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T boundary at Beloc (Haiti): Compared stratigraphic distributions of the boundary markers
EPSL ELSEVIER
Earth
and Planetary
Science
Letters
131 (1995) 255-268
The K/T boundary at Beloc (Haiti): Compared stratigraphic distributions of the boundary markers Hugues Leroux a, Robert Rocchia b, Laurence Froget b, Xavier Orue-Etxebarria Jean-Claude Doukhan a, Eric Robin b
‘,
a Laboratoire Structure et Prop&t& de I’Etat Solide, Uniuersite’ Sciences et Technologies de Lille. 59655 Villenewe d’Ascq Cedex, France ’ Centre des Faibles Radioactivit& CEA-CNRS, 91198 Gif-sur-Yllette Cedex, France ’ Unitlersidad de1 Pais Vasco, Facultad de Ciencias, Apartado 644* 48080 Bilbao, Spain Received
14 December
1994; accepted
after revision
22 February
1995
Abstract At Beloc, Haiti the Cretaceous-Tertiary boundary (KTB) is characterized by a spherule bed containing glass particles. These particles are considered by some authors as remains of tektites resulting from a nearby impact. However, because of the stratigraphic complexity of the Beloc sections the genetic link between the KTB cosmic event and the spherule bed is not obvious. In this paper, we report new data on shocked quartz and Ni-rich spinels at Haitian KTB sites. The detailed stratigraphy of these minerals shows that there is no empty gap in the sedimentary sequence. The first and largest shocked quartz is found in the upper part of the spherule layer. They are abundant and size graded over the 25-30 cm of carbonate-rich sediments overlying this layer. The first Ni-rich spinels, which are also rich in Cr, appear in the carbonate sediments. The size grading of the spherules and shocked quartz and the stratigraphical overlapping of their distributions suggests that these two components were derived from the same event. Although different from typical impact glasses (tektites), Beloc glass particles must be considered as impact-derived products. The enormous fluence of shocked quartz (= lo4 grains/cm’) is consistent with a proximal event. The upper part of the sequence is more complex. A second distribution of shocked quartz associated with Ni-rich spinels of different compositions appears in the layer containing the maximum Ir concentration. We propose that these features, which are not easily explained by a sedimentary artifact, result from a second collisional event.
1. Introduction The Cretaceous-Tertiary boundary (KTB) biological crisis is associated with geochemical and mineralogical anomalies which are attributed by a number of authors to the impact of a large asteroid or comet [l-7]. In Haiti, near the village of Beloc, the KTB is characterized by a 30 cm thick layer of spherules possibly representing the mate0012-821X/95/$09.50 0 1995 Elsevier SSDI 0012-821X(95)00032-1
Science
rial ejected by the alleged impact. The thickness of this presumed ejecta layer suggests that the impact occurred nearby in the Caribbean area. This hypothesis is supported by the existence of a large circular structure of nearly 200 km in diameter centred on the Yucatan peninsula-the Chicxulub crater [8]. A major objection against this scenario has been raised by detailed stratigraphic studies of Beloc K/T sites. In the sec-
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and Planetary Science Letters 131 (1995) 255-268
tions where the K/T transition appears to have been properly recorded and preserved, the spherule layer is separated from the iridium-enriched layer (also containing Ni-rich spinels) by 2.5-35 cm of carbonate-rich Cretaceous sediments. JChanno et al. [9] proposed that this clear stratigraphic separation results from two distinct and unrelated events. However, it remains that the close association of two exceptional deposits is unlikely on statistical grounds. The question that we address in the present paper is the following: Can other impact markers yield significant information? To answer this question we have studied the stratigraphic distribution of shocked quartz, a K/T marker considered as an unequivocal impact-derived product [ 10,111. We have also reanalysed Beloc sections for their Nirich spine1 content using improved sensitivity.
2. Geological
setting
The Beloc sites are located in the southern part of the Haitian peninsula. A general description of the sites has been published by Maurasse et al. [12,131. We have sampled a dozen outcrops over 2.5 km along the road from Carrefour Dufort to Jacmel. In most sections the spherule bed and overlying layers appear to have been disturbed, as suggested by the presence of mud chips, discontinuities, or periodicities within the spherule bed mimicking multiple flows. At the type locality we have observed abundant mud clasts of Maastrichtian age within the spherule bed with at least four alternating spherule size gradings. The sedimentary structure, which is revealed by recent surficial erosion and colour changes, indicates in some places horizontal injection flow. At other places we have even observed stratigraphically distinct spherule beds (suggesting multiple events), which appeared in fact to be of limited horizontal extent. The sedimentary conditions at the Beloc sites were not perfectly quiet. These disturbing conditions were not limited in time to the K/T transition, as is demonstrated by the frequent slumping observed above the boundary. Deposition did not occur on a flat seafloor but probably on or at the base of a
slope. The results discussed below are derived from the detailed study of two adjacent sites (1.50 m apart), Beloc A and H [9]. Although showing crossbedding structures, Beloc A and H seem to have been less disturbed by reworking than other outcrops. The choice of these two sites is justified by the continuity of the sequence, the single grading of the section, and the discovery of a high Ir concentration spike which could not have been preserved if reworking had been active. For these reasons they were selected by JChanno et al. [91 for a precise stratigraphical study of Ir and Ni-rich spinels. Beloc H, the best preserved outcrop, is characterized by a 25 cm thick spherule layer (unit 1) that is size graded; at its base the contact with the underlying Cretaceous limestone is sharp. This contact is used as reference level for sample positioning. The spherule bed sensu strict0 is overlain by 25-35 cm of carbonate-rich sediments containing Cretaceous fossils (unit 2). There is a 1 cm thick grey-greenish clay layer (unit 3) 63-65 cm above the reference level; this clay layer is sometimes barely visible and contains rust-coloured lenses. The maximum Ir and spine1 concentrations have been found in this layer [9l. Above it we find calcareous deposits of the basal Danian. Although very similar to Beloc H, the Beloc A section is affected by slight horizontal variations which make the comparison of previously published data on Ir and spinels [9] with the shocked quartz distribution reported here difficult. Indeed, the samples used for the two studies ([9] and the present one) were collected at not exactly the same place during two different fieldtrips.
3. Shocked quartz as an impact signature 3.1. Significance of shocked quartz
The occurrence of shocked quartz is considered by many authors as one of the most important indicators of shock metamorphism (i.e., of an impact origin) [lO,ll]. Shocked quartz grains found in impact craters display characteristic planar deformation features (PDFs) [10,14-171 which reveal that the rock experienced an intense and
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and Planetary Science Letters 131 (1995) 255-268
short peak pressure (P> 10 GPa, with a high strain rate in the range 103-lo6 s-l). PDFs appear in optical microscopy as pervasive, straight, and very narrow contrasts parallel to the basal plane and rhombohedral planes {10&z]. Their fine structure cannot be resolved by optical miTransmission electron microscopy croscopy. (TEM) is a better adapted technique which provides higher magnification together with a num-
257
ber of other characterization capabilities (diffraction, contrast analysis and X-ray microanalysis). Bohor et al. [7] were the first to report the occurrence of shocked quartz grains, in the continental K/T deposits of Brownie Butte, Montana. This provided major support for the impact hypothesis of Alvarez et al. [I]. This discovery was soon extended to other KTB sites around the world and in 1987 Bohor et al. [18] noted that the
Fig. 1. (a) Optical micrograph (crossed nicols) of a shocked quartz grain of 120 Frn in diameter at level 35-40 cm in the Beloc A section. Two sets of PDFs are clearly visible. fb) TEM bright field micrograph showing two sets of PDFs. These are composed of a high density of small defects, probably tiny dislocation loops or small fluid inclusions. Cc) TEM micrograph, dark field: Thin mechanical twins (with fringe contrast) parallel to the basal plane. Cd) Mosaicism as seen in TEM. This consists of small crystalline domains disoriented relative to each other. The different contrasts reflect their different crystallographic orientations (bright field). (e) Spindle-shaped bands parallel to the (lOi0) planes (bright field). (f) Some of spindle-shaped bands are filled with phyllosilicates (bright field).
