Int J Earth Sci (2000) 89 : 108±129
Springer-Verlag 2000
ORIGINAL PAPER
H.-J. Gawlick ´ F. Böhm
Sequence and isotope stratigraphy of Late Triassic distal periplatform limestones from the Northern Calcareous Alps (Kälberstein Quarry, Berchtesgaden Hallstatt Zone) Received: 24 September 1998 / Accepted: 14 November 1999
Abstract The Kälberstein quarry at Berchtesgaden exposes Carnian-Norian deeper-water Hallstatt limestones. Conodont biostratigraphy, microfacies and stable isotopes of bulk carbonate matrix were investigated. The biostratigraphic results demonstrate a complete succession from the latest Carnian (Tuvalian 3/I) to the late Norian (Sevatian 2). As expected from the periplatform setting of the Hallstatt Zone, calculated mass accumulation rates conform partly to prograding sequences observed on the contemporary Dachstein platform. However, discrepancies exist, mainly for the middle Norian, pointing to an incomplete knowledge of the platform sequences. The sequence stratigraphic framework based on platform data should be complemented with data from the periplatform Hallstatt Zone. Diagenetic alteration of the limestones from Kälberstein quarry is low with a conodont alteration index (CAI)=1.0 throughout the section. Oxygen isotope values ranging from ± 1.2 to + 0.1½ (VPDB) point to stabilization and cementation at very shallow burial depths in contact with seawater in a deeper-water environment. Carbon isotope values display a clear stratigraphic trend with a rapid increase from 3.6 to 4.1½ (VPDB) during the basal Norian (Lacian 1), high values up to 4.2½ during the Lacian 2, and a slow decline starting in Lacian 3 to 2.6½ at the end of the Norian (Sevatian 1±2). These trends are best explained by variations in the global organic carbon/carbonate burial ratio with maximum organic carbon burial during the middle Lacian.
H.-J. Gawlick ´ F. Böhm ()) GEOMAR, Forschungszentrum für Marine Geowissenschaften, Wischofstrasse 1±3, D-24148 Kiel, Germany e-mail:
[email protected] H.-J. Gawlick Montanuniversität Leoben, Institut für Geowissenschaften; Prospektion und Angewandte Sedimentologie, Peter Tunner Strasse 5, A-8700 Leoben, Austria
Key words Carbon isotope stratigraphy ´ Conodont biostratigraphy ´ Sequence stratigraphy ´ Early diagenesis ´ Oxygen isotopes ´ Late Triassic ´ Hallstatt ´ Limestones ´ Northern Calcareous Alps
Introduction During the past few years a huge amount of carbon isotope data has been collected from various periods of earth history from Precambrian to modern times to gain information about variations in the global carbon cycle (e.g. Baud et al. 1989; Druffel and Benavides 1986; Jenkyns and Clayton 1986; Wenzel and Joachimski 1996; Weissert and Mohr 1996). Moreover, global carbon isotope shifts can be of great value for stratigraphic correlations (Jenkyns et al. 1994; Odin et al. 1994), even for shallow-water carbonates (Jenkyns 1995; Saltzman et al. 1998). By now, there are wellfounded carbon isotope curves for most of the Phanerzoic time (Holser et al. 1996). However, very few studies have investigated carbon isotope stratigraphies of the Late Triassic (Lintnerova and Hladikova 1992; Steuber 1989; Bellanca et al. 1995) or Earliest Jurassic (Morante and Hallam 1996; Böhm et al. in press), leaving a large gap in the global carbon isotope history. The latest Carnian to late Norian section of the Kälberstein quarry at Berchtesgaden (Northern Calcareous Alps; Figs. 1, 2) is perfectly suited for isotope investigations: it combines stratigraphic completeness, excellent biostratigraphic control, very mild late diagenetic alteration and a red hemipelagic limestone facies that, due to low sedimentation rates, preserves a carbon isotopic composition unaffected by organic carbon decomposition. With these features the Kälberstein quarry is exceptional among Late Triassic sections worldwide. Unusually high carbon and oxygen isotope values from the Kälberstein quarry were first reported by Gökdag (1974) and compared with data from contem-
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Fig. 1 Outcrop view of the Kälberstein quarry after Gümbel (1861; modified from Rieche 1971). Biostratigraphic ages based on conodonts and lithostratigraphic members are indicated. Note the slightly different attitude of the ªMassiger Hellkalkº compared with the underlying and overlying strata
porary platform limestones. The quarry (Fig. 1) is a famous stop of geological field trips described in numerous guide books (Zankl 1971; Ganss 1979; Hagn Fig. 2 Position of the Kälberstein Quarry (Berchtesgaden) in a tectonic map of the central Northern Calcareous Alps (partly modified after Tollmann 1985 and Gawlick et al. 1994). ST Staufen-Höllengebirgs nappe; BD Berchtesgaden nappe; DD Dachstein nappe; T Southern Tennengebirge nappe; HS Hallstatt sliding sheets; GL GöllLammerzone
1981; Herm et al. 1991) and geological papers (Schafhäutl 1848; 1851; Gümbel 1861; Krumbeck 1938; Rieche 1971; Staudt 1989; Risch 1993). Nevertheless, a precise detailed biostratigraphic framework is still lacking (Risch 1993). In this paper we first present a detailed stratigraphic outline of the Kälberstein section, based on conodonts. Together with facies investigations this allows us to conclude on the palaeogeographic prove-
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nance of the Hallstatt limestones of the Kälberstein, which are part of an allochtonous slide block of the Hallein-Berchtesgaden Hallstatt Zone. We further discuss sequence stratigraphic implications of sediment accumulation rate estimates, derived from the biostratigraphic data. Finally, we discuss the stable isotope record, which displays long-term variations in d13C and shows d18O values typical for condensed deeper-water limestones of the Jurassic Tethys (Marshall 1981).
