SEDIMENTARY ROCKS Source, Slope and Sink SOURCE Area : the composition of the source area exerts a great influence on the nature of the sedimentary rock(s) that can form -- CLIMATE and PLATE SETTING. granite could alter to form clays (from the feldspars) leaving quartz basalt, would alter rapidly to form clays plus various iron-rich compounds SLOPE refers to those processes which move solid material at the surface of the Earth Wind Water Ice In addition to carrying solid particles, water can carry ions in solution. Clays do not have the same composition as the materials from which they were produced. Some ions are carried away in solution. The SINK refers to a place of accumulation of previously transported material Granites in central Texas are weathered chemically and physically. The quartz is transported by the river systems and some eventually end up being deposited along the Texas beaches. Will characteristics of the beach environment be imposed on these grains? Can a geologist infer the SINK from the properties of a sedimentary rock? Diagenesis and Lithification Loose grains are called sediment Sediment can be converted to sedimentary rock if the particles are either cemented together or compacted during burial. As sediment is buried, changes in composition and texture may take place and sedimentary processes merge with metamorphc Classification of Sedimentary Rocks Clastic - particles carried from the source to the sink by slope processes .... grains do not interpenetrate Chemical - ions in solution are łforced˛ to react to precipitate solid material. If you leave a glass of water outside it will evaporate leaving a white film of calcium carbonate....an interpenetrating texture Clastic Sedimentary Rocks n About 75% of the Earth's continental crust is covered by sedimentary rocks Sedimentary rocks form a thin covering About 85% of the sedimentary rocks consist of grains that were transported as solids Grains between 2mm and 1/16mm in diameter are called SAND sized. Coarse Clastics Conglomerates & Breccias > 2mm Sandstones consist of framework grains plus matrix (finer in size) Quartz Arenite - framework grains are Quartz Arkose - framework grains are Feldspar Litharentite - framework grains are rock fragments Grains between 1/16mm and 1/256mm are called SILT Grains less than 1/256 mm are called CLAY SAND, SILT and CLAY are SIZE terms. A sandstone is a sedimentary rock made up of SAND sized grains. SANDSTONES account for about 20% of the Earthąs sedimentary rocks CLAY and SILT sized grains are dominated by clay minerals All clay minerals have a good cleavage. If the cleavage planes are lined up parallel to each other the rock is said to be FISSILE - A SHALE is a fine grained clastic rock with fissility. If the cleavage planes are not oriented in parallel, the rock is said to be non-fissile. A MUDSTONE is a fine grained clastic rock without fissility. SHALES and MUDSTONES are the most abundant sedimentary rocks - ~60%. Chemical Sedimentary Rocks ~15% of the sedimentary rocks are chemical - original transport was as ions in solution. Carbonates - limestones and dolomites - calcium carbonate precipitated or extracted from solution(s)...shells....animal dies and shell transported and eventually deposited. Evaporite Sequences - Carbonates, Sulfates and Salts Mineralogical Maturity Given time, minerals, with the singular exception of quartz, will react to form clays. The more quartz in a coarse sedimentary rock the higher the mineralogic maturity. A feldspar-rich sandstone (Arkose) is immature - not enough time for the feldspar to form clays Shales and mudstones are mineralogically mature - clay minerals Textural Maturity Texturally Mature Coarse Clastics are WELL SORTED (particles are all about the same size) The quartz grains in these rocks tend to be rounded. The more persistent the energy level, the greater the textural maturity Other Properties of Sedimentary Rocks Porosity - the percentage of open space - remember the generalizations about packing spheres The poorer the sorting, the lower the porosity Permeability - a measure of the łinter connectnedness of the pores - permeable units will transmit fluids Sedimentary Structures n Parallel Beds Cross Beds Graded Beds Ripple Marks "Sinks" In broad terms environments of deposition can be viewed as Continental, Transitional (beaches and shorelines) and Marine In Continental settings the supply of sediment may be high but the łpreservation potential˛ is usually low --too many things can happen before the sediment is lithified. Transitional - supply of sediment may be high (at the mouth of a river) and preservation potential is usually high. Marine - preservation potential is high but the supply of sediment may be low. Much of the sedimentary section on Earth accumulated in the Transition Zone - deltas, barrier islands, beaches, etc. Geologists study modern sediments to try and see what imprint the environment of deposition leaves on the sediment and resulting sedimentary rock. Evaporite Basins Climate Structure Evaporate 1 liter of marine water 50% gone : carbonate : 2.8 g/cc 80% gone : sulfate : 2.4 g/cc 90% gone : salt : 2.1 g/cc Repeat Sedimentary Facies Focus on grain size - related to proximity to source area Sea Level Rises : encroachment of marine environment onto continental area (or, land subsides) : TRANSGRESSION Sea Level Falls : encroachment of continental environment onto marine area (or land rises) : REGRESSION