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.

Showing posts with label environmental. Show all posts
Showing posts with label environmental. Show all posts

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
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

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.

Banded-iron formations (BIFs) - Evidence of Oxygen in Early Atmosphere

Our knowledge about the rise of oxygen gas in Earth’s atmosphere comes from multiple lines of evidence in the rock record, including the age and distribution of banded iron formations, the presence of microfossils in oceanic rocks, and the isotopes of sulfur.
However, this article is just focus on Banded Iron Formation.

BIF (polished) from Hamersley Iron Formation, West Australia, Australia

Summary: Banded-iron formations (BIFs) are sedimentary mineral deposits consisting of alternating beds of iron-rich minerals (mostly hematite) and silica-rich layers (chert or quartz) formed about 3.0 to 1.8 billion years ago. Theory suggests BIFs are associated with the capture of oxygen released by photosynthetic processes by iron dissolved in ancient ocean water. Once nearly all the free iron was consumed in seawater, oxygen could gradually accumulate in the atmosphere, allowing an ozone layer to form. BIF deposits are extensive in many locations, occurring as deposits, hundreds to thousands of feet thick. During Precambrian time, BIF deposits probably extensively covered large parts of the global ocean basins. The BIFs we see today are only remnants of what were probably every extensive deposits. BIFs are the major source of the world's iron ore and are found preserved on all major continental shield regions. 

Banded-iron formation (BIF)
consists of layers of iron oxides (typically either magnetite or hematite) separated by layers of chert (silica-rich sedimentary rock). Each layer is usually narrow (millimeters to few centimeters). The rock has a distinctively banded appearance because of differently colored lighter silica- and darker iron-rich layers. In some cases BIFs may contain siderite (carbonate iron-bearing mineral) or pyrite (sulfide) in place of iron oxides and instead of chert the rock may contain carbonaceous (rich in organic matter) shale.

It is a chemogenic sedimentary rock (material is believed to be chemically precipitated on the seafloor). Because of old age BIFs generally have been metamorphosed to a various degrees (especially older types), but the rock has largely retained its original appearance because its constituent minerals are fairly stable at higher temperatures and pressures. These rocks can be described as metasedimentary chemogenic rocks.

                     Jaspilite banded iron formation (Soudan Iron-Formation, Soudan, Minnesota, USA
Image Credits: James St. John

In the 1960s, Preston Cloud, a geology professor at the University of California, Santa Barbara, became interested in a particular kind of rock known as a Banded Iron Formation (or BIF). They provide an important source of iron for making automobiles, and provide evidence for the lack of oxygen gas on the early Earth.

Cloud realized that the widespread occurrence of BIFs meant that
the conditions needed to form them must have been common on the ancient Earth, and not common after 1.8 billion years ago. Shale and chert often form in ocean environments today, where sediments and silica-shelled microorganisms accumulate gradually on the seafloor and eventually turn into rock. But iron is less common in younger oceanic sedimentary rocks. This is partly because there are only a few sources of iron available to the ocean: isolated volcanic vents in the deep ocean and material weathered from continental rocks and carried to sea by rivers.

Banded iron-formation (10 cm), Northern Cape, South Africa.
Specimen and photograph: A. Fraser
Most importantly, it is difficult to transport iron very far from these sources today because when iron reacts with oxygen gas, it becomes insoluble (it cannot be dissolved in water) and forms a solidparticle. Cloud understood that for large deposits of iron to exist all over the world’s oceans, the iron must have existed in a dissolved form. This way, it could be transported long distances in seawater from its sources to the locations where BIFs formed. This would be possible only if there were little or no oxygen gas in the atmosphere and ocean at the time the BIFs were being deposited. Cloud recognized that since BIFs could not form in the presence of oxygen, the end of BIF deposition probably marked the first occurrence of abundant oxygen gas on Earth (Cloud, 1968).
Cloud further reasoned that, for dissolved iron to finally precipitate and be deposited, the iron would have had to react with small amounts of oxygen near the deposits. Small amounts of oxygen could have been produced by the first photosynthetic bacteria living in the open ocean. When the dissolved iron encountered the oxygen produced by the photosynthesizing bacteria, the iron would have precipitated out of seawater in the form of minerals that make up the iron-rich layers of BIFs: hematite (Fe2O3) and magnetite (Fe3O4), according to the following reactions:
4Fe3 + 2O2 → 2Fe2O3
6Fe2 + 4O2 → 2Fe3O4
The picture that emerged from Cloud’s studies of BIFs was that small amounts of oxygen gas, produced by photosynthesis, allowed BIFs to begin forming more than 3 billion years ago. The abrupt disappearance of BIFs around 1.8 billion years ago probably marked the time when oxygen gas became too abundant to allow dissolved iron to be transported in the oceans.
Banded Iron Formation
Source is unknown

