Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/125 Comunicações Geológicas (2012) 99, 2, 27-34 ISSN: 0873-948X; e-ISSN: 1647-581X
Iridium anomaly and extraterrestrial component in the clays at the Cretaceous-Paleogene boundary in Denmark, Spain and New Zealand
Anomalia de irídio e componente extraterrestre das argilas do limite Cretácico – Paleogénico na Dinamarca, Espanha e Nova Zelândia
P. I. Premović1*, B. S. Ilić2
Recebido em 28/09/2011 / Aceite em 11/12/2011 Artigo original Disponível online em Janeiro de 2012 / Publicado em Dezembro de 2012 Original article © 2012 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The Cretaceous-Paleogene boundary clays at Højerup been identified and studied at about 345 sites worldwide. Of these, (Denmark), Caravaca (Spain) and Woodside Creek (New Zealand) show 85 sites, representing all depositional environments, contain an Ir anomalous enrichments of iridium compared with the marine sedimentary anomaly. In general, Ir and other platinum-group elements (PGE: rocks. For the average iridium content of 465 ppb of CI carbonaceous collectively the elements Ru, Rh, Pd, Os, Ir and Pt) are invariably chondrite the estimate of CI chondrite proportions in the decarbonated iridium-rich boundary layers, based on the integrated iridium fluencies, is enriched in the prominent boundary clays. Other trace elements (e. about 25 % at Højerup, 15 % at Caravaca and > 30 % at Woodside Creek. g. heavy metals) are also relatively abundant in these clays. These proportions are most likely too high due to a significant Ir influx Many researchers consider that the KPB impactor formed the from the nearby marine or continental site to these sections. ca. 180 km crater at Chicxulub (Yucatan Peninsula, Mexico, Fig. Keywords: Fish Clay, Caravaca, Woodside Creek, Iridium, 1). It has been suggested that the impactor was a carbonaceous Carbonaceous chondrite. chondrite projectile - probably type CI (Kyte, 1998; Shukolyukov & Lugmair, 1998; Frei & Frei, 2002; Quitté et al., 2003; Trinquier Resumo: As argilas do limite Cretácico – Paleogénico em Højerup et al., 2006), though it is still unclear whether it was a (Dinamarca), Caravaca (Espanha) e Woodside Creek (Nova Zelândia) carbonaceous chondrite or a comet. Indeed, comets are believed to mostram enriquecimentos anómalos de irídio comparados com rochas be primitive bodies with a composition like that of carbonaceous sedimentares marinhas. Para o conteúdo médio de irídio de 465 ppb do chondrites (Gelinas et al., 2004). The anomalous Ir associated, condrito carbonáceo CI, a estimativa das proporções condríticas CI nas camadas descarbonatadas ricas em irídio da zona de limite, baseado nas however, with the prominent boundary clay is consistent with the fluências integradas de irídio, é cerca de 25% em Højerup, 15 % em high Ir content typical of most chondritic meteorites and Caravaca e > 30% em Woodside Creek. Estas proporções demasiado inconsistent with the general proposition of comets. Indeed, a elevadas são provavelmente devidas a influxo significativo de Ir a partir simple calculation shows that in the case of the ice-rich (>70 %) das regiões marinhas ou continentais próximas destas secções. comets the amount of Ir produced by an impact energy for a crater Palavras-chave: Argila, Caravaca, Woodside Creek, irídio, condrito of the Chicxulub size could be less than 0.001 % then that of an carbonáceo. asteroid. In the past many researchers used the iridium concentration and/or integrated amount of Ir to estimate the proportion of 1Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of Niš, P.O. Box 224, 18000 Niš, Serbia. chondritic component in the renowned boundary clays. This paper 2Department of Pharmacy, Faculty of Medicine, University of Niš, 18000 Niš, aims to re-examine this method using comprehensive Ir data for Serbia. the boundary sections at Højerup, Caravaca and Woodside Creek *Corresponding author / Autor correspondente: [email protected] which are available and published by Schmitz (1988). These
