Element profiles and Ir concentration of Cretaceous-Tertiary (K-T) boundary layers at Medetli, Gölpazari, northwestern Turkey

Geochemical Journal, Vol. 37, pp. 681 to 693, 2003 NOTE Element profiles and Ir concentration of Cretaceous-Tertiary (K-T) boundary layers at Medetli, Gölpazari, northwestern Turkey YOJI ARAKAWA, 1 * XIAOLIN LI, 2 MITSURU EBIHARA, 2 ENGIN MERIÇ, 3 IZAVEL TANSEL, 3 SIMAU BARGU, 3 HAYRETTIN KORAL 3 and KUNITERU MATSUMARU 4 1 Geosphere Research Institute, Saitama University, Saitama 338-8570, Japan 2 Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan 3 Department of Geology, Faculty of Engineering, Istanbul University, 34850 Avcilar-Istanbul, Turkey 4 Department of Geology, Faculty of Education, Saitama University, Saitama 338-8570, Japan (Received January 14, 2003; Accepted June 16, 2003) Some element concentrations were measured and element profiles were made for the Cretaceous-Tertiary (K-T) boundary layers in Medetli, Gölpazari, northwestern Turkey. In Medetli region, gray-colored medium to coarse grained sandstone (layer A) is overlain by yellow-colored fine-grained sandstone with intercalated thin goethite-rich layers (layer B). White colored limestone (layer C) overlies on the layer B. The layer A is latest Cretaceous and layer C is early Paleocene, and in layer B (28 37 cm in thickness) fossils are absent, and is regarded as a K-T transitional layer. In several goethite-rich layers, siderophile elements such as Fe, Cr, Ni, Co, and chalcophile element, such as As, Sb, and Zn, are enriched, and have maximum in succession from layer A to C. The enrichment of these elements are common features in most of K-T boundary sediments of the world. However, Ir concentration is relatively low (0.05 0.10 ppb) in the goethite-rich layers. The analyzed Ir concentration is slightly elevated (0.24 ppb) only in the top part of layer A. Iridium may have been diluted during sedimentation and diagenesis. The actual K-T boundary may be situated between the top of layer A and the bottom of layer B. The section from the top part of layer A to layer B is assumed to have been formed during the K-T and its successive events. INTRODUCTION After finding of anomalously high concentrations of the platinum group elements (PGEs) in Cretaceous-Tertiary (K-T) boundary sediments worldwide (Alvarez et al., 1980; Ganapathy, 1980; Kyte et al., 1980; Smit and Hertogen, 1980), asteroid impact has been used to explain the mass extinction at that time. As well as high PGEs concentration, the discovery of abundant spheroidal debris (Smit and Klaver, 1981; Montanari et al., 1983) and shocked minerals (Borhor et al., 1984) in those sediments demonstrated that the asteroid impact is truly a global phenominon. Hildebrand et al. (1991) proposed a model that the Chicxulub crater in Yucatan Peninsula, Mexico is a K-T impact site, and this model was supported by the petrographical and geochemical studies (Blum et al., 1993; Koeberl, 1993) and radiometric age determination of melted rocks (Swisher et al., 1992; Sharpton et al., 1992). More recently the stratigraphy, composition and distribution of K-T impact ejecta have been discussed and summarized (e.g., Kyte et al., 1996; Smit, 1999; Claeys et al., 2002). Even now the studies on the K-T boundary sediments are in still continuing interests and are tried from various points of view. In Turkey upper Cretaceous and Palocene *Corresponding author (e-mail: yaraka@post.saitama-u.ac.jp) 681

