Urban geology: The strange tale of a windowsill
Stephen K Donovan and John WM Jagt (The Netherlands)
Leiden, in the Dutch province of Zuid-Holland, is a city with a fine selection of fossiliferous building stones, mainly Mississippian (Visean, Lower Carboniferous) limestones. which preserve an array of fossils, such as rugose and tabulate corals, brachiopods, bryozoans, molluscs, and crinoids. However, this fine diversity of body fossils is not supplemented by a similar composition of trace fossils. Despite having examined these rocks over many years, when leading student fieldtrips and in collusion with co-workers, SKD has found no evidence of burrows, nor any borings in bioclasts, which can be locally common at some localities where Mississippian strata are exposed (for example, Donovan and Tenny, 2015).
It is therefore of note to recognise an uncommon rock type among the building stones of Leiden that is dominated by burrows and lacks body fossils. This article highlights this distinctive building stone that has engrossed SKD for some years. The street Rapenburg (Fig. 1) in Leiden is a favourite route for building stone tours of the city. Although the dominant building materials are bricks, there are ample rocks to make a visit worthwhile. When SKD has led groups of students from the University of Leiden on geological excursions down the Rapenburg, the start is commonly at the North End Pub (Fig. 1).
This is presumed to be a former bank building and it is sumptuously faced with polished, imported larvikite. Elsewhere in the Rapenburg, the common rock used for facing and ornamental stones at pavement level is grey, Lower Carboniferous (Mississippian) limestone. As there is no outcrop of Upper Palaeozoic limestones in the Netherlands (compare De Mulder et al., 2003), it also represents an import, long before the current international traffic in rocks (Nield, 2014). Available evidence indicates that these limestones were imported from southern Belgium (southern Ardennes, near the cities of Dinant and Namur) at the beginning in the sixteenth century (Van Ruiten and Donovan, 2018). The age of these limestones has been established using the combined evidence provided by their lithology and palaeontology (Donovan, 2016; Donovan and Madern, 2016; Van Ruiten and Donovan, 2018).
The walk is a feast of Mississippian palaeontology, but the end is notable for limestones of a different lithology, palaeontology and age. When you reach the botanical gardens, the Hortus Botanicus (Fig. 1), part of the University of Leiden (Rapenburg 71), go through the entrance gates, but veer almost immediately to the left, before the first doorway. You will find yourself in a broad alley with buildings on either side; the Hortus Botanicus is straight ahead, under a ‘bridge’ in the university offices. It is worth a visit, even if only for a coffee, but geology comes first.
You have just come through the gates from the Rapenburg and stopped. On the left side, the windows have sills of two distinctive rocks types. Past the first door, once again the limestones are Mississippian, containing various fossils including one particularly fine specimen of the tabulate coral colony Michelinia sp (Van Ruiten and Donovan, 2018, fig. 2B-D). But first look at the rather different lithology of the sills of the high windows above head height, on the left side of the first door and near the entrance gate.
These pale, cream- to pale grey-coloured limestones contain fossil burrows (Fig. 2). The contrast in colour between the rock matrix and the infill of the burrows is enhanced when wet, so remember to take a bottle of water when you visit them. Bedding is apparent on a millimetre-scale, trending from left to right, but wavy in places, and shows contrasts in coloration from pale to darker; the latter dark beds and darker infill of burrows are presumed to be glauconitic. The age is ambiguous, but the chalky limestone is most reminiscent of the (lower) Upper Cretaceous and we assume this stratigraphic assignment to be correct (but see below).
Common trace fossils, assigned to the crustacean burrow Thalassinoides isp, are exposed in windowsills above head height at Rapenburg 71. Cut faces of these blocks in the windowsills show darker colouration of irregular, knobbly burrows, some of which are obviously branched; note the irregularity of burrows produced by cut effect. Way-up structures have not been identified, so the original orientation of these blocks remains uncertain. Burrows vary from parallel to sub-parallel to vertical to bedding. The most prominent vertical burrow (Fig. 2E) cross-cuts (and therefore post-dates) a left-to-right burrow just above the centre of the image. Therefore, with careful observation, there are successions apparent in the trace fossils.
Thalassinoides is not an unusual trace fossil in the Upper Cretaceous (for example, Kennedy, 1967; MacEachern et al., 2007; Niebuhr and Wilmsen, 2016) and its common presence in these rocks suggests that deposition was on the shelf rather than deeper water (Ekdale and Bromley, 1984; MacEachern et al., 2007). Similarly, glauconite is a common component of the fill of such burrows (Bromley, 1967, fig. 1). The burrows appear rounded in section, indicating that they were not deformed by the weight of overburden. This suggests that lithification occurred before deep burial. Both this feature and the glaconitization are features that suggest hardground formation, but this is difficult to prove on such a small ‘exposure’, where samples for detailed examination cannot be collected for obvious reasons.
Perhaps the most mystifying question about these windowsills is their provenance. The Netherlands does not have a great diversity of ancient rocks in surface outcrop, which is dominated by Quaternary deposits of varied origins (Van Staalduinen et al., 1979; De Mulder et al., 2003). In addition to Middle Triassic outcrops close to Winterswijk (a province of Gelderland), the most extensive pre-Quaternary succession is the Upper Cretaceous (Campanian-Maastrichtian) of southern Limburg in the south of the country, which is (most notably) the type area for the Maastrichtian stage (Jagt et al., 2003). For many years, SKD has assumed that the Upper Cretaceous windowsills at Rapenburg 71 were from southern Limburg and therefore were probably of Campanian or Maastrichtian age.
However, with his extensive field experience of the geology of the area, JWMJ has never seen such a distinct and well-lithified, glauconite-bearing Thalassinoides-rich limestone. So, like the Mississippian limestones, it must be deduced that these windowsills are, again, an imported building stone, but for the present, at least, of unknown provenance. A potential clue is provided by Reumer (2016, fig. 150), who illustrated an Eocene limestone rich in Thalassinoides from Middelburg (province of Zeeland). Unlike the Leiden occurrence, the burrows and rock are dark and pale grey, respectively, and bedding is not apparent. So, we maintain our provisional assignment of these rocks to the Upper Cretaceous, while recognising that they may be younger.
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