Physical Geology Unit 6 Review: Sedimentary Rocks and Environments

What Are Sedimentary Rocks?

• Form at or near Earth's surface from the accumulation and lithification of sediment, which consists of loose grains of rock, minerals, or organic material
• Sedimentary rocks are one of the three main rock types, along with igneous and metamorphic rocks
• Cover approximately 75% of Earth's land surface but make up only 5% of the Earth's crust
• Sedimentary rocks provide important information about past environments, climate, and life on Earth
• Most sedimentary rocks are deposited in layers, which can be used to determine the relative ages of the rocks and the events that occurred during their formation
  • Principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it
• Sedimentary rocks are generally more porous and less dense than igneous and metamorphic rocks due to the spaces between the sediment grains
• Some common examples of sedimentary rocks include sandstone, limestone, and shale

Types of Sedimentary Rocks

• Clastic sedimentary rocks form from the accumulation and lithification of rock and mineral fragments (clasts) that have been weathered, eroded, and transported
  • Classified based on the size of the clasts: gravel (conglomerate and breccia), sand (sandstone), silt (siltstone), and clay (claystone and shale)
• Chemical sedimentary rocks form from the precipitation of minerals from water, often due to changes in temperature, pressure, or chemical conditions
  • Examples include limestone (calcium carbonate), chert (silica), rock salt (halite), and rock gypsum
• Biochemical sedimentary rocks form from the accumulation and lithification of organic material, such as plant and animal remains
  • Examples include coal (formed from plant remains) and some limestones (formed from the shells of marine organisms)
• Detrital sedimentary rocks are composed of rock and mineral fragments that have been eroded from pre-existing rocks and transported to the site of deposition
• Allochemical sedimentary rocks contain grains that have been chemically precipitated or formed by biological activity, such as ooids and fecal pellets
• Authigenic sedimentary rocks form from minerals that precipitate directly at the site of deposition, such as some evaporites and cements

How Sedimentary Rocks Form

• Weathering breaks down pre-existing rocks into smaller fragments through physical, chemical, and biological processes
• Erosion is the removal and transportation of weathered rock and mineral fragments by water, wind, ice, or gravity
• Deposition occurs when the energy of the transporting medium (water, wind, or ice) decreases, allowing the sediment to settle and accumulate
• Diagenesis is the process by which sediment is transformed into sedimentary rock through compaction, cementation, and recrystallization
  • Compaction reduces the pore space between sediment grains as the weight of overlying sediment increases
  • Cementation is the precipitation of minerals (such as calcite, silica, or iron oxides) from water that binds the sediment grains together
  • Recrystallization is the change in mineral composition or crystal size due to changes in temperature, pressure, or chemical conditions
• Lithification is the combined process of compaction and cementation that converts sediment into solid rock

Sedimentary Structures and Textures

• Bedding is the layering of sedimentary rocks, which reflects changes in the type or size of sediment being deposited over time
  • Bedding planes are the surfaces that separate individual beds or layers
• Cross-bedding is the inclined layering within a bed that forms from the migration of ripples or dunes under the influence of currents or waves
• Graded bedding is a vertical sequence in which the sediment size decreases from the bottom to the top of the bed, reflecting a decrease in the energy of the depositing current
• Ripple marks are small-scale, wave-like features on the surface of a sedimentary bed that form from the action of currents or waves
• Mud cracks are polygonal cracks that form in fine-grained sediments (such as mud or clay) as they dry and shrink
• Sole marks are irregular depressions or protrusions on the bottom surface of a sedimentary bed that form from the interaction between the sediment and the underlying surface
• Fossils are the preserved remains or traces of once-living organisms that can provide valuable information about past environments and life forms

Depositional Environments

• Continental environments include alluvial fans, rivers, lakes, deserts, and glaciers
  • Alluvial fans form at the base of mountain ranges where streams deposit sediment in a fan-shaped pattern
  • Rivers deposit sediment in channels, floodplains, and deltas
  • Lakes accumulate sediment in both deep and shallow water settings, often with seasonal variations in sediment input
  • Deserts are characterized by wind-blown sand (forming dunes) and evaporite deposits (such as salt flats)
  • Glaciers transport and deposit sediment through the action of moving ice and meltwater streams
• Transitional environments include deltas, estuaries, and beaches
  • Deltas form where rivers deposit sediment as they enter a standing body of water (such as a lake or ocean)
  • Estuaries are partially enclosed coastal bodies of water where freshwater from rivers mixes with saltwater from the ocean
  • Beaches are coastal accumulations of sand and gravel that are shaped by waves and currents
• Marine environments include shallow marine (continental shelf) and deep marine (continental slope and abyssal plain) settings
  • Shallow marine environments are characterized by the influence of waves, tides, and currents, and often have a high diversity of life forms
  • Deep marine environments are characterized by low-energy conditions, fine-grained sediment, and a lower diversity of life forms

Interpreting Past Environments

• Sedimentary rocks provide a record of the environments in which they were deposited, allowing geologists to reconstruct past landscapes, climates, and life forms
• Grain size and shape can indicate the energy of the transporting medium and the distance of transport
  • Angular grains suggest short transport distances and/or rapid burial, while rounded grains suggest longer transport distances and/or prolonged exposure to weathering and erosion
• Sedimentary structures (such as cross-bedding, ripple marks, and mud cracks) can indicate the presence and direction of currents, waves, or wind, as well as the water depth and exposure to air
• Fossil content can provide information about the age of the rock, the environment of deposition (marine, freshwater, or terrestrial), and the climate (tropical, temperate, or polar)
• Chemical and isotopic composition of sedimentary rocks can indicate the salinity, temperature, and chemical conditions of the water in which they were deposited, as well as the source of the sediment

Economic Importance of Sedimentary Rocks

• Many sedimentary rocks are important sources of natural resources, such as fossil fuels, building materials, and minerals
• Coal is a combustible sedimentary rock formed from the accumulation and alteration of plant remains in swampy environments
• Oil and natural gas are formed from the burial and heating of organic matter (mostly marine plankton) in fine-grained sedimentary rocks (such as shale)
  • Reservoir rocks (such as sandstone and limestone) are porous and permeable sedimentary rocks that can store and transmit oil and gas
  • Cap rocks (such as shale) are impermeable sedimentary rocks that trap oil and gas beneath them
• Limestone and sandstone are widely used as building stones and aggregate for concrete production
• Gypsum is used in the production of plaster, drywall, and cement
• Some sedimentary rocks (such as phosphorites and evaporites) are important sources of fertilizers and chemicals

Key Concepts and Terms

• Sediment: loose, solid particles of rock, minerals, or organic material that are transported and deposited by water, wind, ice, or gravity
• Weathering: the breakdown of rocks and minerals at or near Earth's surface through physical, chemical, and biological processes
• Erosion: the removal and transportation of weathered rock and mineral fragments by water, wind, ice, or gravity
• Deposition: the settling and accumulation of sediment when the energy of the transporting medium decreases
• Diagenesis: the process by which sediment is transformed into sedimentary rock through compaction, cementation, and recrystallization
• Lithification: the combined process of compaction and cementation that converts sediment into solid rock
• Bedding: the layering of sedimentary rocks, which reflects changes in the type or size of sediment being deposited over time
• Fossils: the preserved remains or traces of once-living organisms that can provide valuable information about past environments and life forms
• Depositional environment: the physical, chemical, and biological setting in which sediment is deposited and accumulates
• Principle of superposition: in an undeformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the one below it