Geol 101 - Sedimentary Rocks

Sedimentary Rocks

Rocks composed of materials deposited on land or in water, then lithified through diagenesis, classified as clastic, biochemical, or chemical based on formation mode.

Clastic Rocks (Detrial)

Formed from solid fragments of pre-existing rocks called clasts, classified primarily by texture (size and shape of grains).

Biochemical Rocks

Form due to a chemical change involving a biologic entity, classified primarily by chemical makeup (minerals and/or plant matter).

Chemical Rocks

Directly precipitate out of water, classified based on mineral content, examples include rock salt, rock gypsum, and limestone.

Fossils

Found only in sedimentary rocks, provide clues about the environment in which the rock formed.

Depositional Environments

Provide information on precursor rocks, transport mechanisms, and deposition conditions, characterized by particle size and shape.

Clast Size

Determines the texture of clastic sedimentary rocks, classified as clay-sized, silt-sized, sand, or gravel based on clast diameter.

Sorting

Refers to the range of clast sizes found in a particular sample.

Well sorted

All clasts are very similar in size.

Moderately sorted

Clast size varies, but not over a broad range.

Poorly sorted

Contains a wide variety of clast sizes.

Bimodal sorting

Two distinctly different clast sizes are present.

Shape

Refers to the presence (or absence) of well-defined corners on individual clasts.

Angular

Individual clasts appear freshly broken with sharp edges and corners.

Sub-angular

Corners and edges of individual clasts are slightly rounded off.

Sub-rounded

Individual clasts lack sharp corners or edges.

Rounded

Individual clasts are roughly spherical with few, if any flat surfaces.

Sedimentary Structures

Larger scale features in clastic rocks formed during deposition.

Bedding

Consists of parallel layers within the rock representing individual applications of sediment over a large horizontal area.

Cross beds

Created when sand is deposited on the upwind and downwind faces of a sand dune to form sets of parallel, non-horizontal beds.

Graded beds

Occur when poorly sorted sediment is suddenly dumped into still water, with larger clasts settling to the bottom first.

Ripple marks

Form when moving water creates small undulations in bottom sediments that are later buried and lithified.

Fissility

Feature limited to mud rocks (shale and mudstone) where shale tends to break along roughly parallel surfaces.

Mud cracks

Form when mud containing clay and silt-sized particles shrinks as it dries, leading to the formation of cracks.

Worm burrows

Tubular structures formed when worms tunnel through soft sediments, indicative of shallow, near shore environments.