It is interesting to note that BIFs reappeared briefly in a few places around 700 millionyears ago,during a period of extreme glaciation when evidence suggests that Earth’s oceans were entirely covered with sea ice. This would have essentially prevented the oceans from interacting with the atmosphere, limiting the supply of oxygen gas in the water and again allowing dissolved iron to be transported throughout the oceans. When the sea ice melted, the presence of oxygen would have again allowed the iron to precipitate.


1. Misra, K. (1999). Understanding Mineral Deposits Springer.
Cloud, P. E. (1968). Atmospheric and hydrospheric evolution on the primitive Earth both secular accretion and biological and geochemical processes have affected Earth’s volatile envelope. Science, 160(3829), 729–736.
James,H.L. (1983). Distribution of banded iron-formation in space and time. Developments in Precambrian Geology, 6, 471–490.

10 of the Best Learning Geology Photos of 2016

A picture is worth a thousand words, but not all pictures are created equal. The pictures we usually feature on Learning Geology are field pictures showing Geological structures and features and many of them are high quality gem and mineral pictures. The purpose is to encourage students and professionals' activities by promoting "learning and scope" of Geology through our blogs.
In the end of 2016, we are sharing with you the 10 best photos of 2016 which we have posted on our page.

P.S: we always try our best to credit each and every photographer or website, but sometimes it’s impossible to track some of them. Please leave a comment if you know about the missing ones.

1. Folds from Basque France

 Image Credits: Yaqub ShahYaqub Shah

2. Horst and Graben Structure in Zanjan, Iran

Image Credits:

3. A unique Normal Fault

4. The Rock Cycle
 rock cycle illustrates the formation, alteration, destruction, and reformation of earth materials, and typically over long periods of geologic time. The rock cycle portrays the collective system of processes, and the resulting products that form, at or below the earth surface.The illustration below illustrates the rock cycle with the common names of rocks, minerals, and sediments associated with each group of earth materials: sediments, sedimentary rocks, metamorphic rocks, and igneous rocks.

Image Credits: Phil Stoffer

5. An amazing Botryoidal specimen for Goethite lovers! 

Image Credits: Moha Mezane 

6. Basalt outcrop of the Semail Ophiolite, Wadi Jizzi, Oman

Image Credits: Christopher Spencer
Christopher Spencer is founder of an amazing science outreach program named as Traveling Geologist. Visit his website to learn from him

7. Val Gardena Dolomites, Northern Italy

8. Beautiful fern fossil found in Potsville Formation from Pennsylvania.
The ferns most commonly found are Alethopteris, Neuropteris, Pecopteris, and Sphenophyllum.

Image Credits: Kurt Jaccoud

9. Snowball garnet in schist

Syn-kinematic crystals in which “Snowball garnet” with highly rotated spiral Si. 

Porphyroblast is ~ 5 mm in diameter.
From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures.

10. Trilobite Specimen from Wheeler Formation, Utah
The Wheeler Shale is of Cambrian age and is a world famous locality for prolific trilobite remains. 

Image Credits: Paleo Fossils

Stratigraphy: Making sense of chaos

What is Stratigraphy?

Stratigraphy- The branch of geology that seeks to understand the geometric relationships between different rock layers (called strata), and to interpret the history represented by these rock layers.

Public Domain Image by the US Dept. of Interior.

Contact- A boundary that separates different strata or rock units.
Steno's Laws of Stratigraphy

Image from J. P. Trap: berømte danske mænd og kvinder, 1868

Nicholas Steno (1638-1686) was a Danish-born pioneer of geology, and is considered to be the father of stratigraphy.

