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 fossil. Show all posts
Showing posts with label fossil. Show all posts

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

Evolution and Extinction

Evolution and Extinction 

Darwin’s Grand Idea 

As a young man in England in the early 19th century, Charles Darwin had been unable to settle on a career but had developed a strong interest in natural history. Therefore, he jumped at the opportunity to serve as a naturalist aboard HMS Beagle on an around-the-world surveying cruise. During the five years of the cruise, from 1831 to 1836, Darwin made detailed observations of plants, animals, and geology in the field and amassed an immense specimen collection from South America, Australia, and Africa. Just before Darwin departed on the voyage, a friend gave him a copy of Charles Lyell’s 1830 textbook, Principles of Geology, which argued in favour of James Hutton’s proposal that the Earth had a long history and that geologic time extended much further into the past than did human civilization.

The Fossil Record

The Fossil Record 

A Brief History of Life 

Based on laboratory experiments conducted in the 1950s, researchers speculated that reactions in concentrated “soups” of chemicals that formed when seawater evaporated in shallow, coastal pools led to the formation of the earliest protein-like organic chemicals (“proto-life”). More recent studies suggest, instead, that such reactions took place in warm groundwater beneath the Earth’s surface or at hydrothermal vents on the sea floor. While the nature of proto-life remains a mystery, an image of early life has begun to take shape, based on detailed analysis of the oldest sedimentary rocks. The fossil record defines the subsequent long-term record of life’s evolution on planet Earth. And, of course, that record is more complete in younger strata.

Taxonomy and Identification of fossils

Taxonomy and Identification of fossils

The study of how to identify and name organisms is taxonomy. Taxonomic classification of fossils follows the same principles used for the classification of living organisms and has a hierarchy of divisions. These principles were first proposed in the 18th century by Carolus Linnaeus, a Swedish biologist.



What Kinds of Rocks Contain Fossils? 

Most fossils are found in sediments or sedimentary rocks. Fossils form when organisms die and become buried by sediment, or when organisms travel over or through sediment and leave imprints or debris. The degree of preservation of a fossil reflects the context of burial. For example, rocks formed from sediments deposited under anoxic (oxygen-free) conditions in quiet water (such as lake beds or lagoons) can preserve particularly fine specimens. In contrast, rocks made from sediments deposited in high-energy environments where strong currents tumble shells and bones and break them up contain at best only small fragments of fossils mixed with other clastic grains. Fossils sometimes occur in volcaniclastic rocks, but they are not found in intrusive igneous rocks and tend to be destroyed by metamorphism.


Correlation of Strata

The need to classify and organize rock layers according to relative age led to the geologic discipline of stratigraphy.

Rocks at different locations on Earth give different "snapshots" of the geologic time column.  At a particular location, the rocks never fully represent the entire geologic rock column due to extensive erosion or periods of non-deposition or erosion.

The thickness of a particular rock layer (representing a particular time period) will vary from one location to another or even disappear altogether.

The process that stratigraphers use to understand these relationships between strata at different localities is known as "correlation".

For example, rocks named Juras (for the Juras Mountains) in France and Switzerland were traced northward and found to overlie a group of rocks in Germany namedTrias.  The Trias rocks in turn, were found to underlie rocks named Cretaceous in England (the chalky “White Cliffs of Dover”).

Based on these relationships, is the Juras older or younger than the Cretaceous?  What are the two possible scenarios?

The location where a particular rock layer was discovered is called a "type locality".  Most of the “type localities” of the geologic time column are located in Europe because this is where the science of stratigraphic correlation started.

The Sedgwick/Murchison Debate

In 1835, Adam Sedgwick (Britain) and Roderick Murchison (Scotland) decided to name the entire succession of sedimentary rocks exposed throughout Europe.  They were geology colleagues and friends, but they had a famous argument over the division between the Cambrian and Silurian in Wales. 

Sedgwick’s topmost Cambrian overlapped with Murchison’s lowermost Silurian.  Eventually the disputed rock layers were assigned the age “Ordovician”.
Rocks Divisions versus Time Divisions

It is important to remember that the rock record is an incomplete representation of real geologic time due to the presence of unconformities.

