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Chapter 7: Other Chemical/ Biochemical and Carbonaceous Sedimentary Rocks. Evaporites : all deposits that are composed of minerals that originally precipitated from saline solutions concentrated by solar evaporation. Found in both marine and nonmarine environments.
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Chapter 7: Other Chemical/ Biochemical and Carbonaceous Sedimentary Rocks
Evaporites: all deposits that are composed of minerals that originally precipitated from saline solutions concentrated by solar evaporation. Found in both marine and nonmarine environments. Gypsum (CaSO4·2H2O) and Anhydrite (CaSO4) Calcium sulfates are deposited dominantly as gypsum. However, gypsum can be altered into anhydrite while still in its general depositional environment and also upon burial to a few hundred meters. Loss of gypsum’s water decreases volume by 38% Anhydrite can be hydrated back to gypsum upon uplift and exposure to low-salinity surface waters. Original depositional structures and textures are distorted due to alternating dehydration and hydration of calcium sulfates.
Nodular Anhydrite: irregularly shaped lumps of anhydrite that are partly or completely separated from each other by a salt or carbonate matrix. Chickenwire structure: a nodular anhydrite that consists of slightly elongated, irregular polygonal masses of anhydrite separated by thin, dark stringers of other minerals such as carbonate or clay.
Laminated anhydrites: consist of thin, nearly white, anhydrite or gypsum laminations that alternate with dark gray or black laminae rich in dolomite or organic material.
Halite: (NaCl) forms as crusts, presumably in shallow water and as very finely laminated deposits in deep water that may reach thicknesses of as much as 1000m
**Precipitation of gypsum increases Mg/Ca favoring dolomitization.** Evaporation > Precipitation + isolation from open ocean = Brine Evaporative Drawdown: Evaporation Periodic overflow Brine level far below sea level Evaporation Complete basin evaporation
Evaporite deposits are also transported and deposited in turbidity flows, etc. Can show grading, cross bedding and ripple marks Salt (Halite) deposits can form diapirs or domes because of density differences. The salt domes are less dense and can pierce overlying sediments often creating hydrocarbon traps.
Siliceous sedimentary rocks (Cherts) Fine-grained, dense, very hard rocks composed predominantly of SiO2 Granular microquartz: consists of nearly equidimentional grains of quartz. Grain sizes range from ~1 to 50 microns Chalcedony: (fibrous silica) sheaf like bundles of radiating extremely thin crystals of about 0.1mm in length Megaquartz: elongated grains greater than 20 microns in length. Opal: hydrated metastable quartz that makes up tests of siliceous organisms.
Origin of Chert • What are the sources of silica? • What mechanism extracted the silica from the water? • Sources: • River input • Volcanic • Halmyrolysis (of oceanic basalts & detrital SiO2 particles) • Pore water reflux
Extraction from seawaters Inorganic extraction is unlikely in unsaturated waters like those of the ocean. However, it may be possible in local basin saturated in SiO2 due to dissolution of volcanics. Biogenic extraction appears to be the only large scale mechanism for silica extraction from the seawater. Diatoms are largely responsible during the present, whereas radiolarians extracted more during the Jurassic and earlier periods. Nodular or other replacement chert are formed during diagenesis where they replace carbonates and and clays.
Iron formation versus ironstone Iron formations are iron-rich deposits that range in age from early Precambrian to Devonian age. They consist of distinctively banded successions 50 – 600m thick, composed of layers enriched in iron alternating with layers rich in chert. Granular iron formation (GIF): have or had coarse, granular textures. Banded iron formation (BIF): have finer grained textures. Ironstones are dominantly Phanerozoic sedimentary deposits that form poorly-banded or nonbanded bedded successions. Ironstones commonly have an oolitic texture and may contain fossils that have been partially or completely replaced by iron. Sedimentary structures (i.e. cross-bedding) are common.
Iron Formation Ironstone (oolitic)
Iron deposit theories: 1. Formed subaerially in the Precambrian prior to an oxidating atmosphere and were subsequently transported to a marine environment. Problem: after the appearance of an oxidating atmosphere, we need another explanation. Iron in the oxidized or ferric (Fe3+) state is much less soluble than iron in the reduced or ferrous (Fe2+) state. 2. Iron was transported as colloids attached to clay particles or organic materials. Problem: it seems unlikely this could account for large quantities of iron transport. 3. Iron-bearing minerals were transported to the ocean were the ferric iron was reduced by anoxic bottom waters and the resulting ferrous iron taken into solution.
