Correlation of Strata
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
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CORRESPONDING ROCK DIVISIONS
(AND ROCK UNITS)
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Eon
Examples: Precambrian/Phanerozoic
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Eonothem
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Era
Examples: Paleozoic/Cenozoic/Mesozoic
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Erathem
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Period
Examples: Cambrian/Ordovician/Silurian
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System
Groups
Formations (The main stratigraphic unit)
Members
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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).
A 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?
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:
ENVIRONMENT OF DEPOSITION
|
EXPECTED LITHOLOGY
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Near shore marine- The energy is high due to rough waters at the water-land interface.
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Coarse sediments, and fossils of robust organisms that can withstand high energy environments.
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Deep ocean- The energy is low due to the general calmness of water away from land.
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Fine sediments, and fossils of more fragile organisms.
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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.
A sedimentary facies is a three-dimensional body of sediment (or rock) that contains lithologies representative of a particular depositional environment. For example,
FACIES
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LITHOLOGIES
|
Floodplain
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Mudstone and shale with interbedded sandstone.
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Ocean basin
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Laminated pelagic clays, cherts, and possible limestone.
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Delta
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Well-sorted, well-rounded, and possibly cross-bedded sandstone.
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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.
A transgression is an advance of the sea over land.
A 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.
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