1: Introduction
Sedimentary rocks are composed of material derived from the weathering and erosion of other, older rocks. They cover the vast majority of the terrestrial surface of the Earth and are an important archive of the history of life on Earth, ancient environments and processes, and are of profound economic and environmental significance. This book is a first step in learning how to "read the rocks” and understand the stories they tell us.
- Summarize the distribution of sedimentary rocks on the surface of the Earth and the relative abundance of the different sedimentary rock types
- Explain why sedimentary rocks provide important information about the ancient environments and ecosystems
- Summarize the economic, industrial, and practical importance of sedimentary rocks
Abundance and distribution
Sediment and sedimentary rocks cover the majority of the surface of the Earth; they cover about 70% of the continents and about 90% of the seafloor. Although widespread, they represent a relatively thin veneer and make up approximately 8% of the crust and less than 1% of our planet's total volume.
The geologic record of sedimentary rocks is skewed toward younger rocks and relatively fine grained rocks deposited in ocean basins and rapidly subsiding areas of sedimentary basins. Although conglomerates and breccias are striking, they form in proximal areas that are not likely to be preserved whereas mudrocks, sandstones and limestones are deposited in more distal areas that are more likely to subside and be preserved as part of a sedimentary basin. Numerous workers have provided estimates of the relative abundance of the different types of sedimentary rocks; although the numbers vary widely, the general patterns agree:
Mudrocks: 50-80%
Sandstones: 10-30%
Carbonates: ~10%
Conglomerates and breccias: ~1%
All others together: ~1%
Historical archive
Sedimentary rocks are deposited at the surface of the Earth and provide an important archive of information about the Earth's past. Their composition and texture contain important clues about tectonics, paleoclimate, and processes that were active at the surface of the Earth when they were deposited. The overwhelming majority of the fossil record is contained in sedimentary rocks and thus they record both the history of the Earth and the history of life on Earth.
Natural resources
Energy Resources
In terms of the economic significance of sedimentary rocks, most people immediately think of energy resources from fossil fuels including coal, oil, and natural gas. As eloquently described in Richard Alley's book "Earth - The Operator's Manual" these energy resources fueled the industrial revolution and are largely responsible for the convenient, comfortable, and cheap energy intensive world that many of us live in today. Although kicking the carbon habit is a challenging but necessary step to safeguard our future, fossil fuels still represent the majority (~79% in 2022) of the US energy mix and will likely continue to contribute to it for decades to come.
Uranium ore can come from a variety of igneous, metamorphic, and sedimentary sources, but well over half of the world's known sources come from sandstones.
Industrial materials
Sediments and sedimentary rocks are important sources of material for the construction of roads and buildings. Some specific examples include:
- Limestone is used to make the binding agent in cement and concrete, building stone and facade, and as a flux in steel making
- Sandstone is used for a building stone and facade
- Gypsum is used for plaster and drywall
- Rock salt is used for road de-icing and a food additive
- Kaolinite and other clays are used for brick and the manufacturing of paper, paint, ceramics, plastics, healthcare products and numerous other applications
- Sand and gravel are used for concrete, filtration, and grading for roads and foundations.
- Pitch is used in asphalt and road construction
Minerals
Sediments and sedimentary rocks are also important hosts for economically important minerals. Banded iron formations are an important source of iron ore, bauxites are an ore for aluminum, and sedimentary phosphorites are an important source of phosphorous.
Water, soil, and geomorphology
Inherently, sedimentary rocks have the potential to be more porous and permeable than crystalline rocks. Consequently, they are very important for groundwater recharge and storage and provide natural filtration as water slowly moves through ore spaces. Given their widespread distribution, the physical characteristics of sediment and sedimentary rocks represent the starting point for soil development; characteristics like mineralogy, grain size, and sorting are important factors that influence the nature of the resulting soils. From a geomorphology perspective, the physical characteristics of sedimentary rocks influences their resistance to erosion which can cause things like karst topography (limestones), susceptibility to mass movement (mudrocks and poorly consolidated materials), and the distribution of ridges, valley, cliffs, etc.
Resources and Readings
- Geology of Uranium Deposits by the World Nuclear Association
Chapter thumbnail shows the Chinle Badlands in Grand Staircase-Escalante National Monument, Utah ( U.S. Department of the Interior via Wikimedia Commons ; public domain ).