Post-Chicxulub depositional and diagenetic history of the northwestern Yucatan Peninsula, Mexico

Sedimentary Geology 183 (2006) 51 – 69 www.elsevier.com/locate/sedgeo

Post-Chicxulub depositional and diagenetic history of the northwestern Yucatan Peninsula, Mexico Mihai Lefticariu a,*, Eugene C. Perry a, William C. Ward b, Liliana Lefticariu c a

Northern Illinois University, Department of Geology and Environmental Geosciences, DeKalb, IL 60115, USA b University of New Orleans, Department of Geology and Geophysics, New Orleans, LA 70148, USA c Indiana University Bloomington, Department of Geological Sciences, Bloomington, IN 47405, USA Received 30 April 2004; received in revised form 7 August 2005; accepted 8 September 2005

Abstract The Chicxulub Sedimentary Basin of the northwestern Yucatan Peninsula, Mexico, which was formed because of the largest identified Phanerozoic bolide impact on Earth, became a site of deposition of dominantly marine carbonate sediments during most of the Cenozoic Era. This is a study of the filling and diagenetic history of this basin and surrounding areas. The study makes use of lithologic, biostratigraphic, petrographic, and geochemical data obtained on core samples from boreholes drilled throughout the northwestern Yucatan Peninsula. The core sample data indicate that: 1) The Chicxulub Sedimentary Basin concentrated the deposition of pelagic and outerplatform sediments during the Paleocene and Eocene, and, in places, during the Early Oligocene, as well, and filled during the Middle Miocene, 2) deeper-water limestone also is present within the Paleocene and Lower Eocene of the proposed Santa Elena Depression, which is located immediately south of the Basin, 3) shallow-water deposits are relatively more abundant outside the Basin and Depression than inside, 4) the autigenic and allogenic silicates from the Paleogene formations are the most abundant inside the Depression, 5) sediment deposition and diagenesis within the Basin also were controlled by impact crater topography, 6) the abundance of the possible features of subaerial exposure increases upward and outward from the center of the Basin, and 7) the formation of replacive low-magnesium calcite and dolomite, dedolomitization, dissolution, and precipitation of vug-filling calcite and dolomite cement have been more common outside the Basin than inside. d 18O in whole-rock (excluding vug-filling) calcite from core samples ranges from 7.14x to + 0.85x PDB. d 13C varies from 6.92x to + 3.30x PDB. Both stable isotopes correlate inversely with the abundance of subaerial exposure features indicating that freshwater diagenesis has been extensive especially outside and at the edge of the Chicxulub Sedimentary Basin. d 18O and d 13C in whole-rock (excluding vug-filling) dolomite ranges from 5.54x to + 0.87x PDB and 4.63x to + 3.38x PDB, respectively. Most dolomite samples have negative d 18O and positive d 13C suggesting that replacive dolomitization involved the presence of a fluid dominated by freshwater and/or an anomalously high geothermal gradient. Most dolomite XRD-determined mole percent CaCO3 varies between 51 and 56. Replacive dolomite is larger, more euhedral, and less stoichiometric inside the Chicxulub Sedimentary Basin than outside. D 2005 Elsevier B.V. All rights reserved. Keywords: Chicxulub; Diagenesis; Calcite; Dolomite; Isotopes

* Corresponding author. Present address: TTL Associates, Inc., 1916 N12th Street, Toledo, OH 43624, USA. Tel.: +1 419 324 2222. E-mail address: [email protected] (M. Lefticariu). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.09.008

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1. Introduction The large Terminal Cretaceous Chicxulub Impact Crater is an important structure of the northwestern Yucatan Peninsula. In marked contrast to many other parts of the world, a Cenozoic section of predominantly carbonate rocks formed inside the sedimentary basin that occupies the impact crater. The present study is the first to identify deeper-water Tertiary deposits outside this basin. The post-impact sedimentary formations have been affected by mostly shallow- and shallowto-medium burial diagenesis in the presence of fresh, saline and mixed (fresh + saline) groundwater. The main purpose of this study is to determine the control exerted by the Chicxulub Impact Crater on sediment deposition and diagenesis during the Cenozoic Era in what is now the northwestern Yucatan Peninsula, Mexico. The study benefits from core samples from UNAM (Universidad Nacional Autonoma de Mexico), PEMEX (Petroleos Mexicanos), ICDP (International Continental Drilling Program), and other cores from wells located throughout the study area. 2. Geologic, hydrogeologic, and hydrogeochemical framework The present structural configuration of the large Yucatan Platform is a result of complex non-impactrelated tectonic events upon which a Terminal Cretaceous bolide impact was superimposed. This tectonic history is unique. The main known structural features of the northwestern Yucatan Peninsula are the Ticul Fault and Chicxulub Sedimentary Basin (Fig. 1). The Ticul Fault has a trend of about N 558 W and a length of approximately 160 km. Its surface expression is the app. 100-km-long NE-facing escarpment of the karstified Sierrita de Ticul (Weidie, 1985). McClain (1997) estimates that the fault formed during the Middle Eocene to Early Late Eocene. The Chicxulub Sedimentary Basin (henceforth Basin) is approximately coincident with the impact crater and is circumscribed by the Ring of Cenotes (henceforth Ring), which is a circular alignment of karstic sinkholes. The Ring marks a zone of high permeability developed in Tertiary carbonate rocks.

