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Sedimentary Environments Lesson

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Step 1: Diagenesis - Soil Formation

Soil is a mix of mineral matter (about 45%), water (25%), air (25%) and organic matter (5%).

Soil begins to form as water (usually rain) enters the ground. Biochemical weathering occurs along the roots of plants, breaking down the minerals in the ground into ions. Physical weathering of the ground is due to roots, worms, and other organisms; break up the ground just beneath the surface, allowing the water and the ions to seep deeper into the Earth. This downward movement of water is called the zone of leaching.

Silts, clays, and ions filtered down from the upper zone begin to clog the pore spaces in the sediments in the deeper portion of the ground. This region is referred to as the zone of accumulation. Chemical weathering of the sediments/bedrock occurs in this area.

Soil Horizons - Distinct horizons reflect soil-forming processes.

Zone of Leaching

Zone of Accumulation


Soil genesis influenced by a number of factors.

Climate - Climate exerts a dominant control on soil development. The climate of a region changes with both latitude and elevation. Lower latitudes typically have wetter climates, so soils in these region are deeply weathered. Desert and Mediterranean climates, both regions with low rainfall amounts, predominate at thirty degrees north and south latitude, and soil horizons are either shallow or non-existent. Temperate zones between 40 to 70 degrees north and south latitude have increasing amounts of rainfall and a correspondingly deep soil development. The poles are deserts, cold ones to be sure, and there is little to no soil development as a result. A similar scenario exists for elevation. The higher the elevation, the less rain (or snow), and the less soil is present

Substrate - Some rocks just weather faster than others. Those that do produce soils more easily.

Slope steepness - Water tends to accumulate on flat ground. Therefore, the flatter the slope the deeper the soil development.

Drainage - The faster an area drains, the less saturation of the ground there is, and less chemical weathering occurs. This, in turn, reduces the amount of soil formation.

Time - The longer the soil has to develop, the deeper the soil will be.

Vegetation - typically, the more vegetation, the more biochemical alteration occurs, and the deeper the soil development.

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