Litologia osadów powierzchniowych południowej części Basenu Bornholmskiego

Abstract

Przedstawiono wyniki uziarnienia. składu mineralnego. zawartości substancji organicznej i wilgotności osadów powierzchniowych południowej części Basenu Bornholmskiego. Scharakteryzowano typy granulometryczne utworów oraz podano zasięg ich występowania. Omówiono współzależności między głębokością morza a typem granulometrycznym i składem mineralnym, zawartością substancji organicznej oraz wilgotnością osadu.

LITHOLOGY OF SEA-FLOOR SEDIMENTS IN SOUTHERN PART OF THE BORNHOLM BASIN

The studies covered southern peripheral part of the Bornholm Basin, delineated by the meridians 150 in the west and 160 in the east, parallel of latitude 54 ̊42' in the north and 30 m isobath, accepted here as southern boundary of this basin, in the south. The sea is up to 71 m deep in this area. The studies on upper 10 - 20 cm layer of the modern sea-floor sediments, concerned their granulometry (130 analyses), mineral composition (59 analyses), content of organic matter (59 analyses) and absolute humidity content (33 analyses).

The following granulometric types of sediments have been identified with reference to the F.P. Shepard (1954) classification: sands and silty sands, sandy silts, silts, clay silts and silty clays. Distribution of individual types of sediments was found to be zonal. Sands cover slopes of the basin down to 53 m depth at the average whereas the remaining sediments occur at larger depths. There is noted a general trend to decrease of size of mineral grains in sediments along with depth at which they occur.

The studies on mineral composition of fine-grained sediments showed omnipresence of illite, chlorite and quartz which form typical mineral assemblage here. Minerals of the smectite group - beidellite. montmorillonite and nontronite - are sporadically present, presumably forming small admixtures to the major mineral assemblage. Quantitative relations between individual minerals are varying. Transition from silty clays to silty sands is connected with decrease in share of grains below 0.062 mm in size and clay minerals at the advantage of quartz, the share of which rises from about 30% to 80% (Table 1).

The studies on mineral composition of sands showed the presence of quartz (85% at the average) and feldspars (8 % at the average) as well as subordinate amounts of rock debris and alterated and heavy minerals. Organic matter is also important here. Its content closely depends on granulometric type of sediment. Correlation coefficient of content of organic matter and the fraction below 0.062 mm equals 0.83 and equation of rectilinear regression has the form: y = 0.09 x+0.09, where), is content of organic matter in per cent, and x -content of fraction below 0.062 mm in per cent.

The above dependence is typical of modern sediments. Absolute humidity content in sediments increases along with content of silt and clay size grains (below 0.062 mm in size) and organic matter. The humidity appears more closely related to content of organic matter than that of fine-grained (below 0.062 mm) fraction (Table 2).

Distribution of individual types or sediments appears mainly related to haline stratification of waters. Sands, practically without organic matter, occur at sea floor in zone of contact of sediments and relatively freshened waters of upper layer (salinity below7.5‰)and saturation with oxigen over 70%. Upper boundary of lower saline layer is usually situated in this basin at about 50 m depth. This layer infills lower parts of the Bornholm Basin and low hydrodynamic activity of its waters is advantageous for sedimentation of particles below 0.062 mm in size and shortage of oxygen - for accumulation of organic matter. The extent of sedimentation of silts and clays (mineral particles below 0.062 mm in size) seems ·to be controlled by internal waves and ascending currents, occurring at the boundary of waters differing in density.