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174K.O. Pope et al.Figure 4. Ramonal North stratigraphic section. (I) 30%–35% greenclay clasts 2–40 mm in diameter (mean 4–6 mm), many clasts haveelongated vesicles; 20% carbonate accretionary lapilli 1–25 mm indiameter, most lapilli are coated with red iron oxide and many havecores of limestone and green clay fragments; 1%–3% angular micriticlimestone clasts 0.5–5 cm in diameter with rare clasts up to 20 cm;40% calcite silt matrix; common arcuate fracture planes with thin claylinings. (II) Weathered top of section; 1% green clay clasts 2–20 mmin diameter; 10% carbonate accretionary lapilli 1–25 mm in diameter;1%–3% angular micritic limestone clasts 0.5–5 cm in diameter; 85%calcite silt matrix; abundant roots.were found in Barton Creek outcrops between Ramonal Southand the town of Allende. This confirms that the Cretaceous isexposed in southern Quintana Roo. Ongoing research in Quintana Roo has recovered a rather diverse Upper Cretaceous faunain the Barton Creek Formation. Several species of bivalves andgastropods are being studied. A new species of the gastropodAporrhais represents the first record for that genus in the Cretaceous of the Gulf Coast and Caribbean Provinces (Vega et al.,2001). Although the stratigraphic range for this genus extendsfrom Lower Cretaceous to Recent, specimens from QuintanaRoo show similarities to species from the Maastrichtian of Montana and Germany.Overlying the Barton Creek Formation at Ramonal South is a2-m-thick section of the Albion Formation spheroid bed, which isoverlain by a 5-m-thick section of the diamictite bed that extendsFigure 5. Alvaro Obregón stratigraphic section. (I) 30% sub-angularto sub-rounded micritic dolomite and limestone pebbles and cobblesup to 25 cm in diameter; 10% green clay clasts 3–20 mm in diameter.(II) 20% sub-angular to sub-rounded micritic dolomite and limestonepebbles and cobbles up to 30 cm in diameter; 5% green clay clasts.These two units are separated by a horizontal layer of orange calciticclay with slickensides (shear plane?).to the surface (Figs. 3 and 7). The upper and lower contacts ofthe spheroid bed are marked by 2–20-cm-thick layers of orange,purple, and green calcitc clay with common slickensides. Theseclay layers appear to be shear planes. There is an abrupt appearance of large micritic dolomite clasts above the upper shear planeand one large dolomite boulder rests immediately on top (Fig. 7).Field tests with HCl indicate that the base of the spheroid bed iscalcitic, and becomes progressively more dolomitic near the top.The diamictite bed matrix is dolomitic.Laboratory AnalysesWe examined 12 thin sections from the top of the BartonCreek Formation and from the spheroid and diamictite beds atRamonal South. Here we describe the textures and key aspectsof the mineralogy. Estimates of the percentages (area) of majorconstituents were made using photomicrographs and percentagecharts. The top of the Barton Creek exhibits abundant iron oxidestaining, secondary coarse calcite filling fractures and vugs,

Chicxulub impact ejecta deposits175and polycrystalline calcite replacement of dolomite rhombs(de-dolomitization) (Fig. 8), all consistent with a subaeriallyweathered surface. The main stratum of the spheroid bed (stratum II, Fig. 3) contains 20% yellowish-green clay with spherulitic devitrification features and relict vesicles typical of alteredglass (Fig. 9). The spheroid bed contains 20% carbonate clastsof various types, including foraminifera fragments (Fig. 9), and3% accretionary lapilli (core-type, Schumacher and Schmincke,1991) composed of aggregates of detrital calcite grains 10–125µm in size (Figs. 9). The matrix of the spheroid bed is composedof detrital calcite grains similar in size and character to that ofthe accretionary lapilli aggregates. Some portions of the matrixare altered to interlocking grains of dolomite, especially near thetop of the bed. Thin section analysis of the diamictite bed revealsthat it is pervasively dolomitized, but contains 10% clay and 20%dolomite clasts ( 0.5 mm) of various types (smaller clasts arecompletely recrystallized) (Fig. 10).Ramonal NorthField ObservationsThe Ramonal North road cut exposes a 3.6 m section of thespheroid bed in a dip in Ucum–La Unión highway 200 m northof Ramonal South (Figs. 4 and 11). Neither the top nor the baseof the spheroid bed is exposed in the road cut, but the AlbionFormation diamictite bed crops out 30 m behind the road cut,suggesting the top of this exposure may be near the upper contactwith the diamictite bed. The base of the Ramonal section lies 13 m below the base of the spheroid bed at Ramonal South.We interpret this elevation difference as a dip in the paleo-karstlandscape, as the exposed surface of the Barton Creek Formationat Ramonal South is sloping toward Ramonal North and this surface is observed to undulate up and down in numerous road cutsalong the Ucum–La Unión highway.Figure 6. Agua Dulce stratigraphic section. (I) 25% sub-angular tosub-rounded micritic dolomite and limestone pebbles, cobbles, andboulders up to 3.2 9.0 m in size; 5%–10% green clay clasts; fewer(5%) cobbles and boulders in the upper part. (II) 10% micritic dolomite clasts 1–15 cm in diameter; 1%–3% green and orange clay clasts0.5–2 cm in diameter; 85% matrix of coarse calcite silt with orangeand purple iron oxide straining (weathered version of stratum I); (III)calcite cemented caliche with large 2–3 mm calcite crystals; (IV)weathered white micritic dolomite overlying pink and tan, thin-bedded, micritic limestone.Laboratory AnalysesWe analyzed 17 thin sections from the Ramonal North spheroid bed. Yellowish-green clay fragments comprise 20% of the bedand contain common relict vesicles and spherulitic devitrificationfeatures typical of altered glass (Figs. 12). No preserved glass wasfound. Calcite accretionary lapilli of diverse types comprise 10%of the bed, and range in size from 0.2 to 5.0 mm, and average 1.0 mm (Figs. 13–15). There are many examples of what aresometimes classified as mantled lapilli—lithic cores coated withfine ash (Fig. 15), and both rim-type and core-type accretionarylapilli—aggregates of fine detrital grains, with or without a finegrained rim (Schumacher and Schmincke, 1991). We refer to alltypes as accretionary lapilli, in part for simplicity, but also becauseit is not always apparent whether the core is an aggregate or lithicfragment when the latter is rounded and has a similar lithology tothe aggregate. Rim-type accretionary lapilli are the most abundanttype (Fig. 13), but core-type are also common (Fig. 14), as areones with limestone (Fig. 15) and clay (altered glass) lithic cores(Fig. 14). The detrital calcite grains in the lapilli aggregates range

