[show abstract][hide abstract] ABSTRACT: Multidisciplinary studies, including stratigraphy, sedimentology, mineralogy and geochemistry, of the new core Mullinax-1 and outcrops along the Brazos River and Cottonmouth Creek, Falls County, Texas, reveal the complex history of the Chicxulub impact, the event deposit and the K–T boundary event. The K–T boundary, as identified by the negative δ13<sup>C></sup> shift, first occurrence of Danian planktic foraminifera and palynomorphs occurs 80 cm above the event deposit in core Mullinax-1. The underlying 80 cm interval was deposited in a shallow low oxygen environment during the latest Maastrichtian, as indicated by high stress microfossil assemblages, small shells and burrows infilled with framboidal pyrite. The underlying event deposit, commonly interpreted as K–T impact tsunami, consists of a basal conglomerate with clasts containing Chicxulub impact spherules, repeated upward fining units of spherule-rich sands, followed by hummocky cross-bedded and laminated sands, which are burrowed by Thalassinoides, Planolites and Ophiomorpha and truncated by erosion. This suggests a series of temporally separated storm events with re-colonization of the ocean floor by invertebrates between storms, rather than a series of waning tsunami-generated waves. The lithified clasts with impact spherules at the base of the event deposit provide strong evidence that the Chicxulub impact ejecta layer predates the event deposit, but was eroded and re-deposited during the latest Maastrichtian sea level lowstand. The original Chicxulub ejecta layer was discovered in a 3 cm thick yellow clay layer interbedded in undisturbed late Maastrichtian clay- and mudstones 40 cm below the base of the event deposit and near the base of planktic foraminiferal zone CF1, which spans the last 300 kyr of the Maastrichtian. The yellow clay consists of cheto smectite derived from alteration of impact glass, as indicated by rare altered glass spherules with similar chemical compositions as reworked spherules from the event deposit and Chicxulub impact spherules from NE Mexico and Haiti. The Brazos sections thus provide strong evidence that the Chicxulub impact predates the K–T boundary by about 300 kyr, consistent with earlier observations in NE Mexico and the Chicxulub crater core Yaxcopoil-1.
[show abstract][hide abstract] ABSTRACT: Synchrotron radiation, collimated to a μm scale was used for the determination of trace elements in micro-tektites and spherule material for the first time. The experimental set-up of the SXRF microprobe at beamline L at HASYLAB at DESY offers a suitable method for performing non-destructive in situ multi-element analysis focusing on spatial trace element distributions and mineral phases of the melted ejecta material from the Cretaceous/Tertiary boundary. The spatial distribution of trace elements was determined in melt inclusions as well as in phase transitions in selected parts of chlorite–smectite spherules and tektite glass material by using a beam with a diameter of 15 μm collimated with a glass capillary for line- and area scans as well as for single point measurements for elements with Z between 19 and 92. The analyzed spherules show alteration features but also zonation and carbonate inclusions, originating from the Chicxulub impact event. These initial results demonstrate the potential of μ-SXRF analysis for the discrimination of alteration and primary signals of the spherules and re-construction of their genetic evolution. It could be shown that the spherules represent a complex mixture of different materials from the subsurface at the Chicxulub impact site.
Spectrochimica Acta Part B Atomic Spectroscopy 01/2007; · 3.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Maastrichtian–Danian strata of the Cauvery basin as well as selected sections of NE-Mexico, Guatemala and Israel record Ba anomalies, away from the Cretaceous/Tertiary boundary (KTB) in addition to common occurrences of geochemical and stable isotopic anomalies across the KTB. Ba anomalies were recorded in monotonous shallow marine sandstones of the Cauvery basin (south India) which contain minor amounts of Ba-orthoclase. Barium anomalies were observed also in shallow marine carbonates in sections of Israel, NE-Mexico and Guatemala. Calculation of excess Ba with reference to PAAS (Post-Archaen Average Australian Shale), comparison of coeval geochemical anomalies, depositional pattern and associated petrographic and mineralogical features of the Cauvery basin revealed that while a first Ba peak was related to detrital influx, the second Ba peak was coincident with sea level fall which in turn may have been influenced by emission of volatile hydrocarbons and resultant climatic changes. In view of intrinsic involvement of Ba in various geochemical processes and occurrence of Ba anomalies in K/T sites distributed around the world (NE-Mexico, Guatemala and Israel), it is suggested that probable causes of such widespread Ba-anomalies should be taken into consideration while analyzing end Cretaceous events. These observations support the views espoused by many workers who have stated that the K/T boundary was also accompanied by many non-catastrophic events that might have contributed to environmental stress on marine fauna, as a result of which selective multi-stage extinctions occurred.
