Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Monday, 20 February 2017

Download Geoscience Books



Geoscience Books:


We are grateful to Qazi Sohail Imran for providing Geoscience books to our community. Qazi is from Islamabad Pakistan and is a Former Research Geophysicist at King Fahd University of Petroleum and Minerals. He is contributing to his oil and gas community with the believe that "Knowledge is power and knowledge shared is power multiplied".Follow the link above the images to download the books. However if the link is not working or you have any other query, just mention it in comment or email us here , we will fix it for you.


1. 
Name: Sedimentology and Stratigraphy by Gary Nichols 
Download Here



.

2. Name: Physical Geology- Earth Revealed. 9th Edition by C.C. Plummer   
     Size: 100MBs
   
     Pages: 670
     Download link: https://goo.gl/inHgv3




3. Name: An Introduction to Structural Geology and Tectonics by Stephen Marshak
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4. Name: 3-D Structural Geology by R.H.Groshong
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5. Name: Structural Geology of Rocks & Regions 3rd Edition - Davis Reynolds
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6. Name: Geological Field Techniques Edited By Angela L. Coe
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7. Name: Seismic Stratigraphy, Basin Analysis and Reservoir Characterisation_Vol37       by Paul C.H. Veeken
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8. Name: General Dictionary of Geology by Alva Kurniawan, John Mc. Kenzie,                  Jasmine Anita Putri
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9. 
Name: The Handy Geology Answer Book
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10.   Name: Sedimentary Basin Formation-Presentation
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11. Name: Facies Models Response to Sea Level Change by Walker and James
       Dowload link: https://goo.gl/SSwsJy



12. Name: AAPG Memoir 33 - Carbonate Depositional Environments
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13. Name: Petroleum Formation and Occurrence by Tissot, B.P. and Welte, D. H        Download link: https://goo.gl/JBxm86

14. Name: Basin Analysis-Principles and Applications by Allen     
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15. Name: Sedimentary Rocks in the Field by Tucker
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16. Name: Exploration Stratigraphy 2nd Edition - Visher
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17. Name: Petroleum Geology of Pakistan by Iqbal B. Kadri
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18. Name: The Geological Interpretation Of Well Logs by Rider
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19. Name: Digital Signal Processing Handbook by Vijay K. Madisetti
      Download link: https://goo.gl/IQZuOm





20. Name: AAPG Memoir 88 - Giant Hydrocarbon Reservoirs of the World
                           Download link: https://goo.gl/KM0opJ
             

21. Name: Deep-Water Processes and Facies Models-Implications for Sandstone               Reservoirs by  Dr. Shanmugam
      Download link: https://goo.gl/a05VyQ

               
      




Friday, 17 February 2017

Earthquake Precursors: Signs Before Earthquakes



Earthquake prediction is the ultimate goal of seismologists. Being able to predict when and where an earthquake will occur could save thousands, if not hundreds of thousands, of lives, over the years. Even after decades of study, earthquake forecasting remains notoriously difficult, however. So what are the signs which occur b
efore 
an earthquake – earthquake precursors – and how useful are they?




About the author (who writes this article): Nusrat Kamal Siddiqui is one of the leading Geoscientists from Pakistan. He has a diverse professional career of being a Petroleum Geologist, Hydrologist and Engineering Geologist, both in Pakistan and overseas. He recently published a book " Petroleum Geology, Basin Architecture and Stratigraphy of Pakistan". Click here for further details about the book.


The Precursors

There are some long-term, medium-term and short-term precursors of seismic activity that cause earthquakes.

The long-term precursors are based on statistical studies and the prediction is probabilistic. The medium-term precursors help in predicting the location of an earthquake to a sufficient degree of accuracy. The short-term precursors of seismic events are indicated by changes in geomagnetic field, changes in gravity field, rising of subsurface temperature and rise in ground radioactivity. Agriculture institutions record subsurface temperature at 20, 50 and 100 cm depth as it is useful for monitoring crop growth. In earthquake-prone areas the temperature starts rising about 700-900 days before the event. This readily available data can be of help.

The short-term precursors are more important as they can be observed by a common man, and happen from a few days before the earthquake to just before it happens. With a reducing lag time these are: rise in water in the wells with increased sediments, sudden increase and decrease in river water flow, disturbance in the reception of radio, television, telephones, water fountains on the high grounds, strange behavior of animals, a sudden jump in the number of deliveries in hospitals and malfunctioning of cell phones. These days cell phones are the most handy and common piece of electronic equipment. A general collapse of this system can be noted by masses, and hence could be a very effective means to take timely mitigation measure. It has been found that about 100 to 150 minutes before the earthquake the cell phones start malfunctioning. However, the humans are very careless by nature and there would be only very few who would be observant enough to note the above precursors.  



It is indeed believed that animals exhibit unusual behavior before an earthquake


In the earthquake-prone areas groups of observant and responsible people (including women - they normally haul the water) may be constituted wherein the list of precursors, in local languages, may be distributed and some training imparted. And this exercise may not 
be left to the authorities, for obvious reasons!

Source: Earthquakes are inevitable, Disasters are not– Mitigation, therefore, is better than Prediction by Nusrat K. Siddiqui



Suggested Readings:

1. A systematic compilation of earthquake precursors
2. Earthquakes: prediction, forecasting and mitigation
3. Earthquake Prediction, Control and Mitigation

