陸棚堆積物の区分と暴風型陸棚における堆積相

Abstract

The criteria for the classification of present shelf sediments are summarized, and the classification of sedimentary environments and the facies of their sediments in a stormdominated shelf are presented.The present shelf sediments are classified by the seven criteria as follows.(1) Are the sediments modern or relict? and when were the sediments supplied? (2) Are the sediments palimpsest or not? (3) By which kind of physical processes were they deposited? (e. g. density currents, flood, tidal currents, geostrophic currents, oceanic currents, tsumanis, etc.)(4) What type of sedimentary environment? (e. g. foreshore, shoreface, inner shelf, outershelf, etc.)(5) What is the grain size? (6) What is the natur e of the constituents? (e. g. authigenic, biogenic, volcanic, clastic, etc.)(7) What is the sedimentary structure or bedform of the sediments? These classification criteria of the present shelf sediments are the key to the interpretation of the environment of ancient sedimentary rocks.The storm-dominated and siliciclastic shelf sediments are divided into nearshor e (foreshore and shoreface), inner shelf and outer shelf facies. Foreshore is the zone of beach and the facies is characterized by well-sorted sand and seaward dipping, low-angle wedge-shaped cross- or parallel-bedding. Upper shoreface is the zone in which longshore bars and troughs are recognized within water depth of less than about 6m, and the facies consists of coarse to pebbly sand with cosets of high -angle tabular or trough cross-bedding of 10 to 100cm thickness. Lower shoreface is the zone which is between 6 to about 20m of water depth, and the facies is comprised of well-sorted fine to very fine sand with amalgamated hummocky cross- or parallel-stratification. Inner shelf is the zone which ranges from 20 to about 70m of water depth, and the facies is made up of interbedded sand and mud. Also, hummocky cross-stratification is recognized in the sand beds deposited in a zone less than about 50m of water depth. Outer shelf is the zone below about 70m of water depth, and the facies is composed of bioturbated mud. The boundary between shoreface and innershelf corresponds to the mean fairweather wave base and the boundary between inner shelf and outer shelf to the mean storm wave base.