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Lithostratigraphy
Gulpen Formation
next sectionVijlen Member – The Vijlen Member is comprised of yellowish grey, glauconitic, fine-grained chalks, with a basal glauconite-rich portion, which locally occurs also higher up in the section. Total thickness generally is between 15 and 25 m, but locally up to c. 100 m (Albers & Felder, 1979).
The most complete section of Vijlen Member sedimentary rocks known to date results from combining sections exposed in outcrop 62D-79 (stratotype) and borehole 62D-168 near Mamelis, c. 300 m southwest of the stratotype (Fig. 1). Here the total thickness amounts to 65-70 m. P.J. Felder & Bless (1994) subdivided this section into seven intervals, numbered 0 to 6 (see also P.J. Felder, 1997). Intervals 0 to 3 were defined in the Mamelis borehole, while intervals 4 to 6 were defined at the stratotype. These units have subsequently been adopted by Keutgen (1996) and correlated with his biozones. Keutgen (1996) recorded many hardground and erosion surfaces in the Vijlen Member. These document the following sequence of genesis: sedimentation break, omission surface, erosion surface, fossil hash, nodular chalk, incipient hardground and hardground.
P.J. Felder & Bless (1994) noted major changes in lateral and vertical composition of bioclast and microfossil contents in the Vijlen Member in the type area (see also P.J. Felder, 1997). South of Mamelis, echinoderm bioclasts predominate and indicate sedimentation in a low-energy setting below storm wave base. North of Mamelis, molluscan bioclasts predominate and glauconite-rich, quartz pebble-bearing beds with abundant clasts of belemnites occur repeatedly, attesting to deposition in a high-energy, shallow subtidal (above storm wave base) to occasionally intertidal environment. The rhythmic succession of belemnite-rich and belemnite-poor intervals in the Mamelis section may reflect rhythmic variations in relative sea level, which may have been responsible for the regional appearance/disappearance of various microfossil taxa or for (occasionally repeated) changes in relative frequency or abundance.
The seven intervals proposed by P.J. Felder & Bless (1994) are as follows. Interval 0 (thickness 3.2 m) is characterised by numerous horizons with quartz pebbles floating in glauconite-rich clayey and limy marls. Belemnites are particularly common at 1.7 m above the Bovenste Bos Horizon. It is correlated with early early Maastrichtian belemnite peaks at the base of the Vijlen Member in the Bovenste Bos and Zeven Wegen sections. Interval 1 (thickness 10.5 m) consists of comparatively indurated, glauconitic clayey and limy marls; belemnites are slightly commoner just below the upper limit. Interval 2 (thickness 10.0 m) comprises soft, glauconite-rich clayey and limy marls, with a twofold subdivision: a lower portion (thickness c. 5.4 m) with numerous quartz pebble horizons and extremely common belemnites, and a higher portion (thickness 4.6 m) characterised by many echinoderms and a few belemnites. This is correlated with the upper lower Maastrichtian (upper sumensis Zone) interval of Aachen-Vaalserstraße. Interval 3 (thickness > 7.3 m) comprises glauconitic clayey and limy marls with scattered quartz pebbles. Belemnites are missing, but echinoderms are common. Interval 4 (thickness 8.5 m) is characterised by at least three glauconite concentration levels with quartz pebbles floating in whitish grey, glauconitic clayey and limy marls, belemnites being common. It is correlated with the upper lower Maastrichtian of Aachen-Hans Böckler Allee. Interval 5 (thickness 12.0 m) consists whitish grey clayey and limy marls with low glauconite content. Belemnites are almost totally missing, except at the upper interval limit where they are slightly commoner. Interval 6 (thickness 11.5 m) is similar to interval 4 in having three glauconite concentration levels with quartz pebbles, floating in whitish grey clayey and limy marls. These levels are between 0.3 and 0.5 m thick, indurated and of a reddish brown colour. Locally they contain small, light grey, rarely greyish black flints (bioturbation flints). As in interval 5, belemnites are virtually absent and are commoner only near the upper limit.
In the stratotype area, the Vijlen Member is capped by the Lixhe Member, which comprises white to yellowish white chalks, with narrow, black-grey small flint bands. The base is developed as the 0.1-0.3 thick Wahlwiller Conglomerate, which is a glauconite-rich molluscan packstone with numerous quartz pebbles and comminuted fossil fragments. Temporary outcrops in the Aachen city area (Friedrichberg, Vaalserstraße, Wilkensberg, Schurzelterstraße and Hans Böckler Allee) were discussed in detail by Keutgen (1996), who documented P.J. Felder & Bless’s (1994) intervals 0 (at Friedrichberg), 1-2 (Vaalserstraße), 3-5 (Schurzelterstraße and Hans Böckler Allee), and 5-6 (Wilkensberg). For the classic Schneeberg locality (northwest of Aachen), Keutgen (1996) documented all of P.J. Felder & Bless’s (1994) intervals outcropping in various fields. According to Robaszynski et al. (1985), the Vijlen Member as exposed at the CPL SA quarry (Haccourt), corresponding to interval 6, was deposited in a relatively offshore setting (c. 20-25 km), at a palaeowater depth of c. 80 m and in a low-energy environment.
