Lower to middle triassic sequence stratigraphy and climatology of the Netherlands, a model

Palaeogeography, Palaeoclimatology, Palaeoecology, 91 (1992): 277-290 Elsevier Science Publishers B.V., Amsterdam

277

Lower to Middle Triassic sequence stratigraphy and climatology of the Netherlands, a model C . J . v a n d e r Z w a n " a n d P. S p a a k b

aNAM, XGS/31, P.O.Box 28000, 9400 HH Assen, The Netherlands bSIPM, EPX/331, Carol v. Bylandtlaan, The Hague, The Netherlands (Received March 19, 1991; revised version accepted August 21, 1991)

ABSTRACT Van der Zwan, C.J. and Spaak, P., 1992. Lower to Middle Triassic sequence stratigraphy and climatology of the Netherlands, a model. Palaeogeogr., Palaeoclimatol., Palaeoecol., 91: 277-290. The biostratigraphic framework for the Triassic used by the Nederlandse Aardolie Maatschappij in the Netherlands has been reviewed and updated. This resulted in the recognition of 5 palynological zones and 8 subzones. Northwest European palynological assemblages have been interpreted in terms of xerophytic and hygrophytic flora-elements, characterising dry and humid climatic conditions. The biostratigraphic zonation, the climatic fluctuations both in the Northwest European Basin and on the northwestern margin of the Tethys, and the cyclic lithostratigraphical development have been integrated. This leads to a climatological/ sequence stratigraphical model for the Lower/Middle Triassic of the Netherlands. During the Late Scythian (Smithian-Early Spathian) monsoonal activity was responsible for the activation of an ephemeral drainage system on the northern side of the Central European Highland. Under the influence of sealevel highstands, the position of the monsoon belt shifted northwards and the northward flowing drainage system was most active, leading to mass transport of erosional products from the highlands into the Germanic Basin. Based on a combination of biostratigraphic data and climatic events, the Lower Triassic sedimentary cycles in the Netherlands can be linked indirectly with the "standard" sequence stratigraphy and to "Milankovitch" orbitally forced cyclicity. For the marine Middle Triassic sediments a more direct link is envisaged.

Introduction The b i o s t r a t i g r a p h i c f r a m e w o r k for the Triassic until recently in use by the N e d e r l a n d s e A a r d o l i e M a a t s c h a p p i j ( N A M ) was established in the early 1970's. Since then n u m e r o u s d e v e l o p m e n t s in the field o f Triassic p a l y n o s t r a t i g r a p h y have t a k e n place, which have been used as a basis for a review o f the interval. This has resulted in a refinement o f the z o n a t i o n . C l i m a t i c fluctuations, b o t h in the N o r t h w e s t E u r o p e a n Basin a n d in the Tethys, the b i o s t r a t i g r a p h i c z o n a t i o n a n d the lithostratig r a p h i c d e v e l o p m e n t have been i n t e g r a t e d leading to a c l i m a t i c / s e q u e n c e s t r a t i g r a p h i c a l m o d e l for the L o w e r to M i d d l e Triassic interval o f the N e t h erlands. T h e Triassic l i t h o s t r a t i g r a p h y south o f the M i d 0031-0182/92/$05.00

N o r t h - S e a H i g h is c h a r a c t e r i s e d by a cyclic sedim e n t a t i o n p a t t e r n (cf. B r e n n a n d , 1975). Cycles c o m p r i s i n g c o a r s e a n d fine clastics f o r m e d d u r i n g the E a r l y Triassic, whereas e v a p o r i t i c a n d fineg r a i n e d clastic sediments a l t e r n a t e d d u r i n g the M i d d l e a n d L a t e Triassic. N o r t h w e s t E u r o p e a n b i o s t r a t i g r a p h i c d a t a , p o i n t to a fairly s y n c h r o n o u s n a t u r e o f the u n i f o r m a n d regionally extensive L o w e r Triassic strata. C o n s e q u e n t l y , one c o u l d speculate t h a t these s t r a t a were n o t derived from a local source, but instead that relatively sudden, large scale geological (cyclic) events g o v e r n e d the s e d i m e n t a t i o n p a t t e r n in the G e r m a n i c Basin. I f this a s s u m p t i o n is correct, these cycles (reflected in the lithology), in c o m b i n a t i o n with the bios t r a t i g r a p h y a n d the climatic fluctuations, offer the possibility to e x t r a p o l a t e the s t r a t i g r a p h i c frame

© 1992 - - Elsevier Science Publishers B.V. All rights reserved

278

to intervals with no or limited biostratigraphic control.