Wednesday, 15 February 2017

Historical Anoxic Event and consequences

Anoxic events in Earth's history

Cretaceous Anoxic Event

Sulphidic (or euxinic) conditions, which exist today in many water bodies from ponds to various land-surrounded mediterranean seas such as the Black Sea, were particularly prevalent in the Cretaceous Atlantic but also characterised other parts of the world ocean. In an ice-free sea of these supposed super-greenhouse worlds, oceanic waters were as much as 200 meters higher, in some eras. During the time spans in question, the continental plates are believed to have been well separated, and the mountains we know today were (mostly) future tectonic events meaning the overall landscapes were generally much lower and even the half super-greenhouse climates would have been eras of highly expedited water erosion carrying massive amounts of nutrients into the world oceans fuelling an overall explosive population of microorganisms and their predator species in the oxygenated upper layers.
Detailed stratigraphic studies of Cretaceous black shales from many parts of the world have indicated that two oceanic anoxic events (OAEs) were particularly significant in terms of their impact on the chemistry of the oceans, one in the early Aptian (~120 Ma), sometimes called the Selli Event (or OAE 1a)  after the Italian geologist, Raimondo Selli (1916–1983), and another at the Cenomanian-Turonian boundary (~93 Ma), sometimes called the Bonarelli Event (or OAE 2) after the Italian geologist, Guido Bonarelli (1871–1951). OAE1a lasted for ~1.0 to 1.3 Myr. The duration of OAE2 is estimated to be ~820 kyr based on a high-resolution study of the significantly expanded OAE2 interval in southern Tibet, China.
  • Insofar as the Cretaceous OAEs can be represented by type localities, it is the striking outcrops of laminated black shale within the various coloured clay-stones and pink and white limestone near the town of Gubbio in the Italian Apennines that are the best candidates.
  • The 1-meter thick black shale at the Cenomanian-Turonian boundary that crops out near Gubbio is termed the ‘Livello Bonarelli’ after the man who first described it in 1891.
More minor oceanic anoxic events have been proposed for other intervals in the Cretaceous (in the Valanginian, Hauterivian, Albian and Coniacian–Santonian stages), but their sedimentary record, as represented by organic-rich black shales, appears more parochial, being dominantly represented in the Atlantic and neighbouring areas, and some researchers relate them to particular local conditions rather than being forced by global change.

Jurassic

The only oceanic anoxic event documented from the Jurassic took place during the early Toarcian (~183 Ma). Because no DSDP (Deep Sea Drilling Project) or ODP (Ocean Drilling Program) cores have recovered black shales of this age there being little or no Toarcian ocean crust remaining in the world ocean the samples of black shale primarily come from outcrops on land. These outcrops, together with material from some commercial oil wells, are found on all major continents and this event seems similar in kind to the two major Cretaceous examples.

Paleozoic

The boundary between the Ordovician and Silurian periods is marked by repetitive periods of anoxia, interspersed with normal, oxic conditions. In addition, anoxic periods are found during the Silurian. These anoxic periods occurred at a time of low global temperatures (although CO2 levels were high), in the midst of a glaciation.
Jeppsson (1990) proposes a mechanism whereby the temperature of polar waters determines the site of formation of downwelling water. If the high latitude waters are below 5 °C (41 °F), they will be dense enough to sink; as they are cool, oxygen is highly soluble in their waters, and the deep ocean will be oxygenated. If high latitude waters are warmer than 5 °C (41 °F), their density is too low for them to sink below the cooler deep waters. Therefore, thermohaline circulation can only be driven by salt-increased density, which tends to form in warm waters where evaporation is high. This warm water can dissolve less oxygen, and is produced in smaller quantities, producing a sluggish circulation with little deep water oxygen. The effect of this warm water propagates through the ocean, and reduces the amount of CO2 that the oceans can hold in solution, which makes the oceans release large quantities of CO2 into the atmosphere in a geologically short time (tens or thousands of years). The warm waters also initiate the release of clathrates, which further increases atmospheric temperature and basin anoxia. Similar positive feedback operate during cold-pole episodes, amplifying their cooling effects.
The periods with cold poles are termed "P-episodes" (short for primo), and are characterised by bioturbated deep oceans, a humid equator and higher weathering rates, and terminated by extinction events for example, the Ireviken and Lau events. The inverse is true for the warmer, oxic "S-episodes" (secundo), where deep ocean sediments are typically graptolitic black shales. A typical cycle of secundo-primo episodes and ensuing event typically lasts around 3 Ma.
The duration of events is so long compared to their onset because the positive feedback must be overwhelmed. Carbon content in the ocean-atmosphere system is affected by changes in weathering rates, which in turn is dominantly controlled by rainfall. Because this is inversely related to temperature in Silurian times, carbon is gradually drawn down during warm (high CO2) S-episodes, while the reverse is true during P-episodes. On top of this gradual trend is overprinted the signal of Milankovic cycles, which ultimately trigger the switch between P- and S- episodes.
These events become longer during the Devonian; the enlarging land plant biota probably acted as a large buffer to carbon dioxide concentrations.
The end-Ordovician Hirnantian event may alternatively be a result of algal blooms, caused by sudden supply of nutrients through wind-driven upwelling or an influx of nutrient-rich meltwater from melting glaciers, which by virtue of its fresh nature would also slow down oceanic circulation.

Archean and Proterozoic

Throughout most of Earth's history, it was thought that oceans were largely oxygen-deficient. During the Archean, euxinia was largely absent because of low availability of sulphate in the oceans, but during the Proterozoic, it would become more common.

Consequences of Oceanic Anoxic Event

Oceanic anoxic events have had many important consequences. It is believed that they have been responsible for mass extinctions of marine organisms both in the Paleozoic and Mesozoic. The early Toarcian and Cenomanian-Turonian anoxic events correlate with the Toarcian and Cenomanian-Turonian extinction events of mostly marine life forms. Apart from possible atmospheric effects, many deeper-dwelling marine organisms could not adapt to an ocean where oxygen penetrated only the surface layers.
An economically significant consequence of oceanic anoxic events is the fact that the prevailing conditions in so many Mesozoic oceans has helped produce most of the world's petroleum and natural gas reserves. During an oceanic anoxic event, the accumulation and preservation of organic matter was much greater than normal, allowing the generation of potential petroleum source rocks in many environments across the globe. Consequently, some 70 percent of oil source rocks are Mesozoic in age, and another 15 percent date from the warm Paleogene: only rarely in colder periods were conditions favourable for the production of source rocks on anything other than a local scale.