Therefore, geologists are careful to distinguish geologic time from the rocks that represent snapshots of geologic time:


Examples: Precambrian/Phanerozoic


          Examples: Paleozoic/Cenozoic/Mesozoic


               Examples: Cambrian/Ordovician/Silurian

                    Formations (The main stratigraphic unit)

Rock divisions, such as the Cambrian System, can be correlated worldwide based on fossils.  In contrast, rock units such as groups, formations, and members are localized subsets of systems.  Rock units depend on the environment of deposition, which varies from one location to another.
Stratigraphic Rock Units

The rock divisions (Eonothem, Erathem, and System) simply divide rocks into the appropriate time eon, era, or period.  Obviously, all Cambrian System rocks are from the Cambrian regardless of their location on Earth's surface.

In contrast, the rock units (Groups, Formations, Members) are localized features (of limited regional extent) that depend on the local environment of deposition. 

The main rock unit of stratigraphy is the formation, a localized and distinctive (easily recognizable) geologic feature (i.e., the Chinle Formation of Late Triassic lake and river deposits in Arizona, Nevada, Utah, and New Mexico).

Different formations are distinguished and correlated based upon lithology (overall rock characteristics), which includes:

1) Composition of mineral grains
2) Color
3) Texture (grain size, sedimentary structures)
4) Fossils

Formations are “clumped” into groups and divided into members.

Datum- In correlation, a datum is a line of equivalent age.

The ideal datum is a stratigraphic marker that is both geographically extensive and represents an instantaneous moment in geologic time.  A good example is a volcanic ash layer that formed by a specific volcanic eruption followed by worldwide dispersal by atmospheric currrents.
Using Fossils for Strata Correlation

Sedimentary rocks that date from the same age can be correlated over long distances with the help of fossils.

Principle of Fossil Correlation- Strata containing similar collections of fossils (called fossil assemblages) are of similar age.  Also, fossils at the bottom of the strata are older than fossils closer to the top of the strata.

Index Fossils- Index fossils are the main type of fossil used in correlation.  To be an index fossil, a fossil species must be:

1) Easily recognized (unique).
2) Widespread in occurrence from one location to another.
3) Restricted to a limited thickness of strata (limited in age range).

The limited life-spans of these organisms allows us to easily constrain the age of rocks in which they occur.

The best index fossils are those that are free floating and independent of a particular sedimentary environment.  For example, organisms that are attached to one particular type of sediment are going to have limited geographic extent and will not be found in many rock types.   By contrast, organisms that are “free floaters” or “swimmers” will have a wider geographic extent and be found in many different rock types (i.e., trilobites).

fossil zone is an interval of strata characterized by a distinctive index fossil.

Fossil zones typically represent packets of 500,000 to 2,000,000 years.  Fossil zones boundaries do not have to correlate with rock formation boundaries.  Fossil zones may be restricted to a small portion of a formation or they may span more than one formation.

A fundamental assumption in fossil correlation is that once a species goes extinct, it will never reappear in the rock record at a later time.

Fossil types that are generally restricted to just one type of sediment are called facies fossils.  They are not very useful in correlation, but are extremely useful for reconstructing paleoenvironments.
  What is a Fossil?

Some examples of fossils are:

1) The preservation of entire organisms or body parts.  This includes the preservation of actual body parts (mammoths in tundra), as well as morphological preservation via the replacement of biological matter by minerals (petrified wood).
A petrified log in Petrified Forest National Park, Arizona, U.S.A.-impressions

2) Casts or impressions of organisms.
Eocene fossil fish Priscacara liops from Green River Formation of Utah

3) Tracks.
Trackways from ''Climactichnites'' (probably a slug-like animal), in the Late Cambrian of central Wisconsin.

4) Burrows.
Thalassinoides, burrows produced by crustaceans, from the Middle Jurassic of southern Israel.

5) Fecal matter (called coprolites).
Carnivorous dinosaur dung found in southwestern Saskatchewan,  USGS Image.
Theories on The Origin of Fossils

At one time, fossils were considered to be younger than the rocks in which they occurred.  People speculated that fossils formed when animals crawled into preexisting rock, died, and became preserved in stone.

Some people interpreted the widespread occurrence of fossilized marine organisms on land as support for a world-wide flood as described in scripture.

Leonardo da Vinci’s (1452 - 1519) Interpretation of Fossils
Self-portrait of Leonardo da Vinci, circa 1512-1515.

Regarding fossils that occur in strata many miles from the sea, da Vinci argued that:

1) The fossils could not have been washed in during a "Great Deluge" because they could not have traveled hundreds of miles in just 40 days.

2) The unbroken nature of the fossils suggest that they were not transported by violent water; instead the fossils represent formerly living communities of organisms that were preserved in situ.