682 Y. Arakawa et al. Fig. 1. Sample location of Medetli K-T boundary in Turkey. Inset map shows the locality of the outcrop. Paleocene limestone crops out forming a crest. sediments are widely distributed and K-T transitional formations are found in several sites. In 1995 and 1996 we have carried out geological field research on K-T transitional formations of several locations along Black Sea region (Matsumaru et al., 1996; Matsumaru et al., 1997). Especially in Medetli, Gölpazari (about 120 km southeast from Istanbul) (Fig. 1), a clear section crops out. In this report, we present the results of chemical analyses of boundary layers, and provide element profiles across the boundary. Iridium and other platinum group element concentrations of some samples were also tried to determine. We compare the results with those of the other K-T boundary sediments of the world. Fig. 2. Schematic cross section of the K-T boundary in Medetli. Layer A is gray-colored medium to coarsegrained sandstone, layer B is yellow-colored finegrained sandstone, and layer C is white-colored limestone. One to several goethite-rich layers (black bar) are intercalated in layer B. Layer A is Maastrichtian and layer C is early Paleocene, and in layer B fossils are absent. STRATIGRAPHY In the Medetli region, upper Cretaceous (Maastrichtian) sandstone of Tarakli Formation is overlaid by lower Paleocene limestone of Selvipinari Formation (Dizer and Meriç, 1983). A schematic cross section is shown in Fig. 2, and an outcrop photo is added in Fig. 3. Layers A and B correspond to the Tarakli Formation and layer C to the Selvipinari Formation. The layer A is Exogyra-bearing sandstone. Layer B does not contain any fossils. In some sites, submarine cave or small channel in which limestone deposed convex downward is seen (Fig. 2). Layer A is composed of coarse to medium grained gray colored sandstone, and Exogyra is found in coarse-grained sandstone layer. Layer B consists of fine-grained

Element profiles of Cretaceous-Tertiary boundary at Medetli, Turkey 683 PETROGRAPHY AND MINERALOGY Fig. 3. A photograph of the outcrop. Labels A, B and C correspond to layer A, layer B and layer C, respectively. yellow colored sandstone and intercalated thin goethite-rich layers. Geothite-rich layers are generally thin (3 mm to 8 mm) and form layers or thin lenses within the fine-grained sandstone. One to several layers of them are parallely intercalated (Fig. 2). Layer C (Selvipinari Formation) is a white-colored limestone which contains Laffittenina bibensis Marie and Mississippina sp. This limestone is more than 20 m thick. The lowest parts of the limestone are all silicified. From the available biostratigraphical data, there may be a small(or major) hiatus at the boundary between layer A and layer B and/or between layer B and layer C. These stratigraphical and sedimentological features are different from those of well-known K-T sites (e.g., Gubbio in Italy, Stevns Klint in Denmark and Woodside Creek in New Zealand), in which only thin clay layer exists as K-T boundary. Because layer B is devoid of fossils, we could not clearly determine the K-T boundary in this section. In this report, we assumed that the layer B is the transitional layer of the K-T time. We collected fresh rock samples and checked thin section of sedimentary rocks under the microscope and tried X-ray diffraction measurement of powdered rock samples. The sandstone in layer A is composed of sub-rounded quartz and feldspars and some rock fragments. The grain size is generally 0.3 0.5 mm. Polycrystalline quartz and feldspars are abundant. In X-ray diffraction, quartz, kaolinaite, illite and smectite were detected. Sandstone in layer B is composed of 0.3 0.4 mm size quartz and feldspar grains, and goethite is also included (though not detected in X-ray measurement). Besides quartz, kaolinite and illite are also detected by X-ray diffraction. The goethite-rich layer within layer B consists of goethite, quartz and feldspar grains. X-ray diffraction showed quartz, kaolinite, illite and goethite from this layer. In the samples we investigated, shocked quartz grains, mineral or glass spherules and Ni-rich spinel (e.g., Kyte and Smit, 1986; Bohor et al., 1987; Izett, 1990; Montanari, 1991; Koeberl, 1994; Robin et al., 1992; Rocchia et al., 1996; Bostwick and Kyte, 1996; Keller et al., 2002) have not been found yet. ANALYTICAL METHODS We have analyzed 24 samples from three sections (section M1, M2 and M4). Major element concentrations for the samples except limestones were measured using X-ray fluorescence (XRF) spectrometer, and those for limestone samples were determined by inductively coupled plasmaatomic emission spectrometer (ICP) at the Saitama University. Detection limits for XRF method are 0.01 wt.% for all major elements, and those for ICP are better than 0.01%. Ba, Sr, Y, Zr and V were measured by ICP method. Chromium, Co, Th, U, Cs, Hf, As, Sb and Sc were measured by instrumental neutron activation analysis (INAA) method, and Rb, Ni, Cu and Zn, and rare earth elements (REEs) were determined by inductively coupled plasma-mass spectrometry (ICP-MASS) method, both by Activation Laboratories Ltd.,

684 Y. Arakawa et al. Table 1. Analytical results for major elements (data in wt.%) Sample SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 (t) MnO MgO CaO Na 2 O K 2 O P 2 O 5 Total Level (cm) Section-M2 MD04 0.01 0.12 0.31 0.02 0.15 49.72 0.01 0.01 0.03 +95 MD03 0.01 0.12 0.06 0.01 0.14 49.92 0.03 0.01 0.03 +15 3-01 0.26 4.50 2.74 0.02 0.16 33.67 0.05 0.22 0.10 2 3-02a* 64.49 0.59 11.37 20.60 0.02 0.20 0.33 0.03 0.42 0.22 98.27 6 3-02b 82.26 0.63 12.18 1.50 0.01 0.34 0.18 0.03 1.16 0.26 98.55 7 3-02c 82.62 0.72 11.55 1.64 0.01 0.18 0.37 0.04 0.62 0.14 97.89 8 3-03 81.28 0.60 11.44 2.43 0.01 0.28 0.66 0.03 0.80 0.24 97.77 9 3-05 81.65 0.81 10.58 3.00 0.01 0.14 0.40 0.03 0.46 0.15 97.23 18 3-06 81.56 0.64 11.77 2.96 0.01 0.17 0.39 0.03 0.47 0.15 98.15 21 3-07 75.79 0.66 11.14 10.98 0.01 0.20 0.30 0.03 0.50 0.25 99.86 25 3-10 75.40 0.93 14.94 5.29 0.02 0.47 0.56 0.11 1.12 0.17 99.01 37 3-12 74.95 0.82 14.00 6.98 0.01 0.44 0.49 0.07 1.13 0.18 99.14 45 3-13 80.10 0.85 13.55 2.21 0.06 0.35 0.62 0.09 1.13 0.19 99.15 65 3-15 79.68 0.85 13.29 1.46 0.01 0.52 0.44 0.10 1.44 0.28 98.07 101 3-18 83.62 0.34 11.03 1.32 0.05 0.28 0.40 0.04 0.75 0.13 97.96 328 Section-M1 MD02 70.32 0.62 11.43 8.18 0.03 0.47 6.33 0.01 0.98 0.20 98.57 1.2 MD00f* 56.85 0.31 7.04 32.80 0.03 0.16 0.31 0.07 0.22 0.26 98.05 2.1 MD00y 0.24 4.18 1.39 0.07 0.23 22.50 0.04 0.16 0.08 2.5 MD01 80.94 1.04 12.86 1.47 0.01 0.52 0.44 0.04 1.70 0.19 99.21 5.3 MD10 0.38 6.61 1.64 0.06 0.35 27.98 0.52 0.85 0.12 100 Section-M4 5-04b* 54.54 0.30 6.50 34.70 0.03 0.16 0.22 0.06 0.26 0.25 97.02 17.4 5-04c 82.08 0.42 9.74 4.40 0.01 0.13 0.60 0.05 0.31 0.19 97.93 17.8 *Goehtite-rich layer; : not determined. Canada. We used standard rock samples of JB-1a (basalt) and JG-1a (granite) (Ando et al., 1987) for all analyses for checking the accuracy. Detection limits for ICP and INAA methods are 3 ppm for Ba, Sr, Y and Zr, 2 ppm for V, 0.5 ppm for Cr, 0.1 ppm for Co, Th and U, 0.2 ppm for Cs and Hf, 1 ppm for As, 0.1 ppm for Sb and Sc. Detection limits for ICP-MASS method are 0.1 ppm for Rb, 5 ppm for Ni, Cu and Zn, and 0.01 ppm for all rare earth elements. Platinum group elements (PGEs) including Ir were determined by neutron activation analysis with NiS fire-assay preconcentration. The detailed procedure appears elsewhere (Li and Ebihara, 2003). Here are some outlines of the analytical procedure. Each powdered sample (1 g) was well mixed with the following fluxes; Na 2 B 4 O 7 (2 g), Na 2 CO 3 (1 g), SiO 2 (0.1 g), Ni (0.1 g) and S (0.1 g). They were fused for 2 hours at 1000 C. After fusion, NiS