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- Volume 9, Issue 4, 1997
Basin Research - Volume 9, Issue 4, 1997
Volume 9, Issue 4, 1997
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On the power law size distribution of turbidite beds
More LessA number of studies have shown that cumulative distributions of turbidite bed thicknesses (as observed in boreholes or outcrops) follow a power law in the sense that the number of beds whose measured thickness is greater than η is proportional to η−β. The purpose of this paper is to investigate the relationship between the distribution of measured thicknesses of turbidite sand beds and the distribution of bed volumes. The distribution of sand bed volumes has practical implications to model hydrocarbon reservoirs in 3D and quantifies how the total amount of sand within a turbidite sequence is distributed among beds of different sizes. If the cumulative distribution of bed volumes v is proportional to v−c the few largest beds account for more and more of the total volume of sand and the sequence becomes more and more ‘punctuated’ as the exponent c decreases below 1. The relationship between the exponents of the distribution of measured bed thicknesses (β)and of bed volumes (c) can be investigated by assuming that the turbidite beds are disc‐like bodies distributed in the 3D space of a basin, and that their thicknesses are sampled on a vertical line that does not in general intersect all the beds. The relationship between β and c can be shown to depend on how bed length scales with bed thickness (as noted by other authors) and on how the beds are distributed in the 3D space of the basin (which is a new result). The main conclusion is that it is not generally possible to make inferences on the distribution of turbidite bed volumes knowing only the distribution of bed thicknesses. On the other hand, if some independent information is available on the geometrical characteristics of the turbidite deposit, the observations of bed thicknesses can be converted to bed volumes, thus providing general information on the 3D characteristics of the sequence.
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Regular spacing of drainage outlets from linear fault blocks
More LessOutlets of river basins located on fault blocks often show a regular spacing. This regularity is most pronounced for fault blocks with linear ridge crests and a constant half‐width, measured perpendicular to the ridge crest. The ratio of the half‐width of the fault block and the outlet spacing is used in this study to characterize the average shape (or spacing ratio) of 31 sets of drainage basins. These fault‐block spacing ratios are compared with similar data from small‐scale flume experiments and large‐scale mountain belts. Fault‐block spacing ratios are much more variable than those measured for mountain belts. Differences between fault‐block spacing ratios are attributed to variability in factors influencing the initial spacing of channel heads and subsequent rates of channel incision during the early stages of channel network growth (e.g. initial slope and uplift rate, precipitation, runoff efficiency and substrate erodibility). Widening or narrowing of fault blocks during ongoing faulting will also make spacing ratios more variable. It is enigmatic that some of these factors do not produce similar variability in mountain belt spacing ratios. Flume experiments in which drainage networks were grown on static topography show a strong correlation between spacing ratios and surface gradient. Spacing ratios on fault blocks are unaffected by variations in present‐day gradients. Drainage basins on the Wheeler Ridge anticline in central California, which have formed on surfaces progressively uplifted by thrust faulting during the last 14 kyr, demonstrate that outlet spacing is likely to be determined during the early stages of drainage growth. This dependency on initial conditions may explain the lack of correlation between spacing ratios of fault blocks and slopes measured at the present day.
Spacing ratios determine the location of sediment supply points to adjacent areas of deposition, and hence strongly influence the spatial scale of lateral facies variations in the proximal parts of sedimentary basins. Spacing ratios may be used to estimate this length scale in ancient sedimentary basins if the width of adjacent topography is known. Spacing ratio variability makes these estimates much less precise for fault blocks than for mountain belts.
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Headless submarine canyons and fluid flow on the toe of the Cascadia accretionary complex
Headless submarine canyons with steep headwalls and shallowly sloping floors occur on both the second and third landward vergent anticlines on the toe of the Cascadia accretionary complex off central Oregon (45 °N, 125° 30′W). In September 1993, we carried out a series of nine deep tow camera sled runs and nine ALVIN dives to examine the relationship between fluid venting, structure and canyon formation. We studied four canyons on the second and third landward vergent anticlines, as well as the apparently unfailed intercanyon regions along strike. All evidence of fluid expulsion is associated with the canyons; we found no evidence of fluid flow between canyons. Even though all fluid seeps are related to canyons, we did not find seeps in all canyons, and the location of the seeps within the canyons differed.