3) The presence of fossil-rich strata separated by fossil-poor strata suggests that the fossils were not the result of a single worldwide flood, but formed during many separate events.
Lateral Variations in Formations

Historically, geologists initially believed that the layer-cake sequence of sedimentary rocks existed worldwide (i.e., that the layers extended indefinitely without change).

By the late 1700’s people began to realize that formations had a limited extent both vertically (up and down) and laterally (horizontally across Earth's surface).

People also began to realize that lithologic variations (changes in texture, color, fossils, etc) can occur laterally within formations themselves.

Today we interpret such variations in the context of modern depositional environments.  For example:



Near shore marine- The energy is high due to rough waters at the water-land interface.

Coarse sediments, and fossils of robust organisms that can withstand high energy environments.

Deep ocean- The energy is low due to the general calmness of water away from land.

Fine sediments, and fossils of more fragile organisms.

Note that the two different lithologies can be deposited simultaneously (representing the same moment in geological time) so long as they are deposited at different locations.

Different lithologies grade laterally into one another in a manner called intertonging.  An example is the way that the Old Red Sandstone of Wales (a terrestrial deposit) grades laterally into marine sediments of Devonshire to the south (both are Devonian).

Intertonging reflects the changes in depositional environments that occur over space and time (lateral and temporal variations).  Often these changes in environment are linked to shoreline migrations resulting from sea-level changes over time.
 Depositional Environments and Sedimentary Facies

Depositonal environments are characterized initially by the sediments that accumulate within them, and ultimately by the sedimentary rock types that form.  For example, a reef environment is characterized by carbonate reef-building organisms.  Ultimately, the sediments become lithified to form fossiliferous limestone.

sedimentary facies is a three-dimensional body of sediment (or rock) that contains lithologies representative of a particular depositional environment.  For example,




Mudstone and shale with interbedded sandstone.

Ocean basin

Laminated pelagic clays, cherts, and possible limestone.


Well-sorted, well-rounded, and possibly cross-bedded sandstone.

Analysis of sedimentary facies helps geologists to reconstruct past geologic environments and paleogeography.
Transgressions vs. Regressions

The sea-level has fluctuated throughout geologic history, and these changes have a profound effect on the geologic rock record.

transgression is an advance of the sea over land.

regression is a retreat of the sea from land area.

A transgressive facies pattern is characterized by:

1. The movement of marine facies landward over terrestrial facies.
2. A fining-upward sequence (the new marine environment is lower energy than the prior terrestrial environment).
3. A basal, erosional unconformity (erosion was more profound before the seas advanced).

A regressive facies pattern is characterized by:

1. The movement of terrestrial facies seaward and over marine facies.
2. A coarsening-upward sequence.
3. An erosional unconformity at the top.

Walther’s Law- Over time, the lateral changes in sedimentary facies due to transgressions and regressions will also produce vertical changes in sedimentary facies:

1. A transgressive facies sequence fines in the direction of the transgression, and also fines upward.
2. A regressive facies sequence coarsens in the direction of the regression, and also coarsens upward.

What causes transgressions and regressions?

1. Worldwide rises and falls in sea level (eustatic changes), perhaps related to climatic change.
2. Tectonic uplift, isostatic rebound, or crustal subsidence.
3. Rapid sedimentation.

It is often difficult or impossible to determine the exact cause of a transgression or regression seen in the geologic record.  The cause may be worldwide or local.  The fact that there is a transgression or regression indicates an “apparent” sea-level change.
 The Stratigraphy of Unconformities

Recall that unconformities represent missing time due to:

1)      Periods of non-deposition.
2)      Periods of erosion.

The main types of unconformities are:
1. Disconformity
2. Angular unconformity
3. Nonconformity
4. Paraconformity

Unconformities vary from one location to another (just like rock formations and sedimentary facies).  In other words, some locations along the unconformity surface will represent more missing geologic time than others.

Unconformities may eventually disappear laterally and transition into a conformable sequence of strata.

Oil companies use large scale, unconformity bounded rock units called sequences to correlate rocks in a process called sequence stratigraphy.

Six major unconformity-bounded sequences are recognized worldwide in the Phanerozoic.  These sequences are not restricted to period or era boundaries.

The major sequences are believed to represent worldwide fluctuations in sea-level.

