GROUNDWATER SILICIFICATIONS IN THE CALCAR EOUS FACIES OF THE TERTIARYPIEDMONT DEPOSITS OF THE ATLAS MOUNTAIN (HAMADA DU GUIR, MOROCCO)

The Tertiary piedmont deposits (Hamada Formations), on the southern ed ge of the Haut-Atlas mountains, form extensive tablelands in the Boude nib area. They consist of two main sedimentary sequences, the Hamada d e Boudenib and the Hamada du Guir, of Eocene and Miocene age. Both seq uences show elastic facies at their base (conglomerates, calcareous sa ndstones, silty clays) and end with thick lacustrine limestones and pe dogenic calcretes are characterised by rather confined facies, palygor skite-rich, with some gypsum in the second sequence. The recent evolut ion of the region is marked by the dissection of the tableland that is lined with high cliffs. The water flaw is mainly through wide karst f eatures as there is no major river on the tableland. Silicifications w hich affect the different facies, form pods of various shape and size, and show an erratic spatial distribution. In the calcareous sandstone s, there are irregularly shaped tubules of about 5 cm in diameter, mor e planar bodies from 5 to 50 cm thick, which frequently display voids lined with translucent silica concretions. The conglomerates display r elatively few silicifications, the more characteristic ones consist of a silica cortex on some Limestone pebble and silica plates fitting cl osely the base of the pebbles. The lacustrine limestones and the calcr etes from the upper part of the formation show frequently well develop ed silicifications. These show very variable shapes; horizontally stre tching layers, interconnected or isolated amoeba-like bodies, thin sla bs, karst micro-breccia, with frequent concretionnary structures, and quartz crystallisations. Limestone nodules remain often included in th ese silicifications. The more argillaceous facies display either small tubules or thin plates formed of translucent concretionnary silica. A s a rule, the importance of the voids and related structures (concreti ons, drusy crystals) has to be noticed in all these silicifications, s ometimes they are also linked with fractures or karst pipes. Petrograp hy of the silica minerals, their relation with the primary structures. their distribution and their succession, give invaluable information on the silicification processes. Microcrystalline and fibrous quartz a re the most common silica minerals, including minor amounts of opal an d euhedral quartz. But micrographic arrangements show clearly that pri mary opal deposits have been more extensive and have recrystallized in to chalcedony, microcrystalline quartz, or even ''flame-like'' quartz. Silica deposits in voids make up an important part of the silica pods . The tubules and thin plates of translucent silica of the argillaceou s facies are formed of laminar chalcedony deposited around voids. Sili ca deposits in voids are also particularly obvious in the sandstones. The pores between the quartz grains are then cemented by fibrous quart z and little opal. Some samples show very large cemented voids that ca nnot be related to the primary porosity of the sandstone. These large voids correspond to the dissolution of the primary calcareous cement, which even led to the collapse of the sandstone fabric. In the limesto nes, there are silicified micro-karst breccia with a very high primary porosity cemented by quartz crystals, and even in the large microcrys talline quartz zones there are numerous void fillings, the primary por osity often exceeding 50%. There is obviously the alternation of silic a deposits and calcite dissolution. Beside the void filling, silicific ations comprise also matrix epigenesis, that is replacement of the car bonate by silica with preservation of most of the limestone structures , without development of voids. Nevertheless, the epigenesis of the li mestone matrix is restricted to the vicinity of the voids. The silicif ications relate to diagenetic processes. The main part of the silica i s formed of void deposits and matrix replacement (epigenesis) on the e dge of the voids. These void deposits give evidence of the feeding sol utions. The regularity of the deposits all around the voids point out to a hydrologic regime characterised by a ground-water our now. Silica originates most probably from alteration of the magnesian clay minera ls along the ground-water path. Regarding the low solubility of silica in surficial waters, high flows are needed in order to renew continuo usly the silica precipitated from solution. This points to a relativel y humid climate at time of silicification, and to relief and incised l andscapes to bring about these high flows.