Depositional sequences geometries of the Triassic series of the Paris. Basin have been elucidated by high resolution correlations between out- crop and ...
Depositional sequences in the Triassic series of the Paris Basin: Geodynamic implications 5. BoUROUIN
»,
R.
FRIEDENrIERG **
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GUíLLocííEAt
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URA 157 du CNRS, (‘entre des Sciences de la Terre, 6 boulevard Gabriel, 21000 DIJON, France ** Géosciences-Rennes; UPR 4661 du CNRS, Université de Rennes 1, 35042 RENNES cedex, France *
ABSTRACT
Depositional sequences geometries of the Triassic series of the Paris Basin have been elucidated by high resolution correlations between outcrop and subsurface data using stacking pattern of genetic units. The Triassic may be subdivided into three transgressive-regressive cycles: Scythian, Scythian-Carnian and Carnian-Lias. The sediments of the Scythian cycle are exclusively fluvial and only record the transgressive trend («Grés vosgien»). The Scythian-Carnian cycle commences with a braided fluvial facies («Conglomérat principal») and is terminated at the base of the «Grés it roseaux». The maximum flooding surface occurs within the marine storm-dominated deposits of Ladinian age (top of the «Calcaires it Cératites»). This cycle may be subdivided mío íhree third order sequences during ihe transgressive trend —-Scythian (oConglomérat principals>-oCouches intermédiaireso), Amsian («Grés it Voltzia»-oComplexe de Volímunster»), and Anisian-Ladinian (‘-
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tures and comprises two phases. The first phase, based on one dimensional (vertical) information, consists in (1) observing sedimentary facies in cores and at outcrops, (2) deducing depositional processes, (3) identifying facies associations and interpreting sedimentary environments, (4) constructing a sedimentological model, (5) distinguishing individual genetic units, (6) obtaining well-log signatures of the genetic units, and (7) establishing the stacking pattern of genetic units. The second phase, of establishing correlations between the various vertical sections produced in phase one, includes: (8) selecting a regional datum, (9) correlating individual genetic units and sequences, and is completed by (10) mapping sequences (Homewood eta!, 1992). In subsurface studies, it is essential to use complete sets of log data to identify and correlate genetie units, especially in continental environments (Bourquin eta!, 1990, 1993). For example, sandstones that consist large amounts of radioactive minerais (potash feldspar, heavy minerals, etc.) may produce high gamma-ray values similar to values obtained from clays. Consequently, the use of gamma-ray and the sonic logs alone may lead to mis-interpretation. Similarly, a density log coupled with a photoelectric factor log is required to distinguish between dolomitie and anhydritic shales. Neutron-porosity, density and photo-electric factor logs, used with high resolution logs (dipmeter or Formation Microscanner) are necessary (1) to determine sedimentary facies, (2) to calibrate cores and outcrops with well-logs and (3) to obtain significant correlations. The results are summarized on an east-west section between Emberménil (east of Nancy) and south-west Paris.
SCYTHIAN CYCLE Only the transgressive part of this cycle is recorded in continental sediments (oGrés vosgien»). This hemi-cycle corresponds to a vertical Figure 2.—Lithostratigraphic units along an E-W sections. a: location of stratigraphic units. The limits between the litht,stratigraphic units are not tirne-lines. See figure 1 for the stratigraphy. 1: sandstones; 2: dolomite; 3: halite; 4: basemenr; 5: lithostratigraphic limits. CH 17: (‘baunoy 17; JAY: Janvry; Mt Méligny; FRV: Francheville. Figura 2—Unidades litostratigráficas en un corte l-¿-W. a: localización de los 37 sondeos correlacionados en el corte, b: geometría de las unidades litostratigráficas. Los límites de las unidades no son líneas-tiempo. Estratigrafía en la figura 1. 1. areniscas; 2. dolomias; 3. halita; 4. basamento; 5 limites litostratigráficos. CH 17: Chaunoy 17; lAY: lanvry; M: Meligny; FRV: ¡tancheville.