Geological and tectonic framework The Kälberstein quarry is a small abandoned quarry on the northwestern outskirts of Berchtesgaden in the Northern Calcareous Alps (Fig. 2). It exposes a succession of red and grey Hallstatt limestones of Carnian and Norian age (Rieche 1971; Risch 1993). These Hallstatt limestones are part of the ªHallein-Berchtesgadener Hallstätter Schollenregionº, an allochthonous tectonic unit formed by several nappe sheets (Fig. 2). These Hallstatt nappe sheets (ªLower Juvavic Nappeº) are thought to be overlain by the ªUpper Juvavicº Berchtesgaden Nappe (e.g. Pichler 1963; Zankl 1971; Tollmann 1985; Langenscheidt 1994). The timing of the Lower Juvavic Nappe emplacement is controversial. A period between the later Late Jurassic and the later Late Cretaceous was formerly discussed by several authors (e.g. Plöchinger 1955, 1976, 1984, 1995; Schweigl and Neubauer 1997; Tollmann 1985; Zankl 1971). New investigations, however, point to an even earlier emplacement during the early Late Jurassic (Gawlick et al. 1999a).
Materials and methods Approximately 45 samples were collected from in situ beds in the quarry during field seasons in 1996 and 1997. Forty-three samples were dissolved in acetic acid and conodonts collected from the residues. Additionally, Kälberstein conodont samples from the collection of Rieche (1971) were reinvestigated. Taxonomic determination and biostratigraphic interpretation of the conodont faunas are based on Orchard (1983), Orchard and Tozer (1997), Krystyn (1980, 1991) and unpublished data of L. Krystyn (pers. commun.). The position of the Norian-Rhaetian boundary is still under discussion (e.g. Krystyn 1987, 1990, 1991; Dagys and Dagys 1994). We follow Krystyn (1990), who places the Norian-Rhaetian boundary at the base of the Stuerzenbaumi zone and thus includes the Reticulatus zone in the Upper Norian (Sevatian 2). The conodont samples are stored at the Institute of Geosciences of the Montanuniversität Leoben. For each conodont sample the conodont alteration index (CAI) was determined according to Epstein et al. (1977) and Rejebian et al. (1987). For the methodology see also Nöth (1991), Königshof (1992) and Bur-
nett et al. (1994). Only acetic acid was used for sample preparation to avoid alterations of the conondonts. The CAI standard for the Northern Calcareous Alps of Gawlick and Königshof (1993), verified by Gawlick et al. (1994), was used for calibration. In this standard surface texture, size of fluor-apatite crystals, colour and thickness of different conodont elements (e.g. of Gondolella, Epigondolella) are taken into account. The standard has been verified and calibrated with Palaeozoic standards from the Appalachians and the Rheinisches Schiefergebirge (standards of A.G. Harris, U.S. Geological Survey, Reston, Virginia, and of P. Königshof, Forschungsinstitut Senckenberg, Frankfurt/ Main) as decribed in Gawlick and Königshof (1993). Twenty-one samples were cut for preparation of thin sections and isotope sampling. Isotope samples were drilled with a dental drill from polished surfaces. Care was taken to avoid drilling cement-filled bioclasts or veins by optical inspection with a microscope. Thus, the isotope samples represent mainly micrite and very small bioclasts. Carbonate powder samples were reacted with 100% H3PO4 at 75 C in an online, automated carbonate reaction device (Kiel Device) connected to a Finnigan Mat 252 mass spectrometer at the University of Erlangen Geological Institute. Isotopic ratios are reported in standard notation in permil relative to VPDB (Vienna Peedee Belemnite). External precision (2s) is 0.05½ for d13C and 0.1½ for d18O based on multiple analyses of standards NBS 19 and IAEA CO1. Isotopic variations (standard deviations) within single samples were determined as 0.11½ for d13C and 0.03½ for d18O by two repeat analyses in two samples. The dolomite content of those samples used for isotope analysis was determined by X-ray diffractometry applying the method described by Tennant and Berger (1957; see also Diebold et al. 1963). Most samples were measured twice with high counting rates. In most whole-rock samples the dolomite content was below detection limits (1±2%; Tennant and Berger 1957). In these cases we additionally analysed insoluble residues after leaching with weak acetic acid. Isotopic alterations during recrystallization and diagenetic stabilization of precursor sediments were calculated with a modified version of the model of Banner and Hanson (1990). Our model is identical to the Banner and Hanson (1990) model, except that the precursor and end product have different mineralogy and therefore different equilibrium fractionations. The model calculates the isotopic and chemical composition of a fluid volume (prescribed by porosity) after complete dissolution of the precursor sediment (aragonite). Next, all dissolved aragonite is reprecipitated as calcite from the resulting fluid with an isotopic composition determined by the fluid composition and equilibrium fractionation factors, partly depending on water temperature (Kim and O©Neil 1997; Romanek et al. 1992; Tarutani et al. 1969). Variations in the fluid/ rock ratio are simulated by varying porosity values.
