Middle and Lower Jurassic


In the Netherlands the uppermost Triassic, and Lower and Middle Jurassic deposits consist of a thick succession of marine claystones, siltstones, marls and some sandstones, which have been placed in the Altena Group. There are only a few general studies on the Lower and Middle Jurassic in the Netherlands (e.g. Haanstra (1963) ; Heybroek (1974) ), but the occurrences in the Roer Valley Graben Herngreen and de Boer (1978) and especially the Achterhoek (ten Dam and Reinhold (1942) ; Faber (1946) ; Gerth (1955) ; Harsveldt, Herngreen and de Boer (1978) Harsveldt (1063) ; Herngreen et al (1983) .; Witte et al (1993) ) has received special attention.
Table F.1 (see pdf) lists a number of selected palynological and micropalaeontological markers for the Early and Middle Jurassic, that are used by the RGD. For additional data on Jurassic biostratigraphy in the North Sea area, see for instance Klingler (1962) ),Herngreen and de Boer (1978) ; Herngreen and de Boer (1974) ; Herngreen et al (1983) .; Cox (1990) and Partington et al (1993) , and references given by these authors.

Geological history
At the end of the Triassic the depositional regime changed significantly. During the Permian and most of the Triassic sedimentation took place in an arid continental or predominantly highly restricted marine setting, in which widespread red-bed or evaporite sedimentation occurred. From the latest Triassic (Rhaetian) on, this environment was replaced by an open-marine system. In the basin, which encompassed most of present-day Western and Central Europe (Wolburg (1969) ; Ziegler (1990) ), a thick blanket of open-marine fines and marls was deposited. On some elevated structural elements Fig. A.4 (see pdf) a thinner, more calcareous succession with occasional iron oolites was deposited. Only rarely did influxes of coarser clastics reach the deeper basinal areas. A few intercalated sandstones, and sandy and oolitic carbonates indicate periods of sea-level lowstand and/or hinterland uplift. Ferruginous horizons and layers of iron oolites (hardgrounds) reflect periods in which deposition ceased and reworking occurred. On the whole, the group displays first a transgressive and then a regressive trend.
The Early to Middle Jurassic marine depositional sequence started with a basin-wide transgression in the Rhaetian, following the second pulse of the Early Kimmerian epeirogenetic uplifts (Ziegler (1982) ). The first pulse, which formed the main unconformity, is situated within the Keuper Formation, below the Red Keuper Claystone or Dolomitic Keuper Member (Wolburg (1969) ; Schroeder (1982) ; see Section E). The basal unit of the Altena Group is the Sleen Formation, a rather thin, marine, fossiliferous, shaly claystone of Rhaetian age. The unit shows a highly uniform development. Detailed correlation suggests that the transgression which deposited the Sleen Formation took place very rapidly. In the basin centre, deposition through the Late Triassic appears to have been more or less continuous (Ziegler (1982) ; Clark-Lowes et al (1987) ; Brown (1990) ). Towards the basin margins and on the intrabasinal highs, the contact between the Rhaetian transgressive sediments and the underlying Triassic is locally clearly unconformable (e.g. Sleen Formation overlying the Muschelkalk or Röt Formation in the eastern Netherlands).
The basin shoaled during the later Rhaetian Wolburg (1969) , but at the start of the Hettangian fully marine conditions returned, resulting in the deposition of a widespread argillaceous limestone bed, which has been taken as the base of the Aalburg Formation. This formation further consists of a monotonous succession of dark, marly clays and silts with intercalated thin limestone beds. It reflects quiet, open-marine depositional conditions which prevailed practically throughout western Europe from the Hettangian till the earliest Toarcian.
The Early Toarcian is marked by a period of high sea level and restricted basin circulation across large parts of Europe, causing anoxic conditions at the bottom of the basin. Under these conditions the Posidonia Shale Formation was deposited. This is a prominent bituminous shaly claystone interval in the Lower Jurassic succession. The unit is one of the most important oil source rocks in southern North Sea area (Cornford, 1990) and the Netherlands (Bodenhausen and Ott (1981) ; Oele et al (1981) ; Clark-Lowes et al (1987) ). In the Late Toarcian well-oxygenated, open-marine conditions resumed in the Northwest European Basin, under which the Werkendam Formation was deposited. This unit consists of a succession of silty claystones with one prominent sandy marlstone intercalation, the Middle Werkendam Member. The formation ranges in age from Toarcian to Late Bajocian, while the Middle Werkendam Member is of mid-Bajocian (basal Dogger-d) age. During the latest Bajocian and Early Bathonian the marine basin shoaled and became smaller. As a result, the continuous open-marine clay deposition, which had characterized the Werkendam Formation, was replaced by shallow-marine, sandy carbonate deposition of the Brabant Formation. Thusfar, four carbonate-marlstone cycles have been recognised, ranging in age from Early Bathonian to Early Oxfordian.
The uplift of the central North Sea area in the Late Aalenian to Bathonian (Central North Sea dome as a result of the ‘Mid-Kimmerian’ tectonic movements; Ziegler (1990) ) restricted the area of deposition of the Altena Group to an E-W belt, which extends from the southern United Kingdom, through the southern Netherlands, into northern Germany. In the axial areas of the Dutch basins in this belt, viz. the Roer Valley Graben, the West Netherlands Basin, the Broad Fourteens Basin and the eastern Central Netherlands Basin (Achterhoek), deposition of marine sandy carbonates and marls persisted into the Callovian and locally into the Oxfordian (Haanstra (1963) ; Herngreen and de Boer (1974) ; Bodenhausen and Ott (1981) ; RGD (1993) ). At the same time continental deposition (Schieland Group; see Section G) resumed after the Mid-Kimmerian event in the Dutch Central (North Sea) Graben rift, during the Middle Callovian and Oxfordian. In the Danish sector of the Central Graben continental deposition started even earlier, during the Bathonian-Bajocian (Michelsen and Wong (1991) ).
The marine environment in which the Altena Group sediments were deposited, retreated completely from the Netherlands after the first pulse of the Late Kimmerian uplift in the Late Oxfordian-Early Kimmeridgian (‘Late Kimmerian I’ pulse of RGD (1990) ; related to the ‘Mid-Kimmerian unconformity’ by NAM and RGD (1980) ).

