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UMD GEOL 342 - Introduction to Sedimentation and Stratigraphy

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Introduction to Sedimentation and StratigraphySedimentary rock interpretationsGEOL 342 Sedimentation and StratigraphyLecture 1: Introduction27 January 2005Assoc. Prof. A. Jay KaufmanIntroduction to Sedimentation and StratigraphyGoals of this course1. To explore the origin and evolution of sediments and sedimentary rocks2. To examine the nature of sedimentary deposits across all time and length scales3. To provide the tools necessary to properly describe features of sedimentary rocks4. To provide practical laboratory and field experience in the description of sedimentary depositsin order to interpret Earth’s long history5. To integrate aspects of sedimentation and stratigraphy into other aspects of the earth sciencesRock d’jourEach lecture will highlight a rock d’jour (rock of the day) that will be described and usedto illustrate important concepts in sedimentary geology. These may show up on later tests, so makesure to take notes on their characteristics and the discussion issues they highlight.Descriptions of sedimentary rocksI. ColorII. CompositionIn contrast to the study of igneous and metamorphic rocks where compositions are oftencharacterized by elemental abundances, sedimentary rocks are most often characterized by theirmineralogy. Describing the composition of a sedimentary rock using bulk chemistry is misleadingbecause it often obscures important genetic distinctions. If necessary, review the characteristics ofminerals introduced in Physical Geology.III. TextureTexture refers to the size, shape and arrangement of grains that make up a sedimentaryrock, of which there are two fundamental types: clastic and crystalline. When rocks containfragments of pre-existing rocks or minerals that were transported as discrete particles they areknown as clastic (from klastos, Gk. For broken). On the other hand, when sedimentary grains areinterlocked or intergrown the texture is called crystalline. Grain size of clasts or crystals are characterized by maximum grain diameters. Variationgrain size is called sorting. The degree of sorting is often a function of the mode of transport of the1materials. For example, while ice is a fluid, it has little ability to sort or round the particles ittransports, so a glacial deposit generally has poorly sorted, angular clasts. Particle size is theprimary basis for classifying clastic sediments and sedimentary rocks, regardless of the mineralogyof the clasts. The shape of grains can also be a diagnostic tool (e.g. sphericity, roundness or angularity),which usually implies something about the distance and mode of transport. Finally, the degree of packing and cementation of a sedimentary rock are importanttextural characteristics.IV. Fossil contentFossils can be characterized as hard parts (i.e. “body fossils” including shells, bones, ortheir replacements), or as traces of organisms (such as tracks, trails, and burrows, knowncollectively as ichnofossils).V. Sedimentary structuresVarious sedimentary structures in hand sample and outcrop yield clues to the origin of therock and its depositional environment. Bedding and stratification are the hallmark of sedimentaryrock. The layers, or strata, are built up from the accumulation of particles of various sizes andarrangements. These particles formed during the breakdown and transport of pre-existing rock, orby precipitation from ions in solution. Sedimentary rock interpretations1. When was it deposited? HISTORY 2. What is its lateral distribution? STRATIGRAPHY23. What rocks were deposited at the same time? CORRELATION4. What were the source rocks and how far away were they? PROVENANCE5. How was the material weathered, broken, and transported? WEATHERING AND TRANSPORTATION6. How was the material deposited and lithified? DEPOSITIONAL SETTING 7. What has happened to the sediment since it was laid down? DIAGENESISCategories of sedimentary rocksSediments and sedimentary rocks are classified into three groups based on the types of particles they contain. These groups include:Clastic sediments form from loose rock and minerals produced by weathering and erosion. Chemical sediment is formed by the precipitation of minerals dissolved in fresh or sea water.Biogenic sediment is mainly made of the remains of plants and animals.Loose sediments are transformed into rock, that is they are lithified, by compaction,recrystallization, and cementation. Together, these changes are commonly referred to as diagenesis.Sedimentary basins Most of the surface of the Earth is covered with sedimentary rocks or soils (over 90%),including some 75% of land areas and 100% of the ocean basins. At the same time, sedimentaryrocks compose only about 5% of lithosphere, so it is a very thin veneer. What are sedimentary basins? 3Sedimentary basins are holes filled with dirt, which was originally weathered rock(igneous, metamorphic, or sedimentary) in mountains that was transported by gravity andaccumulated in the oceans (eventually, although there may have been several waystations along thetraverse). The base level is a horizon below which erosion will not occur, and this is typically sealevel. Accomodation space is the area below the base level where sediments may accumulate. What different types of sedimentary basins are there?1. topographic - low-lying areas of the earth's surface surrounded by higher areas; subaerialand/or subaqueous.2. structural – syn- to post-depositional deformation (folding and faulting)3. sedimentary – a hole in the ground, filled or filling with dirt due to its own loadingSedimentary depositional environmentsDepositional environments include a set of physical, chemical, and biological processes,acting at a certain time, place, and intensity, preserving recognizable set of geometric and physicalattributes. These commonly correspond to some geomorphic entity (e.g., a


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