Sandstone: Formation, Classification, and Depositional Environments

Introduction

Sandstone is a clastic sedimentary rock that  primarily composed of sand-sized mineral particles, rock fragments, and organic materials. It accounts for about 20-25% of all sedimentary rocks and is widely distributed across the Earth's surface. Because of its porosity, permeability, and durability, sandstone is essential for groundwater reservoirs, petroleum storage, and as a construction material.

Formation Processes of Sandstone

Sandstone forms through a series of geological processes over millions of years:

1. Weathering & Erosion

• Mechanical Weathering: Physical breakdown of rocks into sand-sized grains (0.0625–2 mm) due to wind, water, or ice.
• Chemical Weathering: Dissolution of unstable minerals (e.g., feldspar) while resistant minerals (e.g., quartz) remain.

2. Transportation

• Grains are transported by rivers, wind, or ocean currents.
• Rounding and sorting occur as grains collide and abrade during transport.
• Well-rounded grains indicate long transport distances, while angular grains suggest nearby erosion.

3. Deposition

• Sand accumulates in fluvial (river), aeolian (desert), marine (ocean), or deltaic environments.
• Depositional energy (wave/current strength) influences grain size and sorting.

4. Lithification (Diagenesis)

• Compaction: Overburden pressure reduces pore space.
• Cementation: Minerals like silica (SiO₂), calcite (CaCO₃), or iron oxides (Fe₂O₃) precipitate between grains, binding them into solid rock.
• Recrystallization & Replacement: Some minerals may alter over time (e.g., feldspar to clay).

Classification of Sandstone

1. Folk’s Classification (1968) – Based on Composition & Matrix

Grain Types

• Quartz (Q): Highly stable, dominant in mature sandstones (e.g., quartz arenite).
• Feldspar (F): Indicates short transport and arid climates (common in arkose).
• Lithic Fragments (L): Rock pieces (volcanic, metamorphic, or sedimentary) found in immature sandstones.

Matrix vs. Cement

• Matrix: Fine-grained silt/clay filling pore spaces (common in greywacke).
• Cement: Chemical precipitates (silica, calcite, hematite) binding grains.

Folk’s Sandstone Naming Scheme

Grain Composition	Matrix <15%	Matrix >15%
Q >90%, F+L <10%	Quartz Arenite	-
F >25%, Q dominant	Arkose	Subarkose
L >25%, Q+F minor	Litharenite	Sublitharenite
Mixed F + L, high matrix	-	Greywacke

2. Pettijohn’s Classification (1975) – Based on Texture & Tectonics

Type	Key Features	Depositional Setting
Quartz Arenite	>95% quartz, well-sorted	Stable continental shelves, deserts
Arkose	>25% feldspar, angular grains	Rapid erosion (granite mountains)
Lithic Sandstone	>25% rock fragments, immature	Active margins (volcanic regions)
Greywacke	Poorly sorted, clay-rich matrix	Turbidites, submarine fans

 

Occurrence and Depositional Environments 

1. Fluvial (River) Systems

• Braided Rivers: Coarse, cross-bedded sandstone with gravel lenses.
• Meandering Rivers: Finer, well-sorted sandstone with ripple marks and mud layers.
• Example: Kayenta Formation (USA).

2. Aeolian (Desert) Environments

• Dune Fields: Well-sorted, rounded grains with large-scale cross-bedding.
• Interdune Areas: Thin layers of finer sand and silt.
• Example: Navajo Sandstone (USA).

3. Marine (Coastal & Shallow Sea) Settings

• Beaches: Well-sorted, quartz-rich sandstone with heavy minerals.
• Tidal Flats: Laminated sandstone-mudstone alternations.
• Example: St. Peter Sandstone (USA).

4. Deltaic Environments

• River-Dominated Deltas: Coarse, cross-stratified sandstone.
• Wave-Dominated Deltas: Well-sorted, clean sandstone layers.
• Example: Brent Delta (North Sea).

5. Deep Marine (Turbidites & Submarine Fans)

• Turbidity Currents: Poorly sorted greywacke with graded bedding.
• Submarine Canyons: Chaotic, coarse-grained sandstone deposits.
• Example: Martinsburg Formation (Appalachians).

Table of Sandstone Types & Environments

Sandstone Type	Key Minerals	Sorting	Depositional Environment
Quartz Arenite	95% Quartz	Well-sorted	Deserts, Beaches, Stable Shelves
Arkose	>25% Feldspar	Moderate	Mountain Bases, Arid Regions
Litharenite	Rock Fragments	Poor	Volcanic Arcs, River Floodplains
Greywacke	Mixed Grains + Clay	Very Poor	Deep Marine, Submarine Fans

 

Summary :

The sandstone is a geologically important rock that offers insights into Earth’s history through its texture, composition, and depositional environment. Folk’s and Pettijohn’s classifications aid geologists in interpreting sediment sources and transport mechanisms.. From desert dunes to deep-sea turbidites, sandstone forms in diverse environments, making it essential for hydrocarbon exploration, aquifer studies, and architectural use.