made it possible to locate an anomalously high abundance of iridium at the Cretaceous-Tertiary boundary underlining the interface between the 'calcaires a Bryozoaires/calcaires a Algues, of Larcan-Nizan', newly assigned to Danian-Selandian and the underlying 'marnes bleues de Saint-Loup', the top of them being Uppermost Maastrichtian in age (Hariaensis zone and grade-datation of 65.47 Ma). A short gap (subzones P0 to P1b, Lowermost Danian)probably marks, as in Bearn, the base of the Paleocene Larcan-Nizan Bryalgal limestones. (C) Academie des sciences/Elsevier, Paris.

In Oman, the sedimentary basins located along the edges of the Oman Mountain chain show continuous sedimentation at the K/T boundary. Two distinct sedimentary palaeoenvironments have been analysed. In the northwestern flank of the Oman Mountains (near Buraymi), the K/T boundary occurs within hemipelagic deposits; whereas in the Abet basin (southeastern flank), it occurs within confined carbonate platform facies. The negative partial derivative(13)C shift and the positive iridium anomaly, which characterize the global geochemical events of the K/T boundary in marine sections, have been detected in Oman. Moreover this isotopic signature is nor the only event recorded in these sections. Some other important shifts have been recognized in the Upper Maastrichtian and Lower Danian sediments. These shifts are related to some palaeogeographical events such as platform flexuration which correspond to some regional tectonic reajustments.

Lake Télé has a nearly perfect ellipsoidal shape and is surrounded by swamps and barely penetrable flooded forests in the heart of the Congo—Zaire watershed basin, and has intrigued the international scientific community for decades. In June 1992, a first Franco—Congolese multidisciplinary scientific expedition was finally able to reach the lake which is vast (23 km2) and shallow (3 m). Its volume is estimated at 71 × 106m3 and is 40% filled with an organic silt layer a metre thick. Its hydrological exchanges are almost exclusively vertical with very little lateral contribution from the surrounding swamp. This leads to the observation that the waters are very slightly mineralized (< 3 mg 1−1), but are very rich in organic carbon (44% of suspended matter and of total dissolved matter) and are very acidic (pH < 4). In addition, a magnetic anomaly of some hundred nanoTesla from a magnetic body at shallow depth has been detected in the lake's northern half, although its origin remains unknown. Botanical and palynological observations suggest the persistence of a strongly hydromorphic forest environment for at least the last 6600 years. Identified pollen taxa, whether ancient or recent, come mainly from pioneer and colonizing taxa such as Macaranga, which tends to indicate that the forest is continuing to gradually fill in the lake.

The Cretaceous-Tertiary (KIT) boundary is marked by the collision of the Earth with an asteroid or a comet. This event produced high concentrations of iridium and other PGEs in K/T boundary sediments. Although delivered by a very brief event, Ir is observed over a sizeable thickness of sediments simulating a long duration phenomenon (several tens of thousands of pears). In order to understand this point we have analysed rocks from the WT section of El Kef (Tunisia), a site which offers ideal conditions (hi,oh deposition rare and low bioturbation) for a detailed stratigraphic study. The Ir distribution shows a millimetre thick pulse-like feature occurring in perfect coincidence with the beginning of the biological crisis. This feature is superimposed to a diffuse component extending over about two metres of sediment from the upper Maastrichtian to the top of the Globigerina eugubina zone in the Danian. The pulse-like feature is associated with crystals of Ni-rich spinel, a mineral derived from meteoritic material. This pulse, reminiscent of the brevity of the K/ event, results from the prompt deposition of a large amount of debris of the projectile dispersed world-wide as a result of the impact event. As far as the diffuse component is concerned, its duration agrees with the estimated residence time of Ir in sea water. This component is explained by the transit through the oceanic reservoir of the iridium associated with the finely divided fraction of the projectile.

Research Article| April 01, 1998 Extraterrestrial Ni-rich spinel in upper Eocene sediments from Massignano, Italy Olivier Pierrard; Olivier Pierrard 1Centre des Faibles Radioactivités, Laboratoire mixte C.E.A./C.N.R.S., 91198 Gif-sur-Yvette, France Search for other works by this author on: GSW Google Scholar Eric Robin; Eric Robin 1Centre des Faibles Radioactivités, Laboratoire mixte C.E.A./C.N.R.S., 91198 Gif-sur-Yvette, France Search for other works by this author on: GSW Google Scholar Robert Rocchia; Robert Rocchia 1Centre des Faibles Radioactivités, Laboratoire mixte C.E.A./C.N.R.S., 91198 Gif-sur-Yvette, France Search for other works by this author on: GSW Google Scholar Alessandro Montanari Alessandro Montanari 2Osservatorio Geologico di Coldigioco, 62020 Frontale di Apiro, Italy, and Ecole des Mines de Paris, France Search for other works by this author on: GSW Google Scholar Author and Article Information Olivier Pierrard 1Centre des Faibles Radioactivités, Laboratoire mixte C.E.A./C.N.R.S., 91198 Gif-sur-Yvette, France Eric Robin 1Centre des Faibles Radioactivités, Laboratoire mixte C.E.A./C.N.R.S., 91198 Gif-sur-Yvette, France Robert Rocchia 1Centre des Faibles Radioactivités, Laboratoire mixte C.E.A./C.N.R.S., 91198 Gif-sur-Yvette, France Alessandro Montanari 2Osservatorio Geologico di Coldigioco, 62020 Frontale di Apiro, Italy, and Ecole des Mines de Paris, France Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1998) 26 (4): 307–310. https://doi.org/10.1130/0091-7613(1998)026<0307:ENRSIU>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Olivier Pierrard, Eric Robin, Robert Rocchia, Alessandro Montanari; Extraterrestrial Ni-rich spinel in upper Eocene sediments from Massignano, Italy. Geology 1998;; 26 (4): 307–310. doi: https://doi.org/10.1130/0091-7613(1998)026<0307:ENRSIU>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Evidence of a major cosmic event at the end of the Eocene is given by the finding of at least one, possibly two or more, impact horizons containing microtektites, microkrystites, shocked quartz, and unusually high iridium concentrations. We report here the discovery, in the global stratotype of the Eocene-Oligocene boundary at Massignano in Italy, of Ni-rich spinel, a mineral that forms by the interaction of meteoritic bodies with the Earth's atmosphere. A maximum concentration of ≈800 crystals mg−1, corresponding to a flux of 2 × 107 crystals cm−2, is observed in a well-dated layer (35.7 ± 0.4 Ma) that also contains shocked quartz and iridium. The crystals are found clustered in flattened spheroids, the probable relic of their host bodies. The chemical and physical characteristics of the spinel crystals indicate that the spheroids formed by interaction of dust particles in the upper part of the atmosphere rather than by ablation of large objects, as proposed for the spinel-bearing spheroids found at the Cretaceous-Tertiary boundary. The most likely explanation is that the dust particles were generated by a large cometary impact. Whether this impact is the one that produced microtektites and/or microkrystites, or another one that may have occurred shortly after or before, is still questionable. Additional searching for Ni-rich spinel at other upper Eocene sections may help to answer this question. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.