Presentation on theme: "Sedimentology and Sedimentary Processes Virginia T. McLemore."— Presentation transcript:
Sedimentology and Sedimentary Processes Virginia T. McLemore
Sedimentology "The scientific study of sedimentary rocks and of the processes by which they were formed; the description, classification, origin and interpretation of sediments" (Glossary of Geology, AGI, 1974) Study of modern sediments such as sand, mud (silt),and claysediments Understanding the processes that deposit them Studies of ancient sedimentary rockssedimentary rocks
Sedimentary rock types Clastic rocks –particles derived from the weathering and erosion of precursor rocks and consist primarily of fragmental materialweatheringerosion –classified by grain size and compositiongrain size Carbonates –precipitated by a variety of organic and inorganic processes Evaporites –evaporation of water at the Earth's surface Chemical sedimentary rocks (chert, jasperiod)
Principles Uniformitarianism, which states that the sediments within ancient sedimentary rocks were deposited in the same way as sediments which are being deposited at the Earth's surface today Principle of superposition Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.Principle of superposition Principle of original horizontality sediments are deposited at their angle of repose which, for most types of sediment, is essentially horizontalPrinciple of original horizontality
Principles Principle of lateral continuity states that layers of sediment initially extend laterally in all directions unless obstructed by a physical object or topographyPrinciple of lateral continuity Principle of cross-cutting relationships states that whatever cuts across or intrudes into the layers of strata is younger than the layers of strataPrinciple of cross-cutting relationships
Methodology Measuring and describing the outcrop and distribution of the rock unit Descriptions of rock core Sequence stratigraphy (Describes the progression of rock units within a basin )Sequence stratigraphy Describing the lithology of the rocklithology Analysing the geochemistry of the rockgeochemistry
Sediment characteristics Porosity is the volume of voids within a rock which can contain liquids. Permeability is the ability of water or other liquids (e.g. oil) to pass freely through a rock. Roundness refers to the roughness of the surface of the sedimentary grain. Sorting refers to the range of particle sizes in a sediment or sedimentary rock. Matrix is the fine-grained material (usually clays or silt) that is deposited originally with the coarser-grained material
Classification by GRAIN SIZE Gravel > 2mm Sand >1/16 mm < 2 mm Mud <1/16 mm Decide what the relative proportions of each size
Grain Size Distribution
COMPOSITION Gravel Sand Mud
TEXTURE Grain shape (attributes which refer to the external morphology of particles) –surface texture, –roundness –form. is determined by: Grain shape (Bustin, 1995) –internal structure –mineral cleavage characteristics of source rock such as jointing and bedding lithology hardness fracture transport
COLOR Munsell color chart
Any other features? Is there anything outstanding about this rock? Does it have any important minerals or clasts? Anything worth making note of? Any important clasts? Are there any fossils? Any visible sedimentary structures (ie cross bedding)?
Give the rock a NAME [colour][texture][cement][important minerals or clasts] ROOT NAME [with...]
Sediment transported by Suspension load is when sediments are carried in suspension (usually fine-grained sediments that can be carried along easily by the flow) Bed load is when the forward force of the moving current acts more directly on the larger particles at the bottom as it pushes, rolls, and slides them along Saltation is more complex and usually affects sand-sized particles. Here, the particles are sucked up by eddies into the flow, travel with the flow for a while, and then fall back to the bottom
Sediment structures Asymmetrical ripples are ripples that have a gentle slope upstream and a steep slope downstream. Cross-bedding is inclined bedding and commonly forms in alluvial environments. Potholes are rounded depressions caused by swirling currents and eddies. Mud cracks are formed by evaporation on mudflats or in shallow lakes.
Alluvial Fans Alluvial fans are sedimentary deposits that typically form at the margins of a dry basin. They typically contain coarse boulders and gravels and are poorly sorted. Fine-grained sand and silt may be deposited near the margin of the fan in the valley, commonly in shallow lakes. These lakes may periodically dry, and evaporite deposits may result.
Eolian Wind is an effective sorting agent and will selectively transport sand. Gravel is left behind and dust-sized particles are lifted high into the atmosphere and transported great distances. Windblown sand forms dunes that are characterized by well-sorted grains showing large-scale crossbedding.
Glacial Glaciers do not effectively sort the materials that they transport. Common type of resulting deposit is an unstratified accumulation of boulders, gravel, sand, and fine silt for which the term "till" is usually applied.
Rivers Fluvial environments include braided and meandering river and stream systems. River channels, bars, levees, and floodplains are parts (or subenvironments) of the fluvial environment. Channel deposits consist of coarse, rounded gravel, and sand. Bars are made of sand or gravel. Levees are made of fine sand or silt. Floodplains are covered by silt and clay.
