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Unit Structure 7.0 Overview 7.1 Learning Objectives 7.2 Introduction 7.3 Formation of the island 7.4 Hydrogeology of the island 7.5 Aquifers of the island 7.6 Permeability & Storage 7.7 Summary

7.0 Overview This unit will be about the formation of the island of Mauritius, its resulting geological structure and the consequent hydrogeological properties of its subsurface.

7.1 Learning Objectives In this unit, students will learn the following:

1. Historical period during which the island was formed. 2. The four volcanic activity period which formed the island. 3. The structure of the geology of the island with respect to its formation. 4. The hydrogeological structure of each type of geological unit making up the

subsurface of the island. 5. The location and spread of the various aquifers of the island.

7.2 Introduction Mauritius is found on the Mascarene Plateau in the Indian Ocean. This sub-marine plateau was formed by intense undersea activity between 10-15 million years ago.

Location map of Mauritius

The island of Mauritius was formed by volcanic activities, taking place during four distinct phases and giving rise to different types of subsurface formations.

7.3 Formation of the Island (PRIMTAFF, 1998) Except for the beaches, the coral formation of the reefs and limited extents of alluvium (e.g estuary of Black River), Mauritius is entirely of volcanic origin, with almost all the rocks found on the island being basalts or pyroclastics of various kinds. The only non volcanic formations found on the island are recent reefs and beach and dune deposits. These are thin and of very small areal extent. Mauritius was formed as a result of four major volcanic activity periods; Emergence, Older Volcanic Series, Intermediate or Early volcanic series and Younger Volcanic or Late Volcanic Series. Argon-potassium dating in the steep side massif of the older volcanic series supports the theory that these massifs are the eroded remnants of a single, large, shield volcano built above sea level by the extrusion of lavas over a relatively short period of time between about 7.8 and 6.8 million years ago in the early Pliocene. During the second stage of volcanic activity (6.2-5 million years), the island acquired a circular, shield shape. The emerged part of the volcano was a vast dome of 40km diameter and culminating at an elevation of approximately 900m, and having sides with a gentle slope (5o). The summit of the shield stretched out and crumbled, forming a 24km diameter caldera. After a relatively calm period (1.5 million years), during which heavy erosion took place, the bottom of the caldera and of secondary depressions were filled by volcanic flows

between 3.5 1.9 million years. These series outcrop in certain valleys in the east and still more in the south west of the island between Baie Du Cap and Chamarel. They are also visible in certain river gorges (e.g. confluence of GRNW and Profonde). The intermediate phase is believed to have ended about 1.9 million years ago. It was followed by a period of quiescence extending over 1.2 million years. Eruptions during the period 7000000-25000 years, the Younger Volcanic Series, corresponds to the last volcanic activities of the island. The emissions came from 26 craters located along a NNE-SSW axis. The flows from the Younger lava series are compact vesicular olivine basalt with a thickness ranging from 0.2 to 2, though sometimes reaching 5m. Often lateritic soils are to be found in between them. Lava tunnels, which may be locally interlinked, are present in these horizons. The late lavas flooded the topography between about 0.7 to 0.17 million years ago.

Formation of the island

7.4 Hydrogeology of the island The hydrogeology of a region is directly related to the geology of that particular region. This relationship can be directly established from the definition itself of the word hydrogeology. Hydrogeology consists of 2 parts: hydro meaning water and geology meaning the study of rocks. More generally, we refer to hydrogeology as being the study of the distribution and movement of water in the subsurface, i.e, movement of ground water in the soils and rocks of the earths crust. Hydrogeology is also linked to aquifers and groundwater systems; the study of aquifers and groundwater systems with respect to their nature or environment is termed as hydrogeology. Therefore, to understand or have an idea about the behaviour, nature and associated factors of the ground water systems with respect to volcanic subsurfaces, i.e, the hydrogeology of volcanic formations. The geology of the island is basically basalt everywhere. But the three main phases of volcanic activity has given rise to different types of rock with consequently different hydrogeological properties. There are three main mountain ranges which are the Port Louis-Moka Mountain Range, the Black River - Savanne Mountain Complex and the Bambous Mountain Massif. These mountain ranges were formed by very important lava flows some 7 million years ago. In fact they are remnants of an ancient shield volcano that collapsed upon itself forming a caldera. The three mountain ranges can be seen to form a rough discontinuous circle that encircle the central uplands and highlands. They were the walls of this ancient shield volcano, hence constituting the Older Volcanic Series, that first caused the emergence of the island. The Older Volcanic Series rocks are generally massive and any fissures or vesicles that may be present are usually filled with secondary zeolite and calcite crystals. The emissions of lava from Intermediate series are not continuous, for clay deposits are to be found in between the successive flows. Thus the lava flows from the Intermediate series are of relatively low permeability. The rocks of the intermediate and last stages include highly fissured, vesicular and scoriaceous basalts and fine-grained pyroclastics. All important aquifers are found within the basalts of these two series, particularly the Late Volcanics.

7.5 Aquifers of the island Aquifers are considered to be made up of the recent and intermediate lava series, lying above the impermeable ancient series. The topography of the ancient formations influences the location of the ground water reservoir. Structural disposition of the recent and intermediate series, as well as their inter connection with the ancient layers also contribute to give a natural ground water reservoir. Factors found throughout all the terrains act as natural drains and help ground water separation from the reservoir. Isolation of water transmissive layers are often isolated within an aquifer due to presence of the semi-permeable weathered levels of clayish basalt (basalts with high clay content), (Giorgi et al, 1999). The central plateau, is made up of mainly of the recent and intermediate lava flows. The region also receives one of the highest rainfall of the island, between three to four metres of rain annually. The high permeability of the recent lava layers and the presence of the intermediate lavas over a large part of the plateau (constitutes) makes the region a hydrogeological regulator for the high land. It is also clear that underground flows-contribute-to the feed of reservoirs and springs; located downstream. (Giorgi et al, 1999) Mauritius has a ground water systems divided into five main aquifers; namely

Aquifer of Curepipe - Vacoas - Flic-en-flac. Aquifer of Phoenix Beau Bassin Albion/Moka Coromandel Aquifer of Nouvelle France Rose Belle Plaisance Aquifer of Nouvelle Decouverte Plaine des roches Midlands Trou Deau Douce Aquifer of Northern Plains

Secondary aquifers also exist, which are:

Aquifer of Chemin Grenier - Frederica Aquifer of Alluvium and Eluvium Aquifer of Carbonated formations Aquifer characterized by fractures

Aquifers of Mauritius

7.6 Permeability & Storage The water bearing properties of basaltic formations are extremely complex. These rocks present a system of vesicles, scoria, joints, fissures and even lava tunnels which influence permeability and storativity. Thus, the tranmission properties of basaltic aquifers are mainly related to the major openings such as joints and fissures while minor openings influence the storage characteristics. The Older Volcanic Series have poorly developed minor and major structures. Water can permeate only through the rare small fissures of the hard compact basalts, although these fissures are more or less clogged by clayey deposits or by calcites or zeolits of secondary crystallization. Agglomerates and lenses of tuffs are sufficiently compact and clayey to restrict the flow of water. The abundant dykes of the old series form excellent geological barriers against water movement, as they are very compact and occur in many different directions. The Second phase form a highly variable suite of basaltic rocks, including vesicular basalt, agglomerates and tuffs, often deeply weathered. The lavas are generally only slightly permeable because the clayey products of weathering tend to plug the various openings. In some places the

lavas are however very permeable, specially where there are unweathered doleritic basalts. Water tend to circulate not only through fissures and cracks, but also between the different lava flows. The Third phase lavas have a well developed systems of cavities, joints, fissures and scoriaceous zones. Most of these lavas are only slightly weathered. Under favourable conditions this slight weathering enhances permeability rather than reduce it. In some areas tunnels and caverns within doleritic basalts act as excellent water collectors. In the limited alluvial zones of the island, groundwater circulation is normal, some zones being more permeable than others.

7.7 Summary This unit has been describing the formation of the island of Mauritius. It has also highlighted how the geological structure of an island is directly related to its formation and the various types of volcanic activity phases. Finally much emphasis was placed on the hydrogeology of basaltic formations.