Sedimentary Rocks: Formation, Textures, and Types

Comprehensive vocabulary flashcards covering the formation processes, textural classifications, depositional environments, sedimentary structures, and specific rock types detailed in the lecture.

Sedimentary Rocks

Rocks formed from pieces of other rocks, minerals, or living things (sediments) that are deposited in layers and pressed together over time.

Sediments

Solid fragments of rock, minerals, or organic matter created by weathering and erosion from wind, water, or ice.

Lithification

The geological process that transforms loose, unconsolidated sediment into solid sedimentary rock through compaction and cementation.

Clastic (Detrital) Sedimentary Rock

A major category of sedimentary rock made from compacted and cemented broken pieces of preexisting rocks.

Chemical Sedimentary Rock

A category of sedimentary rock formed when dissolved minerals in water precipitate or crystallize as water evaporates or becomes saturated.

Biochemical / Organic Sedimentary Rock

A sedimentary rock formed from the accumulated remains of once-living organisms, such as shells, plant matter, and fossils.

Weathering

The process by which wind, water, ice, and climate break down solid rock into smaller sediment fragments.

Erosion

The process of carrying away and transporting weathered rock fragments and sediment particles from one location to another.

Deposition

The dropping and settling of transported sediments into layers when moving wind, water, or ice slows down.

Compaction

The process where the immense weight of overlying sediment layers squashes and squeezes the deeper sediments together.

Cementation

The process where dissolved minerals such as calcite or silica crystallize from groundwater, acting as a natural glue that binds sediment grains into solid rock.

Well-Sorted Sediment

Sediment in which individual grains are all approximately the same size, typically sorted by sustained fluid motion like beach waves.

Poorly Sorted Sediment

Sediment composed of a wide mixture of diverse grain sizes, such as a combination of mud, sand, and pebbles.

Angular Grains

Sediment pieces with sharp, jagged edges, indicating that they were deposited close to their original source without traveling far.

Rounded Grains

Sediment pieces whose sharp edges have been worn down and smoothed through prolonged tumbling in rivers or ocean waves over long distances.

Alluvial Fan

A depositional environment at the base of mountains where sediment rapidly spreads out, forming large, poorly sorted deposits such as breccia or conglomerate.

Delta

A depositional environment formed where a river enters an ocean or lake, building outward with mixed sand, silt, clay, cross-beds, and organic layers.

Floodplain

Flat land adjacent to a river where overflowing floodwaters deposit fine mud sediments that commonly form shale.

Planar Bedding

A primary sedimentary structure characterized by horizontal, stacked flat layers of sediment resembling pancakes.

Cross-Bedding

A primary sedimentary structure consisting of slanted, inclined layers formed by the directional flow of wind in sand dunes or moving water in rivers.

Ripple Marks

Small, wavy patterns preserved on sediment surfaces created by the gentle flow of water currents or wind.

Mud Cracks

Polygonal fracture structures formed when wet mud dries and contracts, indicating an environment that was once submerged and subsequently dried out.

Graded Bedding

A sedimentary layer characterized by coarse, large grains at the bottom that progressively grade into fine grains at the top, typically created by floods or underwater landslides.

Breccia

A coarse-grained clastic sedimentary rock made of large, sharp, angular rock chunks bound together by a fine-grained mineral cement, indicating little to no transport distance.

Conglomerate

A clastic sedimentary rock made of smooth, rounded river pebbles and gravel cemented in a matrix of hardened sand or clay, indicating deposition by strong currents of water.

Limestone

A sedimentary rock composed mostly of calcium carbonate ($\text{CaCO}_3$) derived from the accumulated shells and skeletons of ancient marine organisms, which fizzes when exposed to acid.

Dolostone

A sedimentary rock composed mostly of the mineral dolomite, formed when magnesium-rich groundwater alters limestone by replacing some calcium ions with magnesium.

Rock Salt (Halite)

A chemical evaporite sedimentary rock with a Mohs hardness of $2.5$ that naturally crystallizes into cubes when ancient saltwater bodies evaporate.

Chert

A hard, microcrystalline sedimentary rock composed of silica ($\text{SiO}_2$) that breaks with a sharp, curved conchoidal fracture and was historically used to fashion arrowheads.

Conchoidal Fracture

A smooth, shell-like, curved break pattern characteristic of brittle, glass-like rocks such as chert, resulting in exceptionally sharp edges.

Rock Gypsum

A very soft chemical evaporite rock ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) that can be scratched with a fingernail, commonly utilized in drywall, school chalk, and plaster casts.

Sandstone

A porous clastic sedimentary rock composed of sand-sized mineral grains (predominantly quartz and feldspar) glued together by mineral cements such as calcite or silica.

Shale

A smooth, fine-grained clastic sedimentary rock formed from compacted clay, mud, and silt that readily splits into thin, flat sheets.

Banded Iron Formations (BIFs)

Ancient sedimentary rock layers consisting of alternating bands of gray iron and red chert, formed when early photosynthetic cyanobacteria produced oxygen that precipitated dissolved ocean iron.

Chalk

A soft, white, porous variety of limestone formed from the accumulated microscopic skeletal remains of marine plankton.

Coal

A combustible biochemical sedimentary rock formed from ancient plant material that accumulated in swamps and underwent millions of years of heat and compaction.