Mieszane zespoły hieroglifowe ze śladami rozmywania wcześniejszymi od śladów przedmiotów

Abstract

MIXED CURRENT MARK ASSEMBLAGES SHOWING SCOUR MARKS ANTEDATING TOOL MARKSSummaryScour marks antedating tool marks prove a decrease of velocity of the “mature” turbidity current (sensu R. G. Walker, 1965). Scour marks are formed due to the erosion caused by turbulent nose of a current transporting sediment in suspension. Tool marks, in turn, are formed by a slower, main body of the current transporting a part of the sediment in bottom traction. The falling down of the objects that leave their marks on the bottom can be considered as the first sign of the forth-coming sedimentation. The mixed current mark assemblages of the Silurian from Northern Poland can be assigned into the following groups: l) assemblages proving a high velocity of both nose and main body of the turbidity current (Table I, Figs. 1 and 2); 2) assemblages proving rather a low velocity of the current nose, while its main body - slower though it was - still flowed relatively fast (Table II, Figs. 3 and 4); 3) assemblages proving a low velocity of both nose and main body of the turbidity current (Table III, Figs. 5 and 6). There are reasonable grounds to suppose that all the tool marks of the Silurian deposits from Northern Poland are a result of tractional movement of similar, or, at least, of comparable objects. The current mark assemblages falling under the first and the third groups indicate a uniform decrease of the current velocity, thus proving the existence of a uniformly inclined, or even horizontal bottom of the basin. These assemblages are no doubt most frequent in the Silurian deposits of Northern Poland. The current mark assemblages belonging to the second group were formed due to rapid slowing down of the turbidity current. Its main body characterized by a greater momentum than that of the current nose, has longer maintained the impetus gained previously. Assemblages that indicate a non-uniform decrease of current velocity prove also an irregularly inclined bottom of a sedimentary basin. It should be kept in mind that every current mark assemblages described in the paper can be formed due to local irregularities in the bottom of the basin. Only current mark assemblages, which are permanent in the long-aged sedimentary series, may be thought to reflect regional conditions.