Villains at the Cretaceous-Tertiary boundary in Neuquén Basin, Argentina.

SCASSO, R.A.; KIESSLING, W.; MEDINA, F.A.; RUIZ, L.

Trelew, Chubut. Argentina

Congreso; ICHNIA, First International Congress on Ichnology,; 2004

International association of ichnology

A strongly bioturbated sandy clastic layer of KT boundary age occurs in a homogeneous neritic shelf mudstone sequence in the Neuqutn Basin of Argentina. This layer, 15-25 cm thick, contains reworked pyroclastic plagioclase, shells, and shark teeth. Sedimentological features, such as an erosive base, abundant rip-up clasts, normal grading and hummocky cross-bedding, are characteristic of tsunami deposits. Nannofossils indicate a late Maastrichtian (CC 26) age of the mudstones until the base of the sandstone, and a Danian age in the overlying mudstones, but the sandy layer itself contains a mixed Maastrichtian-Danian nannoflora. ´The bioturbation gives a mottled appearance to the sandstone and penetrates up to 15 cm in the mudstone below, forming a dense network close to the contact. It consists of horizontal, oblique or rarely vertical, curved to irregular tubes with elliptical sections about 0.5 cm wide, belonging to the ichnogenus Planolites. Sandstone infillings of the burrows show that particles were dragged by burrowing animals from below the contact after the accumulation of the tsunami bed. Post-event bioturbation could also produce a similar pattern, mixing the microhuna accumulated ar the top of the bed with the rest of it, "pushingn the apparent boundary down (even below the sandstone bed). The KT boundary stratigraphy in Neuqutn was dramaticdly modified by endobenthic activity. The careful study of the trace fossils was thus crucial fbr the placement and interpretation of bed boundaries in this high-resolution stratigraphic study.