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T boundary in shallow water carbonates, Denmark
Palaeogeography, Palaeoclimatology, Palaeoecology 154 (1999) 67–85
Benthic extinction and recovery patterns at the K=T boundary in shallow water carbonates, Denmark Eckart Ha˚kansson a,Ł , Erik Thomsen b a
Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen, Denmark b Geological Institute, Aarhus University, Universitetsparken, DK-8000 Aarhus, Denmark Received 30 August 1997; revised version received 10 January 1998; accepted 19 August 1998
Abstract Benthic extinction at the K=T boundary in the Danish Basin is abrupt and indistinguishable from the termination of Maastrichtian White Chalk deposition. The Danian benthic fauna — already fully established in the earliest Danian nannoplankton zone NP1 — is essentially an impoverished Maastrichtian fauna established through direct survival or limited evolution within well-established clades, already present in the Maastrichtian of the Danish Basin. However, recovery of the early Danian community is initially retarded. The transition is illustrated in some detail for the locality, Nye Kløv. Here the lower 2–3 m of Danian strata (corresponding largely to the lower NP1 subzone) contains an extremely impoverished, highly unusual, ‘dead zone’ community dominated by bourgueticrinid crinoids associated with other presumed soft ground specialists and devoid of such important faunal elements as cyclostome bryozoans, brachiopods, calcitic bivalves, etc. Over the next metres the ‘dead zone’ fauna is gradually replaced by more ordinary faunas, and about 6 m above the boundary the characteristic early Danian bryozoan limestone community is fully established. 1999 Elsevier Science B.V. All rights reserved. Keywords: K=T boundary; extinction; recovery; benthos; bryozoa
1. Introduction Denmark boasts a large concentration of complete exposures spanning the Cretaceous=Tertiary boundary. The boundary lies within a marine realm of continuous accumulation below and above the boundary marl, and there is no demonstrable hiatus at the boundary. Uniquely in Denmark, the exposed successions contain very rich, highly diverse autochthonous faunas of benthic invertebrates (Birkelund and Ha˚kansson, 1982), with a combined Ł Corresponding
author. Fax: C45-3532-2499; E-mail: [email protected]
Maastrichtian and Danian diversity of well over 1000 species. Other K=T boundary sections providing a continuous accumulation experience either a faunal depletion towards the boundary, e.g. Zumaia, Spain (Ward et al., 1991), Brazos River, Texas (Hansen et al., 1987, 1993), or allochthonous, fragmented faunas distort the record in part of the succession (e.g. Brazos River, Texas). Two Danish localities stand out as particularly intensely studied: Stevns Klint, the classic type locality for the Maastrichtian=Danian boundary (Desor, 1847), has been in focus for more than a century, while the potential of the much more modest profile exposed at Nye Kløv (Nye Klov of authors) was only realized quite recently (Ha˚kansson
0031-0182/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 0 8 7 - 5
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and Thomsen, 1979; Ha˚kansson and Hansen, 1979). The subject of this paper is concerned primarily with the extinction and recovery patterns at Nye Kløv.
2. Regional geology A pronounced low-stand in global sea level occurred during the late Maastrichtian–early Danian interval of time. In the Danish region this major regressive pulse is evidenced from rapid reduction in the area of White Chalk accumulation, with associated large-scale shifts in facies boundaries. Thus, from a position in central Europe during the early Maastrichtian the southern limit of the White Chalk facies retreated to northernmost Germany by the middle-late Maastrichtian, and by the Cretaceous– Tertiary transition marine deposition was restricted to a narrow sound between the Ringkøbing–Fyn High and the Fennoscandian Shield (Fig. 1). During the early Danian this distribution expanded only slowly, and the areas south of the Ringkøbing–Fyn
High were not marine again until the late Danian (Ha˚kansson and Thomsen, 1979). The narrow marine sound crossing the Danish region during the K=T boundary turnover connected the main oceanic water masses through what were conceivably equally narrow and shallow seaways. To the northwest it was connected to the initial North Atlantic, and to the southeast across Europe via Crimea to the Tethyan realm. The K=T boundary is at present exposed in nine outcrops along a narrow belt across Denmark (Fig. 2), with Stevns Klint located at the southeastern termination of this belt, and Nye Kløv together with three other small exposures in the salt-influenced Thisted Dome at the northwestern termination. Except for a 2–35 cm thick clay-rich layer at the boundary (the ‘Fish Clay’), the Maastrichtian and Danian sediments in the narrow Danish seaway are consistently carbonates deposited in a moderately warm climate. Two main facies types dominate, a pelagic facies composed mainly of calcareous nannofossils and planktonic foraminifera, and a benthic
Fig. 1. Palaeogeography of the Danish region at the time of the K=T boundary (based on Ha˚kansson et al., 1974, and Thomsen, 1995).
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Fig. 2. Geological map of Denmark (based on Ha˚kansson and Pedersen, 1992) with the location of presently accessible K=T boundary sections.
facies dominated by bryozoans set in a matrix also dominated by pelagic organisms. The two main facies types are traditionally termed chalk and bryozoan limestone, respectively; however, across the basin a number of intermediate facies types may be recognized (Bromley, 1979). In the bryozoan limestone fragments of bryozoan colonies and other benthos having calcitic skeletons typically make up more than 50% of the sediment >125 µm (Thomsen, 1976), whereas in chalk these constituents only rarely amount to more than 5% (Ha˚kansson et al., 1974). The Maastrichtian deposits are consistently pelagic White Chalk in which bryozoans may be abundant locally in the shallower parts of the sea (Ha˚kansson et al., 1974; Thomsen, 1974; Larsen and Ha˚kansson, 1998). The Danian deposits are characterized by interchanging benthic and pelagic facies. The pelagic facies dominates in the deeper parts of the depositional system towards the northwest, while benthic facies dominate in the shallower parts towards the southeast. At Nye Kløv the K=T boundary is set exclusively in chalky facies, with
typical, pelagic White Chalk below the boundary and a (?)chemically precipitated ‘chalk’ above; bryozoan limestone only appears several metres above the boundary. As a result of the insignificant overall cementation found in the Danish on-shore carbonates, most Maastrichtian and Danian sediments are easily disintegrated, allowing quantitative studies of the faunas to be made from bulk samples. In general the aragonitic constituents of the faunas have been lost through early diagenetic dissolution. However, their presence is abundantly documented through locally preserved moulds and casts, particularly in the more shallow deposits in the eastern part of the basin, where repetitive episodes of seafloor cementation have led to the formation of hardground levels. Some of the finest hardgrounds are exposed in the Stevns Klint succession, which therefore offers a unique opportunity to investigate the survival and recovery patterns of the aragonitic benthos (Heinberg, 1999). However, the cemented limestone horizons at Stevns Klint are difficult to disintegrate and, in contrast
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to the relatively soft limestones spanning the K=T boundary in the section at Nye Kløv, they cannot be studied quantitatively from bulk samples.
3. The Nye Kløv profile The Nye Kløv locality is situated in a steep palaeocliff on the eastern flank of the Thisted Dome, northwest Jylland (Fig. 2). The boundary succession at Nye Kløv compares closely with that of nearby Kjølby Gaard (Troelsen, 1955; Ha˚kansson and
Hansen, 1979) but exhibits, in addition, a more complete transition into the overlying bryozoan limestone. The exposed succession is about 19 m thick including 7 m of Maastrichtian White Chalk and 12 m of Danian chalks and limestones (Fig. 3) (Ha˚kansson and Hansen, 1979; Ha˚kansson and Thomsen, 1979; Thomsen, 1981; Birkelund and Ha˚kansson, 1982; Surlyk and Johansen, 1984; Ekdale and Bromley, 1984; Johansen, 1987; Keller et al., 1993). Generally, the Maastrichtian=Danian boundary is exposed over a horizontal distance of 30–40 m. The lowermost 5 m of the Maastrichtian is presently covered by talus.
Fig. 3. Stratigraphy and sample locations in the Nye Kløv succession.