Lixhe 1-3 members – The total thickness of these three units is up to 25 m; they comprise white, fine-grained chalks with irregular dark blue-grey to black flint nodules. West of the River Maas a threefold subdivision can be recognised on the basis of flint type and abundance. Albers & Felder (1979) interpreted these members to be fully marine, deposited invariably below wave base, with decreased erosion and sedimentation dynamics, at least temporarily decreased O2 supply of the substrate and, in the eastern part, a pinching out of the euphotic zone on account of increased terrigenous sedimentation, which resulted in zonal subdivision of differing diversities. Zijlstra (1994) considered the Lixhe 3 Member to be a pure coccolithic wackestone, with silt-sized bioclasts, horizontal flint beds and considered it to be homogeneously bioturbated (Planolites, Zoophycos, Chondrites, Bathichnus and Thalassinoides-type deep burrows).
Flint genesis in the upper Gulpen Formation (Lixhe and Lanaye members) was unravelled by Zijlstra (1994), who demonstrated a regular succession of c. 75 thickening-upwards, continuous flint layers. These nodule layers were considered to have formed when detrital skeletal opal dissolved during late diagenesis and concentrated in sites of relatively high early diagenetic authigenic silica polymorph concentration. The highest concentrations of authigenic silica occurred during periods when deposition rate was low, and when sediment resided for a relatively long period in the anoxic redox zone. The rhythmic vertical variation of the flint nodule concentration is held to reflect the influence of the periodic variation of the earth’s orbital parameters (precession index) on climate, oceanography and periodically varying deposition rates. Zijlstra (1994) also attempted to relate the flint-rich sequence of the Lixhe and Lanaye members to the Milankovitch rhythmicity. His conclusion was as follows: the chalk with flint contains 75 flint layers, with individual members containing 20, 15, 20 and 20 flint layers, respectively, and with flint layers forming bundles of five, suggesting that this sequence may be interpreted as E4 (1300 kyr), E3 (413 kyr) and E1 (98 and 126 kyr) eccentricity cycles, and P (20 kyr) precession cycles.
The Gulpen Formation as exposed south of Maastricht reaches a total thickness of some 40 m and contains about 75 precession induced sedimentary cycles. Zijlstra (1994) assumed these cycles to have been deposited during approximately 75 × 20 = 1.5 million years (compare Table 2 here). The increase of the mean cycle thickness from 4 dm at the base to 1 m at the top of the Gulpen Formation sequence would then reflect an increase in mean deposition rate from 2 to 5 cm/kyr. To Zijlstra (1994), this fine-grained chalk with a high flint concentration and symmetrical, thin-bedded, eccentricity cycles reflects a low-energy environment with a low deposition rate. The coarser grained (tuffaceous) chalk, on the other hand, with a lower flint concentration and asymmetrical thick-bedded eccentricity cycles, reflects a high-energy environment with high erosion/deposition rates.
Lanaye Member – This member comprises white, fine-grained chalks with irregular light to dark blue-grey flint nodules. West of the River Maas, south of the St Pietersberg, 23 flint levels are distinguished; east of the river these are less conspicuous and bedding is absent, with only randomly distributed flint nodules occurring. The total thickness amounts to c. 20 m. Albers & Felder (1979) noted that in the southeast the Lanaye Member consisted of fine-grained chalks and in the west of pure biodetrital chalks, which graded into the sedimentation of the Maastricht Formation biocalcarenites.
Villain (1977, p. 7) interpreted the ‘Craie grossière Cr4’ (= Lanaye Member) as a compact biomicrite (with biomicrosparite patches), deposited in an environment still under oceanic influence, with horizontal transport of sediment particles by episodic currents and a palaeowater depth of 40 to 80 m. Liebau (1978) described the depositional setting as a platform environment with minor open oceanic influences (middle sublittoral) and subtropical temperatures.
Zijlstra (1994) considered this unit to represent a pure (97 per cent) coccolithic bioclastic silty, homogeneously bioturbated packstone, with large-scale wavy lamination preserved in places. Well-developed planar-parallel flint nodule layers occur at 0.5-1.5 m interspaces. Nodules are either tubular (formed around crustacean burrows) or platy; traces of shallow burrowing and sediment mixing are common, but poorly preserved. The activity of deep burrowers is preserved as ghost structures in flint.
The Kunrade Limestone facies, which is widely distributed in the Benzenrade-Kunderberg area (Fig. 1), was subdivided by P.J. Felder & Bless (1989) into two bioclast zones, ecozones IV and V, both of late Maastrichtian age on cephalopod evidence. According to these authors, ecozone IV is best correlated with Hofker’s (1966) benthic foraminifer zone F (or possibly the base of zone J); this zone equates with the Lanaye Member (Fig. 3). Ecozone V was equated with the lower half of foraminifer zone J in the Thermae (Valkenburg aan de Geul) and Maastricht-Kastanjelaan boreholes, and with zone H at the ENCI-HeidelbergCement Group quarry. In this correlation, the upper limit of the Kunrade facies, the so-called ‘Koraalbank van Kunrade’ and ‘Oesterlaag van Craubeek’ match the Romontbos Horizon at the base of the Emael Member, suggesting that equivalents of the (remainder of the) Emael, Nekum and Meerssen members (= benthic foraminifer zones I, K, L, and M) are not represented in the Kunrade area (Fig. 3).
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.