Sedimentary Facies and Provenance | Geology Optional Notes for UPSC PDF Download

Summary of Sedimentary Environments

• Definition of Sedimentary Environment

  An area on the Earth's surface where sediments are deposited and can be physically, chemically, and biologically separated from adjacent terrains.

• Types of Sedimentary Environments

  • Erosional Environments

    Areas where sediments are eroded away and are not likely to be preserved in the stratigraphic record.

  • Depositional Environments

    Areas where sediments are deposited and preserved in the stratigraphic record.

  • Equilibrial Environments

    Non-depositional environments that occur on land and sea, where transportation is the dominant mechanism.

• Identification of Ancient Sedimentary Environments

  The best way to identify ancient sedimentary environments is by relating them to their modern counterparts. By comparing the sedimentary products in rocks from ancient environments with those in modern environments, we can extrapolate the characteristics of ancient sedimentary environments.

• Challenges in Reconstruction

  Reconstructing ancient sedimentary environments can be challenging due to inconsistencies and variations in depositional associations over time.

Major Types of Criteria in Recognizing Sedimentary Environments

• Recognition of sedimentary environments involves analyzing physical, chemical, and biological characteristics of rocks.
• Physical characteristics include bedding features, formation contacts, sedimentary structures, and directional properties.
• Chemical analysis focuses on rock composition, major mineral constituents, and authigenic minerals.
• Evidence of solution or nodule accumulation provides additional insights.
• Organic materials, such as floral and faunal assemblages, are crucial for environmental interpretation.

Ancient Sedimentary Environments

• These environments are characterized by genetically related successions of sedimentary rocks.
• Deposits accumulate under specific environmental conditions, recording environmental changes over time and space.
• Primary physical structures in rocks do not solely determine the depositional environment.
• Unique sedimentary structures are specific to certain environments, while others can form in various settings.

Primary Structures and Depositional Facies

• Primary structures reveal the mechanisms of rock formation and relate to processes within specific environments.
• Sedimentary rocks are categorized into depositional facies based on primary structures.
• Depositional facies represent the outcomes of processes in a particular environment.
• Understanding the association and distribution of structures over time aids in environmental interpretation.

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Facies Analysis

• Facies
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  • Facies is a term coined by Gressly in 1838, derived from the Latin word for 'aspect' or 'appearance'.
  • According to Reading (1986), facies can be defined as "a body of rock with specified characteristics."
  • The most common definition of facies, as per Walker (1993), describes it as "A body of rock characterized by a particular combination of lithology, physical and biological structures that distinguish it from surrounding rocks."
  • Sedimentary facies are always unique, with no two being identical.
  • The term "facies" is used in various contexts, such as observational (sandstone facies), genetic (e.g., turbidite facies), environmental (e.g., fluvial facies), and tectofacies (postorogenic facies).
  • Genetic facies are particularly informative as they reveal formative mechanisms, while descriptive facies provide detailed characteristics.
  • Classical sedimentary facies are defined based on sedimentary structures, lithology, texture, and body geometries present in the rock unit.
  • Facies descriptions should emphasize the distinctiveness of each facies within the association, while subfacies account for minor variations.
  • For environmental interpretation, a single facies is insufficient, necessitating the study of facies groups, their temporal distribution, and spatial arrangement to understand the depositional environment thoroughly.

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Paraphrasing the information provided, facies, a term introduced by Gressly in 1838, refers to distinct rock bodies characterized by specific lithological, physical, and biological features that differentiate them from adjacent rocks. Each sedimentary facies is unique, and the term is applied in various contexts such as observational, genetic, environmental, and tectonic. Genetic facies offer insights into formative processes, while descriptive facies focus on detailed characteristics. When defining classical sedimentary facies, sedimentary structures, lithology, texture, and body geometries play crucial roles. Descriptions of facies should underscore their uniqueness within the geologic setting, with subfacies accounting for minor variations. To comprehend the depositional environment accurately, it is essential to analyze facies groups, their temporal and spatial distributions, providing a holistic view of the sedimentary system.

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Facies Associations and Interpretations

• Facies associations are groups of facies that are genetically related and hold environmental significance.
• They are fundamental in understanding sedimentation dynamics and depositional environments.

Facies Association and Facies Succession

• Facies associations occur in distinct successions, showing changes in properties vertically or laterally.
• Successions like coarsening upward indicate progressive changes in grain size and flow energy.

Interpreting Facies Successions

• Each cycle in sedimentary successions represents a specific facies association.
• Analysis of multiple cycles helps in identifying idealized facies associations.

Paleogeographic Interpretations

• Facies associations aid in predicting the geographic distribution of facies in the past.
• Logical assumptions and fossil evidence assist in reconstructing paleogeography.

Significance of Facies Associations

• Facies associations provide insights into past environments and sedimentary dynamics.
• They play a crucial role in both depositional and paleogeographic interpretations.

Understanding Facies Analysis in Geology

Importance of Facies in Geology

• Fossils found in facies are crucial as they remain undamaged during diagenesis, aiding in understanding past environments.
• Walther's Law of the Correlation of Facies emphasizes the significance of facies contacts in deciphering spatial relationships between depositional systems.
• Gradational transitions between facies indicate original adjacency and genetic relationships, while sharp/erosional contacts suggest unrelated facies.

Geological Classification: Facies Analysis

• Facies analysis is a systematic classification scheme used to evaluate rocks based on sedimentation processes and interpret depositional settings.
• It involves unraveling changes in framework through space and time, essential for paleogeography and basin analysis.

