Transcript 1-geo236x
Introduction to Sedimentology and Stratigraphy UNIT -1 What are Sediments Sediments are loose Earth materials (unconsolidated materials) such as sand which are transported by the action of water, wind, glacial ice and gravity. These materials accumulate on the land surface, (such as in river and lake beds), and / or on the ocean floor and form sedimentary rocks. How are sediments formed? Sediments form by weathering of rock. Then they are eroded from the site of weathering and are transported by wind, water, ice and mass wasting. Finally sediment settles out and accumulates after transport: This process is known as deposition. Sedimentation is a general term for the processes of weathering, erosion, transport, and deposition Examples of Erosion Classification of Sediments Sediments are commonly subdivided into three major groups. They are Mechanical, Chemical and Organic Formed by the breakdown of pre-existing rocks and minerals by the process of weathering and erosion Detrital Mineral Grains Common: Quartz, Felspar, Mica and Heavy Minerals Lithic Fragements Polymineral grains or rock fragments Biogenic Particles Shell Fragments, woods, seeds and other parts of land plants Authigenic Minerals Minerals which grow crystals in deposition environment, eg .Glauconite Composition Materials which form within the environment of transport and deposition Mechanical Sediments Mechanical, or clastic, sediments are derived from the erosion of earlier formed rocks on the earth's surface or in the oceans. These are then carried by streams, winds, or glaciers to the site where they are deposited. Streams deposit sediment in floodplains or carry these particles to the ocean, where they may be deposited as a delta. Mechanical Sediments Ocean sediments, especially in the form of turbidites, are usually deposited at the foot of continental slopes. Glaciers carry sediment frozen within the mass of the ice and are capable of carrying even huge boulders (erratics). Chemical sediments Chemical sediments are formed by chemical reactions in seawater. This result in the precipitation of minute mineral crystals. These crystals settle to the floor of the sea and ultimately form a more or less chemically pure layer of sediment. Evaporation in shallow basins results in a sequence of evaporite sediments, which include gypsum and rock salt. Organic sediments Organic sediments are formed as a result of plant or animal actions; for example, peat and coal form by the incomplete decay of vegetation and its later compaction. Deep-ocean sediment known as pelagic ooze consists of the remains of microscopic organisms (mostly foraminifera and diatoms) from the overlying waters as well as minor amounts of windblown volcanic and continental dust. Limestones are commonly formed by the aggregation of calcite shells of animals. The process involved in the formation of Sedimentary Rocks WEATHERING (Weathering is the process that changes bedrock to soil) EROSION (Erosion is the process that removes loose materials from the place of weathering) TRANSPORTATION DEPOSITION DIAGENESIS COMPACTION CEMENTATION Diagenesis Lithification refers to processes that convert loose sediment to hard rock. Diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification. Two of the most important processes involved during diagenesis are compaction and cementation Imagine some rocky outcrops at the side of a valley high in the mountains. Cracks in the rock fill with water when it rains. When it is cold, the water changes to ice. As it forms, the ice expands and makes new cracks in the rock. After this happens a few times, pieces of rock are loosened and fall from the outcrop. They slide downhill and eventually land up in the river flowing along the valley floor. Once in the river, the pieces of rock tend to slide, roll and bounce downstream as they are forced along by the flow of water As they travel downstream, they get broken up into smaller sand grains and mud particles. When the river flows into the sea, all the rock fragments fall to the sea bed as the flow of water slows and stops. Over long periods of time, layers of pebbles, sand and mud build up on the sea bed at river mouths. Water circulates in the tiny spaces between the rock particles in these layers and slowly deposits chemicals that cement the particles together. Sediments such as sand and mud eventually become sedimentary rocks called sandstone and mudstone. Over millions of years, layer upon layer of different sedimentary rocks pile up on top of each other to create rock formations hundreds of metres thick. These formations often lie undisturbed for hundreds of millions of years. Classification of Sedimentary Rocks Clastic sedimentary rocks are composed of fragments of weathered rocks, called clasts, that have been transported, deposited, and cemented together. Clastic rocks make up more than 85 percent of all sedimentary rocks. Chemical sedimentary rocks form from precipitation within the environment of deposition, so their composition depends on the chemical composition of the aqueous solution from which they are deposited (e.g. lake, ocean, groundwater) and chemical conditions (e.g. carbon dioxide, oxygen, pH) Classification of Sedimentary Rocks Biogenic sedimentary rocks result from the accumulation of skeletal remains of plants or animals. The most important types are (1) made up of calcium carbonate (calcareous) e.g. limestone, and shelly sandstone; (2) made up of silica (siliceous) e.g. chert; and (3) made up of plant remains (carbonaceous) e.g. coal. Classification of Sedimentary Rocks What is Sedimentary Geology Sedimentology is the science that deals with the description, classification, and origin of sedimentary rock. It is the study of sediments and sedimentation. Sedimentology = the study of the processes of formation, transport and deposition of material which accumulates as sediment in continental and marine environments and eventually forms sedimentary rocks Stratigraphy = the study of rocks to determine the order and timing of events in Earth history Sedimentary geology » Sedimentology + Stratigraphy What is Sedimentary Geology Stratigraphy focuses on the larger scale strata and Earth history (when and where were sediments/sedimentary rocks formed?) Larger temporal and spatial scales The stratigraphic record is nearly always very incomplete due to a limited preservation potential, that decreases with increasing time scales Sedimentology focuses primarily on facies and depositional environments (how were sediments/sedimentary rocks formed?) Smaller temporal and spatial scales The aim of Sedimentology Description of sediments Origin of sediments Understanding the depositional condition resulting in the formation of a given sedimentary rock Evolution of the sedimentary basin and the sequence Information and understanding of the past depositional and environmental conditions Analysis of Sediments The analysis of sediments requires standard methods if similar deposits from different locations are to be compared and contrasted. The standard methodology includes: Facies Analysis Particles Size Analysis Particle Shape Analysis Lithological Analysis Methods in Sedimentology The methods employed by sedimentologists to collect data and evidence about the nature and depositional conditions of sedimentary rocks include: Measuring and describing the outcrop and distribution of the rock unit Describing the rock formation; thickness; lithology; outcrop; distribution Contact relationships with other formations. Mapping the distribution of the rock unit; or units Stratigraphy Describes the progression of rock units within a basin Describing the lithology of the rock Petrology and petrography: measuring the textures; grain size; grain shape; sorting and composition of the sediment Analyzing the geochemistry of the rocks Isotope geochemistry, including uses of radiometric dating, to determine the age of rocks Methods in Sedimentology A basic understanding of sedimentary processes is essential for many geological and geotechnical engineering applications. Stratigraphers and Sedimentologists must apply their knowledge of how sedimentary successions are deposited and preserved to interpret the geologic history recorded in the rocks. Sedimentology is primarily with the formation of sedimentary rocks. As soon as these beds of rock are looked at in terms of their temporal and special relationships the study becomes stratigraphy.