Biostratigraphy The Triassic biostratigraphy used by NAM prior to this study was defined some two decades ago. Research continued, both within Shell, and in the academic world (especially at Utrecht University: Visscher and Commisaris, 1968; Visscher, 1971; Schuurman, 1977, 1979; Visscher and Brugman, 1981; Van der Eem, 1983; Brugman, 1983, 1986, pers. comm., 1990; Van Buggenum, 1984/1985, 1985). The results of these studies triggered a critical review of NAM's current palynological zonation. Although the database is limited, 8 subzones are now recognised, in addition to the 5 existing zones, ,in line with the results of the above mentioned research in Germany and Northeastern France. The chronostratigraphic position ("ages") of the zones is based on a comparison of western European species with assemblages from the stratotypes in the Alps. The results of this revision are shown in Fig. 1. The revised biostratigraphic frame allows direct and fairly detailed chronostratigraphic interpretations, especially for Middle and Upper Triassic strata. To date, the resolution for the Lower Triassic (reservoir) sequence is limited. However, research on directly correlatable German material (Brugman, 1983, pers. comm., 1990) has shown that also in the Lower Triassic a high biostratigraphic resolution might be possible.

Climatology Variations in vegetation are usually a direct reflection of changes in climate (Good, 1974). The relative proportion of different palynomorphs in an assemblage can be used to recognise different types of vegetation, regional environment and subsequently, climatic conditions. For instance, a relative abundance of fern spores would suggest a "wet" vegetation, whereas a large number of conifer pollen is more characteristic of "dry" conditions. This approach to reconstruct the Triassic climate was first attempted for the Carnian (Visscher and Van der Zwan, 1981). During this

C.J. VAN D E R Z W A N A N D P. S P A A K

age changes in humidity formed the prime parameter controlling the flora distribution, especially in Europe. Based on an integration with sedimentological data (Robinson, 1970), the existence of regions with distinct seasonal rainfall (monsoons) were recognised (Compare Fig. 2). By applying this method, the main climatological intervals for the Lower and Middle Triassic of northwestern Europe were reconstructed (Fig. 4). Superimposed on this scheme and similar to Quaternary climatic changes, numerous smaller scale alternations of relatively drier and more humid periods (not shown on Fig. 3) can be recognised throughout the sequence (compare Simms and Ruffell, 1990). In addition to the "local" northwest European conditions larger scale climatic trends during the Early Triassic are considered, in relation to paleogeographic and paleo-oceanographic conditions. Based on these data a climatic/depositional model was established for the Early and Mid Triassic. Northwest European climatic events Similar to the reconstruction for the Carnian by Visscher and Van der Zwan (1981), Northwest European floras (NAM data; Visscher and Commisaris, 1968; Visscher, 1971; Schuurman, 1977, 1979; Visscher and Brugman, 1981; Brugman, 1983, 1986, pers. comm., 1990; Van Buggenum, 1984/1985, 1985) from the Late Permian to Early Ladinian are characterised by xerophytic palynological assemblages, indicating dry climatic conditions. Within these xerophytic palynological assemblages minor alternations occur with assemblages with a more hygrophytic (humid), but still moderately dry character. Truly hygrophytic floras, however, are found outside this region only, e.g., in the Barents Sea (compare Hochuli et al., 1989). Late Permian Palynological assemblages from the Late Permian of western Europe are characterised by the relative abundance of bisaccate conifer pollen (e.g., Lueckisporites virkeai) suggesting a dry climate. This is in line with the presence of the Zechstein evaporites.