beads were mechanically separated and then dissolved in 6M HCl. PGE precipitates were recovered on a hydrophilic PTFE (polytetrafluoroethylene) filter by filtration. Filters containing PGE residues were irradiated with neutrons at the JRR-4 reactor of the Japan Atomic Energy Research Institute. Irradiations were repeatedly performed twice. The shorted irradiation was performed by using a pneumatic transfer system. Samples including reference chemical standard samples were individually activated for 30 s with a thermal neutron flux of 4.5 10 13 n/cm 2 /s and a Cd ratio of 3.6. After 2 to 5 min cooling, each sample was counted for 300 s for 104m Rh (half live: 4.36 m) with a high-purity germanium semiconductor detector (HPGe) with a relative counting efficiency of 20% and an energy resolution of

Element profiles of Cretaceous-Tertiary boundary at Medetli, Turkey 685 Fig. 4. Profile of Fe concentration of section M2. Sample number 3-02 is goethite-rich layer. 1.80 kev (full width at half maximum, FWHM) at 1330 kev. After short irradiations, all the samples used for short irradiations and relevant reference chemical standard samples were sealed in a quarts tube and irradiated for 6 h with neutrons of a thermal neutron flux of 4.9 10 13 n/cm 2 /s and a Cd ratio of 4.8. Samples were successively counted three times with different cooling periods by two HPGe detectors with a relative energy efficiency of 20% and an energy resolution of 1.69 kev (FWHM) at 1330 kev. Samples were firstly measured for 4000 to 5000 s for 109 Pd (half times: 13.3 h) with a 20 h cooling period. After one week cooling, samples were counted for 6000 to 8000 s for 199 Au and 191 Os (half lives: 3.14 d and 15.4 d, respectively). Samples were finally measured for 10000 to 15000 s for 103 Ru and 192 Ir (half lives: 39.2 d and 73.8 d, respectively) after three weeks cooling. RESULTS AND DISCUSSION Element profiles of boundary sediments Analytical results of element concentrations are shown in Tables 1 3. Figure 4 indicates a profile of Fe concentration of section M2. The concentration is generally low in layers A, B and C, but increase in the goethite-rich layer (3-02a) within layer B. A small peak is also seen in 3-07 sample of layer B. The other major elements (Al 2 O 3, MgO, MnO, Na 2 O and K 2 O) are slightly lower in layer B compared with layer A. These major elements show no increase in concentrations in goethite-rich layers. Figure 5 shows some trace element profiles in the Medetli section M2. Peaks of Ni, Co, Cr, As, Sb and Sc (Fig. 5 and Table 2) are also seen in a goethite-rich layer (3-02a). Small peaks in Ni and Co are found in 3-07. The Cu and Zn concentrations also show similar profiles (Table 2). In section M1, Fe, Ni, Co, As, Zn, Sb and Sc are extremely enriched in goethite-rich layer (MD00f) (Fig. 6 and Table 2). About 23 wt.% of Fe, 29 ppm for Co, 194 ppm for Ni and 580 ppm of As are detected in this layer. These elements are very low in the layers A, C and layer B except for the goethite-rich layers. In section M4, a similar trend is obtained (Tables 1 2). In 5-04 sample (goethite-rich layer) of section M4, 24 wt.% of Fe, 42.5 ppm of Co, 349 ppm of Ni, 443 ppm of Cr and 851 ppm of As are determined. These high concentrations of Fe, Co, Ni and Cu are generally found in K-T boundary layers in the world (e.g., Alvarez et al., 1980; Kyte et al., 1980; Smit and Ten Kate, 1982; Schmitz, 1988). Concentrations of Ni (194 349 ppm) and Co (29 43 ppm) for the goethite-rich layers in this study are in the range

686 Y. Arakawa et al. Table 2. Analytical results for trace element concentrations (in ppm)

Element profiles of Cretaceous-Tertiary boundary at Medetli, Turkey 687 Table 2. (continued) *Goethite-rich layer.

