UNAM scientific shallow-drilling program of the Chicxulub impact crater

A scientific drilling program is being carried out by the National Autonomo us University of Mexico (UNAM) at the southern sector of the Chicxulub impa ct crater in the Yucatan Peninsula, Mexico. Eight boreholes, ranging in dep th from 60 m to 702 m, with a total of 2.62 km of continuos core, were reco vered. 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 stratig raphy 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 up per breccia sequence rich in basement clasts, impact glass, and fragments o f melt (suevite-like breccia) and a lower breccia sequence rich in limeston e, dolomite, and evaporite clasts (bunte-like breccia). Depths of contact b etween 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 th e KIT boundary. In UNAM-7, the contact between the upper and the lower brec cias is at 348.4 m, which yields a thickness of 126.2 m for the suevite-lik e breccia. The rest of the boreholes sampled part of the Tertiary carbonate sequence (similar to 200 m thick), composed mainly of limestones, dolomiti zed carbonates, and calcarenite, with some fossiliferous horizons.