Sedimentary Rocks Lec9 PDF

Sedimentary Rocks & Sedimentary Environments  What are Sedimentary Rocks? Sedimentary rocks: products of mechanical & chemical weathering Sedimentary covers underlying metamorphic basement  From Sediment to Rock PROCESSES: ◼ Weathering(disintegration of bedrock into grai...

Sedimentary Rocks & Sedimentary Environments  What are Sedimentary Rocks? Sedimentary rocks: products of mechanical & chemical weathering Sedimentary covers underlying metamorphic basement  From Sediment to Rock PROCESSES: ◼ Weathering(disintegration of bedrock into grains/sediments) ◼ Erosion (involves abrasion, falling, plucking etc.) ◼ Transportation(gravity, wind, water & ice carry sediments) ◼ Deposition (Sediments settles out of water etc.) ◼ Diagenesis & Lithification (Transformation of loose sediments into solid rock) Abrasion is the process of scraping or wearing away.  From sediment to rock Lithification  From sediment to rock ◼ Lithification – involves: - Compaction (by overburden pressure) - Cementation (Cements: calcite, silica, & iron oxide) ◼ Diagenesis - chemical, physical, & biological changes after sediments deposited  Effects of Diagenesis Effects: - Grains are squeezed together - Reduction in porosity - Cementation  Classifying sedimentary rocks A) Based on textures: ◼ Textures ▪ Clastic – Grains are stuck together by cement ▪ Nonclastic – Have pattern of interlocking crystals Cement are minerals that precipitate from water and fill the space between grains (Ex. of cements -- calcite, silica, & iron oxide)  Clast characteristics ◼ Clast size (diameter of fragments) ◼ Clast composition (mineral grains) ◼ Angularity (Smooth or angular corners?) ◼ Sphericity (shape of a clast; resemblance to a sphere) ◼ Sorting (clast’s size in a rock) same size or variety of sizes in a rock? ◼ Character of cement (quartz or calcite?) Clast characteristics can be used to further classify clastic sed. Rocks.  Classifying sedimentary rocks B) Based on sediment transport: ▪Detrital rocks – sediment transported as solid particles – Detrital rocks have clastic texture ▪Chemical rocks – sediment once in solution  Common Detrital Rocks ▪ Shale – Mud-sized – Most common ▪ Sandstone – Sand-sized – Dominated by quartz ▪ Conglomerate & breccia (Particles > 2mm diameter) – Conglomerate = rounded – Breccia = angular  Shale with plant remains Quartz sandstone (Dominated by quartz)  Conglomerate (Rounded) Breccia (Angular) Particle Size: Detrital Sed. rocks  What does the size of the particles in a detrital rock indicate? What about the rounding of the particles?  Transport distance Closer to source Farther from source Angularity Sorting Transport distance ( Closer or farther from the source?) Transport distance ( Closer or farther from the source?) Common Chemical Rocks 1) Limestone - composed of calcite (CaCO3) and forms in a variety of ways: – Organic limestone: form from reef (coral), broken shells (coquina), & chalk (micro organisms,/plankton shells) Microorganisms secrete/produce CaCO3 to build shells – Inorganic limestone: (CaCO3 precipitates from water) Examples : travertine & oolitic limestone Organic limestone – from Coral reef Corals produce shells/ organic limestone Organic limestone Coquina (shell fragments) Ex. From shells of Molluscs Fossiliferous limestone Inorganic limestone Ex.Travertine - Composed of calcite CaCO3 - Formed by the precipitation of carbonate when CO2 bubbles out of groundwater (groundwater degasses!). Travertine speleothems in a Cave Travertine on the surface (Hot or cold water Springs) Chemical Sedimentary rocks 2) Dolostone – Formed from limestone (CaCO3) – Calcite replaced with dolomite mineral (CaMgCO3)2 Chemical Sedimentary rocks 3) Chert ▪ Biochemical – made from microcrystalline quartz (too small in size) – Processes: – form from shells of silica-secreting plankton (Bio.) – Shell dissolves (forming silica rich gel) – Solidified gel yields Chert Biochemical Chert Formed after deposition of plankton that secrete silica shells. Chemical sedimentary rocks Chert ▪ Chemical A) Replacement Chert - Quartz/silica replaces calcite in limestone (CaCO3) B) Chert (petrified wood) - Silica precipitate as quartz within a wood - Silica replaces the cellulose of the wood C) Chert (Agate) - Rock cavity filled by siliceous matter Replacement Chert Quartz replaces calcite in limestone (CaCo3) Agate Chert (siliceous matter) precipitates inside hollows in a rock.  