Arenaceous Sedimentary Rocks PDF

Arenaceous sedimentary rocks MEDIUM - GRA1NED CLASTIC ROCKS Sandstone Sandstone: is a clastic sedimentary rock composed mainly of sand-sized minerals or rock grains (2 to 1/16 mm di...

Arenaceous sedimentary rocks MEDIUM - GRA1NED CLASTIC ROCKS Sandstone Sandstone: is a clastic sedimentary rock composed mainly of sand-sized minerals or rock grains (2 to 1/16 mm diameter) or consolidated clastic material of sandy size without genetic or mineralogical composition. Composition: Typically quartz , feldspar; lithic or rock fragments, that binds or harden together by calcareous, siliceous chemically precipitate cements or matrix material (fine- grained material less than 64 um, which is present within pore spaces between the framework grains.). Quartz grains evolve from plutonic rock, and also from older sandstones that have been recycled. Feldspar grains are commonly the second most abundant mineral in sandstones. Feldspar can be divided into two smaller subdivisions: alkali feldspars (orthoclase) and plagioclase feldspars. Lithic framework grains or rock fragments are pieces of ancient source rock that formed through weathering from igneous, metamorphic, or sedimentary rock. Accessory minerals make a small percentage of the grains in sandstone. Common accessory minerals include heavy minerals (zircon, tourmaline, rutile , garnet, magnetite, or other dense, resistate minerals derived from the source rock). Cement and matrix: Silica cement can consist of either quartz or opal minerals. Quartz is the most common silicate mineral that acts as cement. In sandstone where there is silica cement present the quartz grains are attached to cement, this creates a rim around the quartz grain called overgrowth. The overgrowth retains the same crystallographic continuity of quartz grain. Opal cement is found in sandstones that are rich in volcanogenic materials, and very rarely is in other sandstones. Calcite cement is the most common carbonate cement. Calcite cement is composed of smaller calcite crystals. Other minerals that act as cements include: hematite, limonite, feldspars, anhydrite, gypsum, barite, clay minerals, and zeolite minerals. Matrix material (fine- grained material less than 64 um, which is present within pore spaces between the framework grains.). Significant of Sandstone: are porous enough to store large quantities, making them good aquifers and petroleum reservoirs. Sandstone was a popular building material from ancient times. It is relatively soft, making it easy to carve. It has been widely used in constructing temples, homes, and paving material. 1  Colors will usually be tan or yellow (from a blend of the clear quartz with feldspar). A predominant additional colorant is iron oxide, which imparts reddish pink to dark red, giving red sandstones. Classification of sandstones 1) Field description (based on grain size, color, sedimentary structures, accessory minerals fossils, depositional environments). 2) Pettijohn et al., (1973) Classification Scheme Pettijohn et al., (1973) classification scheme is based on the mineralogy of framework grains, and on the amount of matrix present in between the framework grains. On the amount of matrix, the sandstones are divided into two major groups. Arenites are texturally "clean" sandstones that are free of or have very little clay matrix (15%). According to composition, arenite is subdivided into Quartz Arenite are sandstones that contain more than 90% of siliceous grains. Grains can include quartz or chert rock fragments. Quartz arenites are texturally mature to supermature sandstones. These pure quartz sands result from extensive weathering. They are commonly deposited in a stable environment, such as aeolian beaches or shelf environments Arkose sandstones are more than 25 percent feldspar. The grains tend to be subrounded and less well sorted than those of pure quartz sandstones. These feldspar-rich sandstones come from rapidly eroding granitic and metamorphic terrains and mostly deposited in fluvial environment. Lithic Arenites are characterized by generally high content of unstable lithic fragments. Examples include volcanic and metamorphic clasts, though stable clasts such as chert are common in lithic arenites. This type of rock contains less than 90% quartz grains and more unstable rock fragments than feldspars. They are commonly immature to submature texturally. They are associated with 2  fluvial conglomerates and other fluvial deposits, or in deeper water.. Fig.2 Classification of sandstones (from Pettijohn et al., 1973). With matrix >15%: Graywacke sandstones are a heterogeneous mixture of lithic fragments and angular grains of quartz and feldspar, with clay matrix (>15%). The grains tend to be poorly sorted and less rounded. Mostly are deposited in deeper water by turbidity current. According to composition, wacke is subdivided into - Quartz Wacke are uncommon because quartz arenites are texturally mature to supermature. - Felspathic Wacke are feldspathic sandstone that contain a matrix that is greater than 15%. - Lithic Wacke is a sandstone that has a matrix greater than 15%. Flysch and molasse: Two other sandstone terms, flysch and molasses. Flysch is commonly referred to the graywacke suite, deposited in large basins, whereas molasse refers to a subgraywacke suite, a product of the erosion of contemporaneous uplifts during orogenic cycles. Tuffs and tuffaceous sandstones: The sandstone is of volcanic origin. Tuff is a stratified rock composed of sand size particles expelled from volcanoes and deposited on the land surface or in water. Depositional environments of sandstones: The principal depositional sites are (i) fluviatile environments, (ii) deserts, (iii) lakes, (iv) deltas, (v) shallow marine and beaches (vi) continental margins and deepwater basins environments. 3 4 5 6