6.3 Sedimentary structures and depositional environments

Sedimentary Structures

Sedimentary structures are physical features preserved in sedimentary rocks that record how and where sediment was deposited. By reading these structures, geologists can reconstruct ancient environments: the speed and direction of currents, whether an area was underwater or exposed to air, and even the climate at the time. Think of them as clues left behind by processes that happened millions of years ago.

Primary Sedimentary Structures

Primary structures form during deposition. They reflect the conditions that existed as sediment was being laid down.

Bedding is the most fundamental sedimentary structure. It forms layers due to changes in sediment composition (mineralogy), grain size (sand vs. silt), or energy of the environment (current velocity). There are several types:

• Parallel bedding consists of horizontal, uniform layers. Common in shale deposited in calm, low-energy settings.
• Wavy bedding has undulating, uneven layers, often found in sandstone where energy levels fluctuated.
• Lenticular bedding contains lens-shaped layers of one sediment type enclosed in another, common in mixed-energy settings like tidal flats.

Cross-stratification produces inclined layers within a bed. These form when bedforms like dunes or ripples migrate in the direction of a current or wind. The angle and shape of the inclined layers tell you about the flow:

• Planar cross-stratification has straight, inclined layers. Typical of aeolian (wind-blown) dunes in desert environments.
• Trough cross-stratification shows curved, concave-upward layers. This is characteristic of fluvial (river) channels where three-dimensional dunes migrate downstream.

Graded bedding shows a vertical change in grain size within a single bed. In normal grading, the coarsest grains are at the bottom and grain size decreases upward. This happens when a current gradually loses energy and progressively finer particles settle out. Turbidites are a classic example: an underwater sediment flow (turbidity current) rushes down a slope, then slows, depositing coarse material first and fine material last.

Secondary Sedimentary Structures

Secondary structures form after initial deposition, either through erosion, deformation, or exposure.

Sole marks are features preserved on the bottom surface of a bed. They form when currents or objects scour into soft sediment below, and the overlying bed fills in the impression. Types include flute casts (scoop-shaped marks from turbulent currents) and groove casts (linear marks from objects dragged along the bottom). Sole marks are useful because they indicate paleocurrent direction, the direction ancient currents were flowing.

Mud cracks (also called desiccation cracks) are polygonal cracks that form when wet mud dries out and shrinks. Finding mud cracks in a rock tells you that surface was exposed to air at some point, which rules out a permanently submerged environment. They're common in settings like playa lakes and tidal flats that periodically dry out.

Ripple marks are small, undulating ridges on a sediment surface created by the interaction of water or wind with loose sediment. Their shape reveals the type of flow:

• Symmetrical ripples have equal slopes on both sides and form from oscillating wave action, like on a beach or shallow shelf.
• Asymmetrical ripples have a steeper side facing downstream and form from unidirectional currents, like in a river or tidal channel.

Both types preserve paleocurrent information and help identify the depositional environment.

Depositional Environments

A depositional environment is the specific geographic setting where sediment accumulates. Each environment has a characteristic combination of physical, chemical, and biological conditions that leave a recognizable signature in the rock record.

Reconstructing Depositional Environments

Geologists use three main lines of evidence to figure out where a rock was originally deposited:

1. Facies associations are groups of sedimentary facies (distinct rock types) that consistently occur together because they formed in related sub-environments. For example, a fluvial facies association might include channel sandstones, floodplain mudstones, and point-bar deposits all stacked together. Other examples include tidal facies associations (estuaries) and deep-marine facies associations (submarine fans).

2. Sedimentary structures record the physical processes at work. Cross-stratification points to deposition by currents or waves. Mud cracks indicate periodic drying. Graded bedding suggests turbidity currents. No single structure is definitive on its own, but the combination narrows down the environment.

3. Rock composition, including mineralogy and texture, reflects both the sediment source and the depositional setting. A quartz-rich, well-sorted sandstone suggests prolonged transport and a high-energy environment like a beach, where less durable minerals have been broken down and removed.

Continental, Transitional, and Marine Environments

Depositional environments are broadly grouped into three categories based on their position relative to the shoreline.

Continental environments are depositional settings on land. They tend to produce coarser-grained sediments with irregular bedding and contain terrestrial fossils (plants, land animals). Examples include:

• Alluvial fans (at mountain fronts, where steep streams spread onto flat terrain)
• Rivers (meandering rivers with point bars; braided rivers with gravel bars)
• Lakes (freshwater settings with fine-grained, laminated sediments)
• Deserts (wind-blown sand dunes producing well-sorted, cross-stratified sandstone)
• Glaciers (poorly sorted deposits called till, plus moraines)

Transitional environments sit between land and open ocean, where continental and marine processes interact. Sediments here alternate between coarse and fine, and fossils include both terrestrial and marine organisms. Examples include:

• Deltas (like the Mississippi River delta, where a river deposits sediment into the sea)
• Estuaries (like Chesapeake Bay, where seawater and river water mix)
• Tidal flats (like the Wadden Sea, with alternating sand and mud layers reflecting tidal cycles)
• Lagoons (sheltered water bodies behind barrier islands)
• Beaches (high-energy shoreline deposits of well-sorted sand)

Marine environments are depositional settings in the ocean. They generally produce finer-grained sediments with more regular bedding and contain marine fossils (shells, fish, plankton). Examples include:

• Continental shelf (shallow marine areas like the Gulf of Mexico, with sand and mud)
• Continental slope (steeper areas like the Atlantic margin, where turbidity currents are common)
• Abyssal plain (deep-sea floor with very fine-grained sediment settling slowly from above)
• Reefs (like the Great Barrier Reef, built by organisms and composed largely of carbonate)