MICROSTRUCTURE AND MINERAL COMPOSITION OF THE EOCENE ROCKS FROM THE BLACK SEA NORTH-WESTERN SHELF

Abstract

The study area is located within the North-Western shelf of the Black sea. Eocene rocks with the local breaks occur on the sediments of Lower Palaeocene and are overlapped by Oligocene rocks. We studied the mineral composition and microstructural features of Eocene carbonate-clay and terrigenous rocks from three wells by X-ray and electron-microscopic methods. Based on the research we distinguished seven main types of microstructures of the rocks: spongy, scaly aggregates in intergranular space, matrix, laminar, turbulent, cellular-domain and domain-spongy. Microaggregates, composed of hydromica, kaolinite and montmorillonite flakes, form spongy microstructure of the rock. Microstructure of scaly aggregates in intergranular space available in terrigenous rocks; their framework is formed mainly of quartz grains of aleuritic dimensionality. Mainly randomly oriented flakes of kaolinite, hydromica and mix-layered phases (hydromica–montmorillonite, chlorite–montmorillonite) form matrix microstructure. Oriented structural ele-ments and considerable adjoining particles are characteristic for laminar microstructure; the main structural elements are the isometric grains of calcite of pelitic dimensionality. Turbulent micro-structure is formed of polymineral aggregates (hydromica, chlorite, montmorillonite and calcite) and single grains of quartz, sometimes pyrite and zircon; microaggregates flow around clastic grains, forming a “turbulence”, which gives the microstructure the form of a frozen turbulent flow. Cellular-domain microstructure is formed of large consolidated microaggregates – domains; they are composed of montmorillonite and hydromica flakes and calcite crystals. Unoriented mi-croaggregates and domains form domain-spongy microstructure. Microaggregates consist of hydromica, chlorite and montmorillonite plates; they form a homogeneous solid small-cellular structural grid, resembling a sponge. Domains are large column-like microaggregates, composed of axially oriented kaolinite flakes. We discovered the connection between rock composition and its microstructure: the micro-structure of scaly aggregates in intergranular space is characteristic for terrigenous rocks, spongy, laminar and cellular-domain – for carbonate rocks, matrix, turbulent and domain-spongy – for clay rocks. We also diagnosed individual structural elements: microaggregates of kaolinite, coccoliths and their fragments, threads of blue-green algae, micropores, microcracks, individual crystals of calcite, pyrite, clinoptilolite.