SEDIMENTOLOGIC AND PETROGRAPHIC ANALYSIS OF UPPER CRETACEOUS IMPACT-RELATED DEPOSITS, LA POPA BASIN, NORTHEASTERN MEXICO

Ejecta-rich strata located at or near the K/T boundary in La Popa basin, NE Mexico, were deposited by tsunami waves and tsunami-induced subaqueous debris flows ensuing from the Chicxulub impact. An ejecta-bearing event deposit occupies a stratigraphic position within the upper 5-15m of the Delgado Sandstone Tongue (late Maastrichtian), on a sharp contact above delta-front and lower shoreface deposits. The event deposit is continuous throughout the basin, but varies laterally and stratigraphically in lithology and thickness (0.25-6.1 m). Internally, the deposit contains interbedded massive, sandstone matrix-supported, polymictic, boulder to cobble conglomerate and graded, coarse-grained sandstone. Conglomeratic units locally occupy a paleovalley that is about 600 m wide with an azimuth of ~145¢ª-325¢ª. The valley-fill succession is as much as 6.1 m thick and contains basal, subangular siltstone blocks (1.5 m length) overlain by 5 graded, pebbly sandstone beds. Conglomeratic strata onlap valley walls, whereas overlying sandstone units (wave-reworked conglomerate) overlap the entire succession. Conglomerate matrix and sandstones contain a distinctive assemblage of grains that includes: (1) bubbly calcite spherules; (2) micrite-coated silicate and sparry carbonate grains interpreted as accretionary lapilli; (3) glass tektites; (4) abundant bioclasts, commonly filled with micrite, but generally lacking micrite coats. Bubbly spherules, glass tektites, and coated grains are interpreted as impact ejecta; spherules are present at the very base of the deposit, indicating that ejecta were deposited at the site prior to reworking by wave and debris-flow processes. Ejecta are concentrated near the base of sandstone units and comprise approximately 10%-100% (locally) of framework grains. We infer conglomerate deposition by multiple subaqueous debris flows within the northwest-trending paleovalley. Overlapping, ejecta-bearing sandstones suggest a more regionally-extensive, instantaneous depositional mode such as tsunami wave(s). The abundant ejecta link both depositional processes with the impact at Chicxulub (800 km to the southeast, on a bearing of 115¢ª). Early arrival of ejecta may have taken place as base surge(s), created by the gravitational collapse of the atmospheric impact column, ran out across the Gulf of Mexico.