![]() ![]() Rock units can also be divided on the basis of fossils into zones that are named after key index fossils. On a large scale, several formations can be combined into one group.Ħ. The thinnest rock unit observable within a formation is called the stratum.ĥ. Formations are typically named after the locality where the rock sequence is best exposed and/or serves as the basis for the classification.Ĥ. In these latter two cases, some arbitrary division must be chosen.ģ. In other cases, however, (b) adjacent units may be of the same lithology and differ only in color such as green shale overlying brown shale, or (c) a rock sequence may be interstratified with two or more lithologies. (a) Sometimes a formation can be defined simply on the basis of rock type. A particular rock unit is grouped into a formation based on a distinctive appearance and/or other characteristics. Figure 9.5 & Box 9.1: The most basic local unit of stratigraphy is the formation. The science of stratigraphy has its roots in the late 1700s and early 1800s as a consequence of the extensive geologic mapping that took place in Great Britian and western Europe during that time.Ģ. Stratigraphy is a sub-science of geology that attempts to organize the rock record into some manageable classification scheme. Rock units can also be correlated using color, mineral composition, chemical composition, sedimentary structures and other criteria.ġ. Fossils are therefore very useful in correlating, or matching up rock sequences in exposures separated by great distances.ĥ. The principle of fossil correlation states that like assemblages of fossils are the same age and therefore strata containing these particular fossils are also the same age. The study of the history of ancient life from the fossil record is called paleontology.Ĥ. As a result, rocks of a certain age contain one set of fossils that differ from fossils in rocks of different ages. This is because life evolved and changed throughout earth history. Figure 9.5: Studies of fossils preserved within the rock record indicate that rocks of different ages contain different types or assemblages of fossils. By correlateing various rock exposures separated by great distances, geologic maps can be constructed and the original geographical extent of the rocks can be estimated.ģ. Correlation involves matching a particular rock unit in one exposure with its counterpart at a different locality using criteria such as color, mineral composition, characteristic fossils etc. Between exposures, rocks were either buried or have been eroded away.Ģ. Rock exposures often are separated by great distances. Figure 9.5: Rock units are not continuously exposed. Rocks of a certain age contain one set of fossils that differ from fossils in rocks of different ages.ġ. ![]() Rocks of different ages therefore contain different types or assemblages of fossils.ģ. Figure 9.4: Life evolved and changed throughout earth history.Ģ. ![]() (c) Principle of original lateral continuity: Strata originally extended in all directions until they thinned to zero or terminated against the edges of the basin of deposition.ġ. ![]() Strata that are inclined or folded must have suffered disturbance subsequent to deposition. (b) Principle of original horizontality: Most strata were initially deposited horizontally. (a) Principle of superposition: In a succession of undeformed strata, the oldest stratum lies on the bottom with successively younger ones above. In 1669, Steno stated three basic principles for chronological analysis of the rock record which have proven to be extremely useful in the study of earth history. This layering of sedimentary rocks is referred to as stratification.ġ) Figure 9.3: Nicholas Steno was an Italian scientist who recognized that many rocks units were deposited in horizontal layers, a phenomenon called stratification. Sedimentary rocks generally occur in layers that reflect the pattern of sedimentation of the original sediments. CHAPTER 9: The Rock Record and Geologic Time Scaleġ. ![]()
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |