UNAM SCIENTIFIC DRILLING PROGRAM OF CHICXULUB IMPACT STRUCTURE - EVIDENCE FOR A 300 KILOMETER CRATER DIAMETER

As part of the UNAM drilling program at the Chicxulub structure, two 7 00 m deep continuously cored boreholes were completed between April an d July, 1995. The Peto UNAM-6 and Tekax UNAM-7 drilling sites are simi lar to 150 km and 125 km, respectively, SSE of Chicxulub Puerto, near the crater's center. Core samples from both sites show a sequence of p ost-crater carbonates on top of a thick impact breccia pile covering t he disturbed Mesozoic platform rocks. At UNAM-7, two;impact breccia un its were encountered: (1) an upper breccia, mean magnetic susceptibili ty is high (similar to 55 x 10(-6) SI units), indicating a large compo nent of silicate basement has been incorporated into this breccia, and (2) an evaporite-rich, low susceptibility impact breccia similar in c haracter to the evaporite-rich breccias observed at the PEMEX drill si tes further out. The upper breccia was encountered at similar to 226 m below the surface and is similar to 125 m thick; the lower breccia is immediately subjacent and is >240 m thick. This two-breccia sequence is typical of the suevite-Bunte breccia sequence found within other we ll preserved impact craters. The suevitic upper unit is not present at UNAM-6. Instead, a >240 m thick evaporite-rich breccia unit, similar to the lower breccia at UNAM-7, was encountered at a depth of similar to 280 m. The absence of an upper breccia equivalent at UNAM-6 suggest s some portion of the breccia sequence has been removed by erosion. Th is is consistent with interpretations that place the high-standing cra ter rim at 130-150 km from the center. Consequently, the stratigraphic observations and magnetic susceptibility records on the upper and low er breccias (depth and thickness) support a similar to 300 km diameter crater model.