Initial results of the UNAM scientific drilling program on the Chicxulub impact structure: rock magnetic properties of UNAM-7 Tekax borehole

Abstract

As part of the initial studies of the UNAM scientific drilling program on the Chicxulub structure we have measured the magnetic susceptibility of cores recovered from the UNAM-7 exploratory borehole. The drilling site is located at 126 km from the center of the structure, south-southeast of Chicxulub Puerto, in Tekax County ·(2.5 km from downtown Tekax in the direction of Ticum). The contact between the Tertiary carbonate sequence and the impact breccia is observed at 222.2 m. Above this contact, there are marls (207 to 215.2 m) and evaporites (215.2 to 222.2 m). Low-field magnetic susceptibility measurements for the contact show a two-step increase from low -1 to Ox10-6 SI values in the carbonates and evaporites to intermediate values of 1 to 10x1Q-6 SI between 222.2 and 226m, followed by an increase to values of about 50-100xl0-6 SI within the breccia. The polymict breccia can be divided into an upper unit of basement clasts and a lower unit of evaporite breccia. The contact between the two breccia units at 348.4 m is marked by a decrease in susceptibility from values around 55x10-6 SI to low values of -1 to Ox10-6 SI. The contrast in susceptibility between the carbonates, evaporites and the breccia and clasts is high enough to provide a simple means to diferentiate and characterize the different lithologies. Because of the heterogenous nature of the breccia, it is difficult to obtain data on bulk magnetic properties for magnetic anomaly modeling. Data obtained from well UNAM-7 allow estimation of statistically characteristic values for the magnetic susceptibility of the breccia, to be used in modeling of the magnetic anomalies. Thickness of the breccia with susceptibility values around 55x10-6 SI is 126.2 m.