The origin of the clay minerals at the Cretaceous/Tertiary boundary in Denmark } Department of Geological Sciences, Case Western Reserve University, Cleveland, Ohio 44106 JAMES L. ARONSON T HUGH T. MILLARD, JR. U.S. Geological Survey, M.S. 424, Denver Federal Center, Denver, Colorado 80225 ELIZABETH GIERLOWSKI-KORDESCH Freie Universität Berlin, Institut fir Palaeontologie, Schwendener Str. 8, D-1000 Berlin 33, West Germany ABSTRACT We cannot reject the impact hypothesis chemically to the somewhat variable chemistry summarily, because shocked quartz with mul- of the microtektite-like spherules in the Fish The distinctive Mg-smectite that is the tiple planar sets is distinct to the K/T bound- Clay, which were taken to represent unaltered predominant clay mineral in the Cretaceous/ ary, and this distinctive shocked quartz has ejecta (Varekamp and Thomas, 1982). Kastner Tertiary (K/T) boundary marl at Stevns Klint, not yet been identified in distal volcanic teph- and others (1984) concluded that this unusual Denmark (that is, the Fish Clay), which has ras or flows. If shocked quartz is of impact Mg-smectite originated as a direct isochemical been previously believed to have formed from origin, then the collision of either a smaller alteration of impact ejecta. If true, then this dis- impact-derived glass, is also present in the asteroid, or a comet, is more consistent with tinctive Mg-smectite would be expected to be a upper of two marls of probable early Danian the relatively small amount of impact-derived significant component in smectite-dominant age at Limhamn Quarry, Sweden, approxi- material in the Fish Clay. Alternatively, if K/T-boundary marls close to Stevns Klint, such mately 8 m above the K/T boundary. This shocked quartz is proven to be formed by as Nye Kl0v and Kj<alby Gaard, and it would be Danian marl is devoid of anomalous iridium. explosive volcanism, then our evidence would expected to be absent in the Danian marls in We contend that this Mg-smectite is the result permit explaining the K/T boundary events as stratigraphic proximity to the K/T boundary. of alteration of a volcanic ash precursor, per- entirely due to volcanism rather than impact. We tested this contention by examining the haps reworked, and that impact debris is a clay fractions from the K/T boundaries and from relatively minor component of the K/T bound- INTRODUCTION Tertiary (Danian) marls enclosed in the chalk ary marls studied herein. K-Ar dates demon- surrounding the K/T boundary at various sec- strate that Caledonian detritai potassic phases After the initial paper by Alvarez and others tions in Denmark and Sweden. Our results sug- (that is, illite) are present in the coarse clay (1980), many studies proposed, or implied, that gest that the Mg-smectite that is the predominant fraction of the Fish Clay. The Fish Clay is thus the Cretaceous/Tertiary (K/T) boundary clay mineral in these K/T boundary clay units is not composed predominantly of volcanic, detritai layer (that is, the boundary clay unit) was derived unique to the K/T boundary. We contend that phases and lesser amounts of impact phases. from impact, based on the anomalous concentra- the Mg-smectite was not derived by bolide In addition, we have discovered subhedral tions of noble metals, and it has been "treated by impact, but rather by alteration of volcanic ash. labradorite in the organic-rich black clay of supporters of the Alvarez hypothesis as direct layer III of the Fish Clay. Therefore, we sug- evidence of the fallout of material of the earth LOCALITIES STUDIED gest that pyroclastic basaltic volcanism was embracing cloud produced by the impact event" the predominant progenitor of the boundary (Hallam, 1987, p. 1238). Hypothetical^, the The K/T boundary layer in the Danish basin clay unit and is responsible for its high Mg boundary clay units could contain detritai, vol- is typically a marl (that is, a clay-dominant content. canic, and impact-derived phases. Kastner and lithology with some calcium carbonate) that is Our results indicate that the main carrier of others (1984) took one of the first steps toward underlain by thick accumulations (=900 m) of Ir in the K/T boundary marls is of submicron identifying a predominant impact-derived phase Cretaceous (Maastrichtian) chalk and overlain size and that Ir is mobile during diagenesis within the type K/T-boundary clay at Stevns by Tertiary (Danian) limestone and chalk. Here- and/or weathering. In addition, our results Klint, Denmark, known as the "Fish Clay," by in, we compare the chemical and mineralogic show that Pt and Au are present in the upper analyzing the carbonate-free <2 |um fraction of compositions of the clay minerals from the K/T Limhamn marl, and, for comparison, in the samples from within and