Geologic characterization of sedimentary rocks and sediments associated to shallows of El Confital Bay (NE of Gran Canaria Island, Canary Islands)

Abstract

The shallows of Fernando, Roquerillos and Zabala are located in the NE of El Confital Bay, in the municipal district of Las Palmas de Gran Canaria. So, Roquerillo is next to La Isleta (UTM Coordinates 28º 09’ 29”N, 15º 26’ 35”W), Zabala is nearer to Barra de Las Canteras (28º 09’ 01”N, 15º 26’ 24”W) and Fernando (28º 09’ 03”N, 15º 27’ 01”W) between them. These shallows appear in depths comprised between -15 and -25m, they form submarine platforms with walls smaller than 4 m and they contain numerous geomorphologies such as erosion caves, holes and corridors. These submarine morphologies together with its varied marine fauna and flora make these shallows are used as sport diving places in Las Palmas de Gran Canaria city. Up to now, geological studies of these shallows have never been made, consequently this is the aim of this research. To carry out this study we needed 12 immersions in the three shallows, 33 samples of rocks have been collected, 16 stratigraphic columns have been lifted, and we have taken 8 samples of near sands. With all these materials, 41 thin sections have been prepared for the petrographic study with the object of identifying microscopically the nature of the grains and its relative abundance counting points. Mineralogical and geochemical studies had been carried out with EMPA and SEM techniques to determine the composition and morphologies of the cements. Except the lower part of Roquerillos where we can find volcanic materials (levels of trachytic-phonolitic felsic pyroclasts), the rest of rocks which compose the three shallows are sedimentary detritic rocks, amongst which layers of sandstone predominate, in a lower proportion, some levels of conglomerate appear, and we can hardly find microconglomerate strata. In the walls where there are some layers of conglomerate and sandstone (Fernando and Zabala), the first are below. The conglomerates consist of felsic rounded stones (trachyte-phonolite), and in a lower proportion, mafic (basanite-basalt). These conglomerates have a matrix of bioclastic microconglomerates and sandstones. The petrographic study of the sandstones of the shallows confirms that in Fernando and Roquerillos litho- clasts grains predominate (data between 88 and 72 %) over the bioclasts (24-10%). The lithoclasts are mainly fragments of felsic rocks, minerals (feldspars) and glass (35-5%), and in lower proportion fragments of mafic rocks, minerals (olivine, clinopyroxene, amphibole and opaque) and glasses (25-2%). The most abundant bioclasts are the rests of fauna (mollusks, bryozoes, equinoderm and foraminifera) with values between 21% and 2% while the flora (red seaweed meshes) between 16% and 1%. Carbon- ated cements are mainly patches of micrite, sparite and microsparite around the grains (bladed) or as mosaic (blocky), but some fibrous silicate crystals of clays and zeolites also appear. The carbonated cement is essentially LCM with concentrations of CO3Mg between 4.8 and 1.2% and values of Sr between 600 and 100 ppm. The porosity of these rocks is intragranular and intergranular with data between 13 and 3%. Due to this, these rocks are defined as volcanic lithoarenites. Referring to Zabala, this shows petrographic features different to its components. So the proportions of lithoclasts and bioclasts are similar, near the 50 % with abundance of felsic (30- 8%) over the mafic (16-5%) and, the rests of flora predominate (31-10%) over the fauna (26-14%). These rocks only have carbonate cements of microsparite and sparite (isopachous and blady) and fibrous (drusy). The porosity is intergranular and intragranular with values between 40% and 6%. Therefore these rocks can be classified as volcanic calcarenites. The petrography of the samples of sands confirms heterogeneity in the results. So, in Fernando bioclasts sometimes predominate with values between 60% and 42% and some other times lithoclasts between 59% and 32%. The values of grain of bioclasts are variable, where the rests of the flora prevail with results between 58 and 10%, and the fauna rests are between 44 and 6%. Equally, the fragments of lithoclasts of felsic rocks abound and show values between 25 and 3%, while mafic components between 17 and 6%. In Roquerillos deposits of gravel-sized rhodolites are plentiful, and the sands which appear occasionally are felsic lithoclasts (about 77%) predominating over the mafic grains. These sands are bioclast poor (around 22%) with more rests of flora than fauna. The sands of Zabala have similar percentages of lithoclasts and bioclasts, about 50% prevailing one or the other depending on the areas. In the lithoclasts volcanic fragments of felsic rocks and minerals, with data between 26% and 16% while fragments of mafic rocks and minerals are comprised between 20% and 8%. Among the bioclasts there are plenty of rests of flora with values between 31 and 20% and the fauna varies between 25% and 12%. Taking the nature of the components of the sedimentary rocks and sands studied in the shallows and its relative abundance into account, an origin analysis can be carried out, locating the source area which the sediments come from. So, the abundance of fragments of felsic rocks, minerals and glasses in the shallows make us think erosion of the Miocene lava and ignimbrites in the cliffs of El Rincón, the gorges closed to the bay (La Ballena and Boca Barranco) and the subtidal outcrops on El Confital Bay. The mafic lithoclasts can come from the erosion of plioquaternary lava-flows and pyroclasts of the same area and also some similar quaternary materials which conform La Isleta. Referring to the grains of bioclasts, these are of marine flora and fauna, of shallow littoral environments. Moreover, the conglomerates and sandstones existence, together with textural parameters of grain rounding (textural maturity) indicate high energy and shallow marine means (beaches and bars). Also, the carbonated cements with Mg and Sr confirm marine origin and their typologies indicate phreatic conditions of the mineral precipitations. Eventually, the data of the nature of the components of the studied sedimentary rocks, their textural, porosity and cementing features described confirm that Fernando and Roquerillos are genetically similar and they seem to be older than Zabal