The Chicxulub crater has an approximate center at Chicxulub Puerto on the present coastline (Fig. 1); it has been imaged by geophysical surveys including gravity, magnetics, magnetotellurics and seismic reflection and refraction. Gravity anomalies delineate a series of nearly concentric anomaly patterns that have been related to crater morphology, which are enhanced in the horizontal gravity anomalies [4,7]. The central sector is characterized by high amplitude, high frequency magnetic anomalies, which are related to the central uplift and the magnetic breccias and melt. On the surface, the buried crater is expressed in the semicircular array of sinkholes referred to as the cenote ring (Fig. 1). Analyses of topographic charts, digital terrain model and GPS measurements indicated the occurrence of a topographic depression related to the cenote circular ring [4,12,16,17].

It has been proposed that Chicxulub ejecta blanket may extend continuously in the peninsula region up to the Albion Island and other sites in Belize [18]. There are no surface outcrops of ejecta deposits and melt inside the crater and the adjacent area. Evidence for proximal ejecta deposits around the crater came from the Universidad Nacional Autónoma de México (UNAM) drilling program in the southern sector [25], where impact breccias were cored in three boreholes situated between 110 and 150 km from crater center, in the southern area adjacent to the crater rim. Outcrops of ejecta deposits reported in the Rio Hondo area of northern Belize and southern Quintana Roo (Chetumal area) may correspond to gravity or secondary flows [18] These outcrops are located at about 360 km radial distance from the crater center. The deposits grouped into the Albion Formation are composed of a ∼1 m thick clay and dolomite spheroid basal layer and a ∼15 m thick coarse diamictite bed. Pope et al. [18] interpret the clay and dolomite spheroids as altered impact glass and accretionary lapilli, respectively. The diamictite contains large accretionary blocks, striated polished cobbles, altered glass and occasional shocked quartz. Ejecta deposits are emplaced on top of the shallow water carbonate platform strata of the Late Cretaceous Barton Creek Formation. Samples from the impact polymictic breccias and melt were first recovered in the exploratory PEMEX drilling program at Chicxulub-1 (C1), Sacapuc-1 (S1) and Yucatan-6 (Y6) boreholes. Coring was intermittent and samples were only available in limited amounts [11,23], which have hampered detailed studies of the breccia sequence. The UNAM drilling program incorporated continuous coring and eight boreholes were drilled in the central and southern sectors of the crater (Table 1). Carbonate dissolution features including underground karst cavities restricted the depth of the drilling/coring efforts and the breccias were not reached in boreholes inside the crater. Drilling outside the crater rim in southern sector was more successful and three boreholes Santa Elena (U5), Peto (U6) and Tekax (U7) cored the breccias [19,25]. Core analyses indicate that breccias are formed by two distinct sequences: an upper breccia rich in basement and melt fragments and a lower breccia rich in carbonate clasts [25]. The upper breccia presents high magnetic susceptibility, low seismic velocities, low density and high porosity and permeability. The lower breccias are characterized by low susceptibility, variable high seismic velocities and higher porosity and permeability. The upper and lower breccias appear similar, with some differences, to the suevitic and Bunte breccias documented in the Ries crater in southern Germany [8].

The area studied is located on the on-land portion between Merida and Valladolid, which is characterized by marked changes in distribution and density of karst features, fracture patterns and topographic landforms. These features have been found to be related to the buried crater through differential compaction of the breccia sequences, changes in crater morphology and groundwater flow patterns [4,7,12,13,16]. The higher number and density of cenotes occur east of the crater rim approximately between the (sinkhole) “cenote” ring and the Valladolid area; this zone extends towards the coast (Fig. 1). Karst features are largely absent in the coastal plain, suggesting a change in groundwater flow and characteristics of the aquifers. The northwestern Yucatan aquifer is largely unconfined, which accounts for the carbonate dissolution and karst features [13]. Near the coastline development of a caliche layer forms an impermeable layer, which has migrated inland as sea level rose during the Holocene, which was recognized by Perry et al. [12] (see also references [13,16]). The high density of cenotes and karst features in the area gives a distinctive surface landform (referred as the “pockmarked terrain” [14]), which has been related to the possibly occurrence of impact breccia deposits at depth. Gypsum and anhydrite dissolution by the deep saline intrusion in the ejecta deposits may account for the characteristic karst topography [14]. The cenotes in the southern group are generally dry, in contrast to the water-filled cenotes in the cenote ring and the groups near the coast. The change in the density distribution of karst features cuts SW–NE diagonally from about 20.5° N, 88.6° W to the coast, parallel to a gravity low and correlating with a magnetic low [4].

