Sedimentary Rocks
Chapter 9: Sedimentary Rocks
Learning Objectives
After reading this chapter and answering the review questions at the end, you should be able to:
Explain the differences between the four kinds of sedimentary rocks: clastic, chemical, biochemical, and organic.
Describe some of the specific kinds of rocks in each of the four categories and the depositional environments in which they form.
Describe the various terrestrial and marine sedimentary depositional environments and explain how the formation of sedimentary basins is related to plate tectonic processes.
Apply your understanding of the features of sedimentary rocks, including grain characteristics, sedimentary structures, and fossils, to the interpretation of past depositional environments and climates.
Explain what groups, formations, and members are in sedimentary rocks and why such terminology is used.
Formation and Characteristics of Sedimentary Rocks
Sedimentary rocks form in layers called beds.
Planar boundaries that separate each bed are called contacts.
Each bed indicates specific conditions under which it formed, such as fluctuations in sea level or previous environmental states (for instance, the presence of coal seams indicating swampy environments).
The processes leading to sedimentary rock formation include:
Weathering and Erosion: The transformation of pre-existing rocks into sediments through breakdown and removal.
Transportation: Movement of sediments or dissolved ions from the site of erosion to a deposition site via various mediums (wind, water, glaciers, mass movement).
Deposition: Occurs when conditions change sufficiently so that sediments can no longer be transported.
Burial: The process where sediments are covered, compacted, and cemented to modify their state.
Lithification: The conversion of compacted sediments into solid sedimentary rock, occurring at depths of hundreds to thousands of meters within Earth.
Compaction: Weight of new sediments compresses existing sediments, reducing pore space.
Cementation: Minerals precipitate from groundwater in pore spaces, binding grains together (common cementing minerals include quartz and calcite, but can also include hematite and clay).
Types of Sedimentary Rocks
There are four main types of sedimentary rocks:
Clastic Sedimentary Rocks: Composed mainly of solid fragments (called clasts) that were transported and then cemented together.
Chemical Sedimentary Rocks: Formed from material transported as ions in solution, often through evaporation leading to mineral precipitation.
Biochemical Sedimentary Rocks: Similar to chemical rocks but involve organisms converting ions into solid structures like calcium carbonate or silica.
Organic Sedimentary Rocks: Contain significant amounts of organic matter, including plant remains.
Clastic Sedimentary Rocks
Lithification Process:
The conversion of sediments to solid rock includes two key processes, compaction and cementation.
Grain Characteristics: Clastic rocks are identified based on clast characteristics such as grain size, shape, and sorting.
Types of Clastic Sedimentary Rocks:
Coarse-Grained Clastic Rocks:
Conglomerate: Clasts larger than 2 mm; formed in high-energy environments; rounded clasts.
Breccia: Also larger than 2 mm; composed of angular clasts; typically found in environments with minimal transport.
Medium-Grained Clastic Rocks:
Sandstone: Comprised of sand-sized grains; includes two subdivisions:
Arenite: "Clean" sandstone with less than 15% silt/clay, further classified based on mineral composition:
Quartz Arenite (mostly quartz), Arkosic Arenite (feldspar > fragments), Lithic Arenite (fragments > feldspar).
Wacke: "Dirty" sandstone with 15-75% fine-grained particles; further classified similarly to arenites.
Fine-Grained Clastic Rocks:
Mudrock: Composed mostly of silt and clay; further classified into:
Shale: Shows laminations and breaks into sheets.
Siltstone, Mudstone, Claystone: Based on dominantly silt, mixed, or clay content, respectively.
Sediment Maturity
Refers to the extent of sediment characteristics modified through prolonged weathering and transport.
Mature sediments (e.g., quartz sandstone, shale) undergo reduction in size, increase in rounding, and improved sorting.
Immature sediments (e.g., wacke, conglomerate) retain less refined characteristics.
Chemical and Biochemical Sedimentary Rocks
Composed of materials transported as ions in solution, with overlap in clastic rocks due to produced cement.
Chemical rocks form from inorganic processes; biochemical rocks involve the role of organisms in material conversion.
Common Examples:
Carbonate Rocks: Dominated by carbonate anions (CO3²⁻), sourced from calcite and aragonite.
Limestone: Can be both chemical and biochemical in origin, predominantly from marine environments with high biological activity.
Chert: Made of cryptocrystalline silica, forming often as nodules or from diatoms and radiolarians depositing their tests.
Evaporites: Form through crystallization of salts from evaporating water in arid conditions.
Organic Sedimentary Rocks
Contain significant organic carbon from plant and animal remains, notably coal formed in swampy, low-oxygen environments where accumulation prevents decay.
Process includes the transformation of peat to lignite, bituminous coal, and eventually anthracite under increasing pressure and temperature.
Depositional Environments and Sedimentary Basins
Accumulate in varied environments both terrestrial and marine, marking different forms of sediment deposition.
Preserved sediments represent significant time laxes and are most often found in basins formed by tectonic activity.
Types of sedimentary basins include trench basins, forearc basins, foreland basins, and rift basins, each associated with different tectonic processes.
Sedimentary Structures and Fossils
Observed characteristics include bedding, cross-bedding, graded bedding, and sedimentary features like ripples and mud cracks. Key principles include:
Original Horizontality: Layers deposited horizontally.
Superposition: Lower layers are older unless disturbed.
Inclusions: Fragments within a layer are older than the layer itself.
Faunal Succession: Fossils can determine the age and environmental context of rocks.
Fossils provide insights into past climates and environmental conditions, supporting interpretations of depositional history.
Groups, Formations, and Members
Stratigraphic units are categorized as formations (distinct beds), groups (series of formations), and members (sub-divisions of formations based on significant lithologies).
Example: The Nanaimo Group is a well-studied formation with various subdivisions reflecting diverse depositional environments over geological time.
Summary of Key Topics
Clastic sedimentary rocks vary from conglomerates to mudstones and are classified based on size, sorting, and composition.
Chemical and biochemical rocks are primarily formed from ions in solution, with limestone being a key example.
Origin of sediments relates closely to their depositional environments, which are vital for understanding geological history and processes.
Review Questions
What are the minimum and maximum sizes of sand grains?
Why can't conglomerate material be deposited by a slow-flowing river?
Describe the two main processes of lithification.
Differentiate between lithic arenite and lithic wacke.
Compare feldspathic arenite with quartz arenite.
Discuss the source rock lithology for quartz sandstone and its weathering and transportation history.
Identify the original source of carbon in carbonate deposits like limestone.
What long-term environmental change on Earth influenced the deposition of banded iron formations?
Define two significant terrestrial and marine depositional environments.
Compare foreland and forearc basins in terms of origin and tectonic settings.
Explain bedding, cross-bedding, graded bedding, and mud cracks as depositional features.
Under what conditions does reverse-graded bedding form?
How is the term formation applied to sedimentary rock classification?
Outline the reasons for dividing some formations into members, particularly in the context of the Nanaimo Group.