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The Maastrichtian part of the sequence consists of poorly lithified White Chalk with a few flint nodule levels. The chalk is highly bioturbated (Ekdale and Bromley, 1991) and bedding planes are normally indistinguishable. It contains about 1% by weight of skeletal material >0.5 mm, with an overall dominance of bryozoans. Aragonitic skeletons are generally not preserved, but moulds of Baculites and Scaphites occur. The Maastrichtian=Danian boundary is marked by a 2–3 cm thick, dark grey clay layer with a thin rusty colouring at the base. The clay, which is sharply delimited downwards, is characterized by a marked increase in iridium content in the lower part (Hansen et al., 1986). Upwards the boundary clay rapidly changes into a 4–5 m thick succession of poorly fossiliferous, whitish grey ‘chalk’ presumed to be dominated by chemically precipitated calcite micrite (Hansen, 1990), at least in the lower part. From about 5 m above the boundary macrofossils, again dominated by bryozoans, gradually become more abundant and between 6 and 8 m bryozoans (>500 µm) make up nearly 10% of the deposits, and this interval thus constitutes a meagre bryozoan limestone. From this level the content of bryozoans decreases, and benthic constituents make up less than
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2% of the sediment in the uppermost, chalky part of the section. On the whole, the Danian succession is only slightly lithified. In the bryozoan limestone proper bioturbation is moderate, and indistinct bedding planes may be seen. Several omission surfaces are found in the bryozoan limestone from which Thallasinoides burrow systems can be traced up to 50 cm into the sediment below. Flint layers and flint nodules are common in the Danian deposits. Compared to the ‘typical’ situation further eastwards in the Danish basin both the Maastrichtian White Chalk and the Danian bryozoan limestone at Nye Kløv have only a fairly moderate content of benthos, both in terms of density and diversity. 3.1. Stratigraphy Investigations of calcareous nannofossils indicate continuous deposition across the boundary with a relatively thick early Danian succession (Fig. 3), which is in keeping with the foraminifera data presented by Keller et al. (1993). The boundary between the NP1 and NP2 zones in the zonation scheme of Martini (1971) is located approximately 8.5 m above the boundary as indicated by the first occurrence
Fig. 4. Comparative lithology and stratigraphy of Nye Kløv and Stevns Klint.
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of Cruciplacolitus intermedius. In the Danish region Perch-Nielsen (1979) subdivided the NP1 zone into two subzones D1 and D2 on the basis of the first occurrence of Placozygus sigmoides. In the Nye Kløv section this species first appears in a sample 270– 280 cm above the boundary. Perch-Nielsen (1979) also subdivided NP2 into two subzones, D3 and D4, with the base of D4 indicated by the first occurrence of Prinsius dimorphosus. In the Nye Kløv section P. dimorphosus appears simultaneously with C. intermedius in a sample ca. 8.5 m above the boundary. Zone D3 therefore is apparently absent. This hiatus conceivably coincides with a marked omission surface, and it is, furthermore, associated with a significant decrease in the content of macrofossils. All Maastrichtian samples contain abundant Nephrolithus frequens and are referred to the uppermost Cretaceous zone CC 26 in the zonation scheme of Sissingh (1977). Compared to the classic section at Stevns Klint (Fig. 4) it appears from nannofossil investigations that (1) both successions are continuous across the boundary, although they differ significantly in accumulation rates, and (2) the most prominent hiati in the two successions (located in the lower part of D2 in Stevns Klint and at the D2=D4 boundary in Nye Kløv) are not correlatable. It should be stressed,
however, that none of these hiati (Fig. 4) corresponds to the hiatus claimed to be present in the very earliest Danian at both Stevns Klint (Schmitz et al., 1992) and Nye Kløv (Keller et al., 1993). The existence of such a regional hiatus has been promoted in a massive array of papers by Keller and Macleod (see e.g. the reference list in Macleod et al., 1997). But, in our opinion, neither the absence in Nye Kløv of foraminifera of particular importance to the stratigraphic framework in the low latitude succession at El Kef, Tunesia (Keller et al., 1993), nor the total absence of planktonic foraminifera in part of the Stevns Klint succession (Schmitz et al., 1992) is sufficient for the invention of a hiatus not supported by any lithological data.
4. Material and methods The present investigation is based on 18 bulk samples covering the uppermost 2 m of the Maastrichtian succession and the whole of the Danian (Fig. 3). Sample spacing is closest in the interval immediately below and above the boundary. The samples, each weighing from 1 kg in the bryozoan limestone to 5 kg in the poorly fossiliferous chalks, were disintegrated through repetitive freezing and
Fig. 5. General faunal composition at Nye Kløv (part of this set of data previously published by Birkelund and Ha˚kansson, 1982).
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Fig. 6. Distribution of cheilostome bryozoans at Nye Kløv (a few clearly recognizable reworked specimens of Maastrichtian species found in the lowermost few centimetres of Danian strata are omitted). Dots indicate recorded presence, solid lines connect identical species group taxa, dashed lines connect closely related species group taxa.
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thawing in a saturated solution of sodium sulphate. The quantitative analyses are based on washed sample residues in the size fraction >500 µm. In the White Chalk and the bryozoan limestone only split samples were analysed. The fossils were sorted into main animal groups and weighed as a measure of their relative importance. The primary emphasis in this study is on cheilostome bryozoans, and this group was sorted further into species, except for the uppermost three samples representing zone D4. The relative importance of individual species in a sample was determined by their specimen count. The unsorted sample residues were finally examined for rare species. The highly unusual combination of very low density and extremely high diversity characterising the bryozoan fauna from most Danish–Maastrichtian level bottom chalk (E. Ha˚kansson, unpubl. data; Ha˚kansson and Thomsen, 1979) implies that many species are very rare. They may be present in one or a few samples and then absent in several others taken nearby, with seemingly random and unpredictable distribution. The many rare species in combination with the low density of the fauna makes it practically impossible in a reasonably sized sample to obtain even the majority of species present at a certain stratigraphic level. Whereas this problem may not be impeding when working at a large-scale level, it becomes eminently pernicious when approaching the level necessary to investigate, for instance, the nature of the collapse of the Maastrichtian White Chalk community towards the K=T boundary. In the case of Nye Kløv, where the entire Maastrichtian succession exposed belongs to the upper part of the topmost Maastrichtian brachiopod zone, we find that the absence of species in individual samples is entirely random, and that all chalk exposed does in fact represent a single, fundamental White Chalk community. We therefore refrain from further differentiation between the individual Maastrichtian samples in this presentation.
5. Results The invertebrate fauna in the washed residues >500 µm from Nye Kløv (Fig. 5) contain octocorals, cheilostome and cyclostome bryozoans, bra-
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chiopods, calcitic bivalves, serpulids, ostracods, and various groups of echinoderms including regular and irregular echinoids, crinoids, asteroids and ophiuroids. Aragonitic forms are not preserved and ammonites are only represented by a few aptychi. In general terms the invertebrate fauna can be characterized as rich to very rich, except for the lowermost 2–3 m of the Danian interval, where an extremely impoverished fauna characterize an initial Danian ‘dead zone’. A similar pattern can be seen in all K=T boundary sections in Denmark (Ødum, 1926), and the development at Nye Kløv can be considered as representative. It is particularly noteworthy that important members of the Dano– Maastrichtian benthic faunas such as brachiopods, calcitic bivalves, serpulids and cyclostome bryozoans, are totally absent from this lowermost Danian ‘dead zone’ and that otherwise fairly subordinate groups, such as crinoids and echinoids, suddenly become very prominent (Fig. 5). Only cheilostome bryozoans are present in all samples, although in strongly varying densities and diversities. We shall discuss below the development of three groups in more detail, namely the cheilostome bryozoans, the brachiopods and the crinoids. These groups were selected because they are important in the Nye Kløv succession and because they clearly react differently to the K=T boundary event. Of the three groups, only the data concerning the brachiopods have previously been published in extenso (Johansen, 1987). 5.1. Bryozoans Bryozoans constitute by far the most conspicuous part of the preserved fauna in the Danian– Maastrichtian sediments in the Danish Basin as, indeed, in much of the North Sea region. As a general rule the cheilostome part of the fauna dominate in terms of diversity, while the cyclostome bryozoans commonly constitute a significant proportion of the bulk density, and in the Danian cyclostomes may even be more abundant than cheilostomes (Berthelsen, 1962). Unfortunately the current systematic state of the cyclostome bryozoans essentially prevents investigation of this important part of the fauna at lower taxonomic levels, hence cyclostomes are here considered merely as a group. At Nye Kløv, bryozoans >500 µm constitute be-
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tween 0.0001 and 9% of the bulk sediment, with a very prominent density peak in the top of the D2 zone and an equally prominent density low in the basal Danian (Fig. 5). Among the cheilostome bryozoans a total of 66 genus level and 203 species level taxa have been identified from Nye Kløv (Fig. 6). The Maastrichtian fauna presently includes 138 species (with 4 species identified only from the Danian of Nye Kløv, but known from Maastrichtian sediments elsewhere in the Danish Basin), whereas the Danian fauna includes 83 species (with 2 species identified only from the Maastrichtian of Nye Kløv, but known from Danian sediments elsewhere in the Danish Basin). Further, 19 of the Maastrichtian species known from Nye Kløv survived the K=T boundary turnover directly (13 of which have thus far been found both below and above the boundary in Nye Kløv), and although an additional 11 Maastrichtian species are found to have very close relations across the boundary (e.g. ‘Membranipora’ pauparata=‘M.’ n. sp. aff. pauparata), the cheilostome bryozoan fauna evidently suffered very severely. Thus, the direct survival rate can be calculated to just below 15%. At the genus level, however, 34 taxa among the 57 found in the Maastrichtian at Nye Kløv are also present in the Danian samples, and when the remaining part of the basin is taken into consideration, all of the genera presently known only from the Danian at Nye Kløv have Maastrichtian representatives elsewhere in the basin. In other words, in the Nye Kløv section no new cheilostome genera appear in the Danian. So even though such figures are not entirely representative for the situation in the whole Danish Basin, it is obvious that the Danian fauna — albeit in itself very rich — should be regarded merely an impoverished version of the extremely rich Maastrichtian fauna. Termination of the Maastrichtian bryozoan fauna at Nye Kløv is abrupt, but within the resolution possible it is indistinguishable from the termination of the White Chalk facies. Accordingly, in terms of ecological groups, it appears that encrusting, rigidly erect and free-living species were affected differently. Thus, of the 18 Maastrichtian free-living species, among the most obvious White Chalk specialists (cf. Thomsen and Ha˚kansson, 1995), none survived the K=T boundary, whereas survival in both other groups amount to just below 20%.
