Stratigraphic and sedimentological observations from seismic data across the Chicxulub impact basin

Abstract

Abstract-Seismic data across the offshore half of the Chicxulub impact crater reveal a 145 kmdiameter post-impact basin to be a thickening of Tertiary sediment, which thickens by ~0.7 sec from the basin margin to the basin center. The basin existed long after the impact and was gradually infilled to its current flat surface. A suite of seismic horizons within the impact basin have been picked on four reflection lines across the crater. They reveal that the western and northwestern parts of the impact basin were filled first. Subsequently, there was a dramatic change in the depositional environment, indicated by an unconformable surface that can be mapped across the entire basin. A prograding shelf sequence downlaps onto this unconformity in the eastern basin. The seismic stratigraphic relationships suggest a marine regression, with sedimentation becoming gradually more passive as sediments fill the eastern part of the impact basin. The central and northeastern parts of the basin are filled last.The onshore hole Yaxcopoil-1 (Yax-1), which was drilled on the flanks of the southern basin, has been projected onto the offshore seismic data to the west of the crater center. Using dates obtained from this onshore well and regional data, approximate ages have been placed on the most significant horizons in the offshore seismic data. Our preliminary interpretation is that the western and northwestern basins were almost entirely filled by 40 Ma and that the marine regression observed in the eastern basin is early Miocene in age. Offshore seismic stratigraphic analyses and onshore data within Yax-1 suggest that the early Paleocene is highly attenuated across the impact basin. The Mesozoic section appears to be ~1 km thicker offshore than onshore. We calculate that, given this offshore thickening, the volume of Mesozoic rocks that have been excavated, melted, or vaporized during impact is around 15% larger than expected from calculations that assume the offshore thickness is equal to that onshore. This has significant consequences for any environmental calculations. The current offset between the K-T boundary outside and inside the crater is ~700 m. However, infilling of basins with sediments is usually accompanied by subsidence, and immediately following the impact, the difference would have been smaller. We calculate the original topographic offset on the K-T boundary to have been between 450 and 700 m, which is in agreement with depthdiameter scaling laws for a mixed target.

Volume 364: Chicxulub: Drilling the K-Pg Impact Crater

“…Morgan and Warner (1999) argue that the head scarp of this terrace zone is analogous to the crater rim in peak-ring craters ( Figure F1), and rings outside the head scarp (Figure F3) suggest that Chicxulub is a multi-ring basin (Morgan et al, 1997;Gulick et al, 2008). The acquired seismic data show that the water was deeper and the Mesozoic sediments thicker in the northeast quadrant of the crater than in the other quadrants (Bell et al, 2004;Gulick et al, 2008) and that lateral variation in the target at the impact site might explain the current crater asymmetry (Collins et al, 2008). Velocities and densities of the rocks that form the peak ring are low (Morgan et al, 2000;Vermeesch and Morgan, 2008;Barton et al, 2010), and a high-resolution velocity model obtained using full-waveform inversion ( Figure F4) shows that the uppermost peak ring is formed from about 100-150 m of rocks with low P-wave velocity (3000-3200 m/s) (Morgan et al, 2011).…”

Post‐impact structural crater modification due to sediment loading: An overlooked process

“…At Chicxulub crater, several small-throw faults restricted to the Cenozoic post-impact succession are observed and are mainly concentrated above the "inner ring" and peak ring (Fig. 3b) and the crater rim (e.g., Bell et al 2004). In detail, the internal post-impact depositional patterns reveal discrete fluxes in relative post-impact vertical movements, with sedimentation initially diminishing the impact-generated relief and subsequent differential compaction creating additional relief that is also filled (black triangles, Figs.…”

“…Continued deposition subsequently created a substantial overburden. For the Chicxulub crater, Bell et al (2004) postulated a spatially progressive infilling. In particular, the western and northwestern parts of the Chicxulub post-impact Cenozoic basin were filled ∼25 million years after the impact, whereas during a major marine regression a shelf progradation took place in the east ∼45 million years after the impact (in Early Miocene).…”

“…Reflector R1 represents the first continuous reflector to have surpassed and covered the impact-induced relief, whereas reflector R2 represents the stratigraphic level above which the inherited original impact structure is minimal. Based on interpolation of the onshore Yaxcopoil-1 borehole stratigraphy and seismic profile ties, a preliminary age of ∼40 Ma (middle Eocene) was assigned close to reflector R2 (Bell et al 2004).…”

“…The Tertiary sediments of the onshore Yaxcopoil-1 borehole are dominated by carbonaceous siltstone to limestone deposits (e.g., Arz et al 2004;Kenkmann et al 2004;Popov et al 2004). Nevertheless, it was argued that the thick offshore post-impact strata may also contain several siliciclastic deposits as revealed from the seismo-stratigraphic depositional patterns, including prominent progradating clinoform buildups (Bell et al 2004). In order to include various alternative possibilities, numerous porosity-depth relationships (Ø 0 and c combinations) ranging between siliciclastic (sand-and shale-dominated alternatives and mixtures), carbonaceous and dolomitic strata were used to decompact the post-impact succession at Chicxulub (Figs.…”