6.3 Sedimentary structures and depositional environments
3 min read•july 22, 2024
Sedimentary structures are the building blocks of Earth's history. From to , these features tell tales of ancient environments and processes. They're like nature's own time capsules, preserving clues about past landscapes and climates.
Depositional environments are the stages where sedimentary stories unfold. By studying and , geologists can reconstruct ancient worlds. From to , each setting leaves its unique signature in the rocks, helping us piece together Earth's past.
Sedimentary Structures
Primary sedimentary structures
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Top images from around the web for Primary sedimentary structures
Herringbone & trough cross-stratified quartzose sandstones… | Flickr View original
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Cross bedding in structurally tilted quartzose sandstones … | Flickr View original
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- Bedding forms layers in sedimentary rocks due to changes in sediment composition (mineralogy), grain size (sand vs silt), or depositional conditions (current velocity)
- consists of horizontal, uniform layers (shale)
- has undulating layers (sandstone)
- contains lens-shaped layers (limestone)
- creates inclined layers within a bed formed by the migration of bedforms like dunes (desert) or ripples (river)
- has straight, inclined layers (aeolian dunes)
- exhibits curved, concave-upward layers (fluvial channels)
- displays a vertical change in grain size within a bed, typically showing a decrease in grain size from bottom to top known as , formed by the settling of sediment from a waning current () or turbidity flow (underwater landslide)
Secondary sedimentary structures
- are erosional features on the base of a bed formed by currents () or objects (, ) moving over a soft substrate, indicating paleocurrent direction and substrate conditions
- () form polygonal cracks in mudstone or siltstone due to the shrinkage and cracking of mud during drying, indicating subaerial exposure and arid conditions (playa lake)
- Ripple marks create undulating surface features formed by the interaction of currents or waves with sediment
- result from wave action (beach)
- are produced by unidirectional currents (river)
- Ripple marks provide information about paleocurrent direction and depositional environment
Depositional Environments
Reconstruction of depositional environments
- Facies associations are groups of sedimentary facies that occur together and are genetically related, reflecting specific depositional environments
- (river)
- (estuary)
- (submarine fan)
- Sedimentary structures, both primary and secondary, provide clues about depositional processes and conditions
- The presence of cross-stratification suggests deposition by currents (fluvial) or waves (coastal)
- Rock compositions, including mineralogy and texture of sedimentary rocks, reflect the source area and depositional environment
- suggest a continental source and high-energy environment (beach)
Continental vs marine depositional environments
- are depositional settings on land
- (mountain front)
- Rivers (meandering, braided)
- (freshwater)
- (sand dunes)
- (moraines)
- Characterized by coarse-grained sediments, irregular bedding, and terrestrial fossils (plants, dinosaurs)
- occur between continental and
- (Mississippi River delta)
- (Chesapeake Bay)
- (Wadden Sea)
- (barrier island)
- Beaches (coastline)
- Characterized by a mix of continental and marine influences, such as alternating coarse and fine sediments, and both terrestrial and marine fossils (crocodiles, sharks)
- Marine environments are depositional settings in the ocean
- (Gulf of Mexico)
- (Atlantic margin)
- (abyssal plain)
- Reefs (Great Barrier Reef)
- Characterized by fine-grained sediments, regular bedding, and marine fossils (shells, fish)
Key Terms to Review (41)
Alluvial fans: Alluvial fans are fan-shaped deposits of sediment formed at the base of mountain ranges, where a river or stream exits a steep slope onto a flatter plain. These formations result from the sudden decrease in water velocity as it spreads out, causing sediment to settle and accumulate in a cone-like shape. Alluvial fans are important indicators of past fluvial processes and can offer insights into sedimentary structures and depositional environments.
Asymmetrical Ripples: Asymmetrical ripples are sedimentary structures characterized by their non-uniform shapes, typically formed by unidirectional flow, such as that seen in rivers or wind-driven environments. These ripples have a steeper slope on one side and a gentler slope on the other, indicating the direction of sediment transport. They provide important clues about the depositional environments and the conditions under which sediments were laid down.
Bedding: Bedding refers to the distinct layers of sedimentary rock that are formed as sediments are deposited over time in a variety of environments. Each layer, or bed, can vary in thickness, composition, and color, providing insights into the conditions under which it was laid down. The study of bedding helps geologists interpret the geological history and past environmental conditions of an area.
