Folk Sandstone Classification (QFL Diagram)

Folk Sandstone Classification (QFL)

Plot framework grains on the standard QFL ternary diagram to determine sandstone provenance and maturity.

The Folk (1974) Sandstone Classification

While igneous rocks are classified by their cooling history and bulk chemistry, sedimentary rocks are classified by their provenance (where they came from) and their maturity (how far they travelled). To systematically classify sandstones, sedimentary petrologists use the definitive Folk Classification system (Folk, 1974), centered around the iconic QFL Diagram.

Unlike geochemical plots, the QFL diagram is based strictly on physical framework grains visible under a petrographic microscope. By determining the relative proportions of Quartz (Q), Feldspar (F), and Lithic fragments (L), a geologist can reconstruct entire ancient landscapes, mapping out vanished mountain ranges and paleo-river systems.

Our interactive Folk Classification Tool automatically normalizes your point-count data and plots the exact position on the QFL ternary diagram. Because mud matrix fundamentally alters the classification (separating "clean" arenites from "dirty" wackes), our tool includes a dedicated matrix parameter that dynamically re-routes the mathematical classification algorithm.

        The Rule of Normalization: The QFL diagram strictly plots framework grains. Cement (like calcite or silica binding the rock together), porosity (empty space), and muddy matrix must be mathematically excluded before plotting. If a rock is 50% Quartz, 10% Feldspar, and 40% Calcite Cement, you ignore the cement and normalize the Quartz and Feldspar to sum to 100%.
    

1. The QFL Apices: What Do They Mean? (Part 1)

The three corners of the diagram are not just minerals; they are indicators of tectonic and environmental history.

Q (Quartz): Quartz is mechanically hard (Mohs hardness 7) and chemically ultra-stable. It survives millions of years of intense weathering, tumbling down rivers, and crashing on beaches. A sandstone composed almost entirely of quartz (a Quartz Arenite) is considered mineralogically mature. It has been heavily reworked, and all the "weak" minerals have been destroyed.
F (Feldspar): Feldspar is abundant in igneous rocks but is chemically unstable at the Earth's surface. In wet climates, it rapidly weathers into clay (kaolinite). If a sandstone contains abundant feldspar (an Arkose), it means the rock is mineralogically immature. It likely formed in a dry, arid climate (where chemical weathering is slow) or was deposited very close to its source rock before it had time to break down.
L (Lithic Fragments): These are microscopic chunks of pre-existing rocks (e.g., a tiny grain of basalt or schist) that haven't broken down into individual minerals yet. High lithic content indicates rapid erosion and deposition, often associated with tectonic uplift and volcanic arcs.

2. Arenites vs. Wackes: The Role of Matrix

The standard QFL triangle shown in textbooks is actually just a 2D cross-section of a 3D tetrahedron, where the fourth vertical axis is the amount of mud matrix (silt and clay sized particles). Folk divided sandstones into three fundamental categories based on this matrix:

Arenites (<15% Matrix): These are "clean" sandstones. The pores between the sand grains are either empty or filled with later chemical cement. They imply deposition in high-energy environments (like a crashing beach or a fast-flowing river) where the fine mud was entirely washed away. They are classified into Arkose, Litharenite, and Quartz Arenite.
Wackes (15% to 75% Matrix): These are "dirty" sandstones. The sand grains are floating in a sea of mud. They imply deposition in low-energy environments or catastrophic, chaotic deposition (like deep-ocean turbidity currents or submarine landslides). They are classified into Quartz Wacke, Feldspathic Wacke, and Lithic Wacke.
Mudrocks (>75% Matrix): If the rock is overwhelmingly mud, it is no longer a sandstone. It is a siltstone or shale.

3. Tectonic Provenance

Perhaps the most powerful application of the QFL diagram is determining provenance (the tectonic setting of the source area), pioneered by Dickinson and Suczek (1979).

If you plot hundreds of sandstone samples from a specific basin, they will cluster in specific zones on the QFL triangle. Quartz Arenites typically plot near the Q apex and originate from Stable Cratons (ancient, flat continental interiors). Arkoses plot near the F-Q axis and originate from Basement Uplifts (where deep granites are thrusted to the surface). Litharenites plot near the L apex and originate from Magmatic Arcs (active volcanoes shedding rock fragments) or recycled orogens (colliding tectonic plates).

The Folk (1974) Sandstone Classification

        The Rule of Normalization: The QFL diagram strictly plots framework grains. Cement (like calcite or silica binding the rock together), porosity (empty space), and muddy matrix must be mathematically excluded before plotting. If a rock is 50% Quartz, 10% Feldspar, and 40% Calcite Cement, you ignore the cement and normalize the Quartz and Feldspar to sum to 100%.
    

1. The QFL Apices: What Do They Mean? (Part 2)

The three corners of the diagram are not just minerals; they are indicators of tectonic and environmental history.

Q (Quartz): Quartz is mechanically hard (Mohs hardness 7) and chemically ultra-stable. It survives millions of years of intense weathering, tumbling down rivers, and crashing on beaches. A sandstone composed almost entirely of quartz (a Quartz Arenite) is considered mineralogically mature. It has been heavily reworked, and all the "weak" minerals have been destroyed.
F (Feldspar): Feldspar is abundant in igneous rocks but is chemically unstable at the Earth's surface. In wet climates, it rapidly weathers into clay (kaolinite). If a sandstone contains abundant feldspar (an Arkose), it means the rock is mineralogically immature. It likely formed in a dry, arid climate (where chemical weathering is slow) or was deposited very close to its source rock before it had time to break down.
L (Lithic Fragments): These are microscopic chunks of pre-existing rocks (e.g., a tiny grain of basalt or schist) that haven't broken down into individual minerals yet. High lithic content indicates rapid erosion and deposition, often associated with tectonic uplift and volcanic arcs.

2. Arenites vs. Wackes: The Role of Matrix

Arenites (<15% Matrix): These are "clean" sandstones. The pores between the sand grains are either empty or filled with later chemical cement. They imply deposition in high-energy environments (like a crashing beach or a fast-flowing river) where the fine mud was entirely washed away. They are classified into Arkose, Litharenite, and Quartz Arenite.
Wackes (15% to 75% Matrix): These are "dirty" sandstones. The sand grains are floating in a sea of mud. They imply deposition in low-energy environments or catastrophic, chaotic deposition (like deep-ocean turbidity currents or submarine landslides). They are classified into Quartz Wacke, Feldspathic Wacke, and Lithic Wacke.
Mudrocks (>75% Matrix): If the rock is overwhelmingly mud, it is no longer a sandstone. It is a siltstone or shale.

3. Tectonic Provenance

Perhaps the most powerful application of the QFL diagram is determining provenance (the tectonic setting of the source area), pioneered by Dickinson and Suczek (1979).

The Folk (1974) Sandstone Classification

        The Rule of Normalization: The QFL diagram strictly plots framework grains. Cement (like calcite or silica binding the rock together), porosity (empty space), and muddy matrix must be mathematically excluded before plotting. If a rock is 50% Quartz, 10% Feldspar, and 40% Calcite Cement, you ignore the cement and normalize the Quartz and Feldspar to sum to 100%.
    

1. The QFL Apices: What Do They Mean? (Part 3)

The three corners of the diagram are not just minerals; they are indicators of tectonic and environmental history.

Q (Quartz): Quartz is mechanically hard (Mohs hardness 7) and chemically ultra-stable. It survives millions of years of intense weathering, tumbling down rivers, and crashing on beaches. A sandstone composed almost entirely of quartz (a Quartz Arenite) is considered mineralogically mature. It has been heavily reworked, and all the "weak" minerals have been destroyed.
F (Feldspar): Feldspar is abundant in igneous rocks but is chemically unstable at the Earth's surface. In wet climates, it rapidly weathers into clay (kaolinite). If a sandstone contains abundant feldspar (an Arkose), it means the rock is mineralogically immature. It likely formed in a dry, arid climate (where chemical weathering is slow) or was deposited very close to its source rock before it had time to break down.
L (Lithic Fragments): These are microscopic chunks of pre-existing rocks (e.g., a tiny grain of basalt or schist) that haven't broken down into individual minerals yet. High lithic content indicates rapid erosion and deposition, often associated with tectonic uplift and volcanic arcs.

2. Arenites vs. Wackes: The Role of Matrix

Arenites (<15% Matrix): These are "clean" sandstones. The pores between the sand grains are either empty or filled with later chemical cement. They imply deposition in high-energy environments (like a crashing beach or a fast-flowing river) where the fine mud was entirely washed away. They are classified into Arkose, Litharenite, and Quartz Arenite.
Wackes (15% to 75% Matrix): These are "dirty" sandstones. The sand grains are floating in a sea of mud. They imply deposition in low-energy environments or catastrophic, chaotic deposition (like deep-ocean turbidity currents or submarine landslides). They are classified into Quartz Wacke, Feldspathic Wacke, and Lithic Wacke.
Mudrocks (>75% Matrix): If the rock is overwhelmingly mud, it is no longer a sandstone. It is a siltstone or shale.

3. Tectonic Provenance

Perhaps the most powerful application of the QFL diagram is determining provenance (the tectonic setting of the source area), pioneered by Dickinson and Suczek (1979).

The Folk (1974) Sandstone Classification

        The Rule of Normalization: The QFL diagram strictly plots framework grains. Cement (like calcite or silica binding the rock together), porosity (empty space), and muddy matrix must be mathematically excluded before plotting. If a rock is 50% Quartz, 10% Feldspar, and 40% Calcite Cement, you ignore the cement and normalize the Quartz and Feldspar to sum to 100%.
    

1. The QFL Apices: What Do They Mean? (Part 4)

The three corners of the diagram are not just minerals; they are indicators of tectonic and environmental history.

Q (Quartz): Quartz is mechanically hard (Mohs hardness 7) and chemically ultra-stable. It survives millions of years of intense weathering, tumbling down rivers, and crashing on beaches. A sandstone composed almost entirely of quartz (a Quartz Arenite) is considered mineralogically mature. It has been heavily reworked, and all the "weak" minerals have been destroyed.
F (Feldspar): Feldspar is abundant in igneous rocks but is chemically unstable at the Earth's surface. In wet climates, it rapidly weathers into clay (kaolinite). If a sandstone contains abundant feldspar (an Arkose), it means the rock is mineralogically immature. It likely formed in a dry, arid climate (where chemical weathering is slow) or was deposited very close to its source rock before it had time to break down.
L (Lithic Fragments): These are microscopic chunks of pre-existing rocks (e.g., a tiny grain of basalt or schist) that haven't broken down into individual minerals yet. High lithic content indicates rapid erosion and deposition, often associated with tectonic uplift and volcanic arcs.

2. Arenites vs. Wackes: The Role of Matrix

Arenites (<15% Matrix): These are "clean" sandstones. The pores between the sand grains are either empty or filled with later chemical cement. They imply deposition in high-energy environments (like a crashing beach or a fast-flowing river) where the fine mud was entirely washed away. They are classified into Arkose, Litharenite, and Quartz Arenite.
Wackes (15% to 75% Matrix): These are "dirty" sandstones. The sand grains are floating in a sea of mud. They imply deposition in low-energy environments or catastrophic, chaotic deposition (like deep-ocean turbidity currents or submarine landslides). They are classified into Quartz Wacke, Feldspathic Wacke, and Lithic Wacke.
Mudrocks (>75% Matrix): If the rock is overwhelmingly mud, it is no longer a sandstone. It is a siltstone or shale.

3. Tectonic Provenance

Perhaps the most powerful application of the QFL diagram is determining provenance (the tectonic setting of the source area), pioneered by Dickinson and Suczek (1979).

The Folk (1974) Sandstone Classification

        The Rule of Normalization: The QFL diagram strictly plots framework grains. Cement (like calcite or silica binding the rock together), porosity (empty space), and muddy matrix must be mathematically excluded before plotting. If a rock is 50% Quartz, 10% Feldspar, and 40% Calcite Cement, you ignore the cement and normalize the Quartz and Feldspar to sum to 100%.
    

1. The QFL Apices: What Do They Mean? (Part 5)

The three corners of the diagram are not just minerals; they are indicators of tectonic and environmental history.

Q (Quartz): Quartz is mechanically hard (Mohs hardness 7) and chemically ultra-stable. It survives millions of years of intense weathering, tumbling down rivers, and crashing on beaches. A sandstone composed almost entirely of quartz (a Quartz Arenite) is considered mineralogically mature. It has been heavily reworked, and all the "weak" minerals have been destroyed.
F (Feldspar): Feldspar is abundant in igneous rocks but is chemically unstable at the Earth's surface. In wet climates, it rapidly weathers into clay (kaolinite). If a sandstone contains abundant feldspar (an Arkose), it means the rock is mineralogically immature. It likely formed in a dry, arid climate (where chemical weathering is slow) or was deposited very close to its source rock before it had time to break down.
L (Lithic Fragments): These are microscopic chunks of pre-existing rocks (e.g., a tiny grain of basalt or schist) that haven't broken down into individual minerals yet. High lithic content indicates rapid erosion and deposition, often associated with tectonic uplift and volcanic arcs.

2. Arenites vs. Wackes: The Role of Matrix

Arenites (<15% Matrix): These are "clean" sandstones. The pores between the sand grains are either empty or filled with later chemical cement. They imply deposition in high-energy environments (like a crashing beach or a fast-flowing river) where the fine mud was entirely washed away. They are classified into Arkose, Litharenite, and Quartz Arenite.
Wackes (15% to 75% Matrix): These are "dirty" sandstones. The sand grains are floating in a sea of mud. They imply deposition in low-energy environments or catastrophic, chaotic deposition (like deep-ocean turbidity currents or submarine landslides). They are classified into Quartz Wacke, Feldspathic Wacke, and Lithic Wacke.
Mudrocks (>75% Matrix): If the rock is overwhelmingly mud, it is no longer a sandstone. It is a siltstone or shale.

3. Tectonic Provenance

Perhaps the most powerful application of the QFL diagram is determining provenance (the tectonic setting of the source area), pioneered by Dickinson and Suczek (1979).

The Folk (1974) Sandstone Classification

        The Rule of Normalization: The QFL diagram strictly plots framework grains. Cement (like calcite or silica binding the rock together), porosity (empty space), and muddy matrix must be mathematically excluded before plotting. If a rock is 50% Quartz, 10% Feldspar, and 40% Calcite Cement, you ignore the cement and normalize the Quartz and Feldspar to sum to 100%.
    

1. The QFL Apices: What Do They Mean? (Part 6)

The three corners of the diagram are not just minerals; they are indicators of tectonic and environmental history.

Q (Quartz): Quartz is mechanically hard (Mohs hardness 7) and chemically ultra-stable. It survives millions of years of intense weathering, tumbling down rivers, and crashing on beaches. A sandstone composed almost entirely of quartz (a Quartz Arenite) is considered mineralogically mature. It has been heavily reworked, and all the "weak" minerals have been destroyed.
F (Feldspar): Feldspar is abundant in igneous rocks but is chemically unstable at the Earth's surface. In wet climates, it rapidly weathers into clay (kaolinite). If a sandstone contains abundant feldspar (an Arkose), it means the rock is mineralogically immature. It likely formed in a dry, arid climate (where chemical weathering is slow) or was deposited very close to its source rock before it had time to break down.
L (Lithic Fragments): These are microscopic chunks of pre-existing rocks (e.g., a tiny grain of basalt or schist) that haven't broken down into individual minerals yet. High lithic content indicates rapid erosion and deposition, often associated with tectonic uplift and volcanic arcs.

2. Arenites vs. Wackes: The Role of Matrix

Arenites (<15% Matrix): These are "clean" sandstones. The pores between the sand grains are either empty or filled with later chemical cement. They imply deposition in high-energy environments (like a crashing beach or a fast-flowing river) where the fine mud was entirely washed away. They are classified into Arkose, Litharenite, and Quartz Arenite.
Wackes (15% to 75% Matrix): These are "dirty" sandstones. The sand grains are floating in a sea of mud. They imply deposition in low-energy environments or catastrophic, chaotic deposition (like deep-ocean turbidity currents or submarine landslides). They are classified into Quartz Wacke, Feldspathic Wacke, and Lithic Wacke.
Mudrocks (>75% Matrix): If the rock is overwhelmingly mud, it is no longer a sandstone. It is a siltstone or shale.