stratigraphic sections are well preserved and distal (paleodistance: >7000 km) to the proposed Chicxulub impact site. An Ir analysis 1. Introduction of the decarbonated KPB samples from Denmark, Spain and New Zealand was carried out by Schmitz using instrumental neutron In the original paper Alvarez et al. (1980) have first reported activation analysis (INAA). Relative error in the precision of the anomalously high Ir concentrations in the Cretaceous-Paleogene analyses ranges from 5 % to 10 %. Total uncertainties (including (KPB) boundary clays at Gubbio (central Italy, Fig. 1), Højerup accuracy errors) were up to 20 %. (the eastern Denmark, Fig. 1) and at Woodside Creek (the For the sake of completeness, the Ir data related to the distal northern part of the South Island of the New Zealand, Fig. 1). KPB sections at Agost (Spain), Flaxbourne River (New Zealand), They proposed an impact of extraterrestrial bolide to explain the Gubbio (Italy), Bidart (France), El Kef and Aïn Settara (Tunisia), elevated Ir content at the KPB. Alvarez et al. also suggested that and in Ocean Drilling Program (ODP) Hole 738C (Kerguelen approximately 60 times the mass of the impactor would have been Plateau, southern Indian Ocean) (Fig. 1) are also briefly discussed. ejected into the atmosphere as impact dust. Almost simultaneously Although the sections studied systematically show an Ir with Alvarez et al., Smit & Hertogen (1980) reported an abundance anomaly, they differ from one another because local anomalous Ir in the boundary clay at Caravaca (southeastern sedimentation conditions under a strong continental influence. Spain, Fig. 1). Since the Alvarez et al. discovery, the KPB has Throughout this paper we make five reasonable postulates: (a) Ir 28 P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34 is wholly located in the non-carbonate fraction of the boundary carbonaceous chondrites typically contain 406.0 - 849.4 ppb Ir clays studied, i. e. the carbonate fraction of this section is (Table 1). So, a little addition of the CC component would be essentially an Ir diluents; (b) the Ir content of the carbonaceous necessary to critically increase the concentration of Ir in a chondrites range from 406.0 ppb to 849.4 ppb (Table 1); (c) the marine sedimentary rock. Thus, Ir is a very sensitive means of average content of Ir in CI chondrites is 465 ppb (Table 1); (d) all examining the CC contribution to the marine KPB clays such as Ir found in the boundary clays originated from the carbonaceous the Fish Clay. Some authors suggested that Ir in the Fish Clay chondrite (CC) impactor, especially of type CI; and (e) assumed was sourced by the Ir from seawater (Goldberg et al., 1986). density of the boundary clays is about 2 g cm-3. However, the average Ir concentration in the decarbonated BH (ca. 100 ppb, see below) represents an enrichment factor of about 1017 compared to seawater (ca. 2×10-15 ppb, Table 1). Moreover, according to these authors, the residence time of Ir in the oceans is about 1 million years. Thus, excess of Ir in the BH could not, therefore, have been derived from the sea reservoir.
Table 1. Concentrations of Ir [ppb] for carbonaceous chondrites, marine sediments and seawater.
Tabela 1. Concentrações de Ir [ppb] para os condritos carbonáceos, sedimentos marinhos e água do mar.
Fig.1. Geographic location of studied KPB clays.
Fig.1. Localização geográfica das argilas KPB estudadas.