Facies Analysis Process

• Facilitating successful implementation of Sequence Stratigraphy, particularly in outcrop studies.
• Extracting an idealized facies model by combining features of recent and ancient sedimentary rocks to characterize each depositional environment.

Key Takeaways

• Facies analysis is crucial for understanding past environments through rock classification and interpreting depositional settings.
• Walther's Law of the Correlation of Facies aids in establishing relationships between depositional systems in space.
• Facies contacts provide valuable insights into the genetic relationships between different depositional environments.

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Facies Model

• A facies model serves as a simplified representation of a sedimentary environment, showcasing the typical distribution of facies over time and space, making it relevant to various geological research areas.
• It consolidates essential characteristics that define a specific depositional system, drawing from a wealth of information gathered from multiple instances of the same depositional system, including both ancient and modern examples to identify recurring patterns.
• The primary goals of facies models are:
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  • To summarize the general attributes of sedimentary environments based on observations from the study area.
  • To derive universally applicable models that can be used across different sedimentary environments.
  • To employ these universal models as a framework for studying new areas.

Classification of Sedimentary Environments

• Characterization of sedimentary environments involves comparing them with their present-day counterparts to understand the underlying processes shaping each environment.
• Sedimentary Environments are broadly categorized into three groups:
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  • Continental Environments: Examples include fluvial, alluvial fan, eolian, and lacustrine environments. The glacial environment, while significant, is primarily erosional in nature.
  • Transitional Environments: These encompass deltaic, estuarine, beach, barrier island & lagoonal, and tidal flat environments.
  • Marine Environments: Categories consist of reef, continental shelf, and continental slope environments.

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Continental Sedimentary Environments:

• Alluvial Fan Fluvial:
  • Breccia, conglomerate, arkose are common rock types found here.
  • Fossils might be present, showcasing past life forms.
• Delta:
  • Rock types include sandstone, siltstone, shale, and coal.
  • Colors are typically brown or red with varying grain shapes.
• Lacustrine:
  • Characterized by deposits ranging from clay to gravel, often showing a fining upward trend.
  • Rock types include sandstone, siltstone, shale, and occasionally coal.

Transitional Sedimentary Environments:

• Desert (Dunes):
  • Mainly composed of quartz arenite (sandstone).
• Lagoon:
  • Contains siltstone, shale, limestone, oolitic limestone, or gypsum.
  • Features like asymmetrical ripples and cross-bedding are common.
• Tidal Flat:
  • Materials can be terrigenous, carbonate, or evaporites.
  • Often associated with unique features like tracks, trails, burrows, and stromatolites.

Composition of Sedimentary Rocks

• Terrigenous
• Color
• Grain size
• Grain shape
• Sorting
• Structures:
  • Poor
  • Inorganic cross-bedding
  • Sedimentary graded structures
  • Bedding
  • Organic or biogenic sedimentary structures
  • Fossils
• Rock Type:
  • P
  • AG

Characteristics of Terrigenous Rocks

• Color: Brown, black, grey, green, red
• Grain size: Clay to sand (coarsening upward)
• Grain shape: Variable, framework, white to tan, few to no grains
• Structures:
  • Sand
  • Rounded or angular grains
  • Good sorting
  • Cross-bedding, symmetrical ripples
  • Track burrows
  • Marine shells

Marine Sedimentary Environments

• Continental Slope and Shelf Rise:
  • Sandstone, litharenite, siltstone, shale, fossiliferous limestone
  • Oolitic limestone, terrigenous or carbonate
  • Gray to brown color
  • Clay to sand composition
• Abyssal Plain:
  • Shale, chert, micrite, chalk, diatomite
  • Black, white, red color
  • Clay composition

Key Sedimentary Environments

• Continental
• Marine
• Abyssal Plain

Sedimentary Structures Summary

Sorting

• In sedimentary rocks, sorting refers to the uniformity of grain size in the rock.
• Well-sorted sediment has grains that are similar in size, while poorly sorted sediment has a wide range of grain sizes.

Inorganic Sedimentary Structures

• These structures are formed through natural, non-biological processes.
• Examples include laminations, graded bedding, and cross-bedding.
• Laminations are thin layers within sedimentary rocks, graded bedding shows a gradual change in grain size within a layer, and cross-bedding results from the migration of ripples or dunes.

Organic or Biogenic Sedimentary Structures

• These structures are created through the activities of living organisms.
• Examples include fossils, tracks, burrows, and marine shells.
• Fossils are the preserved remains of ancient organisms, tracks and burrows are traces left by organisms, and marine shells are the shells of marine creatures.

Poor to Good

• This term refers to the sorting of sedimentary particles, ranging from poorly sorted (a wide range of grain sizes) to well-sorted (uniform grain sizes).

Poor Laminations, Graded Bedding, Cross-bedding

• These sedimentary structures indicate varying degrees of sorting and layering within rocks.
• For example, cross-bedding can be seen in sandstone formations where layers are inclined at an angle to the main bedding plane.

Cross-bedding, Laminations, Marks (Turbidites)

• Turbidites are sedimentary deposits that form from underwater landslides or turbidity currents.
• These deposits often exhibit cross-bedding, laminations, and distinctive marks due to the way they are deposited.

Track, Burrows

• Tracks and burrows in sedimentary rocks are evidence of past animal activity.
• These structures provide valuable insights into the behavior and ecology of ancient organisms.

Corals, Marine Shells

• Corals and marine shells are examples of biogenic sedimentary structures.
• They are formed by living organisms and can be preserved in sedimentary rocks over time.