688 Y. Arakawa et al. Fig. 5. Profiles of Ni, Co, Cr, As and Sb concentrations and Ir concentration for some samples in section M2. of those for the well-known marine K-T boundary sediments (66 1370 ppm for Ni and 7 230 ppm for Co) (e.g., Gilmour and Anders, 1989). Also high concentrations of chalcophile elements, such as As, Sb and Zn, are documented at many K-T boundary sites (e.g., Kyte et al., 1980; Smit and Ten Kate, 1982; Strong et al., 1987; Gilmour and Anders, 1989; Schmitz, 1992). The concentrations of As in the goethite-rich layers of this study (307 851 ppm) are much higher than those in the K-T sediments of the world (7 256 ppm). For other elements, middle to heavy REEs are elevated in the goethite-rich layers (Table 2). Iridium and other platinum group element (PGE) concentrations We attempted to measure the concentrations of Ir and some other PGEs for selected samples including goethite-rich samples. Iridium and Pt concentrations could be measured, but the other elements were lower than the detection limits (Table 3). The iridium concentration is relatively low (0.02 0.24 ppb) compared with those in the typical K-T boundary layers of the world (0.5 87 ppb) (e.g., Gilmour and Anders, 1989; Claeys et al., 2002). Notable is that the Ir concentration is not high in the goethite-rich layer (0.05 0.10 ppb) where some siderophile and chalcophile elements are enriched (Figs. 5 and 6). In two samples of layer A (MD01, 3 10), Ir yields slightly higher concentrations (0.13 0.24 ppb). As for Ir, the Pt concentrations of the Medetli section (except 3-07) are generally low compared with those of other K-T boundary sediments (e.g., Ganapathy, 1980; Kyte et al., 1980; Smit and Hertogen, 1980; Evans et al., 1993). The sample 3-07 indicates Pt concentration of 19.0 ppb and this value is only comparable with those of K-T boundary sediments at Gubbio (Ebihara and Miura, 1996). There is no correlation between Ni and Ir concentrations (Fig. 7). Also No correlation is found between Ir and Pt (Table 3). This may be partly due to differential mobilization of Ir and other PGEs during low temperature diagenesis under reducing condition (Wallace et al., 1990). These low values of Ir concentration particularly for goethite-rich layers are close to the background level of the K-T boundary sediments of the world. These results are not

Element profiles of Cretaceous-Tertiary boundary at Medetli, Turkey 689 Fig. 6. Profiles of Fe, Ni, Co, As and Sb concentrations and Ir concentration for two samples in section M1. MD00f is goethite-rich layer. consistent with those of the other well-known K- T boundary sites (e.g., Gubbio, Stevens Klint, Woodside Creek, Caravaca and Zumaya) where high Ir concentrations (4 580 ppb) and their covariation with Co, Ni and some other elements are found (e.g., Schmitz, 1988; Gilmour and Anders, 1989). In Medetli section Ir concentration as high as in those K-T sites could not be obtained. However, in some marine K-T sites Ir peaks are not necessarily so high (for example, 0.7 ppb for DSDP Site 536; 1.5 ppb for Brazos River, Texas; 0.5 ppb in Mimbral, Mexico) (Rocchia et al., 1996). Iridium is slightly enriched in the uppermost part of layer A (0.24 ppb for MD01) in Medetli section of this study. This value is clearly higher than the background level in this region (0.02 0.05 ppb). Although it is not eliminated that Ir is concentrated all from the terrestrial sources, it is not unlikely that some amounts of Ir was derived from extraterrestrial materials and that the Ir enriched layer