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largest shocked quartz grains are found in North American sections, suggesting a nearby impact. Shocked quartz grains were also found in abundance in Haiti, near Beloc [19]. This abundance, together with the presence of glass-bearing spherules, was immediately taken as having resulted from a very proximal impact. However, no detailed stratigraphic distribution of the shocked quartz has been studied in the Haitan sections. The purpose of the present study is to determine this stratigraphic distribution and to compare it with the distribution of other KTB markers. 3.2. Analytical procedure Shocked quartz was extracted from the sediments in the following way: The material was first disaggregated and dispersed in water by ultrasonic treatment. Small clay particles stayed in suspension in the aqueous solution. The carbonate fraction was dissolved in HCl. The clay-free and carbonate-free residue was dried, and then deposited on a thin glass plate with a drop of benzil alcohol. As the refraction index of benzil alcohol is very close to that of quartz, the quartz grains are easily identifiable under the optical microscope. The shocked grains are identified by the optical contrasts produced by PDFs (Fig. la>. Several quartz grains with well-visible PDFs were selected for detailed TEM characterization. After extraction, they were embedded in epoxy resin and a standard petrographic thin section (30 pm thick) was then prepared by the usual grinding and mechanical polishing (on both faces). Final thinning down to electron transparency ( 5 1 pm) of these samples was achieved by the usual ion-thinning technique with Ar ions accelerated by a high voltage of = 5 kV, with a beam angle of 1.5”. Finally, the thinned samples were ctated with a very thin layer of carbon (= 300 A) to prevent the accumulation of electric charges in the TEM. Investigations were carried out with a Philips CM30 electron microscope operating at 300 kV. As explained below, these TEM investigations have shown that all the quartz grains selected for their optical contrast in benzil alcohol do contain PDFs. This supports the stratigraphic data presented below. Shocked quartz
grains from various levels in the section were also carefully counted and their size measured for statistical analysis (Section 3.4). 3.3. TEM investigations The shock defects consist of multiple sets of straight and narrow bands often extending through the whole grain (Fig. lb). Their thickness varies from 0.1 to 0.5 pm and they are separated from each other by a few micrometres. These bands lies in rhombohedral planes, with {lOi2} and (lOi3) as the most common orientations. Several different microstructures can be distinguished. A number of PDFs contain a high density of small spheroidal defects, exhibiting miniscule contrast (< 10 nm) which cannot be fully resolved. These might consist of tiny dislocation loops and/or bubbles containing fluid or amorphous silica. The TEM observations show a high density of PDFs. All the grains selected by simple optical observation contain shock defects but the density of these defects is much higher than in the optical range. For instance, the mean distance between PDFs of a given set varies from = 0.5 to 2 pm, depending on the observed area. We suppose that the observed optical contrasts correspond to interference phenomena rather than to individual PDFs. This means that all strongly shocked quartz grains are correctly identified by optical means. A simple optical survey, however, is not sufficient to reveal weakly shocked grains with widely spaced PDFs. This may affect the statistical analysis presented in the next section. PDFs in the basal plane are rather rare but they are identical to those already identified in shocked quartz in other impact structures [15-171. They are very narrow (I 100 nm> mechanical twins with partial dislocations in the twin-matrix boundaries (Fig. 1~). The twins are generated by the rapid nucleation and the cooperative motion in adjacent basal planes of partial dislocations. The investigations using TEM always failed to find such twins related to purely endogenous phenomena. Mosaicism (i.e., slight misorientation of small crystalline domains) is pervasive, and seems to
H. Leroux et al. /Earth
result from the intersection of two or more sets of PDFs bounding small square or rectangular shaped domains = 1 km size (Fig. Id). Selected diffraction patterns on areas containing a large number of such domains clearly show slight misorientations (asterism of the diffraction spots). Our TEM investigations reveal another type of defect which was not described earlier. These are spindle shaped, with their long axis roughly parallel to the {lOiO) planes (Fig. le) extending over distances of a few micrometres. Their thickness does not exceed 0.5 pm, and they often start and/or stop on rhombohedral PDFs. Qualitative X-ray microanalyses performed on the material
Level
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inside the spindles show, in addition to silica, relatively large amounts of Ca and Al and lower amounts of K, Fe, and Na oxides. The spindle lamellae might be poorly crystallized (fibrous) phyllosilicates (Fig. If) and therefore they do not represent original PDFs. They could result from open fractures filled with silicate and later altered to phyllosilicates by circulating water-rich fluids. Despite their high degree of alteration, quartz grains from the Beloc K/T sediments exhibit the characteristic signature of meteorite impacts. The shock-induced defects are similar to those already detected in quartz grains from well-charac-
cm
Level 30
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Grain size (pm) Fig. 2. Beloc
H section.
Histograms
of shocked
Concentration (grainsper gramme), mean
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Grain size (pm) grain
sizes at the
levels
size and standard deviation(S.D.)
23-28 cm, 38-42 cm, 48-55 is indicated on each diagram
cm and
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and Planetary Science Letters 131 (1995) 255-268
(a)
Cc)
(b)
Tertiary Unit 3 Cretaceous fossils (Unit 2)
SpEiY’e (Unit 1) ‘2
10 Spherule size
Cretaceous
,
60
I
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X0
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IO0
log (a), distribution
of spherule
I
160
I80
dimensions)(b),
and shocked
quartz (mean size) (cl
cm (above the spherule bed) and +65 cm. Outside this range, few quartz grains are still present, and are without evident shock features. A total of = 2000 quartz grains have been observed and = 1000 shocked grains were used for the microstratigraphic analysis. Together they yield representative trends as shown in Fig. 3. In Beloc H, the most carefully sampled section, the first shocked quartz grains are detected at level +25 cm mixed up with smaller spherules from the underlying bed (Fig. 3). They are not very abundant ( = 30 grains/g sediment) but they are rather large (maximum size = 700 pm, mean
3.4. Microstratigraphy of shocked quartz grains We have determined the number of shocked quartz grains per unit mass of sediment and their sizes in samples from the Beloc A and H sections. A size histogram was evaluated for each level (Fig. 2). This investigation was carried out from level -100 cm to level + 180 cm but shocked quartz grains were detected only between +25 (a)
I
140
(pm)
size (maximum
terised impact structures [15-171 and remain clearly distinct from the microstructures resulting from usual low-strain-rate tectonic deformations.
.
Shocked quartz size
(mm) Fig. 3. Beloc H: Stratigraphic
I20
Cd)
(b)
Tertiary Unit 3 Cretaceous fossils (Unit 2)
SpkT’e (Unit 1) 0 Cretaceous
Fig. 4. Beloc H: Stratigraphic spinels Cd).
IW
200
300
400
500
Iridium
(grlg)
@g/g)
log (a), abundance
of shocked
quartz
.Ol
1
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Shocked quartz
(grains
per gramme
,I
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IO
Ni-rich spinels (numberlmg) of sediment)
(b). iridium
(c), and Ni-rich
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size at this level = 160 pm). Although there is a marked scattering of the grain sizes at all levels, a very clear trend emerges: the mean size roughly decreases linearly from 160 pm, in the lowermost (+25 cm) level containing shocked grains, down to 80 pm at level +58 cm. The concentration of shocked quartz (in number of grains per gramme of sediment) first increases from = 30 grains/g at level +25 cm to = 500 grains/g at level +45 cm, and then it falls rapidly in the samples located between 50 and 58 cm (Fig. 4). No shocked quartz grains are observed between 59 and 62 cm, and then they appear again in level 63-65 cm with a relatively large mean size of 140 pm. This looks like a bimodal distribution. The first distribution starts at the top of the spherule bed and extends up to level +58 cm with a monotonous grading. The mean size of the whole distribution (integrated between levels +25 and +58 cm) is 105 pm. The fluence is = 7500 shocked grains/cm2 for this first distribution. The second distribution is a pulse-like function centred on level 64 cm where JChanno et al. [9] observed the maximum concentrations of Ir and Ni-rich spinels. The fluence of this second distribution is only 400 grains/cm2. We must note that for both distributions optically selected shocked grains represent at least 50% of the total number of quartz grains contained in the investigated samples. In contrast, in levels below 25 cm and above 65 cm quartz grains are scarce-a few grains per gramme-and unshocked. This marked difference suggests that the numerous quartz grains found between levels +25 and +58 cm and considered as unshocked on the basis of optical investigations are also derived from the impact. They might be slightly shocked grains with PDFs hardly identifiable by optical means, As they were not taken into account in Figs. 3 and 4 the fluences given above (7500 and 400 grains/cm2) should be considered as Lower limits. Data from the Beloc A section are quite similar. A smooth size grading is observed in the mean size in the range 23-58 cm (mean size from 80 to 170 pm). A distinct population of large shocked quartz grains is superimposed at level 63-65 cm (mean size = 150 pm). The total flux of shocked grains at the two localities is very similar.
We find 8000 and 600 grains/cm2 for the lower and upper distributions respectively. If we except the fact that, in the Beloc A section, the layer of sediments corresponding to level 58-62 cm of section H was not sampled, the Beloc A and H sections look like twin outcrops-they essentially bear the same information. In order to compare these results with those found at distant localities, we have estimated the size-frequency distribution and the amount of 16
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Mean size = 145 pm
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size
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Fig. 5. Frequency diagrams showing size distribution of shocked quartz grains from the KTB in the Raton Basin and Frenchman Valley. Amount (grains per gramme), mean size and standard deviation (S.D.) is indicated on each diagram.
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and Planetary Science Letters 131 (1995) 255-268
shocked quartz per gramme of sediment in two North American K/T sections (Raton Basin, Coland Frenchman Valley, South orado Saskatchewan). The calculated mean sizes are 140 pm for Raton Basin and 145 Frn for Frenchman Valley (Fig. 5). We have estimated their abundance at 100 grains/g at Raton Basin and 50 grains/g in the Frenchman Valley section. Assuming that the layer containing shocked quartz is 2 cm thick at both localities, one finds fluences of 400 and 200 shocked quartz/cm2 respectively, 20 and 40 times less than in Haiti.
4. Ni-rich spinels as impact signatures The bimodal distribution of shocked quartz led us to reinvestigate the distribution of Ni-rich spinels. Jthanno et al. 191have already published data derived from the study of small quantities of sediments. We report here new data obtained with improved stratigraphical resolution and sensitivity. 4.1. Significance of Ni-rich spinels Ni-rich spinels are magnetic crystals produced in extraterrestrial objects during interaction at high temperature with Earth’s atmosphere. They are found in cosmic spherules [20,21], in meteorite fusion crust [21,22], and in various types of cosmic debris, such as meteoroid ablation droplets discovered in a Jurassic hardground [23] and impact glassy spherules in late Pliocene sediments [24]. They differ from terrestrial spinels by their high Ni content (> 1 wt%), which indicates an undifferentiated source material (meteoritic or mantle material), and by their high iron oxidation state (Fe”+/Fe,,, > 70 at%) resulting from formation under relatively high oxygen fugacities (> 10v4 atm) 121,251. Terrestrial magmas, which are poor in Ni, do not evolve under such high oxygen pressures [26]. This explains why highly oxidized Ni-rich spinels have no counterpart in terrestrial rocks. Therefore, in collisional events they can be considered as specific markers of the projectile in the same way as shocked quartz are specific markers of the target.