5.2. Brachiopods Danian–Maastrichtian brachiopod faunas in the Danish Basin are characterized by a dominance of minute forms adapted to a life on bryozoan skeletons and by secondarily free-living, apedunculate forms adapted to soft bottom conditions (Surlyk, 1972; Surlyk and Birkelund, 1977). Johansen (1987, 1988) has investigated the fauna from Nye Kløv in detail, and the distribution chart presented here (Fig. 7) is based entirely on her data, reorganized to facilitate comparison with the bryozoan data presented above (Fig. 6). Note that in spite of the very peculiar, and in part simply incorrect resume´ published by Macleod et al. (1997, p. 277), the brachiopod fauna from Nye Kløv is indeed very well documented. Most conspicuous is the total absence of brachiopods from the lower 3 m of the Danian and a ‘typical’ Danian fauna is not developed until 6 m into the Danian succession. Of the 46 species group taxa recorded at Nye Kløv, only 6 are found in both Maastrichtian and Danian sediments, whereas another 7 Maastrichtian species have very close relations across the boundary, so evidently the brachiopod fauna was very strongly affected in the K=T turnover and, like the cheilostome bryozoans, their disappearance is abrupt (Surlyk and Johansen, 1984; Johansen, 1987, 1988). A total of 16 genus-level clades have been found in the Maastrichtian part of the succession at Nye Kløv. Of these, 6 become extinct at the K=T boundary, and another 6 have disappeared by the end of the Danian; only 3 new genera appear in the Danian. As a general conclusion it therefore appears that the Danian brachiopod fauna is essentially a slightly impoverished continuation of the Maastrichtian fauna, with pervasive species replacement within well-established clades. However, it should be noted that in terms of ecological groups, some of the most notable White Chalk specialists (hemispherical, apedunculate species, cf. Surlyk, 1972) are among the prominent K=T boundary casualties (Johansen, 1987). 5.3. Echinoderms Echinoids, crinoids, ophiuroids and asteroids are present in most Danian–Maastrichtian environments in the Danish region (Rasmussen, 1950, 1961, 1972).
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Fig. 7. Brachiopod species distribution at Nye Kløv (all data extracted from Johansen, 1987; a few clearly recognizable reworked specimens of Maastrichtian species found in the lowermost Danian strata are omitted). Signatures as in Fig. 6.
In most sediments they play a fairly subordinate role, with only the holasteroid echinoid Echinocorys being locally conspicuous. It is therefore of exceptional interest that echinoderms are totally dominant in the lowermost few metres of Danian strata at Nye Kløv (Fig. 5). Crinoids of the order Bourgueticrinidae, in particular, experience a dramatic bloom in the lowermost 2 m of the Danian succession, where they may account for as much as 80% by weight of the benthic fauna (Fig. 5). Bourgueticrinids are rare in the uppermost Maastrichtian White Chalk, with only a few calices and columnals retrieved from more
than 20 kg of chalk examined from Nye Kløv (Kjaer and Thomsen, 1999). In marked contrast, the lowermost Danian ‘chalk’, sample D1: 3–13 cm above the K=T boundary, has yielded 42 calices and approximately 700 columnals per kg sample, whereas higher in the Danian succession bourgueticrinids again become subordinate, although they never return to the low levels of the Maastrichtian. Associated with the bourgueticrinid bloom is a quite diverse echinoid fauna composed of disintegrated cidaroids and Aerosoma as well as unidentifiable spatangoid fragments. It deserves mentioning that a prominent mem-
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Fig. 8. Bourgueticrinid species distribution around the K=T boundary at Nye Kløv (based on Kjaer, 1992, and Kjaer and Thomsen, 1999).
ber of the initial Danian bourgueticrinid fauna is Democrinus maximus (Kjaer and Thomsen, 1999). The entirely Cainozoic genus Democrinus evolved from Bourgueticrinus through paedomorphosis simply by the loss of the proximal plate in the calyx (Nielsen, 1913; Kjaer and Thomsen, 1999; Fig. 8), and the Nye Kløv species, which is abundant already in the lowermost Danian sample, is thus the earliest known member of the Democrinus clade. The proximal plate is normally formed during early ontogeny by incorporation of a columnal element into the calyx, and once the proximale is formed the addition of new columnals is terminated. Elimination of the proximale in Democrinus therefore, in principle, allows columnal elements to be added throughout life (Kjaer and Thomsen, 1999). This novel potential may reflect selection for a longer stem in a food-depleted environment.
6. The ‘dead zone’ — bryozoan limestone faunal succession
particular limestone interval (it ‘sounds dead when hit with a hammer’), without any reference to the fauna. The faunal condition in the lowermost few metres of the Danian at Nye Kløv is found to be in good accord with the situation at Voxlev. Indeed, the situation with a ‘dead zone’ interval of faunal paucity combined with a highly unusual faunal composition in the initial strata of the Danian subsequent to the boundary clay is valid for boundary sections all along the Danish Basin. Also the petrologic peculiarity of the ‘dead zone’ has a basin wide distribution, probably correlated directly with the occurrence of chemically precipitated micrite. However, only at Nye Kløv the subsequent zone of recovery is preserved in its entirety. Hence the significance of listing the Nye Kløv faunal succession in some detail, since this succession — sample by sample — constitutes the recovery pattern, from absolute ‘nothing’ in the boundary clay to a full-scale bryozoan limestone fauna, all within the basal Danian nannoplankton zone (Figs. 5 and 9). 6.1. ‘Dead zone’ — samples D1–D5
The ‘dead zone’ concept has its roots in the locality Voxlev in eastern Jylland, from where Jessen and Ødum (1923) described a very unusual, highly impoverished fauna in a 2 m interval immediately superseding the boundary clay. However, in spite of a very clear understanding of the faunal peculiarities of their ‘dead layer’, it is an interesting fact that Jessen and Ødum (1923), in their original designation of this phrase, refer solely to the peculiar petrology of this
Sample D1. Bourgueticrinus and Democrinus are overwhelmingly dominant, and together with Aerosoma, spatangoid echinoids and a single species of cheilostome bryozoan, Pavolunulites n. sp. (Fig. 10), constitute the entire skeletonized fauna. Sample D2. Similar to D1, Bourgueticrinus and Democrinus reach their peak dominance, Pavolunulites n. sp. remains dominant among the bryo-
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Fig. 9. Distribution and proportion of the earliest Danian cheilostome bryozoans at Nye Kløv.
zoans, but at this level two more cheilostome species are present, Lekytoglena ampullacea and Danocella hakanssoni (Fig. 10). Samples D3 and D4. Similar to D2; the first sporadic occurrences of additional benthos, serpulids and sponges. Sample D5. Similar to D4; the first Danian representatives of the Pachytecella filiformis=lundgreni complex appear together with the first cyclostome bryozoans. 6.2. Recovery zone — samples D6–D9 Samples D6 and D7. Pronounced changes; most of the pioneer ‘dead zone’ fauna drop to their ordinary Danian level of importance; overall dominance is taken over by echinoids, while bryozoan diversity is increasing; brachiopods and calcitic bivalves reappear. Samples D8 and D9. Bryozoans dominate for the first time in the Danian, with further increase in cheilostome diversity.
6.3. Bryozoan limestone — samples D10 and D11 Samples D10 and D11. Full-scale bryozoan limestone fauna, with cheilostome diversity in excess of 60 species. It should be noted that trace fossils even in the very basal Danian strata constitute convincing evidence that an infauna including unmineralized biota was active at this time (Ekdale and Bromley, 1974), although the ichnodiversity was reduced to no more than 3–4 ichnospecies (R. Bromley, pers. commun., 1997).
