Keuper stratigraphic cycles in the Paris Basin and ... - Science Direct

Sedimentary

Geology ELSEVIER

Sedimentary

Geology

105 (1996) 159-182

Keuper stratigraphic cycles in the Paris Basin and comparison with cycles in other Peritethyan basins (German Basin and Bresse-Jura Basin) Sylvie Bourquin a**,Francois Guillocheau b ClUMR

5561 du CNRS, Centre des Sciences de la Terre, 6 boulevard Gabriel, 21000 Dijon, France I’Gtfosciences-Rennes, UPR 4661 du CNRS, Universite de Rennes I, 35042 Rennes cedex, France Received

1 November

1994; accepted

16 November 1995

Abstract High-resolution sequence stratigraphy of the Keuper, Paris Basin, is used to establish correlations between the basincentre evaporite series and the basin-margin elastics series. The high-resolution correlations show stratigraphic cycle geometries. The Keuper consists of five minor base-level cycles which occur in the upper portion of the Scythian-Camian

major base-level cycle and the lower part of the Camian-Liassic major base-level cycle. The maximum relative rate of subsidence for the base-level fall phase of the Scythian-Camian

major cycle occurs

in the eastern part of the Paris Basin. During the base-level rise phase of the Camian-Liassic major cycle, the area of highest rate of subsidence shifted westwards and northwards. This shift records the first occurrence of an independent Paris Basin which was no longer merely the western margin of the German Basin. Two phases of tectonic movement influenced the distribution of the depocentres. The first is recorded in the ‘Mames irisees infkrieures’ deposits. Faulting controlled evaporite sedimentation and sequence geometries by creating areas of subsidence where halite could accumulate. The second, within the ‘Mames iris&es supkrieures’, induced a general westward and northward tilt of the basin. Concurrent migration of depocentres to the west and north produced an intra-‘Mames irisCes supkrieures’ truncation. Comparison of the stratigraphic records of the Paris Basin and of other Triassic Peritethyan basins (German Basin, Bresse-Jura Basin and South-East Basin) reveals numerous similarities. The coastal onlap curve of the German Keuper (Aigner and Bachmann, 1992) exhibits many similarities with the sequence evolution of the Paris Basin. But the Triassic succession is more complete in the German Basin and more cycles are observed. The major difference between these two basins during the Keuper is that the ‘Mames irisCes inf&ieures’ minor base-level cycle does not occur in the German Basin. In the Bresse-Jura Basin, the major difference concerns the Lettenkohle. One minor base-level cycle is recorded in the Paris Basin while no cycle is observed in the Bresse-Jura Basin.

1. Introduction The Paris Basin is commonly considered a product of a phase of extension during the Permo-Triassic * Corresponding author. 0037-0738/96/$15.00

and Le Pichon, 1982; Curnelle and Dubois, 1986; Perrodon and Zabek, 1991). Triassic strata of the Paris Basin were deposited in an epicratonic Peritethyan basin and exhibit three main lithological subdivisions (Fig. 1): (1) the predominantly continental Buntsandstein (Lower Triassic); (2) the (Brunet

Copyright

SSDlOO37-0738(95)00153-O

0 1996 Elsevier Science B.V. All rights reserved.

S. Bourquin,

160

E Guillocheau/Sedimentar?

Geology

FRANCHEVILLE

105 (1996)

159-382

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Fig. 1. Lithostratigraphic column, sedimentary environment variations and biostratigraphic data (after Bourquin et al., 1995, moditied) of the Triassic succession in the eastern part of the Paris Basin based on the Francheville well. See Fig. 2 for location. (a): Durand and Jurain (I 969); Gall ( I97 I ). (b): Kozur (1972); Adloff et al. (I 982); Khatib-Nguyen (1977).(c): Kozur (1972); Khatib-Nguyen ( 1977). (d): Kozur (1972); Adloff et al. (1984). (e): Kozur (1972): Geisler et al. (1978). (f): Kannegieser and Kozur (1972); Kozur (1993). (g): Schuurmann (1977).

