Download Chapter 7: Other Chemical/ Biochemical and Carbonaceous

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
Evaporite formation and diagenesis.
Evaporation Sequence
Volume of water remaining Evaporite Precipitated
50%
20%
Minor quantities of
carbonate minerals form
Gypsum precipitates
10%
Halite precipitates
5%
Mg & K salts precipitate
**Precipitation of gypsum increases Mg/Ca favoring
dolomitization.**
Evaporation > Precipitation + isolation from open ocean = Brine
Evaporative Drawdown:
Evaporation
Periodic overflow
Complete basin evaporation
Brine level far below sea level
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-bearing sedimentary rocks
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.
Origin of Iron Deposits
Sedimentary Phosphorites
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 finegrained 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.
Principle constituents of oil shales.
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.
Petroleum Traps