As part of the Comisión Federal de Electricidad (CFE) program, exploratory boreholes have been drilled in the eastern sector between Merida and Valladolid (Fig. 2). Macroscopic descriptions and petrographic studies on samples from three boreholes BEM-1, BEH-1 and BEV-4 have been undertaken. The BEM-1, BEH-1 and BEV-4 boreholes are located at increasing radial distance from crater center, along an east-west transect near Merida, Huhi and Valladolid, respectively (Table 1). Boreholes reach depths down to greater than 350 m, and cut the Tertiary carbonate sequence. The Tertiary limestone strata in the zone between Merida and Valladolid are formed by crystalline limestones, dolomitized limestones, clay-bearing limestones, and marls. Subunits can be distinguished by the intercalations of micrites, biomicrites, calcirudites, calcilutites of varying clay contents, dissolution and fracture features, fossil remains and textures. Colors and textures of the carbonate strata vary from white, cream, reddish gray to gray (in BEV-4 borehole, some dolomite horizons are bluish and lutites beneath the breccia sequence show light green colors). The carbonate strata are part of the Icaiché, Chichen Itza and Carrillo Puerto Formations [11]. Beneath 200–250 m depths, the carbonate rocks show effects of deformation and fracturing. In Merida, the carbonate sequence in BEM-1 (Fig. 3) is formed by intercalations of micritic limestones, calcirudists, calcirudites, clay-rich gray limestones between 200 and 300 m. In the area between Merida and Huhi, the sequence is more abundant in white to cream coquina deposits, with fragments of bivalves, gastropods and corals. In parts, it is poorly consolidated with horizons of cream calcarenites towards the bottom. In Huhi area, BEH-1 borehole sequence (Fig. 3) is formed by micritic cream and white limestones and dolomitized gray to reddish gray limestones, which show fragments of macrofossils and in some parts dissolution features. In the area between Huhi and the crater rim, the sequence shows greater relative proportion of calcarenites, with limestones, dolomitized limestones, coquina and some recrystallized coralline structures. In the Valladolid area, the BEV-4 borehole sequence (Fig. 3) is formed down to 250 m by miciritc limestones with some clay-bearing horizons, partly altered and dissolution features and calcirudites, fossiliferous and clay-rich horizons. Carbonate breccias are present between 250 m and 284 m (see below). Beneath 284 m and down to 300 m, the sequence is composed by limestones, dolomitized light reddish limestones with light green lutite horizons.

An interval of about 34 m of carbonate breccias is present in the BEV-4 borehole at the Valladolid area (Fig. 3). The carbonate breccia interval occurs between about 250 and 284 m. Breccias are characterized by abundant clasts of limestone, dolomite, gypsum and anhydrite; the breccia section correlates with the carbonate breccias observed to the south in the Peto and Tekax boreholes [19,25]. Based on results from chemical analyses of deep-water samples in the Ucil cenote (located at about 20.99° N, 88.6°W), Perry and co-workers [14] proposed occurrence of an impact breccia layer at ∼200 m depth below the cenote area to account for the strontium contents of the cenote saline waters. The coring in BEV-4 confirms the presence of the impact breccia layer, which is present beneath the Valladolid area and in the Ucil cenote. The BEV-4 borehole is located at approximately similar radial distances than the Tekax borehole in the southern sector. Impact breccias in the Tekax borehole are formed by two distinct units: an upper unit rich in basement and melt clasts located between 222.2 and about 384 m, and a lower unit rich in carbonate and evaporate clasts beneath 384 m and which grades downward into a sequence of marls and evaporites. In Peto borehole, the upper breccias are between 257 and 283 m, and the carbonate breccias are located approximately between 283 and 540 m. These boreholes lie outside the crater rim, and outside the external gravity anomaly ring [20]. The carbonate breccias rest on carbonate strata (Fig. 3), which may correspond to large displaced Cretaceous blocks (mega-breccia unit) or to the pre-impact Cretaceous target sequence. A similar carbonate sequence is also observed in the Peto borehole, which shows higher abundance of anhydrites and gypsum.

