The Cretaceous–Paleogene Boundary Chicxulub Impact<subtitle>Its Effect on Carbonate Sedimentation on the Western Margin of the Yucatan Platform and Nearby Areas</subtitle>

Abstract

Outcrops and offshore Campeche borehole data clearly document the presence of a carbonate facies succession, including calcareous breccia, on the western Yucatan Platform (Campeche Sound) and the Chiapas-Tabasco Platform. This carbonate sequence is associated with ejecta that contains altered glass, shocked minerals, and accretionary lapilli derived from the Chicxulub impact on the Yucatan Platform. The Cretaceous–Paleogene (K-Pg) boundary sedimentary succession is found at the El Guayal, Bochil, and Chilil outcrops of Tabasco and Chiapas, and offshore Campeche, 300–500 km (186–311 mi) west of the Chicxulub structure center. From base to top, this succession consists of four subunits: (1) carbonate breccia, 40–300 m (131–984 ft) thick, without ejecta; (2) fine- to medium-grained carbonate breccia, 10–20 m (33–66 ft) thick, mixed with sparse ejecta; and (3) siltstone, shale, and carbonate sand facies, 9–30 m (29–98 ft) thick, containing abundant ejecta (altered glass and shocked quartz). This unit culminates in a nearly pure clay layer (2 cm [0.7 in.] thick) with the well-known iridium anomaly at the top. Unit 4 is a conglomeratic breccia ranging from 10 to 20 m (33 to 66 ft) thick containing ejecta that is interbedded with or overlays subunit 3 (the ejecta layer) in some wells. Subunits 1, 2, and 3 are highly dolomitized in offshore Campeche, and the glass in subunit 3 is altered to clay minerals (smectite). Subunits 1 and 2 constitute hydrocarbon reservoir facies, whereas subunit 3 corresponds to the sealing layer of these reservoirs. Regionally, this sequence displays a gradational structure that represents a large debris flow followed by ballistic and clastic sedimentation with materials reworked by currents. Moreover, well logs, areal distribution, and stratigraphic relationships suggest that the thick K-Pg boundary sedimentary succession is a base-of-slope apron deposit. Based on the stratigraphy, sedimentology, and distribution of impact materials in the carbonate sedimentary succession, the following sequence of events can be inferred: megaseismic shaking that induced the collapse of the platform margin and produced the lower breccia facies (subunits 1 and 2); ballistic emplacement of ejected material (carbonate fragments, shocked minerals and glass) that supplied components to subunit 2 and formed the ejecta layer (subunit 3), the latter acting as the seal for Cantarell and neighboring oil fields; and reworking of the ejecta layer and coarser-grained carbonate fragments by the effect of one or more impact-generated tsunami waves to form a conglomeratic breccia (subunit 4) within subunit 3.