Nicholas Steno's observations of rocks layers suggested that geology is not totally chaotic.  Rather, the rock layers preserve a chronological record of Earth history and past life.

He developed three fundamental principles of stratigraphy, now known as Steno's Laws:

1) Law of Original Horizontality– Beds of sediment deposited in water form as horizontal (or nearly horizontal) layers due to gravitational settling.

2) Law of Superposition– In undisturbed strata, the oldest layer lies at the bottom and the youngest layer lies at the top.

3) Law of Lateral Continuity– Horizontal strata extend laterally until they thin to zero thickness (pinch out) at the edge of their basin of deposition.
Other Important Principles of Stratigraphy

4) Law of Cross-Cutting Relationships– An event that cuts across existing rock is younger than that disturbed rock.  This law was developed by Charles Lyell (1797-1875).

5) Principle of Inclusion– Fragments of rock that are contained (or included) within a host rock are older than the host rock.
Unconformity – A surface that represents a very significant gap in the geologic rock record (due to erosion or long periods of non-deposition).
There are 3 main types of unconformities:
1) Disconformity – A contact representing missing rock between sedimentary layers that are parallel to each other.  Since disconformities are parallel to bedding planes, they are difficult to see in nature.

2) Angular Unconformity – A contact in which younger strata overlie an erosional surface on tilted or folded rock layers.  This type of unconformity is easy to identify in nature.
Image provided by FCIT. Original image from Textbook of Geology by Sir Archibald Geikie (1893).
3) Nonconformity – A contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.
4) Paraconformity- A contact between parallel layers formed by extended periods of non-deposition (as opposed to being formed by erosion).  These are sometimes called "pseudo unconformities").
Unconformities VS Bedding Planes
Unconformities represent huge gaps in time!  The nonconformity between the Vishnu Schist and overlying sedimentary layers in the Grand Canyon represents 1.3 billion years of missing rock record.
Bedding planes, or planes separating adjacent sedimentary layers, also represent gaps in the rock record but on a much smaller scale than an unconformity.
Relative Age Dating
Relative age dating is a way to use geometric relationships between rock bodies to determine the sequence of geologic events in an area.  Relative dating is different from absolute dating in which specific dates are assigned to geologic events (we will discuss absolute dating techniques later).
Relative dating is based on the five principles of stratigraphy discussed above.
Historical Perspective on the Origin of Rocks: Werner's Concept of Neptunism

Abraham Werner (1749-1817), a German geologist, proposed that Earth’s crust originated in ocean water through the process of precipitation.  This idea became known as Neptunism, in reference to the Roman God of the sea.

Werner classified rocks into 4 categories, as shown in the diagram below:

Figure by RJR

1. Primitive rock (red)– Granite and metamorphic rock were precipitated from oceans.

2. Transition rock (light brown)– Next, fossil-rich sedimentary rocks were precipitated.  These rocks are tilted due to deposition on the non-horizontal surfaces of primitive rocks.  This aspect of Werner's model was useful for explaining the origin of tilted sedimentary rocks.

3. Secondary rock (dark brown)– Flat lying sedimentary rocks were eventually precipitated.  The secondary rocks were thought to include interlayered basalts, which Werner thought formed by combustion of buried coal layers.

4. Tertiary (or alluvial) rock (yellow)– Finally, after the ocean receded, recent erosion and deposition created a thin veneer of overlying sediment.

Today we know that Werner's basic assumption that granite precipitated from seawater is incorrect.  We also know that basalt is not the product of coal combustion.

Nevertheless, Werner's concept of Neptunism was influential because:

1) Werener was right that some sedimentary rocks, such as limestones, do precipitate from ocean water.

2) Werner was not a catastrophist and did not need to make his interpretation of rock layers consistent with scriptual teachings.

3) Werner’s relative age assignments represents an early attempt to determine Earth's sequential history.
Historical Perspective on the Origin of Rocks: Hutton's Concept of Plutonism

The Scottish geologist James Hutton (1726-1797) argued that granite and basalt by solidification within the earth (as opposed to precipitating in from oceanwater).  This idea is known as Plutonism, in reference to the God of the deep underworld.

