UNAM Scientific Drilling Program of Chicxulub Impact Structure‐Evidence for a 300 kilometer crater diameter

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Abstract
As part of the UNAM drilling program at the Chicxulub structure, two 700 m deep continuously cored boreholes were completed between April and July, 1995. The Peto UNAM‐6 and Tekax UNAM‐7 drilling sites are ∼150 km and 125 km, respectively, SSE of Chicxulub Puerto, near the crater's center. Core samples from both sites show a sequence of post‐crater carbonates on top of a thick impact breccia pile covering the disturbed Mesozoic platform rocks. At UNAM‐7, two impact breccia units were encountered: (1) an upper breccia, mean magnetic susceptibility is high (∼55 × 10−6 SI units), indicating a large component of silicate basement has been incorporated into this breccia, and (2) an evaporite‐rich, low susceptibility impact breccia similar in character to the evaporite‐rich breccias observed at the PEMEX drill sites further out. The upper breccia was encountered at ∼226 m below the surface and is ∼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 well 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 ∼280 m. The absence of an upper breccia equivalent at UNAM‐6 suggests some portion of the breccia sequence has been removed by erosion. This is consistent with interpretations that place the high‐standing crater rim at 130–150 km from the center. Consequently, the stratigraphic observations and magnetic susceptibiity records on the upper and lower breccias (depth and thickness) support a ∼300 km diameter crater model.