Chicxulub is the only known impact structure on Earth with a fully preserved peak ring, and it forms an important natural laboratory for the study of large impact structures and understanding of large-scale cratering on Earth and other planets. Seismic data collected in 1996 and 2005 reveal detailed images of the uppermost crater in the central basin at Chicxulub. Seismic reflection profiles show a reflective layer ~1 km beneath the apparent crater floor, topped by upwardly concave reflectors interpreted as saucer-shaped sills. The upper part of this reflective layer is coincident with a thin high-velocity layer identified by analyzing refractions on the 6 km seismic streamer data. The high-velocity layer is almost horizontal and appears to be contained within the peak ring structure. We argue that this reflective layer is the predicted coherent melt sheet formed during impact, and it may be comparable with the unit known as the Sudbury Igneous Complex at the Sudbury impact structure. The Sudbury Igneous Complex, interpreted as a differentiated impact melt sheet, appears to have a similar scale and geometry, and an uppermost lithological sequence consisting of a high velocity layer at the top and a velocity inversion beneath. This comparison suggests that the Chicxulub impact structure also contains a coherent differentiated melt sheet.

Carbon, oxygen, and hydrogen isotope results from carbonate and silicate fractions of altered core samples from the Yaxcopoil-1 borehole drilled into the 65 Ma Chicxulub impact crater provide constraints on the physico-chemical parameters of the hydrothermal solutions, and their likely origin. Yaxcopoil-1 impactites were initially permeated with calcite and halite at ambient temperature. This was followed by thermal metamorphism (diopside after igneous augite) and widespread Na-K metasomatism (feldspar after igneous plagioclase), which were overprinted by abundant lower-temperature clay and calcite. Silicate fraction isotopic values have δ 18 O SMOW values between 10 and 23‰ indicating important isotopic exchange between impact melt (∼8‰) and Cretaceous limestone (∼26‰). Heavier δ 18 O values occur over depth intervals with intense feldspar alteration (813–833 m and 864–872 m). The δD SMOW values (−34 to −54‰) are chiefly influenced by smectite abundance and roughly mirror δ 18 O values. Carbonate fraction δ 18 O SMOW values (22–30‰) are controlled by calcite contents, and several exceed the limestone signature. Most δ 13 C PDB (−1 to +2‰) values also cluster around that of local limestone, but a number are significantly lighter (down to −7‰). Isotopic and fluid inclusion results indicate hydrothermal fluid temperatures between 270 and 100 °C, high salinities (∼20%), and minor kerogen contents. These data are compatible with mineralogical constraints, which further support an increase in oxidation state with decreasing temperature. Isotopic data point to a saline CO 2 -bearing fluid mixed with small amounts of reduced carbon, and decarbonation and infiltration processes. Combined results are most consistent with a basinal oilfield saline brine that was driven by impact-induced heat.

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An important carbonate breccia interval represents a significant portion of the Cretaceous-Tertiary (K-T) boundary sedimentary succession accumulated under deep-water conditions in the western part of the Yucatan platform, offshore Campeche, during the Chicxulub impact event. The K-T boundary sedimentary succession is located approximately 300 km westward from the center of the Chicxulub structure. This sedimentary succession consists of a single graded deposit subdivided into three main units that from base to top includes: (1) a basal 50 to 300 m-thick, coarsegrained carbonate breccia; (2) a 10 to 20 m-thick, finegrained carbonate breccia, and; (3) a 25 to 30 m-thick, interval of sand and silt to clay-sized constituents, mostly abundant ejecta material. Additionally, a 10-20 m-thick, fine-grained calcareous breccia is recognized within the ejecta material-rich layer (unit 3) in some wells. The K-T boundary sedimentary succession is bounded at its base by a deep-water, shaly-calcareous facies of Upper Maastrichtian age and at its top by similar rocks of lower Paleocene age. Similar sedimentological characteristics and stratigraphic relationships are observed in analog outcrops in the Sierra de Chiapas (El Guayal, State of Tabasco; and Bochil, Chilil and Soyalό, State of Chiapas). Lithoclasts of the calcareous breccias are derived dominantly from platform-interior and platform-margin environments and only a few from deep-water settings. Ejecta material in unit 3 includes: shocked quartz, quartz with ballen structure, shocked plagioclase, altered melt rock, and rare pelitic schist fragments. Wireline log data, distribution, and stratigraphic relationships indicate a base-of-slope apron geometry for the thick carbonate breccia deposit. The stratigraphic architecture and distribution of impact material within the K-T boundary sedimentary succession suggest the following sequence of events and products that probably occurred within a very short time span following the Chicxulub impact: Unusually strong seismic shaking induced the collapse of the platform margin, resulting in an enormous debris flow (units 1 and 2), Arrival and deposition of ballistic impact ejecta (unit 3), and Reworking and deposition, possible induced by tsunami currents (carbonate breccia within unit. Units 1 and 2 represent the most important oil reservoirs at the Campeche Bay oil fields, and unit 3 is the seal layer. Unit 4 is a dark clay bed deposited during a global decrease in ocean productivity following the meteorite impact.

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 (∼2cm[∼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.