2. Results and Discussion Fish Clay. The lowermost Danian Fish clay Member of the Rødvig Formation near the village of Højerup is a classic marine KPB section. The lithology of the Fish Clay within this boundary section characterizes three distinctive layers (from bottom to top): a 3 cm thick (mainly black-to-dark) marl (hereinafter BH) with 0.5 cm-thick basal red (goethite-rich) sublayer, grey-to-brown marl and a light-grey marl (Fig. 2). The red sublayer is underlain by (the latest) Maastrichtian bryozoan-rich limestone (chalk) whereas the top marl is overlain by (early) Danian Cerithium limestone (Schmitz, 1988; Christensen et al., 1973; Schmitz, 1985; Elliott, 1993; Surlyk et al., 2006). Geochemical studies show that the Ir profile (on a whole rock basis) across the Fish Clay column is characterized by a maximum just above the base of BH with an upward gradual decrease (tailing-off) from its maximum (e. g. Schmitz, 1985). This gradual tailing off indicates that the Fish Clay Fig.2. Expanded lithological log of the Fish Clay at Højerup based on Surlyk et al. represent a relatively continuous and complete section. The BH (2006). is considered to constitute the main part of the boundary section, since it contains more than 95 % of the total Ir in the Fig.2. Log litológico expandido das argilas em Højerup baseado em Surlyk et al. Fish Clay (Premović, 2009). The mineralogy of BH is (2006). comparatively simple, smectite and authigenic (mainly biogenic) calcite being the principal components. Geochemical Kyte et al. (1985) estimated that the extraterrestrial component evidence indicates that the BH was deposited under strong in the basal 1 cm part of BH from the measured Ir concentration anoxic conditions but the red sublayer under strong oxic (47.4 ppb) is about 10.5 %, corrected for about 20 % carbonate sedimentation conditions (Premović, 2009). content. According to Grieve (1997), chondrite-normalized relative Premović et al. (2000) inferred that the BH was deposited abundance of Ir indicates that the Fish Clay (on a whole-rock basis) in a shallow marine <100 m water depth at Stevns Klint within contains an admixture of about 10 % chondrite. Trinquier et al. an interval of about 40 years. Wendler & Willems (2002) (2006) have shown that Cr isotopic signature of BH represents a considered that this layer represents the first decades or mixing of the CC matter with terrestrial material in a ratio up to 6.8 centuries of deposition following the KPB impact event. Thus, %. Recently, Osawa et al. (2009) estimated from the measured Ir the average sedimentation rate in the BH is probably about 0.3 concentration (29.9 ppb) that the extraterrestrial material in the mm per year. basal part of BH was diluted to 1/19 (on a whole-rock basis), As pointed out above, the Ir enrichment of the KPB clays assuming that Ir concentration of CC is 470 ppb. Of note, that an is generally regarded as indicative of the presence of a CC impactor mass fraction globally dispersed after the Chicxulub component. Table 1 shows that on average marine sedimentary impact ranges between 22 % (Alvarez et al., 1980) to 50 % rocks usually contain the Ir bellow 1 ppb (Table 1) while (Vickery & Melosh, 1990). Ir anomaly and meteoritic contribution 29
The INAA data (Schmitz, 1988) for Ir in the carbonate-free al., 2000). About 29 - 33 shocked quartz grains per gram are also fraction across the Fish Clay are presented in Fig. 3a. This profile identified in this layer (Morgan et al., 2006). Of note, the total Ir shows that the peak concentration of Ir in the decarbonated BH is flux of the carbonate-free Fish Clay is about 615 ng cm-2 which about 120 ppb. Using these data, we estimate that the corresponds to about 22 % the type CI chondrite. For the average decarbonated BH is derived from about 6 - 30 % CC (Fig. 4a); sedimentation rate in the BH of 0.3 mm yr-1, the average the CI chondrite proportion is about 22 % for an average Ir accumulation rate of Ir in the BH is ca. 3 ng cm-2 yr-1; the average concentration of about 100 ppb. accumulation rate is about 6.5 µg cm-2 yr-1 CI chondrite.