The detailed structure and exact size of the impact crater are still a matter of debate. 3-D gravity models suggest that the Crater is complex and could have both a central uplift and a peak ring (Sharpton et al., 1996). The UNAM 5 core, drilled at Santa Elena, is located south of the Basin within a bowl-shaped depression (Lefticariu, 2004). This buried feature was first contoured by geophysical surveys and the present study also investigates its possible relationships to the larger and deeper Basin. Carbonate and evaporite deposition predominated during the Cretaceous throughout the northern Yucatan Peninsula. This also is the predominant clast lithology within the ejecta blanket. 2.1. Cenozoic lithostratigraphy Cenozoic sedimentation above the impact breccia in the northwestern part of the Yucatan Platform has been dominated by carbonate deposition accompanied maybe by some evaporite formation (Ward et al., 1995; Rebolledo-Vieyra et al., 2000). Within the study area, limestone of Paleocene age has been identified only in core samples (Lefticariu, 2004). The oldest formations to have been mapped at the surface in the northwestern part of the Peninsula are of Eocene age (Lopez-Ramos, 1983; Weidie, 1985; McClain, 1997). In outcrop, the Eocene contains mostly shallow-water marine carbonate, which also is present in core samples from outside the Ring (Lefticariu, 2004). Oligocene deposits have been identified only inside the Ring and, in general, they contain shallow-bank assemblages. It has been proposed that, during the Early Oligocene, most sedimentation occurred inside the Basin and that by Late Oligocene a large portion of the Yucatan Platform had been subaerially exposed (Perry et al., 1995; McClain, 1997; Pope et al., 1996; Rebolledo-Vieyra et al., 2000). The depositional boundary defined by the Basin disappeared by the Middle Miocene. Therefore, the impact crater concentrated mostly the deposition of deep-water sediments and was completely filled in the Miocene [the perched deep basin of Galloway et al. (2000)]. The Mio–Pliocene encountered at the surface contains shallow-water limestone that, outside the Ring, where the Oligocene section is missing, disconformably

Fig. 1. Location of the study area (delimited by the bold line and the northwestern coast); filled circles locate sampled cores and empty ones locate cores situated outside the study area. Y1, Y2, Y4, Y5A, and Y-6 wells were drilled by PEMEX (Petroleos Mexicanos), UNAM wells were drilled under the auspices of Universidad Nacional Autonoma de Mexico, and Yax-1 well is the ICDP (International Continental Drilling Program) well at Yaxcopoil.

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overlies Eocene rocks (Weidie, 1985; Galloway et al., 1991; McClain, 1997). Marine and non-marine Quaternary carbonates rim the Yucatan Peninsula (Ward, 1985). Unlike Florida, extensive clastic deposition was absent from Yucatan during the Cenozoic Era. Several potential unconformities have been identified and dated in the northwestern Yucatan Peninsula. Pope et al. (1996) speak about paleosols of Middle–Late Eocene, Late Eocene–Oligocene, Oligocene, Late Miocene–Pliocene, and Pliocene–Pleistocene age. McClain (1997) proposed three important episodes of subaerial exposure that took place during the Early Middle Eocene, Oligocene, and Early to Middle Miocene. 2.2. Hydrogeology and hydrogeochemistry of the northwestern Yucatan Peninsula The predominantly unconfined carbonate aquifer of the northwestern Yucatan Peninsula contains a freshwater lens, an extensive seawater-derived saline intrusion that has been followed several tens of kilometers inland, and a mixing zone. The aquifer is confined only along the coast where a caliche-like aquitard is maintained (Perry et al., 1989; Marin, 1990). Groundwater flow is preferentially channeled through the Ring and the Ticul Fault (Perry et al., 1995, 2002; Perry and Velazquez-Oliman, 1996). Supersaturation of mixing-zone groundwater with respect to disordered dolomite and groundwater-dolomite equilibrium have been reported (Prinos, 1996; Perry et al., 2002). The study of Perry et al. (2002) revealed that sulfate concentration in groundwater and its correlation with magnesium and strontium indicated that impact breccia-related sulfate phases (anhydrite, gypsum, celestite) contributed ions to water chemistry and in this way could have been involved in dedolomitization processes. 2.3. Diagenetic processes of Cenozoic carbonates The Cenozoic sedimentary carbonates were affected by both syngenetic and epigenetic diagenesis. As a consequence of the latter, dolomite and low-magnesium calcite are the most abundant carbonate phases encountered in the core samples from UNAM wells (Lefticariu, 2004). Karst development is extensive throughout the study area. 2.4. Late Tertiary–Quaternary dolomite In the northern Yucatan Peninsula both penecontemporaneous supratidal dolomitic crusts and post-deposi-