Fossils in rocks

Fossils in rocks are preserved which are studied by the scientists. The fossils are the key to past, by studying fossils provide us the evidence for what happened in the Earth history and when it happened. Fossils can tell us series of things like

  • Correlating same age rock units.
  • The time a rock layered deposited.
  • What were the conditions at time of deposition.
  • What were the environment in which the rock was formed.
  • What happened in the Earth's history.
The word fossil makes numerous individuals consider dinosaurs. Dinosaurs are currently included in books, motion pictures, and TV projects, and the bones of some expansive dinosaurs are in plain view in numerous historical centers. These reptiles were predominant creatures on Earth for well more than 100 million years from the Late Triassic through the Late Cretaceous. Numerous dinosaurs were entirely little, however by the center of the Mesozoic Period, a few species weighed as much as 80 tons. By around 65 million years prior all dinosaurs were wiped out. The purposes behind and the rate of their termination are a matter of exceptional level headed discussion among researchers.

Regardless of the greater part of the enthusiasm for dinosaurs, they frame just a little portion of the a large number of animal groups that live and have lived on Earth. The colossal greater part of the fossil record is commanded by fossils of creatures with shells and minute stays of plants and creatures, and these remaining parts are broad in sedimentary rocks. It is these fossils that are examined by most scientists.

In the late eighteenth and mid nineteenth hundreds of years, the English geologist and designer William Smith and the French scientists Georges Cuvier and Alexandre Brongniart found that stones of the same age may contain the same fossils notwithstanding when the stones are isolated by long separations. They distributed the first geologic maps of vast zones on which shakes containing comparative fossils were indicated. Via cautious perception of the stones and their fossils, these men and different geologists had the capacity perceive rocks of the same age on inverse sides of the English Channel.

William Smith had the capacity apply his insight into fossils in an exceptionally down to earth way. He was a designer building channels in England, which has loads of vegetation and few surface exposures of rock. He expected to comprehend what rocks he could hope to discover on the slopes through which he needed to fabricate a channel. Frequently he could tell what sort of rock was prone to be underneath the surface by inspecting the fossils that had disintegrated from the stones of the slope or by burrowing a little opening to discover fossils. Realizing what rocks to anticipate that permitted Smith will gauge costs and figure out what devices were required for the employment.

Smith and others realized that the progression of life structures saved as fossils is valuable for seeing how and when the stones shaped. Just later did researchers build up a hypothesis to clarify that progression.

This clearly shows how the rocks can be correlated and what time deposited by examining the fossils of the age at which it were deposited. The age of the fossil is regarded as the age of the rock unit in which it is found. Moreover by looking at the fossils of marine and land animals clearly shows the deposition of rock unit under conditions of terrestial and marine.

Types of fossils

What it a fossil? 

The word fossil is derived from the Latin fossilis meaning an object that has been dug up from the ground. Fossils are the evidence for the existence of once-living animals and plants and may be either the preserved remains of an organism or evidence of its activity. 

Types of fossils

Trace fossils

Trace fossils are the preserved impressions of biological activity. They provide indirect evidence for the existence of past life. They are the only direct indicators of fossil behaviour. As trace fossils are usually preserved in situ they are very good indicators of the past sedimentary environment. Trace fossils made by trilobites have provided an insight into trilobite life habits, in particular walking, feeding, burrowing, and mating behaviour.

Chemical fossils 

When some organisms decompose they leave a characteristic chemical signature. Such chemical traces provide indirect evidence for the existence of past life. For example, when plants decompose their chlorophyll breaks down into distinctive, stable organic molecules. Such molecules are known from rocks more than 2 billion years old and indicate the presence of very early plants. 


Coprolites are fossilized animal feces. They may be considered as a form of trace fossil recording the activity of an organism. In some coprolites recognizable parts of plants and animals are preserved, providing information about feeding habits and the presence of coexisting organisms. 

Body fossils 

Body fossils are the remains of living organisms and are direct evidence of past life. Usually only hard tissues are preserved, for example shells, bones, or carapaces. In particular environmental conditions the soft tissues may fossilize but this is generally a rare occurrence. Most body fossils are the remains of animals that have died, but death is not a prerequisite, since some body fossils represent parts of an animal that are shed during its lifetime. For example, trilobites shed their exoskeleton as they grow and these molts may be preserved in the fossil record.

