Triassic

The Germanic Trias was originally defined from outcrops in central Germany where it was subdivided into Buntsandstein, Muschelkalk and Keuper. This tripartite subdivision was essentially lithostratigraphic, although it was often used with a chronostratigraphic connotation. By careful, step-by-step correlations, these units were recognized in the subsurface of northwestern Germany (Boigk (1959) ; Trusheim (1961) ; Trusheim (1963) ; Herrmann et al (1964) ; Röling (1991) ). Correlation was the main purpose, therefore the names of the main subdivisions were maintained despite sometimes pronounced lithological variations. Some traditional names were considered more as sequences than as pure lithostratigraphic units. In the Netherlands, the German nomenclature for Triassic strata is adopted in essence. In the present updated nomenclature, the correlations were extended into the basin-fringe area, where previously the British nomenclature was used Bunter Group; NAM and RGD (1980) . An RGD selection of diagnostic biomarker horizons is presented in Table E.1 (see pdf)

Within the Triassic rocks, two major groups of sediments are recognised, i.e. the Lower Germanic Trias Group, a mainly clastic succession and the Upper Germanic Trias Group, comprising an alternation of clastics, carbonates and evaporites. These groups are virtually equivalent to the Haisborough and Bacton Groups of the British sector of the North Sea region. The boundary between the Lower and Upper Germanic Trias Groups has been taken at the level of the Base Solling Unconformity (also known as ‘H-’ or Hardegsen Unconformity, or Spathian Unconformity), the most prominent break in sedimentation that can be recognised within the Germanic Trias succession. This concept deviates from the classic German tripartite subdivision, since the level of the unconformity is located within the classic Buntsandstein succession. However, a major subdivision based on the Base Solling Unconformity has been advocated by some German authors as well (e.g. Trusheim (1963) ; Dockter et al (1964)

A deviation from the current German stratigraphic concept is the incorporation of the ‘Upper Bröhllschiefer ’ into the Buntsandstein. It is our view that the sedimentation cycle of the Buntsandstein started with these shales, a view also supported by some German authors (e.g. Röhlng (1991) ). Another deviation from the German classification is the assignation of the youngest unit of Triassic age. This unit, the Sleen Formation, of predominantly marine nature, is considered to represent the basal transgression of the Jurassic Altena Group. In the German classification this unit is still assigned to the Keuper (Rhätkeuper).

Geological history

Withdrawal of the sea from the Northern and Southern Permian Basins towards the end of the Permian terminated the cyclic deposition of marine evaporites. Terrestrial conditions returned with the influx of clastic sediments from southern source areas. A complex rift system transecting the Variscan fold belt and these Permian basins was formed Ziegler (1990) . During the latest Permian and Scythian, when the Lower Germanic Trias Group was deposited, initially, wide-spread, uniform subsidence occurred. Later during the Scythian a large, domal swell, called the Netherlands Swell, developed in the main part of the Netherlands onshore. The Cleaver Bank High in the southern North Sea developed as a similar swell. These swells are comparable to the Hunte and Eichsfeld/Altmark swells in Germany. Parts of the Maasbommel High/Peel Block probably also had positive structural relief during the deposition of this group. The Netherlands Swell was flanked by more strongly subsiding areas such as the Off Holland Low, the Ems Low, the West Netherlands Basin and the Roer Valley Graben. Most swells and lows have a general NNE-SSW to N-S orientation, almost perpendicular to the general W-E direction of the axis of the main Southern Permian (-Triassic) Basin. Exceptions are the West Netherlands Basin and the Roer Valley Graben which display a NW-SE trending strike (see Figs. A4 (see pdf) , A5 (see pdf) and A6 (see pdf) in Section A). Initially, fine-grained sedimentation under lacustrine conditions predominated, which led to the deposition of the Lower Buntsandstein Formation. This was followed by a cyclic alternation of sandstones and claystones with a strong fluvial influence, extending from the south far into the basin, forming the Main Buntsandstein Subgroup. At the end of the Scythian, the swells were uplifted considerably, resulting in erosion of the previously deposited formations, reflected by the Base Solling Unconformity with a subcrop pattern as depicted in Figure E.2 (see pdf) Sedimentation resumed during the latest Scythian (Spathian) with widespread deposition of the sandstones and claystones of the Solling Formation, which covered all of the swells. During deposition of the lower part of the Upper Germanic Trias Group the sediments were derived predominantly from southern directions, whereas in the upper part of the group the sediments were derived from the northeast. Since the Netherlands was situated far away from these northern source areas, the latter deposits are generally fine-grained. During the early Anisian, claystones and evaporites of the Röt Formation were laid down in a lacustrine, restricted-marine to evaporitic setting in the basin. Along its fringes, rivers fed sand into the system. During the late Anisian and early Ladinian, carbonates of the Muschelkalk Formation were deposited under marine conditions; temporary restricted conditions resulted in a prominent, wide-spread evaporitic break. In late Ladinian to Norian times, clastics and evaporites of the Keuper Formation were deposited in a predominantly continental setting. The Rhaetian was marked by a return of marine conditions in the area.

