Editing Shale (section)

== Formation ==
The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, shales are typically deposited in very slow moving water and are often found in lakes and [[lagoon]]al deposits, in [[river delta]]s, on [[floodplain]]s and offshore below the [[wave base]].{{sfn|Blatt|Tracy|1996|p=219}} Thick deposits of shale are found near ancient [[Continental margin|continental margins]]{{sfn|Blatt|Tracy|1996|p=219}} and [[Foreland basin|foreland basins]].<ref>{{cite book |last1=Fillmore |first1=Robert |title=Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs |date=2010 |publisher=University of Utah Press |location=Salt Lake City |isbn=9781607810049 |page=222-223, 236-241}}</ref> Some of the most widespread shale formations were deposited by [[epicontinental sea]]s. Black shales<ref name=":0" /> are common in [[Cretaceous]] strata on the margins of the [[Atlantic Ocean]], where they were deposited in [[fault (geology)|fault]]-bounded silled basins associated with the opening of the Atlantic during the breakup of [[Pangaea]]. These basins were anoxic, in part because of restricted circulation in the narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today.{{sfn|Blatt|Tracy|1996|pp=287-292}}

Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow.{{sfn|Potter|Maynard|Pryor|1980|p=8}} [[Flocculation]] is very rapid once the clay encounters highly saline sea water.<ref>{{cite journal |last1=McCave |first1=I.N. |title=Vertical flux of particles in the ocean |journal=Deep Sea Research and Oceanographic Abstracts |date=1975 |volume=22 |issue=7 |pages=491–502 |doi=10.1016/0011-7471(75)90022-4|bibcode=1975DSRA...22..491M }}</ref> Whereas individual clay particles are less than 4 microns in size, the clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called [[Syneresis (chemistry)|syneresis]]).{{sfn|Potter|Maynard|Pryor|1980|p=9}} Clay pelletization by organisms that [[Filter feeder|filter feed]] is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along the [[Gulf Coast of the United States|U.S. Gulf Coast]].{{sfn|Potter|Maynard|Pryor|1980|p=10}}

As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo [[diagenesis]]. This mostly consists of [[compaction (geology)|compaction]] and [[lithification]] of the clay and silt particles.{{sfn|Blatt|Tracy|1996|pp=265-280}}{{sfn|Boggs|2006|pp=147-154}} Early stages of diagenesis, described as ''eogenesis'', take place at shallow depths (a few tens of meters) and are characterized by [[bioturbation]] and mineralogical changes in the sediments, with only slight compaction.<ref name="choquette-pray-1970">{{cite journal |last1=Choquette |first1=P.W. |last2=Pray |first2=L.C. |title=Geologic Nomenclature and Classification of Porosity in Sedimentary Carbonates |journal=AAPG Bulletin |date=1970 |volume=54 |doi=10.1306/5D25C98B-16C1-11D7-8645000102C1865D}}</ref> [[Pyrite]] may be formed in anoxic mud at this stage of diagenesis.<ref name=":0" />{{sfn|Boggs|2006|p=148}} 

Deeper burial is accompanied by ''mesogenesis'', during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as [[clay mineral]] grains) are deformed, and [[pore space]] is reduced.<ref>{{Cite journal |last1=Richardson |first1=Ethan J. |last2=Montenari |first2=Michael |date=2020 |title=Assessing shale gas reservoir potential using multi-scaled SEM pore network characterizations and quantifications: The Ciñera-Matallana pull-apart basin, NW Spain |url=https://www.sciencedirect.com/science/article/abs/pii/S2468517820300010 |journal=Stratigraphy & Timescales |volume=5 |pages=677–755 |doi=10.1016/bs.sats.2020.07.001 |isbn=9780128209912 |s2cid=229217907 |via=Elsevier Science Direct}}</ref> In addition to this physical compaction, chemical compaction may take place via [[pressure solution]]. Points of contact between grains are under the greatest strain, and the strained mineral is more [[soluble]] than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact.{{sfn|Boggs|2006|pp=147-154}} 

It is during compaction that shale develops its fissility, likely through mechanical compaction of the original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric.<ref>{{cite journal |last1=Lash |first1=G. G. |last2=Blood |first2=D. R. |title=Origin of Shale Fabric by Mechanical Compaction of Flocculated Clay: Evidence from the Upper Devonian Rhinestreet Shale, Western New York, U.S.A. |journal=Journal of Sedimentary Research |date=1 January 2004 |volume=74 |issue=1 |pages=110–116 |doi=10.1306/060103740110|bibcode=2004JSedR..74..110L }}</ref> Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations.<ref>{{cite journal |last1=Sintubin |first1=Manuel |title=Clay fabrics in relation to the burial history of shales |journal=Sedimentology |date=1994 |volume=41 |issue=6 |pages=1161–1169 |doi=10.1111/j.1365-3091.1994.tb01447.x|bibcode=1994Sedim..41.1161S }}</ref> Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility (''paper shales'') due to binding of [[hydrocarbon]] molecules to the faces of the clay particles, which weakens the binding between particles.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |pages=398–400}}</ref>

Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of [[cement]] that binds the grains together. Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example, [[smectite]] is altered to [[illite]] at temperatures of about {{convert|55 to 200|C||sigfig=2}}, releasing water in the process.<ref name=":0" /> Other alteration reactions include the alteration of smectite to [[chlorite]] and of [[kaolinite]] to illite at temperatures between {{convert|120 and 150|C||sigfig=2}}.<ref name=":0" /> Because of these reactions, illite composes 80% of [[Precambrian]] shales, versus about 25% of young shales.{{sfn|Boggs|2006|pp=142, 145-154}}

Unroofing of buried shale is accompanied by ''telogenesis'', the third and final stage of diagenesis.<ref name="choquette-pray-1970"/> As erosion reduces the depth of burial, renewed exposure to [[meteoric water]] produces additional changes to the shale, such as dissolution of some of the cement to produce [[secondary porosity]]. Pyrite may be oxidized to produce [[gypsum]].{{sfn|Boggs|2006|pp=147-154}}

{{anchor|Black shale}}
'''''Black shales''''' are dark, as a result of being especially rich in [[oxidation|unoxidized]] [[carbon]]. Common in some Paleozoic and [[Mesozoic]] [[stratum|strata]], black shales were deposited in [[Hypoxia (environmental)|anoxic]], reducing environments, such as in stagnant water columns.<ref name=":0" /> Some black shales contain abundant heavy metals such as [[molybdenum]], [[uranium]], [[vanadium]], and [[zinc]].<ref name=":0" /><ref>R. Zangerl and E. S. Richardson (1963) ''The paleoecologic history of two Pennsylvanian shales'', Fieldiana Memoirs v. 4, Field Museum of Natural History, Chicago</ref><ref name="E.B. Tourtelot 1970, pp. 253-273">{{cite journal|author=J.D. Vine and E.B. Tourtelot|year= 1970|title= Geochemistry of black shale deposits – A summary report|journal= Economic Geology |volume=65|pages=253–273|doi=10.2113/gsecongeo.65.3.253|issue=3}}</ref><ref>{{cite journal|author=R.M. Coveney|year= 1979|title= Zinc concentrations in mid-continent Pennsylvanian black shales of Missouri and Kansas|journal= Economic Geology |volume=74 |pages=131–140|doi=10.2113/gsecongeo.74.1.131}}</ref> The enriched values are of controversial origin, having been alternatively attributed to input from [[hydrothermal]] fluids during or after sedimentation or to slow accumulation from [[sea water]] over long periods of sedimentation.<ref name="E.B. Tourtelot 1970, pp. 253-273"/><ref>R.M. Coveney (2003) "Metalliferous Paleozoic black shales and associated strata" in D.R. Lenz (ed.) ''Geochemistry of Sediments and Sedimentary Rocks'', Geotext 4, Geological Association of Canada pp. 135–144</ref><ref>{{cite journal|author=H.D. Holland|year= 1979|title= Metals in black shales – A reassessment|journal= Economic Geology |volume=70 |pages=1676–1680|doi=10.2113/gsecongeo.74.7.1676|issue=7}}</ref>
<gallery>
File:Shale in Potokgraben.jpg|Shale in Potokgraben, the [[Karawanks]], [[Austria]]
File:MesselShaleSplitting.JPG|Splitting shale ([[Messel Formation|Messel oil shale]]) with a large knife to reveal fossils
File:Shale 8040.jpg|[[Weathering]] shale at a road cut in southeastern [[Kentucky]]
</gallery>
[[Fossil]]s, animal [[Fossil track|tracks]] or [[Burrow fossil|burrows]] and even [[raindrop impressions]] are sometimes preserved on shale bedding surfaces. Shales may also contain [[concretion]]s consisting of pyrite, [[apatite]], or various carbonate minerals.{{sfn|Potter|Maynard|Pryor|1980|pp=22-23}}

Shales that are subject to heat and pressure of [[metamorphism]] alter into a hard, fissile, [[metamorphic rock]] known as [[slate]]. With continued increase in [[metamorphic grade]] the sequence is [[phyllite]], then [[schist]] and finally [[gneiss]].{{sfn|Potter|Maynard|Pryor|1980|p=14}}