was situated in the uppermost part of layer A or bottom of layer B. Iridium and some other related elements may have been diluted during sedimentation and diagenesis. Origin of goethite-rich layers Goethite is a common mineral in K-T boundary sediments worldwide (e.g., Kyte et al., 1980; Schmitz, 1992; Gilmour and Anders, 1989). Generally ferric hydroxide such as goethite and jarosite are considered to represent weathered pyrite or re-precipitated iron from dissolved pyrite (Schmitz, 1992). The ferric hydroxides are also common along small cracks and root channels in the sedimentary rocks, and are considered to anoxically scavenge As and Sb from sea water (e.g., Peterson and Carpenter, 1986). Although the goethite-rich layers in this study do not contain pyrite as prior mineral phase, it is probable that the goethite was formed from dissolved pyrite. In many K-T sites the correlation of As and Sb with Ir was demonstrated, particularly for marine K-T sites (Gilmour and Anders, 1989). In the Medetli section of this study, the correlation is not found.

690 Y. Arakawa et al. Table 3. Analytical results of PGEs for selected samples (data in ppb) Sample Os Ir Ru Rh Pt Pd Level (cm) Weight (g)** Section-M2 3-01 <0.80 0.02 <3.6 <0.20 2.5 <40 2 1.0270 3-02a* <0.75 0.07 <3.2 <0.20 2.5 <46 6 1.0098 3-02b <0.94 0.13 <2.7 <0.20 2.4 <45 7 1.0009 3-07 <0.58 0.08 <2.4 <0.20 19.0 <35 25 1.0157 3-10 0.35 0.13 <2.1 <0.20 0.7 <39 37 1.0059 Section-M1 MD00f* <1.6 0.10 <0.73 <0.20 3.5 <174 2.1 0.3312 MD01 1.13 0.24 <3.1 <0.20 <2.0 <29 5.3 1.0121 Section-M4 5-04b* <1.7 0.05 <8.0 <0.20 <7.2 <169 17.4 1.0012 5-04c <0.66 0.03 <2.8 <0.20 0.4 <51 17.8 1.0083 5-04d* <1.99 0.07 <8.0 <0.20 4.7 <181 18.2 0.7404 Blanks for all procedures <0.40 0.012 1.7 <0.19 <2.0 <56 *Goethite-rich layer; **weight for samples used. Fig. 7. Ir-Ni diagram for nine samples in layer A, and layer B including goethite-rich layer. Arsenic and Sb (and Zn) enrichments in goethiterich layers may be explained by incorporation (or precipitation) as pyrite phase, originally from sea water or continental crust. As stated before, Ni and Co concentrations in the goethite-rich layers are within the ranges of those for well known K-T boundary sediments. Thus the possibility that some amounts of Co, Ni and Fe in the goethiterich layers are of extraterrestrial origin is not excluded. Comparison with other K-T boundary sites Medetli section shows some differences from those of the other K-T sites of the world. In many marine K-T sites, thin (1 2 cm) red clay layer is intercalated between the Cretaceous and Tertiary sedimentary rocks (Gubbio, Caravaca, Stevns Klint, Woodside Creek etc.). In these thin layers the markers for asteroid impact such as enrichment of Ir and other PGEs and some siderophile elements, mineral or glass spherules, shocked

Element profiles of Cretaceous-Tertiary boundary at Medetli, Turkey 691 quartz grain are included. These results have suggested that the K-T extinction event occurred during very short time interval (e.g., Alvarez, 1986; Wolbach et al., 1988; Gilmour and Anders, 1989). Medetli K-T related section consists of about 37 cm of yellow colored sandstone layer including thin several goethite-rich layers and probably the top of the layer A, and this section is not similar to the sections of well-known K-T sites. In recent studies, however, it has been clear that the temporal and stratigraphic differences in markers showing the impact of extraterrestrial materials exist during the K-T time. For example, spherule rich layer is 60 cm