The presence of such minerals in most KTB rocks [6,9,27-311 is therefore a strong argument supporting a major collisional event triggering the K/T extinctions. In most K/T sections Ni-rich spinels are found isolated in the sediment, but it is worth noting that originally they were trapped in rather big bodies up to several hundred micrometres in size [9,30] that are now completely weathered out. However, at least at one K/T site located in the Pacific Ocean [32] these spinelbearing particles have preserved their original morphology and properties of meteoroid ablation droplets. 4.2. Separation and analysis of Ni-rich spinels Because Ni-rich spinels are strongly magnetic minerals they can be easily extracted from a liquid phase with an electromagnet [30]. Magnetic separation was carried out on quantities varying from 100 mg for levels most enriched in spinels (+62 cm) to 10 g for the others. Such large quantities allowed us to reach a sensitivity lo-100 times better than that obtained by JChanno et al. [91. The detection limit depends on the abundance of other magnetic grains but it is generally lower than 0.01 spinel/mg. 4.3. Distributions spinels
and
compositions
of Ni-rich
The Ni-rich spine1 distribution, like the shocked quartz distribution, is bimodal. The first Ni-rich spinels appear in level +30 cm, approximately at the same level as the first shocked quartz ( + 28 cm). The first maximum of spinels (0.4/mg, Fig. 4) is closely associated with the first maximum of shocked quartz. The fluence of this lower distribution is about lo4 spine1 crystals/cm2, which is comparable with the fluence observed at El Kef, Tunisia [30]. The spine1 abundance shows a deep minimum (O.O06/mg) at level +50 cm. The second maximum (35/mg, Fig. 4) occurs at level +62 cm in coincidence with the maximum of Ir and the second maximum of shocked quartz. The fluence of this upper distribution is lo5 spine1 crystals/cm’, comparable with the fluence observed at Caravaca, Spain [30]. The two populations of spinels differ in
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5. Micropaleontological analysis
composition. The number of analysable spinels (i.e., larger than 2-3 pm) is not sufficient to permit an accurate estimate of the composition in individual samples of the lower levels. Therefore, in order to obtain adequate statistics we have grouped all data of the lower distribution (five samples between levels +28 and 48 cm). Fig. 6 shows the histograms of the Cr concentrations in the lower and upper levels (levels between +53 and 63 cm). The lower population of spinels is, on the average, significantly richer in Cr (= 8%) than the upper one (= 4%). Owing to the low abundance of spinels in the intermediate levels ( + 45 to 55 cm> we do not know if this difference results from a continuous compositional variation or from a sharp stratigraphic discontinuity indicating two distinct compositions. In the same interval (28-63 cm> the iridium distribution (Fig. 4) exhibits a monotonous increase, except at level = 45 cm where we observe a small maximum, in coincidence with the first maximum of Ni-rich spinels. The most enriched layer corresponds to level 63 cm.
I
o-2.5
Fig. 6. Histogram
of Cr content
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2.5-5
I
s-1.5
5.1. h4ethods Samples from the Beloc A and H sections were also studied in order to obtain micropaleontological data. The lowest samples of the stratigraphic succession were collected at the top of the spherule bed (Unit 1, see Fig. 3) and the last samples were from 70 cm (above Unit 3). Two different fractions have been separated through the processing of the samples. The first fraction contains the residue between 40 and 100 Frn in size, whereas the second fraction comprises everything above 100 pm. In addition, thin sections have also been prepared and analysed. 5.2. Results All the samples are poor in planktonic foraminifera, even though the specimens are quite well preserved. A total of 30 species have been identified from the uppermost Maastrichtian, in
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in the lower (28-50 cm) and upper (53-63
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of the Ni-rich
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contrast to the 55-60 species found in other sections such as El Kef, Tunisia and those on either side of the Pyrenees in the Basque Country. Only latest Maastrichtian planktonic foraminifera occur at the top of Unit 1 and Unit 2. Most individuals identified in the Beloc samples belong to the family Heterohelicidae and the most abundant species are H. globuloso, P. costulata, G. minuta and G. yaucoensis. No large species (> 500 pm1 characteristic of the uppermost Cretaceous have been found in the samples from the upper part of Unit 1 or from Unit 2. The sample collected immediately above the Ir-enriched layer corresponds to the beginning of the Tertiary (Zone PO) or the G. cretacea Biozone. All the species found in this sample are typical of the uppermost Maastrichtian and almost all of them are now considered by various authors as survivors of the KTB extinction. In the uppermost sample ( + 70 cm>, in addition to rare Maastrichtian species we also principally find species typical of the base of the Tertiary (such as Ch. midwayensis, W. claytonensis, G. conusa and P. eugibina). Based on the presence of this assemblage, this sample should be included in Biozone Pl or the P. eugibina Biozone. 6. Discussion:
Record of impact remains
at Beloc
Let us consider first the lower part of the sedimentary sequence (Units I and 2). An important result of the present stratigraphic study is that the spherules and the first population of shocked quartz are derived from the same event. Indeed, the clear size-graded distribution, slightly overlapping the underlying graded bed of spherules, strongly suggests a single and continuous sequence which started with the deposition of spherules and continued with quartz grains mixed with carbonate-rich sediments. In this hypothesis glass particles must be considered as impact-derived products. Although shocked quartz and glass spherules were produced at the same time, their different stratigraphic distribution indicates that a differential delay occurred between their ejection and their deposition. As particles larger than 100 pm cannot stay for long in the atmosphere
1341 the sole process efficient
at producing this delay, and consequently the size grading, is differential transit through the water column. At Beloc the sections formed in the deep sea (below = 2000 m [12,131), so the particles travelled through 2000 m of water before reaching the seafloor. The glass particles settled first because of their large size (0.05-0.3 mm), followed by the quartz grains (0.05-0.3 mm). According to [35] particles with a specific mass of = 2.5 g cm-’ and sizes of 5, 0.2 and 0.05 mm should settle out under 2000 m of water in 2 h, 1 day and 10 days respectively. Therefore, the deposition of spherules, shocked quartz and spinel-bearing particles must have been completed in less than 10 days, whatever the ejection and dispersion mechanism might have been. As Ni-rich spine1 crystals were originally locked in spheroidal objects of 50-300 pm in size [9,32] it is not surprising to find them mixed with shocked quartz of comparable dimensions and not with the remains of large glass spherules. So, the first 58 cm of the K/T sequence, at least, probably represents the record of a rapid single-shot event. The carbonate-rich Cretaceous sediments mixed with spherules and shocked quartz may stem from the resettling of older seafloor sediments stirred up and put into suspension by the seismic waves or turbiditic flow induced by the impact. The unusual thickness (60 cm) of the layer containing impact remains is not entirely due to the impact-derived fallout: it is partly due to the resedimentation of Cretaceous sediments. Without these redeposited sediments, shocked quartz and Ni-rich spinels would be confined to a layer of millimetres to centimetres in thickness on top of the spherule bed. Biostratigraphic data are consistent with this explanation: in the interval 20-50 cm, where foraminifera fossils are abundant, we have identified only small foraminifera. Large specimens ( > 500 km), which are common in the Cretaceous, are not present. This could be exp!ained by the size-graded deposition of mixed sediments, which would concentrate the largest fossils in the lower part of Unit 1 together with the spherules, the largest impact remains. Finer shocked quartz grains (Unit 2) are associated only with small and more abundant foraminifera.
