Spherule deposits, commonly interpreted as ejecta from the Chicxulub
impact at Yucatan, 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 in 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
ejecta-dispersion 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 (similar to50
wt% SiO2; 5-8 wt% K) is also present. They contain Ti-, Fe-, K-rich
schlieren, Fe-, Mg-rich globules, and rare pm-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.