Sedimentology & Stratigraphy Thanks to Rob Viens for slides
Sedimentology The study of the processes that erode, transport and deposit sediments Sedimentary Petrology The study of the characteristics and origin of sedimentary rocks. Stratigraphy The study of the origin, relationship, and extent of rock layers (strata).
Sedimentary Rocks on Earth Shale Sandstone Siltstone Conglom. Limestone
What do Sedimentary Rocks Record? Deposition Transport Erosion Weathering Sediment Lithification Sedimentary Rocks Metamorphism Igneous Rocks Metamorphic Rocks Crystallization Magma Melting
What do Sedimentary Rocks Record? Source (Provenance) of sediment Erosion and Transport Agent Depositional Environment Paleogeography/Tectonic Setting Diagenesis (what happened after deposition)
Rock Identification is based on: Composition Texture What minerals make up the rock? What is the shape, size and orientation of the mineral grains that make up the rock? Major Difference: Crystalline vs. Clastic
Types of Sedimentary Rocks Detrital Chemical Clastic Texture Organic Crystalline Texture
Detrital Rock Composition Detrital Rocks Lithic Fragments Quartz Clay Minerals Fossil Fragments Iron Minerals Orthoclase (K-spar) Muscovite / Biotite
Note About Sediment Composition: Weathering The sediments that make up sedimentary rocks are produced by: Mechanical & Chemical Weathering
Frost action Mechanical Weathering Mechanic effect of freezing (and expanding) water on rocks Pressure release Removal of overlying rock allows expansion and fracturing Plant growth Growing roots widen fractures Burrowing animals Thermal cycling Large temperature changes fracture rocks by repeated expansion and contraction
Mechanical Weathering
But mostly physical weathering is a matter of things just falling down. So in a sense, gravity, is the primary cause of physical weathering.
Oxidation Chemical Weathering Chemically active oxygen from atmosphere Iron oxides are common result Soil and sedimentary rocks often stained with iron oxides Acid dissolution Hydrogen cations replace others in minerals Carbonic acid from atmospheric CO 2 dissolved in water Sulfuric, hydrofluoric acids emitted by volcanic eruptions Some minerals, such as calcite, may be totally dissolved Human activity, such as mining and burning of fossil fuels, produces acids
Chemical Weathering
Feldspars Chemical Weathering Most common minerals in crust Slightly acidic rain water attacks feldspar Clay minerals produced K +, Na +, Ca ++ ions released into water Other minerals Ferromagnesian minerals Clays, iron oxides, Mg ++ ions produced More complex silicate bonds lead to lower weathering susceptibility Olivine most susceptible, quartz least Warm, wet climatic conditions maximize weathering
Mineral Stability
Products of Weathering Lithic (Rock) Fragments (granite, basalt, schist, etc.) Dissolved Ions (Calcium, Potassium, Sodium, etc.) Rust Minerals (Hematite, Goethite, etc.) Clay Minerals (Bentonite, Montmorillonite, etc.) Residual Minerals (Quartz, Orthoclase, Muscovite, etc.)
Sediment Sediment - loose, solid particles originating from: Weathering and erosion of pre-existing rocks Chemical precipitation from solution, including secretion by organisms in water Classified by particle size Boulder - >256 mm Cobble - 64 to 256 mm Pebble - 2 to 64 mm Sand - 1/16 to 2 mm Silt - 1/256 to 1/16 mm Clay - <1/256 mm Gravel
Distinguishing Characteristics of Clastic Sediments: Grain Size - mud/clay, silt (<0.06mm), fine sand (0.06-0.25mm), coarse sand (0.25-2.0mm), pebbles (2-64mm), cobbles (64-256mm), boulders (>256mm); particle size reflects energy (velocity) of the transport and depositional system.
Transportation From Sediment to Sedimentary Rock Movement of sediment away from its source, typically by water, wind, or ice Rounding of particles occurs due to abrasion during transport Sorting occurs as sediment is separated according to grain size by transport agents, especially running water Sediment size decreases with increased transport distance
Distinguishing Characteristics of Clastic Sediments (cont.): Sorting - Well-sorted sediment indicates prolonged reworking by wind or water; poorly sorted sediment may indicate rapid deposition, or deposition by ice or mass movement. Angularity/Roundness and Shape Well rounded sediment also indicate prolonged reworking by transporting agent; the shape of grains often indicates the transport system, but also may be related to the type of mineral or rock fragment
Interpretation of Composition Detrital Rocks: Source of Sediment (Provenance) Exposure to Weathering (Distance of Transport) (Type of Climate)
Note About What Causes Detrital Rock Color Grain Size Smaller is often darker Composition Carbon - black or brown Quartz - tan, clear, white Orthoclase - orange Depositional Environment Iron on land - red Iron in deep, oxygen poor water - green
Chemical & Organic Sedimentary Rocks (Names based primarily on composition) Composition Calcite Dolomite Quartz (silica) Halite Gypsum Carbon / Plant Remains Basic Rock Name Limestone Dolostone Chert Rock Salt Rock Gypsum Peat or Coal
Limestone Crystalline Limestone Fossiliferous Limestone Chalk
Formation of Limestone Calcite Precipitates in Warm Water Water Under Low Pressure (Shallow) Or It is secreted by biological organisms Clams, Mussels, Scallops, etc. Corals Foraminifera
Limestone Represents: Shallow Tropical Ocean
Or relatively shallow open ocean, i.e. near shore or mid ocean ridges http://geosci.uchicago.edu/~archer/classes/geosci238/
Chalk (Coccolithophores)
Black Sea
Oolitic Limestone
Travertine (Limestone)
Dolostone
Chert (Flint, Jasper, Agate )
Diatom Skeletons
Chert Represents Highly productive areas of the Ocean (diatoms, radiolarians as beds) or near shore (sponges as nodules)
Evaporites: Bonneville Salt Flats, Utah Rock Gypsum Rock Salt
Mediterranean Geology Isolated, Arid, Salty Sea/Lake
Peat and Coal Swamp, Bog