Research Herman Zevenberg: Stratigraphy of the Berg en Terblijt hardground; Stratigraphic correlation

The Meerssen member consists of seven sedimentary cycles (see also Zijlstra et al., 1996) of varying thickness. The precession induced cycles of, on average 20 KA, have been formed due to periodic variations of hydrodynamic energy and of deposition rate (Zijlstra et al., 1996). Each cycle starts with a graded, coarse bioclast grit layer (stormlayer), displaying Hummocky cross stratification. The uppermost part of the laminated storm layers became bioturbated after the storm. These bioturbated layers are only preserved when followed by a extended period of low storm intensity. The boundary between the bioclast grit and the fine carbonates is not well definable. The homogeneous carbonates are commonly capped by a hardground.

5.2 Hardground genesis

The hardgrounds formed when the lithified layer was exposed during a storm. Only when the lithified layer remained exposed during subsequent fairweather conditions (high energy), the layer could develop into a hardground (Zijlstra et al., 1996). During exposure the hardground was bored and encrusted. The inside of the borings and also sometimes the surface of the hardground itself are covered with encrusting benthos. Interesting is the description of Voigt, 1959 of a hardground in the Curfs quarry. He described a hardground not covered by encrusting benthos, but with borings with a black clay covering in the inside.

Using sequence stratigraphy, the hardground can be seen as a transgressive surface. The subsequent fining upward stormlayers are the Transgressive System Tract (TST).

5.3 Curfs Quarry

Fig. 1. Map of the Curfs quarry in Berg en Terblijt with the locations of the sections. From section 4 to 6 is about 150 to 200 meters.

In the Curfs quarry seven, detailed stratigraphic sections (see 4.1 Curfs Quarry and Berg en Terblijt horizon) were measured. The locations are shown in Fig. 1. The Berg en Terblijt horizon (see also Curfs stratigraphy) is used for correlation and is shown horizontally in the correlation of the sections (Fig. 2). It was not possible to make a detailed profile like in the ENCI quarry using photographs. In reality the Berg en Terblijt horizon is not horizontal and does not have a constant thickness, but in the southern part of the Curfs quarry the horizon can be traced as a relatively horizontal surface.

The correlation shown by Jagt et. al. (1996) Fig. 9., seems unlikely. In this correlation the IVf7 part is cut off to the east. No evidence for this is visible in Fig. 2. Near section C6 the Vroenhoven hardground was collapsed and sunken into the underlying carbonate. This probably happened shortly after hardground forming, when the carbonate was still soft and muddy. These collapsed hardgrounds could have led to a wrong correlation in Jagt et. al. (1996), Fig. 9.

Klik Hier voor figuur.

Fig. 2. Correlation of the seven sections measured in the Curfs quarry in Berg en Terblijt. Section from SE at left to NW at right. The scale of a small square is 5 cm. in reality.

In places where the hardground is thick, there is generally a thin bioclast grit layer on top. When the hardground is thin, a thick bioclast grit layer is present. This can be the result of an irregular hardground, where the depressions are filled in by bioclast grit.

In the Curfs quarry there is a claylayer present of up to two centimeters. In some sections only clayflakes are present. These clayflakes rest on top of a graded bioclast grit layer, above the Berg en Terblijt hardground. In some of the sections no claylayer was found. The Berg en Terblijt horizon and the Vroenhoven horizon are easy to follow in the field from one section to the other. The thinner bioclast grit layers are probably local, and in the field it was not possible to follow these layers. Some of the layers showed individual channel like structures (see 4.1 Curfs Quarry and Berg en Terblijt horizon). It was not possible to extend most of the sections until the Vroenhoven horizon, because of the bad exposure in the quarry.

The correlation of the seven sections is shown in Fig. 2. The general pattern is the Berg and Terblijt horizon (except section 3) on top of bioturbated chalks, followed by a graded bioclast grit layer, sometimes with cross stratification (sections 1, 2, 3 and 4). In section 1, carbonate pebbles are present in the bioclast grit layer above the Berg en Terblijt hardground. In Section 3 a hardground was found, but probably not the Berg en Terblijt hardground. 70 Centimeters below the missing part of the section, a complete nautilus of 20 cm. was found. Possibly the Berg en Terblijt hardground was badly developed at section 3.

In sections with a thicker Berg en Terblijt hardground, there is a thinner bioclast grit layer (sections 5, 6 and 7) and claylayers are absent. The exception is section 4; here, probably two hardgrounds are formed on top of each other.
Above the claylayer or, if missing, directly above the Berg en Terblijt horizon, a second graded, fine bioclast grit layer is present (sections 1, 4, 5 and 6). From there until the Vroenhoven horizon fine carbonate occurs with occasional thin coarse bioclast grit layers and burrow structures.

