An older post, Chicxulub – slayer of dinosaurs and slayer of Maya?, was the first of three posts on the importance of the Chicxulub fracture zone. This zone is crucial to my research on changes in cave use, settlement strategies, and climate in the Cochuah region which is located on the border of the greater Chicxulub fracture zone. The first post concerned the effects this Late Cretaceous impact had on the flora and fauna at that time. This second post concerns the geology of the area as it is today and probably during most of the human presence in the area. My final post will concern how the fracture zone has affected the settlement patterns in northern Yucatan from the Middle Formative to the present.
The northern Yucatan plain consists of karst development, composed of uplifted sections of Early and Late Tertiary platform carbonates. A stretch of Quaternary beach and lagoon deposits surround the coast. Sinkholes (cenotes) and caves are very common throughout the area. There are also two fault lines, the Ticul zone directly north of the Puuc hills and the Holbox fault in the east, associated with extensive wetlands. This was pretty much the general view of the geology of the region until the late 1970s when Pemex (the Mexican state-owned oil company) began to search for places to drill oil.
It was Penfield and Camargo who first proposed the existence of a large impact structure on the Yucatan peninsula. They based this on gravity and magnetic data. It was later linked to the K/T boundary mass extinction by Hildebrand and Boyton. The geometry of the impact structure has been mapped by drilling, radiometric and palaeomagnetic dating, and petrologic and geochemical studies. Geophysical studies like gravity and magnetic exploration, rock magnetism, magnetotellurics and heat flow studies have provided information on the shape and dimensions of the Chicxulub structure. Below is a gravity anomaly map of the Chicxulub impact structure. The coastline is shown as a white line. A series of concentric features reveals the location of the crater. White dots represent cenotes.
The overall structure of the Chicxulub multi-ring basin consists of at least four documented topographic rings. Radial positions of the topographic rings follow a square root of 2 spacing rule. The inner ring (the so-called Ring of Cenotes) has a diameter of 170 km, and within this area lie the central basin. The Ring of Cenotes is associated with the largest peripheral gravity-gradient feature in the figure above. A broad gravity low that defines the Chicxulub basin extends roughly 140 km from the center where the fourth broad ring is located. This fourth ring is a fairly discontinuous pattern of local gravity highs. Based on observations of large multi-ring basins on the moon, geologists suggest that the diameter of the Chicxulub multi-ring basin is between 260 and 340 km. The cenote in Sacalaca lies roughly 170 km from the center of the impact and marks more or less the edge of the cenote zone in the southwest.
The vertical structure of the Chicxulub basin differs within the impact crater and in the area outside the Ring of Cenotes. The rocks above the impact feature consist of layers of marl and limestone reaching roughly 1,100 m in depth. These rocks date back as far as the Paleocene. Below these layers lie more than 500 m of andesite glass and breccias and these are only located within the impact feature. Quantities of feldspar, augite, and shocked quartz, normally only found in impact-melt rocks, are present. Outside the Ring of Cenotes the thickness of the Tertiary carbonate rocks are 200-300 m.
Along the edge of the crater is the Ring of Cenotes, which has more than 200 cenotes, ranging between 50 and 500 m in width and 2 and 120 m in depth. There are 3 cenotes/sqkm in a 3 km wide band in the southwestern part of the Chicxulub structure. This decreases to a chain of single cenotes, 3 km apart in the southeast part of the structure. The Ring of Cenotes was formed by post-impact subsidence in the rim of the crater. The subsidence would have occurred during or after the late Tertiary. Seismic studies indicate that no major tectonic movement took place in the impacted area after a few million years. However, some form of tectonism has occurred after the impact since the Ticul fault formed in the Tertiary period.
The area that is of main interest to my research is beyond the Ring of Cenotes. Differential fracturing caused by the thickness of the Tertiary rocks may be another explanation for regional stresses. As mentioned above, the Tertiary strata that overlie the crater floor are three times thicker than the strata outside the rim. Thinner strata are more likely to fracture and the abrupt change in thickness near the Ring of Cenotes make the stresses concentrate in this region. However, stresses occur in a greater area, all the way down to the Cochuah region. The Holbox fracture zone is outside the postulated zone of ring faulting, but geologists do not rule out that the curvature of the Holbox fracture zone may be partly controlled by buried impact structures. If so, the Chicxulub fracture zone covers most of the northern Yucatan peninsula. It is strange that few archaeologists have investigated what this large feature do or did to the acquifer in Yucatan. More on this in the final post.