The mass-age distribution of Phanerozoic sediments

Abstract

summaryThe mass of the Phanerozoic sediments is about 2.1 x 1018metric tons, and between a quarter and one-third of it is distributed on the present continental margins and deep sea floor. The survival rate (surviving mass per unit time of deposition) seems to decrease exponentially with advancing age back to the Carboniferous, beyond which the tail of the distribution holds up and is somewhat irregular. The distribution for the past 300 Ma can be expressed by the equation log s = 10.01 - 0.24t, where s is the survival rate in metric tons per year and t is in units of 100 Ma. For a constant sediment mass with constant probability of destruction, this corresponds to a mean sedimentation rate since Devonian time of 101(}metric tons per year and a half-life for the post-Devonian mass of about 130 Ma. The overall distribution, however, is the sum of the distributions for three major realms, cratonic, marginal and pelagic, each with its own characteristic pattern. The last two account for most of the exponential-looking trend in the later Phanerozoic, but it is not clear (nor, in the case of the pelagic sediments, likely) that they are themselves exponential in character.The surviving masses (per unit time) of the Phanerozoic Systems tend to decrease with advancing age. This trend was revealed by a series of global volumetric estimates for the Devonian through Jurassic Systems (Ronov 1959) and extrapolations to the rest of the Phanerozoic based on a rough correlation between the volumes of the Systems surveyed and their maximum known thicknesses (Gregor 1967). Ronov and his colleagues have lately published (Ronovet al. 1980) complete volumetric estimates for those parts of the Phanerozoic Systems that underlie the global land surface. These estimates (after subtraction of the volcanic component and conversion from volume to mass) are summarized here in Table 3, Column 4. To them, in order to obtain the complete Phanerozoic mass-age distribution, must be added the sediments of the continental margins and deep sea. For convenience of study, the submarine realm can be divided into: 1. 'Passive' continental margins as defined by the US National Academy of Sciences (1979) and by Sclateret al. (1980). Examples are the margins bordering the Atlantic and Arctic Oceans and the Indian Ocean as far east as the Andaman Sea; 2. Marginal basins (Sclateret al. 1980) separated from ocean ridges by tectonic barriers (trenches, arcs, continental crust). They include such regions as the Caribbean and Mediterranean Seas and the marginal basins of the western Pacific; 3. The deep sea floor. The areal distribution of these submarine sedimentary environments, as determined by Sclateret al. (1980) is, in millions of km2: passive margins, 52.2; marginal basins, 26.9; deep sea floor, 281.7.