Cements from Bajo Barreal Formation Sandstones, San Jorge Basin, Patagonia Argentina

IBAÑEZ, L.M.; OVEJERO, R.; FERREIRA, L.; GEORGIEFF, S.M.

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

Upper section of the Bajo Barreal Formation (Turonian) is composed of fine and medium sandstones which show mainly cross trough stratification but also planar, massive and ripple lamination. Occasionally, a silt parallel lamination is observed at the top of sandstone bodies. The fluvial sandstone bodies are well exposed around La Sin Nombre small Lake, where three stratigraphic logs were described. Besides, architectural descriptions in some representative sandstones bodies, fully exposed, and additional short detailed logs separated by tens of meters each were recorded. The sandstones from Bajo Barreal Formation, show several kinds of cements. These cements play an important role in the porosity-permeability characteristics of the reseorvoirs. Characterizes the cements is the main objective of this work and the fundamental descriptions are presented in this abstract. The older cement is the argileous, which could be present in different ways, such as pore lining rims, sometimes associated to clasts dissolution generated porous, followed by HC filling. In these cases, rests of the cement in the middle of the pore space can be observed. Besides, ribbons of smectitic cement around the clasts, showing differential extinction according to its water content, are present. The width of the ribbon is variable, and also the HC impregnation, which offer different colors from grey to yellowish brown. Small quantities of Kaolinitic cement, sticked on some sides of the grains are present as well. The argileous cement is followed by silica. It can be like fibrous chalcedony, commonly impregnated by HC; like microcrystalline silica with sutured contact that grade into ftanite, often associated to the argileous cement and growing toward the centre of the pore space. Colorless rectangular crystals, with low relief, one direction cleavage, weak birefringence and grey interference color can be observed. Thorough X ray diffraction analysis their composition was determined as Clinoptilolite-Heulandite. Their small crystals are mainly disposed perpendicularly to the grains, but they can also be in the centre of the pore without any arrangement. Sometimes these crystals are partially impregnated with HC. Characteristic but less developed crystals of Harmotome and mordenite are present too. Another cements are the carbonate (calcite) and ferruginous-carbonate (siderite?). They can be the main cements in some sandstones levels. Poikilitic spatic calcite shows an excellent rhombohedral cleavage and sometimes presents polysynthetic twin. Microcrystalline calcite is also present. The calcitic cement can growth with acicular and columnar habits around the grains. It can move away the surrounding grains, corrode their perimeter and brake them, and so modify the fabric. The ferruginous-carbonate cement is present in microgranular aggregates of high relief, redish color, high birefringence, bright interference colors and no well defined relationship neither to the clasts nor to the previous cements. The last cement is represented by occlusive zeolite (Harmotomo?). They are long thin colorless columnar crystals whit low relief and birefringence, grey interference color and parallel extinction. They have suffered dissolution, but no HC impregnation.