The importance of fossilized soils in the Denver Basin and Kiowa core.

Paleosols, or fossilized soils, appear throughout the Denver Basin. They can be used by scientists to reconstruct ancient climates and landscapes providing information that enables accurate reconstruction of ancient environments. Additional information derived from paleosols can give clues to episodes of mountain building. The Denver Basin has a large, thick paleosol that appears on the surface in various places and in cored holes. This particular soil will provide information to project scientists on past climates and landscapes as well as helping us understand the timing of what is called the Laramide orogeny when the Rocky Mountains were formed.

The study of paleosols as indicators of past climates and landscapes is relatively new. Paleosols in the geologic record are extremely varied and pose problems for geologists attempting to reconstruct the environments they represented. Fortunately we live on a world that is covered in soil and can compare modern soils to paleosols and thereby generate clues to past environments. For example, we find paleosols in the rock record that look very much like the soils we see today in rainforests, temperate forests, arid climates, and grasslands. This comparison of ancient soils to modern soils has allowed us to make hypotheses about past climates and land surfaces.

It may seem intuitive to compare modern soils to past soils to generate ancient environments. On a finer scale, it is the actual minerals in the soils that allow us to make comparisons. Modern soils that exist in climates with lots of rainfall are made up of different minerals than soils from dry climates. The most useful type of minerals to soil scientists are the clay minerals. Clay minerals, or minerals that contain water, exist today all around us. They are of particular interest to engineers because they tend to swell when saturated with water, ruining the foundations of buildings inappropriately located. To the soil scientist, clay minerals provide clues to how much weathering the soil underwent, how much water moved through the soil, and hence climate. The more water that passes through a soil the more weathering. Different levels of weathering produce different clay minerals. Today the areas of the world that receive the most rainfall have the most weathered soils as well as specific types of clay minerals. Finding those same clay minerals in ancient soils allows us to hypothesize that the climate represented is the same as the climate represented by a similar modern soil.

Landscapes may be reconstructed by much the same methods. If you go to a forest today you will find areas, like bogs, where there is more water close to the surface (the water table) than there is elsewhere. You will also find areas that are well drained with no excess of water. These different landscapes have different types of soils. By looking at the mineralogy and characteristics of wet and dry areas in modern landscapes we can reconstruct small-scale landscapes in the ancient record.

Paleosols also give us information about the uplift of mountains. Soils form on land surfaces that are stable. A stable land surface is one that is not subject to burial by debris from, for example, the uplift of mountains. The longer that a landform is stable, the soil that forms on it will be thicker and more mature. Because the Denver Basin paleosol is so thick, 12 meters or more, it represents an extremely stable landscape that remained undisturbed for millions of years. This paleosol also happens to sit on top of rocks that record the rise of the Rocky Mountains. It sits below more rocks that also record mountain building. Because of the paleosols unique position, above and below rocks that show mountain building, we can hypothesize that there was a major pause in the formation of the Rocky Mountains. This pause is represented by the paleosol.

The scientists involved in the Denver Basin Project hope the paleosol in the Denver Basin will address issues such as what kind of climate does the soil represent, is there enough variation of the paleosol around the entire basin to reconstruct multiple landscapes, and what does the paleosol tell us about the uplift of the Rocky Mountains. Paleosol data from the Kiowa core will be compared with surface outcrops of the paleosol and other data from cores to provide answers to these questions. Amazingly, old dirt helps us reconstruct what the Denver area looked like millions of years ago.

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