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 facies. Show all posts
Showing posts with label facies. Show all posts

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STRATIGRAPHIC CORRELATION, FOSSILS, FACIES, AND SEA LEVEL CHANGE

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:

TIME DIVISIONS
CORRESPONDING ROCK DIVISIONS
(AND ROCK UNITS)

Eon
Examples: Precambrian/Phanerozoic


Eonothem

          Era
          Examples: Paleozoic/Cenozoic/Mesozoic


          Erathem

               Period
               Examples: Cambrian/Ordovician/Silurian

               System
              Groups
                    Formations (The main stratigraphic unit)
                         Members


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).
File:Coprolite.jpg
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:


ENVIRONMENT OF DEPOSITION


EXPECTED LITHOLOGY


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,


FACIES

LITHOLOGIES


Floodplain


Mudstone and shale with interbedded sandstone.

Ocean basin


Laminated pelagic clays, cherts, and possible limestone.

Delta


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.

Interpreting past depositional environments

Sediments accumulate in a wide range of settings that can be defined in terms of their geomorphology, such as rivers, lakes, coasts, shallow seas, and so on. The physical, chemical and biological processes that shape and characterise those environments are well known through studies of physical geography and ecology. Those same processes determine the character of the sediment deposited in these settings. A fundamental part of sedimentology is the interpretation of sedimentary rocks in terms of the transport and depositional processes and then determining the environment in which they were deposited. In doing so a sedimentologist attempts to establish the conditions on the surface of the Earth at different times in different places and hence build up a picture of the history of the surface of the planet.



The concept of facies
The term facies is widely used in geology, particularly in the study of sedimentology in which sedimentary facies refers to the sum of the characteristics of a sedimentary unit. These characteristics include the dimensions, sedimentary structures, grain sizes and types, colour and biogenic content of the sedimentary rock. An example would be crossbedded medium sandstone: this would be a rock consisting mainly of sand grains of medium grade, exhibiting cross-bedding as the primary sedimentary structure. Not all aspects of the rock are necessarily indicated in the facies name and in other instances it may be important to emphasise different characteristics. In other situations the facies name for a very similar rock might be red, micaceous sandstone if the colour and grain types were considered to be more important than the grain size and sedimentary structures. The full range of the characteristics of a rock would be given in the facies description that would form part of any study of sedimentary rocks. If the description is confined to the physical and chemical characteristics of a rock this is referred to as the lithofacies. In cases where the observations concentrate on the fauna and flora present, this is termed a biofacies description, and a study that focuses on the trace fossils in the rock would be a description of the ichnofacies. As an example a single rock unit may be described in terms of its lithofacies as a grey bioclastic packstone, as having a biofacies of echinoid and crinoids and with a Cruziana ichnofacies: the sum of these and other characteristics would constitute the sedimentary facies.

Facies analysis
The facies concept is not just a convenient means of describing rocks and grouping sedimentary rocks seen in the field, it also forms the basis for facies analysis, a rigorous, scientific approach to the interpretation of strata. The lithofacies characteristics are determined by the physical and chemical processes of transport and deposition of the sediments and the biofacies and ichnofacies provide information about the palaeoecology during and after deposition. By interpreting the sediment in terms of the physical, chemical and ecological conditions at the time of deposition it becomes possible to reconstruct palaeoenvironments, i.e. environments of the past. The reconstruction of past sedimentary environments through facies analysis can sometimes be a very simple exercise, but on other occasions it may require a complex consideration of many factors before a tentative deduction can be made. It is a straight forward process where the rock has characteristics that are unique to a particular environment. Hermatypic corals have only ever grown in shallow, clear and fairly warm seawater: the presence of these fossil corals in life position in a sedimentary rock may therefore be used to indicate that the sediments were deposited in shallow, clear, warm, seawater. The analysis is more complicated if the sediments are the products of processes that can occur in a range of settings. For example, crossbedded sandstone can form during deposition in deserts, in rivers, deltas, lakes, beaches and shallow seas: a cross-bedded sandstone lithofacies would therefore not provide us with an indicator of a specific environment. Interpretation of facies should be objective and based only on the recognition of the processes that formed the beds. So, from the presence of symmetrical ripple structures in a fine sandstone it can be deduced that the bed was formed under shallow water with wind over the surface of the water creating waves that stirred the sand to form symmetrical wave ripples. The shallow water interpretation is made because wave ripples do not form in deep water but the presence of ripples alone does not indicate whether the water was in a lake, lagoon or shallow-marine shelf environment. The facies should therefore be referred to as symmetrically rippled sandstone or perhaps wave rippled sandstone, but not lacustrine sandstone because further information is required before that interpretation can be made.