Fig.3. Concentration profiles of Ir (on a carbonate-free basis) in the Cretaceous- Fig.4. The CC contributions for the decarbonated boundary clays at (a) Højerup Paleogene boundary clays at: (a) Højerup (Fish Clay), (b) Caravaca and (c) (Fish Clay), (b) Caravaca and (c) at Woodside Creek. The lowest ( ) and highest Woodside Creek. The samples were analyzed with instrumental neutron activation ( ) CC contributions are based on the Ir concentrations of the carbonaceous (INAA). Relative error in the precision of the analyses ranges from 5 % to 10 %. chondrites (see text). Total uncertainties (including accuracy errors) were up to 20 % (Schmitz, 1988). Fig.4. As contribuições CC para as argilas descarbonatadas do limite em (a) Højerup Fig.3. Perfis de concentração de Ir (numa base sem carbonato) nas argilas do limite (Fish Clay), (b) Caravaca e (c) em Woodside Creek. As contribuições CC mais Cretácico-Paleogénico em: (a) Højerup (Fish Clay), (b) Caravaca e (c) Woodside baixa ( ) e mais elevada ( ) baseiam-se nas concentrações de irídio dos condritos Creek. As amostras foram analisadas com activação neutrónica instrumental carbonáceos (ver texto). (INAA). O erro relativo na precisão das análises varia entre 5 % a 10 %. As incertezas totais (incluindo erros de incerteza) chegaram até 20 % (Schmitz, 1988). Our estimation (judging from the Ir concentrations and A better measure of an Ir anomaly in the boundary clay is integrated Ir fluence) may substantially underestimate the actual the integrated Ir fluence. For the BH, the fluence is estimated at CC contribution to the decarbonated BH. There are two reasons about 305 ng cm-2 after integration in the decarbonated segment for this. First, anomalous amounts of Ir in the decarbonated Fish from 0.5 - 2.0 cm (Fig. 3a). (For comparison, an estimate of the Ir Clay display a gradual decrease from the BH upwards but this fluence for the global impact deposit for the KPB is about 40 - 55 metal is still relatively elevated (up to near 5 ppb) in the light- ng cm-2: Kyte, 2004). This value corresponds approximately to gray marl (Fig. 3a). These stratigraphically extended enrichments about 0.35 - 0.75 g cm-2 CC. These estimates correspond to about of Ir may be explained only partly by the redox-controlled 15 - 30 % CC; the CI contribution is about 25 %. Such a remobilization (Colodner et al., 1992). Second, according to significant proportion of CC is, however, not supported by any Peucker-Ehrenbrink & Hannigan (2000), between about 40 % mineralogical evidence. Indeed, if initially the precursor material and 80 % of the initial Ir budget could have been lost due to of BH contained a high percentage of the ejecta fallout, then surficial weathering of black shales (such as the BH) within diagenetic alteration would have left a residue with high roughly 13, 000 years. Thus, because of the assumed concentrations of the impact markers. To our knowledge, the part remobilization and/or weathering loss of the Ir, the original of BH above the red sublayer contains no altered meteoritic proportion of CC in the carbonate-free part of BH actually could fragments (Bauluz et al., 2000) and only about 0 - 6 shocked be even considerably higher than presented above. The more quartz grains per gram (Morgan et al., 2006). On the other hand, detailed discussion of this issue is beyond the scope of this paper. a simple calculation indicates that the underlying red sublayer Assuming that ejecta fallout (with an average density of -3 (with 15. 2 ppb of Ir, Fig. 3a) contains as low as about 3.3 % of about 2 g cm ) contained 50 % of the CC matter then the BH type CI chondrite material. In sharp contrast, this sublayer is portion in question would contain about 30 - 60 % of this (mostly abundant (about 10 wt %) with the well-preserved impact-related diagenetically altered) fallout. The CC matter (and associated Ir) goethite-rich microspherules (Schmitz, 1985; Graup et al., 1992) could be deposited in the Fish Clay by the direct airborne ejecta and with nano-size goethite grains (Bauluz et al., 2000; Wdowiak fallout settling through the seawater column or they were et al., 2001) interpreted as altered meteorite fragments (Bauluz et transported from the nearby marine or continental site to the Fish 30 P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34
Clay bed. Kyte et al. (1985) reasoned that Ir (and other turn correspond to about 10 - 20 % CC; the CI chondrite siderophiles) in the basal 1 cm thick part of BH is only contribution is about 15 %. representative of primary Ir derived from the initial ejecta fallout Assuming that ejecta fallout (with an average density of deposited during short time (a few days, months or years) after the about 2 g cm-3) contained 50 % of the CC matter then the BW impact. According to these researchers, Ir above this layer is portion in question would contain about 20 - 40 % of this (mostly probably secondary in origin and transported from the original diagenetically altered) fallout. As previously stated for the BH, a nearby site, which increased the primary Ir values. Wolbach et al. high percentage of the ejecta fallout would have left a residue (1988) proposed that secondary Ir is ejecta material eroded from after diagenesis with high concentrations of the impact indicators marine elevated sites to topographic lows already containing some in the BC. The thin red sublayer contains numerous presumably primary fallout. Premović (2009) has suggested that Ir in the Fish impact-derived goethitic microspherules (ca. 10 % of the total Clay was probably fluvially transported from the soil on adjacent weight: Schmitz, 1988), shocked zircons and Ni-rich spinels. land and redeposited in a shallow marine basin at Højerup. This About 19 - 38 shocked quartz grains per gram were identified in author argued that a predominant part of Ir in the BH ultimately the BC (Morgan et al., 2006). came from the CC material associated with ejecta fallout covering By analogy with the Fish Clay, we assume that Ir in the BC nearby coastal soil. Indeed, the gradual decrease of Ir upsection was probably derived from carbonaceous chondritic component across the Fish Clay indicates the single input of Ir probably of ejecta fallout accumulated on nearby adjacent land or marine associated with detrital sedimentation, consisting of smectite, silt area. Indeed, the major carrier of this Ir was probably the and humic materials (Premović, 2009, and references therein). smectite which is most likely of detrital origin (Ortega-Huertas et Consequently, the overestimated proportion of the CC matter in al., 1995). The decarbonated boundary section at Caravaca is the decarbonated BH is probably due to the high Ir (detritus- characterized with a similar stratigraphically extendeded associated) input from the marine or continental site. enrichments (<5 ppb) as the decarbonated Fish Clay, Fig. 3b. Anomalous Ir concentration (on a whole-rock basis) is not Moreover, Rocchia et al. (1987) reported the Ir-rich interval at limited to the Fish Clay but extends about 1 m above and below Caravaca also contains above-background concentrations of Ir the BH (Rocchia et al., 1987). They estimate that the duration of (on a whole-rock basis) over 1.5 m of interval. the enhancement represents about 104 years. This may indicate an extended contribution of Ir derived from an extraterrestrial source The Woodside Creek boundary section. The KPB section at other than the predominant and virtually instantaneous source of Ir Woodside Creek is represented by a reddish (up to 1 cm thick) of the KP impactor. It is also possible that extension of Ir clay-rich layer (BW) and overlying dark to grey-to-brown marls. enrichment originate from a relatively long terrestrial source, e. g. These layers were probably deposited in a shallow marine <500 Deccan Traps volcanism at the end of the Cretaceous period m water depth (Morgan et al. 2006) probably under well- which lasted millions of years. The third explanation is that this oxygenated conditions. BW is underlain by the latest tail represents postdepositional redistribution of Ir out of its Maastrichtian marl, Fig. 3c. original peak position. This topic is, however, out of scope here. The INAA data for Ir (Schmitz, 1988) in the decarbonated fraction of the boundary section