tional dolostone have been described (Schmitz, 1984; Ward, 1985; Back, 1985; Ward and Halley, 1985; Gmitro, 1986). A mixing-zone origin was proposed for post-depositional age dolomite based on textural aspects and carbon and oxygen isotope values. The stable isotope signature is similar to that of Plio–Pleistocene dolomites from the Caribbean and Gulf of Mexico regions described by Budd (1997). 3. Analytical methodology This study is based on core samples coming from both inside the Ring [Y-6, Sierra Papacal, Merida, Motul, UNAM 1, Yax-1 (at Yaxcopoil), UNAM 2 (at Telchaquillo), UNAM 8 (at Huhi), Huhi, and Dzidzantun cores] and outside the Ring [UNAM 5 (at Santa Elena), UNAM 6 (at Peto), UNAM 7 (at Tekax), Cenotillo, Y2]. Their location is shown in Fig. 1. The elevation of the sampled cores was estimated from INEGI maps (1983, 1984). In this study depths are reported in meters below the present mean sea level. Petrographic and scanning electron microscopes were used at Northern Illinois University (JEOL 35 CF and JEOL 7510 LV) and Indiana University Bloomington (FEI Quanta 400 F) to identify the mineral phases and phase relationships and describe crystal habit and surface microtopography. A Siemens D500 X-ray diffractometer was employed at Northern Illinois University (NIU) in order to determine the mineralogical composition of bulk samples, calculate the amount of calcium carbonate in dolomite according to Goldsmith and Graf (1958), and estimate the ordering of the dolomite structure according to Tucker and Wright, 1990. An a-quartz standard (d-spacing of ˚ ) was used for the calibration of dolomite 3.342 A peaks. Microprobe analyses of polished thin sections were performed at the University of Chicago with a Cameca microprobe equipped with 4 wavelength dispersive spectrometers. Internal standards were used for both major and trace elements. Direct comparison demonstrates that mole percent CaCO3 determined in this way closely approximates that obtained by X-ray diffractometry. Carbon and oxygen isotope analyses were performed at NIU on a Finnigan MAT 250 mass spectrometer. The selective acid extraction procedure and subsequent corrections of Al-Aasam et al. (1990), based on the earlier studies of McCrea (1950), Epstein and Mayeda (1953), Craig (1957), and Sharma and Clayton (1965) were used.

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4. Lithofacies and biofacies petrography and succession A comparison of the pre-dolomitization facies present inside the Basin to those from outside is shown in Fig. 2 (modified from Lefticariu (2004)) and summarized in Table 1. The facies, lithologic, and petrographic comparison shows that [see also Lefticariu (2004) for the location of the sampled cores with respect to the impact crater topography]: 1) The sampled formations are no younger than Miocene. 2) No extensive evidence of compaction and stylolite development associated with medium and/or deep burial diagenesis at a bnormalQ geothermal gradient has been found in any of the studied Tertiary rocks. This includes compacted, broken, or interpenetrated grains. However, burial testfilling poikilotopic calcite cement occurs in the pelagic limestone present at the base of the Tertiary deposits from within the basin. 3) Tertiary formations are much thicker inside the Basin than outside. 4) Outside the Basin, the thickest sampled Tertiary occurs in the UNAM 5 core. According to LopezRamos (1983), the Cenozoic formations of the Ticul-1 core (drilled at the town of Ticul, see Fig. 1), which is located outside the Basin, too, are 525 m thick. 5) The Lower Eocene formations sampled by the Yax-1, UNAM 2, and UNAM 5 boreholes have similar thicknesses. 6) The thin Paleocene of the UNAM 7 core overlies an altered impact breccia. 7) The Lower Eocene evaporite of UNAM 6 core was deposited on top of a collapse breccia formed during the Paleocene due to the subaerial exposure of the impact breccia. 8) There is no evidence for siliciclastic input to the Basin by surface streams from the igneous and/or metamorphic terrains of the Yucatan Peninsula. In this respect, the predominant carbonate deposition took place in a siliciclast-starved basin. 9) The oldest rocks that crop out within the study area are of Middle Eocene age. The youngest, which are of Early Miocene age, crop out only within the peak ring. 10) In terms of vertical Tertiary facies transition, a shallowing-upward trend is visible within the Basin. The planktonic/benthic ratio calculated according to Murray (1976) and Gibson (1988)

