TOPIC 7: SEDIMENTOLOGY: Rock from Mud What follows is mainly a glossary of terms and concepts, I hope helpful. The Main Types of Sediment - Clastic, Chemical, Biogenic (= Biochemical). How do you get them? By WEATHERING, TRANSPORTATION, DEPOSITION. Weathering breaks down solids into pieces (Clastics; meaning = broken), and dissolved material. The "solids" can be chemically changed. Thus, feldspar in granite + water + carbon dioxide yields clay, + silicic "acid" + free ions; quartz stays the same chemically, but in smaller pieces. More on weathering, see Topic 19. Transportation generally involves fluids driven by gravity, rivers, air, circulation of oceans, glacier ice. In water, air, sediments are segregated by size (=SORTING), with fast currents carrying gravel, slower carrying sand, weak motions transporting "fines" e.g. silt and clay. Dissolved material is part of the water itself and doesn't settle out; most goes to the ocean. Clastic Sediment - The "broken" pieces of weathered rock. Grain sizes of clastic sediment ranges from boulders to submicroscopic clay flakes. Features of clastic sediments Sorting - the measure of range of particle sizes of sediments. A sediment having a wide range of particle sizes is poorly sorted; if the size range is small, the sediment is well sorted. Particle Shape - mechanical weathered particles broken from bedrock tend to be angular. However, such weathered particles become smooth and rounded if they are transported far by water or air. Rhythmic Layering - a distinctive alternation of parallel layers having different properties such as grain size and sorting. Cross Bedding - beds that are inclined with respect to a thicker, more-or-less horizontal stratum within which they occur. Marine, fluvial, and eolian (wind-blown)sediments may be cross bedded. Occurs in sand bars, dunes, ripples. Graded Bedding - beds in which the largest particles have settled first, followed by successively smaller grains so that the particles are sorted more or less according to size. Grains coarse to fine, bottom to top. Types of Clastic Sedimentary Rocks - Conglomerate - dominant particle size varies from 2 mm (a pebble) to more than 256 mm (a boulder). Sandstone - dominant particle size varies from 1/16 mm to 2 mm (sand). Siltstone - dominant particle size varies from 1/256 mm to 1/16 mm (silt) Shale and Mudstone - particle size is less than 1/256 mm (clay); shale breaks along beds; mudstone can break across beds. Shale has allignment of platy clay particles. Chemical and Biochemical Sediments Chemical Sediment - Components were dissolved, transported in solution, and precipitated chemically. Think of the ocean as a "chemical mix tank", with Inputs of water and chemical ions, and Outputs of water and chemical molecules as crystals. Your Inputs are derived from rock weathering, and river water. Outputs are the same chemicals, reorganized as other minerals like calcite, halite. Sometimes marine organisms do the work of producing minerals, sometimes water evaporation and chemical supersaturation does the work (evaporites). Types of Chemical Sedimentary Rocks - Evaporites:Rock Salt (Halite), Gypsum; Chert (silica). Mineral Deposits include iron and phosphorus deposits. Biogenic (Biochemical) Sediment - a sediment composed mainly of fossil remains. Includes most limestones, cherts. Siliceous Biogenic Sediment: Diatomite - rock from the remains of silica-producing algae (diatoms). Radiolarian chert - composed of the sunken remains of floating (when alive) protozoa called radiolarians; shells of silica. Calcareous Biogenic Sediment: Limestone - rock made of skeletons of carbonate-secreting organisms that precipitate calcite or aragonite Dolostone - often after deposition, part of a limestone deposit is converted to dolomite with the addition of magnesium carried by circulating brines. Other topics: Organic Matter - whenever organic matter is buried, a small portion of it escapes complete decay and oxidation. Coal - large concentrations of plant matter, buried and affected by pressure and temperature. Bituminous coal generally grouped with sedimentary rocks, anthracite with metamorphics. Oil and gas - algae and bacteria buried as organic matter in muds converts at appropriate temperatures and pressures to hydrocarbons. Oil forms between temps of about 65 to 150 deg C, which implies depths of around 6000 to 16000 feet (2-5 km). This zone is called the Oil Window. The Gas Window is a little deeper, involving temps of around 130-220 deg C. Below this range the organics convert to graphite. The above ranges govern where oil and gas are formed, that is the "source rocks". If the oil remains in place, the result is an "Oil Shale", an important oil reserve but one not yet economical ti "mine". Most oil is driven from the source rock by hydrodynamic processes involving pore fluids, and ends up in permeable oil reservoir rocks (porous sandstones, fractured limestones etc.); it can be extracted from these rocks by drilling and pumping wells. Lithification - the process whereby a sediment is converted to sedimentary rock. Usually involves Diagenesis. Diagenesis - all chemical, physical, and biological processes that affect sediment after its initial deposition; compaction, solution, precipitation of minerals as cement, recrystallization. These processes are affected by pressure change, temp change, chem change, and water circulation. Ordinarily pressure increases about 1 psi/foot overburden. Average temperature change is about 30 deg C per kilometer. Sedimentary Facies - The change in sediment character that takes place as we move from one depositional environment to another. A given rock formation can display changes in facies, due to different sedimentary environments existing at the same time in different places. Nonmarine Sedimentary Environments: Stream Sediments - streams are principal agent for transporting sediments. Types of stream environments include the alluvial fan, river channel, flood plain, and delta. Stream deposits are called fluvial deposits. Lake Sediments - sediments on the lakeshore and on the lake floor. Glacial Sediments - sedimentary debris eroded and transported by glacier deposited along the glacier base or released at the glacier margin, as melting occurs. Glacial sediment is commonly called till and consists of unsorted mix of unweathered boulders, pebbles, sand, silt and clay. Glacial meltwater streams can rework the till to produce well-sorted glacier "outwash-stream" gravels and sands. Eolian Sediments - sediments carried by the wind, These sediments tend to be finer than sediments moved by other processes; sand, and "dust". Marine Sedimentary Environments: Estuarine Sediments - Sediments in a semi-enclosed body of coastal water within which seawater is diluted with fresh water. Deltaic Sediments - marine deltas build outward into the sea. Beach Sediments - quartz, the most durable of common minerals in the continental rocks, is the typical component of beach sands. Offshore Sediments - some fine-grained sediment, carried in suspension, reaches the outer shelf (mud). Carbonate Shelves - carbonate biogenic sediments accumulate on the continental shelves wherever the influx of land-derived clastic sediment is minimal (the reefs aren't buried in mud) and the climate and sea surface temperature are warm enough to encourage carbonate -secreting organisms. For example, a broad, flat carbonate shelf surrounds the numerous islands of the Bahamas. Marine Evaporite Basins - ocean basins with restricted circulation that lie in a region of warm climate; evaporation leads to precipitation of soluble substances. Continental Slope and Rise - Gravity-driven "turbidity currents", consisting of dilute muddy mixtures of sediment and water having a density greater than the surrounding water, move down the continental slope and into the deep ocean. The result is deposition of graded beds. Turbidites - the graded beds of sediment due to deposition of the slowing "turbidity current. Turbidity currents are produced by submarine landslides.