Overview
This lecture covers organic sedimentary rocks, focusing on their formation, types, and economic importance, especially regarding fossil fuels like coal and oil shale.
Organic Sedimentary Rocks: Introduction
- Organic sedimentary rocks form from the accumulation and lithification of once-living material (fossils).
- Main types are coal (from land plants) and oil shale (from marine organic matter).
Formation of Coal
- Coal forms from plant and organic material in swampy terrestrial environments over millions of years (process called coalification).
- Stages: peat (partially decayed plant matter) → lignite (soft brown coal) → bituminous coal (dull, harder) → anthracite (shiny, metamorphic).
- Burial, heat, and pressure drive the transformation of organic matter through these stages.
- Most of Earth's coal formed during the time of the dinosaurs, primarily when continents formed the supercontinent Pangea.
Formation of Petroleum and Oil Shale
- Marine organic material accumulates on the ocean floor, mixes with fine-grained sediment, and is buried.
- When the sediment is impermeable, organic matter transforms into petroleum (oil and gas) as it becomes trapped in reservoir rocks, often shale.
- Oil shale is an organic sedimentary rock with pores filled by crude oil and natural gas, forming over millions of years.
Identification and Importance
- Organic sedimentary rocks are identified by their composition (plant or marine organic matter) and visible texture.
- These rocks are economically valuable as sources of fossil fuels (coal, petroleum, natural gas).
Key Terms & Definitions
- Organic Sedimentary Rocks — Rocks formed from the accumulation of organic material.
- Coalification — The process of converting plant material into coal over millions of years.
- Peat — Partially decayed plant matter, an early stage of coal formation.
- Lignite — A soft, brown type of coal, formed from compressed peat.
- Bituminous Coal — A harder, dull coal formed from lignite.
- Anthracite — A shiny, hard coal formed from bituminous coal under heat and pressure (metamorphic).
- Oil Shale — Sedimentary rock containing organic material that yields oil and gas when processed.
Action Items / Next Steps
- Review the coalification process and stages of coal formation.
- Study the properties and formation of oil shale and petroleum.
- Be able to identify organic sedimentary rocks by composition and texture.