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Griesbachian to Smith&n At the end of the Permian the existing floras died out and were replaced by floras palynologically characterised by, apart from bisaccate pollen (e.g., Lunatisporites noviaulensis group) relative large numbers of lycopod spores (e.g., Densoisporites nejburgii). These spores are also believed to reflect relatively dry, well-drained floodplain (possibly saltmarsh) environmental settings. Spathian The floras from this period are characterised by similar lycopod floras as found in the preceding Griesbachian to Smithian interval, together with the relative abundance of hygrophytic (humid) fern spores (e.g., Verrucosisporites jenensis, V. protumulosus), together indicating a moderately dry, poorly

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phytic flora elements, indicating moderately humid to humid environmental settings and a moderately humid to humid climate, at least in the source area of the deposits containing these floras, e.g., the Lettenkohle and the Schilfsandstein and equivalents (compare Simms and Ruffell, 1990). These intervals are separated by intervals characterised by floras with a generally xerophytic (dry) character, as indicated by the pollen Triadispora, Vallasporites ignacii, Patinasporites densus and the Prae/Paracirculina group (Scheuring, 1970). These variations in floral assemblages indicate the continued climatic fluctuations during the remainder of the Triassic.

Climatic and eustatic events in the West Tethys area The climatic and paleogeographic developments in the West Tethys area are thought to have affected the "Central European (Iberian-Bohemian, Variscan) Highland", which was one of the source areas of the lowlands in northwestern Europe ("Rotliegend-Zechstein Depression"). Following this line of reasoning, these developments had an (indirect) impact on the sedimentation patterns in this Northwest European Basin. The relevant trends are shortly reviewed in this section.

Paleogeography From the Late Permian to the Mid Triassic, southwestern Europe and North Africa formed the western margin of the Tethys Ocean (Fig. 2). Paleomagnetic data (Smith et al., 1981) indicate that Pangea gradually drifted to the north (Fig. 2), and that Central Europe was positioned at approximately 15° N, during the Early Triassic (Scythian). In addition, this "Central European Highland" broke up towards the end of the Early Triassic. In the Mid Triassic the fairly continuous landmass, stretching from the Iberian Meseta to the Bohemian Massif, had drowned and individual smaller elevated areas remained, like the Ebro High, the Golf of Lyon High and a reduced LondonBrabant-Armorican Massif (cf. Ziegler, 1982,

C,J. VAN DER ZWAN AND P. SPAAK

1990). These processes continued during the Late Triassic. Monsoons A configuration as described above (a large land area bordering the Tethys Ocean, approximately 15° N of the equator) is similar to the present-day geographic situation at the northern margin of the Indian Ocean. This area is characterised by tradewinds and strong monsoonal rains. Using meteorological parameters similar conditions are modelled by computer for the Cretaceous (Barron et al., 1989) along the northwestern margin of the Tethys Ocean. Based on an idealised Pangean palaeogeography similar monsoonal rains are modelled for the margins of the Tethys Ocean (Kutzbach and Gallimore, 1989). They predicted these conditions to extend inland for no more than 1000 km. The equatorial landmass is modelled to have a yearround dry climate. For the margins of the Tethys Ocean in the Triassic monsoonal conditions were reconstructed based on lithological, sedimentological, palynological and palaeontological data (Robinson, 1973; Visscher and Van der Zwan, 1981; Parrish et al., 1989; Simms and Ruffell, 1990). In view of the northward drift of Pangea and the breaking up of the "Central European Highland" (essential for the build-up of tradewind systems) monsoonal activity probably reached its peak towards the end of the Scythian (Fig. 2). Subsequently during the latest Scythian and Anisian its effect diminished rapidly due to further erosion and transgression of the lowlands surrounding the "Central European Highlands (Islands)". Sealevel fluctuations The "Central European Highland" and the Tethys Ocean towards the southeast were separated by a zone of relatively flat coastal lowlands and the shallow Tethys Shelf. In this zone, sealevel fluctuations as described by Haq et al. (1987), would have caused distinct shifts in the position of the shoreline. Regional climatic conditions in general, and tradewind systems and monsoonal belts in particular, exist because of different pressure buildups above land and water surfaces (Barran et al., i 980). Consequently shifts in the position of the coastline (Gyllenhaal et al., 1991) will result

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LOWER TO MIDDLE TRIASSIC SEQUENCE STRATIGRAPHY AND CLIMATOLOGY OF THE NETHERLANDS

in shifts in the monsoon system. One may speculate that during periods of sealevel highs, the monsoon belt extended well into the latitudes of the "Central European Highland", to the north (Fig. 3).