Flood Plains Rivers commonly meander across a flat flood plain before reaching the sea and depositing a considerable amount of sediment. Rocks formed in a flood plain environment are commonly lenses of "fluvial" sandstone deposited in the meander channel enclosed in a shale deposited on the flood plain.
Lakes Lacustrine environments (or lakes) are diverse; they may be large or small, shallow or deep, and filled with terrigenous, carbonate, or evaporitic sediments. Fine sediment and organic matter settling in some lakes produced laminated oil shales.
Delta Deltas are large accumulations of sediment that are deposited where a river empties into a standing body of water. They are one of the most significant environments of sedimentation and include a number of subenvironments such as stream channels, flood plain beaches, bars, and tidal flats. The deposit as a whole consists of a thick accumulation of sand, silt, and mud. Because of the abundance of vegetation in geologically young deltaic environments, coals of various ranks commonly are associated with these clastic sediments.
Swamps Swamps (Paludal environments) Standing water with trees. Coal is deposited.
Shoreline Beaches, bars, and spits commonly develop along low coasts and partly enclose quiet-water lagoons. Such sediments are well washed by wave action and is typically clean, well-sorted quartz sand. Behind the bars and adjacent to the beaches, tidal flats may occur where fine silt and mud are deposited; evaporites may be present locally. Barrier islands
Shoreline Lagoons are bodies of water on the landward side of barrier islands. They are protected from the pounding of the ocean waves by the barrier islands, and contain finer sediment than the beaches (usually silt and mud). Lagoons are also present behind reefs, or in the center of atolls. Tidal flats border lagoons. They are periodically flooded and drained by tides (usually twice each day). Tidal flats are areas of low relief, cut by meandering tidal channels. Laminated or rippled clay, silt, and fine sand (either terrigenous or carbonate) may be deposited. Intense burrowing is common. Stromatolites may be present if conditions are appropriate.
The continental shelf is the flooded edge of the continent. The continental slope and continental rise are located seaward of the continental shelf. The abyssal plain is the deep ocean floor.
Shallow Marine Shallow seas are widespread along continental margins and were even more extensive during many periods of the geologic past. Sediments deposited in these shallow marine waters from extensive layers of well-sorted sand, shale, limestone, and dolomite, that commonly occur in a cyclic sequence as a result of shifting depositional environments related to changes in sea level. When the rate of evaporation exceeds the rate of water supply, chemicals dissolved in the water may be concentrated and precipitated as beds of gypsum, halite, and more complex salts.
Organic Reef An organic reef is a structure built of the shells and secretions of marine organisms. The framework of geologically young reefs typically is built by corals and algae, but the reef community includes many types of organisms. A highly fossiliferous limestone commonly is the result of these organisms in the rock record. Reworking of reef-derived sediments by wave and biological activities commonly results in a complex group of sedimentary facies that may be referred to as the reef tract. Reefs are wave-resistant, mound-like structures made of the calcareous skeletons of organisms such as corals and certain types of algae.
Deep Ocean The deep oceans contain a variety of sediment types. Adjacent to the continents, a considerable amount of sediment is transported from the continental margins by turbidity currents. As the current moves across the deep-ocean floor its velocity gradually decreases, and sediment carried in suspension settles out. The resulting deposit is a widespread layer of sediment in which the size of grains grade from coarse at the base to fine at the top. Such deep-sea deposits are characterized by sequences of graded beds of these "turbidites". Distant to the continents, dust transported by eolian processes may accumulate as muds. In sediment-starved parts of oceans away from the continents, siliceous ooze formed of the tests of microorganisms called radiolaria accumulate. These sediments form the radiolarian cherts of the rock record.
Facies and depositional environments The facies concept refers to the sum of characteristics of a sedimentary unit, commonly at a fairly small (cm-m) scale Lithology Grain size Sedimentary structures Color Composition Biogenic content Lithofacies (physical and chemical characteristics) Biofacies (macrofossil content) Ichnofacies (trace fossils)
Facies and depositional environments Facies analysis is the interpretation of strata in terms of depositional environments (or depositional systems), commonly based on a wide variety of observations Facies associations constitute several facies that occur in combination, and typically represent one depositional environment (note that very few individual facies are diagnostic for one specific setting!) Facies successions (or facies sequences) are facies associations with a characteristic vertical order Walther’s Law (1894) states that two different facies found superimposed on one another and not separated by an unconformity, must have been deposited adjacent to each other at a given point in time