OBSERVATIONS OF ACCRETIONARY LAPILLI FROM THE CHICXULUB IMPACT EVENT

Accretionary lapilli (AL) are cm-scale rounded accretionary clasts, typically containing a core and concentric zones of accreted ash particles, most commonly associated with volcanic eruptions, and sometimes forming in hypervelocity impact events. AL associated with impact events are not well understood. In this study, AL from the Chicxulub impact event (at the Guayal site, roughly 600 km from the crater center) are investigated using petrographic and backscatter electron images for determining the large-scale and microscopic structures, and chemical composition of the clasts within the lapilli. AL often follow a pattern of (1) a core with many ash-coated clasts that are up to 100 µm in diameter; (2) elongate inclusions within the AL that are tangentially aligned around the spherical core, forming concentric outer rims; (3) a fining-out sequence of particles becoming extremely fine-grained at the outer edge of the AL. The fining-out sequence can sometimes repeat in multiple layers. Clasts are typically composed of carbonate, quartz, glass, and, less commonly, feldspar. Clasts outside of the core are usually < 50 µm in diameter. Clasts can be both well-rounded and very angular, although angular clasts seem to dominate the population. AL retain some porosity, particularly in the outer layers. The composition of clasts within the AL differs from the surrounding matrix, demonstrating either fractionation of the ash or that the source of the surrounding ejecta differs from the AL. The AL have been silicified post-depositionally, but many broken carbonate clasts, feldspars, and some quartz grains appear to be primary. The mechanism of formation has previously been attributed to a ground-hugging pyroclastic density current, as would be typical of a volcanic eruption. However, due to the great distance from the impact event and the constraints on the formation of the AL, we propose a model of aggregation, whereby an ash cloud in the upper atmosphere that would eventually form the well-known global ejecta layer associated with the K/Pg extinction, locally had enough turbulence and density to produce aggregate clasts, which rained in specific areas several hundred km from the impact crater.