Presentation on theme: "Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 7 Sediments."— Presentation transcript:
Classroom presentations to accompany Understanding Earth, 3rd edition prepared by Peter Copeland and William Dupré University of Houston Chapter 7 Sediments and Sedimentary Rocks
Sedimentary rock (a) Rock resulting from the consolidation of loose sediment that has been derived from previously existing rocks and accumulated in layers (detrital or clastic) (b) Rock formed by the precipitation of minerals from solution by either organic or inorganic processes (chemical)
Sedimentary rocks 5 % by volume of the upper crust 75% by area of continental areas Often the only record of geologic events: e.g. The Himalayas will someday be sandstone
Transport and deposition of clastic sediments Movement of sediment by wind, ice or water. Mode of transport produces distinctive deposits.
Transport affects the sediment in several ways Sorting Sorting : measure of the variation in the range of grain sizes in a clastic rock or sediment Well-sorted sediments indicate that they have been subjected to prolonged water or wind action. Poorly-sorted sediments are either not far-removed from their source or deposited by glaciers.
Transport affects the sediment in several ways Roundness Roundness : measure of how rounded the corners are Sphericity Sphericity : measure of how much it is like a sphere Sorting, roundness, and sphericity all increase with amount of transport.
Types of detrital rocks Largely based on the size of the particles, which may be anything. Conglomerate Breccia Sandstone (quartzite, arkose, greywacke) Shale Mudstone Siltstone
Sedimentary structures stratification = bedding = layering Produced due to differences in 1. size of particles 2. kinds of particles
Clues to interpreting sedimentary environment Sedimentary structures Sorting, roundness, sphericity Sequence of beds
Types of chemical sedimentary rocks LimestoneCaCO 3 ChertSiO 2 SaltNaCl, KCl, K 2 SO 4 GypsumCaSO 4 2H 2 O Coalaltered organic debris
Chemical environments: Carbonates Clear water — away from big rivers (or volcanoes) Warm water — subtropical to tropical shallow water, two reasons: Organic:sunlight only penetrates to about 100 m Inorganic: CCD (dissolution CaCO 3 dependant on P)
Chemical environments: Evaporites Restricted environments (Mediterranean Sea, Texas coast) Minerals precipitate according to solubility. Gypsum Halite 50%90% CaSO 4 2H 2 O NaCl
Sedimentary structures Particular structural features can give information about the environment of deposition. Structures also help determine if a bed is right-side-up. — this is important in deformed rocks
Turbidity currents Suspension of water, sand, and mud that moves downslope (often very rapidly) due to its greater density than that of the surrounding water (often triggered by earthquakes) Speed of turbidity currents first appreciated in 1920 — breaking of phone lines in the Atlantic; also gave indication of distance traveled by a single deposit
Other up indicators Cross-bedding Ripple marks Mudcracks Raindrop impressions Fossils (some may have been preserved in growth position)
From sediment to sedimentary rock (lithification) Compaction: reduces pore space Clays and muds are up to 60% water; 10% water after compaction. Cementation: chemical precipitation of mineral material between grains (SiO 2, CaCO 3, Fe 2 O 3 ) binds sediment into hard rock Recrystallization: P and T increase with burial 30°C/km or 1°C/33 m 2