176K.O. Pope et al.Figure 7. Ramonal South outcrop photograph showing the spheroid bed overlaying the Barton Creek Formation and thediamictite bed overlaying the spheroidbed. Note large boulder resting on contactbetween spheroid and diamictite beds.in size from 150 µm to 10 µm, and the outermost rims are typically composed of very fine particles ( 10 µm). Detrital grains arenearly all calcite, but clay fragments (altered glass?) make up 1%–3% of the aggregate. Detrital carbonate clasts ( 100 µm) of various lithologies, including foraminifera tests (Fig. 14), comprise5% of the spheroid bed in thin section and occur in approximatelythe same size range as the accretionary lapilli, although there is nodiscernable lower size limit (there is a continuum between theseclasts and the matrix). The matrix, arbitrarily defined here as thatfraction 100 µm, comprises 60%–70% of the thin sections andis composed mostly of detrital carbonate grains and a few percentclay. The matrix and the lapilli aggregate are essentially the samecomposition.Two samples of the clay (altered glass fragments) fromwithin the spheroid bed were analyzed by X-ray diffractionafter air drying, glycolation, and heating to 550 C, using aPhilips PW1710 X-ray diffractometer at the Geological Surveyof Canada. Both were found to be smectite with traces of mixedlayer smectite-illite.Alvaro ObregónField ObservationsThe road cut at Alvaro Obregón exposes 7.5 m of the diamictite bed (Fig. 5) separated by a sub-horizontal (dips slightlysouth) orange calcitic clay layer a few centimeters thick withslickensides on the upper and lower surfaces. This clay layer ispositioned 2.6 m up from the base of the exposure and extendsthe length ( 100 m) of the road cut (Fig. 16). As at RamonalSouth, we interpret this clay layer as a shear plane. Rare cobblesin the diamictite bed have striated surfaces, polish, and accretionary rinds. There is no upper or lower diamictite bed contactFigure 8. Photomicrograph of thin section from the upper contact ofthe Barton Creek Formation at Ramonal South showing (a) iron oxidestaining; (b) secondary coarse calcite filling cavities; and (c) polycrystalline calcite (light) replacing dolomite (dark cores). Plane polarizedtransmitted light. Field of view: 2.8 mm wide.

Chicxulub impact ejecta deposits177Figure 9. Photomicrograph of thin section from the Ramonal South spheroidbed showing (a) clay fragments withelongated vesicles filled with carbonate matrix; (b) micritic limestone clasts(center arrow foraminifera); (c) accretionary lapilli; (d) matrix of detrital carbonate grains. Plane polarized transmitted light. Field of view: 4.4 mm wide.Figure 10. Photomicrograph of thinsection from the base of the diamictitebed Ramonal South showing (a) clayfragments; (b) recrystallized coarsegrain dolomite (left) and fine grainedlimestone (right) clasts; and (c) coarsegrained dolomite matrix (recrystallized). Plane polarized transmitted light.Field of view: 6.0 mm wide.

178K.O. Pope et al.Figure 11. Hand sample of RamonalNorth spheroid bed showing abundantaccretionary lapilli and clay fragments(altered glass) and common limestoneclasts.Figure 12. Photomicrograph of thin section from the Ramonal North spheroidbed showing a large clay fragments with(a) elongated vesicles filled with carbonate matrix; and (b) spherulitic devitrification features. Plane polarized transmittedlight. Field of view: 4.2 mm wide.exposed at Alvaro Obregón. Tests with dilute HCl indicate thatthe upper stratum is mostly dolomitized, whereas the base is onlypartially dolomitized and contains several micritic limestoneclasts 1–20 cm in diameter and a slightly calcitic matrix.Laboratory AnalysesWe analyzed seven thin sections from the Alvaro Obregóndiamictite bed. The bed contains 10% yellowish-green clayfragments, which like those in the spheroid bed, possess relictvesicles and spherulitic devitrification features typical of alteredglass (Fig. 17). Angular to sub-rounded carbonate clasts 100 µmin size comprise 25% of the thin sections, some of which retainpoorly preserved foraminifera (Fig. 18). Most carbonates arerecrystallized to dolomite, which makes it difficult to discern clastboundaries for the smaller clasts as they merge with the 10–40 µmrecrystallized dolomite matrix. The predominant clast lithology isa light brown dolomite with 20–40-µm-sized interlocking dolomite crystals and indistinct clast-matrix boundaries (Fig. 18).

Chicxulub impact ejecta deposits179Figure 13. Photomicrograph of thin section from the Ramonal North spheroidbed showing (a) accretionary lapillicomposed of coarse carbonate detritalgrains with a very thin, fine ( 10 µm)accretionary rim (rim-type); (b) lithicclast with a spherulitic altered glass rim;and (c) angular limestone clast. Planepolarized transmitted light. Field ofview: 5.9 mm wide.Agua DulceField ObservationsThe road cut at Agua Dulce extends for 500 m and exposesan 8-m-thick sequence of the diamictite bed (Fig. 6) with manylarge micritic dolomite boulders (Fig. 19). The diamictite bedhere is similar to that found at Alvaro Obregón, except for theabundance of large boulders (compare the photos in Figs. 16 and19). Tests with dilute HCl indicate that the diamictite bed clastsand matrix at Agua Dulce are only partially dolomitized. TheAgua Dulce section contains one very large dolomite boulder(9.0 3.2 m) that has highly deformed bedding (Fig. 19). Theupper ( 2 m) part of the diamictite bed has fewer large cobblesand clay clasts, but the change is gradational. The top 1 m ofthe diamictite bed is a weathered horizon capped with well-indurated caliche. Overlying this caliche are over 10 m of thin-beddeddolomite and limestone.Figure 14. Photomicrograph of thin section from the Ramonal Northspheroid bed showing (a) accretionary lapilli composed of coarse carbonate detrital grains (core type); (b) clay clasts with relict vesiclesand flow banding (center), note altered glass fragment with accretionary rind (center); (c) carbonate clasts; and (d) foraminifera test. Planepolarized transmitted light. Field of view: 3.0 mm wide.Laboratory AnalysesWe analyzed seven thin sections from the Agua Dulcediamictite bed. The bed contains 15% yellowish-green clay fragments with the same relict vesicles and spherulitic devitrificationfeatures indicative of altered glass found at Alvaro Obregón(Fig. 20). Angular to sub-rounded carbonate clasts 100 µm insize comprise 25% of the thin sections, some with preservedforaminifera (Fig. 20). Some carbonate clasts are recrystallizedto dolomite, and the previously noted clasts of light brown dolomite with 20–40 µm-sized interlocking dolomite crystals andindistinct clast boundaries are present (Fig. 20). Dolomite clasts

180K.O. Pope et al.Figure 15. Photomicrograph of thin section from the Ramonal North spheroidbed showing a large accretionary lapilliwith (a) a micritic limestone lithic corewith a foraminifera test that is encasedby multiple layers of accreted detritalcalcite grains. Outer dark color is dueto iron oxide staining of outer layers.Plane polarized transmitted light. Fieldof view: 4.3 mm wide.Figure 16. Alvaro Obregón road cutexposing Albion Formation diamictitebed. Note horizontal calcitic clay layer(shear plane?) along the center of theoutcrop. Exposure is 7.5 m high.

Chicxulub impact ejecta deposits181Figure 17. Photomicrograph of thinsection from near the base of the Alvaro Obregón diamictite bed showinga clay fragment (altered glass) with (a)relict vesicles filled with matrix and(b) spherulitic devitrification features.Plane polarized transmitted light. Fieldof view: 1.9 mm wide.Figure 18. Photomicrograph of thinsection from near the base of the Alvaro Obregón diamictite bed showing(a) a clay fragment (altered glass) withelongated vesicles; and (b) lithic clastsof coarse dolomite, one with a foraminifera test, in a mostly fine grained calcite and dolomite matrix. Field of view:4.2 mm wide.