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Such an explanation appears consistent for the lower part of the Beloc section. However, it does not account for the upper part of the sequence (Unit 3, 63-65 cm>. No shocked quartz grains are detected in the interval 58-62 cm (Beloc H). They occur again in the uppermost pulse-like distribution, in association with the maximum Ni-spine1 and Ir concentrations. This particular aspect of the stratigraphic distribution raises some questions: Does this bimodal distribution of shocked quartz and spinels result from a sedimentary artifact? Does it result from a sequential ejecta deposition representative of a single impact event? If not, does it mean that two distinct impact events occurred? A reworking process with horizontal transportation and redeposition of sediments from the lowest part of the sequence is unlikely, although it cannot be definitively excluded. To account for the sequence such a reworking process should have been extremely selective. First, it should have reworked only large shocked quartz (mean size = 150 pm, against 105 pm for the lower distribution) and spine1 crystals, leaving the spherule bed undisturbed. Indeed, the 63-65 cm layer contains essentially spine1 and quartz but no glass. Second, it would have preferentially reworked spine1 crystals, as indicated by the high spinel/shocked quartz ratio in the upper distribution (200 times higher than in the lower one). The chemical compositions of Ni-rich spinels is also inconsistent with a redepositional process. The Cr contents in the upper and lower distributions are significantly different. This point is most constraining as it implies sources with different compositions or/and different formation conditions. It is clear from Fig. 6 that the upper distribution has a distinct origin and cannot simply result from the simple redeposition of the lower population. All these restrictions render unlikely any interpretation based on reworking. A more consistent explanation is that the bimodal distribution results from at least two distinct collisional events. However, because of the very complicated sedimentation conditions which prevailed in the Beloc area, this mechanism, although unlikely, cannot be definitively rejected. The idea that two impacts occurred at the
265
KTB has already been proposed to account for the observations in North American sections 1191. At these continental sites the KTB clay is composed of two distinct layers. The lower claystone mostly containing pieces of what is supposed to be melted silicic target rocks would be the remains of the first impact. The upper layer containing Ni-rich spinels, Ir and shocked quartz would be the fallout layer of a smaller impact following closely behind. This bimodal distribution is instead now interpreted as a sequential ejecta deposition of a single impact 1341.The first distribution would correspond to material ejected along ballistic trajectories, while the upper one would stem from the cloud of vaporised bolide. What we observe in Haiti is quite different from the North American succession: the impact markers are present in the two distributions. In particular, the lower distribution contains a very high amount of shocked quartz (= 8000 grains/cm’) compared with the upper distribution ( = 400 grains/cm’). A second argument supporting a dual impact is the respective mean size of shocked quartz in the two distributions (105 and 150 km). Considering the settling velocity of quartz grains in water 1351, we can estimate that the upper quartz ‘event’ would have occurred at least 1 week after the first (lower) one. This delay is much longer than the expected time of flight in the atmosphere of particles 100-200 pm in diameter (Melosh [33l estimated a time of flight of < 1 day for such particles), whatever the ejection processes. In addition the two layers of shocked quartz are separated by 4 cm of sediment which might correspond to a long time interval, much longer than the expected time of flight of 100-150 pm particles. Therefore, a single origin for the two populations is unlikely. The observations are more easily explained by two distinct collisional events. Indeed, in the North American sections the total thickness of the fireball layer containing shocked quartz, spinels and Ir does not exceed a few millimetres, and makes impossible any observation of the fine stratigraphic structure of the K/T transition. In summary, the stratigraphy of the Beloc sections is not easily interpreted with a single ejecta deposit model, and a sedimentary artifact is un-
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likely. We must consider seriously the possibility that the upper part of the sedimentary sequence results from a distinct impact. The first impact would have occurred in the immediate vicinity of Beloc, in such a way that the associated seismic waves would have put in suspension the latest Cretaceous sediments already deposited on the seafloor, resulting in mixed deposits. The markers of this first event (Ir, Ni-rich spinels, spherules and shocked quartz) would have been dispersed in a = 60 cm thick sequence into which, because of their dilution, Ir and spinels would be barely detectable. The very high amount of shocked quartz ( = 8000 grains/cm21 and impact glass indicates a proximal site as the target material is less widely dispersed than the projectile material. The best candidate for the impact site is the Chicxulub structure (Yucatan peninsula). The upper layer, dominated by the projectile material, would correspond to a second and distal impact event. The scattering of shocked quartz sizes is very large (from 60 to 400 pm) and shows that this upper layer was not mixed much with seafloor sediment. This suggests a more distant event which did not disturb the local sedimentation. As a consequence, the products (shocked quartz, Ir and spinels) derived from this second impact are concentrated in a much thinner layer, with correspondingly lower dilution in carbonate sediments. It must be noted that the thickness (= 2 cm), amount, size distribution and calculated flux of shocked quartz in the Beloc upper level are very similar to those found in North American sections. A possible way to confirm the occurrence of multiple events would be to investigate the stratigraphic distribution of shocked zircons, which have been shown to bear information on their geographical origin [36]. This multiple collision alternative is not so unlikely as it looks. The encounter with the multiple fragments of a disrupted comet remains a possibility which has been recently documented by the fall of the debris of the Shoemaker-Levy comet on Jupiter 1381. Our conclusion that the glassy spherules of Unit 1 result from an impact raises a serious problem regarding the chemical properties of Belot glass. First, the absence of lechatelierite is
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strange if we consider the systematic presence of this amorphous phase in tektites. Another problem is the oxidation state of Fe in Beloc glasses: why are they so highly oxidized (Fe3+/Fe2+= 0.6-0.7, [9]) when compared with usual tektites (Fe3+/FeZtc< O.l)? The low oxidation state of Fe in tektites results from their formation in a rarefied atmosphere and at high temperature (2000-3000 K). Clearly, these two conditions were not fulfilled during the formation of the Beloc glasses. These compositional arguments, together with stratigraphic considerations, led three of us (E.R., L.F. and R.R.) to propose that the Beloc glasses resulted from a volcanic process [9]. The new data reported here on the distribution of shocked quartz enable us to envisage that the Beloc glasses are unusual tektites. It is worth reminding that, in that case, the formation conditions of theses glasses remain mysterious.
7. Conclusion Shocked quartz grains of the Cretaceous-Tertiary (K/T) sections at Beloc exhibit the characteristic signature of asteroid impacts. They show multiple sets of PDFs, mechanical basal twins and mosaicism similar to that found in quartz from well-known impact structures. The different microstructures described in this study reflect different post-shock thermal histories. The original PDFs (amorphous lamellae) recrystallized (a recrystallization probably greatly assisted by the presence of water) during the episodes which followed the shock event. However, PDFs can still be detected optically thanks to the numerous relic fluid inclusions. The detailed stratigraphic distribution of shocked quartz was compared to the other markers (impact glass, Ir and Ni-rich spinels) of the KTB at Beloc. The size grading and the concentration of shocked quartz through the stratigraphic column of the K/T section shows a continuity with glass spherules. The lower part of the 65 cm thick KTB sequence can be considered as the record of a quasi-instantaneous event and the simplest explanation is that all the ingredients found in the sequence, including the spherules,
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are the consequence of the KTB event. The enormous amount of impact remains in the Beloc sections places the first impact in the Caribbean area, and makes likely a link with the Chicxulub crater. The upper part of the K/T succession is complicated and suggests, although a purely sedimentological explanation cannot be excluded, the occurrence of a second and distant impact shortly after the first one.
Acknowledgements
We are indebted to four reviewers (G. Keller, A. Montanari, J. Smit and Anonymous) for their constructive remarks. This work was supported by the CNRS-INSU-DBT programme Changements de I’Emlironnement
Global dans le Passe’.
This is INSU-DBT contribution contribution 1676. [ FA]
734 and CFR
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D. Boclet. L. Froget, B. Lambert, E. Robin, R. Rocchia and L. Turpin, The Cretaceous-Tertiary boundary at Beloc, Haiti: No evidence for an impact in the Caribbean area, Earth Planet. Sci. Lett. 109, 229-241, 1992. Shock metamorphism of [lOI D. StGffler and F. Langenhorst. quartz in nature and experiment: 1 Basic observation, experiment, and theory, Meteoritics. 29, 155-181. 1994. [ill R.A.F. Grieve and L.J. Pesonen, The terrestrial impact cratering record, Tectonophysics 216. l-30. 1992. 1121 F.J.-M.R. Maurasse, Survey of the geology of Haiti: guide to the field excursions in Haiti, Miami Geol. Sot. 25. 1982. and J.J.G. Rigaud, [I31 F.J.-M.R. Maurasse. F. Pierre-Louis Upper Cretaceous to lower Paleocene pelagic calcareous deposits in the southern peninsula of Haiti: their bearing on the problem of the Cretaceous-Tertiary boundary, Trans. 4th Lat. Am. Geol. Congr., pp. 328-337. 1979. of [141 B.M. French and N.M. Short, Shock metamorphism natural materials, Mono Book, Baltimore, 1968. [I51 0. Goltrant. H. Leroux, J.C. Doukhan and P. Cordier. Formation mechanisms of planar deformation features in naturally shocked quartz, Phys. Earth Planet. Inter. 74, 219-240. 1992. [161 H. Leroux, W.U. Reimold and J.C. Doukhan. A TEM investigation of shock metamorphism in quartz from the Vredefort dome, South Africa. Tectonophysics 220. 22% 239. 1994.
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[l] L.W. Alvarez, W. Alvarez, F. Asaro and H.V. Michel, Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208, 1095-1108, 1980. [21 R. Ganapathy. A major meteorite impact on the Earth 65 million years ago: evidence from the Cretaceous-Tertiary boundary clay, Science 209, 921-923, 1980. [31 F.T. Kyte, Z. Zhou and J.T. Wasson, Siderophile-enriched sediments from the Cretaceous-Tertiary boundary, Nature 288, 651-656, 1980. [4] J. Smit and J. Hertogen, An extraterrestrial event at the Cretaceous-Tertiary boundary, Nature 285, 198-200, 1980. [5] K.J. Hsii, Terrestrial catastrophe caused by cometary impact at the end of Cretaceous, Nature 285, 201-203. 1980. [61 A. Montanari. R.L. Hay, W. Alvarez, F. Asaro, H.V. Michel, L.W. Alvarez and J. Smit, Spheroids at the Cretaceous-Tertiary boundary are altered impact droplets of basaltic composition. Geology 11, 668-671, 1983. 171 F.B. Bohor, E.E. Foord, P.J. Modreski and D.M. Triplehorn, Mineralogic evidence for an impact event at the Cretaceous-Tertiary boundary, Science 224, 867-869, 1984. 181 A.R. Hildebrand, G.T. Penfield, D.A. Kring, M. Pilkington. Z. Camargo, S.B. Jacobsen and W.V. Boynton. Chicxulub crater: a possible Cretaceous-Tertiary boundary impact crater on the Yucatan Peninsula, Mexico, Geology 19. 867-871. 1991.