Taxa used in biostratigraphy

No single group of organisms fulfils all the criteria for the ideal zone fossil and a number of different groups of taxa have been used for defining biozones through the stratigraphic record. Some, such as the graptolites in the Ordovician and Silurian, are used for worldwide correlation; others are restricted in use to certain facies in a particular succession, for example corals in the Carboniferous of northwest Europe. Some examples of taxonomic groups used in biostratigraphy are outlined below.

Marine macrofossils

The hard parts of invertebrates are common in sedimentary rocks deposited in marine environments throughout the Phanerozoic. These fossils formed the basis for the divisions of the stratigraphic column into Systems, Series and Stages in the 18th and 19th centuries. The fossils of organisms such as molluscs, arthropods, echinoderms, etc., are relatively easy to identify in hand specimen, and provide the field geologist with a means for establishing the age of rocks to the right period or possibly epoch. Expert palaeontological analysis of marine macrofossils provides a division of the rocks into stages based on these fossils.


These Palaeozoic arthropods are the main group used in the zonation of the Cambrian. Most trilobites are thought to have been benthic forms living on and in the sediment of shallow marine waters. They show a wide variety of morphologies and appear to have evolved quite rapidly into taxa with distinct and recognisable characteristics. They are only locally abundant as fossils.


These exotic and somewhat enigmatic organisms are interpreted as being colonial groups of individuals connected by a skeletal structure. They appear to have had a planktonic habit and are widespread in Ordovician and Silurian mudrocks. Preservation is normally as a thin film of flattened organic material on the bedding planes of fine-grained sedimentary rocks. The shapes of the skeletons and the ‘teeth’ where individuals in the colony were located are distinctive when examined with a hand lens or under a microscope. Lineages have been traced which indicate rapid evolution and have allowed a high-resolution biostratigraphy to be developed for the Ordovician and Silurian systems. The main drawback in the use of graptolites is the poor preservation in coarser grained rocks such as sandstones.


Shelly, sessile organisms such as brachiopods generally make poor zone fossils but in shallow marine, high-energy environments where graptolites were not preserved, brachiopods are used for regional correlation purposes in Silurian rocks and in later Palaeozoic strata. 


This taxonomic group of cephalopods (phylum Mollusca) includes goniatites from Palaeozoic rocks as well as the more familiar ammonites of the Mesozoic. The nautiloids are the most closely related living group. The large size and free-swimming habit of these cephalopods made them an excellent group for biostratigraphic purposes. Fossils are widespread, found in many fully marine environments, and they are relatively robust. Morphological changes through time were to the external shape of the organisms and to the ‘suture line’, the relic of the bounding walls between the chambers of the coiled cephalopod. Goniatites have been used in correlation of Devonian and Carboniferous rocks, whereas ammonites and other ammonoids are the main zone fossils in Mesozoic rocks. Ammonoids became extinct at the end of the Cretaceous.


These also belong to the Mollusca and as marine ‘snails’ they are abundant as fossils in Cenozoic rocks. They are very common in the deposits of almost all shallow marine environments. Distinctive shapes and ornamentation on the calcareous shells make identification relatively straightforward and there are a wide variety of taxa within this group.


This phylum includes crinoids (sea lilies) and echinoids (sea urchins). Most crinoids probably lived attached to substrate and this sessile characteristic makes them rather poor zone fossils, despite their abundance in some Palaeozoic limestones. Echinoids are benthic, living on or in soft sediment: their relatively robust form and subtle but distinctive changes in their morphology have made them useful for regional and worldwide correlation in parts of the Cretaceous.


The extensive outcrops of shallow marine limestones in Devonian and Lower Carboniferous (Mississippian) rocks in some parts of the world contain abundant corals. This group is therefore used for zonation and correlation within these strata, despite the fact that they are not generally suitable for biostratigraphic purposes because of the very restricted depositional environments they occur in.

Marine microfossils
Microfossils are taxa that leave fossil remains that are too small to be clearly seen with the naked eye or hand lens. They are normally examined using an optical microscope although some forms can be analysed in detail only using a scanning electron microscope. The three main groups that are used in biostratigraphy are the foraminifers, radiolaria and calcareous algae (nanofossils): other microfossils used in biostratigraphy are ostracods, diatoms and conodonts.


