Chicxulub central crater structure: Initial results from physical property measurements and combined velocity and gravity modeling

Abstract

Abstract-The Chicxulub crater in Mexico is a nearly pristine example of a large impact crater. Its slow burial has left the central impact basin intact, within which there is an apparently uneroded topographic peak ring. Its burial, however, means that we must rely on drill holes and geophysical data to interpret the crater form. Interpretations of crater structures using geophysical data are often guided by numerical modeling and observations at other large terrestrial craters. However, such endeavors are hindered by uncertainties in current numerical models and the lack of any obvious progressive change in structure with increasing crater size. For this reason, proposed structural models across Chicxulub remain divergent, particularly within the central crater region, where the deepest well is only ~1.6 km deep. The shape and precise location of the stratigraphic uplift are disputed. The spatial extent and distribution of the allogenic impact breccias and melt rocks remain unknown, as do the lithological nature of the peak ring and the mechanism for its formation.The objective of our research is to provide a well-constrained 3D structural and lithological model across the central region of the Chicxulub crater that is consistent with combined geophysical data sets and drill core samples. With this in mind, we present initial physical property measurements made on 18 core samples from the Yaxcopoil-1 (Yax-1) drill hole between 400 and 1500 m deep and present a new density model that is in agreement with both the 3D velocity and gravity data. Future collation of petrophysical and geochemical data from Yax-1 core, as well as further seismic surveys and drilling, will allow us to calibrate our geophysical models-assigning a suite of physical properties to each lithology. An accurate 3D model of Chicxulub is critical to our understanding of large craters and to the constraining of the environmental effects of this impact.

The formation of peak rings in large impact craters

“…Craters on Earth often display internal ring-like structures, but complications and uncertainties due to target heterogeneity, erosion and sedimentation, make it difficult to distinguish peak rings that are genetically linked to their extraterrestrial counterparts (8)(9). Seismic reflection data across the ~200-km diameter Chicxulub multi-ring impact structure revealed it to be the only terrestrial crater with an unequivocal and intact peak ring, with the same morphological structure as peak-ring craters on Venus, Mercury, the Moon, and other rocky bodies (10)(11)(12)(13)(14).…”

Structural uplift beneath the Chicxulub impact structure

“…The data misfit is calculated using both the gravity and travel-time misfit, and there is an extra step to convert between slowness (1/velocity) and density. We have used a suite of measurements [Vermeesch and Morgan, 2004] on core from drill holes within the post-impact sedimentary rocks, Cretaceous target rocks and impact breccias, to determine a best-fit equation for the conversion: density (in kg m -3 ) = -3478 × slowness (in 10 3 s m -1 ) + 3326 [Vermeesch, 2006;Vermeesch et al, 2008]. We have used a relatively large uncertainty of 1 mgal for the gravity (~5 % of the total anomaly) as we do not wish to over-fit the gravity data,…”

“…1) revealed strong changes in seismic velocity across the crater, with a low-velocity-zone (LVZ) beneath the peak ring and a high-velocity-zone (HVZ) close to the crater center [Morgan et al, 2000;Christeson et al, 2001;Vermeesch and Morgan, 2004]. Resolution testing suggested that the final velocity model was robust in areas of good ray coverage , and that the top of the HVZ had a concave upward shape along well-constrained profiles.…”

“…Models of Chicxulub constructed from geophysical data are diverse [e.g. Sharpton et al, 1996;Hildebrand et al, 1998;Vermeesch and Morgan, 2004] in part due to the lack of terrestrial examples and the inherent ambiguity of potential-field modeling, and also because drill holes within the impact basin have penetrated the uppermost crater deposits only. For example, in the model of Sharpton et al [1996] the central uplift is broad, with 4 4 uplift in upper-crustal rocks continuing out to 150 km diameter, and with uplifted lower crust forming a 100-km diameter peak ring (Fig.…”

Three-dimensional gravity modeling of Chicxulub Crater structure, constrained with marine seismic data and land boreholes

“…Data on physical properties have been reported from analyses on samples from borehole cores (e.g., Sharpton et al, 1996;Urrutia-Fucugauchi et al, 1996;Vermeesch and Morgan, 2004;Mayr et al, 2008;Elbra and Pesonen, 2011). Data are however limited to the upper formations of the carbonate sequences and breccias, which have been sampled in the boreholes.…”