The seismic reflection data along the east-west profiles offshore parallel to the coastline show that Tertiary carbonates are characterized by high frequency reflections that can be followed across the basin [2,3,7]. The eastern sector displays different characteristics as compared with the western sector, with a distinct infill history characterized by more complex geometries and clinoform packages [2]. The deep one shows relatively steep westward dipping sigmoid-oblique clinoforms, which, together with the offlap relationships, are characteristic of regression on a prograding slope. The shallow clinoform package shows shallower dips, and its lower part contains discontinuous low-amplitude reflectors with complex geometries. Bell et al. [2] indicate that its lower part is a mass flow deposit and that the dip decrease between the clinoform packages resulted from clinoform/slope system reorientation or development of a gentle slope after the mass wasting. The reflectors above the clinoform packages show clinoform surfaces draping and onlap, indicating a passive sedimentation stage [2].

Results from macroscopic descriptions and petrographic studies on the boreholes located between Merida and Valladolid provide a preliminary stratigraphy for the carbonate sequence and carbonate breccias in the eastern crater sector. Boreholes reach depths to greater than 350 m, and cut the Tertiary carbonate sequence (Fig. 3). Tertiary limestone strata can be correlated in the zone between Merida and Valladolid and are formed by crystalline limestones, dolomites, clay-bearing dolomitized limestones, and marls. Beneath about 200–250 m, carbonates show effects of tectonic deformation and fragmentation. In the Valladolid BEV-4 borehole at 250 m depth an interval of about 34 m of carbonate breccias (Fig. 3), characterized by angular fragments of crystalline limestones, gypsum, anhydrites and dolomites in a carbonate matrix with marked color changes, has been cored. This is the first time, breccias have been documented in the eastern crater rim sector, and they correlate with the carbonate breccias in the Peto and Tekax boreholes in the southern sector.

The Peto and Tekax carbonate breccias are similarly formed by limestone, gypsum, anhydrite and dolomite fragments of the surface target lithologies and they lie beneath the basement and melt rich breccias in the Tekax borehole, showing an inversion of target rock stratigraphy. The carbonate breccias are not observed in the lithological columns of boreholes near the crater center C1, S1 and Y6 boreholes and farther away at the terrace zone in the Yaxcopoil-1 borehole. The Yaxcopoil-1 breccias [26] occur at about 800 m and are formed by a 100 m thick sequence of basement and melt rich breccias lying on top of Cretaceous limestone blocks of the terrace, tilted blocks forming the crater rim (or mega-block breccias). The Yaxcopoil-1 breccias are formed by several subunits with distinct characteristics reflecting their emplacement from hot flow-like bottom units to rework upper deposits. From the analyses of borehole information [10,22,26], it then appears that carbonate breccias are absent inside the crater basin. The basement and melt rich breccias are not present in the BEV-4 borehole, and they have been eroded away in the area.

The carbonate breccias may be compared with the Bunte breccias of the Ries crater. The Bunte breccias have been interpreted in terms of ballistic deposition and a surface flow with large blocks scouring and transporting material, resulting in ejecta and target rock fragments [16]. Altered glass and accretionary clasts have not been reported for the Bunte breccias. Microscopic examination of the BEV-4 breccias does not show occurrence of melt fragments, altered glass or accretionary clasts. Magnetic susceptibility logging of the Peto and Tekax boreholes showed dominantly low values corresponding to mainly diamagnetic minerals (paramagnetic values are also present) [25]. Rare millimeter sized altered melt fragments are however present in the Peto carbonate breccias, indicating the need for further detailed examination of the carbonate breccia sequences in the UNAM and the BEV-4 boreholes.

The satellite radar image of the northern Yucatan peninsula (Fig. 4) shows the surface expression of the buried crater structure. The surface projection of the crater rim is marked by a semi-circular topographic depression that forms above the terrace zone near the rim, which coincides on the surface with the cenote ring and stands out of the flat carbonate terrain in the peninsula. The depression zone is associated with the differential compaction of the impact breccias inside the crater basin as compared with the Mesozoic carbonate sequence outside the crater. The satellite radar image shows the presence of ancient coastlines that reflect past sea level changes. Offshore, the marine seismic reflection profiles show differences in the Tertiary infill sediments between the western and eastern sectors of the crater, with sediments in the eastern sector displaying clinoform features related to regression during relative falling sea level [2]. The sediments were likely deposited from the east. The location and orientation of the ancient coastlines that roughly follow the present coastline (Fig. 4) suggest relatively recent sea level changes that affected carbonate deposition across the carbonate platform, in which particular conditions related to the basin depression of the crater may have been superimposed. Regional surface fracture patterns documented in the peninsula relate to the NW-SE Ticul and NE-SW Holbox fault zones, and may correlate to extensive karstification in the zone, which contrasts with the absence of karst features west and south of the crater rim.

