Principles of Stratigraphy | Geology Optional for UPSC PDF Download

Introduction

• The Earth's surface is made up of rocks and soil. Most of these rocks were formed a long time ago when sediments carried by rivers and streams were deposited in oceans and other water bodies, like rivers and lakes. This process has been happening for millions of years. Over time, the soft sediment became hard rock in layers through a process called consolidation. These layered hard rocks are known as sedimentary rocks, and they make up most of the Earth's rock cover.
• Sometimes, these sedimentary rocks can be pushed down deep into the Earth where they are subjected to high pressure and temperature, causing them to change in composition and structure. When this happens, they become metamorphic rocks. Additionally, igneous rocks, which are formed from molten rock material called magma, can intrude into or be extruded from metamorphic rocks.
• Stratigraphy is the study of these stratified rocks, focusing on their geometric relations, compositions, origins, and age relationships. It helps us understand the Earth's history by interpreting how rocks were formed, the type of sediment involved, and the kind of basin where deposition occurred. By understanding the basic principles of stratigraphy, we can trace the historical evolution of the Earth's surface over time.
• In this unit, we will explore the concepts, historical development, and basic principles of stratigraphy. We will also discuss stratigraphic contacts and unconformities, techniques for collecting stratigraphic data, and the importance of stratigraphy. Once you grasp these principles, you will know how to study rocks and collect them to obtain information about their formation history.

Learning Objectives

Upon completing this unit, you will be able to:

• Define stratigraphy.
• Describe the concepts and historical development of stratigraphy.
• Discuss the basic principles of stratigraphy.
• Identify stratigraphic contacts and unconformities.
• Elaborate on breaks in the stratigraphic record.
• List methods of stratigraphic data collection.
• State the significance of the geological time scale.
• Explain the importance of stratigraphy.

Introduction to Stratigraphy

• Stratigraphy is the study of layered rocks (stratified rocks) that are deposited one on top of the other. This study involves classifying, interpreting, and correlating these rocks in terms of space and time, which helps us understand the geological history of Earth. The term "Stratigraphy" was coined by the French geologist d'Orbigny in 1849. It comes from the Latin word "Stratum" and the Greek word "Graphia," referring to the description of all rock bodies in the Earth's crust and their organization into distinct, useful, and mappable units.
• Stratigraphy plays a crucial role in geology as it provides insights into the Earth's past, the processes that shaped it, and the timing of various geological events. By studying stratified rocks, geologists can reconstruct ancient environments, understand the evolution of life, and make inferences about natural resources and geological hazards.

Concept and Definition

• Stratigraphy is a field within geology that focuses on studying rock layers (strata), their sequence, and how they relate to one another. It provides fundamental principles for interpreting geological events, which is why it's also called historical geology.
• While stratigraphic studies mainly involve sedimentary rocks, they can also include layered igneous rocks like lava flows. Additionally, it looks at how intrusive igneous rocks relate to sediments.
• Stratified rocks serve as a record of past geological events, and stratigraphy aims to interpret these events by studying sediments, sedimentary rocks, and fossils within the rocks. The nature and type of rocks, known as lithology, is a crucial aspect of stratigraphic studies (lithostratigraphy). Another important aspect is the study of fossils in the rock sequence (biostratigraphy), which helps establish a chronological sequence of stratigraphic events (chronostratigraphy).

Historical Development

Stratigraphy has evolved through various discoveries and observations over time. Nicholas Steno, considered the father of stratigraphy, laid the groundwork by proposing the law of superposition and the principles of original horizontality and lateral continuity in 1669. James Hutton later advanced the field with the theory of uniformitarianism, which became a fundamental principle in stratigraphic studies.

Following Steno and Hutton, other notable figures contributed to stratigraphy:

• John Playfair popularized Hutton’s principle of uniformitarianism.
• William Smith introduced the concept of layering in sedimentary rocks and coined the term “strata,” giving a name to the science of stratigraphy. He also created the first geological map of Great Britain.
• Alexandre Brongniart in France elaborated on the concept of layering in sedimentary rocks and the significance of different fossils for dating strata.
• Charles Lyell further developed the idea of uniformitarianism and emphasized the importance of stratigraphic sequence in his book “Principles of Geology.”

In recent years, stratigraphy has advanced with the introduction of radiometric and magnetostratigraphic dating techniques, as well as the use of chemical indicators and isotopes. The International Union of Geological Sciences (IUGS) has established an International Subcommission on Stratigraphic Classification to ensure a uniform approach in stratigraphic studies, adapting principles and applications based on new findings.

Basic Principles of Stratigraphy

Stratigraphy is founded on principles that govern sedimentation processes. Sedimentation involves the accumulation of rock material transported by agents like water, wind, and glaciers, and deposited in a basin. The mechanisms of sedimentation and the accumulation of other stratified rocks, such as lava flows, underpin the principles of stratigraphic studies. There are eight fundamental principles or laws of stratigraphy:

• Order of superposition
• Original horizontality
• Lateral continuity
• Cross-cutting relationships
• Inclusions
• Unconformities
• Fossil succession
• Uniformitarianism and catastrophism

Let's briefly discuss each principle in the order listed above.

Order of Superposition

• When sediments are deposited in a basin, they accumulate layer by layer, with the bottom-most layer being the first to deposit. This process continues as more sediments are added in subsequent layers. Therefore, in a sedimentary sequence, the layers at the bottom are the oldest, and the layers above them are younger.
• This principle, known as the order of superposition, is fundamental to stratigraphy and allows for the determination of relative ages of sedimentary layers. As one moves up a sedimentary sequence, the layers become progressively younger.

Original Horizontality

• Sedimentary rocks are often found in various orientations, such as tilted, dipped at different angles, or even folded. However, it is important to note that they could not have been deposited in these conditions. Sediments in a basin are always deposited horizontally, regardless of the basin's shape. The tilting and folding of these rocks occur after they have been deposited and consolidated.
• Therefore, when studying these rocks, it is essential to envision their original state as being horizontal at the time of deposition. This concept is known as the principle of original horizontality.

Lateral Continuity

• Sediments in a basin are initially spread in all directions during deposition, but they exhibit lateral continuity when traced.
• This means that even if sediments are not visible in a valley due to erosion, they can be found across the valley, indicating that they were originally deposited continuously.
• The lateral continuity extends to the limit of the basin, given sufficient sediment availability.

Cross-Cutting Relationships

• The principle of cross-cutting relationships states that any feature that cuts across a sedimentary sequence is younger than that sequence.
• In other words, all cross-cutting features, such as faults, igneous intrusions like dykes, or erosional features like valleys, occur after the sediment has been deposited.
• This principle helps geologists determine the relative ages of different geological features and understand the sequence of events in the Earth's history.

Inclusions

• Inclusions in sedimentary rocks refer to clasts or fragments of older rocks that are carried by water or other mediums and deposited in a basin.
• These clasts, which can range in size from fine silts to gravel fragments, get consolidated into a rock after deposition.
• In lava flows, inclusions can also occur, and they are known as xenoliths.
• The principle of inclusions states that these fragments are always older than the rock in which they are found, as they are derived from the denudation of pre-existing rocks.

Unconformities

• An unconformity occurs when there is a break in sedimentation during the deposition of sediments, leading to a sedimentary sequence.
• This break can happen due to various reasons such as the unavailability of sediment, filling up of the basin, or uplift of the basin, making it impossible for sedimentation to occur.
• The duration of the break can be short or long.
• In case of a brief break, it might be challenging to identify the unconformity.
• However, in long breaks, unconformities can be recognized by signs of erosion or changes in the angle of inclination of the beds.

Fossil Succession

• Fossils are the remains of past organisms found in most sedimentary rocks. However, the types and species of organisms change over time, leading to differences in fossils present in different rock layers, or strata. This variation in fossils allows geologists to differentiate between beds at various levels.
• By examining the types of fossils in a rock bed, scientists can classify the sequence of rocks and establish the order of their deposition over time. Fossils serve as important indicators for determining the relative ages of rock layers and the succession of life forms throughout geological history.

Uniformitarianism and Catastrophism

• Uniformitarianism is the idea that the processes we see happening on Earth today, like sedimentation and erosion, were the same in the past and have been shaping the Earth over a long period of time. This concept, proposed by James Hutton, suggests that by studying present-day processes, we can understand how the Earth evolved in the past.
• However, there have also been sudden and dramatic changes in Earth’s history due to catastrophic events. So, Earth’s history is a mix of long periods of gradual change (uniformitarianism) and short periods of sudden change (catastrophism). While both processes are part of Earth’s history, major changes are often the result of catastrophism.

Stratigraphic Contacts and Unconformities

Stratigraphic contacts refer to the boundaries between different rock units, indicating where one type of rock ends and another begins. There are various types of stratigraphic contacts, including:

(i) Vertical Depositional Contacts: These occur when beds are deposited one on top of the other. Vertical contacts can be conformable or unconformable:

• Conformable Contacts: These indicate continuous deposition without any perceptible breaks, even if the sequence is tilted or folded.
• Abrupt Contacts: These occur when two different types of rocks, such as sandstone and limestone, are in contact along a fixed line.
• Gradational Contacts: These show a gradual change from one rock type to another over a considerable thickness, such as sandstone gradually becoming limestone due to increasing calcium carbonate content.

(ii) Lateral Depositional Contacts: These occur when strata extend laterally, with varying thicknesses and lithologies. Lateral contacts are typically gradational, showing gradual changes in rock types.

