A dual layer Chicxulub ejecta sequence with shocked carbonates from the Cretaceous-Paleogene (K-Pg) boundary, Demerara Rise, western Atlantic

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doi: 10.1016/j.gca.2008.11.011
Author(s): Schulte, P.; Deutsch, A.; Salge, T.; Berndt, J.; Kontny, A.; MacLeod, K. G.; Neuser, R. D.; Krumm, S.
Author Affiliation(s): Primary:
Universität Erlangen-Nürnberg, GeoZentrum Nordbayern, Erlangen, Federal Republic of Germany
Universität Münster, Federal Republic of Germany
Bruker AXS Microanalysis, Federal Republic of Germany
Universität Karlsruhe, Federal Republic of Germany
University of Missouri, United States
Ruhr Universität Bochum, Federal Republic of Germany
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, 73(4), p.1180-1204. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. Includes appendices. 109 refs.; illus., incl. 4 tables, sketch map
Summary: An up to ≈2-cm thick Chicxulub ejecta deposit marking the Cretaceous-Paleogene (K-Pg) boundary (the "K-T" boundary) was recovered in six holes drilled during ODP Leg 207 (Demerara Rise, tropical western Atlantic). Stunning features of this deposit are its uniformity over an area of 30 km2 and the total absence of bioturbation, allowing documentation of the original sedimentary sequence. High-resolution mineralogical, petrological, elemental, isotopic (Sr-Nd), and rock magnetic data reveal a distinct microstratigraphy and a range of ejecta components. The deposit is normally graded and composed predominantly of rounded, 0.1- to max. 1-mm sized spherules. Spherules are altered to dioctahedral aluminous smectite, though occasionally relict Si-Al-rich hydrated glass is also present, suggesting acidic precursor lithologies. Spherule textures vary from hollow to vesicle-rich to massive; some show in situ collapse, others include distinct Fe-Mg-Ca-Ti-rich melt globules and lath-shaped Al-rich quench crystals. Both altered glass spherules and the clay matrix (Site 1259B) display strongly negative εNdT=65 Ma values (-17) indicating uptake of Nd from contemporaneous ocean water during alteration. Finally, Fe-Mg-rich spherules, shocked quartz and feldspar grains, few lithic clasts, as well as abundant accretionary and porous carbonate clasts are concentrated in the uppermost 0.5-0.7 mm of the deposit. The carbonate clasts display in part very unusual textures, which are interpreted to be of shock-metamorphic origin. The preservation of delicate spherule textures, normal grading with lack of evidence for traction transport, and sub-millimeter scale compositional trends provide evidence for this spherule deposit representing a primary air-fall deposit not affected by significant reworking. The ODP Leg 207 spherule deposit is the first known dual-layer K-Pg boundary in marine settings; it incorporates compositional and stratigraphic aspects of both proximal and distal marine sites. Its stratigraphy strongly resembles the dual-layer K-Pg boundary deposits in the terrestrial Western Interior of North America (although there carbonate phases are not preserved). The occurrence of a dual ejecta layer in these quite different sedimentary environments - separated by several thousands of kilometers - provides additional evidence for an original sedimentary sequence. Therefore, the layered nature of the deposit may document compositional differences between ballistic Chicxulub ejecta forming the majority of the spherule deposit, and material falling out from the vapor (ejecta) plume, which is concentrated in the uppermost part. Abstract Copyright (2009) Elsevier, B.V.
Year of Publication: 2009
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Atlantic Ocean; Carbonate rocks; Carbonates; Cenozoic; Chemical composition; Chicxulub Crater; Cretaceous; Demerara Rise; Deposition; Ejecta; Equatorial Atlantic; Geochemistry; K-T boundary; Leg 207; Lower Paleocene; Major elements; Mesozoic; Metamorphism; Mineral composition; North America; North Atlantic; Northwest Atlantic; ODP Site 1258; ODP Site 1259; Ocean Drilling Program; Paleocene; Paleogene; Petrography; Quenching; Sedimentary rocks; Shock metamorphism; Spherules; Stratigraphic boundary; Tertiary; Textures; Upper Cretaceous; West Atlantic; Western Interior; X-ray diffraction data
Coordinates: N090200 N092800 W0541100 W0544400
N092600 N092600 W0544400 W0544400
N091800 N091800 W0541200 W0541200
Record ID: 2009053806
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands