The climatic and geodynamic histories of onshore and offshore Netherlands, summarised in Figures 13 and 14, largely control the distribution in time and space of the major processes that directly or indirectly influence the hydrogeological (storativity and permeability of pores, faults and fractures) and hydrodynamic conditions (overpressure distributions; flow characteristics, such as no flow conditions, continuous or episodic flow conditions; invasion of water from different origins). Table 2 summarises the main mechanical and thermal forces and processes capable of exerting a controlling influence on fluid flow conditions in onshore and offshore Netherlands during its post-Carboniferous evolution.
Additional processes influencing fluid flow, such as dehydration reactions, chemical diagenesis and gas generation, may have operated simultaneously. The deformation of Zechstein salt during different phases of basin development will have influenced
Time period Area Primary driving force Secondary driving force
Variscan
Early Permian The whole of the area Topography of water table Erosional unloading Area S and SE of Netherlands
Pre- and Early rift The whole of the area Sedimentary loading Southern Permian Basin
– depocentre Sedimentary loading
– basin fringe Sedimentary loading Topography of water table Area S and SE of Netherlands Topography of water table
Main Syn-rift Mesozoic basins Sedimentary loading (local) heating Tectonic forces
Platform/highs Topography of water table Erosional unloading Area S and SE of Netherlands Topography of water table Cooling
Post-rift The entire area Sedimentary loading
Syn-inversion
Subhercynean- Inverted Mesozoic basins Topography of water table Cooling
Laramide Tectonic forces
Erosional unloading Remaining area Sedimentary loading Pyrenean Southern Early Tertiary High Topography of water table
Tectonic forces Erosional unloading
Post-inversion Southern North Sea Basin Sedimentary loading Topography water table Ice loading/unloading Oligocene-Recent Roer Valley Graben Topography of water table Sedimentary loading
Tectonic forces Area southeast of Netherlands Topography of water table
Table 2 Main mechanical and thermal forces and processes that influenced fluid flow conditions during the evolution of onshore and offshore Netherlands
fluid flow and pressure distribution during times of active deformation (e.g. Tuncay and Ortoleva 2001). Fluid density gradients may have influenced fluid flow throughout the basin’s evolution, including seawater intrusions in coastal areas and, in particular, density-driven flow near evaporites of the Zechstein Group and the Upper Germanic Trias Group.
4.1 Tectonic control
Tectonic forces include direct and indirect influences on fluid pressures. The indirect influence involves changes in the permeability and pressure distribution through opening, re-opening and closing of faults and fractures (Chapter 1).
The orientation of the principal stresses has changed repeatedly over time: the orientation of the regional extensional stress was approximately E-W in the Mid Jurassic and NE-SW and NNE-SSW in the Late Jurassic (Nalpas et al. 1995); the direction of the maximum principal compressive stress was approximately N170° in the Late Cretaceous (according to Nalpas et al. 1996), roughly N-S in Late Eocene – Early Oligocene (Nalpas et al. 1996) and has an overall NW-SE orientation at present (Müller et al. 1992, Zoback 1992). During the Early and Late Oligocene the tectonic activity in the Roer Valley Graben suggests WNW-ESE extension. Since Miocene the orientation and the location of the main depocentre indicate a NE-SW extension (Michon et al. 2002). These changes in the direction of the principal stresses will have induced changes in the permeability of the fracture system.
Changes in tectonic stress during the evolution of onshore and offshore Netherlands include gradual long-term changes in tectonic stress regime from an active
extensional stress regime (Triassic to Early Cretaceous) towards a compressional stress regime (Late Cretaceous to present-day), short-term stress fluctuations related to distinct rifting pulses (e.g. Late Jurassic rifting of the Mesozoic basins, Late Oligocene to recent rifting of the Roer Valley Graben) and pulse-like changes in compressive stress during the Late Cretaceous and Eocene – Oligocene. All these changes may have affected storativity and permeability and distributions of overpressure and fluid flow.
Major normal fault displacements in the Netherlands are related e.g. to the Late Kimmerian I rifting pulse. Active fault displacement in a compressional setting occurred in the basin during the Late Cretaceous and Eocene – Oligocene periods.
During active deformation associated with Late Kimmerian I rifting, Late Cretaceous inversion and Eocene – Oligocene uplift and Late Oligocene to Recent rifting, faults and associated fractures may have acted as permeable pathways for fluids.
The maximum overpressure that can be sustained at a certain depth depends on the sum of the least principal stress plus the tensile strength of the medium at that depth (e.g. Du Rouchet 1981, Sibson 1994). Apart from being depth dependent, the minimum principal stress also depends on the tectonic regime (Grauls 1997), hence fluid expulsion through faults and fractures may have occurred at different depths and time intervals and maximum overpressures may have reached different magnitudes during the different tectonic regimes operating during geological evolution of the Netherlands.
4.2 Sedimentary loading
Sedimentary loading may induce porosity reduction by compaction of the rock matrix, development of groundwater overpressures and groundwater flow. The development of overpressures involves competition between increase in sedimentary load and dissipation of groundwater pressures by groundwater flow. Aquifers in hydraulic communication with the ground surface will allow groundwater flow induced by sedimentary loading, and as a consequence reduce or even prevent overpressures building up in the aquifer. Poorly permeable compressible rocks/aquitards are not able to dewater and compact rapidly in response to sedimentary loading and are susceptible to developing overpressured conditions of the groundwater. In general, significant overpressured conditions are unlikely to develop in basins that subside less than 100 m/My, with the exception of parts of the basin with extensive layers of very poor permeability (e.g. Bethke 1986, Harrison and Summa 1991). In contrast, rapid rates of sedimentation will cause groundwater overpressures in low permeable layers (e.g. Osborne and Swarbrick 1997).
The highest long-time average sedimentation rates (Figure 14) occurred in the depocentre of the Southern Permian Basin during the Zechstein (140 m/My) and Early Triassic (early rift sequence; 160 m/My), in the Mesozoic Basins during the Late Jurassic – Early Cretaceous (main syn-rift sequence; 220 m/My), and in the northern offshore of the Netherlands during Pliocene-Quaternary times (400 m/My). The Quaternary sedimentation rate of 400 m/My corresponds to an imposed vertical stress rate of approximately 8 MPa/My (assuming density Quaternary sedimentary rock = 2090 kg m–3).
4.3 Erosional unloading
Erosional unloading may give rise to pore volume increase, decrease of groundwater pressures (development of underpressures) and groundwater flow.
The most important phases of erosional unloading occurred during Late-Jurassic to Early Cretaceous uplift and erosion of the Mesozoic highs (such as Elbow Spit High, Cleaverbank High and Schill Grund High, Texel IJsselmeer High, Winterton High and Peel Block, Figure 12; estimated erosion on Texel IJsselmeer High 1000 m), Late Cretaceous – Early Tertiary inversion and erosion of Mesozoic basins (Broad Fourteens Basin, West Netherlands Basin, Roer Valley Graben, Dutch Central North Sea Graben:
erosion between 750 and 3000 m), and Eocene – Oligocene uplift and erosion of the Southern Early Tertiary High (erosion Broad Fourteens Basin estimated at 500 m).
The maximum amount of inversion-related erosion during Late Cretaceous – Early Tertiary times was reported for the Broad Fourteens Basin: it was 3000 m in 20 million years or less, which corresponds to a minimum rate of erosional unloading in the order of 3.3 MPa/My. The erosional unloading during the Eocene – Oligocene inversion period is estimated at 3.7 MPa/My for the Broad Fourteens Basin (corresponding to a maximum erosion of 500 m in 3 My).
4.4 Glacial loading/unloading
Two Pleistocene periods of ice loading occurred in the Dutch part of the Southern North Sea Basin. At the maximum of the Elsterian glaciation the Scandinavian ice extended across the North Sea as far as the northwest coast of the Netherlands
and during the Middle Saalian ice extended farther south and reached the central parts of onshore Netherlands (e.g. Joon et al. 1990, Laban 1995). Three peaks in the southward extent of the Saalian ice sheet have been recognised at 155,000, 145,000 and at 135,000 years B.P. (Wildenborg et al. 2000). The minimum thickness of the Saalian ice sheet in the northern onshore parts of the Netherlands was estimated by Schokking (1990) at 195 m. Boulton et al. (1993) simulated the climate and the dynamics of ice sheet movement during the Saalian. The results of their study show that the advance and retreat of the ice sheets proceeded very rapidly at velocities of a few tens of metres to 200 - 300 metres per year. It is clear that glacial loading and unloading are very rapid processes of relatively short duration compared to sedimentary loading and unloading.
4.5 Topography of the water table
During different time periods in the Permian to Recent history, a water table was able to establish in subaerial parts of the onshore and offshore Netherlands and in subaerial regions close to the Netherlands southern and southeastern borders.