International Journal of Earth Sciences 06/2005; 94(3):475-489. · 2.26 Impact Factor
[show abstract][hide abstract] ABSTRACT: Glass spherule ejecta deposits in various sections from NE-Mexico, Texas, Guatemala, Belize, and Haiti have been investigated by wave-length dispersive electron microprobe analyses, backscatter-electron imaging, scanning electron-, and transmission electron microscopy, in order to characterize the geochemistry of the ejecta, strewn-field mixing, alteration, fractionation within the strewn field, and distribution mechanisms. Earlier investigations include main and trace elements (Harting, 2004), isotope analyses (Kettrup, 2002) and litho- and biostratigraphic investigations (Keller et al., 2003, 2004). In NE-Mexico and Texas, multiple ejecta layers are exposed up to 14 m below the K/T boundary, with the oldest and original ejecta deposit predating the K/T boundary by about 300,000 years. In Texas, preliminary analyses suggest that the original ejecta, now a higly altered smectite layer, is also interbedded in late Maastrichtian claystone. In Guatemala, Belize and Haiti the spherule layers are generally found above the K/T boundary and apparently eroded and reworked from the older original deposit. Geochemical investigations at all these localities reveal Chicxulub impact glass as the origin for all glass spherule ejecta layers. TEM-studies reveal the nature and extent of geochemical alteration. Although most glass spherules have weathered clay rims, relic glass particles contain relatively fresh unaltered glass. Both geochemical and petrological features are well preserved and permit correlation with Chicxulub basement rocks (e.g. granites, gneises, amphibolites and impact meltrocks). Several distinct silicic phases (Al-Fe-rich glasses) are observed. The range of geochemical compositions for NE-Mexico ejecta is: SiO2: 45-61, TiO: 0.1-0.6, Al2O3: 11-22, FeO: 2-14, MnO: 0-0.07, MgO: 2-11, CaO: 1-9, Na2O: 0-3, K2O: 0.2-4. The mean geochemical composition for Texas impact glass falls well within this range with SiO2: 50.81, TiO2: 0.33, Al2O3: 18.34, FeO: 4.99, MnO: 0.02, MgO: 3.42, CaO: 3.03, Na2O: 0.11 and for K2O: 1.31. These compositional phases are present in all studied outcrops but with variable amounts of FeO-, Al2O3 and CaO in NE-Mexico. These characteristics strongly imply an ejecta origin from mafic and felsic rocks of the Chicxulub-basement. However, the occurrence of different glass phases in the upper (reworked) ejecta layers of NE Mexico strongly suggests post-sedimentary mixing and/or fractionation (weathering) of the ejecta. In contrast, the stratigraphically oldest ejecta layer (300,000 yrs pre-K/T) is geochemically more uniform, which reflects the absence of erosion, mixing and transport, and less weathering.