at Woodside Creek are plotted in The Caravaca boundary section. The sections at Agost, Caravaca Fig. 3c. The peak concentration of Ir of 460 ppb is located in the and El Kef are among the most continuous and complete marine carbonate-free BW which is one of the highest measured to date for sections for the KPB transition. The Caravaca and Agost any KPB interval. Using Schmitz’s data, we estimate that in the boundary sections are located in the Betis Cordilleras decarbonated BW is derived from about 60 % up to bizarre 115 % (southerneast Spain). The KPB section at Caravaca consists of a CC (Fig. 4c); the type CI chondrite input averages about 100 %. ca. 1 cm-thick Ir-rich dark marl (BC) overlain by a grey-to-brown The Ir anomaly appears to be much sharper upward and marl, Fig. 3b. A basal 2-3 mm thick red sublayer of BC enriched downward than at the Danian (Fig. 3a) and Spanish (Fig. 3b) with Ir (ca. 47 - 57 ppb: Tredoux et al., 1989) is marking the sites. Moreover, the decarbonated Ir-rich section at Woodside KPB. The clay association of these marls is dominated by Creek is characterized with high Ir enrichments (20-30 ppb) up to dioctahedral smectite (Ortega-Huertas et al., 1995). According to about 5-6 cm above the BW, Fig. 3c. these researchers, this smectite results from (a) the alteration of Brooks et al. (1984) reported that a carbonate-free boundary marine volcanic rocks or (b) the erosion of continental soil. layer (0.8 cm thick) at another site of Woodside Creeks contains Terminal Maastrichtian–basal Paleogene marls at Caravaca were about 153 ppb of Ir. This value corresponds to about 20 - 40 % deposited in a middle bathyal environment (<500 m depth) (Smit, CC; the CI chondrite contribution is about 30 %. 1999). Abundant presence of goethite in the red sublayer Assuming that ejecta fallout (with an average density of indicates that its deposition probably occurred under well- about 2 g cm-3) contained 50 % of the CC matter then the BH oxygenated conditions. portion in question would contain about >40 % of this (mostly The distribution of Ir in the carbonate-free fraction across diagenetically altered) fallout. As in the case of BH, a high the boundary section at Caravaca is given in Fig. 3b. This profile percentage of the ejecta fallout would have left a residue after is based on the INAA measurements performed by Schmitz diagenesis with high concentrations of the impact indicators in (1988). The Ir values reach a profound peak concentration of the BW. To express precisely, the BW contains numerous about 110 ppb in the decarbonated red sublayer. The main Ir peak goethite-rich and organic microspherules (Schmitz, 1988); and, has tails both upsection and downsection, but the later is much about 2 - 8 shocked quartz grains per gram (Morgan et al., 2006). sharper than in the Fish Clay. As previously stated, the upward Anomalous extraterrestrial Ir is also mostly associated with this tail feature infers a relatively continuous and complete section. layer, Fig. 3c. Using his concentration data, we estimate that the decarbonated BC is derived from ca. 5 - 27 % CC (Fig. 4b); the CI chondrite The Agost boundary section. The Agost boundary section is contribution is about 13 % for an average Ir concentration of similar to the neighbouring Caravaca section in lithology, about 60 ppb. geochemistry and depositional history. As at Caravaca this The integrated Ir fluence of BC is estimated at about 100 ng section is comprised of a dark (about 6-cm-thick) clay with a cm-2. This value corresponds to 0.20 - 0.40 g cm-2 CC. These in basal 2-3 mm-thick red goethite-rich sublayer, Fig. 5. The dark Ir anomaly and meteoritic contribution 31 clay layer is underlain with the latest Maastrichtian marl and boundary clay contains from about 0.25 - 5.0 % CC (Fig. 6b). overlain by the grey-to-brown marl (Molina et al., 2005, and However, the decarbonated Ir-rich interval at Flaxbourn River is references therein). This layer is deposited in the middle bathyal characterized with another high Ir peak (ca. 20 ppb) about 1.5 cm environment, 600-1000 m deep (Coccioni & Galeotti, 1994). below the boundary clay, Fig. 6a.
