Bed-by-bed correlation of trench-plain turbidite sections, Campanian Zementmergel Formation, Rhenodanubian Flysch Zone of the East Alps R. Hesse Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7

lntroduction Tracing individual turbidites for lang distances by means of finger-printing through correlation of detailed measured field sections is one important source of information for the quantitative treatment of the flow process of turbidity currents. Due to their predominant occurrence in the deep sea, turbidity currents are hidden from direct observation and need to be studied by their deposits and the morphology of the depositional environment which they create. Few successful correlation attempts have been reported in the literature. This is because most environments of turbidite deposition do not contain very extensive continuous beds and are not suitable for correlations, not to speak of the painstaking efforts required for correlations. In this chapter a new example of a bed­by-bed correlation is presented from the Upper Cretaceous helminthoid flysch in the Rhenodanubian Flysch Zone (East Alps) in Germany.

Geologie setting The Rhenodanubian Flysch Zone is a 500 km lang, highly deformed Cretaceous flysch belt at the northern margin of the Alps that is on average less than 20 km wide. The Rhenodanubian Flysch has been interpreted as the sedimentary fill of a darrnarrt deep-sea trench located in the northern periphery of the Alpine sector of the Tethyan Ocean (Hesse and Butt 1976; Hesse 1982). lt was connected via the West Carpathians with an active Cretaceous trench/island-arc system in the East Carpathians in a fashion similar to the modern

Puerto Rico trench. The latter, which is presently no Ionger associated with subduction, joins the actively subducting Lesser Antilies island-arc system by swinging southward at its eastern end. Based on regional geologic correlations, the East Alpine-Carpathian Are is thought to have represented a similar tectonic configuration in the Cretaceous and Early Tertiary Tethys ocean (Hsü 1972; Hesse 1982).

Detailed sedimentologic analysis of the Albian Gault Formation of the Rhenodanubian Flysch resulted in one of the first successfullong-distance bed-by-bed correlations of turbidite sections described in the literatme (Hesse 1965). The sandstarre turbidites of the 200m thick formation could be traced over a distance of 115 km parallel to the strike of the flysch belt. The facies of the various Stratigraphie units is remarkably constant parallel to the strike of the flysch belt and at the same time varies considerably over short distances perpendicular to strike giving rise to two separate Stratigraphie columns for the Flysch Zone in Bavaria, one for the Northern or Sigiswang Facies and the other for the Southern or Oberstdorf Facies. The Lower Cretaceous formations are mostly restricted to the Southern Facies. Palaeocurrent directions are largely parallel to strike.

The presence of a more or less horizontal trench abyssal plain (Hesse 1974, 1982) was suggested by: 1. successful bed-by-bed correlations in the Gault Formation; 2. facies constancy parallel to strike; 3. repeated palaeocurrent reversals (from W---jE to E---jW); 4. evidence for deposition below the calcite compensation Ievel (CCL). The trench had tobe tectonically inactive because it collected a 1.5 km thick undeformed sediment sequence over a period of 50-60 m.y. In the present study, eight detailed measured sections of the Campanian Zementmergel Formation (also called Kalkgraben Formation; Hesse 1991) of the Northern Facies are shown to correlate bed-by-bed.

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, RN. Hiscott, N.H. Kenyon, F. Ricci Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

Campanian Zementmergel Formation The Zementmergel Formation of the Northern Facies is 170m thick in section 711 (Fig. 46.1), which is the only complete section with minor, measurable fault displacements. 1t consists of seemingly monotonaus alternations of carbonate turbidites, mixed carbonate-siliciclastic turbidites and carbonate-free hemipelagic claystones. The sandstone-siltstone portion of the turbidites, which represent mostly Tc,d and T d, and rarely Tb-cl or T a-d structure sequences (28 cm combined thickness on average), is made up of biogenic and detrital carbonate and siliciclastic sand and silt. Markov-analysis results (Hesse 1975) showed that it is capped by a thick marlstone turbidite (28 cm thick on average) or sametim es by a thin (12 cm on average) lutitic limestone (Te division). This is followed by a thin green hemipelagic claystone (5 cm on average) deposited below the CCL (Hesse and Butt 1976; Hesse 1991). The latter may be missing due to erosion by the following turbidity current. The lutitic limestone may also form separate, individual turbidites less than 30-40cm thick. Where the fine-grained rock­types are not too intensely recrystallized they contain a nannofossil flora suitable for biostratigraphic dating (e.g. Egger 1992). Palaeocurrent directions are predominantly from west to east, as in the Albian. Bioturbation in the hemipelagic claystone has obliterated any Iamination that may have been present. The turbidites are not affected by bioturbation except in the uppermost 5-lOcm of the marlstone which may contain fecal pellets and green hemipelagic sediment moved downward in burrows of Chondrites sp. These and the meandering grazing traces of Helminthoidea sp., which are found on partings in the top layers of the marlstone, are the most abundant distinct trace fossils; Zoophycos sp. are comparatively rare. From Helminthoidea the name 'helminthoid flysch' is derived for numerous Upper Cretaceous carbonate or mixed carbonate-siliciclastic-turbidite formations of the Alps, northern Apennines and Carpathians which are lithologically equivalent to the Zementmergel Formation (Caron et al. 1981).