Atmospheric effects

A model put forward by Lee Kump, Alexander Pavlov and Michael Arthur in 2005 suggests that oceanic anoxic events may have been characterised by up-welling of water rich in highly toxic hydrogen sulphide gas, which was then released into the atmosphere. This phenomenon would probably have poisoned plants and animals and caused mass extinctions. Furthermore, it has been proposed that the hydrogen sulphide rose to the upper atmosphere and attacked the ozone layer, which normally blocks the deadly ultraviolet radiation of the Sun. The increased UV radiation caused by this ozone depletion would have amplified the destruction of plant and animal life. Fossil spores from strata recording the Permian-Triassic extinction event show deformities consistent with UV radiation. This evidence, combined with fossil biomarkers of green sulphur bacteria, indicates that this process could have played a role in that mass extinction event, and possibly other extinction events. The trigger for these mass extinctions appears to be a warming of the ocean caused by a rise of carbon dioxide levels to about 1000 parts per million.

Ocean chemistry effects

Reduced oxygen levels are expected to lead to increased seawater concentrations of redox-sensitive metals. The reductive dissolution of iron-manganese oxyhydroxides in seafloor sediments under low-oxygen conditions would release those metals and associated trace metals. Sulphate reduction in such sediments could release other metals such as barium. When heavy-metal-rich anoxic deep water entered continental shelves and encountered increased O2 levels, precipitation of some of the metals, as well as poisoning of the local biota, would have occurred. In the late Silurian mid-Pridoli event, increases are seen in the Fe, Cu, As, Al, Pb, Ba, Mo and Mn levels in shallow-water sediment and microplankton; this is associated with a marked increase in the malformation rate in chitinozoans and other microplankton types, likely due to metal toxicity. Similar metal enrichment has been reported in sediments from the mid-Silurian Ireviken event.

Oceanic Anoxic Event (OAE)

What is Oceanic Anoxic Event?

Oceanic anoxic event or anoxic event (anoxic conditions) allude to interims in the Earth's past where parts of seas get to be distinctly exhausted in oxygen (O2) at profundities over a substantial geographic region. Amid some of these occasions, euxinia, waters that contained H2S hydrogen sulphide, developed. Although anoxic event have not occurred for a huge number of years, the land record demonstrates that they happened commonly previously. Anoxic event harmonised with a few mass eradications and may have added to them. These mass eliminations incorporate some that geobiologists use as time markers in biostratigraphic dating. Many geologists trust maritime anoxic event are unequivocally connected to abating of sea flow, climatic warming, and lifted levels of nursery gasses. Scientists have proposed improved volcanism (the arrival of CO2) as the "focal outer trigger for euxinia".

Backgroud of Oceanic Anoxic Event

The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger (1927–1990) and geologist Hugh Jenkyns and arose from discoveries made by the Deep Sea Drilling Project (DSDP) in the Pacific Ocean. It was the finding of black carbon-rich shales in Cretaceous sediments that had accumulated on submarine volcanic plateaus (Shatsky Rise, Manihiki Plateau), coupled with the fact that they were identical in age with similar deposits cored from the Atlantic Ocean and known from outcrops in Europe - particularly in the geological record of the otherwise limestone-dominated Apennines chain in Italy - that led to the realisation that these widespread similar strata recorded highly unusual oxygen-depleted conditions in the world ocean during several discrete periods of geological time.
Sedimentological investigations of these organic-rich sediments, which have continued to this day, typically reveal the presence of fine laminations undisturbed by bottom-dwelling fauna, indicating anoxic conditions on the sea floor, believed to be coincident with a low lying poisonous layer of hydrogen sulphide. Furthermore, detailed organic geochemical studies have recently revealed the presence of molecules (so-called biomarkers) that derive from both purple sulphur bacteria and green sulphur bacteria: organisms that required both light and free hydrogen sulphide (H2S), illustrating that anoxic conditions extended high into the illuminated upper water column.
There are currently several places on earth that are exhibiting the features of anoxic events on a localised scale such as algal/bacterial blooms and localised "dead zones". Dead zones exist off the East Coast of the United States in the Chesapeake Bay, in the Scandinavian strait Kattegat, the Black Sea (which may have been anoxic in its deepest levels for millennia, however), in the northern Adriatic as well as a dead zone off the coast of Louisiana. The current surge of jellyfish worldwide is sometimes regarded as the first stirrings of an anoxic event. Other marine dead zones have appeared in coastal waters of South America, China, Japan, and New Zealand. A 2008 study counted 405 dead zones worldwide.
This is a recent understanding. This picture was only pieced together during the last three decades. The handful of known and suspected anoxic events have been tied geologically to large-scale production of the world's oil reserves in worldwide bands of black shale in the geologic record. Likewise the high relative temperatures believed linked to so called "super-greenhouse events".

Euxinia

Oceanic anoxic events with euxinic (i.e. sulphide) conditions have been linked to extreme episodes of volcanic out-gassing. Thus, volcanism contributed to the buildup of CO2 in the atmosphere, increased global temperatures, causing an accelerated hydrological cycle that introduced nutrients to the oceans to stimulate planktonic productivity. These processes potentially acted as a trigger for euxinia in restricted basins where water-column stratification could develop. Under anoxic to euxinic conditions, oceanic phosphate is not retained in sediment and could hence be released and recycled, aiding continued high productivity.