Fig.5. (a) The Ir concentration profile (on a carbonate-free basis) across the Fig.6. (a) The Ir concentration profile (on a carbonate-free basis) across the Cretaceous-Paleogene boundary section at Agost and (b) the CC contributions for Cretaceous-Paleogene boundary section at Flaxbourne River and (b) the CC the decarbonated boundary section at Agost: lowest ( ) and highest ( ) CC contributions for the decarbonated boundary section at Flaxbourne River: lowest ( ) contributions (see above). and highest ( ) CC contributions (see above).
Fig.5. (a) O perfil de concentração de Ir (numa base sem carbonato) ao longo do Fig.6. (a) O perfil de concentração de Ir (numa base sem carbonato) ao longo do limite Cretácico-Paleogénico em Agost e (b) as contribuições CC para a secção limite Cretácico-Paleogénico em Flaxbourne River e (b) as contribuições CC para a descarbonatada do limite em Agost: contribuições CC mais baixa ( ) e mais secção descarbonatada do limite em Flaxbourne River: contribuições CC mais baixa elevada ( ) (ver acima). ( ) e mais elevada ( ) (ver acima). Smit (1990) analyzed Ir (on a whole-rock basis) across the dark clay. Using his Ir and carbonate content data, we computed Ir The Gubbio boundary section. The classical Bottaccione concentrations in the non-carbonate (clay) fraction. These boundary shales near Gubbio originated at a deep marine bathyal calculated concentrations are plotted vs. stratigraphic height in basin with a water depth over 1500 m (Kuhnt, 1990). A Fig. 5a. The results show that the highest Ir enrichment (ca. 30 stratigraphic profile for the basal part of these shales is shown in ppb) is in the red sublayer with an upward gradual decrease from Fig. 7. We roughly estimate that the CC matter in their this peak. We estimate that the decarbonated sublayer contains decarbonated fraction is <2 %. This estimation is based on the about 4.0 - 7.0 % CC (Fig. 5b). The red sublayer contains peak Ir concentration (ca. 7 ppb: Crocket et al., 1988). These mineralogical evidence of extraterrestrial impact such as Ni-rich researchers also found that the enrichment of Ir (on a whole-rock spinels and diagenetically altered K-feldspar or Fe oxide basis) at Gubbio is not confined to the boundary shales but microspherules (Smit, 1990). extends approximately 2 m above and below the boundary level, corresponding to 3.5 million years. According to Montanari The Flaxbourne River boundary section. Beside at Woodside (1986), the boundary section at Gubbio bears evidence of Creek, the KPB has been identified in over 20 stratigraphic widespread contamination from the surrounding soil. sections in the New Zealand region and one of them is Flaxbourne River (Hollis et al., 2003). Despite the lesser Ir enrichment in the The Bidart boundary section. The boundary section at Bidart is Flaxbourne section (21 ppb) compared to Woodside Creek (54- characterized by a sharp contrast between a marl of the latest 100 ppb), the Flaxbourne section is considered to be more Maastrichtian and a 6-cm-thick layer of dark boundary clay with complete (Strong, 2000; Hollis et al., 2003). The dark-to-black a 2-mm-thick basal (iron-rich) red sublayer, Fig. 8a. This clay of boundary clay (average in thickness about 2 cm) in this section is the lowermost Danian is overlain by brownish clay. The whole a calcareous limestone which is overlain by a dark limestone. This interval is deposited in upper-middle bathyal depth (Alegret et clay in turn is underlain by the latest Maastrichtian marl, Fig. 6. al., 2004). The red sublayer is marked by an Ir anomaly (Rocchia The INAA data for Ir (Strong et al., 1987) in the et al., 1987) and the presence of Ni-rich spinels (Robin & decarbonated fraction of the boundary section at Flaxbourne River Rocchia, 1998). In Fig. 8a, we plot a Ir distribution(on a are plotted in Fig. 6a. The peak concentration of Ir of about 21 ppb carbonate-free basis) across the boundary clay using the Ir and is located in the carbonate-free boundary clay; more than about 85 carbonate content data reported by Rocchia et al. (1987). We % of the total Ir is distributed within the Ir-rich interval of -2 cm roughly estimate that the carbonate-free fraction of the red to +6 cm. Using their data, we estimate that the decarbonated sublayer contains <2 % CC (Fig. 9a). 32 P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34
clay is 55–65 cm thick, with the 2–3 mm (almost carbonate-free) red (goethite-rich) sublayer (Keller et al., 1995); this sublayer contains 5 - 10 % of the total Ir flux and most of Ni-rich spinels (Robin et al., 1991). A stratigraphic column of basal part of this clay is presented in Fig. 9b. Smectite is the main component of the boundary clay at this site (Ortega-Huertas et al., 1998). This clay passes upward into marl of the early Danian (Molina et al., 2006). The Ir maximum of 4 ppb in the red sublayer (Robin et al., 1991) corresponding roughly to <1.5 % (Fig. 9b).