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shows a gradual variation from pelagic-outer shelf (water depth: 100–1000 m) to shallow restricted inner shelf (water depth: b40 m). 11) Pelagic limestone is abundant only inside the Basin. It has the following characteristics: a) It commonly contains planktonic and benthic foraminifera and calcispheres and, in places, is laminated. Coccoliths, especially coccolith plates, occur in the sampled portion of the Yax-1 core. Some of the laminae from the Y6 and Yax-1 cores are smectitic. b) Outside the Basin, laminated limestone with planktonic and benthic foraminifera and calcispheres was encountered only in the UNAM 5 core between 277 (top of impact breccia) and 254 m. It is similar but more dolomitized than that of Paleocene–Early Eocene age present in the Yax-1 core. c) A Paleocene–Eocene age range is proposed for most of the pelagic formations, based on planktonic and benthic foraminifera and coccoliths. According to Lopez-Ramos (1983), deeper-water Oligocene is present in the Y-6 core. The lithostratigraphic similarities between this core and others located within the peak ring and described in Lopez-Ramos (1983) would indicate a similar structural location. d) The common occurrence of pyrite in the pelagic limestone suggests anoxic conditions during pelagic sediment diagenesis. Framboidal pyrite has been identified in the UNAM 5 and Yax-1 cores. The latter also contains bituminous organic matter (Lefticariu, 2004; Elswick, personal communication, 2004). In places, the bitumen occurs as thin laminae parallel to the main rock bedding. e) Chert is present in pelagic limestone inside the Ring, where it occurs as bands and nodules. It also fills planktonic foraminifera tests [testfilling gypsum and calcite are present, too]. The preservation of lamination during silicification, absence of deformation features, and preferential silicification of matrix suggest syngenetic chert formation. It is suggested that the age of the chert is Early through Middle Eocene. 12) In the sampled cores, shallow-water deposits contain: a) large benthic foraminifera, which are common in the shallow open-platform Eocene rocks of Y-6, UNAM 2, and UNAM 8 cores, and Oligocene–Miocene of Y-6 and UNAM 1 cores;

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13)

14)

15)

16) 17) 18) 19)

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b) gastropods, pelecypods, and miliolids, which, inside the Basin, become abundant only toward the top of the Tertiary sections of all sampled cores; and c) intraclastic breccias with peloids and selenite molds, which are more common outside the Basin. Reef facies (scleractinian corals, red algae, and associated benthic foraminifera), is present only in UNAM 1 core, between 110 and 90 m. In addition, fragments of corals have been identified in the ~98 m long Cenotillo core. Both cores are located on topographic highs related to the impact structure. The most restricted shallow-water sedimentary carbonate (fine wackestone with gastropods, pelecypods, ostracods, and miliolids) is present in the UNAM 1 core, above 48 m. This indicates the presence of a shallow, restricted water, possibly back-reef environment at the end of the Late Oligocene in the sector of the Basin where the core is located. The number of generations of cement and frequency of possible subaerial exposure features (subaerial crusts, vugs, karst, dedolomite, and micritized root-hair sheaths) increases outward and upward from the center of the Basin. Gypsum cement is common at the bottom of the Tertiary deposits from outside the Basin. Breccias (collapse, karst, and intraclastic) are more common outside the Basin than inside. The deeper-water limestone is the least dolomitized. The relative abundance of dolostone and, in general, of mineral dolomite is greater outside the Basin than inside.

20) Replacive dolomite is larger and more euhedral inside the Basin than outside. 21) Diagenesis, especially dolomitization, tends to obscure sequence boundaries in the sampled cores. 5. The Santa Elena depression The Paleocene of the core UNAM 5, which is located south of the Basin and between the two western branches of the Ticul Fault [Fig. 1; see also Lefticariu (2004)], is similar to that of the Yax-1 core, but for the gypsum cement and dolomite abundance. The lithofacies and mineral assemblages present between 224 and 140 m (Facies b, Lower Eocene) have not been found yet elsewhere in the Cenozoic of northwestern Yucatan. We attribute this to the presence of a distinct basin, here designated the Santa Elena Depression (henceforth Depression). The unique lithofacies comprises a mixture of shallow and deeper-water intervals bracketed by Paleocene and Lower–Middle Eocene outer-platform carbonate. Lithologically, dolostone and dolomitic limestone with intercalations of carbonaceous shale and siltstone predominate. Besides mixed-layer clay, calcite, and dolomite, in places polyhedral, the shale also contains fine potassium feldspar, quartz, and pyrite, often framboidal. The mineral paragenesis of the 224- to 140-m interval also includes montmorillonite, illite, zeolite, celestite, anhydrite, and gypsum. Chert and variably mineralized wood are present within this interval, too. In general, the carbonate of the northwestern Yucatan Peninsula is silicate-poor. Therefore, the occurrence