Mechanism

Temperatures throughout the Jurassic and Cretaceous are generally thought to have been relatively warm, and consequently dissolved oxygen levels in the ocean were lower than today making anoxia easier to achieve. However, more specific conditions are required to explain the short-period (less than a million years) oceanic anoxic events. Two hypotheses, and variations upon them, have proved most durable.
One hypothesis suggests that the anomalous accumulation of organic matter relates to its enhanced preservation under restricted and poorly oxygenated conditions, which themselves were a function of the particular geometry of the ocean basin: such a hypothesis, although readily applicable to the young and relatively narrow Cretaceous Atlantic (which could be likened to a large-scale Black Sea, only poorly connected to the World Ocean), fails to explain the occurrence of coeval black shales on open-ocean Pacific plateaus and shelf seas around the world. There are suggestions, again from the Atlantic, that a shift in oceanic circulation was responsible, where warm, salty waters at low latitudes became hypersaline and sank to form an intermediate layer, at 500 to 1,000 m (1,640 to 3,281 ft) depth, with a temperature of 20 °C (68 °F) to 25 °C (77 °F).
The second hypothesis suggests that oceanic anoxic events record a major change in the fertility of the oceans that resulted in an increase in organic-walled plankton (including bacteria) at the expense of calcareous plankton such as coccoliths and foraminifera. Such an accelerated flux of organic matter would have expanded and intensified the oxygen minimum zone, further enhancing the amount of organic carbon entering the sedimentary record. Essentially this mechanism assumes a major increase in the availability of dissolved nutrients such as nitrate, phosphate and possibly iron to the phytoplankton population living in the illuminated layers of the oceans.
For such an increase to occur would have required an accelerated influx of land-derived nutrients coupled with vigorous upwelling, requiring major climate change on a global scale. Geochemical data from oxygen-isotope ratios in carbonate sediments and fossils, and magnesium/calcium ratios in fossils, indicate that all major oceanic anoxic events were associated with thermal maxima, making it likely that global weathering rates, and nutrient flux to the oceans, were increased during these intervals. Indeed, the reduced solubility of oxygen would lead to phosphate release, further nourishing the ocean and fuelling high productivity, hence a high oxygen demand - sustaining the event through a positive feedback.
Here is another way of looking at oceanic anoxic events. Assume that the earth releases a huge volume of carbon dioxide during an interval of intense volcanism; global temperatures rise due to the greenhouse effect; global weathering rates and fluvial nutrient flux increase; organic productivity in the oceans increases; organic-carbon burial in the oceans increases (OAE begins); carbon dioxide is drawn down due to both burial of organic matter and weathering of silicate rocks (inverse greenhouse effect); global temperatures fall, and the ocean–atmosphere system returns to equilibrium (OAE ends).
In this way, an oceanic anoxic event can be viewed as the Earth’s response to the injection of excess carbon dioxide into the atmosphere and hydrosphere. One test of this notion is to look at the age of large igneous provinces (LIPs), the extrusion of which would presumably have been accompanied by rapid effusion of vast quantities of volcanogenic gases such as carbon dioxide. Intriguingly, the age of three LIPs (Karoo-Ferrar flood basalt, Caribbean large igneous province, Ontong Java Plateau) correlates uncannily well with that of the major Jurassic (early Toarcian) and Cretaceous (early Aptian and Cenomanian–Turonian) oceanic anoxic events, indicating that a causal link is feasible.

Occurrence

Oceanic anoxic events most commonly occurred during periods of very warm climate characterised by high levels of carbon dioxide (CO2) and mean surface temperatures probably in excess of 25 °C (77 °F). The Quaternary levels, the current period, are just 13 °C (55 °F) in comparison. Such rises in carbon dioxide may have been in response to a great out-gassing of the highly flammable natural gas (methane) that some call an "oceanic burp". Vast quantities of methane are normally locked into the Earth's crust on the continental plateaus in one of the many deposits consisting of compounds of methane hydrate, a solid precipitated combination of methane and water much like ice. Because the methane hydrates are unstable, except at cool temperatures and high (deep) pressures, scientists have observed smaller "burps" due to tectonic events. Studies suggest the huge release of natural gas could be a major climatological trigger, methane itself being a greenhouse gas many times more powerful than carbon dioxide. However, anoxia was also rife during the Hirnantian (late Ordovician) ice age.
Oceanic anoxic events have been recognised primarily from the already warm Cretaceous and Jurassic Periods, when numerous examples have been documented, but earlier examples have been suggested to have occurred in the late Triassic, Permian, Devonian (Kellwasser event), Ordovician and Cambrian.
The Paleocene-Eocene Thermal Maximum (PETM), which was characterised by a global rise in temperature and deposition of organic-rich shales in some shelf seas, shows many similarities to oceanic anoxic events.
Typically, oceanic anoxic events lasted for less than a million years, before a full recovery.

Friday, 3 February 2017

The Messinian Salinity Crisis


You will have heard of The Messinian Salinity Crisis no doubt. From learned articles, geology textbooks, probably lectures at your college or University. Or possibly not. This was not always the hot topic it is now. In fact, the very idea of this happening, was for a while, challenged, even ridiculed. It seemed too incredible that this could happen as it did and Dessication/Flood theories took time to gain traction. But, if you had heard about it, you would remember that The Messinian Salinity Crisis, was a time when the Mediterranean Sea, very much as we know it today, evaporated – dried out, almost completely.



You will have heard of the rates of desiccation, influx and yet more desiccation, repeated in endless cycles over tens, even hundreds of thousands of years. On a human temporal scale, this would have been a long drawn out affair, covering a time hundreds of generations deep, more than the span of Homo sapiens existence. In Geologic terms however, it was a string of sudden events. Of incredibly hot and arid periods followed by rapid ingress of waters, either via spillways through what is now modern day Morocco and the southern Iberian peninsular, or headlong through a breach in the sill between the Pillars of Heracles, the modern day Straights of Gibraltar.

There were prolonged periods of dessication, of desolate landscapes beyond anything seen today in Death Valley or The Afar Triangle. These landscapes were repeatedly transgressed by brackish waters from storm seasons far into the African and Eurasian interiors, or the Atlantic, and these in turn dried out. Again and again this happened. It had to be so because the vast deposits of rock salt, gypsum and anhydrites could not have been emplaced in a single evaporite event. The salt deposits in and around the Mediteranean today represent fifty times the current capacity of this great inland sea. You may have heard too of the variety of salts production, as agglomerating crystals fell from the descending surface to the sea floor, or as vast interconnected hypersaline lakes left crystalline residues at their diminishing margins, as forsaken remnant sabkhas, cut off from the larger basins, left behind acrid dry muds of potassium carbonates – the final arid mineral residue of the vanished waters.