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A TEM study of shocked quartz from the Manson impact structure, in: Manson Impact Structure. Geol. Sot. Am. Spec. Pap., in press, 1994. [181F.B. Bohor, P.J. Modreski and E.E. Foord. Shocked quartz in the Cretaceous-Tertiary boundary clays: evidence for a global distribution, Science 336. 705-709, 1987. boundary 1191 G.A. Izett, Tektites in Cretaceous-Tertiary rocks on Haiti and their bearing on the Alvarez impact extinction hypothesis, J. Geophys. Res. 96, 20879-20905, 1991. components. in: The Sea DO1 D.E. Brownlee, Extraterrestrial 7. C. Emiliani. ed., pp. 733-762, Wiley, New York. 1981. ml E. Robin, P. Bontt. L. Froget, C. JChanno and R. Rocchia. Formation of spinels in cosmic objects during atmospheric entry: a clue to the Cretaceous-Tertiary boundary event. Earth Planet. Sci. Lett. 108, 181-190, 1992. geochemDa E. Doehm and S.V. Margolis. Trace-element istry and mineralogy of the Cretaceous/Tertiap boundary: Identification of extraterrestrial components. Geol. Sot. Am. Spec. Pap. 247, 367-382. 1990. [231 C. JChanno, D. Boclet, Ph. BontC, A. Castellarin and R. Rocchia. Identification of two populations of extraterrestrial particles in a Jurassic hardground of the Southern Alps. Proc. Lunar Planet. Sci. Conf. 18. 62X-630. 1988. L-241S.V. Margolis, Ph. Claeys and F.T. Kyte. Microtektites, microkrystites and spinels from a late Pliocenr asteroid impact in the Southern Ocean. Sciencr 151. 1594-1597, 1991.
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[25] L. Gayraud. E. Robin, R. Rocchia and L. Froget. Preliminary results regarding the Formation conditions of meteoric spin&, in: New Developments Regarding the KT Event and Other Catastrophes in Earth History. pp. 41-42, Lunar Planet. Inst.. Houston, 1994 (LPI Contrib. 825). [26] S.E. Haggerty. Opaque mineral oxides in terrestrial ignous rocks. in: Oxide Minerals 3, D. Rumble. ed.. pp. 101-300. Mineral. Sot. Am.. 1976. [77] J. Smit and K.T. Kyte, Siderophile-rich magnetic spheroids from the Cretaceous-Tertiary boundary in Umbria, Italy, Nature 310, 403-405. 1984. [2X] K.T. Kyte and J. Smit. Regional variation in spine1 compositions: an important key to the Cretaceous/Tertiary event, Geology 14.485-487, 1986. [29] B.F. Bohor. E.E. Foord and R.. Ganapathy. Magnesioferrite from the Cretaceous-Tertiary boundary, Caravaca. Spain, Earth Planet. Sci. Lett. 81. 57-66. 1986. [30] E. Robin, D. Boclet, Ph. Bonte. L. Froget. C. Jehanno and R. Rocchia. The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122), Earth Planet. Sci. Lett. 107, 715-721, 1991. [31] R. Rocchia. D. Boclet. Ph. Bonte, L. Froget, B. Galhrun. C. Jehanno and E. Robin, lridium and other element
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distributions. mineralogy and magnetostratigraphy near the Cretaceous-Tertiary boundary in Hole 761C, ODP Sci. Rep. 122, 753-762, 1992. E. Robin. L. Froget, C. Jehanno and R. Rocchia, Evidence for a K/T impact event in the Pacific Ocean, Nature 363. 615-617, 1993. H.J. Melosh, Impact Cratering: A Geologic Process, Oxford University Press, 1989. R.M. Pollasto and B.F. Bohor, Origin and clay clay mineral genesis of the Cretaceous/Tertiary boundary unit, western interior of North America. Clays Clay Miner. 41. 7-25. 1993. J.B. Brun-Cottan, Vertical transport of particles within the ocean, in: The Role of Air-Sea Exchange in Geochemical Cycling, P. Buat-Minard, ed., pp. 83-111, Reidel, 1986. T.E. Krogh, S.L. Kamo and B.F. Bohor, Fingerprinting the K/T impact site and determining the time of impact by U-Ph dating of single shocked zircons from distal ejecta. Earth Planet Sci. Lett. 119, 425-429, 1993. C.S. Shoemaker, E.M. Shoemaker and D.H. Levy, IAU Circ. 5725. 1993. T.J. Ahrens. T. Takata, J.D. O’KeeFe and G.S. Orton, Impact of comet Shoemaker-Levy-9 on Jupiter, Geophy. Res. Lett. 21. 1087-1090. 1994.
Earth
and Planetary
Science
Letters
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The K/T boundary at Beloc (Haiti): Compared stratigraphic distributions of the boundary markers Hugues Leroux a, Robert Rocchia b, Laurence Froget b, Xavier Orue-Etxebarria Jean-Claude Doukhan a, Eric Robin b
‘,
a Laboratoire Structure et Prop&t& de I’Etat Solide, Uniuersite’ Sciences et Technologies de Lille. 59655 Villenewe d’Ascq Cedex, France ’ Centre des Faibles Radioactivit& CEA-CNRS, 91198 Gif-sur-Yllette Cedex, France ’ Unitlersidad de1 Pais Vasco, Facultad de Ciencias, Apartado 644* 48080 Bilbao, Spain Received
14 December
1994; accepted
after revision
22 February
1995
Abstract At Beloc, Haiti the Cretaceous-Tertiary boundary (KTB) is characterized by a spherule bed containing glass particles. These particles are considered by some authors as remains of tektites resulting from a nearby impact. However, because of the stratigraphic complexity of the Beloc sections the genetic link between the KTB cosmic event and the spherule bed is not obvious. In this paper, we report new data on shocked quartz and Ni-rich spinels at Haitian KTB sites. The detailed stratigraphy of these minerals shows that there is no empty gap in the sedimentary sequence. The first and largest shocked quartz is found in the upper part of the spherule layer. They are abundant and size graded over the 25-30 cm of carbonate-rich sediments overlying this layer. The first Ni-rich spinels, which are also rich in Cr, appear in the carbonate sediments. The size grading of the spherules and shocked quartz and the stratigraphical overlapping of their distributions suggests that these two components were derived from the same event. Although different from typical impact glasses (tektites), Beloc glass particles must be considered as impact-derived products. The enormous fluence of shocked quartz (= lo4 grains/cm’) is consistent with a proximal event. The upper part of the sequence is more complex. A second distribution of shocked quartz associated with Ni-rich spinels of different compositions appears in the layer containing the maximum Ir concentration. We propose that these features, which are not easily explained by a sedimentary artifact, result from a second collisional event.
1. Introduction The Cretaceous-Tertiary boundary (KTB) biological crisis is associated with geochemical and mineralogical anomalies which are attributed by a number of authors to the impact of a large asteroid or comet [l-7]. In Haiti, near the village of Beloc, the KTB is characterized by a 30 cm thick layer of spherules possibly representing the mate0012-821X/95/$09.50 0 1995 Elsevier SSDI 0012-821X(95)00032-1
Science
rial ejected by the alleged impact. The thickness of this presumed ejecta layer suggests that the impact occurred nearby in the Caribbean area. This hypothesis is supported by the existence of a large circular structure of nearly 200 km in diameter centred on the Yucatan peninsula-the Chicxulub crater [8]. A major objection against this scenario has been raised by detailed stratigraphic studies of Beloc K/T sites. In the sec-
B.V. All rights reserved
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tions where the K/T transition appears to have been properly recorded and preserved, the spherule layer is separated from the iridium-enriched layer (also containing Ni-rich spinels) by 2.5-35 cm of carbonate-rich Cretaceous sediments. JChanno et al. [9] proposed that this clear stratigraphic separation results from two distinct and unrelated events. However, it remains that the close association of two exceptional deposits is unlikely on statistical grounds. The question that we address in the present paper is the following: Can other impact markers yield significant information? To answer this question we have studied the stratigraphic distribution of shocked quartz, a K/T marker considered as an unequivocal impact-derived product [ 10,111. We have also reanalysed Beloc sections for their Nirich spine1 content using improved sensitivity.
2. Geological
setting
The Beloc sites are located in the southern part of the Haitian peninsula. A general description of the sites has been published by Maurasse et al. [12,131. We have sampled a dozen outcrops over 2.5 km along the road from Carrefour Dufort to Jacmel. In most sections the spherule bed and overlying layers appear to have been disturbed, as suggested by the presence of mud chips, discontinuities, or periodicities within the spherule bed mimicking multiple flows. At the type locality we have observed abundant mud clasts of Maastrichtian age within the spherule bed with at least four alternating spherule size gradings. The sedimentary structure, which is revealed by recent surficial erosion and colour changes, indicates in some places horizontal injection flow. At other places we have even observed stratigraphically distinct spherule beds (suggesting multiple events), which appeared in fact to be of limited horizontal extent. The sedimentary conditions at the Beloc sites were not perfectly quiet. These disturbing conditions were not limited in time to the K/T transition, as is demonstrated by the frequent slumping observed above the boundary. Deposition did not occur on a flat seafloor but probably on or at the base of a
slope. The results discussed below are derived from the detailed study of two adjacent sites (1.50 m apart), Beloc A and H [9]. Although showing crossbedding structures, Beloc A and H seem to have been less disturbed by reworking than other outcrops. The choice of these two sites is justified by the continuity of the sequence, the single grading of the section, and the discovery of a high Ir concentration spike which could not have been preserved if reworking had been active. For these reasons they were selected by JChanno et al. [91 for a precise stratigraphical study of Ir and Ni-rich spinels. Beloc H, the best preserved outcrop, is characterized by a 25 cm thick spherule layer (unit 1) that is size graded; at its base the contact with the underlying Cretaceous limestone is sharp. This contact is used as reference level for sample positioning. The spherule bed sensu strict0 is overlain by 25-35 cm of carbonate-rich sediments containing Cretaceous fossils (unit 2). There is a 1 cm thick grey-greenish clay layer (unit 3) 63-65 cm above the reference level; this clay layer is sometimes barely visible and contains rust-coloured lenses. The maximum Ir and spine1 concentrations have been found in this layer [9l. Above it we find calcareous deposits of the basal Danian. Although very similar to Beloc H, the Beloc A section is affected by slight horizontal variations which make the comparison of previously published data on Ir and spinels [9] with the shocked quartz distribution reported here difficult. Indeed, the samples used for the two studies ([9] and the present one) were collected at not exactly the same place during two different fieldtrips.