Deposition of the Upper Germanic Trias Group was influenced by increasing tectonic instability and by the first halokinetic movements of Zechstein salts during the Middle and Late Triassic. This is reflected by the large variations in thickness and distribution of the Upper Germanic Trias Group. To a certain extent the NNE-SSW orientated highs still continued to influence the depositional patterns, but a revival of NW-SE-oriented, fault-controlled depocentres occurred as well. At the end of the Middle Triassic, the first movements related to the Early Kimmerian tectonic phase occurred. These epeirogenetic movements showed an intermittent activity during the Late Triassic Wolburg (1969a) Wolburg (1969b) . In the Netherlands they resulted in the gentle upwarping of extensive areas, including the Netherlands Swell. As a result of these movements and consequent erosion, the Altena Group may rest unconformably on Röt, Muschelkalk or lowest Keuper sediments, and locally even on sediments of the Late Permian Zechstein Group.

Seismic facies

The seismic appearance of the Lower and Upper Germanic Trias Groups is very characteristic (See figure E.3 (see pdf) ). The high to extremely high continuity of the reflectors is most striking. This is particularly visible in the Lower Germanic Trias Group, where the thickness of the units is fairly constant over large areas and faulting is rare in the centre of the basin. As a result of the onset of halokinesis of the Zechstein salts at the end of the Early Triassic, these conditions changed gradually as younger units were deposited. The base of the Lower Germanic Trias Group is marked by a clear and usually high-amplitude reflector. This event separates the high-velocity evaporites and carbonates of the Zechstein from the lower-velocity shales of the Lower Buntsandstein. The Lower Buntsandstein has a characteristic, low-amplitude, transparant seismic appearance. Going up the Lower Germanic Trias, the first high-amplitude reflector results from the two superimposed reflections of the top and bottom of the Volpriehausen Sandstone Member. Seismic modelling experiments have shown that this strong ‘soft-kick’ reflector owes its character to the fact that in general the thickness of this member roughly coincides with the tuning thickness for common seismic wavelets. Immediately above the Volpriehausen reflector a slight angular unconformity is observed locally. This is related to the Base Solling Unconformity. In most places, however, the reflectors continue to be parallel up the section.

The Upper Germanic Trias shows more closely spaced, high-amplitude and high-continuity reflectors, which are related to contrasts between clastics, evaporites and carbonates. Low-frequency events, generated by the Muschelkalk, prevail. The reflectors within the Keuper Formation usually have a higher frequency, but they show high-amplitude and high-continuity in common with the other reflectors of the Upper Germanic Trias Group. The contact of the Triassic and the Lower Jurassic Altena Group is marked by a very prominent, high-amplitude double reflector.

Lithostratigraphic descriptions and type sections

A litho-chronostratigraphic section of the Triassic formations in the Netherlands is shown in Figure E.4 (see pdf) During the preparation of Section E, new evidence became available on the absolute ages of the Permo-Triassic of northern Europe Menning (1994) . The ages for the Triassic, as presented in that publication, were used for the time scale in Figure E.4. (see pdf) In particular, a duration of 7 Ma for the Anisian seems more likely than the 1.5 Ma in the Harland et al (1990) time scale.

A generalised correlation of the main lithological units of the Triassic in the Netherlands with those in the neighbouring countries is presented in Figure D.5 (see pdf) For a detailed correlation with the German Muschelkalk and Keuper lithostratigraphy, the reader is referred to NAM and RGD () ; Enclosures 11 and 13), with this change that the Main Keuper Evaporite Member correlates with only the upper part of the Unterer Gipskeuper (Region der Anhydrite und grauen Tonmergel).

Regional correlation Regional lithostratigraphic correlation chart of the Triassic for the Netherlands and neighbouring countries Regional lithostratigraphic correlation chart of the Triassic for the Netherlands and neighbouring countries
(increase size by clicking on the picture)
Structural element Triassic to Liassic structural elements Triassic to Liassic structural elements
(increase size by clicking on the picture)
Chrono-stratigraphy Triassic litho-chronostratigraphic chart Triassic litho-chronostratigraphic chart
(increase size by clicking on the picture)