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Results Stratigraphy and facies The Hallstatt Limestones of the Kälberstein quarry comprise a stratigraphic succession from the lower Tuvalian to the higher Sevatian (Fig. 1). The section (Fig. 3) starts in the eastern part of the quarry with the late Tuvalian Roter Bankkalk Member (for a definition of the Hallstatt Limestone members see Tollmann 1985, p 48). Conodonts from the lowest exposed
beds (sample BE 1/97) prove Tuvalian 3/1. The section continues with bedded reddish and grey, partly nodular limestones of the Tuvalian 3/2 (samples BE 17/97, 2a/96, 2/96, 1/96, 25/97, 16/97, 26/97, 27/97, 3/96). Thin sections show a biomicrite rich in ostracods, filaments and conodonts, rare radiolarians and
Fig. 3a,b Stratigraphic section of the Kälberstein quarry with sample positions, conodont biostratigraphy and sedimentary thicknesses. a Overview; b detail of the Carnian-Norian boundary
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crinoids, and rarely small parautochthonous lithoclasts. Only moderate bioturbation is visible. In its upper part the Roter Bankkalk Mb. becomes increasingly grey. In bed H2 (Fig. 3b) resedimentation is confirmed by conodonts (BE 16/97). The uppermost beds of this facies are dated as Lacian 1/1 (BE 18/97, 3/97). These Lacian beds still show the facies characteristics of the underlying Roter Bankkalk. However, for their Lacian age they are assigned to the Massiger Hellkalk Mb. (Tollmann 1985). A brachiopod coquina of up to 1.2 m thickness (Halobia styriaca Lumachelle, higher Lacian 1) overlies this basal Massiger Hellkalk with an angular unconformity. Fissures filled with Halobia shells reach down from the unconformity to the Tuvalian 3/2. Dip and strike of the lumachelle and the overlying Massiger Hellkalk are different from the underlying beds. The lumachelle is overlain by the Massiger Hellkalk Mb. of the higher Lacian 1, reddish-grey to grey massive biomicrites with ostracods, rare crinoids, conodonts, holothurians, radiolarians, filaments and juvenile ammonoids. Rarely parauthochthonous lithoclasts occur (BE 4/96). In the Lacian 2 the Massiger Hellkalk becomes increasingly grey. Intercalated monomictic to oligomictic breccia layers with mostly angular clasts, up to 5 cm large, are common (BE 9/97, 5/96, 6/96, 7/96). Most breccia components are parautochthonous intraclasts derived from the Massiger Hellkalk, i.e. biomicrites with ostracods, conodonts, recrystallized radiolarians, filaments, partly crinoids and juvenile ammonoids. Only in the Lacian 1/Lacian 2 transition zone are older clasts from the Tuvalian/Lacian boundary (BE 5/96) documented by conodonts. The breccia matrix is identical to the typical Massiger Hellkalk. The higher Lacian 2 and the Lacian 3 bring a return to reddish colours. The microfacies shows biomicrites with conodonts, juvenile ammonoids, ostracods, rare crinoids and foraminifers (BE 8/96). The Lacian 3 is characterized by a reddish, bedded, strongly bioturbated, biogene-rich limestone with conodonts, juvenil ammonoids, recrystallized radiolarians, few ostracods and pelagic crinoids (BE 24/97, 19a/97, 19/97, 9/96, 20/97). In the Lacian 3 the first hardgrounds are established as a consequence of reduced sedimentation rates. Additionally, the bioclast content increases. The bedding attitude of the Lacian-3 deposits successively approaches the dip and strike of the upper Tuvalian strata. Sedimentation of the Hangendrotkalk Mb. starts in the basal Alaunian 1 (BE 10/96). Fissures occur at the base of this member, probably reaching down to the Lacian 3 (BE 20/97). They are filled with red limestones of the higher Alaunian 1 (BE 10a/96). The Hangendrotkalk of the Alaunian 1 is characterized by reddish-brown well-bedded nodular biomicrites with ostracods, pellets, conodonts and rare radiolarians and small parautochthonous lithoclasts (up to 1 mm in size).