3. Shocked quartz as an impact signature 3.1. Significance of shocked quartz
The occurrence of shocked quartz is considered by many authors as one of the most important indicators of shock metamorphism (i.e., of an impact origin) [lO,ll]. Shocked quartz grains found in impact craters display characteristic planar deformation features (PDFs) [10,14-171 which reveal that the rock experienced an intense and
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short peak pressure (P> 10 GPa, with a high strain rate in the range 103-lo6 s-l). PDFs appear in optical microscopy as pervasive, straight, and very narrow contrasts parallel to the basal plane and rhombohedral planes {10&z]. Their fine structure cannot be resolved by optical miTransmission electron microscopy croscopy. (TEM) is a better adapted technique which provides higher magnification together with a num-
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ber of other characterization capabilities (diffraction, contrast analysis and X-ray microanalysis). Bohor et al. [7] were the first to report the occurrence of shocked quartz grains, in the continental K/T deposits of Brownie Butte, Montana. This provided major support for the impact hypothesis of Alvarez et al. [I]. This discovery was soon extended to other KTB sites around the world and in 1987 Bohor et al. [18] noted that the
Fig. 1. (a) Optical micrograph (crossed nicols) of a shocked quartz grain of 120 Frn in diameter at level 35-40 cm in the Beloc A section. Two sets of PDFs are clearly visible. fb) TEM bright field micrograph showing two sets of PDFs. These are composed of a high density of small defects, probably tiny dislocation loops or small fluid inclusions. Cc) TEM micrograph, dark field: Thin mechanical twins (with fringe contrast) parallel to the basal plane. Cd) Mosaicism as seen in TEM. This consists of small crystalline domains disoriented relative to each other. The different contrasts reflect their different crystallographic orientations (bright field). (e) Spindle-shaped bands parallel to the (lOi0) planes (bright field). (f) Some of spindle-shaped bands are filled with phyllosilicates (bright field).
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largest shocked quartz grains are found in North American sections, suggesting a nearby impact. Shocked quartz grains were also found in abundance in Haiti, near Beloc [19]. This abundance, together with the presence of glass-bearing spherules, was immediately taken as having resulted from a very proximal impact. However, no detailed stratigraphic distribution of the shocked quartz has been studied in the Haitan sections. The purpose of the present study is to determine this stratigraphic distribution and to compare it with the distribution of other KTB markers. 3.2. Analytical procedure Shocked quartz was extracted from the sediments in the following way: The material was first disaggregated and dispersed in water by ultrasonic treatment. Small clay particles stayed in suspension in the aqueous solution. The carbonate fraction was dissolved in HCl. The clay-free and carbonate-free residue was dried, and then deposited on a thin glass plate with a drop of benzil alcohol. As the refraction index of benzil alcohol is very close to that of quartz, the quartz grains are easily identifiable under the optical microscope. The shocked grains are identified by the optical contrasts produced by PDFs (Fig. la>. Several quartz grains with well-visible PDFs were selected for detailed TEM characterization. After extraction, they were embedded in epoxy resin and a standard petrographic thin section (30 pm thick) was then prepared by the usual grinding and mechanical polishing (on both faces). Final thinning down to electron transparency ( 5 1 pm) of these samples was achieved by the usual ion-thinning technique with Ar ions accelerated by a high voltage of = 5 kV, with a beam angle of 1.5”. Finally, the thinned samples were ctated with a very thin layer of carbon (= 300 A) to prevent the accumulation of electric charges in the TEM. Investigations were carried out with a Philips CM30 electron microscope operating at 300 kV. As explained below, these TEM investigations have shown that all the quartz grains selected for their optical contrast in benzil alcohol do contain PDFs. This supports the stratigraphic data presented below. Shocked quartz
grains from various levels in the section were also carefully counted and their size measured for statistical analysis (Section 3.4). 3.3. TEM investigations The shock defects consist of multiple sets of straight and narrow bands often extending through the whole grain (Fig. lb). Their thickness varies from 0.1 to 0.5 pm and they are separated from each other by a few micrometres. These bands lies in rhombohedral planes, with {lOi2} and (lOi3) as the most common orientations. Several different microstructures can be distinguished. A number of PDFs contain a high density of small spheroidal defects, exhibiting miniscule contrast (< 10 nm) which cannot be fully resolved. These might consist of tiny dislocation loops and/or bubbles containing fluid or amorphous silica. The TEM observations show a high density of PDFs. All the grains selected by simple optical observation contain shock defects but the density of these defects is much higher than in the optical range. For instance, the mean distance between PDFs of a given set varies from = 0.5 to 2 pm, depending on the observed area. We suppose that the observed optical contrasts correspond to interference phenomena rather than to individual PDFs. This means that all strongly shocked quartz grains are correctly identified by optical means. A simple optical survey, however, is not sufficient to reveal weakly shocked grains with widely spaced PDFs. This may affect the statistical analysis presented in the next section. PDFs in the basal plane are rather rare but they are identical to those already identified in shocked quartz in other impact structures [15-171. They are very narrow (I 100 nm> mechanical twins with partial dislocations in the twin-matrix boundaries (Fig. 1~). The twins are generated by the rapid nucleation and the cooperative motion in adjacent basal planes of partial dislocations. The investigations using TEM always failed to find such twins related to purely endogenous phenomena. Mosaicism (i.e., slight misorientation of small crystalline domains) is pervasive, and seems to
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result from the intersection of two or more sets of PDFs bounding small square or rectangular shaped domains = 1 km size (Fig. Id). Selected diffraction patterns on areas containing a large number of such domains clearly show slight misorientations (asterism of the diffraction spots). Our TEM investigations reveal another type of defect which was not described earlier. These are spindle shaped, with their long axis roughly parallel to the {lOiO) planes (Fig. le) extending over distances of a few micrometres. Their thickness does not exceed 0.5 pm, and they often start and/or stop on rhombohedral PDFs. Qualitative X-ray microanalyses performed on the material
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inside the spindles show, in addition to silica, relatively large amounts of Ca and Al and lower amounts of K, Fe, and Na oxides. The spindle lamellae might be poorly crystallized (fibrous) phyllosilicates (Fig. If) and therefore they do not represent original PDFs. They could result from open fractures filled with silicate and later altered to phyllosilicates by circulating water-rich fluids. Despite their high degree of alteration, quartz grains from the Beloc K/T sediments exhibit the characteristic signature of meteorite impacts. The shock-induced defects are similar to those already detected in quartz grains from well-charac-
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(a)
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and shocked
quartz (mean size) (cl
cm (above the spherule bed) and +65 cm. Outside this range, few quartz grains are still present, and are without evident shock features. A total of = 2000 quartz grains have been observed and = 1000 shocked grains were used for the microstratigraphic analysis. Together they yield representative trends as shown in Fig. 3. In Beloc H, the most carefully sampled section, the first shocked quartz grains are detected at level +25 cm mixed up with smaller spherules from the underlying bed (Fig. 3). They are not very abundant ( = 30 grains/g sediment) but they are rather large (maximum size = 700 pm, mean
3.4. Microstratigraphy of shocked quartz grains We have determined the number of shocked quartz grains per unit mass of sediment and their sizes in samples from the Beloc A and H sections. A size histogram was evaluated for each level (Fig. 2). This investigation was carried out from level -100 cm to level + 180 cm but shocked quartz grains were detected only between +25 (a)
I
140
(pm)
size (maximum
terised impact structures [15-171 and remain clearly distinct from the microstructures resulting from usual low-strain-rate tectonic deformations.
.
Shocked quartz size
(mm) Fig. 3. Beloc H: Stratigraphic
I20
Cd)
(b)
Tertiary Unit 3 Cretaceous fossils (Unit 2)
SpkT’e (Unit 1) 0 Cretaceous
Fig. 4. Beloc H: Stratigraphic spinels Cd).
IW
200
300
400
500
Iridium
(grlg)
@g/g)
log (a), abundance
of shocked
quartz
.Ol
1
600
Shocked quartz
(grains
per gramme
,I
1
IO
Ni-rich spinels (numberlmg) of sediment)
(b). iridium
(c), and Ni-rich
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size at this level = 160 pm). Although there is a marked scattering of the grain sizes at all levels, a very clear trend emerges: the mean size roughly decreases linearly from 160 pm, in the lowermost (+25 cm) level containing shocked grains, down to 80 pm at level +58 cm. The concentration of shocked quartz (in number of grains per gramme of sediment) first increases from = 30 grains/g at level +25 cm to = 500 grains/g at level +45 cm, and then it falls rapidly in the samples located between 50 and 58 cm (Fig. 4). No shocked quartz grains are observed between 59 and 62 cm, and then they appear again in level 63-65 cm with a relatively large mean size of 140 pm. This looks like a bimodal distribution. The first distribution starts at the top of the spherule bed and extends up to level +58 cm with a monotonous grading. The mean size of the whole distribution (integrated between levels +25 and +58 cm) is 105 pm. The fluence is = 7500 shocked grains/cm2 for this first distribution. The second distribution is a pulse-like function centred on level 64 cm where JChanno et al. [9] observed the maximum concentrations of Ir and Ni-rich spinels. The fluence of this second distribution is only 400 grains/cm2. We must note that for both distributions optically selected shocked grains represent at least 50% of the total number of quartz grains contained in the investigated samples. In contrast, in levels below 25 cm and above 65 cm quartz grains are scarce-a few grains per gramme-and unshocked. This marked difference suggests that the numerous quartz grains found between levels +25 and +58 cm and considered as unshocked on the basis of optical investigations are also derived from the impact. They might be slightly shocked grains with PDFs hardly identifiable by optical means, As they were not taken into account in Figs. 3 and 4 the fluences given above (7500 and 400 grains/cm2) should be considered as Lower limits. Data from the Beloc A section are quite similar. A smooth size grading is observed in the mean size in the range 23-58 cm (mean size from 80 to 170 pm). A distinct population of large shocked quartz grains is superimposed at level 63-65 cm (mean size = 150 pm). The total flux of shocked grains at the two localities is very similar.