7. Discussion and conclusions Our investigations have confirmed previous findings that the benthic communities of the northwestern European Maastrichtian White Chalk thrived and flourished right up to the K=T boundary (e.g. Ha˚kansson and Thomsen, 1979; Birkelund and Ha˚kansson, 1982; Surlyk and Johansen, 1984; Jo-
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Fig. 10. ‘Dead zone’ cheilostome bryozoans from Nye Kløv. All magnifications ð25. 1 D Pachythecella filiformis (sample D5). 2 D Lekythoglena ampullacea (sample D2). 3 D Pavolunulites n. sp. (sample D1). 4, 5 D Danocella hakanssoni (sample D2).
hansen, 1987). For unknown reasons Macleod et al. (1997) in their broad account of the Cretaceous– Tertiary biotic transition claim that bryozoans, the dominant group in all these communities, experienced a long-term decline prior to the K=T boundary (stated to be the ‘latest Maastrichtian’ in the abstract and ‘throughout the Upper Cretaceous’ in the discussion, but not mentioned at all in the bryozoan chapter). If anything, the bryozoan faunas of both basinal and coastal facies in northwestern Europe experienced an increasing trend in the late Maastrichtian. At the K=T boundary most major benthic groups investigated from the White Chalk environment, most notably bryozoans, brachiopods (Surlyk and Johansen, 1984; Johansen, 1987), and bivalves (Heinberg, 1999), were subject to very substantial extinctions. In all groups the extinction is taxonomically neutral although, in bryozoans and brachiopods, with a preference for the highly adapted, free-living chalk
specialists. As a result, the fully recovered early Danian fauna in the Danish Basin is a somewhat impoverished, within-basin continuation of the Maastrichtian fauna. When, for some reason, a depositional facies disappears, it is a textbook fact, that the fauna specifically adapted to this particular facies will also disappear. So when the unique pelagic White Chalk facies, which had a continuous history of ca. 25 Ma in northwestern Europe, was finally terminated by the abrupt deposition of the ‘Fish Clay’ and its equivalents, it can come as no surprise that the highly adapted chalk benthos disappeared with it. In keeping with this, the faunal collapse at the close of the Cretaceous Period in this region, by the finest resolution possible, remains inseparable from the termination of White Chalk deposition. However, only part of the fauna shows up again (to a large extent evolved into new, closely related species), when seemingly comparable sediment types reappear af-
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ter the very brief boundary clay interval. Indeed, while the planktonic biotic system in the Danish Basin, according to e.g. Perch-Nielsen et al. (1982) and Keller et al. (1993), was essentially continuous across the K=T boundary, to be gradually replaced by Tertiary biota above the boundary, the benthic system characterizing the Maastrichtian chalk sea never reappeared. This is in stark contrast to the near-instant return of a full-scale chalk benthic system seen after earlier marly intervals in the Danish Basin, such as the late Maastrichtian Kjølby Gaard Marl. In the benthic communities the K=T boundary recovery in the Danish Basin differs from these previous chalk disruptions in three important aspects: (1) it is notably retarded, (2) it leads to a somewhat less complex benthic community system, (3) it terminates particular ecological groups, and (4) it includes extensive species replacement. One obvious reason for the delay is the peculiar composition of the basal Danian ‘chalk’, with a notable content of euhedral to subhedral calcite crystallites, which are likely to be chemical precipitates (Hansen, 1990). Hence recovery at this time was arguably delayed due to highly adverse conditions through most or all of zone D1. By the same token, the very nature of the earliest Danian seafloor may have been considerably more soupy than ordinary chalk, due to a high proportion of abiological grains (precipitated crystallites and reworked coccoliths), which did not reach the seafloor in the form of faecal pellets. The extreme benthic depletion characterizing the first few metres of Danian sediments at Nye Kløv has been recorded at all Danish K=T boundary sections. However, since the incoming (early) Danian taxa are ecological specialists with demonstrable late Maastrichtian predecessors within the Danish Basin, isolated pockets of less adverse conditions must have been available through the ‘dead zone’ interval. So the proverbial refuge exists, and in all probability located in close connection to the Danish Basin. In conclusion, therefore, the recovery pattern found for the benthos in the Danish region should be characterized as predominantly migrational rather than evolutionary, with a pronounced local signature. When discussing the Cretaceous=Tertiary boundary on a global scale, it is important to distinguish between the two potentially unrelated elements: (1) the over-all transition from a Mesozoic to a Cain-
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ozoic biosphere, and (2) the existence of a specific boundary ‘event’. As stated above the dramatic collapse of the Maastrichtian benthic community in the Danish Basin is inseparable from the termination of the Maastrichtian depositional regime but, at the same time, it is also coincident with the deposition of the Ir-bearing boundary clay. Our data alone, therefore, do not allow the above-mentioned distinction, since they are obviously part of (1) and entirely compatible with (2). When comparing the extinction and recovery patterns for the bryozoans and brachiopods in the Danish Basin to the patterns for other benthic and nektonic groups, such as rudists and inoceramid bivalves (Kauffman and Johnson, 1996) and ammonites (Ward, 1995), as well as the globally more uniform developments found for the various planktonic groups (see e.g. papers in Macleod and Keller, 1996), it becomes apparent that no single pattern can cover all groups. In fact, the more we learn about the faunal particulars surrounding the K=T boundary discussion, the more complex is the scenario needed to include all observations, and the more prominent does the prospect of a multicausal complex of events become (e.g. Birkelund and Ha˚kansson, 1982; Ward, 1995; Macleod et al., 1997). Curiously, a high proportion of the recovered early Danian fauna in the Danish Basin and neighbouring areas disappeared for good by the end of the Danian, when the sedimentary regime in the region changed from biogenic carbonates to predominantly clastic. Danian strata in northwestern Europe thus constitute a ‘Mesozoic’ faunal refuge for the marine benthos, providing the faunal background for persistent arguments, that the Danian ‘represents the last of the Cretaceous’ (e.g. Rasmussen, 1965; Voigt, 1985).
Acknowledgements We are grateful to Richard Bromley, Copenhagen, for supplying very constructive comments on the content of the manuscript while undertaking the essential task of improving its language. Søren Bo Andersen and Carsten Kjaer, Aarhus helped with echinoderm taxonomy.
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Johansen, M.B., 1988. Brachiopod extinctions in the Upper Cretaceous to lowermost Tertiary Chalk of Northwest Europe. Rev. Esp. Paleontol., Numero Extraordinario, Paleontology and Evolution: Extinction Events, pp. 41–56. Kauffman, E.G., Johnson, C.C., 1996. Maastrichtian extinction patterns of Caribbean Province Rudistids. In: Macleod, N., Keller, G. (Eds.), Cretaceous–Tertiary Mass Extinctions. W.W. Norton and Co., New York, pp. 231–273. Keller, G., Barrera, E., Schmitz, B., Mattson, E., 1993. Gradual mass extinction, species survivorship, and long-term environmental changes across the Cretaceous–Tertiary boundary in high latitudes. Geol. Soc. Am. Bull. 105, 979–997. Kjaer, C.R., 1992. Crinoider omkring Kridt=Tertiærgrænsen I Danmark. Unpubl. Thesis, University of Aarhus, 176 pp. Kjaer, C.R., Thomsen, E., 1999. Heterochrony in bourgueticrinid sea-lilies related to the K=T boundary event. Paleobiology 25, 29–40. Larsen, N., Ha˚kansson, E., 1998. Micro bio-facies mosaics across latest Maastrichtian bryozoan mounds in Denmark. Smithsonian Contribution to Biology (in press). Macleod, N., Keller, G., 1996. Cretaceous–Tertiary Mass Extinctions. W.W. Norton and Co., New York, 575 pp. Macleod, N., Rawson, P.F., Forey, P.L., Banner, F.T., BoudagherFadel, M.K., Brown, P.R., Burnett, J.A., Chambers, P., Culver, S., Evans, S.E., Jeffery, C., Kaminski, M.A., Lord, A.R., Milner, A.C., Milner, A.R., Morris, N., Owen, E., Rosen, B.R., Smith, A.B., Taylor, P.D., Urquhart, E., Young, J.R., 1997. The Cretaceous–Tertiary biotic transition. J. Geol. Soc. London 154, 265–292. Martini, E., 1971. Standard Tertiary and Quaternary