In the higher Alaunian 1 and in the Alaunian 2 the sedimentary succession consists of mostly red, rarely grey, well-bedded nodular limestones. In parts of the succession lithoclasts are common, mostly in connection with hardgrounds. Conodonts, ostracods, recrystallized radiolarians, juvenile ammonoids, rarely crinoids and foraminifers occur (BE 32/97, 31/97, 21/97, 30/97, 12/96, 22/97, 29/97, 28/97). The Alaunian 3 is characterized by red and reddish-grey, bedded, partly nodular micritic limestones. In some parts of the succession, again mostly in red layers above hardgrounds, parautochthonous lithoclasts are common (BE 23/97, 11/96, 13/96, 14/96, 17/96). They are probably derived from the upper Alaunian succession. Conodonts, ostracods, radiolarians and foraminifers are typical components in this upper part of the Hangendrotkalk. The red nodular limestones often show an intense stylolitization. The grey layers of the succession usually have higher bioclast and lower lithoclast contents. The Alaunian/Sevatian boundary is characterized by an abrupt change in facies to the massive or thickbedded grey micritic limestones of the Hangendgraukalk, i.e. reddish-grey to grey limestones with conodonts, ostracods, crinoids and rare parautochthonous lithoclasts (BE 7/97, 6/97, 16/96, 15/96). The Halobia salinaria lumachelle interupts this monotonous series well within the Sevatian 1. Only the highest exposed beds at the top of the quarry in its western part can be attributed to the Sevatian 2 (BE 15/96). Mass accumulation rate estimates Based on the biostratigraphic framework given above, we estimate mass accumulation rates for each member (Table 1). The timescale is based on the assumption of equal duration of ceratite subzones within the framework of radiometrically dated tie-points (Gradstein et al. 1995). The duration of the Norian stage, comprising 16 subzones, is given by Gradstein et al. (1995) as 11.1 Ma. The Carnian (10 subzones) has a duration of 6.6 Ma and the Rhaetian (6 subzones) of 3.9 Ma. The uncertainty of the absolute ages of the Late Triassic stage boundaries is approximately 4.5 Ma, but the uncertainty in the relative duration of the individual stages is probably much less (Gradstein et al. 1995). This results in a mean subzone duration of 0.7 0.2 Ma for the Late Triassic, with the given error range representing an upper limit. Of course, the equal duration of ceratite subzones is an unproven assumption and can only provide a rough approximation. Besides interpolating ages between stage boundaries, Gradstein et al. (1995) also use equal subzone durations in the Triassic±Middle Jurassic interval to better confine stage boundary ages within the limits provided by radiometric data. Their confidence in this approach is based on Late Cretaceous ammonite zones dated by 40Ar/39Ar that deviate
113 Table 1 Mass accumulation rate estimates for the Kälberstein section calculated from measured sedimentary thicknesses, biostratigraphic ranges, a grain density of 2.7 g/cm3 and a porosity of 10%. A mean duration of Carnian to Rhaetian ceratite subzones of 0.7 million years is assumed (Gradstein et al. 1995)
Sedimentary interval
Thickness (m)
Subzones
Duration (Ma)
Accumulation rate (kg/cm2/Ma)
Tuvalian 3/2 Lower Lacian 1/1 Lacian 1/2±Lacian 2/3 Lacian 3 Alaunian 1±Alaunian 3/1 Alaunian 3/2 Sevatian 1 Tuvalian 3/2±Sevatian 1
15 ca. 0.5 42 16 18 14 10 90
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