3. Tectonic Provenance

Perhaps the most powerful application of the QFL diagram is determining provenance (the tectonic setting of the source area), pioneered by Dickinson and Suczek (1979).

The Folk (1974) Sandstone Classification

        The Rule of Normalization: The QFL diagram strictly plots framework grains. Cement (like calcite or silica binding the rock together), porosity (empty space), and muddy matrix must be mathematically excluded before plotting. If a rock is 50% Quartz, 10% Feldspar, and 40% Calcite Cement, you ignore the cement and normalize the Quartz and Feldspar to sum to 100%.
    

1. The QFL Apices: What Do They Mean? (Part 7)

The three corners of the diagram are not just minerals; they are indicators of tectonic and environmental history.

Q (Quartz): Quartz is mechanically hard (Mohs hardness 7) and chemically ultra-stable. It survives millions of years of intense weathering, tumbling down rivers, and crashing on beaches. A sandstone composed almost entirely of quartz (a Quartz Arenite) is considered mineralogically mature. It has been heavily reworked, and all the "weak" minerals have been destroyed.
F (Feldspar): Feldspar is abundant in igneous rocks but is chemically unstable at the Earth's surface. In wet climates, it rapidly weathers into clay (kaolinite). If a sandstone contains abundant feldspar (an Arkose), it means the rock is mineralogically immature. It likely formed in a dry, arid climate (where chemical weathering is slow) or was deposited very close to its source rock before it had time to break down.
L (Lithic Fragments): These are microscopic chunks of pre-existing rocks (e.g., a tiny grain of basalt or schist) that haven't broken down into individual minerals yet. High lithic content indicates rapid erosion and deposition, often associated with tectonic uplift and volcanic arcs.

2. Arenites vs. Wackes: The Role of Matrix

Arenites (<15% Matrix): These are "clean" sandstones. The pores between the sand grains are either empty or filled with later chemical cement. They imply deposition in high-energy environments (like a crashing beach or a fast-flowing river) where the fine mud was entirely washed away. They are classified into Arkose, Litharenite, and Quartz Arenite.
Wackes (15% to 75% Matrix): These are "dirty" sandstones. The sand grains are floating in a sea of mud. They imply deposition in low-energy environments or catastrophic, chaotic deposition (like deep-ocean turbidity currents or submarine landslides). They are classified into Quartz Wacke, Feldspathic Wacke, and Lithic Wacke.
Mudrocks (>75% Matrix): If the rock is overwhelmingly mud, it is no longer a sandstone. It is a siltstone or shale.

3. Tectonic Provenance

Perhaps the most powerful application of the QFL diagram is determining provenance (the tectonic setting of the source area), pioneered by Dickinson and Suczek (1979).

4. How to Properly Point-Count

To use this diagram accurately, data must be collected using the Gazzi-Dickinson point-counting method. When looking through a petrographic microscope, you use a mechanical stage to move the thin section in a precise grid. At each stop, whatever grain is directly under the crosshairs is identified and tallied. You must count at least 300 to 400 framework grains to achieve statistical significance before plugging the data into our calculator.

5. Frequently Asked Questions (FAQ)

What is the difference between an Arkose and a Subarkose? ▼

It is purely a matter of maturity (Quartz content). Both rocks are rich in feldspar relative to lithics. However, a Subarkose is more mature, containing between 75% and 95% Quartz. An Arkose is less mature, containing less than 75% Quartz, indicating it has undergone much less weathering or transport.

Why did my classification output 'Mudrock'? ▼

You entered a matrix percentage of 75% or higher. According to Folk (1974), a sedimentary rock must have at least 25% sand-sized framework grains to be considered a sandstone (wacke). Above 75% mud, the rock is officially a mudrock, shale, or siltstone, and the QFL diagram is no longer applicable.

6. Authoritative References and Outbound Resources

Society for Sedimentary Geology (SEPM): The premier organization for sedimentary petrology. Visit SEPM.
United States Geological Survey (USGS): For a vast repository of geological maps and basin provenance studies, visit the USGS.
Geological Society of America (GSA): For classic papers on sandstone provenance (e.g., Dickinson), visit the GSA.

Folk Sandstone Classification (QFL Diagram)