DEPOSITIONAL ENVIRONMENTS OF SANDY SEDIMENTS OF THE ISHIKARI COASTAL PLAIN
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概要
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The Ishikari plain is a typical coastal plain which is divided into major four geomorphic units (Ishikari beach, Ishikari sand dunes, Bannaguro sand bars and Momijiyama sand dunes) (Fig. 1). The purpose of this investigation is to clarify the difference of four geomorphic units in term of depositional environments and to distinguish grain-size distributions of sandy deposits in the different localities of the same type geomorphic unit. In order to know the grain-size distribution and its relation to depositional environments, the author analyzed 66 samples using grain-size parameters and Q-mode factor analysis. The results are summarized as follows: 1. As for the mean-size of sediments, there are no distinct differences in the other three geomorphic units, except sands of the present beach having the largest value among them (Tables 1 and 2). Sands of the Momijiyama sand dunes are more poorly sorted than those of the other three environments. This fact means that sand deposits are getting to be more poorly sorted inlandwards in the Ishikari plain (Tables 1 and 2). Although skewness and kurtosis usually indicate the difference of depositional environments, it is difficult to distinguish Ishikari sand dunes from Monijiyama's (Table 2). Grain-size parameters merely show the trends of grain-size distributions and depositional environments. Thus, grain-size parameters are not useful to indicate the detailed characteristics of the geomorphology and the division of depositional environments. 2. Appling a Q-mode factor analysis, three basic kinds of energy which are responsible for the deposition of sediments are postulated (High-energy: Factor I, Medium-energy : Factor II, Low-energy: Factor III) (Fig. 5). These three eigenvalues account for 98% of total variability (Table 3). In the Ishikari plain Factor II type sediments occur in the noutheastern part, while Factor III type sediments occur in the southwestern part (Figs. 4 and 5). The area of the strong westerly winds estimated by the wind-shaped trees and the wind data obtained at the meteorological stations correspond with the regional distributions of depositional environments estimated from the types of energy (Fig. 5). This relationship implies that depositional environments at a given locality in the Ishikari coastal plain are deeply influenced by the strong westerly winds and strong wave actions. Therefore, it can be concluded that the Shakotan and the Takashima peninsulas which are located to the west of the Ishikari coast play an important role on the Ishikari coast, because they block the strong westerly wind from the southern part of the Ishikari coast.
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The Association of Japanese Geographers | 論文
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