Continental environments: Continental environments refer to land-based settings where sedimentary processes occur, shaping the Earth's surface and influencing the types of sediments that accumulate. These environments can include rivers, lakes, deserts, and glacial areas, each with distinct sedimentary structures that reflect the conditions of their formation. Understanding these environments is essential for interpreting past geological events and reconstructing ancient landscapes.
Continental shelf: The continental shelf is the submerged area of land that extends from the shoreline out to the continental slope, typically characterized by shallow waters. This region is crucial for sediment deposition, as it serves as a significant site for sediment accumulation due to the influence of waves, tides, and currents. It supports diverse ecosystems and is often rich in natural resources, making it a key zone for both biological and geological studies.
Cross-stratification: Cross-stratification is a sedimentary structure characterized by the arrangement of sediment layers at an angle to the main bedding plane. This feature often forms due to processes like current or wind action, where sediments are deposited in a series of inclined layers that differ from the horizontal layering typical in sedimentary rocks. It provides vital insights into the depositional environments and helps geologists interpret ancient environmental conditions.
Deep-marine facies association: A deep-marine facies association refers to a collection of sedimentary deposits and features that are characteristic of deep ocean environments, typically found at depths greater than 200 meters. This term encompasses a variety of sediment types, such as turbidites and pelagic sediments, and reflects the processes of sedimentation that occur in low-energy, deep-water settings. Understanding these associations is crucial for interpreting past marine environments and the geological history of ocean basins.
Deep-sea plain: A deep-sea plain is a vast, flat, and relatively featureless area of the ocean floor, typically found at depths greater than 3,000 meters (about 9,800 feet). These plains are formed by the accumulation of sediments that settle from the water column, creating a smooth surface. They are significant for understanding sedimentary processes and the geological history of ocean basins.
Deltas: Deltas are landforms created at the mouth of a river where it meets a body of water, such as an ocean, sea, or lake. They form from the accumulation of sediment that is deposited as the river slows down and loses energy when it enters standing water. Deltas are dynamic environments that evolve over time due to processes of erosion and deposition, playing an important role in shaping landscapes, influencing sedimentary structures, and contributing to fluvial landforms.
Deserts: Deserts are arid regions characterized by low precipitation, often less than 10 inches annually, and extreme temperature variations between day and night. These environments support specialized ecosystems and geological features shaped by limited water availability, making them crucial for understanding sedimentary processes and landform development.
Desiccation cracks: Desiccation cracks are fractures that occur in sediment or soil due to the process of drying and shrinkage when moisture is lost. These cracks typically form in environments where sediments are exposed to fluctuating water levels, such as riverbeds, lake shores, or arid landscapes. The presence of desiccation cracks indicates past dry conditions and can provide insights into sedimentary processes and environmental changes.
Estuaries: Estuaries are coastal areas where freshwater from rivers meets and mixes with saltwater from the ocean. These unique environments are characterized by varying salinity levels and are often rich in nutrients, making them vital habitats for a diverse range of plants and animals. Estuaries serve as crucial transitional zones between land and sea, influencing sedimentary processes and depositional environments.
Facies associations: Facies associations refer to a group of related sedimentary facies that occur together in a specific depositional environment, indicating the conditions under which sediments were deposited. Understanding these associations helps geologists interpret the history of sedimentary environments and reconstruct past geological settings, revealing insights into ancient landscapes and their evolution over time.
Flute casts: Flute casts are sedimentary structures formed by the scouring of sediment by turbulent water flow, typically seen in river or marine environments. They are characterized by elongated, scoop-shaped depressions that indicate the direction of current flow and provide valuable information about past hydrodynamic conditions. Flute casts serve as indicators of high-energy environments where sediment was actively moved and shaped by flowing water.
Fluvial facies association: A fluvial facies association refers to a group of sedimentary features and rock types that are formed by river-related processes, showcasing the characteristics of sediment deposition in river systems. This association provides insights into the dynamics of river environments, including the variations in flow regimes, sediment supply, and channel morphology, which are essential for understanding ancient depositional settings.
Glaciers: Glaciers are large masses of ice that form from the accumulation and compaction of snow over long periods. These slow-moving rivers of ice can significantly shape the landscape through processes like erosion and deposition, which directly relate to sedimentary structures and environments created by glacial activity. As glaciers advance and retreat, they carve out valleys, transport sediments, and leave behind unique geological features, influencing the Earth's surface and sedimentation processes.