'Forams' (the common abbreviation of foraminifers) are single-celled marine organisms that belong to the Protozoa Subkingdom. They have been found as fossils in strata as old as the Cambrian, although forms with hard calcareous shells, or ‘tests’, did not become well established until the Devonian. Calcareous forams generally became more abundant through the Phanerozoic and are abundant in many Mesozoic and Cenozoic marine strata. The calcareous tests of planktonic forams are typically a millimetre or less across, although during some periods, particularly the Paleogene, larger benthic forms also occur and can be more than a centimetre in diameter. Planktonic forams make very good zone fossils as they are abundant, widespread in marine strata and appear to have evolved rapidly. Schemes using forams for correlation in the Mesozoic and Cenozoic are widely used in the hydrocarbon industry because microfossils are readily recovered from boreholes and both regional and worldwide zonation schemes are used.


These organisms form a subclass of planktonic protozoans and are found as fossils in deep marine strata throughout the Phanerozoic. Radiolaria commonly have silica skeletons and are roughly spherical, often spiny organisms less than a millimetre across. They are important in the dating of deep-marine deposits because the skeletons survive in siliceous oozes deposited at depths below the CCD. These deposits are preserved in the stratigraphic record as radiolarian cherts and the fossil assemblages found in them typically contain large numbers of taxa making it possible to use quite high resolution biozonation schemes. Their stratigraphic range is also greater than the forams, making them important for the dating of Palaeozoic strata.

Calcareous nanofossils

Fossils that cannot be seen with the naked eye and are only just discernible using a high-power optical microscope are referred to as nanofossils. They are microns to tens of microns across and are best examined using a scanning electron microscope. The most common nanofossils are coccoliths, the spherical calcareous cysts of marine algae. Coccoliths may occur in huge quantities in some sediments and are the main constituent of some fine-grained limestones such as the Chalk of the Upper Cretaceous in northwest Europe. They are found in fine-grained marine sediments deposited on the shelf or any depths above the CCD below which they are not normally preserved. They are used biostratigraphically in Mesozoic and Cenozoic strata.

Other microfossils
Ostracods are crustaceans with a two-valve calcareous carapace and their closest relatives are crabs and lobsters. They occur in a very wide range of depositional environments, both freshwater and marine, and they have a long history, although their abundance and distribution are sporadic. Zonation using ostracods is applied only locally in both marine and non-marine environments. Diatoms are chrysophyte algae with a siliceous frustule (skeleton) that can occur in large quantities in both shallow-marine and freshwater settings. The diatom frustules are less than a millimetre across and in some lacustrine settings may make up most of the sediment, forming a diatomite deposit. They are only rarely used in biostratigraphy. Conodonts are somewhat enigmatic tooth like structures made of phosphate and they occur in Palaeozoic strata. Despite uncertainty about the origins, they are useful stratigraphic microfossils in the older Phanerozoic rocks, which generally contain few other microfossils. Acritarchs are microscopic spiny structures made of organic material that occur in Proterozoic and Palaeozoic rocks. Their occurrences in Precambrian strata make them useful as a biostratigraphic tool in rocks of this age. They are of uncertain affinity, although are probably the cysts of planktonic algae, and may therefore be related to dinoflagellates, which are primitive organisms found from the Phanerozoic through to the present day and also produce microscopic cysts (dinocysts). Zonation based on dinoflagellates is locally very important, especially in non-calcareous strata of Mesozoic and Cenozoic ages: the schemes used are generally geographically local and have limited stratigraphic ranges.

Terrestrial fossil groups used in biostratigraphy

Correlation in the deposits of continental environments is always more difficult because of the poorer preservation potential of most materials in a subaerial setting. Only the most resistant materials survive to be fossilised in most continental deposits, and these include the organo-phosphates that vertebrate teeth are made of and the coatings of pollen, spores and seeds of plants. Stratigraphic schemes have been set up using the teeth of small mammals and reptiles for correlation of continental deposits of Neogene age. Pollen, spores and seeds (collectively palynomorphs) are much more commonly used. They are made up of organic material that is highly resistant to chemical attack and can be dissolved out of siliceous sedimentary rocks using hydrofluoric acid. Airborne particles such as pollen, spores and some seeds may be widely dispersed and the occurrence of these aeolian palynomorphs within marine strata allows for correlation between marine and continental successions. However, although palynomorphs can be used as zone fossils, they rarely provide such a high resolution as marine fossils. Identification is carried out with an optical microscope or an electron microscope after the palynomorphs have been chemically separated from the host sediment using strong acids.

