AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Biostratigraphic, Lithologic, and Seismic Characteristics of the K/Pg Boundary Chicxulub Impact Deposit, Deep-Water Gulf of Mexico
(1) Marathon Oil Company, Houston, TX.
The Cretaceous / Paleogene (K/Pg) boundary deposit has been identified in 16 industry wells penetrating the Cretaceous in the deep-water Gulf of Mexico. Examination of publicly released log and biostratigraphic data revealed that the deposit, ranging in thickness from 10 to 200 m (30-600’), can be readily identified by both its distinctive log character and its unique microfossil assemblage.
Initially it was noted to contain a mixture of lower Maastrichtian to upper Campanian nannofossils with scattered occurrences of older (Coniacian to Aptian) species with no discernible succession of biostratigraphic events, matching the “Cretaceous-Tertiary cocktail” described by Bralower and others (1998). Typically, the shale unit above the deposit contains middle to lower Paleocene nannofossils and foraminifera along with rare to frequent reworked Cretaceous specimens. Within the deposit foraminifera are rare while nannofossils, although common, are usually fair to poorly preserved, possibly a result of the deposit’s micritic composition. The unit also contains mollusk debris and sand. A significant change in fossil content and preservation is noted below the deposit, with the sediment ranging in age from lower Campanian to Aptian and possibly older, indicative of an unconformity.
The log character of the deposit is characterized by low gamma ray and medium to high resistivity values typical of a limestone. Variations in resistivity appear to be related to porosity changes. Nuclear and acoustic logs indicate a low porosity, generally less than 10%, and high velocity deposit. Nuclear magnetic resonance logs show predominantly microporosity and bound fluids. In most wells the deposit exhibits minimal change in character, although there are examples where the deposit can be subdivided into subunits. This overall homogeneity is similar to the 150 m thick K/Pg deposit in the Penalver Formation of Cuba. The top of the unit can be either sharp or gradational, while the base of the unit is relatively sharp.
The deposit has long been recognized seismically as an extensive high amplitude reflector with the misnomer of “Middle Cretaceous Unconformity” (MCU). When clearly imaged it has a parallel couplet of high-amplitude reflectors with little interior character, and often truncates deeper horizons on paleo-highs. This high amplitude is due to the high velocity micrite bounded by lower velocity clastics.