The Nature of Sedimentary Rocks Sedimentary rocks are composed of: Fragments of other rocks Chemical precipitates Organic matter or biochemically produced materials
The Nature of Sedimentary Rocks Sedimentary rocks are common at the Earth s surface Cover ~75% of the continents Cover nearly all of the ocean floor Easily eroded Occur in distinct layers (strata)
The Nature of Sedimentary Rocks Layers are easily identified Majors layers (formations) easily recognized over large distances Smaller layers within a formation are separated by bedding planes Each bedding plane was once surface of the earth at that locality.
Fig. 5.1. Layered sedimentary rocks exposed in the Grand Canyon, AZ
The Nature of Sedimentary Rocks Sedimentary rocks contain evidence of their environment of formation Animal and plant fossils Bedding planes indicate environment and mode of transport of sediment Texture consists of particle aggregates or crystalline precipitate minerals
Types of Sedimentary Rocks Sedimentary rocks are classified by texture and composition Texture defines 2 major categories Clastic Chemical and Biochemical
Clastic Sedimentary Rocks Made of rock & mineral fragments or clasts Clasts are broken and worn particles transported by water, wind or ice Clastic rocks are subdivided by grain size
Fig. 5.4. Grain size ranges for classification of common clastic sedimentary rocks
Clastic Sedimentary Rocks Grain size is controlled by: Size of grains in source rock Carrying capacity of transport process
Clastic Sedimentary Rocks Common clastic sedimentary rocks Conglomerate larger than 2mm; smooth & well rounded; high energy environment Sandstone 1/16 to 2mm; usu. quartz; rounded grains tell of long transportation; high to med. energy environment Siltstone, Shales fine to v. fine; low energy environment
Conglomerate
Sandstone
Shale
Chemical/Biochemical Sedimentary Rocks Formed by a process that takes ions from solution to form a solid Chemical Sediments Precipitates from water by an inorganic process Biochemical Sediments Formed during the growth of some organism
Chemical/Biochemical Sedimentary Limestone Rocks Direct chemical precipitate from warm sea water crystalline limestone Chemical precipitate- springs, caves- travertine Biochemical from corals, shells, algae skeletal lmst, coquina, chalk, micrite
Skeletal limestone
Chemical/Biochemical Sedimentary Rocks Chert - microcrystalline quartz Formed on ocean floor below areas of high biologic productivity; deep marine Evaporites strickly inorganic Rock salt - halite Gypsum calcium sulfate
Sedimentary Structures Stratification Distinct layers having variations in texture, color or physical properties Lamina, strata, beds, formations Each reflects same thing - Some change that occurs during process of deposition Seasonal, climate, or tectonic
Sedimentary Structures Cross-bedding Individual layers are inclined in reference to the top or bottom of the larger bed Formed by movement of sand waves or dunes Inclination shows flow direction and right-side-up
TOP BOTTOM Fig. 5.9. Cross-bedding formed by dune migration. Flow direction and top of bed indicated.
Sedimentary Structures Graded bedding Progressive change in grain size upward through a bed Fining upward Commonly formed by turbidites Subaqueous flows of muddy water Coarsest particles settle first
Sedimentary Structures Surface impressions Indicate the top of a sedimentary bed Preserve features indicating past environment Ripple marks Mud cracks Fossils (relative age of rock)
Sedimentary Systems Systems include: Weathering (erosion) Transportation Deposition Lithification (compaction & cementation) Final sedimentary rock records the processes that produced it
Fig. 5.13. Idealized diagram of major sedimentary systems.
End of Chapter 5
Sedimentary Systems & Plate Tectonics Plate tectonics plays a major role Sediment deposition & geographic distribution Sediment source areas ex. mountain belts - Controls the topography and structure of continents in general
Tectonic settings and their associated sedimentary sequences
Continental Environments Sediment produced by weathering & erosion of distant highlands Transported by streams, wind or glaciers Thin deposits on stable platform or thick at continental margins Most sediment carried to the sea
Continental Rift Valleys Sequences of conglomerates & sandstones Proximity to source area Wedge shaped deposits Lake or evaporite deposits may form Dependent on climate
Convergent Plate Boundaries Subsiding basins associated with folded mountains accumulate clastic sediments Shallow seas may form in basins Marine limestones, shales or evaporites Turbidites associated with trenches Sediment from ocean floor is scraped off to form a melange
Tectonic settings and their associated sedimentary sequences