Pre- and Early rift phase

In general, the Permian to Middle Jurassic sediments are a conformable megasequence, in most areas bounded at the base by Saalian and at the top by Mid Kimmerian regional unconformities. Thermal contraction of the lithosphere after the Saalian tectonic phase – associated with

regional uplift, non-deposition and vulcanism – induced subsidence of the Southern Permian Basin (Van Wees et al. 2000, Ziegler 1990a). The Permian to Middle Jurassic sediments were deposited in the E-W trending Southern Permian Basin, a foreland basin located north of the London–Brabant Massif and the Rhenish Massif. These massifs, part of the northern rim of the Variscan orogenic belt (Geluk et al. 1996), were a sediment source for the Southern Permian Basin. The pre- and early rift sedimentary deposits thin towards these massifs. Time-dependent variations in sedimentation rate occurred in addition to lateral variations caused by the pronounced differences in synsedimentary subsidence of the Southern Permian Basin. Its depocentre was located in the northeastern part of offshore Netherlands.

0 100 km

Figure 12 Late Jurassic–Early Cretaceous structural units in onshore and offshore Netherlands. Location of cross section presented in Figure 19

In the Late Permian, accumulation of terrestrial Rotliegend clastic (Slochteren Formation) and desert lake deposits (Silverpit Formation) was followed by deposition of marine Zechstein evaporites, carbonates and clays. The maximum long-time average sedimentation rate of the Upper Rotliegend Group is 80 metres per million years (80 m/My). Using sequence and cyclicity analysis, Yang and Nio (1993) calculated sedimentation rates for third-order stratigraphic sequences of the Upper Rotliegend Group of 60 - 110 m/My. The maximum long time average sedimentation rate of the Zechstein Group is estimated at 140 m/My for a present-day thickness of the Zechstein Group of 1000 m in basinal areas not affected by major halokinesis. In such areas, the Zechstein Group comprises up to five evaporite cycles. Potential deposition rates of halite can be extremely fast in comparison to other marine deposits (carbonates:

5 cm per thousand years (5 cm/ky), gypsum and anhydrite: 50 cm/ky, and halite:

5000 cm/ky; Einsele 1992). Non-evaporite deposition probably prevailed during most of the Zechstein period (Einsele 1992). Rotliegend and Zechstein deposits are still present in large part of the original sedimentation area. Subsequent structural development of the northern part of the area has been strongly influenced by the thick halite deposits present there. It has been suggested (e.g. Remmelts 1996) that active salt displacements are associated with periods of increased tectonic activity.

The Southern Permian Basin continued to subside during the Triassic. During the Early and Middle Triassic the Roer Valley Graben was the main feeder system of sediment (Ziegler 1990a); sediment also came from the London-Brabant Massif, which remained active during the Early Triassic (Geluk et al. 1996). The build-up of tensional stresses in the Triassic (Ziegler 1990a) induced the differential tectonic subsidence of subbasins within the Southern Permian basin (Off Holland Low, Ems Low, West Netherlands Basin and Roer Valley Graben) and the development of swells (Netherlands Swell, Cleaver Bank High) (e.g. Geluk and Röhling 1998). The Triassic axes of differential subsidence have a N-S and NNE-SSW orientation. The Hardegsen tectonic event induced uplift and caused deep erosion of the sediments of the Lower Germanic Trias Group on the swells. The first halokinetic movement of Zechstein salts started during Early Triassic and continued into the Neogene (Remmelts 1996). The Early Kimmerian tectonic phase – in the Late Triassic Carnian – caused rapid subsidence of a number of fault-bounded structures, such as the Central North Sea Graben, Broad Fourteens Basin and Ems Low, while regional subsidence and sedimentation resumed during subsequent Norian times (Geluk et al. 1996).

The Lower Germanic Trias Group is composed of lacustrine claystones and sandstones of aeolian and fluvial origin, and the Upper Germanic Trias Group consists mainly of lacustrine to shallow marine claystones, carbonates and evaporites (Figure 14). During the Triassic the long-time sedimentation rates calculated for the shifting depocentres decreased from 160 m/My (Lower Buntsandstein Formation) and 85 m/My (Main Buntsandstein and Röt Formations) to 40 m/My (Keuper and Muschelkalk Formations).