Just under six million years ago, Geologic processes isolated what was left of the ancient Tethys ocean, the sea we know as the Mediterranean, home to historic human conflicts and marine crusades of Carthage, Rome, Athens and Alexandria, a Sea fringed by modern day Benidorm, Cyprus, Malta and Monaco. At a time 5.96 million years ago – evaporation outpaced replenishment. Indeed, just as it does today, but without the connecting seaway to replenish losses. Inexorable tectonic activity first diverted channels, then – sealed them. Cut off from the Atlantic in the West, water levels fell, rose briefly and fell again, and again. The mighty Nile - a very different geophysical feature of a greater capacity than today, and the rivers of Europe cut down great canyons hundreds and thousands of metres below present Eustatic sea and land surface levels, as seismic cross sections show in staggering detail. The cores taken at depth in the Mediterranean, show Aeolian sands above layers of salt, fossiliferous strata beneath those same salts, all indicating changing environments. The periods of blackened unshifting desert varnished floors and bleached playas, decades and centuries long, were punctuated often by catastrophic episodes, with eroded non conformable surfaces of winnowed desert pavement, toppled ventifracts, scours and rip up clasts. Species of fossilised terrestrial plant life, scraping an arid existence have been found, thousands of meters down, in the strata of the Mediterranean sea floor.

 


There is much evidence too, in the uplifted margins of Spain, France, and Sicily, of those hostile millennia when the sea disappeared. Incontrovertible evidence, painstakingly gathered, analysed and peer reviewed, demonstrates via the resources of statistical analysis, calculus and geophysical data that the Messinian Salinity Crisis was a period during the Miocene wherein the geology records a uniquely arid period of repeated partial and very nearly complete desiccation of the Mediterranean Sea over a period of approximately 630,000 years. But for the Geologist, the story doesn’t end there. The Geologists panoptic, all seeing third eye, sees incredible vistas and vast panoramas. Of a descent from the Alpine Foreland to the modern day enclave of Monaco, gazing out southwards from a barren, uninhabited and abandoned raised coast to deep dry abyssal plains, punctuated by exposed chasms, seamounts and ridges, swirling and shifting so slowly in a distant heat haze. A heat haze produced by temperatures far above any recorded by modern man and his preoccupation with Global Warming. An unimaginable heat sink would produce temperatures of 70 to 80 degrees Celsius at 4000M depth within the basins. 




Looking down upon this Venusian landscape, the sun might glint on remaining lakes and salt flats so very far away and so very much farther below. Hills and valleys, once submerged, would be observed high and dry – from above, as would the interconnecting rivers of bitter waters hot enough to slowly broil any organism larger than extremophile foraminifer. All this, constantly shimmering in the relentless heat. Only the imagination of the geologist could see the vast, hellish, yet breathtaking landscape conjured up by the data and the rock record. And finally, the Geologist would visualise a phenomenon far greater in scope and magnitude than any Biblical flood – The Zanclean Event.
Also known as The Zanclean Deluge, when the drought lasting over half a million years was finally ended as the Atlantic Ocean breached the sill/land bridge between Gibraltar and North West Africa. Slowly perhaps at first until a flow a thousand times greater than the volumetric output of the Amazon cascaded down the slopes to the parched basins. Proximal to the breach, there would be a deafening thunderous roar and the ground would tremor constantly, initially triggering great avalanches above and below the Eustatic sea level as the far reaching and continuous concussion roared and rumbled on, and on, and on. For centuries great cataracts and torrents of marine waters fell thousands of metres below and flowed thousands of kilometers across to the East. Across to the abyssal plains off the Balearics, to the deeps of the Tyrrhenian and Ionian seas, into the trenches south of the Greek Islands and finally up to the rising shores of The Lebanon. The newly proximal waters to the final coastal reaches and mountains that became islands, must have had a climatological effect around the margins of the rejuvenated Mediterranean. Flora and Fauna both marine and terrestrial will have recolonised quickly. Species may have developed differently, post Zanclean, on the Islands. And in such a short period, there must surely have been earthquakes and complex regional depression and emergence. Isostacy compensated for the trillions of cubic meters of transgression waters that now occupied the great basins between the African and Eurasian plates, moving the land, reactivating ancient faults and within and marginal to the great inland sea, a region long active with convergent movements of a very different mechanism.
Hollywood and Pinewood have yet to match the imagination of the Earth Scientist, of the many chapters of Earths dynamic history held as fully tangible concepts to the men and women who study the rocks and the stories they tell. The movies played out in the mind of the geologist are epic indeed and – as we rightly consider the spectre of Global Warming, consider too the fate of future populations (of whatever evolved species) at the margins of the Mediterranean and the domino regions beyond, when inexorable geologic processes again isolate that benign, sunny holiday sea. Fortunately, not in our lifetime, but that of our far off descendants who will look and hopefully behave very differently from Homo Sapiens.

Note: This blog is written and contributed by Paul Goodrich. You can also contribute your blog or article on our website. See guidelines here.


Sunday, 29 January 2017

Why perform strain analysis?

Why perform strain analysis?

Why do we perform strain analysis?. It can be important to retrieve information about strain from deformed rocks. First of all, strain analysis gives us an opportunity to explore the state of strain in a rock and to map out strain variations in a sample, an outcrop or a region. Strain data are important in the mapping and understanding of shear zones in orogenic belts. Strain measurements can also be used to estimate the amount of offset across a shear zone. It is possible to extract important information from shear zones if strain is known. 
In many cases it is useful to know if the strain is planar or three dimensional. If planar, an important criterion for section balancing is fulfilled, be it across orogenic zones or extensional basins. The shape of the strain ellipsoid may also contain information about how the deformation occurred. Oblate (pancake-shaped) strain in an orogenic setting may, for example, indicate flattening strain related to gravity-driven collapse rather than classic push-from-behind thrusting. 
The orientation of the strain ellipsoid is also important, particularly in relation to rock structures. In a shear zone setting, it may tell us if the deformation was simple shear or not. Strain in folded layers helps us to understand fold-forming mechanism(s). Studies of deformed reduction spots in slates give good estimates on how much shortening has occurred across the foliation in such rocks, and strain markers in sedimentary rocks can sometimes allow for reconstruction of original sedimentary thickness. 