Fig. 2. Proposed pre-dolomitization facies succession in and stratigraphic correlation between the sampled cores. Facies a: Wackestone or mudstone with planktonic and benthic foraminifera, calcispheres, and sponge spicules; coccoliths have been identified only inside the Basin; Facies b: Mudstone with planktonic and benthic foraminifera with intercalations of shale and wackestone with benthic foraminifera, echinoderms, ostracods, gastropods, pelecypods and peloids; Facies c: Wackestone with large benthic foraminifera, echinoderms, red algae, and pelecypods, alternating with mudstone with planktonic foraminifera, sponge spicules, calcispheres, benthic foraminifera, and echinoderms; Facies d: Wackestone or grainstone with large benthic foraminifera, echinoderms, and red algae; Facies e: Wackestone or grainstone with benthic foraminifera, red algae, and echinoderms with intercalations of wackestone or mudstone with benthic foraminifera, pelecypods, gastropods, ostracods, peloids, and pellets; Facies f: Wackestone, grainstone, or mudstone with benthic foraminifera, red algae, echinoderms, gastropods, pelecypods, peloids, oysters, ostracods, and bryozoans; and Facies g: Framestone with scleractinian corals, red algae, and benthic foraminifera; pib: polymictic impact breccia; bepib: breccia with elements of polymictic impact breccia; Ka: Cretaceous limestone with planktonic foraminifera; Kb: Cretaceous limestone with gypsum, anhydrite, and halite; Kc: Cretaceous limestone and dolostone with benthic foraminifera, anhydrite and gypsum; 1: Paleocene, 2: Lower Eocene, 3: Middle Eocene, 4: Upper Eocene, 5: Lower Oligocene, 6: Upper Oligocene, 7: Lower Miocene. Chronologic limits are based on planktonic and benthic foraminifera, and coccoliths (Yax-1 core). The suggested original depositional environments of each facies are: Facies a=pelagic, Facies b=pelagic or outer-platform (outer shelf) with intercalations of shallow restricted water (inner shelf), Facies c=outer-platform (outer shelf) alternating with shallow open-platform (middle shelf), Facies d=shallow open-platform (middle shelf), Facies e=shallow open-platform (middle shelf) with intercalations of shallow restricted water (inner shelf), Facies f=shallow restricted water (inner shelf), and Facies g: reef.

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Table 1 Facies and petrographic characteristics of Cenozoic deposits from the study area Feature

Thickness of Cenozoic deposits (also see Ward et al. (1995)) Lithology

*Proposed pre-dolomitization lithofacies Original depositional environment and age Trends in facies succession

Minerals present

Cement

Dolomite

**Replacive dolomite texture Replacive dolomite mean crystal size (Am) Features of dedolomitization

Area Inside the Ring

Outside the Ring

Up to ~1100 m (Y-6 core)

215 m (UNAM 7 core) to 335 m (UNAM 5 core) Predominant: dolostone, limestone, dolomitic limestone Minor: evaporite, clay Predominant: skeletal wackestone Minor: skeletal grainstone, skeletal mudstone

Predominant: limestone, dolomitic limestone, dolostone, chert Minor: clay Predominant: skeletal wackestone and mudstone Minor: skeletal grainstone, framestone Predominant: pelagic, outer-platform (Paleocene–Eocene) Minor: shallow water (Eocene–Miocene) Shallowing-upward, from pelagic to shallow water limestone, more gradual toward the center of the Ring Predominant: low-magnesium calcite, dolomite, microquartz (chert) Minor: smectite, pyrite, gypsum, phosphates Predominant: low-magnesium calcite (grain-rimming, including pendular and meniscus, pore-filling, syntaxial) Minor: gypsum (pore-filling), chert (pore-filling) Predominant: replacive Minor: detrital, polyhedral (UNAM 2 core), cement Predominant: planar-s Minor: planar-e, non-planar 135 Hollow dolomite crystals

Predominant: shallow water (Eocene–Miocene) Minor: pelagic or outer-platform (UNAM 5 core: Paleocene–Early Eocene) Except UNAM 5 core (shallowing-upward) no easily distinguishable long-term trend visible because of diagenesis Predominant: dolomite, low-magnesium calcite Minor: smectite, illite, mixed-layer clay, celestite, zeolite, chert, quartz, potassium feldspar (UNAM 5 core), gypsum, anhydrite, pyrite Predominant: dolomite (pore-filling, grain-rimming), low-magnesium calcite (pore-filling, syntaxial) Minor: gypsum (pore-filling)

Predominant: replacive Minor: cement, polyhedral (UNAM 5 core) Mostly planar-s and non-planar 54 Hollow dolomite crystals, dolomite partly replaced by calcite

*Cf. Dunham (1962) and **Cf. Sibley and Gregg (1987).

of most silicate minerals in the UNAM 5 core could be explained by siliciclastic input from volcanoes, impact breccia, or in situ metamorphic and/or igneous formations to a warm, saline lake subjected to marine intrusions. This input also could explain the availability of Fe for pyrite formation. So far, the lithology and petrography of the other sampled cores offers no conclusive evidence of altered ash layers. The proximity of the impact breccia, occurrence of detrital sphene at 223.10 m and of altered suevite at 167.90 m suggests suevite reworking into deeper-water Lower Eocene. The available lithologic data on the Ticul core (elevation: 30 m), which also is located within the Depression but north of the Ticul Fault, show the presence of Paleocene–Eocene limestone with intercalations of marls between about 390 and 180 m (Lopez-Ramos, 1983). It is suggested that the marly

intervals are similar to the shale of the UNAM 5 core. The exact origin of the Depression remains to be ascertained but three hypotheses could explain its formation: 1) This depression is a sub-basin of the larger and deeper Basin and both of them are located within the same impact crater; 2) The Depression coincides with an impact crater distinct from the Chicxulub Impact Crater; and 3) The Depression formed after the Basin, due to slumping, crater wall failure, and/or larger-scale tectonic processes. The occurrence at the bottom of the Cenozoic section of deeper-water limestone similar to that of Paleocene–Lower Eocene age from Yax-1 core, the fact that the Lower Eocene deposits have roughly the same thickness as those of the Yax1 and UNAM 2 cores, and lack of conclusive evidence to support multiple impact breccia layers would be more consistent with either the first or third hypothesis.

