2.3 Geology: Age, Lithology and Structural Setting
Table 5 summarizes the geological characteristics of each of the mega aquifer systems through two attributes: age and lithology. The aquifer units included in these systems consist mainly of sediments, among which sandstones and sands are predominant, in several cases in combination with carbonates. Most of these aquifer units form, together with interbedded clays and shales, a complex multilayer sedimentary aquifer system. The age of these sediments varies, but in most cases, their deposition spans a very long period, that began long ago. Millions of years of deposition (in some cases even 600 million years) resulted in thick sedimentary sequences, as indicated in Table 4.
Table 5 – Geological characteristics of the mega aquifer systems (after Margat and Van der Gun, 2013; modified).
* ‘Infra-Cambrian’ refers to the Late Ediacaran and Early Cambrian intervals between circa 585-530 million years ago (Al-Husseini, 2010).
The mega aquifer systems are embedded in structural geologic units that favor the accumulation of thick sequences of sediments over large areas. Such settings include sedimentary basins, such as the Congo, Paris and Tarim basins. Other typical settings of mega aquifer systems are rifted depressions in which sediments accumulate (e.g., California’s Central Valley and the North China Plain Aquifer System), and platforms covered by thick blankets of sediments deposited in piedmont, alluvial or glacial plain environments (e.g., High Plains Aquifer System, West Siberian Basin). Mixed setting types also occur. This characterization may give the impression of rather simple structural features of the mega aquifer systems but these are often much more complex.
In the first place, complexity occurs because many of these sedimentary basins consist of several subbasins, with different degrees of hydraulic interconnection. For instance, the:
- Dakhla, Kufra and Northern Sudan Platform subbasins in the Nubian Aquifer System;
- Great Western Erg and Great Eastern Erg in the North-Western Sahara Aquifer System;
- many subbasins included in the Russian Platform Basins;
- subbasins of the Great Artesian Basin (Surat, Eromanga, Carpentaria and part of the Clarence-Moreton geological basins);
- four subbasins of the Canning Basin (Kidson Basin, Willara Basin, Broome Platform and Fitzroy/Gregory Basin); and the,
- downfaulted Central Valley trough in California includes three distinct zones (Sacramento Valley, Sacramento-San Joaquin Delta and San Joaquin Valley).
In the second place, there is complexity because the mega aquifer systems often contain a large number of aquifers and aquitards, producing considerable lithological and hydraulic diversity. Examples are the:
- Arabian Aquifer System;
- Paris Basin; and the,
- Atlantic and Gulf Coastal Plains Aquifer System (consisting of several distinct zones: Gulf Coastal Plain, Mississippi Embayment, Florida, Atlantic Coastal Plain, each including a variety of aquifers).
Among all the mega aquifer systems considered, the High Plains Aquifer System seems to be the simplest, in terms of structure and lithostratigraphic variation.