Relation between Chemical Composition and Sedimentation Rate of Pacific Ocean-Floor Sediments Deposited since the Middle Cretaceous: Basic Evidence for Chemical Constraints on Depositional Environments of Ancient Sediments

Chemical characteristics of Pacific marine sediments were examined for different environments operating since the middle Cretaceous. As a result of the examinations, a procedure for evaluating depositional environments of ancient sediments in orogenic belts is proposed. The concentration of hydrogenous (authigenic) elements such as Mn, Co, and Ni in sediments has a relationship to the bulk sedimentation rate. Similarly, a new indicator which is a measure of the sedimentation rate of terrigenous (lithogenic) materials in sediments is proposed. This indicator is expressed as the weight ratio of a hydrogenous element to$TiO_{2}$. Because the sedimentation rate of terrigenous materials gradually decreases with distance from land, the ratio varies according to depositional environments. Geochemical examination of Pacific DSDP materials has shown that the distribution of element ratios ($MnO/TiO_{2}, Co/TiO_{2}$, and others) in surface sediments of the Pacific has not changed since the middle Cretaceous. As an application of the procedure, the author: (1) estimated sedimentation rates of ancient sedimentary rocks on land, (2) assessed the depositional environment of geosynclinal cherts, and (3) examined subduction process at the Japan Trench. The results demonstrate that ophiolite suites, including cherts in orogenic belts, are not always oceanic crust segments generated at a mid-ocean ridge; therefore, the plate tectonic orogenic model based on a hypothesis of offscraping mechanism at a plate convergence should be re-examined.