Graded bedding: Graded bedding is a sedimentary structure characterized by a gradual change in grain size within a single layer, typically transitioning from coarser material at the bottom to finer material at the top. This phenomenon often indicates the energy conditions during deposition, revealing important information about past environments and sedimentary processes. Graded bedding is commonly associated with turbidity currents and can help classify sedimentary rocks based on their formation processes and depositional settings.
Groove casts: Groove casts are sedimentary structures that form when sediment fills in the grooves or furrows on the surface of a rock or sediment layer. These structures are indicative of the movement of water, wind, or other forces that create linear depressions, which then become filled with sediment during subsequent deposition. Groove casts provide valuable insight into past environmental conditions and the processes that shaped sedimentary environments.
Lagoons: Lagoons are shallow bodies of water that are typically separated from larger bodies of water, such as oceans or seas, by a barrier such as a sandbar or coral reef. These unique environments play a critical role in sedimentary processes and serve as important habitats for various species, linking coastal ecosystems with the open sea and influencing sediment deposition patterns.
Lakes: Lakes are large bodies of freshwater or saltwater that are surrounded by land, playing a crucial role in the Earth's hydrological cycle and various ecological systems. They can be formed through various geological processes such as glacial activity, tectonic movements, and volcanic activity, influencing the surrounding sedimentary structures and depositional environments. Lakes also serve as important habitats for wildlife and impact regional climate and water resources.
Lenticular bedding: Lenticular bedding refers to a sedimentary structure characterized by the presence of lens-shaped layers of sediment that are typically formed in environments with varying current strength. These beds often indicate changes in energy conditions, reflecting how sediment was deposited and later modified by natural processes. This bedding type can reveal important information about past environments and sedimentary dynamics, making it essential for understanding depositional environments.
Load casts: Load casts are sedimentary structures that form when sediment is displaced by the weight of overlying material, creating impressions or indentations in the underlying layers. These structures provide insight into the depositional environments and sedimentary processes, revealing how weight and pressure can shape sedimentary features. Load casts are often associated with soft sediments and can indicate changes in loading conditions, which helps geologists understand the dynamics of sediment accumulation and compaction.
Marine environments: Marine environments refer to the vast ecosystems found in oceanic and coastal regions, encompassing a wide range of habitats such as coral reefs, open oceans, and estuaries. These environments play a crucial role in sedimentation processes, influencing the types and distribution of sedimentary structures formed by both biological and physical processes. Understanding marine environments helps in deciphering the geological history recorded in sedimentary rocks.
Mud cracks: Mud cracks are polygonal features that form in sediment when it dries and contracts, resulting in a series of interconnected fractures. These structures are essential indicators of past environmental conditions, particularly relating to the drying and wetting cycles of sedimentary environments, showcasing the influence of climatic conditions on sediment deposition.
Normal grading: Normal grading refers to a sedimentary structure characterized by a gradual change in grain size within a sedimentary layer, typically transitioning from coarser grains at the bottom to finer grains at the top. This grading is indicative of the depositional processes that occurred in a specific environment, often reflecting changes in energy conditions or sediment supply over time.
Parallel bedding: Parallel bedding refers to a sedimentary structure characterized by layers of sediment that are deposited in horizontal or nearly horizontal planes. This type of bedding typically forms in environments with relatively low energy, allowing sediments to settle uniformly and create distinct layers that run parallel to each other. It is an important indicator of the depositional environment, reflecting processes such as sediment transport and settling rates in water bodies like rivers, lakes, and shallow seas.
Planar cross-stratification: Planar cross-stratification is a sedimentary structure characterized by flat, inclined layers that form within a sedimentary deposit, typically as a result of sediment transport and deposition processes. This structure provides important insights into the flow dynamics of the environment in which the sediments were deposited, often indicating current flow direction and sedimentary processes like sorting and layering.
Quartz-rich sandstones: Quartz-rich sandstones are sedimentary rocks primarily composed of quartz grains, typically formed from the lithification of sand-sized particles. These rocks are significant indicators of their depositional environments, revealing clues about past geological processes and settings where they were formed, such as riverbeds, beaches, and deserts.