S. Bourquin, E Guillocheau/Sedimentary

marine Muschelkalk (Middle Triassic); and (3) the mainly continental Keuper (Upper Triassic). The aim of this paper is to show the geometries of stratigraphic sequences in the Keuper of the Paris Basin. High-resolution sequence stratigraphy, analysis of well-log and core data from 300 wells across the entire basin, and outcrops in the eastern part of the basin allow correlations between the evaporite series, located mainly in the centre of the basin, and the elastic series of the margins. This paper also compares the Keuper stratigraphic records across the West European Craton (i.e., German, Bresse-Jura and South-East Basins). 2. Geological setting The Triassic of the Paris Basin is subdivided into three major base-level cycles (Fig. 1): the Scythian, Scythian-Camian, and Camian-Liassic cycles (Guillocheau, 1991). The Scythian cycle is essentially fluvial (‘Gres vosgien’ Formation). The ScythianCamian cycle starts with braided fluvial facies (‘Conglomerat principal’ Formation) and ends at an erosional surface at the base of the ‘G&s a roseaux’ fluvial deposits. The maximum flooding surface of the Scythian-Carnian cycle occurs within the marine storm-dominated deposits of Ladinian age (top of the ‘Calcaires a ceratites’ Formation). The’ CarnianLiassic cycle commences with the fluvial deposits of the ‘Gres a roseaux’ Formation. A maximum flooding surface occurs in the Lower to Middle Toarcian marine deposits (‘Schistes cartons’ Formation). The Triassic series studied (Fig. 1) comprise the upper portion of the Scythian-Camian cycle and the lower part of the Camian-Liassic cycle. Classically, two areas of sedimentation have been distinguished in the Keuper of the Paris Basin. These are an eastern area consisting essentially of halitic or anhydritic coastal sabkha deposits and a western area dominated by fluvial deposits (Dubois and Umbach, 1974; Courel et al., 1980; Matray et al., 1989). The Saint-Martin de Bossenay fault divides these two areas (Bourquin and Guillocheau, 1993, Fig. 2). 3. Procedure The correlations established in the Keuper of the Paris Basin are based on high-resolution sequence

Geology 105 (1996) 159-182

161

stratigraphy, where stratigraphic cycles of different scales and durations are recognized and correlated. Both correlations and cycle ranks are based on the identification of the smallest stratigraphic units recorded in well logs, and their groupings into larger scales by stacking pattern analysis. The smallest units, termed genetic sequences, are several metres to several tens of metres thick and represent time spans of tens of thousands to several hundred thousand years (Cross, 1988; Homewood et al., 1992; Eschard and Doligez, 1993). Genetic sequences are defined as package of strata representing a complete base-level cycle of sediment accumulation (Busch, 1971) with no specification as to base-level cycle origin, the cycle duration or cycle symmetry. They are recognized by regular, recurring patterns of vertical successions of facies in each environment. A genetic sequence is a time-bounded rock unit because it consists of numerous facies tracts that were linked spatially and temporally during sediment accumulation. The genetic sequences are the stratigraphic record of sediment accumulation during a cycle of base-level rise to fall, i.e., increasing and decreasing sediment accommodation (Cross et al., 1993). Within the predominantly continental Keuper strata in the Paris Basin, base-level rise to fall turnaround surface can be correlated easily across the basin, In such instance, each genetic sequence is bounded by base-level rise to fall turnaround surfaces and record a complete cycle of base-level fall and rise. Typical well-log signatures of genetic sequences in the numerous continental environments are illustrated in Fig. 3. The facies successions were defined in few wells where cores were used to calibrate the well-log signatures. Vertical successions of the genetic sequences at each well are used to define successively larger scale stratigraphic cycles. By successive smoothing of the facies transitions and successions, and by observing whether the stacking arrangement of genetic sequences are successively more landward (or deeper or thicker) or more seaward (or shallower or thinner or more amalgamated) different scales of stratigraphic cycles are identified. In increasing scale and duration, these are stratigraphic cycles are termed genetic sequence set, minor base-level cycle and major base-level cycle. The base-level cycles are the stratigraphic record of sediment accumulation during a cy-

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