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Based on the available lithostratigraphic data, the following Paleogene depositional history of the Depression is suggested: 1) deposition of pelagic and outer-platform marine carbonate (Paleocene), 2) deposition of outer-platform marine carbonate alternating with deposition of shale, fine carbonate, and evaporite during isolation as saline lake (Early Eocene), and 3)

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deposition of shallow-water marine sediments (Middle Eocene). 6. Dolomite petrography and dedolomitization More than one type of dolomite is present: polyhedral (UNAM 5 core, between 212 and 140 m, and

Fig. 3. d 18O and d 13C in whole-rock (excluding vug-filling) dolomite [modified form Lefticariu (2004)]. The field of the Neogene–Quaternary dolomite from the Gulf of Mexico and Caribbean regions, including the northern Yucatan Peninsula, is outlined. Sample depth appears next to each symbol.

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UNAM 2 core in the dolostone immediately overlying the pelagic limestone), replacive (most abundant type), pore-filling, and possibly detrital, as well (in pelagic limestone). Replacive dolomite is cloudy. Aspects of cloudy centers and clear rims are present. The matrix was preferentially dolomitized and rock texture in thin section changes from planar-e to planar-s and non-planar as matrix dolomitization progresses. In general, replacive dolomitization was pervasive, non-mimetic. The pore-

filling dolomite cement is limpid and larger than the replacive dolomite. Drusy cement textures are common. Partially dissolved dolomite crystals are present in cores from outside the Ring. However, strong evidence for extensive dolomite recrystallization (increase in crystal size associated with non-planar crystal boundaries) has still to be found, despite the abundance of dissolution features. Dedolomitization has affected the Cenozoic dolomite. Features such as hollow dolomite crystals and

Fig. 4. d 18O in whole-rock (excluding vug-filling) dolomite versus depth [modified form Lefticariu (2004)]. Facies type appears next to each symbol (see Fig. 2 for facies description).

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dolomite rhombs partially replaced by calcite are more abundant outside the Ring than inside. 7. Major and trace element patterns in carbonate The fragments of calcitic foraminiferal tests included in the replacive dolomite present within the Middle Eocene of the UNAM 2 core and Upper Oligocene of the UNAM 1 core have no more than 2.82 mol% MgCO3. Taking into account the fact that the coexisting

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undolomitized grains are altered and made up of diagenetic low-magnesium calcite it is suggested that freshwater diagenesis preceded dolomitization in these core samples. Most dolomite mol% CaCO3 ranges between 51 and 56 and does not correlate with the ordering index. The dolomite from inside the Ring is less stoichiometric than that from outside. The fine dolomite, often polyhedral, present in the Lower Eocene of the UNAM 5 core is the most stoichiometric. In general, the concen-

Fig. 5. d 13C in whole—rock (excluding vug—filling) dolomite versus depth [modified form Lefticariu (2004)]. Facies type appears next to each symbol (see Fig. 2 for facies description).

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trations of Na, Sr, Mn, and Fe are similar to those of the Pliocene–Pleistocene dolomite from the Gulf of Mexico and Caribbean regions described in Budd (1997). 8. Stable isotope geochemistry The stable isotope geochemistry of dolomite is different from that of anywhere else bordering the Caribbean Sea or Gulf of Mexico. Most dolomite samples have negative d 18O and positive d 13C.

d 18O and d 13C in whole-rock (excluding vugfilling) dolomite ranges from 5.54x to +0.87x PDB and 4.62x to + 3.38x PDB, respectively (Figs. 3–5). d 18O values are unlike those of the Neogene–Quaternary dolomite from the Gulf of Mexico and Caribbean regions described by Budd (1997). Oxygen isotope values also are different from those of the dolomite present in the Motul and Cenotillo cores, which were studied by Gmitro (1986).

Fig. 6. d 18O versus d 13C in whole-rock (excluding vug-filling) calcite from UNAM cores [modified form Lefticariu (2004)]. Sample depth (meters below the present mean sea level) appears next to each symbol. Note the similar carbon and isotope values of the calcite from the pelagic limestone of UNAM 2 and UNAM 5 cores.

