Sea level variations, global sedimentation rates and the hypsographic curve

Abstract

Sea level changes during the Neozoic have been estimated by two different methods. The first involves measuring the amount of present-day land area which was flooded during the past, and using the present-day hypsographic curve to estimate the amount of sea level rise necessary to produce this flooding. The second involves the estimation of the changing volume of mid-oceanic ridges through time, and estimating sea level changes after having allowed for isostatic adjustment. A difference in sea level of 170 m is obtained from the two methods for the Cretaceous (80-100 m.y.B.P.). This is equivalent to a difference in continental flooding of 24 Mm 2, using the present-day hypsographic curve.We attempt to explain this difference firstly by allowing for the fact that the present-day ocean basins have more sediment in them than did the Cretaceous ocean basins. This produces a sea level change in the opposite direction to that produced by the reduction in mid-ocean ridge volume since the Cretaceous. Secondly, we suggest another large factor in producing the difference is that the present-day hypsographic curve is not the correct one to use when studying sea level stands in the Cretaceous. Present-day average continental heights are closely related to continental areas. Accepting this principle, if continents are joined together in the past, their average height must be greater, and so their hypsographic curve must be steeper. A given sea level rise would produce less continental flooding during times of continental aggregation than it would today.

Rates of continental erosion and mountain building

“…Calculations show that the stretching which has occurred during the past 80 My has caused the sea level to increase by about 28 m (Harrison, 1988). Continental break-up of Pangea has occurred with some regularity since the original break started 180 My aga (Harrison et al, 1981). Therefore the total sea-Ievel change during this time due to continental stretching has been about 60 m. These figures assurne that the stretched continental crust, as it sub si des below sea level, becomes covered with sediments such that the surface remains dose to sea level.…”

“…It is possible to determine the shape of the hypsographic curve for past times using the formulation given by Harrison et al (1981). In this paper an empirical formula is given for continental hypsography, for which the base is assumed to be at the -200 m contour.…”

Scale dependence of hypsometric integrals: An analysis of southeast African basins

“…For instance, there was a re-evaluation of continental hypsometry (Harrison et al, 1983;Cogley, 1985) and an assessment of the relevance of global and continental hypsometry to long-term sea-level change and the issue of changes in continental freeboard through geologic time (Harrison et al, 1981;Schubert and Reymer, 1985;Wyatt, 1986). More recently, there has been a diverse range of applications including testing numerical models of landscape evolution (Hancock and Willgoose, 2001), assessing the role of lithology in influencing basin morphometry , examining the effect on hypsometry of differences in precipitation and runoff (Masek et al, 1994;Montgomery et al, 2001), evaluating the controls over the hypsometry of glaciated terrains (Brocklehust and Whipple, 2004), assessing the relative role of lithology and tectonics (Lifton and Chase, 1992), inferring the relationship between tectonics and denudation (Ohmori, 1993), and characterising tectonic activity through its hypsometric signature (Azor et al, 2002;Chen et al, 2003).…”

Continental size, eustasy and sediment yield

“…Use of the data sets allows hypsographic curves for the modern continents to be constructed, curves which can be used to predict changes in sediment yield. The fact that all the modern continental blocks show similar normalized hypsographic curves (in the sense of Harrison et al, 1981) suggests that the average normalized hypsographic curve ( Fig. 1) can be used to predict the area/height distribution of former continental blocks (Wyatt, 1987), assuming no major %Area change in the total volume of continental crust; an assumption that seems reasonable for most of the Phanerozoic (Taylor & McLennan, 1985).…”

“…In practice, to try to eliminate the effect of climatic change, it is preferable to compare results from the calculations with known deposits before the onset of major glaciation. For much of geological history, however, the continental blocks were different, and calculations need to be made using the normalized hypsographic curve (Harrison et al, 1981;Wyatt, 1987).…”