Seismic character
The Altena Group has a very characteristic seismic appearance. The base of the group is marked by a very prominent high-amplitude reflector. It separates the underlying Triassic, which is characterised by continuous, moderately high-amplitude, predominantly parallel reflectors, from the Sleen and Aalburg Formations, which are represented by a rather thick interval of a transparant character. Some internal reflectors may be observed in the basal parts, but in the upper parts of the Aalburg Formation these are mostly discontinuous as a result of their low amplitudes. The top of the Aalburg Formation interval is marked by the reflection of the Posidonia Shale Formation. It con-sists of one or two highly continuous, very high-amplitude, low-frequency reflections, which stand out very prominently in the otherwise practically transparant Altena Group interval. The seismic character of the Werkendam Formation resembles that of the Aalburg Formation, although some internal reflections may occur. The Brabant Formation, the occurrence of which is restricted to the Broad Fourteens Basin, the West Netherlands Basin and especially the Roer Valley Graben, is characterised by a number of parallel, high-amplitude, low-frequency reflections.
The top of the Altena Group is formed by an often angular unconformity. Usually the Altena Group is clearly unconformably overlain by one of the Upper Jurassic or Lower Cretaceous formations. In the most complete successions the contact with the Schieland Group is practically concordant, and may be difficult to discern with confidence. In the inverted axial zones of most Late Jurassic-Early Cretaceous basins the Altena Group is in places clearly unconformably overlain by the Chalk Group or the Lower North Sea or Middle North Sea Group.
Note that the Sleen Formation of Rhaetian age formally belongs to the Triassic. In Figure E.1 (see pdf) the biomarkers selected for that age are indicated. The Lower Graben Formation (SLCL) is described in Section G, although age-wise it forms part of the Middle Jurassic (Callovian).
Regional correlation Regional lithostratigraphic correlation chart of the Early and Middle Jurassic for the Netherlands and neighbouring countries Regional lithostratigraphic correlation chart of the Early and Middle Jurassic for the Netherlands and neighbouring countries
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Structural element Triassic to Liassic structural elements Triassic to Liassic structural elements
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Chrono-stratigraphy Early and Middle Jurassic litho-chronostratigraphic chart Early and Middle Jurassic litho-chronostratigraphic chart
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References See References Lower and Middle Jurassic

Van Adrichem Boogaert, H.A. & Kouwe, W.F.P., 1993-1997. [Stratigraphic unit]. In: Stratigraphic Nomenclature of the Netherlands.
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