"Milankovitch" (orbitally forced) climatic cycles Variations in the Earth's orbit around the Sun and the associated insolation have a regular frequency pattern (e.g., Berger, 1980) and are demonstrated to be the driving mechanism behind climatic variations through time (Hays et al., 1976; Berger, 1980; Hilgen and Langereis, 1990). The present-day frequencies of these "Milankovitch" cycles range from 0.4 Ma to 0.02 Ma. Through time these frequencies decrease but they are likely to remain in the same order of magnitude (Berger et al., 1990). Cyclicity analysis on wireline logs over the Lower Triassic section in Dutch wells (compare Chang-Shu and Baumfalk, and pets. comm., 1991) indicate durations for the Volpriehausen, Detfurth and Hardegsen reservoir cycles of 0.4 or 0.8 Ma. These values are the same or a multitude of the duration of the "0.4 Ma" orbital eccentricity cycle and this match may point to a causal relationship. In this context Park and Oglesby (1991) made computer models for the Cretaceous demonstrating significant climatic variations caused by Milankovitch rhythms. They were also inferred as driving force behind fluvial sedimentation (Olsen, 1990). The sealevel fluctuations, the changing paleogeography, the assumed long term trend in the monsoon activity and the "Milankovitch" climatic cyclicity are likely to have left a distinct impact on the sedimentation pattern in the lowlands bordering the Central European Highland to the north (Fig. 3). If this assumption is correct, it offers a way to link the northwest European sedimentary cycles with the sequence stratigraphy defined in the marine settings of the Barents Sea and the Tethys Ocean (Haq et al., 1987). This can explain the uniform and synchronous nature of the Triassic strata recorded throughout the Northwest European Basin from the Netherlands to Poland. The inferred link with the sequence stratigraphy (Haq et al., 1987), together with the climatic events in the Northwest European Basin and in the Tethys, and the biostratigraphic data will form the basis

287

for the sequence stratigraphic model presented below. Sequence stratigraphic model for the Dutch Lower and Middle Triassic In this section a tentative stratigraphic frame is presented (Fig. 4) based on a general climatic and environmental model. With this model the Dutch sedimentary cycles are speculatively tied in with the "global" sequence stratigraphy (Haq et al., 1987). Although it is realised that this global nature has not been proven. The overall pattern of the Triassic depositional sequences is very consistent in the Netherlands and NW Europe. Future dating will probably slightly shift individual sequence boundaries, but are unlikely to change the overall pattern. For this reason the "global" sequence stratigraphy (Haq et al., 1987) has been used as frame to tie in Triassic cyclic strata. This allows "dating" of sediments when no biostratigraphy is available and the use of numerical ages (e.g., for burial reconstructions) in a consistent manner.

Late Permian-Early ScTthian In the Northwest European Basin, the base of the German Lower Trias is taken at the base of the Tatarian, at the base of the major sequence boundary in Haq et al.'s (1987) Sequence Stratigraphy Chart (Base UAA-I.I, Fig. 3), indicating a major change in lithostratigraphical and environmental setting. This Late Permian age for the base of the Lower Germanic Trias is supported by limited palynological evidence (Visscher, 1971) and by data of Harland et al. (1982). During the Late Permian to Early Scythian, fine-grained clastic floodplain deposits of the Lower Buntsandstein Formation accumulated in the "Rotliegend-Zechstein Depression" north of the Central European Highland. This highland was formed by a Variscan mountain range, probably extending from Iberia in the southwest to Bohemia or Poland in the northeast (cf. Ziegler, 1982, 1990). It protected NW Europe from the initial activity of the monsoon belt present in the south (Fig. 2).

