TSUNAMI BACKWASH DEPOSITS WITH CHICXULUB IMPACT EJECTA AND DINOSAUR REMAINS FROM THE CRETACEOUS-PALAEOGENE BOUNDARY IN THE LA POPA BASIN, MEXICO

The La Popa basin in NE Mexico features outstanding, continuous 3D exposures of the Cretaceous-Palaeogene (K-Pg) boundary event deposit in shallow shelf environments pierced by salt stocks. In the area to the south-east of the El Papalote diapir, the K-Pg deposit consists of two superimposed sedimentary units and erosively overlies upper Maastrichtian sand-siltstones with soft-sediment deformation structures. The basal unit 1 is an up to 8 m thick chaotic bed that discontinuously fills incised gutters and channels. Besides abundant silicic and carbonate ejecta spherules from the Chicxulub impact, this part includes sandstone boulders and abundant shallow-water debris (e.g., mud clasts, algae, bivalves, gastropods, vertebrate remains). Unit 1 is conformably overlain by unit 2. Distal to the diapir, unit 2 consists of a cm to dm-thick conglomeratic, bioclast and spherule-bearing sandstone bed. Closer to the diapir, unit 2 becomes a m-thick series of four to eight conglomeratic to fine-grained graded sandstone beds rich in shell debris and ejecta spherules. Unit 2 is conformably overlain by structureless sandstone beds that may mark the return to the pre-event depositional regime. The sedimentary characteristics of the K-Pg deposit support an origin by a short-term multiphase depositional event. The occurrence of soft-sediment deformation structures (e.g. liquefaction) below the event deposit suggests that earthquakes were the first to occur at La Popa. Then, shelf collapse and strong backflow from the first tsunami waves may have triggered erosion and deposition by violent ejecta-rich hyperconcentrated density flows (unit 1). Subsequently, a series of concentrated density flows resulting from tsunami backwash surges may have deposited unit 2. The specific depositional sequence and the Fe-Mg-rich as well as Si-K-rich composition of the ejecta spherules both provide a critical link to the deep marine K-Pg boundary sites in the adjacent Burgos basin in NE Mexico and other K-Pg boundary sites. The presence of diverse dinosaur and mosasur bones and teeth in the event deposit is the first observation of such remains together with Chicxulub ejecta material. These findings indicate that dinosaurs lived in the area during the latest Maastrichtian and suggest that the tsunami waves not only eroded deltas and estuaries but the coastal plain as well.