Strain in one dimension

Two elongated belemnites in Jurassic limestone in the Swiss Alps. The different ways that the two belemnites have been stretched give us some two-dimensional information about the strain field: the upper belemnite has experienced sinistral shear strain while the lower one has not and must be close to the maximum stretching direction.
One-dimensional strain analyses are concerned with changes in length and therefore the simplest form of strain analysis we have. If we can reconstruct the original length of an object or linear structure we can also calculate the amount of stretching or shortening in that direction. Objects revealing the state of strain in a deformed rock are known as strain markers. Examples of strain markers indicating change in length are boudinaged dikes or layers, and minerals or linear fossils such as belemnites or graptolites that have been elongated, such as the stretched Swiss belemnites shown in Figure above. Or it could be a layer shortened by folding. It could even be a faulted reference horizon on a geologic or seismic profile. The horizon may be stretched by normal faults or shortened by reverse faults, and the overall strain is referred to as brittle strain. One-dimensional strain is revealed when the horizon, fossil, mineral or dike is restored to its pre-deformational state.

Strain in two dimensions

Reduction spots in Welsh slate. The light spots formed as spherical volumes of bleached (chemically reduced) rock. Their new shapes are elliptical in cross-section and oblate (pancake-shaped) in three dimensions, reflecting the tectonic strain in these slates.
In two-dimensional strain analyses we look for sections that have objects of known initial shape or contain linear markers with a variety of orientations (Figure first). Strained reduction spots of the type shown in Figure above are perfect, because they tend to have spherical shapes where they are undeformed. There are also many other types of objects that can be used, such as sections through conglomerates, breccias, corals, reduction spots, oolites, vesicles, pillow lavas (Figure below), columnar basalt, plutons and so on. Two-dimensional strain can also be calculated from one-dimensional data that represent different directions in the same section. A typical example would be dikes with different orientations that show different amounts of extension.
Section through a deformed Ordovician pahoe-hoe lava. The elliptical shapes were originally more circular, and Hans Reusch, who made the sketch in the 1880s, understood that they had been flattened during deformation. The Rf/f, center-to-center, and Fry methods would all be worth trying in this case.
Strain extracted from sections is the most common type of strain data, and sectional data can be combine to estimate the three-dimensional strain ellipsoid.

Changes in angles 

Strain can be found if we know the original angle between sets of lines. The original angular relations between structures such as dikes, foliations and bedding are sometimes found in both undeformed and deformed states, i.e. outside and inside a deformation zone. We can then see how the strain has affected the angular relationships and use this information to estimate strain. In other cases orthogonal lines of symmetry found in undeformed fossils such as trilobites, brachiopods and worm burrows (angle with layering) can be used to determine the angular shear in some deformed sedimentary rocks. In general, all we need to know is the change in angle between sets of lines and that there is no strain partitioning due to contrasting mechanical properties of the objects with respect to the enclosing rock.

If the angle was 90 degree in the undeformed state, the change in angle is the local angular shear. The two originally orthogonal lines remain orthogonal after the deformation, then they must represent the principal strains and thus the orientation of the strain ellipsoid. Observations of variously oriented line sets thus give information about the strain ellipse or ellipsoid. All we need is a useful method. Two of the most common methods used to find strain from initially orthogonal lines are known as the Wellman and Breddin methods, and are presented in the following sections.

The Wellman method 

Wellman’s method involves construction of the strain ellipse by drawing parallelograms based on the orientation of originally orthogonal pairs of lines. The deformation was produced on a computer and is a homogeneous simple shear. However, the strain ellipse itself tells us nothing about the degree of coaxiality: the same result could have been attained by pure shear.
This method dates back to 1962 and is a geometric construction for finding strain in two dimensions (in a section). It is typically demonstrated on fossils with orthogonal lines of symmetry in the undeformed state. In Figure above a we use the hinge and symmetry lines of brachiopods. A line of reference must be drawn (with arbitrary orientation) and pairs of lines that were orthogonal in the unstrained state are identified. The reference line must have two defined endpoints, named A and B in Figure above b. A pair of lines is then drawn parallel to both the hinge line and symmetry line for each fossil, so that they intersect at the endpoints of the reference line. The other points of intersection are marked (numbered 1–6 in Figure above b, c). If the rock is unstrained, the lines will define rectangles. If there is a strain involved, they will define parallelograms. To find the strain ellipse, simply fit an ellipse to the numbered corners of the parallelograms (Figure above c). If no ellipse can be fitted to the corner points of the rectangles the strain is heterogeneous or, alternatively, the measurement or assumption of initial orthogonality is false. The challenge with this method is, of course, to find enough fossils or other features with initially orthogonal lines typically 6–10 are needed.

The Breddin graph 

The data from the previous figure plotted in a Breddin graph. The data points are close to the curve for R=2.5.
We have already stated that the angular shear depends on the orientation of the principal strains: the closer the deformed orthogonal lines are to the principal strains, the lower the angular shear. This fact is utilized in a method first published by Hans Breddin in 1956 in German (with some errors). It is based on the graph shown in Figure above, where the angular shear changes with orientation and strain magnitude R. Input are the angular shears and the orientations of the sheared line pairs with respect to the principal strains. These data are plotted in the so-called Breddin graph and the R-value (ellipticity of the strain ellipse) is found by inspection (Figure above). This method may work even for only one or two observations. 
In many cases the orientation of the principal axes is unknown. In such cases the data are plotted with respect to an arbitrarily drawn reference line. The data are then moved horizontally on the graph until they fit one of the curves, and the orientations of the strain axes are then found at the intersections with the horizontal axis (Figure above). In this case a larger number of data are needed for good results.