Biozone and zone fossil in biostratigraphy

A biostratigraphic unit is a body of rock defined by its fossil content. It is therefore fundamentally different from a lithostratigraphic unit that is defined by the lithological properties of the rock. The fundamental unit of biostratigraphy is the biozone. Biozones are units of stratigraphy that are defined by the zone fossils (usually species or subspecies) that they contain. In theory they are independent of lithology, although environmental factors often have to be taken into consideration in the definition and interpretation of biozones. In the same way that formations in lithostratigraphy must be defined from a type section, there must also be a type section designated as a stratotype and described for each biozone. They are named from the characteristic or common taxon (or occasionally taxa) that defines the biozone. There are several different ways in which biozones can be designated in terms of the zone fossils that they contain.
Interval biozones These are defined by the occurrences within a succession of one or two taxa. Where the first appearance and the disappearance of a single taxon is used as the definition, this is referred to as a taxon-range biozone. A second type is a concurrent range biozone, which uses two taxa with overlapping ranges, with the base defined by the appearance of one taxon and the top by the disappearance of the second one. A third possibility is a partial range biozone, which is based on two taxa that do not have overlapping ranges: once again, the base is defined by the appearance of one taxon and the top by the disappearance of a second. Where a taxon can be recognised as having followed another and preceding a third as part of a phyletic lineage the biozone defined by this taxon is called a lineage biozone (also called a consecutive range biozone).
Assemblage biozones In this case the biozone is defined by at least three different taxa that may or may not be related. The presence and absence, appearance and disappearance of these taxa are all used to define a stratigraphic interval. Assemblage biozones are used in instances where there are no suitable taxa to define interval biozones and they may represent shorter time periods than those based on one or two taxa. 
Acme biozones The abundance of a particular taxon may vary through time, in which case an interval containing a statistically high proportion of this taxon may be used to define a biozone. This approach can be unreliable because the relative abundance is due to local environmental factors. The ideal zone fossil would be an organism that lived in all depositional environments all overthe world and was abundant; it would have easily preserved hard parts and would be part of an evolutionary lineage that frequently developed new, distinct species. Not surprisingly, no such fossil taxon has ever existed and the choice of fossils used in biostratigraphy has been determined by a number of factors that are considered in the following sections.

Rate of speciation

The frequency with which new species evolve and replace former species in the same lineage determines the resolution that can be applied in biostratigraphy. Some organisms seem to have hardly evolved at all: the brachiopod Lingula seems to look exactly the same today as the fossils found in Lower Palaeozoic rocks and hence is of little biostratigraphic value. The groups that appear to display the highest rates of speciation are vertebrates, with mammals, reptiles and fish developing new species every 1 to 3 million years on average. However, the stratigraphic record of vertebrates is poor compared with marine molluscs, which are much more abundant as fossils, but have slower average speciation rates (around 10 million years). There are some groups that appear to have developed new forms regularly and at frequent intervals: new species of ammonites appear to have evolved every million years or so during the Jurassic and Cretaceous and in parts of the Cambrian some trilobite lineages appear to have developed new species at intervals of about a million years. By using more than one species to define them, biozones can commonly be established for time periods of about a million years, with higher resolution possible in certain parts of the stratigraphic record, especially in younger strata.

Depositional environment controls

The conditions vary so much between different depositional environments that no single species, genus or family can be expected to live in all of them. The adaptations required to live in a desert compared with a swamp, or a sandy coastline compared with a deep ocean, demand that the organisms that live in these environments are different. There is a strong environmental control on the distribution of taxa today and it is reasonable to assume that the nature of the environment strongly influenced the distribution of fossil groups as well. Some environments are more favourable to the preservation of body fossils than others: for example, preservation potential is lower on a high-energy beach than in a low-energy lagoon. There is a fundamental problem with correlation between continental and marine environments because very few animals or plants are found in both settings. In the marine environment the most widespread organisms are those that are planktonic (free floating) or animals that are nektonic (free-swimming lifestyle). Those that live on the sea bed, the benthonic or benthic creatures and plants, are normally found only in a certain water depth range and are hence not quite so useful. The rates of sedimentation in different depositional environments are also a factor in the preservation and distribution of stratigraphically useful fossils. Slow sedimentation rates commonly result in poor preservation because the remains of organism are left exposed on the land surface or sea floor where they are subject to biogenic degradation. On the other hand, with a slower rate of accumulation in a setting where organic material has a higher chance of preservation (e.g. in an anoxic environment), the higher concentration of fossils resulting from the reduced sediment supply can make the collecting of biostratigraphically useful material easier. It is also more likely that a first or last appearance datum will be identifiable in a single outcrop section because if sediment accumulation rates are high, hundreds of metres of strata may lie within a single biozone.

Mobility of organisms

The lifestyle of an organism not only determines its distribution in depositional environments, it also affects the rate at which an organism migrates from one area to another. If a new species evolves in one geographical location its value as a zone fossil in a regional or worldwide sense will depend on how quickly it migrates to occupy ecological niches elsewhere. Again, planktonic and nektonic organisms tend to be most useful in biostratigraphy because they move around relatively quickly. Some benthic organisms have a larval stage that is free-swimming and may therefore be spread around oceans relatively quickly. Organisms that do not move much (a sessile lifestyle) generally make poor fossils for biostratigraphic purposes.

Geographical distribution of organisms

Two environments may be almost identical in terms of physical conditions but if they are on opposite sides of the world they may be inhabited by quite different sets of animals and plants. The contrasts are greatest in continental environments where geographical isolation of communities due to tectonic plate movements has resulted in quite different families and orders. The mammal fauna of Australia are a striking example of geographical isolation resulting in the evolution of a group of animals that are quite distinct from animals living in similar environments in Europe or Asia. This geographical isolation of groups of organisms is called provincialism and it also occurs in marine organisms, particularly benthic forms, which cannot easily travel across oceans. Present or past oceans have been sufficiently separate to develop localised communities even though the depositional environments may have been similar. This faunal provincialism makes it necessary to develop different biostratigraphic schemes in different parts of the world.

Abundance and size of fossils

To be useful as a zone fossil a species must be sufficiently abundant to be found readily in sedimentary rocks. It must be possible for the geologist to be able to find representatives of the appropriate taxon without having to spend an undue amount of time looking. There is also a play-off between size and abundance. In general, smaller organisms are more numerous and hence the fossils of small organisms tend to be the most abundant. The problem with very small fossils is that they may be difficult to find and identify. The need for biostratigraphic schemes to be applicable to subsurface data from boreholes has led to an increased use of microfossils, fossils that are too small to be recognised in hand specimen, but which may be abundant and readily identified under the microscope (or electron microscope in some cases). Schemes based on microfossils have been developed in parallel to macrofossil schemes. Although a scheme based on ammonites may work very well in the field, the chances of finding a whole ammonite in the core of a borehole are remote. Microfossils are the only viable material for use in biostratigraphy where drilling does not recover core but only brings up pieces of the lithologies in the drilling mud.

Preservation potential

It is impossible to determine how many species or individuals have lived on Earth through geological time because very few are ever preserved as fossils. The fossil record represents a very small fraction of the biological history of the planet for a variety of reasons. First, some organisms do not possess the hard parts that can survive burial in sediments: we therefore have no idea how many types of worm may have existed in the past. Sites where there is exceptional preservation of the soft parts of fossils (lagerstatten) provide tantalising clues to the diversity of lifeforms that we know next to nothing about. Second, the depositional environment may not be favourable to the preservation of remains: only the most resistant pieces of bone survive in the dry, oxidising setting of deserts and almost all other material is destroyed. All organisms are part of a food chain and this means that their bodies are normally consumed, either by a predator or a scavenger. Preservation is therefore the exception for most animals and plants. Finally, the stratigraphic record is very incomplete, with only a fraction of the environmental niches that have existed preserved in sedimentary rocks. The low preservation potential severely limits the material available for biostratigraphic purposes, restricting it to those taxa that had hard parts and existed in appropriate depositional environments.


Paleontology is the study of fossils. Fossils are any remains of the organism lived and preserved on this Earth by millions of years ago. Any organism from large to micro level are the part of paleontology. The history of earth is preserved in these fossils. Our generation haven't seen any dinosaurs but it is the fact lived million years ago. These fossils are the key to study earth from its beginning. There were many creatures lived on this earth which are extinct but their fossils preserved and discovered reveals the history. These remains are studied under paleontology
Types of paleontology includes vertebrates and invertebrates

Vertebrates paleontology

Vertebrates are organism that have backbone. Backbone presence shows that organism have skeletal remains. Skeleton is the hard part of an organism which are preserved through years and are now studied. As the soft parts of organism are decayed through time but hard skeleton preserved are discovered.


There are many organism which do not have backbone hence do not have skeleton in them but there are hard parts of an organism which are preserved. Shell remains which are made of mostly silicates and calcium are preserved. The remaining body can be assumed and studied.

This study helps in examining and interpreting creature habitat, body functions and food consumption.