ELUCIDATING THE AGGREGATION PROCESSES OF THE CHICXULUB PROXIMAL EJECTA BLANKET: A VOLCANOLOGICAL APPROACH

Aggregation processes in explosive volcanic eruptions are well-studied, while formation and emplacement of impact-generated aggregates remain poorly understood. Particle aggregation involves collision and adhesion of suspended particles in turbulent hot gas mixtures with binding agents (e.g., water, electrostatic forces). Aggregation can affect ash dispersal and needs to be considered for ash plume forecasting, modeling, and potential climatic effects. Impact-generated aggregates provide insights into the dynamics of proximal impact ejecta blankets. Aggregates were observed at large impact structures, such as Chicxulub (200 km, 66.05 Ma, Mexico), and Sudbury (250 km, 1.85 Ga, Canada).

Petrographic and geochemical investigations were conducted on impact-generated accretionary clasts (ACs), found at two K–Pg sites from the Chicxulub proximal ejecta blanket: (1) Albion Formation (Fm), 330–360 km southeast from Chicxulub, and (2) El Guayal, 520 km southwest. These ACs were compared with accretionary pellets found in ignimbrite deposits from Tenerife (Canary Islands, Spain).

Albion Fm ACs (1–10 mm) are mainly composed of carbonate material, as shown by high CaO contents (72–98 wt%). They appear as angular lithic cores surrounded by a fine-grained rim or as unstructured aggregates. El Guayal ACs (0.5–2 cm) are composed of a core (ash, rock, or mineral, 100–400 μm) surrounded by several millimeter-sized concentric laminations, very similar to accretionary pellets observed in Tenerife ignimbrite deposits. El Guayal ACs are SiO₂-rich (~85 wt%), while the host matrix is SiO₂-poor (23.7 wt%) and CaO-rich (52.4 wt%). Three AC types are identified in the proximal ejecta: AC1 (poorly structured), AC2 (core with concentric laminations), and AC3 (angular millimeter-sized lithic core with fine-grained rim). This classification scheme can be used as a solid framework to describe ACs from impact structures other than Chicxulub.

Albion Fm ACs likely formed at the basal part of high-energy-and-density flows, above pre-impact Cretaceous dolomite, where electrostatic charging of fine particles occurred. At El Guayal, the energy of the density current decreases with increasing distance. Air and water are entrained inside the current, forming a co-ignimbrite-like ash cloud where ACs formed, similarly to formation of accretionary pellets in ignimbrite from Tenerife.