Evaporites Test Flashcards
Evaporites Test
Introduction?
Today’s highlights: Introduction & early work Mineralogy Depositional environments & evaporite sequences Marine Sabkha (coastal supra- & intertidal) Shallow (lake, lagoon, shelf, & basin) to deep Non-marine Sabkha (continental) & saline lakes Soil Caves Salt domes (diapirs) Evaporites Spotted Lake, Canada; [Citation] Sounds Intro Evaporites are chemical sediments precipitated out of solutions by evaporative concentration of dissolved salts Economic importance Essential component of many oilfields (cap rock or structural traps) Useful in paleoclimate studies Restricted to arid areas Less evaporites are precipitated at present compare to other geological intervals Tucker, 2001 Major evaporite deposits with their age modern env. Evaporites are precipitated where temperature are very high, relative humidity is low and evaporation far exceeds precipitation Seawater contains ~35 g/L dissolved ions River water contains solutes in far smaller concentrations [Link] Brooks/Cole, 2005 Early Work There is a predictable order of precipitation from the least soluble (calcite) to the most soluble (K+Mg salts) compounds in the seawater: Seawater evaporation experiment: Usiglio (1849) K and Mg chlorides/sulfates Halite: NaCl Gypsum: CaSO4· 2H2O; anhydrite: CaSO4 Calcite/aragonite: CaCO3 The least soluble compounds precipitate out first Lake water evaporative concentration studies: Hardie & Eugster (1970) As evaporative concentration proceeds the ions with the higher concentration (at the beginning of precipitation) will progressively decrease in concentration while that of the other ions will progressively increase This brine concentration bas been dubbed a “chemical divide”. Hardie & Lowenstein, 2003 More than 100 evaporite minerals (marine & non-marine) Most consist of highly soluble ions (Na, Cl) Main evaporite minerals = gypsum, anhydrite, halite; Subordinately: Mg & K salts, natron, and borax Some evaporite rocks are mono-mineral (gypsum, halite, etc.) Mineralogy Marine evaporites Non marine evaporites halite NaCl halite, gypsum, anhydrite sylvite KCl epsomite MgSO4· 7H2O carnallite KMgCl3· 6H2O trona Na3(HCO3)(CO)3· 2H2O kainite KMgClSO4· 3H2O mirabilite Na2SO4·10H2O anhydrite CaSO4 thenardite Na2SO4 gypsum CaSO4· 2H2O blödite Na2Mg(SO4)2· 4H2O polyhalite K2MgCa2(SO4)4·2H2O gaylussite Na2Ca(CO3)2·5H2O kieserite MgSO4· H2O glauberite Na2Ca(SO4)2 Halite: NaCl - cubic crystals (sometimes hooper-type), granular, or massive colorless or white, sometimes exotically colored Main mineral of modern salt lakes and saline pans Marine and non-marine evaporite deposits Different colors depending on various lattice disorder or presence of impurities e.g., K+ gives a blue color Isotropic in thin section (as it is cubic), shows good cleavage, and often fluid inclusions; Hooper crystals (crater like): appear when a crystal grows faster at the edges of each face than at the center Happens when the growth is fast or/and not enough material to fill in the inner part Halite Cl- Na+ hooper pink halite crystals [Citation] Fournier blue halite [Citation] C Warren, 2006 In evaporites, precipitation of halite leads to the developing of: Chevron fabric/texture (A) Rafts (B) opper crystals (C) TP structures (D) B sedstrat.voices.wooster.edu D Holdoway, 1978 microscope view Gypsum Gypsum: CaSO4· 2H2O - monoclinic; prismatic crystals or aggregates when accumulates in massive beds;
Flexible but inelastic (meaning it ultimately breaks)?
Marine and non-marine evaporite One of the most common mineral in sedimentary environments [Citation] a alternating gypsum layers [Citation] Spruzeniece gypsum crystal (cleavage visible) gypsum quarry (CO); [Citation] Commons Gypsum (a) varieties carry different names (used especially in the mineral trade) Selenite (b): named after Greek for moon; is transparent and colorless, shows pearl luster, and occurs mostly as fibrous aggregates; satin spar (c): is translucent, silky, and forms compact aggregates composed of fibrous crystals It gives a platy of light up and down the fibrous crystals Alabaster (d): is a massive, fine grained gypsum, extensively used for inside sculptures selenite alabaster [Citation] satin spar a b c d e Precipitates as: Desert roses: rosette shaped gypsum; Bottom-growth: prismatic and twinned selenitic gypsum. 1 cm e Shearman, 1978 Anhydrite: CaSO4 - orthorhombic, fibrous, massive, or nodular Colorless to pale blue or violet if transparent, white or brown if impurities are present within crystal lattice Marine and non-marine evaporite Forms in restricted sub-aqueous and deep burial environments by dehydration of gypsum at >22ºC mean annual temperature; Loss of water in gypsum formation of anhydrite with a reduction in volume Generates fractures and cavern as the rock shrinks Exposed to water conversion to gypsum, which creates a volume increase that destroy original Anhydrite anhydrite, Naica [Citation] Railsback Ca [Citation] CaSO4· 2H2O CaSO4 The nodular texture of anhydrite is refered to as: enterolithic: beds of anhydrite irregularly contorted and buckled chicken-wire: closely packed nodules within the sediment formed by displacive precipitation of anhydrite; palmate (palm frond shape): pseudomorphs of anhydrite after selenite (during diagenesis) selenitic gypsum anhydrite www.columbia.com [ 0.5 m enterolithic texture; West, 2005 0.5 m [ chicken-wire texture; West, 2005 0.5 m [ increase of depth + precipitation - temperature increases Tucker, 2001 Gypsum vs Anhydrite The texture of gypsum-anhydrite vary considerably, depending on their precipitation environment and diagenetic history Cross polarized microscope view Anhydrite Gypsum Tucker, 2001
Gypsum: weak?
birefringence and low relief;
Anhydrite: higher relief?
and moderate birefringence Trona Trona: Na3(HCO3)(CO3)· 2H2O - monoclinic, forming well- developed columnar and fibrous crystals or massive aggregates Non-marine evaporite Deposited in the latest stage of a salt lake evolution Mined as the primary source of sodium carbonate Uses: manufacturing of glass, paper, detergents, and food additive; fertilizer (potash) Associated with halite [Citation] Klimitz Searles Lake, CA Death Valley, CA; trona pinnacles Borax: hydrous sodium borate Na2B4O5(OH)4· 8H2O - monoclinic, forming well-developed prismatic crystals or earthy aggregates Non-marine evaporite Uses: detergent, cosmetics, enamel, many others Borax [Citation] Hupperichs Kirka borax mine, Turkey (OVERVIEW) Evaporite Deposition Evaporite deposits are commonly cyclic Few modern sites of evaporite precipitation, none of them scale to what existed in the past 5 different environments (see figure at right) in which deposition of evaporites take place in: Marine: deep (shabka/lake/lagoon; 1-2-3) Shallow water (intracratonic/rift basin; 4) Non-marine: takes place within sediment (sabkha; 2) or in very shallow to desiccated saline pans (5) Has taken place on the floor of deep-marine basins following evaporative drawdown and extreme desiccation EX: Mediterranean basin Depositional Environments Tucker, 2001 1 2 3 4 5 Sabkha • Sabkha is the Arabic word for salt flat • Extensive, barren (not necessary), salt-encrusted, and periodically flooded, coastal and inland mudflats in arid regions • Broad coastal intertidal and supra tidal flats • Precipitation of evaporites happens within the sediment from pore waters derived from seawater/groundwater recharge and seawater flood recharge Abu Dhabi coast [Citation] Al Bahra Tunisia, Landsat 7 view Gypsum is the most common precipitate in sabkhas Crystal forms are: Desert roses Selenite Twinned crystals All 3 forms from <1 mm to >25 cm When gypsum crystal are replaced by anhydrite (high evaporation) then: chicken-wire & enterolithic textures form sabkha lagoonal lagoonal evaporite sequence shallow marine-playa-shallow marine Sabkha Cyclicity Cyclicity is common in sabkha evaporite sequences; it is controlled by sea-level changes; Typical facies are nodular (chicken-wire) and enterolithic anhydrite Key features to identify = shallow water and intertidal sedimentary structures contained within associated carbonates Sabkha cycles are developed along the shorelines of carbonate ramps and rimmed shelves Modern sabkha environments are known from: Trucial Coast of the Arabian Gulf Texas Southern Mediterranean coast of Tunisia and Egypt Ancient sabkha of Permian age have been recognized in the Delaware Basin of Texas and New Mexico Prothero & Schwab, 2014 Marine Shallow Evaporites • Shallow marine includes depositional environments like lakes, lagoons, shelfs, and large basins • Different