SEDIMENTARY ROCKS Sedimentary rocks are rocks that have formed from (1) lithification of any type of sediment, (2) precipitation from solution, or (3) consolidation of the remains of plants and animals. A. : The collective name for loose, solid particles that originate from: and of preexisting rocks. Chemical precipitation from solution, including secretion by organisms in water. Rock Cycle in Earth s Crust Relationship of Transported Particle Size to Water Velocity 1. Types of Sediment are classified by their size (Wentworth Classes). This generalized graph shows the water velocity needed to maintain, but not start movement. Variations occcur due to differences in particle density and shape. a) : A rounded particles coarser than 2 mm in diameter. i. : Range from 2 to 64 mm : Range from 64 to 256 mm i : Coarser than 256 mm Angular fragments larger than 2 mm in diameter are called. b) grains: i. 1/16 mm to 2 mm Feels gritty between the fingers. ESC 101: Professor Vorwald Sedimentary Rock Notes Page 1 of 9
c) grains: i. 1/256 mm to 1/16 mm Too small to see without magnification i Doesn t feel gritty between fingers but feels gritty between teeth. d) i. The finest sediment Less than 1/256 mm is the term used for wet silt or clay. 2. Transportation of Sediment a) : The grinding away of sharp edges and corners of sand and gravel as rivers, glaciers, or waves cause particles to hit and scrape against one another b) : The process by which sediment grains are selected and separated according to: (1) size, (2) density, or (3) shape. Rounding and Sorting # Well sorted and well rounded gravel # Angular and poorly sorted gravel Degrees of Sorting - Well sorted to poorly sorted. Sorting by a stream - Horizontal sorting 3. : a) When transported material settles. b) Also includes organic and biological extraction of sediment from solution c) Factors that affect rate of deposition: ( ) i. Sediment Size: Sediment Density: i Sediment Shape: 4. : The general term for a group of process that convert loose sediment into sedimentary rock. a) Process of lithification, i. : Weight of overlying sediment ( ) packs loose sediment grains tightly together ESC 101: Professor Vorwald Sedimentary Rock Notes Page 2 of 9
: Precipitation of cement around sediment grains binds them into a firm, coherent rock. a) Pore space is. b) Common cements include: 1. Calcium carbonate ( ) 2. Silica: 3. (less common) B i : Minerals may precipitate from solution, without passing through the loose sediment stage. Types of Sedimentary Rocks 1. Sedimentary Rocks (from land-derived or terrigenous sediments) a) A sedimentary rock is said to have a detrital texture when it consists of sediment grains bound by cement into a rigid framework. i. Pore spaces are not completely filled. ii Rock fragments can be identifiable pieces of rock, or individual mineral grains. iii Clay minerals are also considered fragments. b) Detrital sedimentary rocks are classified by the of the particles they contain. i. : Cemented rounded gravel i : Distinguished from conglomerate by angular fragments and is not common. : a) Medium grained b) : The fine-grained silt and clay found in spaces between the sand grains and makes up a substantial volume of the rock. c) Types of sandstone Sandstone: More than 90% of grains are quartz. Because Sand grains have been transported great distances the are well rounded and feldspar and other less resistant minerals have weathered away. : More than 25% of sand grains are feldspar. Most grains are coarse and angular. ESC 101: Professor Vorwald Sedimentary Rock Notes Page 3 of 9
( gray-whacky More than 15% of volume is a fine-grained matrix and consist of quartz, feldspars of sedimentary, volcanic, and metamorphic rocks. Most probably were transported by dense masses of sediment-laden water flowing downslope along the sea floor (called turbidity currents) iv : Composed mostly of silt grains and does not split into thin layers or have laminations. v : a Fine-grained. b Noted for (splitting into very thin layers. c) Actually contains both silt and clay. Because clay is 2/3 the volume, it is often classified by its smallest sized particles. d) is rock composed of predominantly clay-sized particles. e) is often used for all fine-grained detrital rocks. 2. Sedimentary Rocks: Rocks formed by inorganic chemical processes. Formed from of minerals from water. All have a texture. a) Examples include evaporating seawater or from a saline lake. Formed from b) Contain as part of their chemical composition. The two main types of carbonates are and. i. Most limestones are formed either directly or indirectly by the action of living things. Limestones formed completely as a result of inorganic chemical process do not make up a great volume of sedimentary rocks However, limestone can be precipitated directly as the result of inorganic processes and are included in the group of chemically formed sedimentary rocks by many geologists Types of limestone formed by inorganic chemical processes include: ESC 101: Professor Vorwald Sedimentary Rock Notes Page 4 of 9
limestone: Cementation of sand-sized spheres (oolites or ooids) inorganically precipitated in warm, shallow seawater. : Forms from fresh water in caves and in hot springs when droplets of carbonate-rich water lose CO. The Formation of Travertine in Caves. A. Aqueous solution of water, Ca ions, and bicarbonate ions seeps into caves. Dripstone forms. B. Limestone bedrock is dissolved by acidic rain near Earth s surface C. A stalactite is an aggregate of calcite crystals that have been chemically precipitated 2 : Precipitated in the fresh water of a continental spring or lake. Dolostone (also called dolomite after its main mineral constituent) a Forms from limestone as calcium in calcite is partially replaced by magnesium as water solutions move through the limestone. Mg + 2 CaCO CaMg(CO ) + Ca ++ ++ 3 3 2 Magnesium calcite dolomite calcium in in solution Solution b. Mg-rich brines from surface evaporation trickle through existing limestone layers. c. Chemical reactions at the boundary between seawater and fresh underground water. 3. (Organically formed) Sedimentary Rocks. a) Limestones (Biochemical) i. These are carbonate rocks (contain CO as part of their chemical composition) 3 i Precipitated through the actions of organisms (such as algae and corals) on continental shelves in warm, shallow water. These rocks have a crystalline texture and contain fossil remains of the organisms still in their growth positions. b) Limestones (Skeletal) Formed from wave-broken fragments of shells, corals, and algae. i. : Coarse-grained with recognizable fossils shell fragments : fine-grained, light colored, and porous from marine organisms (plankton). ESC 101: Professor Vorwald Sedimentary Rock Notes Page 5 of 9
c) : Forms from the compaction of plant material that has not completely decayed. i Environment of deposition: Shallow swamps in a temperate or tropical climate that have rapid plant growth and water with a low oxygen content. ii Several varieties are recognized on the basis of the type of original plant material and the degree of compaction (peat, lignite, bituminous) C Features found within sedimentary rock Usually form during or shortly after deposition 1. Significance a) Important because they provide clues of past environments and methods of sediment transportation. b) May reveal the upward direction of deposition allowing for interpretation of the geometry of folded and faulted rocks in tectonically active regions. 2 Types of structures: a) : A series of visible layers within a rock. i Most are originally deposited as, and the oldest layer is usually on the bottom with the layers becoming younger upward ( ). : Nearly flat surface of deposition separating two layers of rock. : Boundary surfaces between two different types or ages of rocks. These are usually bedding planes. b) : i A layer with a change in particle size (coarse to fine at the top. ii Most likely to occur in a turbidity current. c) (Figure 6.28 on page 145 of your text) A series of thin, inclined layers within a larger bed of rock. These layers form a distinct angle to the horizontal. Most common in sandstone i. Development of Cross-Bedding in Wind-Blown Sand (1) Sand is deposited in inclined layers on the downwind side of a dune (2) The second dune covers first. (3) Cross-bedding changes direction as the wind direction shifts. ESC 101: Professor Vorwald Sedimentary Rock Notes Page 6 of 9
d) i ii Development of Cross-Bedding by a Water Current (1) The current fills in a depression in the river or sea floor bottom with sediment. (2) Continued sedimentation often covers the first set of cross-beds with another set. (3) The cross-bedding remains in the same direction since the current doesn t change direction. Polygonal cracks formed in very fine-grained sediment as it dries. Only form in environments where sediment is exposed above water. e) Small ridges formed on the surface of sediment by moving wind or water. i. Symmetrical Ripples: From oscillating (back and forth) motion of waves. Asymmetrical Ripples: Formed by water currents Steeper sides in the down-current direction f) : Any trace of plants or animals preserved in rock. i. Hard parts of organisms are most likely to be fossilized. Must be covered by a protective layer (usually sediment) before it decays. i Original bone or shell is rarely preserved. iv. Body Fossils (1) Unaltered remains: Remains of organisms (e.g, bone, shells, teeth, mummification, freezing, preservation in amber) - They keep their original composition and structure) (2) Altered remains: Change in structure as a result of permineralization, recrystallization, replacement, carbonization D 1. A body of rock of considerable thickness with characteristics that distinguish it from adjacent rock units. 2. Usually composed of one or more sedimentary rock beds. 3. Often based on rock type. 4. Criterion for distinguishing and naming a formation is some visible characteristic that makes it recognizable. For example: A sequence of limestone beds may have different fossils in the lower half than in the upper half. It would be divided into two formations based on the fossil content. ESC 101: Professor Vorwald Sedimentary Rock Notes Page 7 of 9
G Environments of Deposition 1. Sedimentary rocks allow geologists to determine ancient environments of deposition. The type of sedimentary rock, type of sorting and rounding of the grains, sedimentary structures and fossils present are clues used. 2. Continental Environments: a Glacial Environments are characterized by an unsorted mix of unweathered boulders, cobbles, pebbles, sand, silt, and clay. b Alluvial Fans i Broad fan-shaped piles of sediment deposited as rivers flow from mountains onto broader plains. ii Often are characterized by coarse sandstones and cross-bedding c River Channel and Floodplains i River deposits typically are cross-bedded and have ripple marks ii Floodplains have thin-bedded shales characterized by mudcracks and fossil animal footprints. d Lakes (Often referred to as lacustrine deposits) i Thin-bedded shales ii Often contain fish fossils 3. Shallow Marine Environments Deltas i. A body of sediment deposited when a river flows into a sea or lake. Generally made of thick sequences of siltstone and shale i Characterized by low-angle cross-bedding cut by coarser channel deposits Beach, Barrier Island, Dune i. Well-sorted quartz sand with rounded grains Cross-bedding i Tropical regions have carbonate sands Lagoon i. Semi-enclosed, quiet bodies of water between a barrier island and a mainland Fine-grained, dark shale i Many fossils of marine organisms Shallow Marine Shelves i. Widespread deposits of sandstone, siltstone, and shale deposits as sediments are transported offshore May contain symmetrical ripples and cross-bedding Reefs i. Massive limestones Fossils of corals, coralline algae, and many other marine organisms ESC 101: Professor Vorwald Sedimentary Rock Notes Page 8 of 9
4. Deep Marine Environment Deep sea floor Shales and graywacke sandstone deposited by turbidity currents Characterized by graded bedding and asymmetrical current ripple marks ESC 101: Professor Vorwald Sedimentary Rock Notes Page 9 of 9