2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 12
Presentation Time: 4:30 PM

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


SHIPLEY, K.W.1, ASCHOFF, J.L.1, LAWTON, T.F.1, GILES, K.A.1 and VEGA, F.J.2, (1)Institute of Tectonic Studies, New Mexico State Univ, Las Cruces, NM 88003, (2)Instituto de Geología, Ciudad Universitaria, Circuito Exterior, Delegación Coyóacan, 04510, shipkw@hotmail.com

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.