We find 8000 and 600 grains/cm2 for the lower and upper distributions respectively. If we except the fact that, in the Beloc A section, the layer of sediments corresponding to level 58-62 cm of section H was not sampled, the Beloc A and H sections look like twin outcrops-they essentially bear the same information. In order to compare these results with those found at distant localities, we have estimated the size-frequency distribution and the amount of 16
’
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Frenchman
’
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!
Valley
With 134 shocked
grains
Mean size = 145 pm
0 60
20
3 6
’
100
140
’ ’ ’ ’
180
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260
Grain
size
’ ’ ’
’ ’ ’
Raton
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(pm)
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Basin
’ ’ ’ ’ ’
16 With 127 shocked grains Mean size = 130 pm
60
100
140
160
Grain
220
size
260
300
340
360
(Km)
Fig. 5. Frequency diagrams showing size distribution of shocked quartz grains from the KTB in the Raton Basin and Frenchman Valley. Amount (grains per gramme), mean size and standard deviation (S.D.) is indicated on each diagram.
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shocked quartz per gramme of sediment in two North American K/T sections (Raton Basin, Coland Frenchman Valley, South orado Saskatchewan). The calculated mean sizes are 140 pm for Raton Basin and 145 Frn for Frenchman Valley (Fig. 5). We have estimated their abundance at 100 grains/g at Raton Basin and 50 grains/g in the Frenchman Valley section. Assuming that the layer containing shocked quartz is 2 cm thick at both localities, one finds fluences of 400 and 200 shocked quartz/cm2 respectively, 20 and 40 times less than in Haiti.
4. Ni-rich spinels as impact signatures The bimodal distribution of shocked quartz led us to reinvestigate the distribution of Ni-rich spinels. Jthanno et al. 191have already published data derived from the study of small quantities of sediments. We report here new data obtained with improved stratigraphical resolution and sensitivity. 4.1. Significance of Ni-rich spinels Ni-rich spinels are magnetic crystals produced in extraterrestrial objects during interaction at high temperature with Earth’s atmosphere. They are found in cosmic spherules [20,21], in meteorite fusion crust [21,22], and in various types of cosmic debris, such as meteoroid ablation droplets discovered in a Jurassic hardground [23] and impact glassy spherules in late Pliocene sediments [24]. They differ from terrestrial spinels by their high Ni content (> 1 wt%), which indicates an undifferentiated source material (meteoritic or mantle material), and by their high iron oxidation state (Fe”+/Fe,,, > 70 at%) resulting from formation under relatively high oxygen fugacities (> 10v4 atm) 121,251. Terrestrial magmas, which are poor in Ni, do not evolve under such high oxygen pressures [26]. This explains why highly oxidized Ni-rich spinels have no counterpart in terrestrial rocks. Therefore, in collisional events they can be considered as specific markers of the projectile in the same way as shocked quartz are specific markers of the target.
The presence of such minerals in most KTB rocks [6,9,27-311 is therefore a strong argument supporting a major collisional event triggering the K/T extinctions. In most K/T sections Ni-rich spinels are found isolated in the sediment, but it is worth noting that originally they were trapped in rather big bodies up to several hundred micrometres in size [9,30] that are now completely weathered out. However, at least at one K/T site located in the Pacific Ocean [32] these spinelbearing particles have preserved their original morphology and properties of meteoroid ablation droplets. 4.2. Separation and analysis of Ni-rich spinels Because Ni-rich spinels are strongly magnetic minerals they can be easily extracted from a liquid phase with an electromagnet [30]. Magnetic separation was carried out on quantities varying from 100 mg for levels most enriched in spinels (+62 cm) to 10 g for the others. Such large quantities allowed us to reach a sensitivity lo-100 times better than that obtained by JChanno et al. [91. The detection limit depends on the abundance of other magnetic grains but it is generally lower than 0.01 spinel/mg. 4.3. Distributions spinels
and
compositions
of Ni-rich
The Ni-rich spine1 distribution, like the shocked quartz distribution, is bimodal. The first Ni-rich spinels appear in level +30 cm, approximately at the same level as the first shocked quartz ( + 28 cm). The first maximum of spinels (0.4/mg, Fig. 4) is closely associated with the first maximum of shocked quartz. The fluence of this lower distribution is about lo4 spine1 crystals/cm2, which is comparable with the fluence observed at El Kef, Tunisia [30]. The spine1 abundance shows a deep minimum (O.O06/mg) at level +50 cm. The second maximum (35/mg, Fig. 4) occurs at level +62 cm in coincidence with the maximum of Ir and the second maximum of shocked quartz. The fluence of this upper distribution is lo5 spine1 crystals/cm’, comparable with the fluence observed at Caravaca, Spain [30]. The two populations of spinels differ in
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5. Micropaleontological analysis
composition. The number of analysable spinels (i.e., larger than 2-3 pm) is not sufficient to permit an accurate estimate of the composition in individual samples of the lower levels. Therefore, in order to obtain adequate statistics we have grouped all data of the lower distribution (five samples between levels +28 and 48 cm). Fig. 6 shows the histograms of the Cr concentrations in the lower and upper levels (levels between +53 and 63 cm). The lower population of spinels is, on the average, significantly richer in Cr (= 8%) than the upper one (= 4%). Owing to the low abundance of spinels in the intermediate levels ( + 45 to 55 cm> we do not know if this difference results from a continuous compositional variation or from a sharp stratigraphic discontinuity indicating two distinct compositions. In the same interval (28-63 cm> the iridium distribution (Fig. 4) exhibits a monotonous increase, except at level = 45 cm where we observe a small maximum, in coincidence with the first maximum of Ni-rich spinels. The most enriched layer corresponds to level 63 cm.
I
o-2.5
Fig. 6. Histogram
of Cr content
I
2.5-5
I
s-1.5
5.1. h4ethods Samples from the Beloc A and H sections were also studied in order to obtain micropaleontological data. The lowest samples of the stratigraphic succession were collected at the top of the spherule bed (Unit 1, see Fig. 3) and the last samples were from 70 cm (above Unit 3). Two different fractions have been separated through the processing of the samples. The first fraction contains the residue between 40 and 100 Frn in size, whereas the second fraction comprises everything above 100 pm. In addition, thin sections have also been prepared and analysed. 5.2. Results All the samples are poor in planktonic foraminifera, even though the specimens are quite well preserved. A total of 30 species have been identified from the uppermost Maastrichtian, in
I
7.5-10
263
B
28-50cm
-
53 - 63cm
I
I
10-12.5 12.5-15 15-17.5
in the lower (28-50 cm) and upper (53-63
I
I
17.5-20 20-22.5
cm) populations
of the Ni-rich
spinels.
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contrast to the 55-60 species found in other sections such as El Kef, Tunisia and those on either side of the Pyrenees in the Basque Country. Only latest Maastrichtian planktonic foraminifera occur at the top of Unit 1 and Unit 2. Most individuals identified in the Beloc samples belong to the family Heterohelicidae and the most abundant species are H. globuloso, P. costulata, G. minuta and G. yaucoensis. No large species (> 500 pm1 characteristic of the uppermost Cretaceous have been found in the samples from the upper part of Unit 1 or from Unit 2. The sample collected immediately above the Ir-enriched layer corresponds to the beginning of the Tertiary (Zone PO) or the G. cretacea Biozone. All the species found in this sample are typical of the uppermost Maastrichtian and almost all of them are now considered by various authors as survivors of the KTB extinction. In the uppermost sample ( + 70 cm>, in addition to rare Maastrichtian species we also principally find species typical of the base of the Tertiary (such as Ch. midwayensis, W. claytonensis, G. conusa and P. eugibina). Based on the presence of this assemblage, this sample should be included in Biozone Pl or the P. eugibina Biozone. 6. Discussion:
Record of impact remains
at Beloc
Let us consider first the lower part of the sedimentary sequence (Units I and 2). An important result of the present stratigraphic study is that the spherules and the first population of shocked quartz are derived from the same event. Indeed, the clear size-graded distribution, slightly overlapping the underlying graded bed of spherules, strongly suggests a single and continuous sequence which started with the deposition of spherules and continued with quartz grains mixed with carbonate-rich sediments. In this hypothesis glass particles must be considered as impact-derived products. Although shocked quartz and glass spherules were produced at the same time, their different stratigraphic distribution indicates that a differential delay occurred between their ejection and their deposition. As particles larger than 100 pm cannot stay for long in the atmosphere
1341 the sole process efficient
at producing this delay, and consequently the size grading, is differential transit through the water column. At Beloc the sections formed in the deep sea (below = 2000 m [12,131), so the particles travelled through 2000 m of water before reaching the seafloor. The glass particles settled first because of their large size (0.05-0.3 mm), followed by the quartz grains (0.05-0.3 mm). According to [35] particles with a specific mass of = 2.5 g cm-’ and sizes of 5, 0.2 and 0.05 mm should settle out under 2000 m of water in 2 h, 1 day and 10 days respectively. Therefore, the deposition of spherules, shocked quartz and spinel-bearing particles must have been completed in less than 10 days, whatever the ejection and dispersion mechanism might have been. As Ni-rich spine1 crystals were originally locked in spheroidal objects of 50-300 pm in size [9,32] it is not surprising to find them mixed with shocked quartz of comparable dimensions and not with the remains of large glass spherules. So, the first 58 cm of the K/T sequence, at least, probably represents the record of a rapid single-shot event. The carbonate-rich Cretaceous sediments mixed with spherules and shocked quartz may stem from the resettling of older seafloor sediments stirred up and put into suspension by the seismic waves or turbiditic flow induced by the impact. The unusual thickness (60 cm) of the layer containing impact remains is not entirely due to the impact-derived fallout: it is partly due to the resedimentation of Cretaceous sediments. Without these redeposited sediments, shocked quartz and Ni-rich spinels would be confined to a layer of millimetres to centimetres in thickness on top of the spherule bed. Biostratigraphic data are consistent with this explanation: in the interval 20-50 cm, where foraminifera fossils are abundant, we have identified only small foraminifera. Large specimens ( > 500 km), which are common in the Cretaceous, are not present. This could be exp!ained by the size-graded deposition of mixed sediments, which would concentrate the largest fossils in the lower part of Unit 1 together with the spherules, the largest impact remains. Finer shocked quartz grains (Unit 2) are associated only with small and more abundant foraminifera.