[show abstract][hide abstract] ABSTRACT: The late Maastrichtian to early Danian at Mishor Rotem, Israel, was examined based on geochemistry, bulk rock and clay mineralogies, biostratigraphy and lithology. This section contains four red clay layers of suspect impact or volcanic origin interbedded in chalk and marly chalks. PGE anomalies indicate that only the K/T boundary red layer has an Ir dominated PGE anomaly indicative of an impact source. The late Maastrichtian red clays have Pd dominated PGE anomalies which coincide with increased trace elements of terrigenous and volcanogenic origins. Deccan or Syrian-Tur-key arc volcanism is the likely source of volcanism in these clay layers. Glauconite, goethite and translucent amber spherules are present in the clay layers, but the Si-rich spherules reported by Rosenfeld et al. [l989] could not be confir-med. The absence of Cheto smectite indicates that no altered impact glass has been present. The red layers represent condensed sedimentation on topographic highs during sea level highstands. In the Negev area, during the late Maas-trichtian, the climate ranged from seasonally wet to more arid conditions during zones CF3 and CF2, with more humid wet conditions in the latest Maastrichtian zone CF1 and in the early Danian, probably linked to greenhouse conditions. Planktic foraminifera experienced relatively high stress conditions during this time as indicated by the low species rich-ness and low abundance of globotruncanids. Times of intensified stress are indicated by the disaster opportunist Guem-belitria blooms, which can be correlated to central Egypt and also to Indian Ocean localities associated with mantle plume volcanism. Marine plankton thus support the mineralogical and geochemical observations of volcanic influx and reveal the detrimental biotic effects of intense volcanism. Evolution des paléo-environnements durant le Maastrichtien terminal et la limite Crétacé-Tertiaire dans la Téthys orientale (Negev, Israël) : minéralogie, géochimie, biostratigraphie et renouvellements fauniques Mots clés. Résumé. – A Mishor Rotem, Israël, l'intervalle Maastrichtien terminal-Danien basal a été étudié par le biais d'analyses géochimiques, minéralogiques, biostratigraphiques et lithologiques. Cette section est caractérisée par des craies et des marnes crayeuses au sein desquelles, quatre couches rouges argileuses ont été mises en évidence. L'analyse des élé-ments du groupe du platine (EGP) a montré que seule la couche rouge correspondant à la limite Crétacé-Tertiaire était dominée par l'Iridium (Ir) et, donc caractéristique d'un impact météoritique. Les trois autres niveaux rouges, situés dans le Maastrichtian terminal (Zone CF1), sont caractérisés par un assemblage de EGP au sein duquel le palladium (Pd) était dominant, indiquant une origine volcanique. Les Trapps du Deccan ou l'arc volcanique syrio-turc pourraient en être la source. De plus, l'abondance d'autres éléments-traces reflète un apport terrigène important. De nombreuses sphérules glauconitiques, de goethite et des goutelettes d'ambre ont été observées dans les quatre couches, mais la présence de sphérules enrichies en Si (tectites), indiquées par Rosenfeld et al. , n'a pu être confirmée. De plus l'absence de smectite bien cristallisée de type Cheto, caractéristique de l'altération des verres d'impacts parle en faveur de l'absence de ces derniers. Ces couches argileuses rouges indiquent une sédimentation réduite sur des paléo-reliefs sous-marins, durant des périodes de haut niveau marin. Le Maastrichtien tardif était caractérisé par des climats à saisons contrastées, tendant à l'aridité (Zone CF3 et CF2), devenant plus humide durant le Maastrichtien terminal (Zone CF1) et le Paléo-cène basal, ce changement étant probablement lié à l'établissement de conditions à effet de serre. Les assemblages de foraminifères planctoniques sont caractérisés par une faible diversité indicative de conditions de stress assez élevé. Ces périodes de stress coincident avec des blooms du genre Guembelitria, un genre oportuniste « désastre », qui peuvent Manuscrit déposé le 27 août 2003 ; accepté après révision le 28 juillet 2004. E:\sgf\SGF-1-2005\pages\SGF 1-2005.vp lundi 31 janvier 2005 16:13:25 Composite Trame par dØfaut être corrélées jusque dans des sections coevalentes localisées en Egypte centrale mais aussi dans l'océan Indien, carac-térisées par une activité volcanique liée à un point chaud. De ce fait, l'étude des assemblages de foraminifères plancto-niques confirme les observations géochimiques et minéralogiques indiquant un influx volcanique important et met en lumière les effets négatifs d'un volcanisme intense sur la biosphère.
[show abstract][hide abstract] ABSTRACT: The investigation of eight Cretaceous-Tertiary (K/T) sections in Mexico, based on major and trace element, platinum group element (PGE), stable isotope, and multivariate statistical analysis, reveals a complex depositional history across the Chicxulub and K/T boundary events. At the biostratigraphically determined K/T boundary, a minor but significant Ir-dominated PGE anomaly (0.2–0.8 ng/g) is present in most sections. This Ir anomaly originated from an impact event and is always stratigraphically and geochemically decoupled from the underlying spherule-rich ejecta deposit related to the Chicxulub event. In all sections examined, one to three glass spherule ejecta layers and one or two chondrite-dominated PGE anomalies are separated by a bioturbated siliciclastic deposit and/or hemipelagic marl, which indicates the occurrence of at least two impact events separated by a considerable amount of time. In addition, bentonite layers and Pt and Pd-dominated PGE anomalies below and above the K/T boundary indicate volcanic activity. Above the K/T boundary, reduced bioproductivity is documented by a decrease in the biogenically bound fraction of nutrients and fluctuating ratios of immobile elements (e.g., Ti/Zr). Variations in detrital elements reflect changes in the depositional environment. Carbon and oxygen isotope and trace element distribution patterns indicate a gradually changing climate during the latest Maastrichtian, an abrupt change at the K/T boundary, and a slight recovery during the lowermost Paleocene.
