Scripta Geologica, 08 (Special Issue 2012)J.W.M. Jagt; E.A. Jagt-Yazykova: Stratigraphy of the type Maastrichtian – a synthesis

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Maastricht Formation

Valkenburg Member – In the western part of southern Limburg this member consists of poorly indurated, white-yellowish to yellowish-grey, fine- to coarse-grained chalks with greyish brown flint nodules of varying size. In the east, this sequence changes into an alternation of poorly and more intensely indurated chalk beds, which are part of the so-called ‘Kunrade Limestone’. Here flints do not occur everywhere; where they do, they are crumbly, light grey nodules. The total thickness increases from west to east. At the ENCI-HeidelbergCement Group quarry it amounts to c. 2.5 m, while just east of Valkenburg aan de Geul it is c. 45 m.

Zijlstra (1994) noted the occurrence of depressions that are several tens of metres wide and decimetres-deep at the top of the Lanaye Member, filled with coarse-grained phosphatic/glauconitic and pyritic bioclastic sand, representing the base of a fining-upward cycle. Depositional lamination was shown to be virtually entirely destroyed by bioturbation, and the sand to contain skeletal remains, reworked chalk, and low concentrations of sand-sized extrabasinal quartz and heavy mineral grains. At the ENCI-HeidelbergCement Group quarry, this member shows a fining-upward trend, with an upper cycle of 1.5 m in thickness, having a rather fine-grained, slightly lithified (proto-hardground), pure carbonate top with poorly developed flint nodules around spreiten and Thalassinoides-type burrows. Of note is Zijlstra’s (1994) observation that the glauconitic cycles of the Valkenburg Member at the ENCI-HeidelbergCement Group quarry change laterally towards the south into cycles with flint nodule layers very similar to those of the Lanaye Member. This correlation is corroborated by analyses of bioclast contents.

Gronsveld Member – In the west this unit comprises poorly indurated, white-yellowish to yellowish-grey, fine- to coarse-grained chalks. In the lower portion small, light to dark greyish-brown flint nodules of varying sizes and shapes occur; in the higher portion they are arranged in more or less regular beds of light-grey to greyish-blue nodules. Towards the east the upper portion is missing. The chalks change into a cyclic alternation of less and more indurated chalk beds, which are part of the so-called ‘Kunrade Limestone’. Total thickness varies between 4.5 and c. 10 m. According to Zijlstra (1994), the lower part of this unit also consists of fining-upward cycles with a phosphatic, glauconitic/pyritic bioclastic sand at the base, the sand of the lowermost cycle being characterised by well-developed wavy lamination. Wavy laminated sediment at the base of these cycles changes upwards via (sub)horizontally laminated sediment towards lithified homogeneously bioturbated, fine-grained, purer carbonate sedimentary rock at the top. The upper part of this member consists of well-sorted, bioclastic, fine-grained sandstone with low-angle, large-scale wavy lamination (hummocky stratification), with flint nodules forming laterally restricted curvi-planar layers.

Schiepersberg Member – In the west this unit is comprised of poorly indurated, white yellowish, fine- to coarse-grained, homogeneous chalks with numerous regular beds and randomly distributed, light-grey to bluish-grey flint nodules. Towards the north the flints disappear. The homogeneous chalk changes into an alternation of chalk beds of varying induration, and are part of the so-called ‘Kunrade Limestone’. Total thickness varies between 5 and 6 m.

Emael Member – In the west this member comprises poorly indurated, white-yellowish and yellowish-brown, fine- to coarse-grained, homogeneous chalks, in the lower portion with numerous light grey flint nodules. Typical are especially large, regular flat and pipe-shaped flint bodies. In the east, between Valkenburg aan de Geul and Benzenrade-Kunraderberg, these homogeneous chalks change into an alternation of more and less indurated chalk beds, which form the highest part of the so-called ‘Kunrade Limestone’. Total thickness varies between c. 5 and c. 7.5 m.

Prior to 1975, the Valkenburg, Gronsveld, Schiepersberg, and Emael members were referred to as units Ma-Mb (sensu Uhlenbroek, 1912), which Villain (1977) considered to represent a gravelly intrabiomicrosparite, with regional currents constant enough to horizontally displace sediment particles over the entire platform, at shallow palaeowater depths of 20 to 40 m and free from oceanic influence. Sediment reworking resulted in their homogenisation over depths of some decimetres, resulting in a relatively firm sea floor and clear waters. Liebau (1978) typified the setting as middle sublittoral, with subtropical temperatures and characterised by the occurrence of seagrass communities.