Elliptical objects and the Rf/f-method 

Objects with initial circular (in sections) or spherical (in three dimensions) geometry are relatively uncommon, but do occur. Reduction spots and ooliths perhaps form the most perfect spherical shapes in sedimentary rocks. When deformed homogeneously, they are transformed into ellipses and ellipsoids that reflect the local finite strain. Conglomerates are perhaps more common and contain clasts that reflect the finite strain. In contrast to oolites and reduction spots, few pebbles or cobbles in a conglomerate are spherical in the undeformed state. This will of course influence their shape in the deformed state and causes a challenge in strain analyses. However, the clasts tend to have their long axes in a spectrum of orientations in the undeformed state, in which case methods such as the Rf/f-method may be able to take the initial shape factor into account.
The Rf/f method illustrated. The ellipses have the same ellipticity (Ri) before the deformation starts. The Rf–f diagram to the right indicates that Ri=2. A pure shear is then added with Rs=1.5 followed by a pure shear strain of Rs=3. The deformation matrices for these two deformations are shown. Note the change in the distribution of points in the diagrams to the right. Rs in the diagrams is the actual strain that is added. Modified from Ramsay and Huber (1983).
The Rf/f-method was first introduced by John Ramsay in his well-known 1967 textbook and was later improved. The method is illustrated in Figure above. The markers are assumed to have approximately elliptical shapes in the deformed (and undeformed) state, and they must show a significant variation in orientations for the method to work.
The Rf/f-method handles initially non-spherical markers, but the method requires a significant variation in the orientations of their long axes.
The ellipticity (X/Y) in the undeformed (initial) state is called Ri. In our example (Figure above) Ri=2. After a strain Rs the markers exhibit new shapes. The new shapes are different and depend on the initial orientation of the elliptical markers. The new (final) ellipticity for each deformation marker is called Rf and the spectrum of Rf-values is plotted against their orientations, or more specifically against the angle f' between the long axis of the ellipse and a reference line (horizontal in Figure above). In our example we have applied two increments of pure shear to a series of ellipses with different orientations. All the ellipses have the same initial shape Ri=2, and they plot along a vertical line in the upper right diagram in Figure above. Ellipse 1 is oriented with its long axis along the minimum principal strain axis, and it is converted into an ellipse that shows less strain (lower Rf-value) than the true strain ellipse (Rs). Ellipse 7, on the other hand, is oriented with its long axis parallel to the long axis of the strain ellipse, and the two ellipticities are added. This leads to an ellipticity that is higher than Rs. When Rs=3, the true strain Rs is located somewhere between the shape represented by ellipses 1 and 7, as seen in Figure above (lower right diagram). 
For Rs=1.5 we still have ellipses with the full spectrum of orientations ( 90 to 90 ; see middle diagram in Figure above), while for Rs=3 there is a much more limited spectrum of orientations (lower graph in Figure above). The scatter in orientation is called the fluctuation F. An important change happens when ellipse 1, which has its long axis along the Z-axis of the strain ellipsoid, passes the shape of a circle (Rs=Ri,) and starts to develop an ellipse whose long axis is parallel to X. This happens when Rs=2, and for larger strains the data points define a circular shape. Inside this shape is the strain Rs that we are interested in. But where exactly is Rs? A simple average of the maximum and minimum Rf-values would depend on the original distribution of orientations. Even if the initial distribution is random, the average R-value would be too high, as high values tend to be over represented (Figure above, lower graph). 
To find Rs we have to treat the cases where Rs >Ri and Rs <Ri separately. In the latter case, which is represented by the middle graph in Figure above, we have the following expressions for the maximum and minimum value for Rf:
In both cases the orientation of the long (X) axis of the strain ellipse is given by the location of the maximum Rf-values. Strain could also be found by fitting the data to pre-calculated curves for various values for Ri and Rs. In practice, such operations are most efficiently done by means of computer programs.
The example shown in Figure above and discussed above is idealized in the sense that all the undeformed elliptical markers have identical ellipticity. What if this were not the case, i.e. some markers were more elliptical than others? Then the data would not have defined a nice curve but a cloud of points in the Rf/f-diagram. Maximum and minimum Rf-values could still be found and strain could be calculated using the equations above. The only change in the equation is that Ri now represents the maximum ellipticity present in the undeformed state. 
Another complication that may arise is that the initial markers may have had a restricted range of orientations. Ideally, the Rf/f-method requires the elliptical objects to be more or less randomly oriented prior to deformation. Conglomerates, to which this method commonly is applied, tend to have clasts with a preferred orientation. This may result in an Rf–f plot in which only a part of the curve or cloud is represented. In this case the maximum and minimum Rf-values may not be representative, and the formulas above may not give the correct answer and must be replaced by a computer based iterative retrodeformation method where X is input. However, many conglomerates have a few clasts with initially anomalous orientations that allow the use of Rf/f analysis.

Center-to-center method 

The center-to-center method. Straight lines are drawn between neighbouring object centers. The length of each line (d') and the angle (a') that they make with a reference line are plotted in the diagram. The data define a curve that has a maximum at X and a minimum at the Y-value of the strain ellipse, and where Rs= X/Y.
This method, here demonstrated in Figure above, is based on the assumption that circular objects have a more or less statistically uniform distribution in our section(s). This means that the distances between neighboring particle centers were fairly constant before deformation. The particles could represent sand grains in well-sorted sandstone, pebbles, ooids, mud crack centers, pillow-lava or pahoe-hoe lava centers, pluton centers or other objects that are of similar size and where the centers are easily definable. If you are uncertain about how closely your section complies with this criterion, try anyway. If the method yields a reasonably well-defined ellipse, then the method works.
The method itself is simple and is illustrated in Figure above: Measure the distance and direction from the center of an ellipse to those of its neighbours. Repeat this for all ellipses and graph the distance d' between the centers and the angles a' between the center tie lines and a reference line. A straight line occurs if the section is unstrained, while a deformed section yields a curve with maximum (d' max) and minimum values (d' min). The ellipticity of the strain ellipse is then given by the ratio: Rs =( d' max)/(d' min).

The Fry method

The Fry method performed manually. (a) The centerpoints for the deformed objects are transferred to a transparent overlay. A central point (1 on the figure) is defined. (b) The transparent paper is then moved to another of the points (point 2) and the centerpoints are again transferred onto the paper (the overlay must not be rotated). The procedure is repeated for all of the points, and the result (c) is an image of the strain ellipsoid (shape and orientation). Based on Ramsay and Huber (1983).
A quicker and visually more attractive method for finding two-dimensional strain was developed by Norman Fry at the end of the 1970s. This method, illustrated in Figure above, is based on the center-to-center method and is most easily dealt with using one of several available computer programs. It can be done manually by placing a tracing overlay with a coordinate origin and pair of reference axes on top of a sketch or picture of the section. The origin is placed on a particle center and the centers of all other particles (not just the neighbours) are marked on the tracing paper. The tracing paper is then moved, without rotating the paper with respect to the section, so that the origin covers a second particle center, and the centers of all other particles are again marked on the tracing paper. This procedure is repeated until the area of interest has been covered. For objects with a more or less uniform distribution the result will be a visual representation of the strain ellipse.The ellipse is the void area in the middle, defined by the point cloud around it (Figure above c). 
The Fry method, as well as the other methods presented in this section, outputs two-dimensional strain. Three-dimensional strain is found by combining strain estimates from two or more sections through the deformed rock volume. If sections can be found that each contain two of the principal strain axes, then two sections are sufficient. In other cases three or more sections are needed, and the three-dimensional strain must be calculated by use of a computer.

