Insights Into Marls From Optical and Back-Scattered Electron Petrography: An Example From the Outer Carpathians (Poland)

Abstract

Abstract: This paper is a study of grain provenance, depositional processes, and the relationship of those processes to composition and diagenesis of fine-grained rocks, based on high-resolution petrography. The marly successions deposited in siliciclastic flysch and related to the paleotectonics of the northern domain of the Tethys Ocean (Outer Carpathian Basin) were examined. The following marls were studied: Upper Jurassic marls from the Vendryně Formation (Goleszow Marls); Upper Cretaceous Siliceous-Fucoid, Wegierka, Weglowka, Frydek, Jasienica, and Żegocina Marls; Eocene Łącko, Budzow, Zembrzyce, and Leluchow Marls; and Oligocene Sub-Cergowa and Grybow Marls. These marls are products of the redeposition of shelf sediments into deeper parts of the basin. Knowledge of their diagenetic history can contribute to the evaluation of the petroleum potential of the Polish Outer Carpathians. Thin-section analysis by optical and electron back-scattered microscopy (FESEM-BSE) highlights the similarity of all these marls with respect to composition, grain size, sedimentary features, and diagenetic changes. They differ only in the quantitative proportion of constituents, the relative extent of key microfacies, and diagenetic effects. The diagenetic modifications recognized in the rocks studied are characteristic of the processes operating in the sedimentary environment and during burial diagenesis. Thin-section petrography of the marls reveals fine, mostly silt-size siliciclastic material and bioclasts supported by matrix. Siliciclastic particles comprise mostly quartz, micas, feldspars, glauconitic nodules, and lithoclasts. Calcareous and siliceous bioclasts, mostly microfossils (foram and sponge spicules, and a few radiolarians and diatoms) and coccospheres, are variably preserved; they may occur as calcite, silica, and clay pseudomorphs. Additional components include peloids and stringers of brown organic matter. Millimeter-scale fabric analysis provides detailed insight into the processes of transport and deposition. Five microfacies are identified: parallel-laminated, wavy and lenticular-laminated, homogeneous exhibiting preferred orientation of elongated components, bioturbated, and bioclast-bearing. The range of microfacies suggests that the marl sediment was deposited while currents were active. The depositional settings identified range from traction, reworking by bottom currents, supply by nepheloid flow (indistinctly bounded laminae), possible eolian input (laminae a few grains thick), and biogenic reworking of sediments. Back-scattered electron images reveal that the matrix consists of coccolithophorid remains with varying abundances of clay, silica, and carbonate cements. Five varieties of matrix are distinguished: 1) carbonate–clay–silica bearing, 2) carbonate–clay supported, 3) carbonate–silica supported, 4) clay–silica supported, and 5) clay dominated. Clay appears as rosette and lamellar aggregates, silica as nanoquartz (microspheres, clusters, coatings), quartz overgrowths, and occasionally opal CT as lepispheres and aggregates moss-like in appearance, and calcite as overgrown coccoliths, pseudomicrite, micrite, and microspar. Pyrite framboids are present in some samples. Intrabiotic and intergranular cements are developed. Also present are partially dissolved coccolith placoliths, silica- and clay-replaced micarb, and grains similar in appearance to volcanic glass shards that now consist of calcite and quartz. Bioclasts may have been a source for calcite and silica cements. Volcanic glass may have been a source for authigenic silica and clay minerals; this is suggested by the close spatial juxtaposition of silica microspheres and authigenic clay (cauliflower-like aggregates). The authigenic origin of the clay in these marls is also suggested by the presence of overgrown coccoliths because recrystallization of shields is reduced when clay is present, so that the overgrowths must have been formed in an environment where clay minerals were scarce initially. Despite diagenetic changes, the presence of pyroclasts is still recognizable. There are quartz crystal chips, shard-shaped grains, volcanic rock fragments, and minerals (biotite, apatite, zircon). The marls studied could be described as impure fine-grained detrital limestones chalky in type, with evidence of redeposition and possible volcanic-ash input. The marl depositional system within the Outer Carpathian Basin was associated with tectonic movements which initiated chalk-type facies, and this was particularly widespread in the Upper Cretaceous and Paleocene.