5.4 ENCI Quarry

At the north end of the ENCI quarry concession (situation summer 1998), a detailed overall section together with four sections were measured. The exact spot was determined using a GPS device and has coordinates 175991-315260 (Dutch topographic map coordinate system). The section is located in the middle of the Meerssen member. At this location bioherm-like structures and hardgrounds were found. Because of the irregular structure of the hardgrounds, the exact stratigraphic position was difficult to determine, but by counting the major hardgrounds from the Caster horizon below, the section was determined to be IVf-3 and IVf-5 (see ENCI stratigraphy).

The numbers in the hardgrounds in Fig. 3 correspond with the counted hardgrounds from the Caster horizon upwards. These numbers do not correspond to the stratigraphy from Felder. The upper surface of hardground II is the boundary between Felder's IVf-2 and IVf-3. The top of hardground V is seen as the boundary between IVf-4 and IVf-5. The boundary between IVf-3 and IVf-4 is difficult to place in the section because of the irregular hardgrounds suggesting locally heavy erosion. Most likely, this boundary must be placed at the top of hardground IV, which is present in sections E2 and E3. In sections E1 and E4, the hardground is eroded or did not form.

An interesting detail are the burrows from underneath in hardground IV, between sections E1 and E2. This can only occur when the hardground was hollowed by erosion and if organisms could live in the hollow. It was not possible to investigate the three dimensional extension of this 'cave'.

Also interesting is the find of many large fragments of corals, on the bottom of the large fillings west of section E2 and directly west of section E3. The good preservation suggests derivation from nearby reefs. The following species of coral were found: Columnastrea fallax, Montastrea arachnoides, Actinelia elegans, Diploctenium cordatum, and some unidentified species. Some large bivalves, sponges, and a rudist were found as well.

Klik Hier voor figuur

Fig. 3. Detailed profile of the middle part of the Meerssen member showing the numbered hardgrounds and locations of the sections.

In Fig 4., six deposition stages (A to F) are indicated in the correlation of the four stratigraphic sections (E1 to E4) measured in the ENCI quarry. The number of the hardground measured from the Caster horizon is also indicated, like in Fig. 3. Below, the six deposition stages are described in detail. Even within the described units, much variation can occur.

Stage A: The fine carbonate is completely bioturbated and is deposited under calm conditions. Hardground II formed during a period of non or slow-deposition.
Stage B: The cross-bedded infilling on hardground II indicates sediment transport from the east. Thereafter, a fine bioclast grit layer (sections E3 and E4) and subsequently fine chalk (sections E1 and E2) were deposited. In section E3, no hardground III is present, but the hardground was correlated (unsure) with a layer of fine carbonate. In sections E1, E2, and E4, hardground III formed at the top of stage B.
Stage C: This stage is only present in sections E3 and E4, and consists of bioclast grit layers. In section E3 the top of the bioclast grit layers are crossbedded. Possibly the thin bioclast grit layer between hardgrounds III and IV in section E2 also belongs to stage C.
Stage D: Very thick and irregular hardground, which is present only in sections E2 and E3. Possibly this bioherm like hardground was the substrate of the corals found in Stage E. Between sections E1 and E2 at the point where the hardground stops, the hardground has burrows from underneath, indicating exposure of the underside.
Stage Ea: Very coarse graded bioclast grit layers, filling the irregularities in hardground IV. Between sections E1 and E2, deposits of stage Ea are below hardground IV, because significant erosion took place under hardground IV before deposition of stage Ea. The parts of a broken-off hardground found in the filling near section E3 also indicate a high energy environment. Moreover, in section E3, an one cm. claylayer was found, which might indicate a calm period between the stormlayers. The claylayer was lens shaped (see also 4.2 ENCI Quarry). On the bottom of the large fillings west of section E2 and directly west of section E3, many large fragments of corals were found. The good preservation suggests derivation from nearby reefs. The filling with bioclast grit seems to have come from the east (see the stormlayers, Hummocky cross stratification), and is more than one meter thick in section E2. During calmer periods, several crossbedded sets with changing directions (herringbone structures) were deposited in section E1, possibly due to tides. In the top of the fossil hash fillings, some plain beds were deposited during a calm period; in section E3 even a thin hardground formed. The large fillings with bioclast grit could have been deposited in a short timespan.
Stage Eb: Only present in section E1. A second set of crossbedded sets (Hummocky cross strat.). Storm deposit.
Stage F: In sections E1 and E3, the stage starts with a graded bioclast grit layer with Hummocky cross stratification in E1. Then fine carbonate was deposited with hardground V on top, except in section E2, where the hardground rests directly on the top of stage E.

Above Stage F first uncontinuous bioclast grit layers were deposited in sections E2 and E4. The bioclast grit layers are overlain by fine white carbonates, which were deposited directly on hardground V in sections E1 and E3. At the top of the section above and west of section E1 (see Fig. 3.) a sinkhole (karst) is present, filled with overlying Oligocene clay.