Strain in three dimensions

Three-dimensional strain expressed as ellipses on different sections through a conglomerate. The foliation (XY-plane) and the lineation (X-axis) are annotated. This illustration was published in 1888, but what are now routine strain methods were not developed until the 1960s.
A complete strain analysis is three-dimensional. Three dimensional strain data are presented in the Flinn diagram or similar diagrams that describe the shape of the strain ellipsoid, also known as the strain geometry. In addition, the orientation of the principal strains can be presented by means of stereographic nets. Direct field observations of three-dimensional strain are rare. In almost all cases, analysis is based on two-dimensional strain observations from two or more sections at the same locality (Figure above). A well-known example of three-dimensional strain analysis from deformed conglomerates is presented in below heading. 
In order to quantify ductile strain, be it in two or three dimensions, the following conditions need to be met:
The strain must be homogeneous at the scale of observation, the mechanical properties of the objects must have been similar to those of their host rock during the deformation, and we must have a reasonably good knowledge about the original shape of strain markers.
The strain must be homogeneous at the scale of observation, the mechanical properties of the objects must have been similar to those of their host rock during the deformation, and we must have a reasonably good knowledge about the original shape of strain markers.
The second point is an important one. For ductile rocks it means that the object and its surroundings must have had the same competence or viscosity. Otherwise the strain recorded by the object would be different from that of its surroundings. This effect is one of several types of strain partitioning, where the overall strain is distributed unevenly in terms of intensity and/or geometry in a rock volume. As an example, we mark a perfect circle on a piece of clay before flattening it between two walls. The circle transforms passively into an ellipse that reveals the two-dimensional strain if the deformation is homogeneous. If we embed a coloured sphere of the same clay, then it would again deform along with the rest of the clay, revealing the three-dimensional strain involved. However, if we put a stiff marble in the clay the result is quite different. The marble remains unstrained while the clay around it becomes more intensely and heterogeneously strained than in the previous case. In fact, it causes a more heterogeneous strain pattern to appear. Strain markers with the same mechanical properties as the surroundings are called passive strain markers because they deform passively along with their surroundings. Those that have anomalous mechanical properties respond differently than the surrounding medium to the overall deformation, and such markers are called active strain markers
Strain obtained from deformed conglomerates, plotted in the Flinn diagram. Different pebble types show different shapes and finite strains. Polymict conglomerate of the Utslettefjell Formation, Stord, southwest Norway. 
An example of data from active strain markers is shown in Figure above. These data were collected from a deformed polymictic conglomerate where three-dimensional strain has been estimated from different clast types in the same rock and at the same locality. Clearly, the different clast types have recorded different amounts of strain. Competent (stiff) granitic clasts are less strained than less competent greenstone clasts. This is seen using the fact that strain intensity generally increases with increasing distance from the origin in Flinn space. But there is another interesting thing to note from this figure: It seems that competent clasts plot higher in the Flinn diagram (Figure. above) than incompetent(“soft”) clasts, meaning that competent clasts take on a more prolate shape. Hence, not only strain intensity but also strain geometry may vary according to the mechanical properties of strain markers. 
The way that the different markers behave depends on factors such as their mineralogy, preexisting fabric, grain size, water content and temperature-pressure conditions at the time of deformation. In the case of Figure above, the temperature-pressure regime is that of lower to middle greenschist facies. At higher temperatures, quartz-rich rocks are more likely to behave as “soft” objects, and the relative positions of clast types in Flinn space are expected to change. 
The last point above also requires attention: the initial shape of a deformed object clearly influences its postdeformational shape. If we consider two-dimensional objects such as sections through oolitic rocks, sandstones or conglomerates, the Rf/f method discussed above can handle this type of uncertainty. It is better to measure up two or more sections through a deformed rock using this method than dig out an object and measure its three-dimensional shape. The single object could have an unexpected initial shape (conglomerate clasts are seldom perfectly spherical or elliptical), but by combining numerous measurements in several sections we get a statistical variation that can solve or reduce this problem.
Three-dimensional strain is usually found by combining two-dimensional data from several differently oriented sections.
There are now computer programs that can be used to extract three-dimensional strain from sectional data. If the sections each contain two of the principal strain axes everything becomes easy, and only two are strictly needed (although three would still be good). Otherwise, strain data from at least three sections are required.

Deformed quartzite conglomerates

Quartz or quartzite conglomerates with a quartzite matrix are commonly used for strain analyses. The more similar the mineralogy and grain size of the matrix and the pebbles, the less deformation partitioning and the better the strain estimates. A classic study of deformed quartzite conglomerates is Jake Hossack’s study of the Norwegian Bygdin conglomerate, published in 1968. Hossack was fortunate he found natural sections along the principal planes of the strain ellipsoid at each locality. Putting the sectional data together gave the three dimensional state of strain (strain ellipsoid) for each locality. Hossack found that strain geometry and intensity varies within his field area. He related the strain pattern to static flattening under the weight of the overlying Caledonian Jotun Nappe. Although details of his interpretation may be challenged, his work demonstrates how conglomerates can reveal a complicated strain pattern that otherwise would have been impossible to map. 
Hossack noted the following sources of error:
  • Inaccuracy connected with data collection (sections not being perfectly parallel to the principal planes of strain and measuring errors).  
  • Variations in pebble composition.  
  • The pre-deformational shape and orientation of the pebbles.  
  • Viscosity contrasts between clasts and matrix.  
  • Volume changes related to the deformation (pressure solution).  
  • The possibility of multiple deformation events.
Credits: Haakon Fossen (Structural Geology)