O sedymentacji dolnokredowej rudy żelaza z otworu wiertniczego Trzemżal 1

Abstract

ON LOWER CRETACEOUS SEDIMENTATION OF IRON ORE IN TRZEMŻAL 1 BORE-HOLEThe author presents the results of his investigation of a Lower Cretaceous ore-bearing series from the Trzemżal 1 bore-hole drilled in 1959 near the Mogilno structure in Western Poland (Fig. 1). On the basis of macroscopic examinations he distinguished in the Lower Cretaceous 6 lithological rock complexes, establishing the boundary between these complexes on top of ore strata (Fig. 2).The iron ore appearing in the Lower Cretaceous consists of ferruginous gravel beds containing numerous oolites and pseudo-oolites built chiefly of iron hydroxides and oxides and of numerous minor ferruginous rock fragments. On the basis of macroscopic and microscopic studies the author subdivided in detail the latest ore seam deposited at the depth from 1270.0 to 1270.75 m.; this ore bed starts the 6th sedimentation cycle of this Lower Cretaceous deposit (Fig. 3).In this seam the author distinguishes 8 lithological strata consisting of alternating layers of ferruginous sandstones, fine-gravelly and oolitic ores and banded ores as well as ores with argillaceous thin striae. In both the lithological and petro-graphical description the author paid much attention to the structure of the ferruginous oolites and the pseudo-oolite structures calling attention to the occurrence of "pseudo-piezolitic" forms of up to 12 mm. size, and to the character of the lithological boundaries between the individual laminae.On the basis of the detailed lithological and petrographical characteristic of the ore-bearing series the author draws his conclusions on the sedimentary environment of the Lower Cretaceous during the sedimentation of the ore-bearing series. This environment has been markedly mobile and well aerated, due to which oolites of iron hydroxides and oxides developed.The factors promoting the formation of ore deposits were:1) changes in the water regime of the sedimentary basin, connected with oscillations of the Lower Cretaceous sea;2) changes in the conditions of sedimentation of the environment (pH, Eh., facies);3) the influence of the mobile salt dome of the Mogilno structure.Presumably the precipitation of iron compounds from the solution went forth in the shape of Fe+++ (oxides and hydroxides); this conclusion is in agreement with N. M. Strachow's (1953) theory and with the opinon expressed by H. Harder (1957). The author ascribed the. origin of the piezolitic and concretionary forms and·the deformation of the oolites to diagenetic processes.