At the end of the Triassic the depositional environment changed from the previously continental to restricted marine towards open-marine. The Late Triassic transgression covered most of the highs in Northwestern Europe (Ziegler 1990a) and thick open-marine clays of the Altena Group (Sleen, Aalburg, Posidonia Shale, Werkendam and Brabant Formations) were deposited. Restricted conditions occurred during the

Toarcian (Posidonia Shale Formation). The long-time average sedimentation rates of the Altena Group deposits vary between 3 and 34 m/My.

Thermal domal uplift of the central part of the North Sea area, which is associated with the Mid-Kimmerian tectonic phase (Glennie and Underhill 1998, Underhill and Partington 1993), decreased the area of deposition of the Altena Group. The present-day occurrence of the Altena Group is restricted to the Late Jurassic–Early Cretaceous basins (Figure 12) because of later erosion.

Claystone/Mudstone

Figure 13 Lithostratigraphy, tectonic events and associated heat flow history in onshore and offshore Netherlands (From Verweij 1999).

Chronology after Harland et al. (1990) and for Permian and Triassic, Menning (1995); stratigraphy and lithology after Van Adrichem Boogaert and Kouwe (1993-1997); Tertiary stratigraphy also after Vinken (1988); intrusives and vulcanism after Latin et al. (1990a-b); evolution of surface heat flow for southeastern flank of Roer Valley Graben after Veld et al. (1996)

Claystone/Mudstone Sand/Sandstone Marl Limestone Dolomite Chalk Rocksalt Anhydrite Coal Argillaceous Bituminous Unconformity

Chrono- logy Period Tertiary

Quaternary Cretaceous Jurassic Triassic Permian Carboniferous

2.4 65 143 208 251 296 3634.8 23.3 35.4 56.5 97 157 178 229 241 265 305 318 333

Time (Ma)

Stratigraphy Groups Upper North Sea Middle North Sea Lower North Sea Chalk Rijnland Scruff; Schieland Niedersachsen* Altena Upper Germanic Trias Lower Germanic Trias Zechstein Upper Rotliegend Lower Rotliegend Limburg FarneCarboniferous Limestone

NSN Depositional environmentS -Also glacigiene Shallow marine, fluvial, paralic, lacustrine Predominantly marine; paralic Marine Shallow to bathyal, open, marine -Moderately to fairly deep marine -Coastal to shallow and fairly deep marine -Continental to paralic; shallow marine Paralic to restricted marine* -Shallow, open, marine -Marine, anoxic -Shallow to fairly deep, open, marine Alternating shallow, restricted, marine Lacustrine, fluvial, aeolian Playa lake Perimarine to shallow, highly restricted marine Perimarine to deep, highly restricted marine Aeolian Fluvial (wadi), sheetflood -Fluvial -Deltaic and fluvial -Marine, deltaic, lacustrine ParalicShallow, mainly open, marine Lake Playa lake

Playa lake, floodplain

Sedimentation rates m/My 050100150200400 LatitudeSN 10º 0º10º 20º30º 40º50º Mean ocean surface temperature (ºC)

Climate Humidity 0102030 Alternating arid and humid Humid Subhumid Humid Seasonally wet Seasonally wet Seasonally wet Arid to semi-aridArid to seasonally wet Arid Arid to semi-arid Humid

Simplified Lithology

NS Figure 14Lithostratigraphy and associated history of depositional environments, sedimentation rates and climatic conditions (From Verweij 1999). Depositional environment based on Van Adrichem Boogaert and Kouwe (1993-1997) and Vinken (1988); Climate compiled from Frakes (1979), Hallam (1985), Yang and Nio (1993), Zagwijn (1975), Ziegler (1990a); mean ocean surface temperature compiled from Glennie (1990; Figure 2.7) and Welte et al. (1997; Figure 1.5); sedimentation rates compiled from different sources Caston (1977), Dronkert et al. (1989), Geluk et al. (1996), Sørensen et al. (1997), Van Adrichem Boogaert and Kouwe (1993-1997); Vinken (1988), Yang and Nio (1993)