PETROLOGICAL, GEOCHEMICAL AND MAGNETIC CHARACTERIZATION OF CRETACEOUS-PALEOGENE BOUNDARY SECTIONS, GULF OF MEXICO

We present initial results of a petrological, geochemical and magnetic study of El Mimbral and La Lajilla sections that span the Cretaceous-Paleogene (K-Pg) boundary in northeastern Mexico because of their relationship to the Chicxulub crater in the Yucatan platform and for investigate the nature, origin, stratigraphic relations and age of the impact ejecta deposits. The K-Pg boundary is preserved between hemipelagic marls and limestones rocks of the Mendez (Maastrichtian) and Velasco (Paleocene) formations. Sections are situated about 1000 km away from Chicxulub and their K-Pg deposits are part of the proximal ejecta deposits and the complex channelized siliciclastic units. These units were separated in two parts, with a basal coarse poorly graded spherulitic bed some 0.2 to 1 m thick and a second part with several sandstone siltstone beds. Using field observations and analytical data, detailed columns for the two localities we prepared samples collected across stratigraphic profiles for petrological, geochemical and magnetic rock analyses. X-ray fluorescence analyses on whole rock were complemented with previous data obtained for the Mimbral section. Further, detailed analyses were concentrated in the ejecta material.

Magnetic rock´s measurements include susceptibility, remanent and isothermal magnetization and remanent coercivity. Magnetic hysteresis loops and IRM and back-field demagnetization were measured for samples of spherulitic bed.

The spherulitic bed is characterized by rich Fe-Mg, chlorite and rich Si-Al-K glass spherules and carbonate accretionary lapilli spherules. The silicic component spherules are altered to calcite or chlorite-smectite, with some retaining glass cores. Spherules have been shown to contain Fe-Mg bubbly spherules, Fe-Ti-K schlieren and micrometer size metallic inclusions documenting a compositional range of mafic to intermediate rocks, which relate to the target stratigraphy in Yucatan with the thick surface carbonate platform sediments and the granitic and metamorphic basement. Sixty individual spherules were separated from the two section beds. They display different morphologies, degrees of alteration, surface colors and sizes, with vesiculated globular spherules. They often form aggregates fractured and deformed spherules appear almost altered.