Albers & Felder (1979) characterised the ‘Kunrader Kalkfazies’ as a cyclic alternation of highly indurated, silicified calcisiltites and less indurated biocalcarenites. The latter generally contain a higher glauconite content and terrigenous component. Cross-bedding has been demonstrated and bioturbation occurs commonly, especially in glauconite-rich portions. In comparison with the Maastricht facies, a less diverse fauna occurs. Rich thallophyte assemblages are known, in particular seagrass and many washed-in terrestrial plants, some of which have also been recorded from various levels within the Maastricht Formation west of the River Maas (see, for example, van der Ham & van Konijnenburg-van Cittert, 2003; van der Ham & Dortangs, 2005; van der Ham et al., 2001, 2003, 2004, 2010).

The depositional setting was interpreted as fully marine, invariably above wave base in the euphotic zone, the proximity of land masses being demonstrated by strong terrigenous influence (land plants), which explains decreased coral growth and slightly less diverse biocoenoses. Ostracod faunas suggest decreased hydrodynamics in a lagoon-like setting near a flat coastline and a low hinterland.

Nekum Member – This unit comprises poorly indurated, white-yellowish, coarse-grained, homogeneous chalks, in the lower part with a few randomly distributed greyish brown flint nodules. Locally coarse-grained fossil hash lenses and beds occur, which are characterised by high numbers of holasteroid echinoids and ostreid bivalves. The total thickness varies between c. 7 and c. 15 m. The chalks are medium- to coarse-grained biocalcarenites (mainly packstones and grainstones; gravelly intrabiomicrosparite according to Villain, 1977), with an indurated calcarenite resting upon the Laumont Horizon. Flint nodules in the lower part of this member (the highest in situ occurrence of flints in the type Maastrichtian) have a crypto/microcrystalline texture and are often associated with concentrations of large skeletal grains. Nodules are tubular when related to bioturbation. The upper part of the member comprises porous, fine-grained carbonate sands, with undulating erosion surfaces. Sand lenticles resting on such erosion surfaces may show tangential cross bedding; the Kanne Horizon represents an undulating erosion surface overlain by coarse-grained bioclastic sand.

Meerssen Member – In the west this member comprises a poorly indurated, white yellowish, coarse- to very coarse-grained chalks with clearly developed hardgrounds and fossil hash layers. These lenses and layers comprise to a large extent bryozoan remains and large foraminifera. Total thickness varies between c. 15 and 20 m.

Zijlstra (1994) observed that the upward-coarsening of grain size and the increase of average bed thickness indicated a gradual increase of average hydrodynamic energy and deposition rates. The most strongly silicified/lithified layers formed when deposition rate was nil, that is, when hydrodynamic energy increased and the consequent increase of erosion equalled the relative sea level rise. During a further increase of hydrodynamic energy, previously lithified sediment was eroded during storms and wavy beds formed. A hardground, that is, a bored, encrusted and mineralised rocky sea bottom, formed when the sediment that was eroded during a storm was not redeposited after the storm, so that the previously lithified layer was continuously exposed.

Villain (1977, p. 8) described this unit as a gravelly intrabiomicrosparite, deposited ‘sous une tranche d’eau réduite (15 à 2 mètres), une agitation supérieure à celle du Mb permet le déplacement de particules plus grosses (...) déposées en stratification obliques sous les énergies maximales du Md inférieur; elle favorise la prolifération de Lithothamniées dès le Mc, et de Polypiers solitaires au Md.’ Liebau (1978) typified these sedimentary rocks as high-energy deposits, with a high production of carbonate detritus leading to the establishment of a broad, shallow, well-lit, warm carbonate platform with rich phytal association. Water temperatures are held to have risen to 20-25° C allowing the growth of scleractinian corals, especially in the lower/middle portion of this member (see also Sprechmann, 1981). Hofmann (1996), on the basis of microborings, concluded that such traces could be ascribed to endolithic algae, thus documenting a euphotic to maximally disphotic depositional environment. Zijlstra (1994) also noted the extreme thickness of the uppermost portion of this member and suggested that this may have been caused by rapid increase of local subsidence rate related to increased tectonic activity connected with Deccan Trap volcanism. Van Harten (1972) also pointed out that deposition of the upper Meerssen Member could have occurred in deeper water, in contrast to the continuous shallowing trend up to halfway this member.

Albers & Felder (1979) characterised the Maastricht tuffaceous facies as biocalcarenites and biocalcirudites, with rare cross-bedding and occasionally with channels. Biocoenoses show a high diversity of tropical-subtropical, warm water faunas, mainly consisting of bivalves, and, in comparison with the Kunrade facies, increased numbers of scleractinians, echinoids and brachiopods. Related to substrate consistency, these biodetritus chalks contain numerous representatives of burrowing endobenthos and, with increased hardground development in the Meerssen Member, epibenthos became more dominant. The rich microfaunas show a high diversity with moderate abundance and rapid evolutionary rates, sharply separated from conditions that prevailed during deposition of the Gulpen Formation. These authors interpreted the depositional setting to have been fully marine, tropical-subtropical, invariably or generally above wave base in the euphotic zone, very strongly decreased suspension, with rich biocoenoses of high diversity and an active biochemical cycle in the formation of exo- and endoskeletons.