(iii) Intrusive Contacts: These occur when igneous rocks intrude sedimentary rocks, such as sills, dykes, or batholiths:

• Sills: Intrusions that are parallel to the bedding plane and concordant with the sedimentary sequence.
• Dykes: Intrusions that cut across the sedimentary sequence and are discordant with the bedding.

(iv) Fault Contacts: These occur when a sedimentary sequence is faulted after deposition and consolidation, disrupting the sequence along a plane that cuts across the bedding. Fault contacts bring rocks of different kinds and ages into contact along a discordant plane.

Unconformities represent gaps in the geological record where deposition was interrupted, often due to erosion or non-deposition. These can be identified through various features, such as changes in rock type, orientation, or the presence of intrusive or fault contacts.

Unconformities and their Identification

Unconformities are breaks in the geologic record that indicate a period of non-deposition or erosion between two layers of sediment. There are several types of unconformities, each with distinct characteristics.

1. Angular Unconformity:

• Definition: An angular unconformity occurs when two sets of strata are at an angle to each other, rather than being parallel.
• Formation: This type of unconformity forms when older rocks are tilted and deformed during a period of uplift and non-deposition, followed by the deposition of younger sediments over the tilted beds.
• Identification: Angular unconformities are easily recognizable in the field due to the angular relationship between the two sets of strata.

2. Nonconformity:

• Definition:. nonconformity occurs when an igneous or metamorphic rock is overlain by a sedimentary rock, or vice versa.
• Contact Surface: The contact surface in a nonconformity is an erosional surface that may represent a long period of hiatus.
• Recognition: Nonconformities are easily recognized because the two sets of rocks are fundamentally different.

3. Disconformity:

• Definition:. disconformity is characterized by an erosional surface where there is no angular difference between the underlying and overlying beds.
• Erosion Indicators: Erosion in a disconformity may be indicated by the presence of a pebble or boulder bed, or the development of a palaeosol (layer of old soil).
• Recognition Challenges: Disconformities are more challenging to recognize because clear indications of a break in deposition are rare. They can be identified by comparing the underlying and overlying beds in terms of rock type or fossil content.

4. Paraconformity:

• Definition: Paraconformity occurs when the bedding planes of the lower and upper sets of rocks are parallel, with no apparent evidence of a break or erosion indicating a period of non-deposition.
• Recognition Difficulty: Paraconformities are difficult to recognize because they mark a period of non-deposition without leaving any evident traces.
• Identification Method: The primary way to identify a paraconformity is by examining the fossil content in the two strata, which may indicate the duration of non-deposition.

How to Collect Stratigraphic Data?

To comprehend the history and characteristics of a rock formation, it is essential to gather stratigraphic data systematically. This helps in understanding how the rocks were deposited and the changes that occurred after their deposition. Additionally, it provides insights into the nature of the basin where the deposition took place. Let us discuss the equipment and techniques required for the collection of data.

Equipment Needed

Collecting stratigraphic data in the field requires some basic equipment. The necessary tools include:

• Field Diary: To record observations and data systematically.
• Measuring Tape: For measuring distances and dimensions accurately.
• Hammer and Chisels: To collect rock samples and make necessary excavations.
• Brunton Compass or Clinometer Compass: For measuring geological features and orientations.
• Pencils: For making notes and sketches.
• Haversack: To carry equipment and collected samples.
• Hand Lens: For examining rock samples and features closely.

It is also advisable to carry a topographical map of the area, if available, to aid in navigation and understanding the geological context.

Recording the Attitude of the Beds

The attitude of the beds refers to the angle and direction at which the rock layers are inclined. This is an important aspect to record as it helps in understanding the geological history and structure of the area.

Recording the Attitude of the Beds

• Start the study and data collection from a chosen point on a topographical or base map.
• Record the dip and strike of the beds using a Brunton compass or a clinometer compass.
• If the beds are horizontal, look for a ravine, gorge, or valley to observe the sequence of beds.
• If the beds are inclined, data collection can be done in flat terrain.
• In dipping terrain, record the direction and amount of the true dip.

Choosing a Traverse

• Select a traverse for studying the sequence along or across the true dip.
• Avoid traversing along the strike of the beds, as it follows the same bed.
• When moving along the true dip, remember that older beds dip towards younger beds, following the natural stratigraphic order.
• If traversing across the dip, record the sequence from younger to older beds accordingly.

Measuring Thickness

• Measure the thickness of individual beds using a measuring tape at a right angle to the bedding plane for true thickness.
• In dipping strata of flat or sloping terrain, calculate true thickness from apparent thickness using the sine formula.
• True thickness is the perpendicular distance between lower and upper bedding planes, while apparent thickness is measured at an oblique angle to the bedding plane.

Recording Lithological Characters

• Systematically record the lithological characters of each measured unit in the field diary.
• Include details such as rock type (e.g., limestone, sandstone, shale, conglomerate), bedding nature (thin bedded, thick bedded), sedimentary structures (cross-bedding, ripple marks), and other features like color and grain size.

Recording and Collecting Fossil Data

• While examining individual rock units, look for fossils present in the rocks, both large fossils visible to the naked eye and smaller ones requiring a hand lens.
• Carefully extract different types of fossils using a hammer and chisel.
• If rocks do not show surface fossils, collect rock samples for laboratory analysis to extract and study microfossils.
• Properly label rock samples to indicate the specific bed from which each sample was collected.

Preparing a Stratigraphic Column

• The main goal of studying and collecting data along a traverse is to create a stratigraphic column.
• A stratigraphic column represents the interpreted sequence of rock layers as they were originally deposited, and it is made to scale based on the thickness of individual beds.
• Different rock types are indicated using a specific code of hachuring. For instance:
  • Limestone is represented by brickwork patterns.
  • Sandstone is shown with dot patterns.
  • Siltstone is indicated by horizontal broken lines and dots.
• This coding helps in visually distinguishing between different rock types in the stratigraphic column.

Stratigraphic Correlation

Stratigraphic correlation involves comparing the rock types from different traverses and matching them based on their characteristics. Several traverses are conducted in an area to collect stratigraphic data, and a stratigraphic column is prepared for each traverse. Once multiple columns are ready, the next step is to correlate them by identifying individual units based on their lithology, fossil content, or both. The thickness of each unit may vary across different stratigraphic columns, contributing to the overall understanding of the basin's stratigraphic setting.

Importance of Stratigraphy

• Stratigraphy is a fundamental branch of geology that reveals the Earth's history over time, from its formation to the present day. It provides insights into how land and sea were distributed at different periods and offers data on the evolution and diversification of organisms throughout history. While stratigraphic studies mainly focus on sedimentary rocks, they also include layered igneous rocks such as lava flows, metamorphic rocks, and intrusive igneous rocks.
• Stratigraphic research enables the classification of rocks into mappable units with temporal control, forming the basis for understanding the Earth's history and evolution. Economically, stratigraphy is crucial for locating oil and gas reserves, as these resources are typically found in stratified sedimentary rocks. The application of stratigraphic concepts has significantly aided the identification of petroleum reservoir traps.
• Beyond its role in studying the Earth's history, stratigraphy is also utilized in archaeology, where the law of superposition helps identify different stages of human cultural evolution. By understanding the distribution of land and sea and other geographical information at various points in time, stratigraphy, along with fossil evidence, provides valuable data on past climates and environments.

Geological Time Scale

• In stratigraphy, the vertical sequence of rock layers is indicative of the passage of time, with older beds being overlain by younger ones. Based on fossils, geological time is classified into units that are grouped into larger units, similar to how we classify time into seconds, minutes, hours, and days. However, instead of using absolute time in thousands or millions of years, relative time is used based on changes in fossil content.
• A geologic time scale has been prepared with the smallest time unit being an age. Multiple ages constitute an epoch, several epochs make up a period, and many periods together form an era. The largest unit, an eon, comprises multiple eras. The divisions used for geological classification of time, in descending order, are Eon, Era, Period, Epoch, and Age.

Activity

Draw diagrams showing the abrupt and gradational types of contacts in geology.

Arrange the following eons and eras in ascending order:

• Phanerozoic eon, Archaean eon, and Proterozoic eon.
• Mesozoic era, Palaeozoic era, and Cenozoic era.

Summary

• Stratigraphy is a field within Geology that examines the sequence of rocks concerning the time of their deposition.
• The focus is mainly on stratified rocks, including sedimentary rocks and lava flows, but it also involves the study of metamorphic and other igneous rocks concerning the time of their formation.
• Stratigraphic studies are based on several principles, such as the order of superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, unconformities, fossil succession, uniformitarianism, and catastrophism.
• Normal contacts occur in stratified rocks that are continuous both vertically and laterally. However, there can be breaks in deposition, leading to unconformable contacts. Contacts may also be faulted, or an igneous rock may create an intrusive contact.
• Unconformities can take various forms, including angular unconformity, non-conformity, disconformity, paraconformity, or diastem.
• Stratigraphic studies can be conducted with simple equipment by taking traverses, recording attitudes, measuring sections, collecting fossils, and preparing a stratigraphic column.
• Stratigraphy aids in uncovering the history of the Earth and its organisms over time and assists in the location and exploration of minerals, particularly fossil fuels.
• Time in stratigraphy is divided into eons, eras, periods, epochs, and ages based on significant events, especially in the evolution of organisms as reflected in the fossil record. A standard scale is prepared and periodically modified, and the absolute age of some rocks can be determined using radiometric methods.