This concept of plutonism was supported by basalt melting/cooling experiments Sir James Hall conducted in 1792.  These experiments showed that the basalts form by the solidification of liquid magma.

Hutton viewed tilted strata as having been initially deposited horizontally, and then were subsequently deformed (tilted and folded) by the forces of Earth's internal heat engine.  He would argue that these forces gave rise to mountains.

Furthermore, he suggested that the mountains eroded to produce the sedimentary rocks we find in the rock record.

Hutton viewed the earth continually recycling itself with a balance between destruction and rejuvenation.  Mountains are created, eroded, and reformed.

Hutton’s ideas were not well received by people in the early 1800’s because he was a poor writer, and because his science was anti-catastrophic and did not support the scriptures.

Mass Flow

What is Mass flow?

Mixtures of detritus and fluid that move under gravity are known collectively as mass flows, gravity flows or density currents. A number of different mechanisms are involved and all require a slope to provide the potential energy to drive the flow. This slope may be the surface over which the flow occurs, but a gravity flow will also move on a horizontal surface if it thins downflow, in which case the potential energy is provided by the difference in height between the tops of the upstream and the downstream parts of the flow.

Debris flows

Debris flows are dense, viscous mixtures of sediment and water in which the volume and mass of sediment exceeds that of water. A dense, viscous mixture of this sort will typically have a low Reynolds number so the flow is likely to be laminar. In the absence of turbulence no dynamic sorting of material into different sizes occurs during flow and the resulting deposit is very poorly sorted. Some sorting may develop by slow settling and locally there may be reverse grading produced by shear at the bed boundary. Material of any size from clay to large boulders may be present. Debris flows occur on land, principally in arid environments where water supply is sparse (such as some alluvial fans) and in submarine environments where they transport material down continental slopes and locally on some coarse-grained delta slopes. Deposition occurs when internal friction becomes too great and the flow ‘freezes’. There may be little change in the thickness of the deposit in a proximal to distal direction and the clast size distribution may be the same throughout the deposit. The deposits of debris flows on land are typically matrix-supported conglomerates although clast-supported deposits also occur if the relative proportion of large clasts is high in the sediment mixture. They are poorly sorted and show a chaotic fabric, i.e. there is usually no preferred orientation to the clasts, except within zones of shearing that may form at the base of the flow. When a debris flow travels through water it may partly mix with it and the top part of the flow may become dilute. The tops of subaqueous debris flows are therefore characterised by a gradation up into better sorted, graded sediment, which may have the characteristics of a turbidite.

Turbidity currents

Turbidity currents are gravity-driven turbid mixtures of sediment temporarily suspended in water. They are less dense mixtures than debris flows and with a relatively high Reynolds number are usually turbulent flows. The name is derived from their characteristics of being opaque mixtures of sediment and water (turbid) and not the turbulent flow. They flow down slopes or over a horizontal surface provided that the thickness of the flow is greater upflow than it is downflow. The deposit of a turbidity current is a turbidite. The sediment mixture may contain gravel, sand and mud in concentrations as little as a few parts per thousand or up to 10% by weight: at the high concentrations the flows may not be turbulent and are not always referred to as turbidity currents. The volumes of material involved in a single flow event can be anything up to tens of cubic kilometres, which is spread out by the flow and deposited as a layer a few millimetres to tens of metres thick. Turbidity currents, and hence turbidites, can occur in water anywhere that there is a supply of sediment and a slope. They are common in deep lakes, and may occur on continental shelves, but are most abundant in deep marine environments, where turbidites are the dominant clastic deposit. The association with deep marine environments may lead to the assumption that all turbidites are deep marine deposits, but they are not an indicator of depth as turbidity currents are a process that can occur in shallow water as well. Sediment that is initially in suspension in the turbidity current starts to come into contact with the underlying surface where it may come to a halt or move by rolling and suspension. In doing so it comes out of suspension and the density of the flow is reduced. Flow in a turbidity current is maintained by the density contrast between the sediment-water mix and the water, and if this contrast is reduced, the flow slows down. At the head of the flow turbulent mixing of the current with the water dilutes the turbidity current and also reduces the density contrast. As more sediment is deposited from the decelerating flow a deposit accumulates and the flow eventually comes to a halt when the flow has spread out as a thin, even sheet.

