6.8: "High-Energy" and "Low-Energy" Depositional Environments
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"High-Energy" and "Low-Energy" Depositional Environments
Flowing water is the dominant natural force causing erosion and deposition on Earth. The faster the water moves, the higher the energy in a physical setting. As flowing water increases in speed, the more it may become turbulent, increasing its ability to lift and move particles. Fast moving water can carry materials of different sizes ranging from boulders and gravel to finer materials (sand, silt, and clays). Flowing water also sorts sediments by size and density. High-energy environments include river channels, beach and shallow offshore environments with high wave action, and wave-battered coral reefs (Figures 6-21 to 6-22). Fast flowing water from waves and currents may let larger materials settle and be deposited while finer materials are carried away and deposited in quieter water settings or what are considered low-energy environments. (Figures 6-23 to 6-24). Low-energy environments on land include most lakes and swamps, and low-energy conditions exist in protected bays and lagoons, and in deeper-water setting in ocean in locations not significantly impacted by wave and strong current action.
Different sedimentary environments have different energy characteristics that may change from time to time. The forces of energy in a stream will increase as the volume of water increases, such as during flood. For most of a year, a stream will may be a calm environment, that changes during a flood, or during a flood season. The same is true of beach and offshore bar environments. As wave energy increases, the greater the amount of energy translates into shoreline erosion and the moving of sediments to quieter and deeper offshore settings. Wave action separates sand from courser and finer fractions, building up or eroding beaches with changing conditions. A beach or offshore region can remain basically calm, relatively low energy for years until a hurricane comes along, and the setting becomes "high energy." One big storm event can move more sediments in a few days that might have moved for decades or even centuries. For example, Hurricane Camille did this to the coast of Alabama and Mississippi in 1969.
Deep-water environments far from shore tend to be low energy environments. However, in regions along continental margins quiet conditions can be suddenly disrupted by the rapid influx of sediments caused by massive underwater landslides or the effects of major storms on the nearby continental shelves.
High-Energy Depositional Environments
Coarse-grain sediments dominate. Weather (climate), currents, and wave energy are variable factors in "high-energy" environmental settings. Sediments are constantly being deposited or eroded in these settings.
|Figure 6.21. Beaches||Figure 6.22. Coral reefs|
Low-Energy Depositional Environments
Fine grained sediments dominate. Slow-moving currents prevent coarse-grained sediment from migrating into in low-energy settings. Fine materials can be carried long distances before they can settle out in the absence of waves and currents.
|Figure 6.23. Lake (lacustrine) and swamp environments.||Figure 6.24. Tidewater marsh and estuary/lagoon settings.|
Sedimentary deposits preserve aspects of the energy levels of the locations in which they were deposited.
In general, the particle size of sediments is larger in sedimentary deposit deposited in high-energy environments. Fine-grained sediments tend to erode in high energy environments, and tend to be deposited in low energy environments. In addition, the higher energy environments tend to have higher dissolved oxygen and nutrient concentrations, which influences the kind of organisms that live in such environments.