Causes for the formation of cenotes (sinkholes) in post-impact strata of the Chicxulub crater, Yucatán Peninsula, Mexico

The 66 Ma Chicxulub impact crater, Mexico, is obscured by hundreds of meters thick horizontal carbonate strata, which in age are as young as Pliocene. The spatial density of karst-induced sinkholes, known as cenotes, is maximal at, and aligns with, the onshore portion of the buried crater margin, forming a distinct semicircle, about 170 km in diameter. The causal relationship between the presence of the buried crater margin and the formation of the partial cenote ring has remained elusive since the discovery of the Chicxulub crater, by now some 30 years ago. Earlier hypotheses, by which the cenotes formed due to subsurface collapse of either impact breccia or porous reef complexes lined with the crater margin have received little support. However, it is well known that ground water flow of the northwestern Yucatán aquifer is channelled in post-impact carbonate rock below the cenote ring. This calls for the presence of prominent structural discontinuities in carbonate strata above the buried crater margin.

We addressed cenote formation in the realm of the Chicxulub crater by scaled analogue experiments and by mapping the locations and surface outlines of some 6500 cenotes using imagery embedded in ArcGIS Pro. The outlines are mostly elongate, suggesting that cenotes formed by preferential dissolution of carbonate rock at planar structural discontinuities. This interpretation is corroborated by the overall shape-preferred orientation of their outline long axes in E-W direction, throughout the northern portion of the Yucatán peninsula. Interestingly, long axes deviate from this trend for many of the cenotes defining the partial ring. Such directional departure points to local perturbation of deformation, and thus stresses, preceding cenote formation above the crater margin. Physical experiments using photo-elastic materials as analogues for continental crust were designed to explore to what extent far-field compressive stresses, imparted by plate convergence at the Middle-America Trench, may account for the perturbations in carbonate rock above the crater margin.

Long-term isostatic relaxation of crust below large impact craters is an alternative hypothesis for the formation of concentric faults, potentially localizing at crater margins and thus, generating the partial cenote ring. Using two-layer analogue experiments scaled to the physical conditions on Earth and modelling the deformational behaviour of lower and upper crust following crater formation, we explored the structural and kinematic consequences of crustal relaxation by systematically varying initial depths and diameters of crater floors. Model results indicate that Chicxulub-size craters do indeed develop concentric faults at crater margins by accomplishing differential displacement between uplifting crater floors and subsiding peripheral areas. Interestingly, crater floors retained structural coherence during uplift, which aligns with the paucity of cenotes within the respective ring at Chicxulub. Based on the scaling of our experiments, the duration of isostatic relaxation translates to natural time scales of at least tens of thousands of years. Although isostatic relaxation of impacted crust may not solely account for the origin of a structurally and karst-controlled cenote ring at Chicxulub, concentric faults generated by this mechanism may propagate with time through post-impact strata, driven by far-field stresses.