Ancient coastal lagoon in Maspalomas dune field (Gran Canaria). Evidences from sedimentology, vegetation and historical data.

Abstract

Maspalomas is the only area covered by active dunes in Gran Canaria. This dune field is located on top of a fan delta formed in the embouchure of Fataga-Maspalomas ravine, just on the southernmost part of Gran Canaria Island. This fan delta is formed by alluvial deposits, which can be observed in several outcrops throughout the interdune areas of the dune field. The western boundary of the dune area is limited by the watercourse of this ravine, which at the present time is artificially channelized and ends at the Charca de Maspalomas. This is a small (c. 3 ha) and shallow (1-2m depth) coastal lagoon, which is the result of groundwater water seepage from inland and infiltrations of seawater through the sand bar. Detailed cartography of the different underlying materials beneath the dunes has allowed differentiation between eolianitic, alluvial and lagoonal materials. Present paper deals with the lagoonal type of deposits, which have been found on small spots all through an area of c. 4 ha located about 200 m north- east the Charca de Maspalomas. Some of these spots can be found in between a small grove of native vegetation. Superficially these deposits are constituted by grayish sandy silt, and some surface sedimentary structures formed by flowing water can be observed. Desiccation cracks are also present, as well as fossils of terrestrial gastropods, such as Hemicycla cf ethelema (Mabille, 1882) and Canariella sp. The upper layer of these deposits is slightly compacted, and it can be disaggregated under finger pressure. The upper 60 cm of the vertical sequence is formed by alternating layers of medium-fine sands 20-30 cm thick and thin layers of clayed sandy silt only 1-2 cm thick. This sequence indicates the fluctuation between ponding processes associated to superficial runoff during rainy periods, and aeolian processes which generates the arrival of large amount of sand from the nearby dunes. The whole deposits has been interpreted to be an ancient coastal lagoon, where the inputs of sand have progressively contributed to fill it up, so that at present times this lagoon has completely disappeared. Why this ancient lagoon could have disappeared while the Charca de Maspalomas is still a functional coastal lagoon? To answer this question several factors should be considered. First the relative location of both lagoons, so that the one which is closer to the dunes receives much more sand than the other one. Furthermore, it has to be considered the westward movement of the dunes, thus it is not strange the silting up of the eastward lagoon due to the displacement of the dunes. Other factors are related to freshwater inputs to this area. Some of them are the construction of several hundreds of wells and galleries in the period 1900-1960, and the building of four large dams in 1950-73 along the Fataga ravine. All these hydraulic works were carried out in order to guarantee water supply, initially for the agriculture and later on for the tourist resorts, but they have also led to a drastic decrease of runoff in the lower part of the ravine. In fact, the dunes movement is the reason that explains how the silty surface area has been preserved: The aerial photographs evolution shows that in the decades of 1950 and 1960 most of the area was covered by mobile dunes, and the actual grove was not present at all. Nevertheless, by mid 60s some vegetation began to appear. In the 70s some hotels and touristic resorts were built in this area. Even though it has not been proved, it is very reasonable to assume that sediment from nearby dunes was used as raw material during construction, and therefore a huge amount of sand was eliminated before it could reach the Charca de Maspalomas. These hotels were dismantled in 1989, and since the 90s it is clearly visible the vegetation growth. Nowadays this small patch of vegetation has evolved into a very dense small forest that includes many different species, such as Phoenix canariensis, Juncus acutus and Typha dominguensis. It has to be focused that this type of vegetation has very high requirements of freshwater, and this is the only place in the whole dune field where T. dominguensis may be found. The colonization of this particular area by this type of vegetation indicates that groundwater level is very close to surface, which could also be related to the pre-existence of an old lagoon in this place. In fact, there are historical documents which indicate that there were two coastal lagoons in Maspalomas at the beginning of XIX century. Probably the best one comes from Webb and Berthelot (1838), who made a very detailed chart of Gran Canaria where some irregular hills are drawn over the coastal plain in the southernmost end of the island, which clearly corresponds with the dunes. In the center of this plain there are two coastal lagoons named “Lagunas de Maspalomas”, both fed by Maspalomas ravine after channel bifurcation. To summarize, this is first time since 1838 that this ancient coastal lagoon in Maspalomas dune field is mapped.