SEDIMENTOLOGIC CHARACTERISTICS OF CHICXULUB EJECTA IN BELIZE AND ADJACENT MEXICO

During the Chicxulub impact, the Yucatan platform was covered by a blanket of pulverized Upper Cretaceous carbonate debris comprising a continuous, direct-ejecta layer. This layer forms the modern Cretaceous-Tertiary (K-T) boundary interval. In northern Belize and adjacent Quintana Roo, Mexico, this boundary interval is called the Albion formation (~ 15 m). In central Belize, there is an equivalent, unnamed diamictite unit (~ 15 m). Within 1-meter square grids, we performed in-situ grain-size frequency measurements at Albion Island, Belize (360 km from crater center), in Quintana Roo, Mexico (340 km from crater center), and in Armenia, Belize (475 km from crater center). At Albion Island, these grain-size analyses show that ejecta matrix (i.e., components < 0.5 cm in diameter) ranges from 71 to 90 volume % and the maximum clast size per square meter (MCS/sm) ranges from 10 to 30 cm (- 6.64 to - 8.23 phi). In Quintana Roo, Mexico, ejecta matrix ranges from 64 to 79 volume % and MCS/sm ranges from 13 to 30 cm (- 7.02 to - 8.23 phi). At Albion Island and in Quintana Roo, the higher matrix content and correlative lower MCS/sm may relate to higher stratigraphic position within the ejecta layer. Albion Island and Quintana Roo grid data, plotted in 0.5-cm bins, show a break from continuous size distribution (i.e., having at least one particle in each size bin) to discontinuous size distribution at approximately 11 to 13 cm (- 6.78 to - 7.02 phi). We noted that larger grains (i.e., boulders ranging in diameter from < 1 to 9 m) also occur at Albion Island and in Quintana Roo, but these were not part of our in-situ measurements. At the Armenia section, in central Belize, the lower part of Albion formation-equivalent diamictite ranges from 71 to 91 % matrix and MCS/sm ranges from 24 to 35 cm (- 7.91 to - 8.45 phi). An upper, winnowed and possibly reworked interval (~ 6 m), contains only 37 % matrix and has a MCS/sm of 18 cm (- 7.49 phi). In the Armenia section's lower part, the continuous-discontinuous break in grain-size distribution occurs at approximately 4 cm (- 5.32 phi). Larger grains (< 35 cm) are apparently absent at Armenia. Review of the ejecta-curtain's main sedimentologic characteristics shows important differences between beds and laterally with distance from impact, which apparently relate to a complex pattern of ejecta deposition.