«Topics: Paleocene/Eocene-boundary section in a succession of deep-water turbidites and hemipelagites Tectonic unit: Rhenodanubian Flysch Zone ...»
Hans Egger, Juliane Fenner, Claus Heilmann-Clausen, Fred Rögl, Birger Schmitz
Paleocene/Eocene-boundary section in a succession of deep-water turbidites and hemipelagites
Rhenodanubian Flysch Zone
Rhenodanubian Group, Anthering Formation
Upper Paleocene to Lower Eocene
Upper part of calcareous nannoplankton Zone NP9 to upper part of Zone NP10
Outcrops in the Kohlbachgraben near Anthering
E 013° 01′ 17″, N 47° 53′ 19″
Heilmann-Clausen & Egger, 1997, Egger, Heilmann-Clausen & Schmitz (2000), Crouch et al. (2001), Egger et al. (2003), Huber et al. (2003), Egger & Brückl (2006), Iakovleva & Heilmann-Clausen (2007), Egger, Heilmann-Clausen &Schmitz (2009) From the carpark at the Reinthal inn it is an approx. 10 minutes walk on a small road to the first outcrop of the section, which is located along the course of the Kohlbach creek (no trail!). We examine the section (Fig. A1.9) walking up-stream from the lower Eocene (NP10) to the uppermost Paleocene (NP9).
The Anthering section is located about 18 km to the north of the Untersberg section as the Anthering and Untersberg sections are separated by the thrust between the Northern Calcareous Alps and the Rhenodanubian Flysch zone, the original palinspastic distance between them must have been much greater than at present. However, reliable data on this distance are lacking.
The 250 m thick upper Paleocene to lower Eocene deposits of the Anthering section, spanning calcareous nannoplankton Zones NP9 and NP10. These sediments comprise the youngest part of the Rhenodanubian Group. This group was deposited on the continental rise to the south of the European plate, which was the main source for the siliciclastic detritus entering the basin. The section is composed of calcareous mud-turbidites with intervening hemipelagic claystone indicating a deposition below the calcite compensation depth. The general sedimentary record of the Anthering-section is typical for an abyssal plain facies. Paleo-water depth estimations by Butt (1981), using foraminifera assemblages, range between 3000 to 5000 m.
Berichte Geol. B.-A., 86 (ISSN 1017-8880) – CBEP 2011, Salzburg, June 5th – 8th In the Eocene part (Anthering Formation) of the section, the turbidite succession is characterized by the predominance of graded silty marlstone, which form about 85 % of the succession (Anthering Formation). Occasionally, these turbiditic marlstone layers overlie silty to sandy beds deposited from the same turbidity current. The turbidites usually display basetruncated Bouma-sequences. Turbidites displaying complete Bouma-sequences are very rare. Single turbidite layers can reach thicknesses up to 2 m. The finegrained sand-fraction represents, on average, 5 % of the sedimentary rocks and exceptionally up to 10 %. The fine-grained (silty-clayey) sediment displays carbonate contents of 29 % to 53 %. The clay fraction is dominated by smectite.
Common intercalations of hemipelagic claystone occur between the individual mud-turbidite beds. The hemipelagic claystones prove a position of the basin-floor below the local calcite compensation depth. They are devoid of carbonate and display sharp contacts to the turbiditic marls. Usually the claystones show a greenish to greyish colour (0.15 wt% organic carbon on average) with a large number of dark spots as indications of intensive bioturbation. Only in the middle part of the section (outcrop E and one layer in outcrop D) darkgrey homogeneous claystones with abundant pyrite framboids and relatively high contents of organic carbon (0,94 wt% on average) occur. These black shales indicate an oxygen deficient environment at the basin floor. As they occur together with Figure A1.9 ▲ bentonite layers, volcanism might have led to eutrophic condi- Location of outcrops and biostratigraphy of tions and high plankton productivity responsible for the anoxic the Anthering sectionsections near Anthering conditions.
In the lowermost Eocene (Subzone NP10a) at the Anther- Figure A1.10 ▼ ing section, 23 layers of altered volcanic ash (bentonites) Photograph of Outcrop B
This value is ca. six times higher than the hemipelagic sedimentation rate in the Paleocene (Egger et al., 2009b).
The increased rate of hemipelagic sedimentation at the CIE suggests a high input of siliciclastic suspension into the basin. At the level of the CIE clay mineral assemblages of hemipelagic claystone display a distinct increase of smectite and kaolinite at the expense of illite and chlorite (Egger et al., 2002). This indicates a decrease of bedrock erosion in the adjoining land areas. Well-developed smectitic soils with a mixture of kaolinite are mostly restricted to subtropical climates with a well-marked dry season (see Thiry, 2000 for a review). During the rainy season continental erosion of such areas is very pronounced (see van der Zwan, 2002, for a review) and will result in a strong increase in hemipelagic sedimentation rates (Schmitz et al., 2001).
Enhanced erosion of land areas around the CIE-interval can also be inferred from the composition of calcareous nannoplankton assemblages. Whereas, in general, reworked Cretaceous species form only 2 – 3 % of the Figure A1.13 ▲ calcareous nannoplankton Log of outcrop E showing positions of bentonassemblages of the Antherites and immobile eleing section, substantial Crement-concentrations of taceous admixtures are presbentonites ent in many samples from across the CIE (Fig. A1.15).
The oldest nannoplankton assemblage showing a high percentage ( 50 %) of reworked specimens originates from a turbidite bed 22 m below the onset of the CIE. Three metres above the onset of this geochemical marker, the youngest assemblage with a similar percentage of reworked Cre- Figure A1.14 ► taceous specimens has been Two bentonite layers at found. outcrop E
Stop A1/2 Anthering Section
Figure A1.15 ▲ Lithostratigraphy percentages of redeposited Cretaceous nannoplankton and stable isotope record of oxygen and carbon across the CIE-interval at Anthering.
