Fe-rich and K-rich mafic spherules from slumped and channelized Chicxulub ejecta deposits in the northern La Sierrita area, NE Mexico

TitleFe-rich and K-rich mafic spherules from slumped and channelized Chicxulub ejecta deposits in the northern La Sierrita area, NE Mexico
Publication TypeJournal Article
Year of Publication2003
AuthorsSchulte, P, Stinnesbeck, W, Stüben, D, Kramar, U, Berner, Z, Keller, G, Adatte, T
JournalInternational Journal of Earth Sciences
KeywordsChicxulub, Ejecta, Iron-rich, K-T boundary, Mafic composition, Mexico, Spherules

Spherule deposits, commonly interpreted as ejecta from the Chicxulub impact at Yucatn, Mexico, are present in many K-T (Cretaceous-Tertiary) sections. Geological mapping of the northern La Sierrita area, NE Mexico, revealed the presence of (1) multiple spherule deposits embedded in late Maastrichtian marls, which are folded or disaggregated (breccia-like). They are up to 6 m thick, locally present in two outcrop areas, and show limited lateral continuity. These deposits consist of mm– cm sized spherical to drop-shaped vesiculated spherules, angular to filamentous (ejecta-) fragments and abundant carbonate. They are interpreted as primary ejecta fallout deposits that have been affected by subsequent local slumps-slides, liquefaction, and debris flows; welded components suggest an initial ground surge-like ejectadispersion mode. (2) A spherule deposit, 10–60 cm thick that constitutes the base of a channelized sand-siltstone deposit at, or close to, the K-T boundary and is characterized by wide lateral continuity. It is of similar petrologic composition to deposit (1), though slightly enriched in terrigeneous detritus, thus reflecting influx from proximal shelf areas. It is interpreted to result from debris flows and turbidite currents, though no size sorting and abrasion of ejecta has been observed. Petrological, mineralogical, and geochemical criteria suggest that ejecta components from both types of spherule deposits are similar and originated from the Chicxulub impact, with multiple deposits produced by subsequent remolding, reworking, and redeposition. Spherules and fragments have an Fe- (25–30 wt%), Al-, Mg-rich and Si-poor (<25 wt% SiO2) composition, and are altered to chlorite and iron-oxides, though rare K-rich mafic glass (~50 wt% SiO2; 5–8 wt% K) is also present. They contain Ti-, Fe-, K-rich schlieren, Fe-, Mg-rich globules, and rare m-sized metallic and sulfidic Ni-, Co-rich inclusions. Carbonate as clasts and within spherules and fragments shows textures indicative of quenching and/or liquid immiscibility. Although potential ejecta fractionation and alteration make accurate evaluation difficult, this composition suggests an ejecta origin mainly from mafic lithologies and carbonaceous sediments, in addition to a contribution from intermediate felsic rocks and the possibility of meteoritic contamination.  PDF