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Such an explanation appears consistent for the lower part of the Beloc section. However, it does not account for the upper part of the sequence (Unit 3, 63-65 cm>. No shocked quartz grains are detected in the interval 58-62 cm (Beloc H). They occur again in the uppermost pulse-like distribution, in association with the maximum Ni-spine1 and Ir concentrations. This particular aspect of the stratigraphic distribution raises some questions: Does this bimodal distribution of shocked quartz and spinels result from a sedimentary artifact? Does it result from a sequential ejecta deposition representative of a single impact event? If not, does it mean that two distinct impact events occurred? A reworking process with horizontal transportation and redeposition of sediments from the lowest part of the sequence is unlikely, although it cannot be definitively excluded. To account for the sequence such a reworking process should have been extremely selective. First, it should have reworked only large shocked quartz (mean size = 150 pm, against 105 pm for the lower distribution) and spine1 crystals, leaving the spherule bed undisturbed. Indeed, the 63-65 cm layer contains essentially spine1 and quartz but no glass. Second, it would have preferentially reworked spine1 crystals, as indicated by the high spinel/shocked quartz ratio in the upper distribution (200 times higher than in the lower one). The chemical compositions of Ni-rich spinels is also inconsistent with a redepositional process. The Cr contents in the upper and lower distributions are significantly different. This point is most constraining as it implies sources with different compositions or/and different formation conditions. It is clear from Fig. 6 that the upper distribution has a distinct origin and cannot simply result from the simple redeposition of the lower population. All these restrictions render unlikely any interpretation based on reworking. A more consistent explanation is that the bimodal distribution results from at least two distinct collisional events. However, because of the very complicated sedimentation conditions which prevailed in the Beloc area, this mechanism, although unlikely, cannot be definitively rejected. The idea that two impacts occurred at the
265
KTB has already been proposed to account for the observations in North American sections 1191. At these continental sites the KTB clay is composed of two distinct layers. The lower claystone mostly containing pieces of what is supposed to be melted silicic target rocks would be the remains of the first impact. The upper layer containing Ni-rich spinels, Ir and shocked quartz would be the fallout layer of a smaller impact following closely behind. This bimodal distribution is instead now interpreted as a sequential ejecta deposition of a single impact 1341.The first distribution would correspond to material ejected along ballistic trajectories, while the upper one would stem from the cloud of vaporised bolide. What we observe in Haiti is quite different from the North American succession: the impact markers are present in the two distributions. In particular, the lower distribution contains a very high amount of shocked quartz (= 8000 grains/cm’) compared with the upper distribution ( = 400 grains/cm’). A second argument supporting a dual impact is the respective mean size of shocked quartz in the two distributions (105 and 150 km). Considering the settling velocity of quartz grains in water 1351, we can estimate that the upper quartz ‘event’ would have occurred at least 1 week after the first (lower) one. This delay is much longer than the expected time of flight in the atmosphere of particles 100-200 pm in diameter (Melosh [33l estimated a time of flight of < 1 day for such particles), whatever the ejection processes. In addition the two layers of shocked quartz are separated by 4 cm of sediment which might correspond to a long time interval, much longer than the expected time of flight of 100-150 pm particles. Therefore, a single origin for the two populations is unlikely. The observations are more easily explained by two distinct collisional events. Indeed, in the North American sections the total thickness of the fireball layer containing shocked quartz, spinels and Ir does not exceed a few millimetres, and makes impossible any observation of the fine stratigraphic structure of the K/T transition. In summary, the stratigraphy of the Beloc sections is not easily interpreted with a single ejecta deposit model, and a sedimentary artifact is un-
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H. Leroux rr al. / Enrth and Planefury S&we
likely. We must consider seriously the possibility that the upper part of the sedimentary sequence results from a distinct impact. The first impact would have occurred in the immediate vicinity of Beloc, in such a way that the associated seismic waves would have put in suspension the latest Cretaceous sediments already deposited on the seafloor, resulting in mixed deposits. The markers of this first event (Ir, Ni-rich spinels, spherules and shocked quartz) would have been dispersed in a = 60 cm thick sequence into which, because of their dilution, Ir and spinels would be barely detectable. The very high amount of shocked quartz ( = 8000 grains/cm21 and impact glass indicates a proximal site as the target material is less widely dispersed than the projectile material. The best candidate for the impact site is the Chicxulub structure (Yucatan peninsula). The upper layer, dominated by the projectile material, would correspond to a second and distal impact event. The scattering of shocked quartz sizes is very large (from 60 to 400 pm) and shows that this upper layer was not mixed much with seafloor sediment. This suggests a more distant event which did not disturb the local sedimentation. As a consequence, the products (shocked quartz, Ir and spinels) derived from this second impact are concentrated in a much thinner layer, with correspondingly lower dilution in carbonate sediments. It must be noted that the thickness (= 2 cm), amount, size distribution and calculated flux of shocked quartz in the Beloc upper level are very similar to those found in North American sections. A possible way to confirm the occurrence of multiple events would be to investigate the stratigraphic distribution of shocked zircons, which have been shown to bear information on their geographical origin [36]. This multiple collision alternative is not so unlikely as it looks. The encounter with the multiple fragments of a disrupted comet remains a possibility which has been recently documented by the fall of the debris of the Shoemaker-Levy comet on Jupiter 1381. Our conclusion that the glassy spherules of Unit 1 result from an impact raises a serious problem regarding the chemical properties of Belot glass. First, the absence of lechatelierite is
Lrttrrs
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strange if we consider the systematic presence of this amorphous phase in tektites. Another problem is the oxidation state of Fe in Beloc glasses: why are they so highly oxidized (Fe3+/Fe2+= 0.6-0.7, [9]) when compared with usual tektites (Fe3+/FeZtc< O.l)? The low oxidation state of Fe in tektites results from their formation in a rarefied atmosphere and at high temperature (2000-3000 K). Clearly, these two conditions were not fulfilled during the formation of the Beloc glasses. These compositional arguments, together with stratigraphic considerations, led three of us (E.R., L.F. and R.R.) to propose that the Beloc glasses resulted from a volcanic process [9]. The new data reported here on the distribution of shocked quartz enable us to envisage that the Beloc glasses are unusual tektites. It is worth reminding that, in that case, the formation conditions of theses glasses remain mysterious.
7. Conclusion Shocked quartz grains of the Cretaceous-Tertiary (K/T) sections at Beloc exhibit the characteristic signature of asteroid impacts. They show multiple sets of PDFs, mechanical basal twins and mosaicism similar to that found in quartz from well-known impact structures. The different microstructures described in this study reflect different post-shock thermal histories. The original PDFs (amorphous lamellae) recrystallized (a recrystallization probably greatly assisted by the presence of water) during the episodes which followed the shock event. However, PDFs can still be detected optically thanks to the numerous relic fluid inclusions. The detailed stratigraphic distribution of shocked quartz was compared to the other markers (impact glass, Ir and Ni-rich spinels) of the KTB at Beloc. The size grading and the concentration of shocked quartz through the stratigraphic column of the K/T section shows a continuity with glass spherules. The lower part of the 65 cm thick KTB sequence can be considered as the record of a quasi-instantaneous event and the simplest explanation is that all the ingredients found in the sequence, including the spherules,
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are the consequence of the KTB event. The enormous amount of impact remains in the Beloc sections places the first impact in the Caribbean area, and makes likely a link with the Chicxulub crater. The upper part of the K/T succession is complicated and suggests, although a purely sedimentological explanation cannot be excluded, the occurrence of a second and distant impact shortly after the first one.
Acknowledgements
We are indebted to four reviewers (G. Keller, A. Montanari, J. Smit and Anonymous) for their constructive remarks. This work was supported by the CNRS-INSU-DBT programme Changements de I’Emlironnement
Global dans le Passe’.
This is INSU-DBT contribution contribution 1676. [ FA]
734 and CFR
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[33] [34]
[Xi]
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