Geochimica et Cosmochimica Acta 01/2005; 69(10):2559-2579. · 3.88 Impact Factor
[show abstract][hide abstract] ABSTRACT: Abstract Yaxcopoil-1 (Yax-1), drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely 65 Myr ago and caused the mass extinction at the Cretaceous-Tertiary (K/T) boundary. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic, and iridium) that revealed that the Chicxulub impact predates the K/T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1 and is corroborated by magnetostratigraphic chron 29r and characteristic late Maastrichtian stable isotope signals. These results were first presented in Keller et al. (2004). In this study, we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic, and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K/T boundary and that the sediments between the breccia and the K/T boundary were deposited in a normal marine environment during the last 300,000 years of the Cretaceous.
[show abstract][hide abstract] ABSTRACT: Yaxcopoil-1 (Yax-1), drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely 65 Myr ago and caused the mass extinction at the Cretaceous-Tertiary (K/T) boundary. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic, and iridium) that revealed that the Chicxulub impact predates the K/T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1 and is corroborated by magnetostratigraphic chron 29r and characteristic late Maastrichtian stable isotope signals. These results were first presented in Keller et al. (2004). In this study, we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic, and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K/T boundary and that the sediments between the breccia and the K/T boundary were deposited in a normal marine environment during the last 300,000 years of the Cretaceous.
[show abstract][hide abstract] ABSTRACT: Many K/T sections all over the world contain impact spherules supposed related to the Chicxulub event. This study focus on ejecta layers in NE-Mexican profiles. We carried out systematic XRF and synchrotron radiation measurements on such spherules at the HASYLAB and ANKA facilities as well as microprobe analyses (CAMECA SX50). Area scans on tektite-like material of the Bochil section reveal a pronounced zonation in the inner part, dominated by Ba and Sr whereas secondary CaCO3 dominates in the altered margin. The composition of the spherules from the Mesa-Juan Perez section differ significantly from the Beloc (Haiti) and Bochil tektite glasses. At Mesa-Juan Perez, spherules are either extremely rich in Fe and Ca or consist of smectite, some of those carry carbonate inclusions. Yttrium, La and Ce are zoned within the smectite with concentrations below the detection limit and up to 20 æg/g The Ca-rich inclusions are enriched in Y (up to 35 æg/g) and La (18 æg/g) and, compared to the surrounding smectite, also in Ce (up to 34 æg/g). The Ce enrichment in spherules from the Mesa-Juan Perez section indicates impact-melted carbonates of the Yucatan carbonate platform as possible precursor rocks. Recent investigations focus on the chemistry of melt rock samples from the PEMEX wells Yucatan-6 and Chicxulub-1: Their average composition (mean of 250 data points in wt-percent ) is 61.6 for SiO2, 0.16 for TiO2, 18.07 for Al2O3, 0.01 for Cr2O3, 1.98 for Na2O, 1.5 for FeO, 0.05 for MnO, 0.01 for NiO, 0.31 for MgO, 9.14 for K2O, 3.44 for CaO, and 0.01 for SO2. These results are in some cases comparable to the geochemistry of ejecta glasses, e.g. from Beloc (Haiti).