The Chicxulub ejecta blanket may extend continuously in the Yucatan peninsula up to the ejecta outcrops of Albion Island and other sites in Belize [18]. There are no surface outcrops of ejecta deposits and melt inside the crater and the adjacent area. Carbonate breccias reported [11,23] in oil exploration boreholes Y1 (135 km), Y5A (138 km), Y2 (160 km) and Y4 (205 km) further support a continuous ejecta blanket. Deposits of proximal ejecta or continuous ejecta blanket from large impact craters are relatively rare. Besides the Ries crater ejecta, other possible example is the Onaping Formation of the Sudbury crater in eastern Canada. The diamictite unit from the Albion Formation in Belize has been compared to the Sudbury Onaping Formation, where the Onaping represents a crater fill deposit and the Albion diamictite represents the distal portion of the continuous ejecta blanket [18], at considerable distance from the crater rim. Apparent similarities may derive from their formation related to the turbulent collapse of the ejecta curtain [18]. The ejecta blanket was deposited after deposition of the ballistic deposits represented by the basal spheroid bed. The Ries Bunte breccias and the Chicxulub carbonate breccias drilled close to the crater rim immediately outside the basin structure may represent proximal ejecta deposits emplaced in the first instants of crater formation from surface lateral ejecta curtains and (perhaps?) collapse of the ejecta cloud prior to emplacement of the (suevite-like) basement and melt rich breccias.

Thickness of the carbonate breccia unit in the Valladolid area is small compared to those documented in UNAM boreholes in the southern sector (around 200–400 m) and in the Y5A borehole (breccia interval between 400 and 900 m). The reduced thickness in the Valladolid BEV-4 breccia section may relate to significant erosion that removed the basement and melt rich breccias and a significant portion of the lower carbonate breccias. Studies suggest that back surge of the sea into the crater resulted in reworking and erosion of the breccia deposits, possibly also affecting the crater rim morphology [5]. If the rim remained above sea level [22], it may have been exposed to erosion and slope collapses. Conditions after the crater formation and ejecta deposition and reestablishment of marine carbonate deposition are yet poorly constrained [5,10,13,22,24]. The thin unit of reworked basement and melt-rich breccias in the Peto borehole, the short interval of carbonate breccias in Valladolid BEV-4 borehole and the reworked breccias in the upper subunit of Yaxcopoil-1 borehole support significant erosive processes at the crater rim area.

Studies of topography, karst landforms and subsurface stratigraphy have been used to locate and investigate major crater features including the topographic rim [4,9,12,16]. In the eastern area, where borehole information has been lacking, the topographic rim has been inferred at distances of about 120–130 km [16,17] and 180 km [7]. The apparent extreme erosion of the ejecta deposits at BEV-4 borehole could be related to a relative slightly higher elevation of the Cretaceous basement associated as suggested to the topographic rim in the eastern crater sector [4,16]. Further drilling in the area is needed to investigate the subsurface stratigraphy, lateral extension of the ejecta deposits and relation to the Cretaceous sequence.

This is the first report on the occurrence of carbonate breccias close to the rim in the eastern sector of the crater (120 km away from crater center), which are interpreted as part of the Chicxulub proximal ejecta deposits. The Valladolid breccias correlate with the carbonate Bunte-type breccias cored in Peto and Tekax boreholes south of the crater rim and at similar crater radii distance. Contact between the breccias and the Tertiary carbonates lies at about 250 m. Thickness of the Valladolid breccias is ∼34 m and thinner than that observed at other sites, which may indicate significant erosion of the ejecta deposits around the crater rim before reestablishment of carbonate deposition. Active dissolution processes of evaporite minerals by deep saline intrusion in the area have been documented from chemical analyses of water samples in cenotes and related to presence of breccia deposits [9]. The region east of the crater rim appears different from regions to the south and west, which is reflected in the karstic features with high density and scattered distribution of cenotes. The buried crater, through differential compaction of the impact breccias and radial faulting/fracturing, controls the topography, groundwater flow and development of karstic features (cenote ring and circular topographic depression). Further study of the post-impact sequence inside and outside the crater rim will permit to better understand the crater morphology, stratigraphy and Tertiary development of the Yucatan carbonate platform.