Facies associations
The characteristics of an environment are determined by the combination of processes which occur there. A lagoon, for example, is an area of low energy, shallow water with periodic influxes of sand from the sea, and is a specific ecological niche where only certain organisms live due to enhanced or reduced salinity. The facies produced by these processes will be muds deposited from standing water, sands with wave ripples formed by wind over shallow water and a biofacies of restricted fauna. These different facies form a facies association that reflects the depositional environment. When a succession of beds are analysed in this way, it is usually evident that there are patterns in the distribution of facies. For example, beds of the bioturbated mudstone occur more commonly with (above or below) the laminated siltstone or the wave rippled medium sandstone? Which of these three occurs with the coal facies? When attempting to establish associations of facies it is useful to bear in mind the processes of formation of each. Of the four examples of facies just mentioned the bioturbated mudstone and the wave rippled medium sandstone both probably represent deposition in a subaqueous, possibly marine, environment whereas medium sandstone with rootlets and coal would both have formed in a subaerial setting. Two facies associations may therefore be established if, as would be expected, the pair of subaqueously deposited facies tend to occur together, as do the pair of subaerially formed facies. The procedure of facies analysis therefore can be thought of as a two-stage process. First, there is the recognition of facies that can be interpreted in terms of processes. Second, the facies are grouped into facies associations that reflect combinations of processes and therefore environments of deposition. The temporal and spatial relationships between depositional facies as observed in the present day and recorded in sedimentary rocks were recognised by Walther. Walther’s Law can be simply summarised as stating that if one facies is found superimposed on another without a break in a stratigraphic succession those two facies would have been deposited adjacent to each other at any one time. This means that sandstone beds formed in a desert by aeolian dunes might be expected to be found over or under layers of evaporates deposited in an ephemeral desert lake because these deposits may be found adjacent to each other in a desert environment. However, it would be surprising to find sandstones formed in a desert setting overlain by mudstones deposited in deep seas: if such is found, it would indicate that there was a break in the stratigraphic succession, i.e. an unconformity representing a period of time when erosion occurred and/or sea level changed.

Facies sequences/successions
A facies sequence or facies succession is a facies association in which the facies occur in a particular order. They occur when there is a repetition of a series of processes as a response to regular changes in conditions. If, for example, a bioclastic wackestone facies is always overlain by a bioclastic packstone facies, which is in turn always overlain by a bioclastic grainstone, these three facies may be considered to be a facies sequence. Such a pattern may result from repeated shallowing-up due to deposition on shoals of bioclastic sands and muds in a shallow marine environment. Recognition of patterns of facies can be on the basis of visual inspection of graphic sedimentary logs or by using a statistical approach to determining the order in which facies occur in a succession, such as a Markov analysis. This technique requires a transition grid to be set up with all the facies along both the horizontal and vertical axis of a table: each time a transition occurs from one facies to another (e.g. from bioclastic wackestone to bioclastic packstone facies) in a vertical succession this is entered on to the grid. Facies sequences/sucessions show up as higher than average transitions from one facies to another.

Facies names and facies codes
Once facies have been defined then they are given a name. There are no rules for naming facies, but it makessense touse namesthatare more-or-lessdescriptive, such as bioturbated mudstone, trough crossbedded sandstone or foraminiferal wackestone. This is preferable to Facies A, Facies B, Facies C, and so on, because these letters provide no clue as to the nature of the facies. A compromise has to be reached between having a name that adequately describes the facies but which is not too cumbersome. A general rule would be to provide sufficient adjectives to distinguish the facies from each other but no more. For example, mudstone facies is perfectly adequate if only one mudrock facies is recognised in the succession. On the other hand, the distinction between trough crossbedded coarse sandstone facies and planar crossbedded medium sandstone facies may be important in the analysis of successions of shallow marine sandstone. Facies schemes are therefore variable, with definitions and names depending on the circumstances demanded by the rocks being examined. The names for facies should normally be purely descriptive but it is quite acceptable to refer to facies associations in terms of the interpreted environment of deposition. An association of facies such as symmetrically rippled fine sandstone, black laminated mudstone and grey graded siltstone may have been interpreted as having been deposited in a lake on the basis of the facies characteristics, and perhaps some biofacies information indicating that the fauna are freshwater. This association of facies may therefore be referred to as a lacustrine facies association and be distinguished from other continental facies associations deposited in river channels (fluvial channel facies association) and as overbank deposits (floodplain facies association). It can be convenient to have shortened versions of the facies names, for example for annotating sedimentary logs. Miall suggested a scheme of letter codes for fluvial sediments that can be adapted for any type of deposit. In this scheme the first letter indicates the grain size (S for sand, G for gravel, for example), and one or two suffix letters to reflect other features such as sedimentary structures: Sxl is cross laminated sandstone, for example. There are no rules for the code letters used, and there are many variants on this theme (some workers use the letter ‘Z’ for silts, for example) including similar schemes for carbonate rocks based on the Dunham classification. As a general guideline it is best to develop a system that is consistent, with all sandstone facies starting with the letter ‘S’ for example, and which uses abbreviations that can be readily interpreted. There is an additional graphical scheme for displaying facies on sedimentary logs: columns alongside the log are used for each facies to indicate their vertical extent. An advantage of this form of presentation is that if the order of the columns is chosen carefully, for example with more shallow marine to the left and deeper marine on the right for shelf environments, trends through time can be identified on the logs.