25 Sedimentary Rock Identification

Sedimentary Rock Identification

Introduction

Rocks are made of minerals. Rocks can be a mixture of different kinds of minerals, a mixture of many grains of the same kind of mineral, or a mixture of different grains of rocks. When you split a rock into very small pieces, the pieces are different from each other. For example, when you break granite apart, you get small pieces of quartz (clear), feldspar (pink or white), and mica (black). When you split a mineral into pieces, you still have pieces of the same mineral. If you break a big chunk of quartz into smaller pieces, you still have pieces of quartz.

There are three basic rock types: Igneous, sedimentary, and metamorphic.

Sedimentary Rocks

Sedimentary rocks (layered rocks) are made by the deposition of particles carried in air or water and by the precipitation of chemicals dissolved in water. These particles and chemicals come from the weathering (breaking apart in place) and erosion (carrying away and breaking apart while moving) of rocks on the Earth’s surface. Boulders, rocks, gravel, sand, silt, clay, and mud are carried by water currents in streams, rivers, lakes, and oceans. These particles are deposited in stream beds, shores, lake and ocean bottoms, and deltas where rivers empty into lakes and oceans. These particles are cemented together and hardened to form the sedimentary rocks called conglomerate, sandstone, siltstone, shale or claystone, and mudstone.

Chemicals that are leached or dissolved from other rocks are carried invisibly in streams and oceans. When these invisible chemicals reach a lake or ocean, and the water evaporates, the chemicals are left behind in evaporite deposits. Salt around salt lakes and limestone from sea bottoms are examples of these chemical sedimentary rocks. Plants and animals are sometimes buried and preserved in the finer grained sedimentary rocks, such as shale or limestone.

Sedimentary rocks are derived from pre‑existing rocks by weathering and erosion. The resulting particles settle out of water or air (clastic rocks such as sandstone and mudstone) or the resulting chemicals precipitate from concentrated solutions (non‑clastic rocks such as limestone and salt).

Clastic Sedimentary Rocks

How Clastic Sedimentary Rocks are Formed:

1. Pre‑existing rock undergoes chemical and mechanical weathering by roots, acid rainwater, gravity, wind, and water.
2. The broken particles are carried through water or air until they settle out in a lower area when the current wasn’t fast enough to carry the particles.
3. Quartz is the most stable and has the greatest resistance to the mechanical and chemical abrasion during erosion, so most sand size grains are quartz.
4. Feldspar alters to clay with chemical weathering and erosion, so arkose, which is sandstone with more than 25% feldspar, indicates the sediment was deposited close to the source rock and was not in transport long.
5. Grain size and shape and composition can indicate the composition, distance, and height of the source rock.
6. Textures & structures (ripple marks, cross‑bedding, sorting, etc.) and size, shape, and composition can indicate the environment of deposition.

Characteristics:

1. Soft, compared to igneous rocks.
2. Occur in layers or beds from a few millimeters thick to 100 feet thick, most commonly 1‑5 ft. thick.
3. Granular and gritty if composed of sand and silt‑sized particles; sand is often rounded, sometimes angular.
4. Sedimentary structures (cross‑bedding, mud cracks, ripple marks, worm trails and burrows, fossil shells) are not usually visible in hand specimens, but are noticeable in outcrops.
5. Color is not usually significant, because as little as 3% hematite (iron oxide) gives a rich red color. Some pinkish sandstones get their color from feldspar.
6. Fossils are more common in shales than sandstones, because of the higher current activity in sandstones.

Non-Clastic Sedimentary Rocks

How Non-Clastic Sedimentary Rocks are Formed:

Formed by chemical precipitation from a concentrated solution in water as salt, gypsum, or limestone.

Characteristics:

1. Soft, because they are composed of soft minerals such as halite, gypsum, calcite. They can easily be scratched with steel or a copper penny.
2. Commonly fine‑grained and homogeneous.
3. Fossils are common in limestone.
4. Limestone fizzes in dilute hydrochloric (HCl) acid, because it is composed of the mineral calcite, CaC03.
5. Some limestone contains chert, which is very, very hard silica (like flint). It typically weathers to brown on the surface and occurs in nodules and occasionally replaces fossil shells.