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There is no significant statistical correlation among dolomite d 18O, d 13C, and depth. Organic carbon species, which tend to shift carbonate d 13C to low values, contributed relatively little to the isotopic composition of dolomite. Some of the most negative dolomite d 18O were encountered in the scattered dolomite from the pelagic limestone of UNAM 2 and UNAM 5 cores and dolomitic samples with features of subaerial exposure, for which d 13C also is low (Figs. 4 and 5). The polyhedral dolomite from UNAM 5 core has

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positive d 13C and d 18O varying between 2.33x and 1x PDB. d 18O and d 13C in whole-rock (excluding vug-filling) calcite ranges from 7.14x to + 0.85x PDB and 6.92x to + 3.30x PDB, respectively (Figs. 6–8). The calcite samples from the pelagic limestone of the UNAM 2 and UNAM 5 cores form a distinct group with a relatively narrow range of stable isotope values (Fig. 6). Statistical correlations among d 18O, d 13C, and depth, that are significant at 95% confidence level and above, are present in calcite samples from inside the

Fig. 7. d 18O in whole-rock (excluding vug-filling) calcite from UNAM cores versus depth [modified form Lefticariu (2004)]. Facies type appears next to each symbol (see Fig. 2 for facies description).

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Ring (UNAM 1, UNAM 2, and UNAM 8 cores) but not in cores from outside the Ring. In general, d 18O and d 13C are directly correlated. Stable isotope values decrease toward the surface (Figs. 7 and 8). A common evolution with respect to d 18O is apparent above the depth at which the calcite oxygen isotope trend of UNAM 1 core intersects that of the UNAM 2 core, which is closer to the edge of the Basin (Fig. 7). Stable isotope trends in calcite indicate that freshwater diagenesis was less pervasive during the time that the impact crater acted as a distinct, deeper depositional basin.

Furthermore, they also suggest that the last extensive diagenetic events involving freshwater affected a nearly horizontal carbonate platform. The participation of organic carbon species in the formation of replacive lowmagnesium calcite increased toward the surface. The vug-filling carbonate cement is the most common in the UNAM 5 core. The following trends are present (Fig. 9): 1) Among multiple calcitic vug linings (d 18O: to + 1.73x PDB, d 13C: 7.30x to

5.06x 0.03x

Fig. 8. d 13C in whole-rock (excluding vug-filling) calcite from UNAM cores versus depth [modified form Lefticariu (2004)]. Facies type appears next to each symbol (see Fig. 2 for facies description).

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Fig. 9. d 18O versus d 13C in the vug-filling carbonate cement of the UNAM 5 core [modified form Lefticariu (2004)].

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PDB), the younger generations are depleted in heavy stable isotopes compared to the older ones. 2) A very strong direct correlation is present between d 18O and d 13C in calcite cement. 3) The vug-lining dolomite (d 18O: 2.11x to 0.08x PDB, d 13C: 0.33x to +1.20x PDB) is enriched in d 18O compared to the predominant replacive dolomite, which makes up most of the host rock. 4) Where two generations of vug-lining dolomite are present, the older generation is depleted in d 18O and d 13C than the younger one.

9. Suggested mechanisms and timing of dolomitization and other diagenetic events Petrographic and geochemical data suggest that most replacive dolomite and dolomite cement formed in the shallow subsurface in the presence of a mixed fluid dominated by freshwater and/or an unusually high geothermal gradient. Unless proved as altered syndepositional hydrocarbon seeps to the floor of the Basin, the bitumen in the Eocene of the Yax-1 core strongly suggests a higher-thannormal geothermal gradient during organic matter diagenesis.

Table 2 The main depositional and diagenetic events that took place during the Cenozoic in the study area Event order

1 (oldest) 2

3

4

5

6

7 (youngest)

Significant processes

Suggested age

Inside the Chicxulub Basin

Outside the Chicxulub Basin and Santa Elena Depression

Santa Elena Depression

Pelagic sedimentation and debris flows Pelagic sedimentation. Chert formation

Impact breccia dissolution. Shallow-water deposition Shallow-water deposition of evaporite and/or carbonate. Precipitation of early polyhedral dolomite from evaporated seawater

Pelagic and outer platform sedimentation. Chert formation. Dolomite formation at the edge Pelagic and outer-platform sedimentation. Sparse dolomite formation away from the basin edge Shallow-bank sedimentation with episodes of subaerial exposure, freshwater diagenesis, and dolomitization at the edge Shallow open-platform and shallow, more restricted water sedimentation, patchy reefs growth above crater peaks. Replacive dolomitization at the edge Basin filling with shallow, restricted water deposits (Middle Miocene). Extensive freshwater diagenesis and karst formation. Precipitation of vug-filing carbonate cement. Sparse replacive dolomite formation

Shallow-open platform and shallower, more restricted water deposition. Subaerial exposure

Pelagic and outer-platform sedimentation Outer-platform marine sedimentation alternating with saline lake deposition and siliciclastic input. Polyhedral dolomite precipitation from warm, saline waters. Chert formation Shallow-open platform and shallower, more restricted water deposition. Subaerial exposure

Subaerial exposure and freshwater diagenesis. Replacive dolomitization

Subaerial exposure and freshwater diagenesis. Replacive dolomitization

Late Eocene through Early Oligocene

Subaerial exposure and freshwater diagenesis. Extensive replacive dolomitization Shallow, restricted water deposition. Extensive replacive dolomitization. Precipitation of vug-filling carbonate cement Basin filling with shallow, restricted water deposits (Middle Miocene). Extensive freshwater diagenesis and karst formation. Precipitation of vug-filing carbonate cement. Sparse replacive dolomite formation