[show abstract][hide abstract] ABSTRACT: CSDP core Yaxcopoil-1 was drilled to a depth of 1,511 m within the Chicxulub crater. An organic-rich marly limestone near the base of the hole (1,495 to 1,452 m) was deposited in an open marine shelf environment during the latest Cenomanian (uppermost Rotalipora cushmani zone). The overlying sequence of limestones, dolomites and anhydrites (1,495 to 894 m) indicates deposition in various carbonate platform environments (e.g., sabkhas, lagoons). A 100-m-thick suevite breccia (894–794 m) identifies the Chicxulub impact event. Above the suevite breccia is a dolomitic limestone with planktic foraminiferal assemblages indicative of Plummerita hantkeninoides zone CF1, which spans the last 300 ky of the Maastrichtian. An erosional surface 50 cm above the breccia/dolomite contact marks the K/T boundary and a hiatus. Limestones above this contact contain the first Tertiary planktic foraminifera indicative of an upper P. eugubina zone P1a(2) age. Another hiatus 7 cm upsection separates zone P1a(2) and hemipelagic limestones of planktic foraminiferal Zone P1c. Planktic foraminiferal assemblages of Zone Plc to P3b age are present from a depth of 794.04 up to 775 m. The Cretaceous carbonate sequence appears to be autochthonous, with a stratigraphic sequence comparable to late Cretaceous sediments known from outside the Chicxulub crater in northern and southern Yucatan, including the late Cenomanian organic-rich marly limestone. There is no evidence that these sediments represent crater infill due to megablocks sliding into the crater, such as major disruption of sediments, chaotic changes in lithology, overturned or deep dipping megablocks, major mechanical fragmentation, shock or thermal alteration, or ductile deformation. Breccia units that are intercalated in the carbonate platform sequence are intraformational in origin (e.g., dissolution of evaporites) and dykes are rare. Major disturbances of strata by the impact therefore appear to have been confined to within less than 60 km from the proposed impact center. Yaxcopoil-1 may be located outside the collapsed transient crater cavity, either on the upper end of an elevated and tilted horst of the terrace zone, or even outside the annular crater cavity. The Chicxulub site thus records a large impact that predates the K/T boundary impact and mass extinction.
International Journal of Earth Sciences 07/2003; · 2.26 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Yaxcopoil-I core was drilled approximately 60 km to the south of the centre of the Chicxulub impact crater at the inner flank of the structure (Smit et al., 2002). Stratigraphical investigations of our group (Stinnesbeck et al., this volume) revealed different lithotypes of the pre- and post-K/T sequence within the sampled core interval. First chemostratigraphical investigations were carried out on 60 bulk samples from the breccia unit (794.65--894.51 m), dolomites below (894.51--977.83 m) and dolomites and marly limestones above (794.65--773.5 m) by means of XRF. The dolomites below the breccia unit are characterized by very constant CaO (43.5--45.2%), extremely low Rb (1--5 mug/g) and Zr (˜5 mug/g) contents, indicating low input of terrigenous material. The suevitic dyke breccias, intercalated in the dolomite, do not contrast in their element contents of terrigenous origin (Rb, Zr and Ti) compared to the surrounding dolomites, while Sr concentrations fluctuate rhythmically between 150--320 mug/g. The low Sr contents support microscopic results suggesting secondary dolomitisation as well as an autochthonous origin for the suevitic dyke breccias. The breccia unit between 794.65--894.51 m is characterized by low CaO (˜5--25%) and variable contents of TiO_2 (0.06--0.83%), Fe_2O_3(0.06--10.14%), Zr (3--127 mug/g), Ba (
[show abstract][hide abstract] ABSTRACT: We examined the lithology, stratigraphy and mineralogy of the KT
transition in the Yaxcopoil-1 hole which was drilled on the southern
inner flank of the Chicxulub impact crater, approximately 60 km from its
center. The100m thick suevite breccia that marks the Chicxulub impact
ends abruptly with an undulose and erosive upper contact and is overlain
by 60cm of dolomitic limestones with small cross- and flaser-bedding and
numerous small disconformities. The lower 25cm of this interval are
unfossiliferous due to intense dolomitization. At the top of this
interval is an erosive contact overlain by a 3-4mm thick glauconitic
layer that indicates a significant hiatus. The first Danian planktic
foraminifera are present 2cm above the glauconitic layer (P. eugubina)
in a micritic limestone that marks an open marine environment. Small
early Danian planktic foraminifera are abundant in the next 5cm and also
indicate the P. eugubina zone with a benthic assemblage indicative of
middle to outer neritic environments. Mottled sediments and bioturbation
by Chondrites and Thalassinoides are common. Middle to outer shelf
evironments persisted into the Danian P1c zone, when shallow carbonate
platform conditions where reestablished. The lithologic succession
above the suevite breccia reflects normal conformable and homogenous
deposition characterized by low energy environments and sea level
changes. There is no evidence of significant detrital input, reworked
breccia or altered glass and the sediments above the breccia are
surprisingly unaffected by post impact events, such as mass sliding and
slumps, which would be expected to be considerable immediately following
the Chicxulub impact. The age of the suevite breccia is difficult to
establish in the absence of microfossils. However, this impact breccia
is temporally and lithologically separated from the earliest Danian by
normal marine sedimentation that exhibits periods of bioturbation,
hardgrounds, erosion and the formation of glauconite layers, all of
which indicate that sediment deposition occurred over an extended time
period which may have preceded the KT boundary. These data indicate that
the underlying suevite breccia, which marks the Chicxulub impact, may
predate the KT boundary, similar to the impact glass spherule layers in
[show abstract][hide abstract] ABSTRACT: The Yaxcopoil-1 CSDP hole was drilled on the southern inner flank of the
Chicxulub impact crater, approximately 60 km from its center.