calcareous nannoplankton zonation. Proc. II Planktonic Conference, Rome 1970 (2), pp. 739–785. Nielsen, K.B., 1913. Crinoiderne i Danmarks kridtaflejringer. Dan. Geol. Unders. II (26), 111 pp. Ødum, H., 1926. Studier over Daniet i Jylland og pa˚ Fyn. Dan. Geol. Unders. II (45), 306 pp. Perch-Nielsen, K., 1979. Calcareous nannofossil zonation at the Cretaceous=Tertiary boundary in Denmark. In: Birkelund, T., Bromley, R.G. (Eds.), Cretaceous–Tertiary boundary events I. University of Copenhagen, Copenhagen, pp. 78–91. Perch-Nielsen, K., McKenzie, J., He, Q., 1982. Biostratigraphy and isotope stratigraphy and the ‘catastrophic’ extinction of calcareous nannoplankton at the Cretaceous=Tertiary boundary. Geol. Soc. Am. Spec. Pap. 190, 353–371. Rasmussen, H.W., 1950. Cretaceous Asteroidea and Ophiuroidea with special reference to the species found in Denmark. Dan. Geol. Unders. II (77), 134 pp. Rasmussen, H.W., 1961. A monograph on the Cretaceous Crinoidea. K. Dan. Vidensk. Selsk., Biol. Skr. 12 (1), 416 pp. Rasmussen, H.W., 1965. The Danian affinities of the Tuffeau de Ciply in Belgium and the ‘Post-Maastrichtian’ in the Netherlands. Mededel. Geol. Sticht. N.S. 17, 33–38. Rasmussen, H.W., 1972. Lower Tertiary Crinoidea, Asteroidea and Ophiuroidea from northern Europe and Greenland. K. Dan. Vidensk. Selsk., Biol. Skr. 19, 83 pp. Schmitz, B., Keller, G., Stenvall, O., 1992. Stable isotope and fo-
E. Ha˚kansson, E. Thomsen / Palaeogeography, Palaeoclimatology, Palaeoecology 154 (1999) 67–85 raminiferal changes across the Cretaceous–Tertiary boundary at Stevns Klint, Denmark: arguments for long-term oceanic instability before and after bolide-impact event. Palaeogeogr., Palaeoclimatol., Palaeoecol. 96, 233–260. Sissingh, W., 1977. Biostratigraphy of Cretaceous calcareous nannoplankton. Geol. Mijnbouw 56, 37–65. Surlyk, F., 1972. Morphological adaptations and population structures of the Danish chalk brachiopods (Maastrichtian, Upper Cretaceous). K. Dan. Vidensk. Selsk., Biol. Skr. 19 (2), 57 pp. Surlyk, F., Birkelund, T., 1977. An integrated stratigraphical study of fossil assemblages from the Maastrichtian White Chalk of northwestern Europe. In: Kauffman, E.G., Hazel, J.E. (Eds.), Concepts and Methods in Biostratigraphy. Dowden, Hutchinson & Ross, Stroudsburg, PA, pp. 257–281. Surlyk, F., Johansen, M.B., 1984. End-Cretaceous brachiopod extinctions in the chalk of Denmark. Science 223, 1174–1177. Thomsen, E., 1974. Maastrichtian and Danian facies pattern on the Ringkøbing–Fyn High. Bull. Geol. Soc. Den. 23, 118–123. Thomsen, E., 1976. Depositional environment and development of Danian bryozoan biomicrite mounds (Karlby Klint, Den-
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mark). Sedimentology 23, 485–509. Thomsen, E., 1981. Revised definition of the Danian . Bull. Inst. Geol. Bassin Paris, Mem. Hors Ser. 2, 82–99. Thomsen, E., 1995. Kalk og kridt I den danske undergrund. Aarhus Geokompendium 1, 31–67. Thomsen, E., Ha˚kansson, E., 1995. Sexual versus asexual dispersal in clonal animals: examples from cheilostome bryozoans. Paleobiology 21 (4), 496–508. Troelsen, J.C., 1955. Globotruncana contusa in the white chalk of Denmark. Micropaleontology 1 (1), 76–82. Voigt, E., 1985. The Bryozoa at the Cretaceous–Tertiary boundary. In: Nielsen, C., Larwood, G.P. (Eds.), Bryozoa: Ordovician to Recent. Olsen and Olsen, Fredensborg, pp. 329–342. Ward, P.D., 1995. After the fall: lessons and directions after the K=T debate. Palaios 10, 530–538. Ward, P.D., Kennedy, W.J., MacLeod, K.G., Mount, J.F., 1991. Ammonite and inoceramid bivalve extinction patterns in Cretaceous=Tertiary boundary sections of the Biscay region (southwestern France, northern Spain). Geology 19, 1181– 1184.
Benthic extinction and recovery patterns at the K=T boundary in shallow water carbonates, Denmark Eckart Ha˚kansson a,Ł , Erik Thomsen b a
Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen, Denmark b Geological Institute, Aarhus University, Universitetsparken, DK-8000 Aarhus, Denmark Received 30 August 1997; revised version received 10 January 1998; accepted 19 August 1998
Abstract Benthic extinction at the K=T boundary in the Danish Basin is abrupt and indistinguishable from the termination of Maastrichtian White Chalk deposition. The Danian benthic fauna — already fully established in the earliest Danian nannoplankton zone NP1 — is essentially an impoverished Maastrichtian fauna established through direct survival or limited evolution within well-established clades, already present in the Maastrichtian of the Danish Basin. However, recovery of the early Danian community is initially retarded. The transition is illustrated in some detail for the locality, Nye Kløv. Here the lower 2–3 m of Danian strata (corresponding largely to the lower NP1 subzone) contains an extremely impoverished, highly unusual, ‘dead zone’ community dominated by bourgueticrinid crinoids associated with other presumed soft ground specialists and devoid of such important faunal elements as cyclostome bryozoans, brachiopods, calcitic bivalves, etc. Over the next metres the ‘dead zone’ fauna is gradually replaced by more ordinary faunas, and about 6 m above the boundary the characteristic early Danian bryozoan limestone community is fully established. 1999 Elsevier Science B.V. All rights reserved. Keywords: K=T boundary; extinction; recovery; benthos; bryozoa
1. Introduction Denmark boasts a large concentration of complete exposures spanning the Cretaceous=Tertiary boundary. The boundary lies within a marine realm of continuous accumulation below and above the boundary marl, and there is no demonstrable hiatus at the boundary. Uniquely in Denmark, the exposed successions contain very rich, highly diverse autochthonous faunas of benthic invertebrates (Birkelund and Ha˚kansson, 1982), with a combined Ł Corresponding
author. Fax: C45-3532-2499; E-mail: [email protected]
Maastrichtian and Danian diversity of well over 1000 species. Other K=T boundary sections providing a continuous accumulation experience either a faunal depletion towards the boundary, e.g. Zumaia, Spain (Ward et al., 1991), Brazos River, Texas (Hansen et al., 1987, 1993), or allochthonous, fragmented faunas distort the record in part of the succession (e.g. Brazos River, Texas). Two Danish localities stand out as particularly intensely studied: Stevns Klint, the classic type locality for the Maastrichtian=Danian boundary (Desor, 1847), has been in focus for more than a century, while the potential of the much more modest profile exposed at Nye Kløv (Nye Klov of authors) was only realized quite recently (Ha˚kansson
0031-0182/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 0 8 7 - 5
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and Thomsen, 1979; Ha˚kansson and Hansen, 1979). The subject of this paper is concerned primarily with the extinction and recovery patterns at Nye Kløv.
2. Regional geology A pronounced low-stand in global sea level occurred during the late Maastrichtian–early Danian interval of time. In the Danish region this major regressive pulse is evidenced from rapid reduction in the area of White Chalk accumulation, with associated large-scale shifts in facies boundaries. Thus, from a position in central Europe during the early Maastrichtian the southern limit of the White Chalk facies retreated to northernmost Germany by the middle-late Maastrichtian, and by the Cretaceous– Tertiary transition marine deposition was restricted to a narrow sound between the Ringkøbing–Fyn High and the Fennoscandian Shield (Fig. 1). During the early Danian this distribution expanded only slowly, and the areas south of the Ringkøbing–Fyn
High were not marine again until the late Danian (Ha˚kansson and Thomsen, 1979). The narrow marine sound crossing the Danish region during the K=T boundary turnover connected the main oceanic water masses through what were conceivably equally narrow and shallow seaways. To the northwest it was connected to the initial North Atlantic, and to the southeast across Europe via Crimea to the Tethyan realm. The K=T boundary is at present exposed in nine outcrops along a narrow belt across Denmark (Fig. 2), with Stevns Klint located at the southeastern termination of this belt, and Nye Kløv together with three other small exposures in the salt-influenced Thisted Dome at the northwestern termination. Except for a 2–35 cm thick clay-rich layer at the boundary (the ‘Fish Clay’), the Maastrichtian and Danian sediments in the narrow Danish seaway are consistently carbonates deposited in a moderately warm climate. Two main facies types dominate, a pelagic facies composed mainly of calcareous nannofossils and planktonic foraminifera, and a benthic
Fig. 1. Palaeogeography of the Danish region at the time of the K=T boundary (based on Ha˚kansson et al., 1974, and Thomsen, 1995).
