A cratonic sequence (also known as megasequence , Sloss sequence or supersequence ) in geology is a very large-scale lithostratigraphic sequence in the rock record that represents a complete cycle of marine transgression and regression on a craton (block of continental crust ) over geologic time. They are geologic evidence of relative sea level rising and then falling (transgressing and regressing), thereby depositing varying layers of sediment onto the craton, now expressed as sedimentary rock . Places such as the Grand Canyon are a good visual example of this process, demonstrating the changes between layers deposited over time as the ancient environment changed.
7-469: Cratonic sequences were first proposed by Laurence L. Sloss in 1963. Each one represents a time when inland seas deposited sediments across the craton. The top and bottom edges of a sequence are each bounded by craton-wide unconformities (time gaps in the rock record). The unconformities indicate when the seas receded and sediment was eroded rather than deposited. These sequences may in part represent eustatic (global) change in sea level; however, when
14-510: A regional geological feature is a stub . You can help Misplaced Pages by expanding it . This article about stratigraphy is a stub . You can help Misplaced Pages by expanding it . Laurence L. Sloss Laurence L. Sloss (August 26, 1913 – November 2, 1996) was an American geologist. He taught geology at Northwestern University from 1947 until his retirement in 1981. He was president, Geological Society of America (GSA), with his tenure beginning in 1980. The GSA's Laurence L. Sloss Award
21-624: A whole, these sequences are large-scale cycles in sedimentary rock records that indicate broad patterns of environmental change over geologic time – specifically marine transgression and regression . He was awarded the William H. Twenhofel Medal from the American Association of Petroleum Geologists in 1980 and the Geological Society of America's Penrose Medal in 1986. This biographical article about an American geologist
28-518: Is named in his honor. He was also president of the Society for Sedimentary Geology and American Geosciences Institute . Sloss received his bachelor's degree at Stanford University and his Ph.D. at the University of Chicago in 1937. He is known as a pioneer in the discipline of sequence stratigraphy , and for his descriptions of cratonic sequences or "Sloss sequences" in ancient North America. As
35-519: The beginning of the Cambrian Period . For North America, from oldest to youngest, they are the Sauk , Tippecanoe , Kaskaskia , Absaroka , Zuñi , and Tejas sequences . Attempts to identify equivalent cratonic sequences on other continents have met with only limited success, suggesting that eustasy (total global sea-level change) is unlikely to be the sole responsible mechanism. This article about
42-449: The continents; conversely, when spreading rates decline, the ridges subside, and the seas drain from the cratons. It is also possible that other mechanisms, such as dynamic topography related to mantle mass anomalies, and intraplate stress related to episodes of contractional and extensional tectonics , play a part by causing significant tectonic uplift and subsidence across the craton. There have been six cratonic sequences since
49-498: The proper names are used they usually refer to relative sea level changes on the North American continent . The most likely causes of these cycles is change in mid-ocean ridge volume, which is related to seafloor spreading rates. When Earth's mid-ocean ridges spread rapidly, the ridges tend to be longer than usual; also, the greater heat elevates the lithosphere over the ridges. This elevated lithosphere displaces seawater onto
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