Geology Final: Sedimentary Rocks

Sedimentary rocks

Rocks formed by the accumulation and lithification of sediment.

Three main types of sedimentary rocks

Clastic, chemical, and organic.

Processes contributing to sediment formation

Weathering, erosion, transport, deposition, and lithification.

Grain size

The diameter of sediment particles; indicates energy of the depositional environment.

Grain shape

The amount of transport—angular grains mean less transport; rounded grains mean more.

Sorting in sedimentary rocks

The uniformity of grain sizes; well-sorted = same size, poorly sorted = mixed sizes.

Bedding in sedimentary rocks

Layers of sediment deposited over time, reflecting changes in environment.

Graded bedding

Layers where particle size decreases from bottom to top; indicates settling from suspension.

Cross beds

Inclined layers within horizontal beds; formed by wind or water currents.

Mud cracks

Cracks formed by drying mud; indicates exposure to air and wet-dry cycles.

Mudstone

A fine-grained clastic rock made of clay or silt-sized particles.

Arkose sandstone

Sandstone with at least 25% feldspar; indicates short transport from granite source.

Quartz arenite

Pure quartz sandstone; well-sorted and rounded—indicates long transport.

Greywacke

Sandstone with mixed grains and rock fragments in a clay matrix; poorly sorted.

Conglomerate

Coarse-grained clastic rock with rounded gravel-sized clasts.

Sedimentary provenance

The origin or source of the sediment.

Chemical sedimentary rocks

Rocks formed by precipitation of minerals from water.

Inorganic limestone

Limestone formed by direct precipitation of calcium carbonate from water.

Organic limestone

Limestone formed from shells and skeletons of marine organisms like reefs and foraminifera.

Chert

A silica-rich rock formed from diatoms and radiolarians.

Evaporites

Rocks like halite and gypsum formed by evaporation of salty water.

Coal

Organic sedimentary rock formed from compressed plant material in swampy environments.

Difference between sources of oil and coal

Oil forms from microscopic marine organisms; coal forms from terrestrial plants.

Lithification

The process of turning sediment into rock.

Two main steps of lithification

Burial/compaction and cementation.

Compaction

Pressure squeezes out water and reduces pore space in sediment.

Cementation

Minerals precipitate between grains and bind them together.

Depositional environment

A location where sediment is deposited (e.g., river, delta, reef, deep sea).

Sedimentary facies

A body of rock with specific characteristics representing a particular environment.

Features of river depositional environments

Coarse-grained sediments, cross-bedding, mud cracks, and channel structures.

Features of marine depositional environments

Finer sediments, fossils, graded bedding from turbidity currents.

Sedimentary basin

A low area on Earth's surface where sediments accumulate over time.

Flexural basin

A basin formed by the bending of the lithosphere under tectonic load, like mountain belts.

Rift basin

A basin formed by the stretching and thinning of the crust, often found at divergent boundaries.

Marine transgression

A rise in sea level where deeper marine environments move landward.

Marine regression

A fall in sea level where shallower environments replace deeper ones.

Rock pattern during transgression

Fining upward sequence: sandstone → shale → limestone.

Rock pattern during regression

Coarsening upward sequence: limestone → shale → sandstone.

Sedimentary rock from a beach environment

Quartz arenite (well-sorted, well-rounded sandstone).

Sedimentary feature formed in a desert

Cross bedding (from wind-blown sand dunes).

Rock from a deep marine environment

Chert or shale (very fine-grained sediments).