immediately surround- boundary marls to those from the lesser known Permian Kupferschiefer Shale at amounts ing the Fish Clay. As was shown in Kastner and marls stratigraphically above the boundary. The comparable to those of the K/T boundary. It others' study as well as in previous studies (Chris- analytical methods are described in Appendix 1. tensen and others, 1973; Rampino and Reynolds, is thus possible that some or much of the Pt The classic and perhaps the best known 1983), smectite is the predominant clay mineral and Au at the K/T boundary is derived from K/T-boundary marl in Scandinavia, the Fish in the Fish Clay, and Kastner and others (1984) nonimpact sources similar to those that Clay (10-15 cm thick), is exposed along the cliffs asserted that its major-element composition is apparently contributed to the parts-per-billion at Stevns Klint, Denmark. The sections at Lim- unique and unlike that of smectite in the litera- concentrations of Pt and Au in the Permian hamn Quarry, Stevns Klint, Nye Kl0v, Kjdlby ture. It is rich in magnesium, and it is similar Kupferschiefer Shale. Gaard, and the Danian Quarry in Denmark and Geological Society of America Bulletin, v. 101, p. 702-710, 3 figs., 4 tables, May 1989. 702 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/5/702/3380692/i0016-7606-101-5-702.pdf by guest on 24 September 2021 ORIGIN OF CLAY MINERALS AT CRETACEOUS/TERTIARY BOUNDARY, DENMARK 703 Sweden are described in various papers (see The occurrence of these marls within this lime- which are shown in Figure 1. The actual K/T Birkelund and Bromley, 1979). K/T boundary stone, and its elevation at approximately -50 m, boundary marl is layers II-V, layer I being the marls were collected at Stevns Klint, Nye Kl0v, permits us to correlate with either the lower 8 m uppermost Maastrichtian chalk and layer VI and Kjolby Gaard. A Danian marl was collected of Danian limestone or with the lower of the two being the lowermost Danian chalk and lime- 10 m above the K/T boundary at the Dania middle Danian mound sequences. Although Hol- stone. Layer II is a light gray marl, <0.5 cm thick. Quarry near Manager, Jutland. land and Gabrielson noted the presence of two Layer III is composed of two units: a basal thin In addition to the marl collected from Dania, distinct marls, 1 and 2 m below the K/T bound- (0.5 cm thick) red-orange clay containing coarse two non-K/T boundary marls (=5 cm thick) ary at Limhamn, which would be close to the spheroidal pyrite overlain by fissile black clay were collected from the north wall at the -60 m quarry floor (-60 m along the west wall), it is without obvious continuous lamination. We did level from Limhamn Quarry, Limhamn, Swe- unlikely that our marls are the Maastrichtian not analyze the red-orange clay in this study. den. The stratigraphy at Limhamn has been de- marls, on the basis of elevation and facies associa- Hansen and others (1986, 1988) described this scribed recently by Holland and Gabrielson tion. We therefore call the two marls collected layer as consisting of filled burrows. Shocked (1979). The K/T boundary at Limhamn is a the "lower" and "upper" marls, respectively, of quartz has been found within the thin (1-3 mm), hardground, and the Maastrichtian is almost probable early Danian age. We note that two rust-colored, spheroidal-pyrite layer at the base completely under water; only the uppermost 3 m marls are present in the eastern wall approxi- of layer III (B. F. Bohor and G. A. Izett, 1988, are exposed presently along the west wall, mately 200 m east of the sampled location in a written commun.) for which reason Bohor and according to Holland and Gabrielson. We were similar stratigraphic sequence, but we did not Izett have referred to this rust-colored layer as the confined to collecting from the eastern half of the check the correlation between these two "impact layer." Ir increases 50 times from layer II quarry. The marls we collected are enclosed in exposures. through layer III (Kastner and others, 1984). chalky bioclastic micritic limestone, which con- Bulk samples of the K/T boundary marls were Apparently, layer II was deposited before tains fragments of branching bryozoans and cor- collected from Nye Kiev (2-3 cm thick) and shocked quartz. We place the boundary between als. The bedding is discontinuous and displays a Kjelby Gaard (3-5 cm thick), and bulk samples layers III and IV at the first upward appearance complex interfingering geometry. On a macro- of the Danian marls were collected from the of obvious and continuous lamination in layer scopic scale, these beds appear to be original Limhamn Quarry (<5 cm thick) and from the IV; the color of layer IV changes upward contin- depositional slopes that dip gently as much as Dania Quarry (3-4 cm thick).