Subaerial exposure and freshwater diagenesis. Extensive replacive dolomitization Shallow, restricted water deposition. Extensive replacive dolomitization. Precipitation of vug-filling carbonate cement Basin filling with shallow, restricted water deposits (Middle Miocene). Extensive freshwater diagenesis and karst formation. Precipitation of vug-filing carbonate cement. Sparse replacive dolomite formation

Late Early Oligocene through Late Oligocene

Paleocene Early Eocene

Middle Eocene through Late Eocene

Late Oligocene through Early Miocene

Middle Miocene through Pleistocene

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Replacive and vug-filling dolomite crystal surface topography data [compared to crystal microtopography data of Lasaga (1998), Arvidson and Mackenzie (1999), and Kessel et al. (2000)] indicate that the dolomitizing fluid was at near-saturation with respect to dolomite and that dolomitization was a surface-controlled process. Polyhedral dolomite could have precipitated from a warm, hypersaline fluid. The following can be said about the timing of dolomitization: 1) The scattering of the oxygen isotope values suggests multiple dolomitization events, whose effects in time overlap partly. 2) The occurrence of massive dolostone below the Lower/Middle Eocene boundary outside the Chicxulub Sedimentary Basin and Santa Elena Depression indicates a dolomitizing event associated with significant relative sea-level fluctuations of correspondent age. Within the Basin, the Lower Eocene is dolomitic only close to the edge (UNAM 2 core). 3) The most important replacive dolomitization event took place after the Middle Eocene and it is suggested that it was associated with a high-amplitude relative sea-level fluctuation that took place during the Early Oligocene. The extensive dolomitization of the Middle Eocene formations at the edge of the Basin and those from outside the Basin was preceded by freshwater diagenesis. 4) The last significant dolomitization event took place after the Late Oligocene, as indicated by the replacive dolomite from the formations of this age present in the Y-6, and UNAM 1 cores. It could be associated with an Early Miocene relative sea-level fluctuation as, in general, dolomite is not abundant within the Lower Miocene of the Y-6 core and at the top of the Upper Oligocene from the UNAM 1 core. 5) In most cases, vugs and karst formed after extensive dolomitization, which would constrain the vug-filling dolomite and in general the vug-filing carbonate cement to be not older than the Early Oligocene. Table 2 presents a brief depositional and diagenetic history of the Cenozoic formations of northwestern Yucatan Peninsula, including dolomite formation. With respect to the timing of the main glacio-eustatic events the present study takes into account the eustatic sea-level curve of Abreu and Anderson (1998), as well as the unconformities proposed by Pope et al. (1996) and McClain (1997).

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10. Conclusions The present study explains: 1) how and when carbonate sediment deposition and diagenesis took place within the Chicxulub Sedimentary Basin, which formed because of the largest known Phanerozoic bolide impact; 2) why deeper-water Lower Tertiary sedimentary carbonate and silicate-rich intervals occur outside the Basin; and 3) why the d 18O of Tertiary dolomite is so low in the northwestern Yucatan Peninsula. During the Tertiary Period, the Chicxulub Impact Crater acted as a complex basin of sedimentation that concentrated the deposition of most pelagic and outer-platform sediments until the Late Early Oligocene. The control exerted by the impact structure on sediment deposition and diagenesis within this basin continued through the Early Miocene. The Chicxulub Sedimentary Basin, which roughly occupies the impact structure and is circumscribed by the Ring of Cenotes, filled in the Middle Miocene. Shallow-water sedimentation predominated both inside the Basin during the Late Oligocene–Miocene and outside the Basin during most of the Cenozoic, with the notable exception of the Santa Elena Depression, where the shallow-water deposition commenced only in the Middle Eocene. During the Early Eocene the Depression filled with both deep marine and saline lake deposits subjected periodically to siliciclastic input. The structural complexity of the crater coupled with the effects of diagenesis prevents a precise calculation of the sedimentation rate within and subsidence of the various sectors of the Basin. However, the similarity between the thickness of the Lower Eocene deposits of the Yax-1 and UNAM 2 cores suggests a minimum mean sedimentation rate during this time interval of 2.15 cm/1000 years for the portion of the Basin where the two cores are located. This also indicates that the final structural configuration of the Basin was attained after the Early Eocene. Based on the available data a maximum subsidence of 380 m is suggested for the Basin, within the peak ring, and consequently a maximum depositional depth at the beginning of the Paleocene not greater than 720 m. Lithologic, petrographic, and geochemical data indicate that formation of diagenetic low-magnesium calcite and dolomite took place more frequently outside the Chicxulub Sedimentary Basin than inside. Extensive replacive dolomitization took place after the Middle Eocene and was preceded by freshwater diagenesis. Sediment deposition and diagenesis was controlled by both the distance from the depocenter and crater topography (rings, peaks).

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