A type 1 sequence boundary is defined to be a sequence boundary "characterized by subaerial exposure and concurrent subaerial erosion associated with stream rejuvenation, a basinward shift of facies, a downward shift in coastal onlap, and onlap of overlying strata". Similarly, a type 2 sequence boundary is "marked by subaerial exposure and a downward shift in coastal onlap landward of the depositional-shoreline break; however, it lacks both subaerial erosion associated with stream rejuvenation and a basinward shift in facies" So the main distinction between type 1 and type 2 sequence boundaries is the amount of subaerial exposure. Type 2 sequence boundaries display, in contrast to type 1 sequence boundaries, only little subaerial exposure.
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Several Palaeozoic to neoproterozoic sedimentary basins onlap the Musgrave Block, and are exposed mostly around its edges. These were derived from erosion of the Musgrave Block basement during the neoproterozoic to Permian.
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Reading (1996), pp. 20–21 The situation in which coastlines move in the direction of the continent is called transgression. In the case of transgression, deeper marine facies are deposited over shallower facies, a succession called onlap. Regression is the situation in which a coastline moves in the direction of the sea.
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Image taken at Station C2 of the Apollo 14 landing site. Rock width is about 1.5 m. The fillet is characterized by an onlap contact with the adjacent rock and by a shallow or concave profile. Associated morphologies are thin pockets of dust present on top of the boulder and rock fragments either laying on top of, or buried by, the fillet.
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Hallam's approach is qualitative and relies on regional scale observations from exposed geologic sections and estimates of the areas of flooded continental interiors. Exxon's approach relies on the interpretation of seismic profiles to determine the extent of coastal onlap in subsequently buried sedimentary basins. Sea level rise since the Last Glacial Maximum. Global or eustatic sea level has fluctuated significantly over Earth's history.
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As plate convergence from the Ouachita orogeny began and progressed from the north to the south, offlap and onlap stratigraphic geometries were produced. The orogeny contributed a large amount of sediments from clastic rocks. These were deposited as thick, subaqueous deltaic systems which gradually filled the modern day midland basin over time. The Ouachita Orogeny thrust belt caused regional compression which led to overthrust structures, faulting and folding throughout the region.
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An overstep is a geological form that has a deposition of a stratum across inclined, progressively older rocks.Alistair F. Pitty Structure and Relief 1985 Page 28 "An overstep is a geological term describing the deposition of a stratum across inclined, progressively older rocks, as seen on both flanks of the Market Weighton axis (figure 9). In South East England, the Lower Greensand is another very ..." An onlap is a more general term than overstep, in which the younger beds overlap onto successively older beds.
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The basement is composed of foldbelts that were > deformed in Late Carboniferous–Permian time during collision of the Siberian > and Kazakhstan continents with the Russian craton. The basement also > includes several microcontinental blocks with a relatively undeformed > Paleozoic sedimentary sequence. The sedimentary succession of the basin is > composed of Middle Triassic through Tertiary clastic rocks. The lower part > of this succession is present only in the northern part of the basin; > southward, progressively younger strata onlap the basement, so that in the > southern areas the basement is overlain by Toarcian and younger rocks.
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These volcanoes are observed to approximate features observed elsewhere on Mars, including Hecates Tholus on the Elysium Rise; Tyrrhena Patera; Hadriaca Patera; and Apollinaris Patera. Later volcanic activity on the Thaumasia Plateau occurred in the Hesperian in association with a profusion of sheet lavas covering Syria Planum, Solis Planum, Sinai Planum, and Thaumasia Planum, emanating from the eastern flanks of the Arsia Mons region. In Thaumasia Planum and against the Coprates Rise, these sheet lavas were observed to onlap underlying plains materials and their associated ridged morphologies. Further volcanism occurred to the south of the Thaumasia highlands in Daedalia Planum.
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Further basinwards, the transitional crust is replaced by the oceanic crust characterized by a prominent gravity high and a well-imaged Moho reflection on seismic lines. The northwest of the Murray ridge is a major plate boundary transform fault which formed in the Late Oligocene/Early Miocene in response to plate reorganization following the opening of the Gulf of Aden. Onlap of the Miocene and younger stratigraphy on the southeast side of the ridge confirms the age of the ridge and resultant hanging wall uplift. Motion along the Murray ridge bounding fault has resulted in the development of northwest-southeast-trending faults, folds and shale diapirs in the hanging wall.
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267 The bathymetry of these basins show a general deepening towards the west, with major turbidite deposition in the Ainsa Basin and farther west. Subsequent ongoing inversion of the basins show a similar trend, with compressional phases becoming younger from east to west. While the onlap and erosion in the Clamosa area started in the early Eocene, around 49 Ma, the western portion experienced this phase terminating around the end of the Eocene, approximately at 35 Ma.Barnolas & Gil Peña, 2001, p.31 In the Jaca Basin, to the west of the Ainsa and Tremp Basins, during the Middle Eocene, flysch was deposited in an underfilled basin setting,Teixell et al.
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Analysis of regional 2D seismic lines and 3D seismic volumes by John Underhill, a geologist at the University of Edinburgh, led to the counterproposal that withdrawal of Upper Permian (Zechstein Supergroup) salt at depth was in fact a better explanation. Underhill found that all layers of rock down to the Permian (with an age of about 250 million years) are synclinically folded, and that sediments of Tertiary age at the crater onlap its sides and thicken into its axis, suggesting that the salt was moving (a process called halokinesis) while Tertiary sediments were being laid down. In 2007, Underhill continued to present evidence that he argues does not support the impact hypothesis. After analyzing seismic data over a wide region, he proposed that Silverpit was just one of many similar features related to the withdrawal of the Permian-age Zechstein salt.
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