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Fig. 2. Geological map of Denmark (based on Ha˚kansson and Pedersen, 1992) with the location of presently accessible K=T boundary sections.
facies dominated by bryozoans set in a matrix also dominated by pelagic organisms. The two main facies types are traditionally termed chalk and bryozoan limestone, respectively; however, across the basin a number of intermediate facies types may be recognized (Bromley, 1979). In the bryozoan limestone fragments of bryozoan colonies and other benthos having calcitic skeletons typically make up more than 50% of the sediment >125 µm (Thomsen, 1976), whereas in chalk these constituents only rarely amount to more than 5% (Ha˚kansson et al., 1974). The Maastrichtian deposits are consistently pelagic White Chalk in which bryozoans may be abundant locally in the shallower parts of the sea (Ha˚kansson et al., 1974; Thomsen, 1974; Larsen and Ha˚kansson, 1998). The Danian deposits are characterized by interchanging benthic and pelagic facies. The pelagic facies dominates in the deeper parts of the depositional system towards the northwest, while benthic facies dominate in the shallower parts towards the southeast. At Nye Kløv the K=T boundary is set exclusively in chalky facies, with
typical, pelagic White Chalk below the boundary and a (?)chemically precipitated ‘chalk’ above; bryozoan limestone only appears several metres above the boundary. As a result of the insignificant overall cementation found in the Danish on-shore carbonates, most Maastrichtian and Danian sediments are easily disintegrated, allowing quantitative studies of the faunas to be made from bulk samples. In general the aragonitic constituents of the faunas have been lost through early diagenetic dissolution. However, their presence is abundantly documented through locally preserved moulds and casts, particularly in the more shallow deposits in the eastern part of the basin, where repetitive episodes of seafloor cementation have led to the formation of hardground levels. Some of the finest hardgrounds are exposed in the Stevns Klint succession, which therefore offers a unique opportunity to investigate the survival and recovery patterns of the aragonitic benthos (Heinberg, 1999). However, the cemented limestone horizons at Stevns Klint are difficult to disintegrate and, in contrast
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to the relatively soft limestones spanning the K=T boundary in the section at Nye Kløv, they cannot be studied quantitatively from bulk samples.
3. The Nye Kløv profile The Nye Kløv locality is situated in a steep palaeocliff on the eastern flank of the Thisted Dome, northwest Jylland (Fig. 2). The boundary succession at Nye Kløv compares closely with that of nearby Kjølby Gaard (Troelsen, 1955; Ha˚kansson and
Hansen, 1979) but exhibits, in addition, a more complete transition into the overlying bryozoan limestone. The exposed succession is about 19 m thick including 7 m of Maastrichtian White Chalk and 12 m of Danian chalks and limestones (Fig. 3) (Ha˚kansson and Hansen, 1979; Ha˚kansson and Thomsen, 1979; Thomsen, 1981; Birkelund and Ha˚kansson, 1982; Surlyk and Johansen, 1984; Ekdale and Bromley, 1984; Johansen, 1987; Keller et al., 1993). Generally, the Maastrichtian=Danian boundary is exposed over a horizontal distance of 30–40 m. The lowermost 5 m of the Maastrichtian is presently covered by talus.
Fig. 3. Stratigraphy and sample locations in the Nye Kløv succession.
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The Maastrichtian part of the sequence consists of poorly lithified White Chalk with a few flint nodule levels. The chalk is highly bioturbated (Ekdale and Bromley, 1991) and bedding planes are normally indistinguishable. It contains about 1% by weight of skeletal material >0.5 mm, with an overall dominance of bryozoans. Aragonitic skeletons are generally not preserved, but moulds of Baculites and Scaphites occur. The Maastrichtian=Danian boundary is marked by a 2–3 cm thick, dark grey clay layer with a thin rusty colouring at the base. The clay, which is sharply delimited downwards, is characterized by a marked increase in iridium content in the lower part (Hansen et al., 1986). Upwards the boundary clay rapidly changes into a 4–5 m thick succession of poorly fossiliferous, whitish grey ‘chalk’ presumed to be dominated by chemically precipitated calcite micrite (Hansen, 1990), at least in the lower part. From about 5 m above the boundary macrofossils, again dominated by bryozoans, gradually become more abundant and between 6 and 8 m bryozoans (>500 µm) make up nearly 10% of the deposits, and this interval thus constitutes a meagre bryozoan limestone. From this level the content of bryozoans decreases, and benthic constituents make up less than
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2% of the sediment in the uppermost, chalky part of the section. On the whole, the Danian succession is only slightly lithified. In the bryozoan limestone proper bioturbation is moderate, and indistinct bedding planes may be seen. Several omission surfaces are found in the bryozoan limestone from which Thallasinoides burrow systems can be traced up to 50 cm into the sediment below. Flint layers and flint nodules are common in the Danian deposits. Compared to the ‘typical’ situation further eastwards in the Danish basin both the Maastrichtian White Chalk and the Danian bryozoan limestone at Nye Kløv have only a fairly moderate content of benthos, both in terms of density and diversity. 3.1. Stratigraphy Investigations of calcareous nannofossils indicate continuous deposition across the boundary with a relatively thick early Danian succession (Fig. 3), which is in keeping with the foraminifera data presented by Keller et al. (1993). The boundary between the NP1 and NP2 zones in the zonation scheme of Martini (1971) is located approximately 8.5 m above the boundary as indicated by the first occurrence
Fig. 4. Comparative lithology and stratigraphy of Nye Kløv and Stevns Klint.
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of Cruciplacolitus intermedius. In the Danish region Perch-Nielsen (1979) subdivided the NP1 zone into two subzones D1 and D2 on the basis of the first occurrence of Placozygus sigmoides. In the Nye Kløv section this species first appears in a sample 270– 280 cm above the boundary. Perch-Nielsen (1979) also subdivided NP2 into two subzones, D3 and D4, with the base of D4 indicated by the first occurrence of Prinsius dimorphosus. In the Nye Kløv section P. dimorphosus appears simultaneously with C. intermedius in a sample ca. 8.5 m above the boundary. Zone D3 therefore is apparently absent. This hiatus conceivably coincides with a marked omission surface, and it is, furthermore, associated with a significant decrease in the content of macrofossils. All Maastrichtian samples contain abundant Nephrolithus frequens and are referred to the uppermost Cretaceous zone CC 26 in the zonation scheme of Sissingh (1977). Compared to the classic section at Stevns Klint (Fig. 4) it appears from nannofossil investigations that (1) both successions are continuous across the boundary, although they differ significantly in accumulation rates, and (2) the most prominent hiati in the two successions (located in the lower part of D2 in Stevns Klint and at the D2=D4 boundary in Nye Kløv) are not correlatable. It should be stressed,
however, that none of these hiati (Fig. 4) corresponds to the hiatus claimed to be present in the very earliest Danian at both Stevns Klint (Schmitz et al., 1992) and Nye Kløv (Keller et al., 1993). The existence of such a regional hiatus has been promoted in a massive array of papers by Keller and Macleod (see e.g. the reference list in Macleod et al., 1997). But, in our opinion, neither the absence in Nye Kløv of foraminifera of particular importance to the stratigraphic framework in the low latitude succession at El Kef, Tunesia (Keller et al., 1993), nor the total absence of planktonic foraminifera in part of the Stevns Klint succession (Schmitz et al., 1992) is sufficient for the invention of a hiatus not supported by any lithological data.
4. Material and methods The present investigation is based on 18 bulk samples covering the uppermost 2 m of the Maastrichtian succession and the whole of the Danian (Fig. 3). Sample spacing is closest in the interval immediately below and above the boundary. The samples, each weighing from 1 kg in the bryozoan limestone to 5 kg in the poorly fossiliferous chalks, were disintegrated through repetitive freezing and
Fig. 5. General faunal composition at Nye Kløv (part of this set of data previously published by Birkelund and Ha˚kansson, 1982).
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Fig. 6. Distribution of cheilostome bryozoans at Nye Kløv (a few clearly recognizable reworked specimens of Maastrichtian species found in the lowermost few centimetres of Danian strata are omitted). Dots indicate recorded presence, solid lines connect identical species group taxa, dashed lines connect closely related species group taxa.
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thawing in a saturated solution of sodium sulphate. The quantitative analyses are based on washed sample residues in the size fraction >500 µm. In the White Chalk and the bryozoan limestone only split samples were analysed. The fossils were sorted into main animal groups and weighed as a measure of their relative importance. The primary emphasis in this study is on cheilostome bryozoans, and this group was sorted further into species, except for the uppermost three samples representing zone D4. The relative importance of individual species in a sample was determined by their specimen count. The unsorted sample residues were finally examined for rare species. The highly unusual combination of very low density and extremely high diversity characterising the bryozoan fauna from most Danish–Maastrichtian level bottom chalk (E. Ha˚kansson, unpubl. data; Ha˚kansson and Thomsen, 1979) implies that many species are very rare. They may be present in one or a few samples and then absent in several others taken nearby, with seemingly random and unpredictable distribution. The many rare species in combination with the low density of the fauna makes it practically impossible in a reasonably sized sample to obtain even the majority of species present at a certain stratigraphic level. Whereas this problem may not be impeding when working at a large-scale level, it becomes eminently pernicious when approaching the level necessary to investigate, for instance, the nature of the collapse of the Maastrichtian White Chalk community towards the K=T boundary. In the case of Nye Kløv, where the entire Maastrichtian succession exposed belongs to the upper part of the topmost Maastrichtian brachiopod zone, we find that the absence of species in individual samples is entirely random, and that all chalk exposed does in fact represent a single, fundamental White Chalk community. We therefore refrain from further differentiation between the individual Maastrichtian samples in this presentation.
5. Results The invertebrate fauna in the washed residues >500 µm from Nye Kløv (Fig. 5) contain octocorals, cheilostome and cyclostome bryozoans, bra-
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chiopods, calcitic bivalves, serpulids, ostracods, and various groups of echinoderms including regular and irregular echinoids, crinoids, asteroids and ophiuroids. Aragonitic forms are not preserved and ammonites are only represented by a few aptychi. In general terms the invertebrate fauna can be characterized as rich to very rich, except for the lowermost 2–3 m of the Danian interval, where an extremely impoverished fauna characterize an initial Danian ‘dead zone’. A similar pattern can be seen in all K=T boundary sections in Denmark (Ødum, 1926), and the development at Nye Kløv can be considered as representative. It is particularly noteworthy that important members of the Dano– Maastrichtian benthic faunas such as brachiopods, calcitic bivalves, serpulids and cyclostome bryozoans, are totally absent from this lowermost Danian ‘dead zone’ and that otherwise fairly subordinate groups, such as crinoids and echinoids, suddenly become very prominent (Fig. 5). Only cheilostome bryozoans are present in all samples, although in strongly varying densities and diversities. We shall discuss below the development of three groups in more detail, namely the cheilostome bryozoans, the brachiopods and the crinoids. These groups were selected because they are important in the Nye Kløv succession and because they clearly react differently to the K=T boundary event. Of the three groups, only the data concerning the brachiopods have previously been published in extenso (Johansen, 1987). 5.1. Bryozoans Bryozoans constitute by far the most conspicuous part of the preserved fauna in the Danian– Maastrichtian sediments in the Danish Basin as, indeed, in much of the North Sea region. As a general rule the cheilostome part of the fauna dominate in terms of diversity, while the cyclostome bryozoans commonly constitute a significant proportion of the bulk density, and in the Danian cyclostomes may even be more abundant than cheilostomes (Berthelsen, 1962). Unfortunately the current systematic state of the cyclostome bryozoans essentially prevents investigation of this important part of the fauna at lower taxonomic levels, hence cyclostomes are here considered merely as a group. At Nye Kløv, bryozoans >500 µm constitute be-
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tween 0.0001 and 9% of the bulk sediment, with a very prominent density peak in the top of the D2 zone and an equally prominent density low in the basal Danian (Fig. 5). Among the cheilostome bryozoans a total of 66 genus level and 203 species level taxa have been identified from Nye Kløv (Fig. 6). The Maastrichtian fauna presently includes 138 species (with 4 species identified only from the Danian of Nye Kløv, but known from Maastrichtian sediments elsewhere in the Danish Basin), whereas the Danian fauna includes 83 species (with 2 species identified only from the Maastrichtian of Nye Kløv, but known from Danian sediments elsewhere in the Danish Basin). Further, 19 of the Maastrichtian species known from Nye Kløv survived the K=T boundary turnover directly (13 of which have thus far been found both below and above the boundary in Nye Kløv), and although an additional 11 Maastrichtian species are found to have very close relations across the boundary (e.g. ‘Membranipora’ pauparata=‘M.’ n. sp. aff. pauparata), the cheilostome bryozoan fauna evidently suffered very severely. Thus, the direct survival rate can be calculated to just below 15%. At the genus level, however, 34 taxa among the 57 found in the Maastrichtian at Nye Kløv are also present in the Danian samples, and when the remaining part of the basin is taken into consideration, all of the genera presently known only from the Danian at Nye Kløv have Maastrichtian representatives elsewhere in the basin. In other words, in the Nye Kløv section no new cheilostome genera appear in the Danian. So even though such figures are not entirely representative for the situation in the whole Danish Basin, it is obvious that the Danian fauna — albeit in itself very rich — should be regarded merely an impoverished version of the extremely rich Maastrichtian fauna. Termination of the Maastrichtian bryozoan fauna at Nye Kløv is abrupt, but within the resolution possible it is indistinguishable from the termination of the White Chalk facies. Accordingly, in terms of ecological groups, it appears that encrusting, rigidly erect and free-living species were affected differently. Thus, of the 18 Maastrichtian free-living species, among the most obvious White Chalk specialists (cf. Thomsen and Ha˚kansson, 1995), none survived the K=T boundary, whereas survival in both other groups amount to just below 20%.
5.2. Brachiopods Danian–Maastrichtian brachiopod faunas in the Danish Basin are characterized by a dominance of minute forms adapted to a life on bryozoan skeletons and by secondarily free-living, apedunculate forms adapted to soft bottom conditions (Surlyk, 1972; Surlyk and Birkelund, 1977). Johansen (1987, 1988) has investigated the fauna from Nye Kløv in detail, and the distribution chart presented here (Fig. 7) is based entirely on her data, reorganized to facilitate comparison with the bryozoan data presented above (Fig. 6). Note that in spite of the very peculiar, and in part simply incorrect resume´ published by Macleod et al. (1997, p. 277), the brachiopod fauna from Nye Kløv is indeed very well documented. Most conspicuous is the total absence of brachiopods from the lower 3 m of the Danian and a ‘typical’ Danian fauna is not developed until 6 m into the Danian succession. Of the 46 species group taxa recorded at Nye Kløv, only 6 are found in both Maastrichtian and Danian sediments, whereas another 7 Maastrichtian species have very close relations across the boundary, so evidently the brachiopod fauna was very strongly affected in the K=T turnover and, like the cheilostome bryozoans, their disappearance is abrupt (Surlyk and Johansen, 1984; Johansen, 1987, 1988). A total of 16 genus-level clades have been found in the Maastrichtian part of the succession at Nye Kløv. Of these, 6 become extinct at the K=T boundary, and another 6 have disappeared by the end of the Danian; only 3 new genera appear in the Danian. As a general conclusion it therefore appears that the Danian brachiopod fauna is essentially a slightly impoverished continuation of the Maastrichtian fauna, with pervasive species replacement within well-established clades. However, it should be noted that in terms of ecological groups, some of the most notable White Chalk specialists (hemispherical, apedunculate species, cf. Surlyk, 1972) are among the prominent K=T boundary casualties (Johansen, 1987). 5.3. Echinoderms Echinoids, crinoids, ophiuroids and asteroids are present in most Danian–Maastrichtian environments in the Danish region (Rasmussen, 1950, 1961, 1972).
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Fig. 7. Brachiopod species distribution at Nye Kløv (all data extracted from Johansen, 1987; a few clearly recognizable reworked specimens of Maastrichtian species found in the lowermost Danian strata are omitted). Signatures as in Fig. 6.
In most sediments they play a fairly subordinate role, with only the holasteroid echinoid Echinocorys being locally conspicuous. It is therefore of exceptional interest that echinoderms are totally dominant in the lowermost few metres of Danian strata at Nye Kløv (Fig. 5). Crinoids of the order Bourgueticrinidae, in particular, experience a dramatic bloom in the lowermost 2 m of the Danian succession, where they may account for as much as 80% by weight of the benthic fauna (Fig. 5). Bourgueticrinids are rare in the uppermost Maastrichtian White Chalk, with only a few calices and columnals retrieved from more
than 20 kg of chalk examined from Nye Kløv (Kjaer and Thomsen, 1999). In marked contrast, the lowermost Danian ‘chalk’, sample D1: 3–13 cm above the K=T boundary, has yielded 42 calices and approximately 700 columnals per kg sample, whereas higher in the Danian succession bourgueticrinids again become subordinate, although they never return to the low levels of the Maastrichtian. Associated with the bourgueticrinid bloom is a quite diverse echinoid fauna composed of disintegrated cidaroids and Aerosoma as well as unidentifiable spatangoid fragments. It deserves mentioning that a prominent mem-
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Fig. 8. Bourgueticrinid species distribution around the K=T boundary at Nye Kløv (based on Kjaer, 1992, and Kjaer and Thomsen, 1999).
ber of the initial Danian bourgueticrinid fauna is Democrinus maximus (Kjaer and Thomsen, 1999). The entirely Cainozoic genus Democrinus evolved from Bourgueticrinus through paedomorphosis simply by the loss of the proximal plate in the calyx (Nielsen, 1913; Kjaer and Thomsen, 1999; Fig. 8), and the Nye Kløv species, which is abundant already in the lowermost Danian sample, is thus the earliest known member of the Democrinus clade. The proximal plate is normally formed during early ontogeny by incorporation of a columnal element into the calyx, and once the proximale is formed the addition of new columnals is terminated. Elimination of the proximale in Democrinus therefore, in principle, allows columnal elements to be added throughout life (Kjaer and Thomsen, 1999). This novel potential may reflect selection for a longer stem in a food-depleted environment.
6. The ‘dead zone’ — bryozoan limestone faunal succession
particular limestone interval (it ‘sounds dead when hit with a hammer’), without any reference to the fauna. The faunal condition in the lowermost few metres of the Danian at Nye Kløv is found to be in good accord with the situation at Voxlev. Indeed, the situation with a ‘dead zone’ interval of faunal paucity combined with a highly unusual faunal composition in the initial strata of the Danian subsequent to the boundary clay is valid for boundary sections all along the Danish Basin. Also the petrologic peculiarity of the ‘dead zone’ has a basin wide distribution, probably correlated directly with the occurrence of chemically precipitated micrite. However, only at Nye Kløv the subsequent zone of recovery is preserved in its entirety. Hence the significance of listing the Nye Kløv faunal succession in some detail, since this succession — sample by sample — constitutes the recovery pattern, from absolute ‘nothing’ in the boundary clay to a full-scale bryozoan limestone fauna, all within the basal Danian nannoplankton zone (Figs. 5 and 9). 6.1. ‘Dead zone’ — samples D1–D5
The ‘dead zone’ concept has its roots in the locality Voxlev in eastern Jylland, from where Jessen and Ødum (1923) described a very unusual, highly impoverished fauna in a 2 m interval immediately superseding the boundary clay. However, in spite of a very clear understanding of the faunal peculiarities of their ‘dead layer’, it is an interesting fact that Jessen and Ødum (1923), in their original designation of this phrase, refer solely to the peculiar petrology of this
Sample D1. Bourgueticrinus and Democrinus are overwhelmingly dominant, and together with Aerosoma, spatangoid echinoids and a single species of cheilostome bryozoan, Pavolunulites n. sp. (Fig. 10), constitute the entire skeletonized fauna. Sample D2. Similar to D1, Bourgueticrinus and Democrinus reach their peak dominance, Pavolunulites n. sp. remains dominant among the bryo-
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Fig. 9. Distribution and proportion of the earliest Danian cheilostome bryozoans at Nye Kløv.
zoans, but at this level two more cheilostome species are present, Lekytoglena ampullacea and Danocella hakanssoni (Fig. 10). Samples D3 and D4. Similar to D2; the first sporadic occurrences of additional benthos, serpulids and sponges. Sample D5. Similar to D4; the first Danian representatives of the Pachytecella filiformis=lundgreni complex appear together with the first cyclostome bryozoans. 6.2. Recovery zone — samples D6–D9 Samples D6 and D7. Pronounced changes; most of the pioneer ‘dead zone’ fauna drop to their ordinary Danian level of importance; overall dominance is taken over by echinoids, while bryozoan diversity is increasing; brachiopods and calcitic bivalves reappear. Samples D8 and D9. Bryozoans dominate for the first time in the Danian, with further increase in cheilostome diversity.
6.3. Bryozoan limestone — samples D10 and D11 Samples D10 and D11. Full-scale bryozoan limestone fauna, with cheilostome diversity in excess of 60 species. It should be noted that trace fossils even in the very basal Danian strata constitute convincing evidence that an infauna including unmineralized biota was active at this time (Ekdale and Bromley, 1974), although the ichnodiversity was reduced to no more than 3–4 ichnospecies (R. Bromley, pers. commun., 1997).
7. Discussion and conclusions Our investigations have confirmed previous findings that the benthic communities of the northwestern European Maastrichtian White Chalk thrived and flourished right up to the K=T boundary (e.g. Ha˚kansson and Thomsen, 1979; Birkelund and Ha˚kansson, 1982; Surlyk and Johansen, 1984; Jo-
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Fig. 10. ‘Dead zone’ cheilostome bryozoans from Nye Kløv. All magnifications ð25. 1 D Pachythecella filiformis (sample D5). 2 D Lekythoglena ampullacea (sample D2). 3 D Pavolunulites n. sp. (sample D1). 4, 5 D Danocella hakanssoni (sample D2).
hansen, 1987). For unknown reasons Macleod et al. (1997) in their broad account of the Cretaceous– Tertiary biotic transition claim that bryozoans, the dominant group in all these communities, experienced a long-term decline prior to the K=T boundary (stated to be the ‘latest Maastrichtian’ in the abstract and ‘throughout the Upper Cretaceous’ in the discussion, but not mentioned at all in the bryozoan chapter). If anything, the bryozoan faunas of both basinal and coastal facies in northwestern Europe experienced an increasing trend in the late Maastrichtian. At the K=T boundary most major benthic groups investigated from the White Chalk environment, most notably bryozoans, brachiopods (Surlyk and Johansen, 1984; Johansen, 1987), and bivalves (Heinberg, 1999), were subject to very substantial extinctions. In all groups the extinction is taxonomically neutral although, in bryozoans and brachiopods, with a preference for the highly adapted, free-living chalk
specialists. As a result, the fully recovered early Danian fauna in the Danish Basin is a somewhat impoverished, within-basin continuation of the Maastrichtian fauna. When, for some reason, a depositional facies disappears, it is a textbook fact, that the fauna specifically adapted to this particular facies will also disappear. So when the unique pelagic White Chalk facies, which had a continuous history of ca. 25 Ma in northwestern Europe, was finally terminated by the abrupt deposition of the ‘Fish Clay’ and its equivalents, it can come as no surprise that the highly adapted chalk benthos disappeared with it. In keeping with this, the faunal collapse at the close of the Cretaceous Period in this region, by the finest resolution possible, remains inseparable from the termination of White Chalk deposition. However, only part of the fauna shows up again (to a large extent evolved into new, closely related species), when seemingly comparable sediment types reappear af-
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ter the very brief boundary clay interval. Indeed, while the planktonic biotic system in the Danish Basin, according to e.g. Perch-Nielsen et al. (1982) and Keller et al. (1993), was essentially continuous across the K=T boundary, to be gradually replaced by Tertiary biota above the boundary, the benthic system characterizing the Maastrichtian chalk sea never reappeared. This is in stark contrast to the near-instant return of a full-scale chalk benthic system seen after earlier marly intervals in the Danish Basin, such as the late Maastrichtian Kjølby Gaard Marl. In the benthic communities the K=T boundary recovery in the Danish Basin differs from these previous chalk disruptions in three important aspects: (1) it is notably retarded, (2) it leads to a somewhat less complex benthic community system, (3) it terminates particular ecological groups, and (4) it includes extensive species replacement. One obvious reason for the delay is the peculiar composition of the basal Danian ‘chalk’, with a notable content of euhedral to subhedral calcite crystallites, which are likely to be chemical precipitates (Hansen, 1990). Hence recovery at this time was arguably delayed due to highly adverse conditions through most or all of zone D1. By the same token, the very nature of the earliest Danian seafloor may have been considerably more soupy than ordinary chalk, due to a high proportion of abiological grains (precipitated crystallites and reworked coccoliths), which did not reach the seafloor in the form of faecal pellets. The extreme benthic depletion characterizing the first few metres of Danian sediments at Nye Kløv has been recorded at all Danish K=T boundary sections. However, since the incoming (early) Danian taxa are ecological specialists with demonstrable late Maastrichtian predecessors within the Danish Basin, isolated pockets of less adverse conditions must have been available through the ‘dead zone’ interval. So the proverbial refuge exists, and in all probability located in close connection to the Danish Basin. In conclusion, therefore, the recovery pattern found for the benthos in the Danish region should be characterized as predominantly migrational rather than evolutionary, with a pronounced local signature. When discussing the Cretaceous=Tertiary boundary on a global scale, it is important to distinguish between the two potentially unrelated elements: (1) the over-all transition from a Mesozoic to a Cain-
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ozoic biosphere, and (2) the existence of a specific boundary ‘event’. As stated above the dramatic collapse of the Maastrichtian benthic community in the Danish Basin is inseparable from the termination of the Maastrichtian depositional regime but, at the same time, it is also coincident with the deposition of the Ir-bearing boundary clay. Our data alone, therefore, do not allow the above-mentioned distinction, since they are obviously part of (1) and entirely compatible with (2). When comparing the extinction and recovery patterns for the bryozoans and brachiopods in the Danish Basin to the patterns for other benthic and nektonic groups, such as rudists and inoceramid bivalves (Kauffman and Johnson, 1996) and ammonites (Ward, 1995), as well as the globally more uniform developments found for the various planktonic groups (see e.g. papers in Macleod and Keller, 1996), it becomes apparent that no single pattern can cover all groups. In fact, the more we learn about the faunal particulars surrounding the K=T boundary discussion, the more complex is the scenario needed to include all observations, and the more prominent does the prospect of a multicausal complex of events become (e.g. Birkelund and Ha˚kansson, 1982; Ward, 1995; Macleod et al., 1997). Curiously, a high proportion of the recovered early Danian fauna in the Danish Basin and neighbouring areas disappeared for good by the end of the Danian, when the sedimentary regime in the region changed from biogenic carbonates to predominantly clastic. Danian strata in northwestern Europe thus constitute a ‘Mesozoic’ faunal refuge for the marine benthos, providing the faunal background for persistent arguments, that the Danian ‘represents the last of the Cretaceous’ (e.g. Rasmussen, 1965; Voigt, 1985).
Acknowledgements We are grateful to Richard Bromley, Copenhagen, for supplying very constructive comments on the content of the manuscript while undertaking the essential task of improving its language. Søren Bo Andersen and Carsten Kjaer, Aarhus helped with echinoderm taxonomy.
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