lower than the Ir rich layer in Haiti K-T section, and several peaks of Ir enrichment were found in 30 cm section in Brazos River, Texas (e.g., Rocchia et al., 1996). Separated existence of spherule-rich layer and Ir-rich layer was documented from the Mimbral section, Mexico, and thus an extended period of time from spherule deposition to Ir precipitation was proposed (Stinnesbeck et al., 1993). However, more recent studies have shown that these complex sedimentological features are explained as a result of single impact at Yucatan Peninsula (e.g., Smit et al., 1996; Smit 1999). The Medetli section, particularly the top of layer A and layer B, may be explained as a result of the same event. In this study, some other markers or evidence for the impact of extraterrestrial materials (such as shocked quartz with characteristic planar deformation features, mineral or glass spherules etc.) have not been found yet. The actual K-T boundary may be situated between the top of layer A and the bottom of layer B. Though the possibility that the real K-T boundary may have been eroded is not eliminated, the top of layer A and layer B of the Medetli section seem to have been formed during the K-T and its related time. CONCLUSION Cretaceous-Tertiary (K-T) transitional layers in Medetli, Gölpazari, northwestern Turkey show different stratigraphical and chemical profiles comparable to well-known K-T boundary layers of the world. Late Maastrichtian gray colored sandstone layer (layer A) is overlain by yellow colored sandstone layer (layer B) with thin goethite-rich layers. Lower Paleocene limestone (layer C) overlies layer B. Layer B (28 37 cm in thickness) is devoid of fossils. In goethite-rich layers some siderophile elements (Fe, Ni, Co, Cr) and chalcophile elements (As, Sb and Zn) are enriched and these enrichments are comparable with those in the boundary sediments of well-known K-T sites. However, Ir concentration is very low (0.05 0.10 ppb) in the layers. Iridium concentration is slightly elevated only in the top part of layer A (0.24 ppb), and this value is clearly higher than the background level (0.02 0.05 ppb) in this region. The actual K-T boundary is situated probably between the top of layer A and the bottom of layer B. In the Medetli region, the deposition of the top of layer A and the layer B are considered to have been formed by the K-T and its successive events. Acknowledgments We thank Mr. H. Matsubara for K-T sample collection in Medetli region. Constructive reviews by Profs. P. Claeys and B. Schmitz are also acknowledged for their comments that substantially improved the manuscript. We also thank Prof. Koeberl (Associate Editor) for his careful and critical comments on the manuscript and editorial handling. This work was supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan. REFERENCES Alvarez, L. W., Alvarez, W., Asaro, F. and Michel, H. V. (1980) Extraterrestrial causes of the Cretaceous- Tertiary extinction. Science 208, 1095 1108. Alvarez, W. (1986) Toward a theory of impact crises. Eos Trans. American Geophys. Union 67, 649 658. Ando, A., Mita, N. and Terashima, S. (1987) 1986 values for fifteen GSJ rock reference samples, Igneous rock series. Geostandards Newsletter 11, 159 166. Blum, J. D., Chamberlain, C. P., Hingston, M. P., Koeberl, C., Marin, L. E., Schuraytz, B. C. and Sharpton, V. L. (1993) Isotopic composition of K/T boundary impact glasses with melt rock from the Chicxulub and Manson impact structure. Nature 364, 325 327.

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