A note on the correction of ocean floor depths for sediment loading

Abstract

The measured physical properties of sediments recovered in a number of deep‐sea drill holes in the North Atlantic have been used to derive expressions for the correction to basement depths required to account for loading by sediments. The average density and porosity over a given thickness of sediment show an approximate linear dependence on sediment thickness. This behavior can be explained as primarily the result of compaction. If the compaction of the sediments is neglected in correcting basement depths for sediment loading, errors of a few hundred meters in the corrected depths can be made. A sediment loading correction based on the observed variation of density with depth can give corrected depths to better than 100 m, with remaining uncertainties due to variability in sediment properties and rebound effects on the measured densities. A relation between sediment thickness and vertical travel time was obtained from sonic velocity measurements on samples from drill holes, sonobuoy wide‐angle reflection experiments, and direct correlation of seismic reflectors with downhole lithology. The sediment loading correction can then be expressed directly in terms of the vertical travel time measured along seismic reflection profiles. The loading correction obtained here for the North Atlantic may not be applicable in areas with different sedimentary environments. When direct measurements of physical properties in drill holes are not available in such areas, it is shown that it is preferable to assume a compaction curve for the appropriate lithology rather than to use observed seismic velocities together with a velocity‐density relationship.

Crustal flow beneath Iceland

“…Average sediment density is estimated using r = (1 À f)r s + fr w , where solid grain density r s = 2.7 Mg m À3 and average porosity determined by integrating the standard empirical relation f = f 0 exp (Àz/l s ) with depositional porosity f 0 = 0.6 and compaction length scale l s = 2 km. This scheme for r agrees with measurements from North Atlantic sediment cores by LeDouaran and Parsons [1982] over their sample range of 0 < D s < 2 km. Calculations performed after interpolating data sets in Figures 1a and 1b onto an isotropic 3 km mesh using continuous curvature splines [Smith and Wessel, 1990].…”

Influence of continental growth on mid‐ocean ridge depth

“…It gives us sediment thickness based on oceanic plate age and latitude. With the calculated sediment thickness, we use Le Douaran and Parsons [] model for sediment loading. This introduces a downshift in depth as expected due to isostasy from the increase loading.…”

“…This introduces a downshift in depth as expected due to isostasy from the increase loading. After accounting for these using a sediment loading model [ Müller et al ., ] and airy isostasy model [ Le Douaran and Parsons , ], the predicted bathymetry (Figure a, green line) shifts ∼2 km deeper and compares favorably with actual depths of ridges and basins (Figure a, red line, 70°E).…”

Measuring dynamic topography: An analysis of Southeast Asia

“…Assuming Airy isostasy, the sediment correction, d s , is given by where S is sediment thickness and other parameters are given Table 1. Using deep drilling sites in the North Atlantic Ocean, Le Douaran and Parsons [1982] derive an empirical loading correction for a sediment pile which is up to 2 km thick. In their scheme, the average density of sediment is given by …”