Low- and medium-density turbidity currents

The first material to be deposited from a turbidity current will be the coarsest as this will fall out of suspension first. Therefore a turbidite is characteristically normally graded. Other sedimentary structures within the graded bed reflect the changing processes that occur during the flow and these vary according to the density of the initial mixture. Low- to medium-density turbidity currents will ideally form a succession known as a Bouma sequence, named after the geologist who first described them. Five divisions are recognised within the Bouma sequence, referred to as ‘a’ to ‘e’ divisions and annotated Ta, Tb and so on. 
  • Ta: This lowest part consists of poorly sorted, structureless sand: on the scoured base deposition occurs rapidly from suspension with reduced turbulence inhibiting the formation of bedforms. 
  • Tb: Laminated sand characterises this layer, the grain size is normally finer than in ‘a’ and the material is better sorted: the parallel laminae are generated by the separation of grains in upper flow regime transport.
  • Tc: Cross-laminated medium to fine sand, sometimes with climbing ripple lamination, form the middle division of the Bouma sequence: these characteristics indicate moderate flow velocities within the ripple bedform stability field and high sedimentation rates. Convolute lamination can also occur in this division. 
  • Td: Fine sand and silt in this layer are the products of waning flow in the turbidity current: horizontal laminae may occur but the lamination is commonly less well defined than in the ‘b’ layer. 
  • Te: The top part of the turbidite consists of finegrained sediment of silt and clay grade: it is deposited from suspension after the turbidity current has come to rest and is therefore a hemipelagic deposit. 
Turbidity currents are waning flows, that is, they decrease velocity through time as they deposit material, but this means that they also decrease velocity with distance from the source. There is therefore a decrease in the grain size deposited with distance. The lower parts of the Bouma sequence are only present in the more proximal parts of the flow. With distance the lower divisions are progressively lost as the flow carries only finer sediment and only the ‘c’ to ‘e’ or perhaps just ‘d’ and ‘e’ parts of the Bouma sequence are deposited. In the more proximal regions the flow turbulence may be strong enough to cause scouring and completely remove the upper parts of a previously deposited bed. The ‘d’ and ‘e’ divisions may therefore be absent due to this erosion and the eroded sediment may be incorporated into the overlying deposit as mud clasts. The complete Ta to Te sequence is therefore only likely to occur in certain parts of the deposit, and even there intermediate divisions may be absent due, for example, to rapid deposition preventing ripple formation in Tc. Complete Ta-e Bouma sequences are in fact rather rare.

High-density turbidity currents

Under conditions where there is a higher density of material in the mixture the processes in the flow and hence of the characteristics of the deposit are different from those described above. High-density turbidity currents have a bulk density of at least 1.1 g/cm 3. The turbidites deposited by these flows have a thicker coarse unit at their base, which can be divided into three divisions. Divisions S1 and S2 are traction deposits of coarse material, with the upper part, S2, representing the ‘freezing’ of the traction flow. Overlying this is a unit, S3, that is characterised by fluid-escape structures indicating rapid deposition of sediment. The upper part of the succession is more similar to the Bouma Sequence, with Tt equivalent to Tb and Tc and overlain by Td and Te: this upper part therefore reflects deposition from a lower density flow once most of the sediment had already been deposited in the ‘S’ division. The characteristics of high-density turbidites were described by Lowe, after whom the succession is sometimes named.

Grain flows

Avalanches are mechanisms of mass transport down a steep slope, which are also known as grain flows. Particles in a grain flow are kept apart in the fluid medium by repeated grain to grain collisions and grain flows rapidly ‘freeze’ as soon as the kinetic energy of the particles falls below a critical value. This mechanism is most effective in well-sorted material falling under gravity down a steep slope such as the slip face of an aeolian dune. When the particles in the flow are in temporary suspension there is a tendency for the finer grains to fall between the coarser ones, a process known as kinetic sieving, which results in a slight reverse grading in the layer once it is deposited. Although most common on a small scale in sands, grain flows may also occur in coarser, gravelly material in a steep subaqueous setting such as the foreset of a Gilbert-type delta.