The discovery of unique low density Chicxulub granite, during the drilling project into the Chicxulub crater's peak ring mountains, is not the only geological surprise from the supposed meteor impact crater. How were these special granites formed?
In the 2004 drilling project there was more geology surprises.
The Chicxulub Scientific Drilling Project (CSDP) between 2001 and 2002 drilled on land into an outer part of the Chicxulub Crater, Mexico. The drilling project was financed by ICDP and the National University of Mexico (UNAM) and was coordinated by UNAM.
It was an area already geological known, surveyed and surface drilled, so there should have been few surprises.
But there were geological and theory surprises and questions raised. This has happened with all the Chicxulub Crater drilling projects in the past and will happen in the future. Why? Because a good theory predicts. Because a good geology theory predicts.
Chicxulub Crater drilling 2002
Below are notes from The Chicxulub Scientific Drilling Project (links to PDF).
The Yaxcopoil-1 (Yax-1) borehole was planned to core continuously into the lower part of the post-impact carbonate sequence, the impact breccias, and the displaced Cretaceous rocks. The drill site at Hacienda Yaxcopoil was selected based on integration of gravity, magnetics, magnetotelluric and offshore seismic surveys, pre-existing boreholes of PEMEX and UNAM programs.
The CSDP recovered core from the lower Tertiary carbonate sequence, impact breccias, and overlying Cretaceous carbonates, down to the depth of 1511m. Core recovery was 98.5%.
The Tertiary carbonates were cored between 404m and 795m. Impact breccias were cored between 795m and 895m, presenting an unexpectedly thin sequence of impactites, given the greater thickness of such breccias in wells located towards and outward the crater center relative to Yax-1.
Beneath the impact breccias, a sequence of carbonate rocks (limestones, dolomites, and anhydrite) were recovered. The dip of the carbonates varies from being sub-horizontal to up to 60 degrees, and these rocks contain thin dikes of breccia and melt.
Impact breccias have been divided into six units which, from top to bottom are [suevites, chocolate-brown melt breccias, glass-rich variegated suevites, green monomicitics-autogene melt breccias, variegated polymicitic and allogenic clast melt breccias]
The Cretaceous sedimentary rocks below about 895m are cut with dikes of polymict breccias and display zones of monomict brecciation. They appear to represent “megablocks” displaced by the impact event. Anhydrite layers, the thickness of which varies from a few centimeters to up to 15m, and make some 27% of the megablocks. Organic-rich layers and oil-bearing units are present at depths between 1410 and 1455m
Chicxulub Scientific Drilling Project surprises or questions
- 1 The proportion of basement material within the impact breccias is higher than observed at other craters, opening exciting possibilities for investigations of excavation models and the nature of the Yucatán crust.
- 2 The impact breccia layers are thinner than predicted, with implications for crater structure, breccia emplacement, and crater formation.
- 3 Although there are layers of anhydrite in the target rocks, there is a surprising lack of evidence for anhydrite in the impact breccias.
- 4 Several different analyses document the importance of post-impact hydrothermal alteration within the core samples.
- 5 Recovered blocks of Cretaceous and impactites provide valuable material for laboratory analyses, despite the effects of subsequent alteration.
- 6 The origin of the thick carbonate sequence beneath the breccias (>600 m) has provoked an interesting debate and requires further study.
- 7 Initial studies of the impactite and early Paleocene stratigraphy have provided interesting age constraints and information on sequence completeness (for example the identification of the 29r/29n chron boundary just above the appearance of the first Danian fossils), provoking a debate about the age of the Chicxulub impact.