Chemical Sedimentary rocks 4) Evaporite – Evaporation triggers deposition of precipitates – Ex: rock salt & rock gypsum Chemical Sedimentary rocks 5) Coal – Organic origin (Biochemical sedimentary rock) – Black combustible rock – (50 – 90%) Carbon Others: Oxygen, nitrogen , hydrogen, sulfur, silica Various stages in coal formation (in order): ▪ 1. Plant material ▪ 2. Peat (partially altered plant material) ▪ 3. Lignite ▪ 4. Bituminous  Stages of coal formation  Stages of coal formation Coal deposited in layers It is a sedimentary rock Sedimentary Environments Where and how sediment accumulates 1) Continental environments – Glacial deposits(glaciers move sediment of any size, Till) – Mountain stream deposits: dominated by turbulent stream erosion & deposition from mountain valleys – Alluvial fan deposits (water stream) : arid regions, water stream deposits sediments (wedge shaped/alluvial fan) – Wind (Aolian deposits) – Ex. Sand dune – River, Lake Glacial (deposits: till at the end of glacier) France Mountain stream (deposits: boulders and Cobbles Alluvial fan sediments near mountain front  2) Marine environments ▪ Marine shallow deposits (~ 200 meters) ▪ Marine Deep deposits (seaward of continental shelves) 3) Transitional sedimentary environments Shoreline ▪ Tidal flats (flat coastal areas, covered and exposed by tides) Deposits: mostly muddy ▪ Lagoons (protected bodies of quite water) Deposits: Calcite mud, Calcite sand ▪ Deltas (where rivers empties into the sea) Lagoon – protected bodies of quite water - Carbonate environment - Water hosts abundance of organisms Deltas: where stream enters an ocean or lake  Sedimentary basins Basins - are places where sediments accumulate - form where the surface of the Earth’s lithosphere sinks  Sedimentary basins ◼ Types of basins: 1) Rift basins - divergent (pull-apart) plate boundaries. Cause: Extensional tectonics  Sedimentary basins ◼ Types of basins: 2) Intercontinental basins- initially were rift zones (failed rifts) - forms in interiors, far from margins  Sedimentary basins ◼ Types of basins: 3) Foreland basins- form when lithos. flexes in response to thickened crust due to collision of plates  Sedimentary basins ◼ Types of basins: 4) Passive margin basins- Cause: subsidences along the edges of continents - form far from active plate boundaries Transgression & regression What causes the succession of sediments in sedimentary basins? ◼ Transgression – if sea level rises - shore migrates inland ◼ Regression- if sea level falls - shore migrates seaward Transgression and regression  Sedimentary structures What are sedimentary structure? Why study sedimentary structures? Sedimentary structures are “Layering of sedimentary rocks”. Sed. structures are important clues to understand sedimentary environment in which sedimentary beds are deposited. Ex. Ripple marks - indicate layers/sediments were deposited in a current.  Sedimentary structures Types: 1) Ripple marks - Elongated ridges formed on a bed - formed perpendicular to the direction of current flow - few centimeters high 2) Dunes (mega ripples) - large elongated ridges - Tens of centimeters high 3) Bedding planes - boundary between beds Bedding Planes (boundary between beds) Ripple Marks (on modern beach) Elongated ridges formed on a bed Ripple Marks (Preserved on bedding) 4) Cross-bedding - Inclined curving surfaces/laminations in ripples or dunes - Sediment moves up the gentle side of a ripple or dune. - Sediment piles up, then slips down the steep face. Cross Bedding 5) Graded Bedding Grading- fine grain at the top and coarse at the bottom - Caused by Turbidity current - deposit called Turbidite 6) Mud Cracks – are bed surface markings Process: - Mud layer dries up and cracks after deposition - Cracks form approx. hexagonal plates