Chicxulub crater stratigraphy
The Chicxulub crater is a 145 km wide depression in subsurface of the northwestern Yucatan Peninsula of Mexico. It is thought to be the result of an asteroid impact approximately 65.5 Ma., dating at around the same time as the Cretaceous-Paleogene (K/T) extinction event. This mass extinction resulted in the loss of roughly 75% or plant and animal life on Earth, including non-avian dinosaurs. The impact crater formed a sedimentary basin that allowed for deposition and diagenesis to occur throughout the Cenozoic Era.
Evidence of impact
Seismic images have described the structure as a circular depression, complete with annular troughs and peak ring, similar in structure to craters seen on the Moon and Mars. Though the impact occurred mostly in deep water, the peak rings are granitic, suggesting the asteroid penetrated deeper into the Earth's crust than previously thought.
Most animal groups survived and diversified, such as fish, insects, mammals and birds. However, fossil groups such as ammonites, rudist bivalves and most famously, the dinosaurs became extinct as a result of the aftermath of this impact. Iridium is rare on Earth though abundant in meteorites. Around the world, iridium is concentrated along single horizons of the K/T boundary. The Alvarez hypothesis suggests that the iridium was widely deposited as scattered impact debris. Though shocked quartz is rare at most K/T boundary rocks around the world, it can be found in K/T rocks in and around North America. This is also a strong indicator that this is also the impact region. Tektites, also known as glass spherules are preserved in clays along the K/T boundary. The glass forms as the rock is melted at the point of impact. The molten material is thrown into the atmosphere, quickly cools and is rained down to the Earth's surface. Sites in the Caribbean, which is thought to be mostly quiet water show evidence of disturbed sediment at the end of the Cretaceous. Other sites in Texas and Mexico also exhibit tsunami deposits in the rock record. Because these areas are located on a passive margin, they typically would not be bombarded by tsunamis. A celestial collision nearby would explain such deposits.
Seismic imaging has revealed the underlying structure of the basin, with which depositional sequences can be deduced. The crater floor has a composition of both impact breccias and melt rocks. During the Paleocene immediately following the impact, shallow-water deposition of pelagic sediment began to infill the basin. Sparse reef growth developed within a shallow platform until the early Miocene. From the middle Miocene through Pleistocene, restricted water deposits filled the basin to its current flat surface. Horizon 6, is a continuous, parallel reflector. It onlaps the horizons in the east and west regions. This horizon marks the most recent stage of deposition as the crater is infilled with sediment. Subaerial exposure and freshwater diagenesis led to replacive dolomite and karsting. On the northern Yucatan peninsula, the perimeter of the crater can be identified by cenotes at the surface.
Western Annular Trough
Horizon 1 is interpreted as the K/T boundary. Low-amplitude reflectors in horizon 3 show toplap that extends from west, suggesting overall progradation to the east. The tops of these clinoforms indicate a sea-level of roughly 100m. Early to middle Eocene sedimentation continued to deposit in shallow-water. Clinoform geometry suggest a marine regression occurred with sedimentation decreasing in the western trough. Carbonate and evaporite deposits resulted from subaerial exposure of the carbonate platform. The upper part of Horizon 4 displays a truncation surface that marks a major boundary as well as a change in depositional environment. Chaotic reflectors in the seismic images can be interpreted as mass flow deposits during a regression. Using dates obtained from well data, the western and northwestern basins were filled entirely by 40 Ma.
Eastern Annular Trough
The eastern trough shows a different depositional history than the western trough. The facies characterization suggests a delta system. A prograding shelf sequence downlaps onto the unconformity in the eastern annular trough. Above this unit, a regression can be seen by offlapping clinoforms. During the late Eocene to early Oligocene, the exposed surface underwent freshwater diagenesis, which led to extensive dolomitization. Horizon 5 is a series of western-dipping clinoforms downlaping onto horizon 4. Chaotic geometries suggest a mass flow resulting from the drop in sea-level.
- Barton, P.J., Christeson, G.L., Grieve, R.A.F., Gulick, S.P.S., Morgan, J.V., Urrutia-Fucugauchi, J. (2013) Geophysical characterization of the Chicxulub impact crater. Retrieved from https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/rog.20007
- Bell, C., Hampson, G.J., Morgan, J.V., Trudgill, B. (2004) Stratigraphic and sedimentological observations from seismic data across the Chicxulub impact basin. Retrieved from https://onlinelibrary-wiley-com.ezproxy.lib.uh.edu/doi/epdf/10.1111/j.1945-5100.2004.tb01130.x
- Lefticariu, L., Lefticariu, M., Perry, E.C., Ward, W.C. (2006) Post-Chicxulub depositional and diagenetic history of the northwestern Yucatan Peninsula, Mexico. Retrieved from https://www-sciencedirect-com.ezproxy.lib.uh.edu/science/article/pii/S0037073805003118