Lom coal basin (NW Bulgaria) – preliminary data on palaeoecology and sedimentology

Lom coal basin is located in NW Bulgaria in the west subsided part of the Lom depression. Lom basin is filled in with thick sedimentary succession of Neogene age, comprising alluvial sand, silt and clay, interbedded with coal seams, and covered by loess deposits of Quaternary age. Neogene deposits have been divided into two formal lithostratographic units – Archar Formation (pure sand of alluvial origin, Pontian age) and Brusartsi Formation (clay, silt and sand with coal beams in the lower part, Dacian-Early Romanian age). The previous studies of Lom basin sediments were focussed on coal characteristics and no detailed sedimentological studies were published. The diatom presence in clays from Brusartsi Fm is established for the first time during our study. The present study aims to characterize sediments of Brusartsi Formation and palaeoecology in the Lom basin based on data from sedimentological studies(include grain-size analysis, petrographic study of thin-sections under the polarizing microscope), and mineralogical studies of the fine sand fraction in immersion eugenol, and X-ray diffraction analysis, and diatom studies. Taxonomic structure studies were made using scanning electron microscope (SEM) Philips 515 at Freie Universität Berlin. Thus preliminary studies of sediments from 5 boreholes drilled during 2007-2008 and kindly provided by ENEMONA, were carried out.
   Neogene sedimentary succession in the boreholes studied is represented by deposits of Brusartsi and Archar Formations. The full thickness of Brusartsi Fm. has been drilled and only the upper parts of the Archar Fm. Sediments drilled from Archar Formation are thick up to 10 m - light gray and almost white sand, very well sorted (mainly quartz and less feldspars, some epidote, garnet and sphene; rare ore minerals are present). Sediments from Brusartsi Formation are varying from 70 to 105 m in thickness. They are represented by thick 2-5 up to 22 m clays and silty clays, gray and gray-greenish in color, and with massive or laminated structure. Some of clays contain significant amount of diatoms up to 50% of the rock volume and in some samples 1 mm thick layers are extremely composed of diatoms. Main rock forming minerals are chlorite (clinochlore), smectite (montmorilonite), illite, quartz and feldspar according to data from X-Ray diffraction analysis. Clay particles are oriented parallel to sedimentation surface and their composition evidenced for chiefly detrite origin. Sand is medium to very well sorted, medium to coarse grained, light grey in colour. These sands show lower mineralogical and structural maturity compared to those from Archar Fm. Well
sorted sand from Brusrtsi Formation is composed of relatively equal amounts of quarts, plagioclase and potassium feldspar, and very few grains of garnet, amphibole, sphene and epidote. Pyrite represents opaque minerals.
  The diatom flora is freshwater one. The planktonic representatives of genus Aulacoseira Thw. have the highest abundance – and they compose the rockforming complex. On some levels there are periphytic (epiphytic) forms, belonged to genera Fragilaria Lyngbye sensu lato, Tetracyclus Ralfs, Navicula Bory sensu lato, Cymbella Ag. sensu lato, Eunotia Ehr., Amphora Ehr. The most abundant species is Pinnularia nobilis var. neogena (Grun.) Cl. It can be considered as biostratigraphic marker for Late Miocene-Pliocene age. Based on our SEM investigation on the frustules of P. nobilis var. neogena high stages of dissolution of the frustules were determined. Dissolution occurred progressively and centripetally, and the final stage was the corroded silica matrix of the central area.
   The grain-size composition and mineralogy of clay minerals led to the conclusion that sedimentation occur in a relatively shallow broad basin with low hydrodynamics. Results from diatom analysis confirm this conclusion and indicate that the basin was eutrophic freshwater lake. The temperature regime was similar to the lakes of the moderate latitudes.