288

¢.J. VAN DER ZWAN AND P. SPAAK

Late Scythian (Smithian-Early Spathian)

Latest Scythian (Late Spathian)-Early Anisian

In the Late Scythian (Fig. 2) the northwestern Tethys area passed through the optimum geographical belt for the development of monsoon systems. In addition, the Central European Highland and the coastal lowlands towards the Tethys still formed a continuous land area, essential for the development of such a system. During that time, three regionally correlatable cycles of finegrained clastics and coarser potential reservoir sandstones were deposited (the Volpriehausen, Detfurth and Hardegsen Members of the main Buntsandstein Formation) in the Northwest European Basin. Tentatively, these cycles are thought to be related to variations in the erosional effect of the monsoonal activity in the source area (Central European Highland). One may speculate that during periods with (third order) sealevel highstands (Fig. 3), the monsoon belt extended well into this "Central European Highland", causing significant (coarser) erosion, on the southern flank, but also on the northern flank of this source area, followed by transport north into the Northwest European Basin (Fig. 2). Evidence from the Volpriehausen, Detfurth and Hardegsen Members support the idea that these cyclic sandstones were deposited as distal alluvial fans that built out progressively across the arid flood plain from the southern margin of the basin only. There is no southward migration of clastics evident from the region of the Mid-North Sea High. The heavy minerals found in these sediments are consistent with a southern source area (Massif Central, France; Van der Baan, pers. comm., 1989). Biostratigraphic data indicate that the main Buntsandstein Formation was deposited in a relatively short timespan of 1.8 Ma (Smithian-Early Spathian), which is represented by only one cycle in the standard sequence stratigraphy (UAA-I.3; Fig. 4). We tentatively link the whole of the main Buntsandstein Formation, with the period of sealevel high in this cycle (244.2-242.4 Ma). The three main (reservoir) alternations within this formation are possibly related to higher order climatic and possibly sealevel fluctuations (Fig. 4), which are thought to be linked with "Milankovitch" (eccentricity) cycles.

Towards the end of the Scythian the Central European Highland, as part of the Pangean Continent, continued to drift north and started to break up into various isolated highs. Consequently, it not only moved away from the centre of the monsoon belt, but also lost its function as a large source area. This process is already reflected in the diminished lateral extent of the overlying sandbodies of the Main Buntsandstein Formation relative to the Volpriehausen Sandstone. During the Late Spathian-Early Anisian, the alternation of coarse and fine clastic sediments gradually ceased and instead evaporitic-clastic cycles were formed (Figs. 3 and 4). In this interval ($6157 Subzone, Fig. 4) the R6t Formation was deposited, which consists mainly of an alternation of fine clastics and evaporites. Coarse clastics are limited to the West Netherlands Basin and to the East Netherlands (the Soiling Sandstone), consistent with the reduced erosional effect of the monsoons and the diminishing source area referred to above. We feel that the origin of the evaporitic-clastic cycles can best be explained by the declining effect of the shifting monsoonal belt, so prominent in the "pre-R6t" period. Accordingly, the finegrained clastics are now thought to be deposited during sealevel highstands (Fig. 3), when the sea occasionally flooded the Central European Basin first via the Polish Trough and later in the Anisian via the Burgundy Trough and Hessian Depressian (cf. Ziegler, 1982, 1990), creating a relatively large inland sea. Additionally, the humid monsoon belt extended into the by now much eroded Central European Highland, causing the input of relatively finer clastic erosion products into the surrounding lowlands. During periods of sealevel lows, the inland sea became isolated from the Tethys and the humid monsoon belt moved to the area south of the Central European Highland. Under those conditions evaporites accumulated in large salt lakes. Consequently the two R6t evaporitic intercalations are thought to have been deposited during sealevel lowstands. For practical reasons they are linked with the bases of the next two cycles on the Sequence Chart (UAA-1.5 and 2.1). This

LOWER TO MIDDLE TRIASSIC SEQUENCE STRATIGRAPHY AND CLIMATOLOGY OF THE NETHERLANDS

ties in reasonably well with the local climatic and palaeogeographic conditions. Evaporites did not form until the very end of the moderately dry Spathian Substage and were mainly deposited during the dry Anisian. The occurrence of the Main R6t Evaporite in the moderately dry Spathian may be explained by the rather generalised palynological dataset, obscuring higher frequency climatic fluctuations of the same frequency (0.4 Ma) as those observed for the Volpriehausen, Detfurth and Hardegsen reservoir cycles. The Main R6t Evaporite would then be linked to another order Milankovitch eccentricity cycle, preceding the main dry period of the Anisian. A regional extensive unconformity, though very minor in terms of sediment column, separates the R6t Formation from the underlying main Buntsandstein Formation. Biostratigraphically, the R6t Formation falls within the Spathian to Lower Anisian. The main Buntsandstein Formation reaches into the Lower Spathian as well (see above). Consequently, in areas not disturbed by salt tectonics, the amount of time between the base R6t and the top of the main Buntsandstein Formation is very short (0.6 1.2 Ma; depending on the lithostratigraphical units present)! Late Anisian Early Ladinian

During the Late Anisian, Central Europe moved away from the monsoonal into a dry climatic belt between 20 and 40 ° N. The former Central European Highland had disappeared and only isolated highs remained. In this setting sealevel fluctuations had a more direct influence on the sedimentary processes (Fig. 3). During periods of sealevel highs the sea invaded the Northwest European Basin and marls and claystones were deposited in marginal marine and coastal environments. During sealevel lowstands, the watermasses became isolated from the Tethys Ocean and evaporites could accumulate, under prevailing dry conditions. This model is adopted for the whole Late AnisianEarly Ladinian period (i.e., German Upper Trias). The base of the Muschelkalk Formation falls within the Anisian ($635 Zone; see also Brugman, 1986) and is the most "transgressive horizon" of the Dutch Triassic. It is related to the Maximum

289

Flooding Surface (MFS) of Cycle UAA-2.1 (Fig. 4). The Muschelkalk Salt has been linked with the base of Cycle UAA-2.2, which is of Anisian age ($6353 Zone).

Conclusions The palaeogeographic and palaeoclimatic evolution had a distinct impact on the depositional conditions in the northwestern Tethyan realm and indirectly influenced the sedimentation pattern in the Northwest European Basin. Based on a combination of biostratigraphic data and climatic events, the Lower Triassic sedimentary cycles in the Netherlands can be linked indirectly with the sequence stratigraphy of Haq et al. (1987) and to Milankovitch astronomical cyclicity. For the marine Middle Triassic sediments a more direct link is envisaged. During the Late Scythian (Cycle UAA-I.3; Smithian-Early Spathian) and under the influence of sealevel highstand, monsoonal activity was responsible for the activation of an ephemeral drainage system on the northern side of the Central European Highland. Superimposed on this event, orbitally forced climatic fluctuations are thought to have caused changes in the drainage system, leading to mass transport of erosional products from the highlands into the German Basin. Based on the composition of the palynomorph assemblages alternations of humid and dry climatic zones can be recognised in Northwestern Europe. Evaporites accumulated during the dry periods. In the Triassic of the Netherlands we now recognise 5 palynological zones and 8 subzones. Recent studies on German material, indicate that a higher resolution may be possible.

Acknowledgements The authors are indebted to the Nederlandse Aardolie Maatschappij, Assen, and Shell/Internationale Petroleum Maatschappij, The Hague, for permission to publish this paper. Further thanks are due to D. Van der Baan, Oman; D. Van Wijhe and J. Mabillard, Assen; P. Ziegler, D. Diederix, P. Brugman, W.M.L. Schuurman and R. Eckert, The Hague, and W. Sissingh, London, for their

290 valuable contributions and stimulating discussions. W e g r a t e f u l l y a c k n o w l e d g e J. V a n K a m p e n , A s s e n , for making the diagrams.

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