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stacking of channels. These channels belong to straight to braided systems. The top of the sequence is characterized by the «Conglomérat principal» basal erosional surface. This sequence is dominated by a tendency of base-level falí. This cycle doesnt exis’t on the western part of the basin because alí the sediments pinch out laterally, displaying onlap truncation over the «1-faute-Marne» fault (Figs. 2, 3).
SCYTHIAN-CARNIAN CYCLE TRANscRassívn l’ART OF SCYTIItAN-CARNIAN CYCLIL
The transgressive part of this second order cycle may be subdivided into higher order sequences. However, the exact order of these sequences, third or fourth order, is very difficult to establish because of poor biostratigraphic information. Whereas Buntsandstein may be subdivided into two sequences (Friedenberg, 1994), only one sequence is recorded by the Muschelkalk.
The «Conglomératprincipa/x’-«Couches intermédinires» sequence
Ibis sequence is highly asymmetrie anawas dominated by a rising base-level. The basal unconformity is very well marked: it is a major erosional surface, cnt across locally tilted underlying sediment of the «Grés vosgien». Both the oConglomérat principal» and «Couches intermédiaires» exhibit lateral facies changes when traced away from the outcrops in the east (Figs. 3, 4). The «Conglomérat principal» was deposited by proximal braided channels (Durand, 1978, Courel et a!, 1980) and grades westward into muddier deposits. In contrast the «Couches intermédiaires», which in tbe east consists of lacustrine coarse-grained sheet-flood deposits grades westwards into a distal braided channel. These facies Figure 3.—Location of the wells and taults (after Perrodon & Zabek. 1990) for the E-W seetions of the Buntsandstein and the Muschelkalk. M: Méligny; FRV: Franeheville: SJD: SaintJtist-Sauvage Figure 3: Localizacin de sondeos y fallas (sego Perrondon & Zabek, 1990) en el corte E-W del Buntsandstein y el Musehelkalk. M: Méligny; MIV: Francheville; SJD: Saintiusl-Sauvage. Figura 3.—Localización de sondeos y fallas (según Perrodon &Zabek, 1991) en el corte E-W del Buntsandstein y el Muschelkalk. M: Meligny; FRV: Francheville, 5 J D: Saint-Just-Sauvage.
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changes are located over major basement faults such as the Meuse and Metz faults and suggest a reversal of the tendency (Friedenberg, 1994). The boundary between these two formations has been termed the oCouches violettes» and corresponds to paleosois of several meters thickness.
TI-te «Crés ñ Voltzia»-«Couches de Vo/Imunter» sequence
Tbc lower boundary of this sequence corresponds to a by-pass zone overlain in the east by more proximal facies (braided deposits) than the underiying ones. This sequence 15 again highly asymmetric aud dominated by a trend of base-level rise. Ihe sequence boundary records a major change in facies zonation; the depositional profile dips westward, Le. the most proximal facies are located to the east (Figs. 3, 4). In the castern part, the «Grés it Voltzia» was produced by a fluvial system (Durand, 1978) which evolved (Fig. 5) from braided, to straight, and then mainly anastomosing channels and finally a distal alluvial plain (Friedenberg, 1994). This paltogeographic trend has lcd to misunderstanding of the stratigraphic record in the west where the distal anastomosing channel facies are mixed with overlying Muschelkalk sediments. The cnd of this base-level risc is marked by the evaporitie coastal plain deposits of the Lower Muschelkalk (oComplexe de Volímunster») which pinch out latcrally and display an onlap truncation on the oGrés it Voltzia» over thc Haute-Mame fault.
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Ihis third sequence shows a general evolution to more marine envtronments (Middlc to Uppcr Muschelkalk). The maximum flooding is reach at the top of the «Calcaires it Cératites» deposits (Figs. 3,6,7). Figure 4.—E-W sectioti of the Buntsandstein between Emberménil and Saint Dizier: depositional sequence geometries-relationship between tinie-lines and facies (after Friedenberg, 1994). See figure 3 lot ~
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-Beaumont dolomite-«Marnes irisées s-upéri cures» sequence This lower boundary of this sequence corresponds to the base of the fluvial «Grés it roseauxo and is marked by a major change in facies from marineinfluenced evaporitie coastal plain to non-marine fluvial deposits. This surface is not erosional and diere is no evidente of any ineised valley. A non erosional valley-shaped depression has been formed due to local flexure where the Saint-Martin de Bossenay, Seine and l3ouchy Faults converge (Figs. 8,9). This depression is filled with lenticular fluvial sediments (Sainte-Colombe-Voulzie Sandstones) indicating a basal onlap overlain by the «Grés it roseaux». The sequence starts with the deposition of alluvial plain sediments of anastomosing and meandering channels («Grés it roseaux», Palain, 1966, Courel et a!, 1980) during a period of base-level rise. These sediments grade vertically into the marine dcposits of the «Dolomie de Beaumont» over an interval of a few metres. The landward lateral equivalent of the maximum fiooding surface observed in the marine deposits of the «Dolomie de Ecaumonto occurs within clayey coastal plain deposits (within l’igtcre 6.—L-W Muschelkalk section between Emberménil and Saint-Just-Sauvage: depositional sequenee geonetries-relationshsp bet~een time-Unes anó facies. See figure 3 for ihe location. Figura 6.—Corte L-W del Muschelkalk entre Emberménil y Saint dust Sauvage: geometría de las -ectiencias deposicionales y relación entre líneas-tiempo y facies. Localización en la Figura 3.
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by sediments lying parallel to the surface (Figs. 8, 9). This discontinuity is recorded within the «Mames irisées supérieures», j.c. in evaporitic coastal plain deposits where it separates the deposits of anbydritic and dolomitie coastal plain sediments. Westward, it is a conformable surface overlain by onlapping dolomitie coastal plain deposits which ended the depositions of braided channels or of the dolomitic paleosols in the extreme west of the Chaunoy Formation (Figs. 8,10). The upper part of the «Mames irisées supéricures» is made up of dolomitic coastal plain facies alone grading upwards into the restricted marine deposits of the Rhaetian Sandstones which display some limited open marine influences (Figs. 8, 9). Correlations show that the Rhaetian boundary is a diachronous facies boundary between a coastal plain and a restricted marine environment within a transgressive trend (Figs. 8, 9, 10). The regressive trend is recorded in the upper part of the Rhaetian, characterized by the continental deposits of the «Argiles de Levallois» (Al Khatib, 1976).
COMPARISON OF THIRD ORDER TRIASSIC SEQUENCES IN THE PARIS AND GERMAN BASINS The third order Triasic sequence described aboye may be compared with the sequences descibed by Aigner & Bachmann (1992). Figure 8.—F-W section of ~tÚtfrr-vrrlrn4wifld
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CONCLUSIONS High resolution correlation (genetie stratigraphy) is a fundamental tool for a geodynamic study of sedimentary basins. The Triassic series is divided into nine third order sequences separated by unconformities or more fmequently by their correlative conformities. These appears to have been a lack of major extensional movements during the first phase of formation of the Paris Basin. The similarity of local tectonie events in the Paris Basin and in other Triassic peritethyan basins (Aigner & I3achmann, 1992, Dromart eta!, 1992) may have been induced by tectonic events on the scale of the west European Craton. Furthermore, these results demonstrate the complex evolution of the Paris Basin which appears to have been multiphased, with numerous fluctuations in the ráte of subsidence (Guillocheau, 1991) and with no real extensional phase during 4w Triassic.
Depositional sequences in tite Triassic series of tite Paris Basin
361
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J.C., MIT