depositional settings different evaporite minerals, sequences, and textures •Most ancient and modern (Marion Lake, Australia) shallow evaporites are dominated by bottom-growth gypsum deposits [Citation] Tibbetts • Largest ancient evaporite deposits of the world appear to be barred, shallow basins origin: • Zechstein (N. Europe) - Permian • Delaware Basin (W Texas) – Permian • Elk Point Basin (W Canada) – Silurian • No modern equivalent for this type of evaporite deposits www.zechsteinmagnesium.com halite halite halite Zechstein evaporites www.beg.utexas.edu anhydrite claystone claystone limestone claystone anhydrite salt/claystone claystone/salt dolomite limestone anhydrite salt anhydrite salt/claystone 475 m van Gostel, 2008 1000 m USDE-WIPP, 2009 • to produce a thick, regionally extensive evaporite sequence, a basin must periodically be isolated from the open ocean; • lateral persistence of the laminae signifies uniform conditions, shallow depth, no waves, paleoclimate significance; • all basin deposits are very well laminated gypsum - anhydrite (± calcite, halite, and/or organic matter) deposits that generate repeated vertical cycles • evaporite lamina are planar or contorted/buckled Castile Fm, NM [ 10 cm Delaware Basin (Permian) Kirkland et al., 2000 Tucker, 2001 halite gypsum carbonates Delaware Basin Castile Fm (NM) Kirkland et al., 2000 Marine Deep Evaporites • The ”deeper” version of the barred basins • Halite and gypsum deposits can reach >2 km in thickness • Ancient deposits lack regionally extensive laminations, • Do contain transported size-graded clasts of gypsum and anhydrite + broken masses of slumped evaporite • It appears most deep deposits were filled with shallow water • Large sill would allow repeated inflow of seawater down into the basin where it would then evaporate • Several evaporite cycles are recognized in saline giants deposits • Deposition was controlled by sea-level changes Prothero & Schwab, 2014 Deep Shallow anhydrite carbonate ± organic laminae Sediments with normal marine fauna halite/anhydrite laminae layered halite massive potash salts subaerial adapted from Tucker, 2001 Increasing salinity playa shallowing basin anoxic basin open marine basin • Late Miocene evaporites from the Mediterranean Basin were erroniouslly interpreted as deep water/subaerial in origin • New evidence shows Mediterranean was subjected to substantial drawdown causing salts to precipitate in sabkha, saline pans, and saline lakes • Formed during the Messinian Salinity Crisis Gibraltar Non-Marine: Sabkha • Continental sabkha is the part of a playa mudflat complex • Brine seepage ascends through capillary action • it sits within a sedimentary transition from the basin margin to the lowest parts of the brine-saturated basin floor (salt pan) • Modern continental sabkha: • Bristol Dry Lake and Death Valley (CA) • Salars (salt playas) developed in intra-montane depressions and deserts of South America Kendall, 1992 ephemeral/perennial lake (salt pan) Bristol Dry Lake (CA); [Citation] Death Valley, CA courtesy NASA [Citation] de Waele • Atacama Desert is the driest place on Earth (15 mm/year = 0.59 in/yr) • Persisted over the last 3 Ma Non-Marine: Saline Lakes • Non-marine evaporites accumulate in closed basins (lake or sea) with interior drainage but no external outlets • Interior drainage is RARE! • Develops in arid and semi-arid regions • Evaporite precipitation resemble a bull’s eye pattern • Carbonates and sulfates at the bottom and around the rim of the ancient lake • Halite along with other K+Mgsalts near the lake center/top of the sequence Tucker, 2001 saline lake in the Middle East [Citation] Suthren GRF in the I-80 road cut Buchheim and Eugster, 1998 • Ancient non-marine evaporites are not particularly common in the stratigraphic record, some though are truly spectacular: • The Green River Formation (GRF) (Eocene, WY) has LARGE beds of trona and halite exceed 50 m in thickness and covers several km2 • Lacustrine sequence alternated with more traditional deposits + anoxic bottoms pristine fossils • Modern ephemeral and perennial saline lakes/seas occur in many desert areas: • Great Salt Lake (UT), Mono Lake (CA), and Dead Sea (Israel) • Spectacular gypsum deposits are found in many caves around the world: • Lechuguilla, Carlsbad Caverns (NM), Naica (Mexico), Frasassi (Italy), Mammoth (KY), Pulpi (Spain) • Variety of morphologies: • Unspectacular rafts, flowers, and crystals (mm to meters) from vapor phase precipitation Non-Marine: Caves gypsum rafts Park’s Range Cave, NM Lechuguilla Cave, NM Mammoth Cave, KY Giant gypsum crystals in caves from Naica Mine Mexico) • Haliteis ductile • Flows when tectonically stressed during burial • Massive halite beds can undergo plastic deformationsalt domes(aka diapirs) • Salt domes can influence surface topography and litho-stratigraphy • Process referred to as salt tectonics • Truncated layers punctured by the rising salt form perfect oil traps • Buoyant oil trapped beneath impermeable salt Salt Domes (Diapirs) britanica.com salt domes [Please refer to the diagram or figure in the lecture material.]