A. spp. percentages of the genus Apectodinium in the dinoflagellate assemblages (Egger et al. 2009b) Most of the reworked specimens consist of species with a long stratigraphic ranges (Watznaueria barnesae, Micula staurophora, Retecapsa crenulata, Cribrosphaerella ehrenbergii, Eiffellithus turriseiffelii). Biostratigraphically important species that were found in all of the counted samples include Broinsonia parca, Arkhangelskiella cymbiformis (small specimens), Calculites obscurus, Lucianorhabdus cayeuxii and Eiffellithus eximius whilst Marthasterites furcatus, Eprolithus floralis and Lithastrinus grillii were found only occasionally. This assemblage suggests that predominantly lower to middle Campanian Berichte Geol. B.-A., 86 (ISSN 1017-8880) – CBEP 2011, Salzburg, June 5th – 8th deposits were reworked at the end of the Paleocene. Probably, the erosional area was the North-Helvetic shelf at the southern European Plate where the Middle Eocene is resting with an erosional unconformity on the Upper Cretaceous.
Substantial reworking of the Cretaceous started already in the latest Paleocene. At Anthering, the uppermost 20 m of the Paleocene succession are formed by the thickest turbidites (up to 5 m) of the entire section. The siliciclastic sandfraction in the turbidites forms around 30 % of the rocks in this part of the section (Altlengbach Formation). This suggests that a sea-level drop took place shortly before the onset of the CIE. This is consistent with data from the Atlantic region (HeilmannClausen, 1995; Knox, 1998; Steurbaut et al., 2003; Pujalte and Schmitz, 2006;
Schmitz and Pujalte, 2007). The synchroneity of this sea-level drop in the Atlantic and Tethys regions indicates a eustatic fluctuation. Starting with the onset of the CIE, mainly fine-grained suspended material came into the basin and caused an increase in hemipelagic sedimentation rates by a factor of 5 or 6. Such an increase associated with decreasing grain-sizes has already been reported from P/E-boundary sections elsewhere and interpreted as an effect of a climate change at the level of Figure A1.16 ▲ the CIE, affecting the hydrological cycle Distribution of the genera Apectodinium and Wetzeliella at Anand erosion (Schmitz et al., 2001).
thering. Apectodinium is shown as percentage of organic-walled microplankton. One fragmentary, possible specimen of A. augustum was recorded in outcrop N.
DINOFLAGELLATE CYSTSGeneral characteristics Dinoflagellate cysts are present in all samples from the Anthering section. Preservation varies from good to moderate.
There is a tendency to better preservation in samples from turbidites than in hemipelagic samples, perhaps due to sea-floor oxidation during slow, hemipelagic sedimentation.
Common genera and species occurring throughout the section are Apectodinium spp., Spiniferites spp., Areoligera spp. + Glaphyrocysta spp. (generally 2 – 15%), Polyspaeridium zoharyi (usually 1 – 5%), Homotryblium tenuispinosum (usually 1 – 5%), Operculodinium cf. centrocarpum Figure A1.17 ▲ (mostly 4 – 12%), and Phthanoperidinium Apectodinium augustum. Left: specimen from Anthering, Outcrenulatum (mostly 1 – 3%). Lingulodinium crop J. Right: specimen from the CIE interval in Denmark (lomachaerophorum occurs sporadically and wermost Ølst Formation, Viborg-1 borehole).
Stop A1/2 Anthering Section
usually amounts to less than 1 %. The overall composition of the dinoflagellate assemblages allows a simple subdivision of the section into three parts: The lower and upper intervals are characterized by a generally low dominance and relatively high species richness. These two intervals are separated by a middle interval coinciding with the CIE (outcrops J and JA). There the genus Apectodinium is dominant and reaches abundances up to 69 % in hemipelagic samples (Fig. A1.16). Below and above this interval Apectodinium usually accounts for 5 – 20 % of the dinoflagellate assemblages. The genus Apectodinium includes several intergrading species, the Apectodinium-plexus of Harland (1979). In spite of the strong dominance, the species richness remains relatively high within the CIE interval.
Quantitative dinoflagellate cyst data from hemipelagic layers of outcrop J reveal a 10-fold to 40-fold increase in the total number of cysts within the CIE interval (where Apectodinium dominates) (up to ca.
40.000 cysts/g) relative to pre-CIE samples. Above the CIE, counts reveal fluctuations in cyst numbers, but with a general trend towards smaller numbers of cysts.
Paleoecology Relying on information from modern cyst production (e.g., Dale, 1996), the Anthering section must have been deposited below neritic waters, or waters that originated in the neritic zone. The genus Impagidinium, which today is purely oceanic, is present in several samples (especially in outcrop N), but usually rarer than 1 – 2%. Such low occurrences indicate the neritic/oceanic boundary zone (Dale, 1996). Neritic cysts are today transported over long distances with currents, and are deposited in various basinal parts of the Atlantic Ocean (e.g., Dale, 1996). The continuous presence of Polysphaeridium zoharyi and Homotryblium tenuispinosum is evidence of a rather constant, and significant, mixing of the water masses at Anthering. Polysphaeridium zoharyi today mainly characterizes equatorial lagoons (Dale, 1996), and the extinct Homotryblium is a dominant form in several well-documented inner neritic, probably lagoonal settings of various ages (e.g., Köthe, 1990; Sluijs et al., 2005).