W sprawie zbadania granicy między proterozoikiem i kambrem w Polsce

Abstract

Omówiono historię badań granicy między proterozoikiem i kambrem w minionych 30 latach. Wskazano na trudności związane z ustaleniem stratotypu zawierającego tę granicę. Proponowane stratotypy: chiński i syberyjski nie spełniają warunków, gdyż w obu stwierdzono nieciągłości sedymentacyjne lub erozyjne. Dokonano krytycznego podsumowania wyników polskich badań.W profilach polskich suma czasu, której odpowiadają luki erozyjne i sedymentacyjne, może przewyższać sumę czasu, której odpowiadają zachowane osady. Zatem wymowa fauny w tych profilach nie może być jednoznaczna. Omówiono warunki, którym powinna odpowiadać sekwencja osadów proterozoiku - kambru dla ustalenia stratotypu. Zaproponowano obszar, na którym takie badania można by przeprowadzić On the research of the Proterozoic/Cambrian boundary in PolandThis paper discusses the investigations of the Proterozoic/Cambrian boundary carried out in the past 30 years. An outline of the international scientific efforts on this problem in various countries has been presented, among others those realized within the International Geological Correlation Program (Project no. 29) and the bilateral Polish-Soviet cooperation. Essential references from 1977 -1987, presenting the results of international investigations, have been indicated. It results from this review that the proposed Chinese stratotype is hardly correlative with other divisions, and the attractive Siberian stratotype contains the proven sedimentary disconformity between the Proterozoic and Cambrian. The duration of this sedimentary gap or synsedimentary erosion is also being constantly discussed. It results as well that the rock sequences from the continental shelfes or - more general from the shallow basins with mobile bottoms are useless for solving the Proterozoic/Cambrian boundary problem. Sedimentary gaps and erosion can appear there not only directly at the discussed boundary but also both above and below it. Similar difficulties occur in the Polish attempts at solving the problem. It should be stressed that the stratigraphic divisions and correlations of complexes from below the deposits with Holmia were being submitted to considerable changes (e.g. the position of the Klimontów stage, and Bazów, Kotuszów and Jasień beds). Considering the unequivocal lithostratigraphic correlation along the Mielnik - Szlinokiemie (Suwałki) cross-section (J. Znosko, 1965), it should be stated that at least five sedimentary gaps occur in the rock sequence from the Wisznice Beds (the Sławatycze Series after K. Lendzion) to the Baltic Beds (the Radzyń Series and partly the Kaplonosy Series after K. Lendzion). The gaps are also evident in the correlation chart by K. Lendzion (B. Areń, K. Lendzion, 1978, Tab. 1) as well as in a supposedly continuous sequence of the Vendian - Lower Cambrian in the Polish part of the Old Platform (B. Areń et al., 1979; K. Lendzion, 1983a, Figs. 2-4, 8). A question arises if the total time of the sedimentary and erosional gaps was not longer than the time of sedimentation of the preserved rock complexes. In this connection it should be noted that the significance of primitive, skeletal fauna found in these deposits cannot be unequivocal. It is not unlikely that this fauna may have existed actually longer than it results from the preserved fragments of rock succession. The extents determined so far may turn out to be "shortened" - that is apparent in relation to their real durations. It results from the above that for the discussed Proterozoic/Cambrian boundary such profiles should be taken into account which represent a quiet, pelitic, i.e. uniform and uninterrupted sedimentation. The rock sequences formed in such a way should comprise a long interval, several hundred or even over a thousand metres thick, in which the questionable boundary should be contained. A pelitic, uniform rock series confirms the quiet, pelagic conditions of sedimentation in an environment distant from the shore. A danger of existence of the sedimentary and erosional gaps in such a succession is negligible. Pelitic character of the sediments prove a slow sedimentation that lasted a sufficiently long time. The best profiles for these investigations should be the profiles of the Vendian and Cambrian with uniform, pelitic development which often caused a wrong classification of the "Riphean" rocks into Cambrian or vice versa. This type of rocks contains hardly any macrofossils but often contains Acritarchs, stratigraphic value of which is distinctly higher than that of trilobites. Acritarchs should become a biostratigraphic standard with which a significance of other fossils could be compared. Taking up this problem the author refers to an idea evolved at the same subject at the Jubilee Session of the 40th Anniversary of the National Geological Institute in 1960. The idea of studies on the contact between the Lower Cambrian and its "Riphean" substratum is not a new one. It had been announced by J. Samsonowicz who pointed out the location where the Bazów borehole has been later drilled in order to precise the character of this contact. This borehole did not realize our hopes although it did not lose its fundamental value for the Cambrian problems. Referring to the Lower Cambrian in the Bazów borehole investigations should be planned by means of several boreholes, 300-500 m deep, situated farther south, i.e. towards an area of domination of the "Riphean" rocks lying directly under the Miocene. There exist sufficiently good maps of the pre-Miocene substratum to situate these boreholes.