![]() Figure 5.2 – Two seismic images of an accretionary prism near the Nankai trough offshore Japan. The seismic character of the layering lets you know important tectonic and sedimentary history along this profile. ![]() Towards the bottom, the seismic reflectors are very discontinuous as the oceanic crust has no internal structure. Below these regions, and in the middle of the seismic profile, the layering is disrupted and non-continuous this probably indicates that these sediments were deformed shortly after deposition and before they were lithified as the sediment was scraped off the downgoing oceanic crust. This pattern implies the sediment was deposited in relatively quiet, low-energy water. Layering is flat and parallel to the ocean floor. If you look closely at Figure 5.2, the sedimentary reflectors are relatively flat and show layering in the Kumano Basin (top left side of the image or NW end highlighted in green in Figure 5.2b) as well as to the right edge to the SE. Are they thick or thin? Also, what is their shape – flat or curved? Are the reflectors continuous? If so, they may be well-bedded. So, when looking at a seismic profile, make observations and ask yourself: Do you see areas without many reflectors? This may be related to its lithology. Just like grain size is important when describing sedimentary rocks, so are the features in a seismic response. Some call this change in seismic response a seismic facies. Image credit: A) Thomas Ronge and IODP, CC BY B) Lindy Newman and IODP, CC BY C) MARUM, CC BY-SA.Īn important tool for stratigraphy is to observe seismic characteristics below the surface. C) An IODP core repository at the University of Bremen in Germany others are located at Texas A&M University in Texas, Rutgers University in New Jersey, and Kochi University in Japan. B) Scientists studying recovered core during Expedition 378 in the South Pacific. Figure 5.1 – A) The JOIDES Resolution, one of the research vessels used by the IODP. If you want to propose a new formation or group, there are strict guidelines set up by the International Commission of Stratigraphy. Sometimes several formations are lumped together to form a group. A formation is a series of sedimentary beds distinct from other beds above and below and is thick enough to be shown on geological maps. Geologists subdivide stratigraphic columns into formations. If a seismic survey is repeated over the same area, this gives another dimension of time. Geologists use the patterns of reflectivity to help identify the rock types, deformation features, and where there might be water or petroleum and other characteristics in the subsurface. Previous seismic surveys were only collected in a line, but now they are typically collected in grids to get three-dimensional data below the earth’s surface. Another way to study stratigraphy is to conduct seismic surveys by sending sound waves into the seafloor or ground and monitoring how the waves reflect back to the surface (Figure 5.2). The cores are then stored at facilities around the world for scientists to request samples from (Figure 5.1c). Geologists can then study the collected sediment (Figure 5.1b) or send instruments into the drill hole to measure the geologic properties of the surrounding sediment. Sediment cores are mostly collected from the ocean floor by organizations like the International Ocean Discovery Program ( IODP) using a ship dedicated to drilling (Figure 5.1a). Stratigraphy is mainly studied through outcrop observations, the collection of sediment cores, and seismic surveys. As you learned in Chapters 2 and 4, one of the primary goals of studying sedimentary rocks is to determine their depositional environment stratigraphy is no different. Stratigraphy is the area of geology that deals with sedimentary rocks and layers and how they relate to geologic time it is a significant part of historical geology.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |