Stromatolity lanwirnu górnego z północno-wschodniej części obniżenia podlaskiego

Abstract

Omówiono pozycję stratygraficzną, typy morfologiczne (makrostruktury), cechy diagenetyczne oraz zmienność następstwa pionowego stromatolitów z osadów węglanowych lanwirnu górnego północno­wschodniej części obniżenia podlaskiego. Wyróżniono trzy zasadnicze typy morfologiczne stromatolitów: kolumienkowy, kopułowy i gładki. Stwierdzono, że identyczna mikrostruktura tych form wskazuje na biologiczne uwarunkowania ich powstania.UPPER LLANDVIRN STROMATOLlTES FROM NORTH-EASTERN PART OF THE PODLASlE DEPRESSION (NE POLAND) In north-eastern part of the podlasie Depression, stromatolites occur in grained carbonate Upper Llandvirn rocks (Figs. 1 - 3). The rocks represent rich bryozoan-trilobite biosparites monotonous in textural composition (Table I, Figs. 4 - 6) and interpreted as formed in a regressional phase. Sequences of these rocks are characterized by intense non-carbonate (ferruginous) mineralization, numerous sedimentary discontinuities of subaqueous origin, and small thickness, indicative of a low rate of sedimentation. The studies showed that the Upper Llandvirn stromatolite assemblage comprises forms of the types of smooth stromatolite layers (Table Ill, Fig. 11), columnar (polygonal) stromatolite layers (Table II, Figs. 7 -9; Table III, Fig. 10), and isolated clumps (Table Ill, Fig. 12) and single domes (Table III, Figs. 13,14; Table IV, Figs. 15-17). The structures are built of carbonates (micrite and skeletal grains), iron hydroxides and oxides, and subordinate amounts of terrigenous, mainly clay material. The studied stromatolites were found to be characterized by identical microstructural character, which suggests biological control of their origin. Structures of the biogenic stromatolites were subjected to processes of ferruginous and carbonate cementation, replacement by iron compounds, infilling of fissures and voids with iron compounds and carbonates, and microsparitization, at early diagenetic stage. An early lithification and dehydration were also connected with origin of fine syneresis cracks in the structures, mainly following lamination. The bulk of originally open cracks became subsequently infilled with mineral matter, mainly iron com-pounds and/or microsparite. Intense development of Upper Llandvirn stromatolites has been facilitated by very low rate of sedimentation, quick lithification (leading to origin of hard grounds) and low effectiveness of rework-ing of sediment by penetrating organisms. It is assumed that the stromatolites were originating in vast shoal areas in subtidal zone of the epicontinental basin.