The post-rift period is the only time period without any subaerial exposure, and hence without any topography-induced flow. The main periods of topography-induced flow are indicated in Table 3. Supra-regional topography-induced flow systems with recharge areas in the highs south of the Netherlands border have probably been active repeatedly and for long periods of time: e.g. during the Permian, the Late Jurassic – Early Cretaceous main syn-rift period and from Miocene to the present-day (Table 3). Regional flow systems with recharge areas in onshore and offshore
Netherlands probably developed in distinct intrabasinal highs, such as the main syn-rift highs, the Late Cretaceous inverted basins, and the Southern Early Tertiary High. In Quaternary times local and regional groundwater flow systems developed in onshore and offshore Netherlands in addition to the supra-regional Ardenno-Rhenish groundwater flow system (Verweij 1990a). The overall topography of the subaerial parts of the Netherlands North Sea Basin was dominated by the flat topography of the deltaic fan system that developed from the Late Tiglian. The local and regional topography-induced groundwater flow systems in such a flat lowland area will generally have limited depths of penetration. The Elsterian and Saalian glaciations modified the topography of the lowland area. During the Elsterian glaciation, deep valleys, locally reaching depths of more than 300 m were eroded in the northern parts of onshore Netherlands and the adjacent offshore areas (Zagwijn 1989, Laban 1995).
Glacial lake sediments subsequently filled these valleys. The Saalian glaciation remodelled the ground surface topography: glacier tongue basins and ice-pushed ridges were formed (Joon et al. 1990). The basins were filled in during the Eemian interglacial. In the Weichselian the old Saalian relief was levelled further. However the present-day surface topography in the centre and east of onshore Netherlands is still dominated by the glacial features of Saalian origin. These positive glacial features act as recharge areas for local and regional topography-induced groundwater flow systems.
The repeated changes in sea level in Quaternary times exerted an important influence on the regional extent and depth of penetration and on the groundwater velocities of the topography-induced groundwater flow (e.g. Elderhorst and Zijl 1992, Oostrom et al. 1993).
The type, magnitude and areal distribution of the identified driving forces and processes for fluid flow systems have changed continuously during the geological evolution of the Netherlands (Tables 2 and 3). During each tectonostratigraphic stage different forces have acted simultaneously on the fluids (Table 2), largely because of the differential subsidence and uplift history of the structural units. Different fluid flow systems have probably coexisted and interacted laterally and vertically in the onshore and offshore Netherlands. From the foregoing it is clear that the different geological histories for the different structural elements (Figure 18) will have induced different fluid flow histories for different locations: for instance different fluid flow histories can be expected for the Mesozoic basins in comparison with the platforms/highs, but also for the different basins (Roer Valley Graben and Central North Sea Graben, Tables 2 and 3).
Subaerial exposure Positive topographical features/highs Climate Flow system related to uplift and
erosion/tectonic phase
Saalian London-Brabant Massif, Rhenish Massif Tropical, semi-arid 1. Supra-regional
London-Brabant Massif Arid to semi-arid
London-Brabant-Massif Tropical, arid to semi-arid 2. Supra-regional and regional Hardegsen Netherlands Swell, Cleaverbank High
Late Kimmerian I Elbow Spit High, Cleaverbank High, Subtropical, seasonally wet 3. Supra-regional
Schill Grund High, Texel IJsselmeer High and regional
Elbow Spit High, Cleaverbank High, Schill Grund High, Texel IJsselmeer High Winterton High, Peel Block
London-Brabant Massif
Late Kimmerian II Elbow Spit High, Cleaverbank High, Schill Grund High, Texel IJsselmeer High Winterton High, Peel Block
London-Brabant Massif
Subhercynian - Broad Fourteens Basin, West Netherlands Subtropical, seasonally wet 4. Regional Laramide Basin, Central Netherlands Basin,
Inversion Lower Saxony Basin, Central North Sea Graben, Roer Valley Graben
Pyrenean Inversion Broad Fourteens Basin/ Warm, subhumid 5. Regional Southern Early Tertiary High
Late Tertiary - Ardennes - Rhenish Shield Cold,cool-warm temperate, 6. Supra-regional,
Recent alternating arid & humid regional and
Uplift local
Ardennes - Rhenish Shield
Table 3 Main periods of topography-induced flow of groundwater in onshore and offshore Netherlands