Mikrofacje osadów węglanowych arenitu w północno-wschodniej części obniżenia podlaskiego

Abstract

 Przedstawiono wyniki badań mikrofacjalnych skał węglanowych arenigu północno-wschodniej części obniżenia podlaskiego, których skondensowane miąższościowo sekwencje reprezentują osady regresywnej fazy arenidzkiego cyklu sedymentacyjnego. Zróżnicowanie składu oraz cech tekstualnych i strukturalnych skał umożliwiło wydzielenie szeregu typów mikrofacjalnych, które w następstwie pionowym tworzą trzy odrębne genetycznie kompleksy: zailonych biomikrytów ramienionogowych, sparytów oraz biosparytów trylobitowo-ramienionogowych. Kompleks pierwszy odpowiada wiekowo górnej części arenigu dolnego (latorp B_I), natomiast dwa pozostałe – arenigowi górnemu (wołchow B_II). Przeprowadzono próbę interpretacji środowisk sedymentacji osadów poszczególnych  kompleksów mikrofacjalnych.

MICROFACIES OF CARBONATE ARENIGIAN ROCKS IN NORTH-EASTERN PART OF THE PODLASIE DEPRESSION (NE POLAND)

The paper presents microfacies characteristics of carbonate Arenigian rocks from north-eastern part of the Podlasie Depression, established on the basis of petrographic, paleontological and sedimentological studies. The studies covered core material from drillings made by the Geological Institute in the years 1961–1976 (Fig. 1): there were made 250 multi-component chemical analyses of the rocks as well as 150 polished sections and about 800 thin sections, a part of which were coloured with alizarin S.

Rock assemblage of Arenigian carbonates forms a clearly separate lithological complex, built of dolomitic marls of the Lower Arenigian (Latorp B₁) in lower part and dolomites and glauconitic

limestones of the Upper Arenigian (Volkhov B_II) in the upper. The sequences overlay in sedimentary continuity a packet of Lower Arenigian glauconitite, markedly different in development, and are overlain by marly limestones (biointrasparites) of the Lower Llanvirnian (Kunda B_III), with which they contact along a clear subaqueous sedimentary discontinuity (Fig. 2).

Rock assemblage of Arenigian carbonates and underlaying packet of Lower Arenigian glauconitite form a separate sedimentary cycle of the transgressive-regressive character. Upper, carbonate member of that cycle, as shown by succession of microfacies types in the vertical, corresponds to sediments of a regressive phase (B. Szymański, 1971, 1973). Carbonate rocks formed during that phase of the Arenigian cycle are characterized by markedly reduced thickness and the presence of numerous intra- and inter-layer sedimentary discontinuities of subaqueous origin. The latter, along with the wealth of glauconite and phosphates, indicate low-rate and nonuniform subsidence of basin floor.

A number of microfacies types have been identified in the sequence of grained carbonate rocks.

Taking into account differences in thickness ratios between the types in individual parts of the section, they are grouped in three separate complexes: 1 – clay brachiopod biomicrites (Lower Arenigian),

2 – sparites and 3 – trilobite-brachiopod biosparites (Upper Arenigian).

Variability in composition of grainy skeletal material in strata of individual microfacies complexes indicates marked ecological contrast of their biotopes. This contrast is, especially well visible in the case of assemblages of two groups of benthic organisms, i.e. brachiopods and trilobites. Taking this into account, the studied Arenigian carbonate sequences were divided into two separate biofacies assemblages in wide sense: lower – brachiopod, and upper – trilobite-brachiopod.

Gradual increase of content of grainy skeletal material upwards in the section, replacement of initially predominating micrite by sparite and the accompanying decrease in share of clay material in sediments indicate gradual increase of role of biogenic factor in the process of lithogenesis in the course of sedimentation of Arenigian carbonates in the studied area. This evidences gradual cease of activity of alimentary areas, especially in the Late Arenigian.

Variability in composition of rocks of individual microfacies complexes in the studied sections reflects succession of microfacies in the vertical. Sedimentary environments of these microfacies may be regarded as related to neritic zone in an epicontinental basin with high energy of waters (above waving base). A marked concentration of skeletal grains (except for or with negligible share of those of reef-forming organisms – corals, algae, bryozoans) seems to indicate sedimentation of the deposits in a belt of nearshore shoals (banks), initially (in the Early Arenigian – Latorp B_I) in their external, shoreward and sheltered part (clay brachiopod biomicrites) and thereafter (in the Late Arenigian – Volkhov B_II) in central or inner part (sparites and trilobite-brachiopod biosparites).

At the early diagenetic stage, Arenigian carbonate rocks became widely subjected to intense although nonuniform dolomitization and recrystallization in the studied area. These processes resulted in changes of original composition and sedimentary textures. The former affected especially sediments of clay brachiopod biomicrite complex and sparites, and the latter – mainly sparites.