UNAM Scientific Shallow-Drilling Program of the Chicxulub Impact Crater

Abstract

A scientific drilling program is being carried out by the National Autonomous University of Mexico (UNAM) at the southern sector of the Chicxulub impact crater in the Yucatan Peninsula, Mexico. Eight boreholes, ranging in depth from 60 m to 702 m, with a total of 2.62 km of continuos core, were recovered. A high recovery rate of up to 99% (overall average recovery rate for the eight boreholes is 87%) allows us to investigate in detail the stratigraphy of the impact lithologies and the Tertiary carbonate sequence. Three of the boreholes (UNAM-5, UNAM-6, and UNAM-7, with core recovery rates from 89 to 99%) sampled impact breccias that were classified in two units-an upper breccia sequence rich in basement clasts, impact glass, and fragments of melt (suevite-like breccia) and a lower breccia sequence rich in limestone, dolomite, and evaporite clasts (bunte-like breccia). Depths of contact between the Tertiary carbonate sequence and the impact breccias are 332.0 m in UNAM-5, 222.2 m in UNAM-7, and 282.8 m in UNAM-6, giving the depth to the K/T boundary. In UNAM-7, the contact between the upper and the lower breccias is at 348.4 m, which yields a thickness of 126.2 m for the suevite-like breccia. The rest of the boreholes sampled part of the Tertiary carbonate sequence (~200 m thick), composed mainly of limestones, dolomitized carbonates, and calcarenite, with some fossiliferous horizons.

Geochemistry of drill core samples from Yaxcopoil‐1, Chicxulub impact crater, Mexico

“…However, the platform sediments were drilled (data summarized in LopezRamos 1975;Ward et al 1995), and the sedimentary and crystalline basement rocks occur in form of clasts in the impactites drilled by PEMEX and UNAM (Hildebrand et al 1991;Sharpton et al 1992Sharpton et al , 1996Schuraytz et al 1994;Ward et al 1995;Urrutia-Fucugauchi et al 1994;Kettrup et al 2000;Rebolledo-Vieyra et al 2000;Claeys et al 2003). The crystalline basement rocks comprise quartz chlorite schists, mica schists, dark-colored schists, quartzite, granitic gneisses, granites, and volcanic rocks, e.g., rhyolites (e.g., Sharpton et al 1992Sharpton et al , 1996Schuraytz et al 1994;Kettrup et al 2000;Claeys et al 2003).…”

“…For the UNAM drill cores (UNAM 1-UNAM 7 = U1-U7; see Fig. 1 for U5 and U7) stratigraphic, petrographic, chemical, and geophysical data are available (Urrutia-Fucugauchi et al 1994;Corrigan et al 1998;Rebolledo-Vieyra et al 2000).…”

“…Nevertheless, the preserved core samples made it possible to confirm the impact origin and the age of the Chicxulub crater (e.g., Hildebrand et al 1991;Sharpton et al 1992;Swisher et al 1992). In the 1990s, a shallow core drilling program initiated by Universidad Nacional Autónoma de México (UNAM) was started in the outer part and the ejecta blanket of the Chicxulub crater (Urrutia-Fucugauchi et al 1994;Rebolledo-Vieyra et al 2000). For the UNAM drill cores (UNAM 1-UNAM 7 = U1-U7; see Fig.…”

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

“…This shows a lack of ferrimagnetic minerals in this borehole, indicating that the absence of the melt-rich breccias. Only the lower breccias are present, with composition similar to that of the carbonate target lithologies (Rebolledo-Vieyra et al, 2000). UNAM-5 exhibits a similar behavior than that of UNAM-7 for the upper breccias; but we could not estimate the contact with the lower breccias, because this borehole did not reached this unit.…”

“…1 and 2). Details of the drilling and coring methods employed in the drilling program, including core diameters and lithological description, are given in Rebolledo-Vieyra et al (2000). Cubic samples were cut from the cores every 50 cm, depending on the preservation conditions of the cores, starting at the last meter of the upper (melt and basement-rich) breccias and ending approximately 15 m above the contact between the upper breccias and the carbonate sequence, averaging 30 samples for each borehole.…”