Sedimentary structures are the building blocks of Earth's history. From bedding to ripple marks, 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 facies associations and rock compositions, geologists can reconstruct ancient worlds. From rivers to reefs, each setting leaves its unique signature in the rocks, helping us piece together Earth's past.
Sedimentary Structures
Primary sedimentary structures
- Bedding forms layers in sedimentary rocks due to changes in sediment composition (mineralogy), grain size (sand vs silt), or depositional conditions (current velocity)
- Parallel bedding consists of horizontal, uniform layers (shale)
- Wavy bedding has undulating layers (sandstone)
- Lenticular bedding contains lens-shaped layers (limestone)
- Cross-stratification creates inclined layers within a bed formed by the migration of bedforms like dunes (desert) or ripples (river)
- Planar cross-stratification has straight, inclined layers (aeolian dunes)
- Trough cross-stratification exhibits curved, concave-upward layers (fluvial channels)
- Graded bedding displays a vertical change in grain size within a bed, typically showing a decrease in grain size from bottom to top known as normal grading, formed by the settling of sediment from a waning current (turbidite) or turbidity flow (underwater landslide)
Secondary sedimentary structures
- Sole marks are erosional features on the base of a bed formed by currents (flute casts) or objects (groove casts, load casts) moving over a soft substrate, indicating paleocurrent direction and substrate conditions
- Mud cracks (desiccation cracks) 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
- Symmetrical ripples result from wave action (beach)
- Asymmetrical ripples 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
- Fluvial facies association (river)
- Tidal facies association (estuary)
- Deep-marine facies association (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
- Quartz-rich sandstones suggest a continental source and high-energy environment (beach)
Continental vs marine depositional environments
- Continental environments are depositional settings on land
- Alluvial fans (mountain front)
- Rivers (meandering, braided)
- Lakes (freshwater)
- Deserts (sand dunes)
- Glaciers (moraines)
- Characterized by coarse-grained sediments, irregular bedding, and terrestrial fossils (plants, dinosaurs)
- Transitional environments occur between continental and marine environments
- Deltas (Mississippi River delta)
- Estuaries (Chesapeake Bay)
- Tidal flats (Wadden Sea)
- Lagoons (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
- Continental shelf (Gulf of Mexico)
- Slope (Atlantic margin)
- Deep-sea plain (abyssal plain)
- Reefs (Great Barrier Reef)
- Characterized by fine-grained sediments, regular bedding, and marine fossils (shells, fish)