On the landward facing limb of the second landward vergent anticline a robust cold seep community occurs at the canyon’s inflection point. This seep is characterized by chemosynthetic vent clams, tube worms and extensive authigenic carbonate. Fluids for this seep may utilize high‐permeability flow paths either parallel to bedding within the second thrust ridge or along the underlying thrust fault before leaking into the overriding section. Two seaward facing canyons on the third anticlinal ridge have vent clam communities near the canyon mouths at approximately the intersection between the anticlinal ridge and the adjacent forearc basin. No seeps were found along strike at the intersection of the slope basin and anticlinal ridge. We infer that the lack of seepage along strike and the presence of seeps in canyons may be related to fluid flow below the forearc basin/slope unconformity (overpressured by the impinging thrust fault to the west?) directed toward canyons at the surface.
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Slope basins, headless canyons, and submarine palaeoseismology of the Cascadia accretionary complex
Authors Brian G. McAdoo, Daniel L. Orange, Elizabeth Screaton, Homa Lee and Robert KayenA combination of geomorphological, seismic reflection and geotechnical data constrains this study of sediment erosion and deposition at the toe of the Cascadia accretionary prism. We conducted a series of ALVIN dives in a region south of Astoria Canyon to examine the interrelationship of fluid flow and slope failure in a series of headless submarine canyons. Elevated head gradients at the inflection point of canyons have been inferred to assist in localized failures that feed sediment into a closed slope basin. Measured head gradients are an order of magnitude too low to cause seepage‐induced slope failure alone; we therefore propose transient slope failure mechanisms. Intercanyon slopes are uniformly unscarred and smooth, although consolidation tests indicate that up to several metres of material may have been removed. A sheet‐like failure would remove sediment uniformly, preserving the observed smooth intercanyon slope. Earthquake‐induced liquefaction is a likely trigger for this type of sheet failure as the slope is too steep and short for sediment flow to organize itself into channels. Bathymetric and seismic reflection data suggest sediment in a trench slope basin between the second and third ridges from the prism’s deformation is derived locally. A comparison of the amounts of material removed from the slopes and that in the basin shows that the amount of material removed from the slopes may slightly exceed the amount of material in the basin, implying that a small amount of sediment has escaped the basin, perhaps when the second ridge was too low to form a sufficient dam, or through a gap in the second ridge to the south. Regardless, almost 80% of the material shed off the slopes around the basin is deposited locally, whereas the remaining 20% is redeposited on the incoming section and will be re‐accreted.
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Volumes & issues
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Volume 36 (2024)
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Volume 35 (2023)
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Volume 34 (2022)
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Volume 33 (2021)
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Volume 32 (2020)
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Volume 31 (2019)
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Volume 30 (2018)
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Volume 29 (2017)
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Volume 28 (2016)
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Volume 27 (2015)
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Volume 26 (2014)
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Volume 25 (2013)
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Volume 24 (2012)
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Volume 23 (2011)
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Volume 22 (2010)
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Volume 21 (2009)
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Volume 20 (2008)
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Volume 19 (2007)
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Volume 18 (2006)
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Volume 17 (2005)
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Volume 16 (2004)
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Volume 15 (2003)
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Volume 14 (2002)
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Volume 13 (2001)
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Volume 12 (2000)
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Volume 11 (1999)
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Volume 10 (1998)
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Volume 9 (1997)
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Volume 8 (1996)
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Volume 7 (1994)
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Volume 6 (1994)
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Volume 5 (1993)
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Volume 4 (1992)
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Volume 3 (1991)
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Volume 2 (1989)
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Volume 1 (1988)