Sedimentary Facies and Provenance | Geology Optional for UPSC PDF Download

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.
• 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 occurring on land and sea.
• Transportation is the dominant mechanism.

Identification of Ancient Sedimentary Environments

• Ancient environments are identified by comparing them with modern counterparts.

• Analysis involves studying sedimentary products in rocks from ancient and modern settings.

Challenges in Reconstruction

• Inconsistencies and variations in depositional associations over time complicate reconstruction.

Major Criteria for Recognizing Sedimentary Environments

Physical Characteristics

• Bedding features, formation contacts, sedimentary structures, and directional properties.

Chemical Analysis

• Rock composition, major mineral constituents, and authigenic minerals.
• Evidence of solution or nodule accumulation.

Biological Characteristics

• Organic materials, such as floral and faunal assemblages.
• Crucial for environmental interpretation.

Ancient Sedimentary Environments

• Characterized by genetically related successions of sedimentary rocks.
• Deposits record environmental changes over time and space.
• Unique sedimentary structures are specific to certain environments.

Primary Structures and Depositional Facies

• Reveal mechanisms of rock formation.
• Categorized into depositional facies based on primary structures.
• Depositional facies represent the outcomes of processes in specific environments.

Facies Analysis

Definition of Facies

• Term introduced by Gressly in 1838, derived from the Latin word for "aspect" or "appearance."
• Defined as "a body of rock with specified characteristics" (Reading, 1986).
• Described as "a body of rock characterized by a particular combination of lithology, physical and biological structures" (Walker, 1993).

Key Aspects of Facies

• Sedimentary facies are unique, with no two being identical.

Applied in various contexts

• Observational (e.g., sandstone facies).
• Genetic (e.g., turbidite facies).
• Environmental (e.g., fluvial facies).
• Tectonic (e.g., post-orogenic facies).

Facies Descriptions

• Emphasize uniqueness within geologic settings.
• Subfacies account for minor variations.
• Analysis of facies groups, their temporal distribution, and spatial arrangement is essential for understanding depositional environments.

Facies Associations and Interpretations

Facies Associations

• Groups of genetically related facies with environmental significance.
• Fundamental in understanding sedimentation dynamics and depositional environments.

Facies Succession

• Vertical or lateral changes in facies properties.
• Examples: Coarsening upward indicating progressive changes in grain size and flow energy.

Paleogeographic Interpretations

• Facies associations predict geographic distribution of facies in the past.
• Fossil evidence aids in reconstructing paleogeography.

Significance of Facies Associations

• Provide insights into past environments and sedimentary dynamics.
• Crucial for depositional and paleogeographic interpretations.

Facies Analysis in Geology

Key Concepts

• Fossils in facies remain undamaged during diagenesis, aiding environmental interpretation.

Walther's Law of the Correlation of Facies

• Gradational transitions indicate genetic relationships.
• Sharp/erosional contacts suggest unrelated facies.

Facies Analysis Process

• Systematic classification of rocks based on sedimentation processes.
• Essential for paleogeography and basin analysis.
• Idealized facies models combine ancient and modern sedimentary rock features.

Facies Models

Definition

• Simplified representation of a sedimentary environment.
• Highlights typical distribution of facies over time and space.

Goals

• Summarize general attributes of sedimentary environments.
• Derive universally applicable models.
• Provide a framework for studying new areas.

Classification of Sedimentary Environments

Continental Environments

• Examples: Fluvial, alluvial fan, eolian, lacustrine, and glacial environments.

Transitional Environments

• Examples: Deltaic, estuarine, beach, barrier island & lagoonal, tidal flat environments.

Marine Environments

• Examples: Reef, continental shelf, and continental slope environments.

Key Characteristics of Sedimentary Environments

1. Continental Environments
Alluvial Fan & Fluvial

• Rocks: Breccia, conglomerate, arkose.

Delta

• Rocks: Sandstone, siltstone, shale, coal.
• Features: Brown/red colors, varying grain shapes.

Lacustrine

• Rocks: Sandstone, siltstone, shale, occasional coal.
• Features: Fining upward deposits.

2. Transitional Environments

Desert (Dunes):

• Rocks: Quartz arenite (sandstone).

Lagoon:

• Rocks: Siltstone, shale, limestone, gypsum.

Tidal Flat:

• Rocks: Terrigenous, carbonate, evaporites.
• Features: Tracks, trails, burrows, stromatolites.

Marine Environments:

• Continental Slope and Shelf Rise: Rocks: Sandstone, litharenite, siltstone, shale, limestone.
• Abyssal Plain: Rocks: Shale, chert, micrite, chalk, diatomite.

Sedimentary Structures

Sorting

• Well-sorted sediment: Uniform grain size.
• Poorly sorted sediment: Wide range of grain sizes.

Inorganic Structures

• Laminations, graded bedding, cross-bedding.

Organic/Biogenic Structures

• Fossils, tracks, burrows, marine shells.

Turbidites

• Features: Cross-bedding, laminations, marks.

Corals and Marine Shells

• Biogenic structures preserved in sedimentary rocks.