Sedimentary Rocks: Formation and Types

• Sediments get deposited in layers commonly known as beds, or strata.
• Lithification: The process of loose grains becoming hard rock

Lithification Steps

• Burial: Previously deposited sediments are buried by more sediments.
• Compaction: Sediments get pressed together under pressure, forcing out the liquid from in-between.
• Cementation: Minerals precipitate from the solution around the grains which bind them together.

Sedimentary Rock Textures

• Display rounded grains/fragments that touch, with cement in-between, but do not interlock.

Common Mineral Cements

• Quartz
• Calcite
• Hematite

Clastic Sedimentary Rocks

• Formed when other rock pieces (clasts) bind together
• Conglomerate, Breccia, Sandstone, and Shale are examples of Clastic Sedimentary Rocks.

Non-Clastic Sedimentary Rocks

• Bio-Clastic Rocks: Formed from once-living organisms (shells, plants)
  • Examples: Limestone (with shell fragments), Coal
• Chemical Sedimentary Rocks: Formed from mineral precipitation out of the water
  • Examples: Limestone formations in caves (stalactites, stalagmites), rock Salt (or halite), gypsum)

Sedimentary Rock Characteristics

• Grain size
• Sorting: How well the sediments are sorted
• Roundness: How rounded to angular the grains are
• Composition of grains (quartzose, arkosic, lithic)
• Grain size, sorting, roundness, and composition determine deposition and transport history.

Bedding and Sedimentary Structures

• Parallel Bedding: Layers are parallel to each other.
• Graded Bedding: Layers with a gradation in grain size from large at bottom to fine at top.
• Cross Bedding: Inclined, or dipping, layers within a horizontal bed.
• Current (Asymmetric) Ripples: Formed by unidirectional water or wind currents.
• Wave (Symmetric) Ripples: Formed by oscillating (back and forth) currents.
• Mud Cracks & Rain Drop Impressions: Indicate wetting and drying of sediment surfaces.
• Trace Fossils: Biological activity indications, like footprints or burrows.

Weathering

• Physical Weathering: Mechanical breakdown of large rocks into smaller pieces.
• Chemical Weathering: Breakdown of rocks through chemical reactions.
• Joints: Natural rock fractures facilitate weathering by allowing water and other agents to penetrate.

Physical Weathering Mechanisms

• Freeze-Thaw: Water freezes in cracks, expands, and breaks the rock.
• Thermal Expansion: Repeated heating and cooling cause rocks to expand and contract.
• Abrasion: Rocks are worn down by friction and impact.
• By Organisms: Plant roots and boring animals break rocks apart.

Chemical Weathering Mechanisms

• Dissolution: Minerals completely dissociate in water or acid.
• Hydrolysis: Water reaction changes the mineral composition (feldspar → clay).

Goldich Weathering Series

• “opposite” order of Bowen's reaction series
  • Minerals formed at highest temperatures (olivine, pyroxene; mafic rocks) weather the fastest
  • Minerals formed at the lowest temperatures (quartz; felsic rocks) weather slowest
• Erosion and Transport: Sediment movement by wind, water, ice, or gravity.

Environments

• Glacial Environments: Characterized by unsorted, more angular sediments.
• Eolian Environments: Wind-blown sediments, well-sorted and rounded with large cross beds.
• Fluvial Environments: River and stream deposits, often layered and sorted with cross bedding.
• Lacustrine Environments: Lake deposits, fine-grained and thinly layered (laminated).
• Marine Environments: Ocean deposits; carbonate shells in clear, tropical, warm water; clastic in areas that are near rivers or colder.
• Deposition: Sediments settle in new locations, known as basins

Basins

• Sediments are deposited in different basins:
  • Continental Basins
  • Ocean Basins
  • Rift Basins
  • Foreland Basins

Relative Sea Level

• Height of the sea relative to the land.
• Changes caused by tectonic activity or changes in water volume.
• Transgression: Sea level rises, causing marine sediments to cover terrestrial sediments.
• Regression: Sea level falls, exposing marine sediments to erosion.