182K.O. Pope et al.are not as abundant as at Alvaro Obregón, and micritic limestoneclasts are common. The carbonate matrix is finer-grained at AguaDulce (less altered) than at Alvaro Obregón, and small clastboundaries are more distinct.Four clay separates (altered glass fragments) were analyzedby X-ray diffraction and are dominated by smectite with traces ofmixed layered smectite-illite.NORTH-CENTRAL BELIZEThe discovery of Chicxulub impact ejecta along the BelizeMéxico border prompted our search for more distal ejectadeposits further south, in Belize. The preservation of Chicxulubejecta on Albion Island, an uplifted block of Cretaceous carbonate platform sediments associated with the Río Hondo fault zone(Ocampo et al., 1996; Pope et al., 1999), suggests that other juxtaposed fault-blocks of Cretaceous and Tertiary platform carbonates in north-central Belize may preserve remnants of Chicxulubejecta. Nevertheless, our reconnaissance of Cretaceous blocksnear Gallon Jug, Laminai, and Yalbac (Fig. 1) produced no evidence of impact ejecta. Thus, if Chicxulub ejecta deposits existedin this region, they have apparently been stripped by erosion.Late Cretaceous and early Tertiary platform carbonates arewell known in central Belize (Ower, 1928; Dixon, 1956; Flores1952a, 1952b; Cornec, 1985; King et al., 2004). Marine sedimentation is thought to be discontinuous across the K-T boundary(Flores, 1952b), but the stratigraphic relationship between Cretaceous and Tertiary strata in Belize has not been studied in muchdetail. Upper Cretaceous Barton Creek Formation outcrops withrudist fragments were identified by Flores (1952a, 1952b) alongthe base of the Maya Mountains (Fig. 1). Paleocene sedimentshave not been previously identified in this region, but outcrops ofshallow marine Paleocene carbonates are found 35 km northeastof Belmopan (Flores 1952a, 1952b). Lower Eocene limestonesoutcrops of the El Cayo Group are common along the northernflanks of the Maya Mountains (Flores 1952a, 1952b). A majoruplift of the Maya Mountains began in the early Tertiary involving the reactivation of the northern boundary fault of the MayaMountains (Fig. 1) (Bateson and Hall, 1977).We examined stratigraphic sections along the northernflanks of the Maya Mountains in the vicinity of juxtaposed Cretaceous and Tertiary blocks shown on the geologic map of Belize(Cornec, 1986). The complex tectonism and the thick tropicalvegetation made if difficult to trace contacts between sections.The upper contact of the Barton Creek Formation in this region isa highly irregular karst surface with relief of 10–30 m and exposures of caves and rubble-filled solution cavities. The BartonCreek carbonates near the upper contact are a coarse, recrystallized limestone with abundant calcite-filled fractures and vugs,but grade down-section into dolomitized limestones. Goodexposures of the weathered top of the Barton Creek are found atPook’s Hill 2 (PH2), Hummingbird Highway 1 (HH1), and SanAntonio 1 (SA1) (Fig. 1). At 15 locations we found the BartonCreek Formation overlain by 10–30 m of pebbles, cobbles, andFigure 19. Agua Dulce road cut with large dolomite boulders in the Albion Formation diamictite bed. All have major axis roughly parallel tothe ground surface. Upper two boulders both 6–7 m long, lower one is9 m (outline in white, black lines mark extreme folding in relict beds).boulders supported in a matrix of red clay and coarse calcitesilt. The coarse size fraction ( 1 cm) of this unit is a mixture ofangular to rounded clasts, but sub-rounded clasts dominate. Clastsizes range from sub-millimeter grains to 2 m diameter boulders,but most clasts are 30 cm in diameter. The clast lithology isa diverse mixture of carbonates, including micritic limestone,coarse crystalline limestone, laminated dolomite, breccia, andcalcareous mudstone. The coarse crystalline limestone is themost common lithology, and is probably derived from the weathered Barton Creek Formation. We interpret this unit as a regolithor saprolite resulting from the karst weathering of the BartonCreek Formation, but we recognize that its genesis may well becomplex and not solely the product of in situ weathering giventhe diverse clast lithologies.Tertiary sediments overly the Barton Creek regolith atPook’s Hill 2 (PH2, Fig. 1), but the contact is not well exposedand is marked by an abrupt change in the lithology of clastseroding out of the bulldozed upper slopes of a hill. These Tertiary sediments are a cream-c

to Chicxulub is the Johnson quarry site (Fig. 2), which contains Albion Formation diamictite deposits 330 km from the center of Chicxulub. The combined Albion Formation exposures in Belize and Quintana Roo indicate