Phosphorites are rocks that contain more than 15% P2O5 or 6.5% phosphorus. Phosphorites have textures that resemble limestones. They may be made up of peloids, ooids, bioclasts and clasts that are now composed of apatite. Apatite: a calcium phosphate with the common varieties of fluorapatite, chlorapatite, and hydroxapatite. Most are carbonate hydroxyl fluorapatites (a.k.a.: francolite)(Ca10(PO4,CO3)6F2-3)in which up to 10% carbonate ions can be substituted for phosphate ions.
Types of Phosphorite deposits: • Bioclastic: composed largely of vertebrate skeletal fragments • Nodular: spherical to irregularly shaped nodules, with or without internal structure, often containing grain, pellets or fossils. • Pebble-bed: the sandstone equivalent—composed of nodules, fragments or phosphatic fossils that have been mechanically concentrated by reworking of earlier formed phosphate deposits • Guano: Bird and bat excrement that has been leached to form an insoluble residue of calcium phosphate.
Origin of phosphorites • Inhibition of organic mater decay due to reducing conditions at ocean floor. • Interstitial water exhalation • Phosphatization: where phosphate replaces skeletal and carbonate grains during diagenesis.
Carbonaceous sedimentary rocks: Coal, oil shale, and bitumens
Kinds of organic matter in sedimentary rocks: Humus: plant organic matter that accumulates in soils Peat: humic organic matter that accumulates in water where stagnant anaerobic conditions prevent total oxidation and bacterial decay. Sapropel: fine organic matter that accumulates in lakes, lagoons or marine basins where oxygen levels are low owing to poor circulation. Kerogen: altered sapropel found in oil shales
Classification of carbonaceous sedimentary rocks: (Coals, oil shales and asphaltic substances) Coals: carbonaceous sediment composed most often of the remains of spores, algae, fine plant debris and noncarbonaceous ash. Peat: Unconsolidated, semicarbonized plant remains with high moisture content. Not true coal.
Lignite: (brown coal) coal with high moisture content and commonly retain many of the structures of the original woody plant fragments. Bituminous: coals that are hard, black coals with a higher carbon content than lignite and commonly display alternating bright and dull bands.
Anthracite: hard, black, dense coal commonly containing more than 90% carbon. Anthracite is hard and shiny and breaks with conchoidal fracture. Cannel coal and boghead coal are nonbanded, dull, black coals that also break with concoidal fracture but have bituminous ranking. Bone coal is a very impure coal containing high ash content.
Coals originate in climates that promote plant growth under depositional conditions that favor preservation of organic matter…Think swamp. For thick coal deposits to form, these conditions must last for a geologically long period of time. Compaction and loss of volatiles (ash) accompany deep burial of plant debris. Thirty meters of original peat may produce only one meter of coal. Carbon content increases with burial depth because of the increase in temperature with depth. Anthracite formation requires temperatures in excess of 200°C.
Oil Shale: fine grained sedimentary rocks from which substantial quantities of oil can be derived by heating. The principal nonkerogen constituents are calcite, dolomite, ankerite, siderite and various amounts of siliciclastic silt for carbonate-rich oil shales; and fine-grained quartz, feldspar, clay and/or chert for silica-rich oil shale. Cannel shale is an oil shale that consists predominately of organic matter that completely encloses other mineral grains. The amount of oil that can be extracted from oil shales through heating and retorting ranges between 10 and 150 gallons of oil per ton of rock. Oil shales form in environments where organic matter is abundant and anaerobic conditions exist. Oil shales are deposited in lacustrine and marine environments.
Petroleum: is NOT a sedimentary rock but a carbon-rich, organic substance that accumulates predominantly in sandstones and carbonate rocks. Petroleum forms from plant and animal organic matter by a complex maturation process during burial that involves initial microbial alteration and subsequent thermal alteration that forms a complex organic substance called kerogen. Kerogen subsequently undergoes additional thermal degradation (cracking) at burial depths exceeding 1000m and temperatures of about 50° - 120°C to form liquid petroleum. Liquid petroleum may subsequently be cracked at temperatures ranging from about 150° to 200°C to form natural gas.