The Aïn Settara boundary section. This section (about 50 km far from the El Kef site) is similar to the El Kef section in lithology, geochemistry and deposition history (Dupius et al., 2001); a stratigraphic column of basal part of this section is shown in Fig. 8c. According to these researchers, (almost) carbonate-free jarositic red sublayer is a few mm thick contains 3 ppb of Ir; this corresponds roughly <1 % of the CI chondrite matter. We roughly estimate that the red sublayer contains <1 % CC (Fig. 9c).
Fig.7. Expanded lithological log across the Cretaceous-Paleogene boundary section at Gubbio (adopted from Premović, 2009).
Fig.7. Log litológico expandido ao longo do limite Cretácico-Paleogénico em Gubbio (adaptado de Premović, 2009).
Fig.9. The CC contributions for the decarbonated boundary section at: (a) Bidart, (b) El Kef and (c) Aïn Settara: lowest ( ) and highest ( ) CC contributions (see above).
Fig.9. Contribuições CC para a secção descarbonatada do limite em: (a) Bidart, (b) El Kef e (c) Aïn Settara: contribuições CC mais baixa ( ) e mais elevada ( ) (ver acima). ODP Hole 738. Schmitz et al. (1991) analyzed Ir (on a whole-rock basis) across a 1m-thick-clay rich boundary section in ODP Hole 738. They found that Ir concentrations are the highest (18 ppb) in the dark boundary clay (lamina) a few centimeters above the base of the clay-rich layer. Using their Ir and carbonate content data, we computed Ir concentrations in the non-carbonate (clay) fraction. These calculated concentrations in Fig.8. The Ir concentration profile (on a carbonate-free basis) across the Cretaceous- Fig. 10a are plotted vs. stratigraphic height. The results show that Paleogene boundary section at: (a) Bidart, (b) El Kef and (c) Aïn Settara. the highest Ir enrichment (ca. 60 ppb) is in the dark lamina. The
Fig.8. O perfil de concentração de Ir (numa base sem carbonato) ao longo do limite Ir concentrations decrease relatively sharply for about 1.5 cm Cretácico-Paleogénico em: (a) Bidart, (b) El Kef e (c) Aïn Settara. upsection and then remains nearly constant over a plateau of tens of centimeters. We roughly estimate that the decarbonated
fraction of the dark lamina contains between 5 % and 15 % CC El Kef boundary section. The base of the El Kef boundary section (Fig. 10b). No shocked quartz or impact-derived microparticles in the central Tunisia has been officially designated as the were reported to be found in the boundary section. Schmitz et al. boundary global stratotype section and point (GSSP) for the KPB (1991) concluded that the clay fraction of this section studied is (Cowie et al., 1989; Molina et al., 1996). At El Kef the boundary predominantly derived from locally derived material. Ir anomaly and meteoritic contribution 33
and endless patience for our effort to make this report as complete as possible.
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