Geology of the Netherlands

This part of the syllabus homes in to specifics and particularities of the Geology of the Netherlands. The country occupies a very much filled sedimentary basin, known as the North Sea Basin. This basin has formed owing to tectonic subsidence (providing the accommodation space) and it has been filled up because of sediment delivery (the trapping of the sediments, is accommodated by the subsidence). Although the rates of subsidence have varied, and some parts have sunken more than others (depocentre vs. basin shoulder and rim areas), the sedimentary basin has been functional since the Carboniferous (Late Paleozoic). Subsidence has continued during the Mesozoic and Cenozoic and still continues to day.

Sediment delivery and basin filling have equally been more or less continuous: we find good amounts of sedimentary rock of all the time periods since the Carboniferous, although the environments in which sedimentation took place have greatly varied. These variations were due to slow global climatic and sea-level developments related to the plate tectonic process that caused continents and oceans to reposition and reconfigure themselves over geological time. This brought us tropical swamp forest, intra-continental desert and very salty evaporative marine environments (Carboniferous, Permian, Triassic), then carbonate rich seas (Jurassic, Cretaceous, Paleogene), than subtropical coastal marine environments (Neogene), then ice ages alternating with temperate conditions such as to day (Pleistocene, Holocene)).

Chapter setup
The previous chapter has touched upon the concepts of basins trapping sedimentary rock, plate tectonics moving continents and reconfiguring global ocean. Climatic, geomorphological and environrnental differences. Think of tropical vs. frosty weathering, subtropical soil formation vs. that below boreal pine forest, spring snowmelt oversized rivers vs. all year round brooks with groundwater recharge, temperate ‘Wadden’ seas vs. desert coast sabkha salt pans. It took a thematic take in presenting the subject, where this chapter does it chronologically.
The book of choice in this course – Geology of the Netherlands (Wong et al. 2007) – conveniently presents the geological situation on pages 5-195, covering the Geological development in general, and then going in eight chapters period-by-period from ‘Silesian’ (=Paleozoic) to ‘Tertiary (=Paleogene +Neogene) and ‘Quaternary’ (the youngest 2.7 Ma). The book, published by KNAW and coming from the mining school and leaning to NAM (Netherlands’ oil-gas exploitation enterprise) and Delft University of Technology gives relative great attention to what in working practice is regarded the deep subsurface, say the geology at 500 m to 5 km depth in most parts of the Netherlands, as formed in the Paleozoic and Mesozoic and of importance for gas, oil, deep geothermal and so on.

Towards the rim of the North Sea Basin and outside it, in the countries border region with Belgium and Germany, the Paleozoic and Mesozoic units occur in shallower positions and crop out (and coal beds were in reach of 20th cy mining). These old deposits (or ‘strata’) with a longer and thus complex geological history that made them lithify to such degrees that we now refer to them as bed rock rather than sediment beds, are covered in the first part of the chapter.
The overlying deposits, forming the shallow subsurface and originating from the younger geological periods (Cenozoic = Tertiary + Quaternary) of the last 66 Million years are covered in the second chapter. These are unconsolidated, i.e. not yet lithified deposits. The shallow substrate of the Netherlands is mined for sand (concrete; harbor expansion, coastal nourishment) and clay (baked bricks), hosts the groundwater systems that are within human reach, include the levels that we use for piling foundations and coffer dam bottom ceiling in construction works and the most shallow deposits that we use for agriculture (cheese making) and safety (coastal defense) and recreation (downtown shopping centres) and routing (of water, of traffic, of goods, of the internet). Just as the deep subsurface, the shallow subsurface is rather variable, especially its Pleistocene landforms.

Up to 30 meter of onlap of Holocene coastal and marsh and river deposits occurs in seaward and inland low lying parts of the country. Holocene thickness drops to near zero in the southern, central and easterly provinces which have a sandy soil and a geomorphology inherited from the last couple of ice ages mainly. These shallow deposits are covered at a relatively basic level by the Wong et al. 2007 book. Therefore, the syllabus and the lecture will include additional figures. These are taken from two text books in Dutch: De ondergrond van Nederland by De Mulder et al. (2003) produced at TNO Geological Survey of the Netherlands; and De vorming van het land by Stouthamer et al. (2015) produced at Utrecht University. These books summarized the heterogeneous body of literature produced for the country, and have treat the shallow subsurface in depth (puns intended). Furthermore, the chapter closes with a section that provides entry point to map data on the subsurface of the Netherlands as offered in digital form via the website www.dinoloket.nl/en as a main national resource.