Lithological, sedimentological and stratigraphic investigations of this
core between a depth of 775m and 1511m identified 10 major bio-and
lithostratigraphic units. The lower part of the core up to 894m contains
a sequence of limestones, dolomites and anhydrites of late early to late
Cretaceous ages. These lithologies indicate deposition in various
carbonate platform (e.g. sabkha, lagoons) and open marine shelf
environments. Anhydrites make up for approximately 25% of the section
(157m of >615m), less than expected from published records. A 100m
thick suevitic breccia is present between 794-894m, with the top 15m
reworked by ocean water. Dolomitic limestones and limestones overlie the
suevite. At 25cm above the breccia/dolomite contact the first early
Danian P. eugubina Zone planktic foraminifers are present and indicate
deposition in a quiet open marine shelf environment. Upsection, the
limestones of planktic foraminiferal Zone P1c indicate more proximal
conditions with debris flows from nearby shallow carbonate platform
environments.These data suggest that the Cretaceous sediment sequence is
largely autochthonous, with a stratigraphical sequence comparable to
Cretaceous sediments known from outside the Chicxulub crater in northern
and southern Yucatan. Our investigation revealed no evidence for major
disruption of sediments, chaotic changes in lithology, overturned or
deep dipping megablocks, or major mechanical fragmentation, shock
alteration or ductile deformation. Breccia units intercalated in the
carbonate platform sequence show conformable and sometimes gradational
contacts to under- and overlying non-brecciated sediments and appear to
be of intraformational origin (e.g. dissolution of evaporites), rather
than a consequence of instant shaking, shearing and sliding of impact
megablocks. No glass spherules or altered glass shards are present in
any of these layers and no basement rocks or other exotic clasts were
detected. Only a single dyke was recognized at a depth of 1399m, in
addition to two dykes of suevitic breccia (at 915m and 909m) close to
the contact between Cretaceous sediments and overlying suevite. The
contact between suevite and overlying hemipelagic carbonates is abrupt.
The absence of a gradational contact and transitional lithologies is
surprising, considering the enormeous amount of compaction, mass sliding
and subsequent shaking which is expected to have accompanied the
post-impact period in and around the Chicxulub crater.