The thick marlstone cap of the carbonate turbidites represents redeposited nannofossil ooze and other biogenic components originally deposited on the slope above the CCL, to which terrigenaus (carbonate and siliciclastic) sediment was admixed. Where no terrigenaus sediment reached the slope, pure carbonate ooze was deposited, which formed the lutitic Iimestone caps of the turbidites after redeposition.

Result of the correlations and conclusion Cumulative thickness variations show a trend towards thinner sections at the northern margin of th,e Flysch Zone (i.e. sections 703, 704, 706 and 711, Fig. 46.1) compared to sections in a more interior position (i.e. sections 705 and 707, Fig. 46.1), although section 708 (Fig. 46.1) (near northern margin) is also relatively thick. The maximum map-distance between northern and southern sections is only 2 km, which corresponds to 4-5 km on a palinspastic reconstruction. The maximum downcurrent distance (parallel to the predominant palaeocurrent direction) between the correlated sections (i.e. sections 703 and 711, Fig. 46.1) is 40km.

The successful correlation of Zementmergel sections demonstrates that the deposition of very extensive sheet-like turbidites occurred several times in the Cretaceous of the Flysch Zone and was not restricted to the Gault Formation, reinforcing the evidence for the trench-plain interpretation.

Heferences Caron, C., Hesse, R., Kherkhove, C., Homewood, P., Van

Stuijvenberg, ]., Tasse, N. and Winkler, W. 1981. Comparaison preliminaire des flyschs a Helmintholdes sur trois transversales des Alpes. Eclogae geologicae Helvetiae, 74, 369-378.

Egger, H. 1992. Zur Geodynamik und Paläogeographie des Rhenodanubischen Flysches (Neokom-Eozän) der Ostalpen. Zeitschrift der deutseben geologischen Gesellschaft, 143, 51-65.

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Hesse, R. 1965. Herkunft und Transport der Sedimente im bayerischen Flyschtrog. Zeitschrift der deutschen geologischen Gesellschaft, 116, 403-426.

Hesse, R. 1974. Long-distance continuity of turbidites: Possible evidence for an Early Cretaceous trench abyssal plain in the East Alps. Geological Society of America Bulletin, 85, 859-870.

Hesse, R. 1975. Turbiditic and non-turbiditic mudstone of Cretaceous flysch sections of the East Alps. Sedimentology, 22, 387-416.

Hesse, R. 1982. Cretaceous-Palaeogene Flysch Zone of East Alps and Carpathians: Identification and plate-tectonic significance of 'dormant' and 'active' deep-sea trenches in the Alpine-Carpathian Are. In: Leggett, ].K. (ed.) Trench­Forearc Geology. Geological Society of London, Special Publication, 10, 471-494.

Hesse, R. 1991 . Flysch-Zone. In: Hesse, R. and Stephan, W. (eds) Geologische Karte von Bayern 1:25 000. Erläuterungen zum Blatt Nr. 8234 Penzberg, pp. 20-74.

Hesse, R. and Butt, A.A. 1976. Paleobathymetry of Cretaceous turbidite basins of the East Alps relative to the calcite compensation Ievel. Journal of Geology, 84, 505-533.

Hsü, K. 1972. Alpine flysch in a Mediterranean setting. Reports of the 24th International Geological Congress, 6, 67-74.

Fig. 46.1. Bed-by-bed correlation of sections 703- 711 of the Campanian Zementmergel Formation, helminthoid flysch facies of the Rhenodanubian Flysch Zone (Northern Facies) of the East Alps. Thieker beds that have been correlated between sections are tied by correlation lines in the figure. For convenience of data presentation and because of the ease of identification, the most prominent thiek bed in the middle of the Stratigraphie column of the formation has been used as reference Ievel. Inset (A). Geologie map of the central portion of the Rhenodanubian Flysch Zone in Bavaria with locations of the measured sections. Tickmarks on bee-line between sections 703 and 711 are 10 km apart, except in the west and in the east, where the distance is 1 km. For abbreviation of Stratigraphie names see inset (B). Inset (B). Stratigraphie table for the Rhenodanubian Flysch Zone (after Hesse 1982) with abbreviations used in inset (A). Inset (C). Outline of the Alps with position of measured sections 703 and 711.

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