The first day of the Cenozoic

Abstract

Highly expanded Cretaceous-Paleogene (K-Pg) boundary section from the Chicxulub peak ring, recovered by International Ocean Discovery Program (IODP)-International Continental Scientific Drilling Program (ICDP) Expedition 364, provides an unprecedented window into the immediate aftermath of the impact. Site M0077 includes ∼130 m of impact melt rock and suevite deposited the first day of the Cenozoic covered by <1 m of micrite-rich carbonate deposited over subsequent weeks to years. We present an interpreted series of events based on analyses of these drill cores. Within minutes of the impact, centrally uplifted basement rock collapsed outward to form a peak ring capped in melt rock. Within tens of minutes, the peak ring was covered in ∼40 m of brecciated impact melt rock and coarse-grained suevite, including clasts possibly generated by melt-water interactions during ocean resurge. Within an hour, resurge crested the peak ring, depositing a 10-m-thick layer of suevite with increased particle roundness and sorting. Within hours, the full resurge deposit formed through settling and seiches, resulting in an 80-m-thick fining-upward, sorted suevite in the flooded crater. Within a day, the reflected rim-wave tsunami reached the crater, depositing a cross-bedded sand-to-fine gravel layer enriched in polycyclic aromatic hydrocarbons overlain by charcoal fragments. Generation of a deep crater open to the ocean allowed rapid flooding and sediment accumulation rates among the highest known in the geologic record. The high-resolution section provides insight into the impact environmental effects, including charcoal as evidence for impact-induced wildfires and a paucity of sulfur-rich evaporites from the target supporting rapid global cooling and darkness as extinction mechanisms.

Keywords: Chicxulub impact crater; Cretaceous–Paleogene; peak ring; suevite; tsunami.

Fig. 1.

(A) Seismic reflection image shown in depth with full waveform velocities overlain; line runs from southeast to northwest, including the location of Site M0077, and radially outward across the annular trough. The suevite interval within M0077, the focus of this paper, is shown in red, which maps to a low-velocity zone beneath the crater floor. The map in Inset shows the location of crater rings, drill sites (in the text), the seismic image, and the direction that ocean waters reentered the crater after formation. Expansion shows (B) representative core images in stratigraphic order with depths, (C) lithologic units, and (D) lithology.

Fig. 2.

Key events within the first day of the Cenozoic based on numerical modeling, geophysical data, and IODP Expedition 364 drilling. The figure includes 2 perspectives: a westerly oriented radial profile crossing inner crater rim and shallow shelf and a northeasterly oriented radial profile that crosses the opening in crater rim into the Gulf of Mexico. (A) Approaching 12-km-sized impactor over the preimpact target of the Yucatán peninsula. (B) A 100-km-wide transient crater and remnant of the impact plume consisting of vaporized/fragmented limestones, evaporites, and granitic basement rocks (timescale and geometry based on ref. 5). (C) Collapse of the transient crater with upward formation of a central uplift starting to undergo dynamic collapse (timescale and geometry based on refs. , , and 44). (D) Morphology after central uplift collapse and peak ring formation (based on refs. and 9). Initial ocean resurge is depicted entering the crater with timescale based on a dam break model and undergoing MWIs. (E) Ocean resurge completes cresting the peak ring where Site M0077 was drilled. (F) Settling of debris within the now flooded crater to form the bulk of the suevite deposit that blankets the peak ring, with a zoomed-in view of processes (including seiches) and deposits capping the peak ring. (G) Tsunami entering crater from returning rim-wave tsunami and shelf collapses, with a zoomed-in view of the peak ring K-Pg deposits (timescale based on ref. 34). Rx, rocks.

Fig. 3.

Combined analysis of the larger-clast size portion of the suevite using line-scan images of the Hole M0077A split cores; A to E show data from visual line logging in orange and machine learning analysis in blue. F shows downhole sonic log from Site M0077 over the same interval. Interpreted intervals are shown for MWI deposits, resurge of ocean waters cresting peak ring at Site M0077, and start of settling in the now-flooded crater to generate the resurge deposit.

Fig. 4.

Core 40 from Site M0077 data. (A1 and A2) Scanning electron microscope images of charcoal fragments. (B1 and B2) Reflected light microscope images of charcoal fragments at 1,600× magnification. (C) Line-scan image of Core 40 showing the Transition Layer (Unit 1G) and the uppermost suevite (Unit 2A). (D) Total PAH data and the dominant PAH observed in the cross-bedded layer, perylene, both shown as micrograms per gram of total organic carbon (TOC). (E) Charcoal counts showing concentrations just above the interpreted tsunami and near the top of Unit 1G. (F) Zoomed-in view of line-scan image of the cross-bedded interval at the top of Unit 2A interpreted as being deposited by a tsunami. (G) Unwrapped CT scan of same portion of core as F. (H) Line-scan image of higher-energy deposits beneath the tsunami interpreted as seiches. (I) Unwrapped CT scan of same portion of core as H.