[show abstract][hide abstract] ABSTRACT: The K/T boundary is long known as one of a few mass extinctions in earth history. The impact of a big meteorite at the Chicxulub on the northern Yucatan peninsula in Mexico is discussed to have triggered the faunal mass extinction and the rapid change of the palaeoenvironmental conditions near the K/T boundary. Tectite material, especially spherules are explained from many of the sections in correlation to the K/T-boundary event. This rare, glassy or alterated material is extremely variable in its major element chemistry, morphology and stratigraphic position in K/T transitions worldwide. For the first time, we perfom trace element analysis on tectites from the K/T boundary using synchrotron radiation XRF (SRXRF). Measurements were performed at the Hamburger Strahlungssynchrotronlabor HASYLAB at DESY (Hamburg, Germany) and at the ANKA (Karlsruhe, Germany) with polychromatic and monochromatic excitation, respectively collimating the beam to 15 æm by capillary optics. Based on results from SRXRF microprobe determinations, these structures are to be interpreted as mixing of several melts with different chemical composition. The different components may represent melts from different sediment layers and possibly of basement material excavated by the Chicxulub impact. Igneous rocks with andesitic composition in cores at Chicxulub are considered to be impact melt rocks and are correlated mainly by the composition of major elements with the glass spherules found in the surrounding. Our investigations show that it is possible to trace elements with high sensitivity and a high spatial resolution. Some of the samples show clearly zonation and alteration parts, as well as carbonate inclusions, triggered by the Chicxulub impact event. In general, the results from the SRXRF show that the tectite material have different trace element patterns, formed by mixing of melts with different chemical composition derived from different sediment layers and possibly of basement material excavated by the Chicxulub impact. There is no evidence at the moment that there is a homogeneous origin in the sample material or distribution in the investigated sections. The enrichment of Ce in spherules from the Mesa-Juan Perez section indicates a possible origin from the Yucatan carbonate platform generated by the Chicxulub impact event near the K/T-boundary. Area scans from tektite material of the Bochil section show a clearly zonation in the inner part, dominated by Ba and Sr as well as a alteration margin dominated by secondary CaCO3. Glassy material of the Beloc (Haiti) section is characterised by a homogeneous trace element distribution but shows characteristic differences between Ca-rich and Ca-poor glass. Moreover there is no similarity to material from other sections investigated. A clear differentiation between alteration rims, non-alterated material and mixing of different source materials can be shown by space resolved trace element determination in æm scale of schlieren structures and inclusions.(see also Schulte et al. this volume)
[show abstract][hide abstract] ABSTRACT: Proximal ejecta deposits in sections from NE Mexico (Rancho Nuevo, La Sierrita, El Peñon, El Mimbral) have been investigated by backscattered electron imaging, wave-length dispersive electron microprobe analyses, and cathodoluminiscence, in order to characterize target lithologies, and ejecta mixing, fractionation, and distribution mechanisms. Additional investigations included magnetic properties (Kontny et al, this meeting) and trace element analyses (Harting et al, this meeting). Petrological features of these ejecta deposits are extraordinarily well preserved. They consist of mm-cm sized vesiculated spherical to drop-shaped spherules and angular to filamentous (ejecta-) fragments, as well as carbonate clasts, marl clasts, and rare benthic foraminifera floating in a carbonaceous matrix. Occasionally, spherules and fragments show welding-amalgamation features and enclose other components, thus resulting in a foam-like texture. An origin from the Chicxulub impact is suggested by geographical proximity and morphologically similarity to spherules found in other K-T sites in North to Central America and the Atlantic. The far distribution of such coarse-grained, foamy, and fragile ejecta-clasts as well as welding features suggest ignimbrite-like transport mechanisms or nearby secondary impacts. Several silicic ejecta phases have been observed that occur as distinct phases, even within one ejecta particle with textures indicative of liquid immiscibility: (1) Fe- (25-35 wt%), Mg- (10-15 wt%) rich phases with 60 wt%) rich andesitic glasses. In addition to these silicic phases, abundant carbonate characterizes all studied ejecta deposits. It occurs within spherules and fragments and as clasts and globules, and shows textures indicative of either liquid immiscibility and/or quenching (`feathery calcite'). Quenched carbonates are enriched in Fe and Mg (up to one wt%) and are characterized by dark red-brown luminescence, in contrast to the carbonaceous matrix, which shows bright luminescence colors. Within all phases, but mainly in (1), various inclusions have been observed: (a) Globules enriched in Fe and Mg, (b) schlieren, rich in Ti, K, Fe, (c) garland-shaped Ti-rich lamellae, (d) dendritic and skeletal crystals of Ti-Fe oxides, (e) hematite crystals with a Ni-content up to 0.4 wt%, as well as goethite and rutile crystals, (f) rare mum-sized Co-, Ni-, Fe-rich metallic or sulfidic particles. These compositional phases are present in all studied outcrops, but their individual amount varies with prevailing Fe-rich phases at Rancho Nuevo and La Sierrita and Fe-, K-rich and silicic phases at El Peñon and El Mimbral. These characteristics imply an origin of the ejecta from mafic lithologies and carbonaceous sediments, in addition to contribution from felsic rocks. The occurrence of different compositional phases in single ejecta layers and even within individual ejecta particles suggests strong fractionation effects and/or negligible mixing of different melt phases. The presence of metallic Fe, Ni and Co may indicate that additional contamination by meteoritic material occurred.