Magnetostratigraphy of the Cretaceous/Tertiary boundary and early Paleocene sedimentary sequence from the Chicxulub Impact Crater

Abstract

We report on the magnetostratigraphy of the Chicxulub crater impact breccias and first 15 meters of the Paleocene sedimentary sequence recovered in three boreholes of the UNAM Scientific Drilling Program. Three geomagnetic polarity zones are documented in the impact breccias and sedimentary sequence, which span from chron 29R to 28N. For the 15 m interval they represent ∼2.5 Ma, which yields low apparent sedimentary rates for boreholes UNAM-5 (110 km from the center of the crater) and UNAM-7 (127 km from the center of the crater). The carbonate sedimentary sequence can be associated to a shallow basin depositional environment. In these boreholes the thickness between the 29R and the 29N chrons is just 0.5 m, suggesting that during the 100 ka from the K/T boundary to the polarity transition sediments were not deposited or eroded. Within borehole UNAM-6 (152 km from the center of the crater) it appears that sediments containing chron 29N are missing, the lack of the upper breccias, the long duration of a reversal event within the base of the sequence and low apparent sedimentary rate of 3.3 m/Ma, suggests a hiatus within the impact breccias and the basal Paleocene sedimentary sequence. Magnetic susceptibility logs confirm absence of the upper breccias at UNAM-6 borehole. Magnetic susceptibility values increase towards the base of the sequence, suggesting that basement and melt clasts were subjected to a low temperature hydrothermal alteration.

Physical properties of the Yaxcopoil‐1 deep drill core, Chicxulub impact structure, Mexico

“…The magnetic mineralogy of the impactite units, however, indicates much higher susceptibilities and, thus, higher quantities of a magnetic fraction, whereby the carriers are MD ⁄ PSD magnetite and PSD ⁄ SD pyrrhotite. These susceptibilities of impactites from the Yax-1 core are mostly, however, considerably higher than those reported for breccias from the UNAM cores (e.g., Urrutia-Fucugauchi et al 1996, 2004bPilkington et al 2004;Rebolledo-Vieyra and Urrutia-Fucugauchi 2006). As the magnetic susceptibility of rocks, in general, depends primarily on its magnetic mineral content, the variations in magnetic fraction and its distribution as well as variations in impact lithologies can explain this difference between drilled locations.…”

“…1996, 2004b; Pilkington et al. 2004; Rebolledo‐Vieyra and Urrutia‐Fucugauchi 2006). As the magnetic susceptibility of rocks, in general, depends primarily on its magnetic mineral content, the variations in magnetic fraction and its distribution as well as variations in impact lithologies can explain this difference between drilled locations.…”

Quantifying the Release of Climate‐Active Gases by Large Meteorite Impacts With a Case Study of Chicxulub

“…The distribution of anhydrite within the target is also important, and data from the Yaxcopoil‐1 drill hole (which is in the southwest direction and just outside the excavation cavity) indicate that the ~600 m thick sedimentary section is late Albian to Campanian in age (Belza et al, ) of which ~ 27% is anhydrite (Kenkmann et al, ). To the south and southwest of the crater the lower half of the sedimentary sequence is lower Cretaceous in age and ~60% anhydrite (López‐Ramos, ; Rebolledo‐Vieyra & Urrutia‐Fucugauchi, ). Using the assumption that the upper half of sedimentary sequence is 75% calcite and 25% anhydrite and the lower half is 60% anhydrite and 40% calcite, the ejected masses become 425 ± 60 Gt CO 2 and 325 ± 60 Gt S (Table S3).…”

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“…UNAM Well U5 shows Cenozoic rocks above suevite, and in Well U7, suevitic breccia overlies melt-poor lithic breccia, composed mainly of sedimentary clasts rich in evaporitic material (Urrutia-Fucugauchi et al, 2008). The upper breccias have high magnetic susceptibilities, low seismic velocities, low density, and high porosities and permeabilities; in contrast, the lower breccias show low susceptibilities, variable seismic velocities, and lower porosities and permeabilities (Urrutia-Fucugauchi et al, 1996;Rebolledo-Vieyra and Urrutia-Fucugauchi, 2006). In Well U6, Cenozoic rocks directly overlie this melt-poor lithic breccia, with an erosional contact between them ( Figure F6C).…”