Reefs: Reefs are underwater structures formed by the accumulation of calcium carbonate, primarily from coral organisms, and can serve as significant habitats for marine life. They play a crucial role in coastal protection, biodiversity, and the overall health of marine ecosystems, showcasing complex sedimentary structures that indicate various depositional environments.
Ripple marks: Ripple marks are small, wave-like patterns formed on the surface of sedimentary rocks, created by the movement of water or wind over loose sediments. These structures provide important clues about past environmental conditions and sedimentary processes, often indicating the direction of sediment transport and the energy of the environment in which they formed.
Rivers: Rivers are natural flowing watercourses, typically freshwater, that flow towards an ocean, sea, lake, or another river. They are crucial components of the hydrological cycle and play a significant role in shaping landscapes through erosion and sediment deposition, thus creating various sedimentary structures and influencing depositional environments.
Rock compositions: Rock compositions refer to the specific minerals and elements that make up a rock, determining its characteristics, classification, and behavior within various geological contexts. Understanding rock compositions helps in identifying sedimentary structures and depositional environments, as different compositions reveal the conditions under which the rocks formed and the processes that influenced their development.
Slope: Slope refers to the angle or steepness of a surface, particularly in the context of landforms and sedimentary processes. It plays a crucial role in influencing how sediment is deposited and eroded, affecting the formation of various sedimentary structures. Understanding slope is key to recognizing how landscapes evolve and how materials interact within different depositional environments.
Sole marks: Sole marks are unique sedimentary structures formed on the bottom of a sedimentary layer, typically created by the movement of sediment and the action of currents. These features can include scratches, grooves, or impressions left by objects or organisms as sediments settle and compact over time. Sole marks provide valuable insights into past environmental conditions and the dynamics of sediment transport.
Symmetrical ripples: Symmetrical ripples are sedimentary structures formed in environments with oscillatory flow, typically seen in shallow water settings. These features are characterized by their wave-like shape, where the ripples rise and fall symmetrically, indicating the influence of waves or currents acting in both directions. Symmetrical ripples provide insight into past environmental conditions, sediment transport processes, and can help in interpreting the energy levels of depositional environments.
Tidal facies association: A tidal facies association refers to a distinctive arrangement of sedimentary structures that are formed in coastal environments influenced by tidal processes. These associations help to characterize specific depositional settings, including variations in sediment composition, grain size, and structural features that arise from the interplay of tidal forces and sediment transport. Understanding tidal facies associations allows geologists to interpret ancient environments and predict the distribution of resources like hydrocarbons and aquifers.
Tidal flats: Tidal flats are coastal areas that are regularly exposed and submerged by the tides, forming a unique environment characterized by sedimentary deposits. These flats play a critical role in coastal ecosystems, serving as transition zones between land and water, where various sediment types accumulate through the interplay of tidal forces and wave action. Tidal flats are essential for understanding sedimentary structures and the processes that shape depositional environments.
Transitional Environments: Transitional environments are zones where terrestrial and marine processes interact, leading to unique sedimentary conditions and depositional features. These areas, like deltas and estuaries, play a critical role in the sedimentary record by capturing sediment from both land and sea, often resulting in diverse geological formations and ecosystems. Understanding these environments helps in interpreting past geological events and ecological changes.
Trough cross-stratification: Trough cross-stratification refers to a sedimentary structure characterized by inclined layers or sets of strata that form within the confines of a trough-like feature. These structures typically indicate deposition by currents, such as rivers or tidal flows, and reveal important information about the energy conditions and flow directions in ancient environments. Recognizing trough cross-stratification helps geologists interpret past sedimentary environments and understand the dynamics of sediment transport and deposition.
Turbidite: A turbidite is a sedimentary deposit formed by the rapid flow of sediment-laden water down a slope, typically in underwater environments like continental slopes and deep-sea basins. These deposits are characterized by their distinct layering and grading, which result from the change in flow velocity as the turbulent flow slows down, allowing larger particles to settle out first, followed by smaller particles. Turbidites provide crucial insights into past geological processes and depositional environments.
Wavy bedding: Wavy bedding refers to a sedimentary structure characterized by undulating or ripple-like patterns in layered sediments. These patterns form due to various processes, including wave action in shallow marine environments and currents, which influence how sediments settle and accumulate over time. Wavy bedding is an important indicator of depositional environments, helping geologists interpret past conditions and understand sedimentary processes.