VOLUME 46 (2)/November 2017 125th Anniversary of Acad. Strashimir Dimitrov; 70th Anniversary of the Geological Institute, Bulgarian Academy of Sciences

Crushed-rock aggregates from 11 railway ballast-producing quarries in Bulgaria have been investigated. The test materials consist of igneous and sedimentary rocks of different ages: basaltic andesite, trachyte, diabase, andesitic tuff, quartz-cemented sandstone, dolomite and five varieties of limestone. The standard laboratory tests (Los Angeles, micro-Deval and point load) and in-situ test (Schmidt hammer) were carried out for determination of the following mechanical properties of aggregates: resistance to fragmentation; wear resistance; strength; and rock hardness. Results show that Los Angeles coefficient (LA) values range from 11.9% to 28.4%. The micro-Deval coefficient (MDE) varies between 3.7% and 22.4%. The point load strength index (IS(50)) is between 4.0 MPa and 8.8 MPa. The Schmidt hammer rebound value (SHV) ranges from 34.4 to 60.2. The possibility of predicting the Los Angeles and micro-Deval coefficients from the Schmidt hammer rebound value and the point load strength index was studied. Regression analysis shows a strong correlation between Los Angeles coefficient and point load strength index (coefficient of determination R2=0.93), a good correlation between the Los Angeles coefficient and the Schmidt hammer rebound value (R2=0.62) and moderate correlation between the micro-Deval coefficient and the Schmidt hammer rebound value (R2=0.51).

Modelling of radionuclide transport in the geosphere is a key issue of the evaluation of safety of low- and intermediate-level radioactive waste (LILW) repositories. In Bulgaria, due to the operation of Kozloduy Nuclear Power Plant (NPP), a building of such facility is ongoing. The migration of ⁹⁰Sr was evaluated by simulation of flow and chemical modelling, performed with coupled numerical code HP1. The adsorption properties of the clays were implemented in the code by surface complexation reactions between strontium and iron oxides. The results were compared with another model performed with computer code HYDRUS 1-D, in which the adsorption properties of the medium were evaluated with the relevant for strontium distribution coefficient, K_d.

This brief overview presents the study of metasomatic processes and rocks in Bulgaria. The very beginning of metasomatic studies coincides with the first steps of Bulgarian geology, with the discovery and description of minerals that are basic for altered rocks. Subsequently, the processes of spilitization and uralitization, the formation of fluorite-silicated rocks during intensive metasomatic alteration of marbles, were characterized. Around and after 1950, the increase in mapping and geological and mining activities created prerequisites for rapid development of the study of metasomatic processes, and especially of wall-rock alterations as their particular cases. Many new data filled in the pages of geological reports and the first dissertations. This was also the time of characterization of new metasomatic processes and metasomatic products for Bulgaria: potassium feldspatization and granitization, propylites and secondary quartzites, etc.
   Nowadays, the following metasomatic families have been established in Bulgaria: skarn, quartz-feldspar metasomatic rocks, greisen, beresite, secondary (hydrothermal) quartzite, propylite, argillisite, aceite, quartz-sericite and quartz-adularia-sericite. The products of potassium feldspatization and granitization, as well as rodingites, can also be included.
   The Geological Institute “Strashimir Dimitrov” (Bulgarian Academy of Sciences) is the cradle of the metasomatic doctrine in Bulgaria. Taking the best of world science, the metasomatic direction in our country has created its own image. Collaboration with prominent specialists from leading countries have helped Bulgarian metasomatists grow and show their knowledge at a number of geological forums in Bulgaria and abroad, as well as in theoretical and practical aspects taking part in exploration works in other countries. Proof of growth are the monographs issued; the large number of scientific and popular articles; the defended 17 PhD and four DSc theses; the new, found in Bulgaria, minerals, rocks, deposits and occurrences of ore and non-metallic raw materials with different applications.
   This overview examines in a detailed way the different directions in researches on metasomatic processes in Bulgaria: geochemical, mineralogical-petrological, structural-tectonic, theoretical and experimental modelling. The most important scientific and applied contributions are noted, and the problems in the study of metasomatic processes are briefly discussed.
   Research on metasomatic rocks (especially wall-rock alterations) and results are widely used in the discovery of a number of ore and raw material deposits, as well as in the proper direction of geological exploration and mining in promising areas.

This review treats the concepts concerning the age of granitoid magmatism in southern Bulgaria and considers the age assessments of the large granite intrusions in the metamorphic complexes known as “South Bulgarian granites”. In terms of the available contributions, this analysis encloses a long period, from 1836, the time of Ami Boué, the first geologist who visited Bulgarian land, to 2000. The review discusses both ideas and geochronological data on a large time-span, from the Archean to the “Tertiary”. The emphasis is on the progress of Bulgarian researchers’ views that had been made prior to the first pioneering attempts at radiological determinations (i.e., using He and Pb quantitative methods), and especially after the advent of modern equipment for radioisotope dating, based on various isotope systems (e.g., K-Ar, U-Th-Pb, Rb-Sr) and mathematical models of lead isotopes (Pb-Pb) following the basic model of “plumbotectonics” and the fission track method.
   The bulk of radioisotope data (more than 300 dates on feldspar and 500 dates on galena and other ore minerals) have been made by using the Pb-Pb method, which was introduced by Blagoy Amov and improved by the same worker via his “dynamic model of a continuous lead-isotope evolution” in the Earth’s crust. From 1969, when pegmatites that are genetically linked to granite intrusions of southern Bulgaria were dated as “Tertiary”, until 2000, all of the above-mentioned methods were tested. These methods confirmed the “Tertiary” ages of the granitoids of the Rhodope Massif and their difference in age from the Hercynian granitoids of the Srednogorie Zone. Owing to this, two groups of granitoids of dissimilar ages, previously referred to as “South Bulgarian granites”, were determined. This distinction was corroborated by varieties of mineralogical, petrological and geochemical studies, which were carried out by collaborators from the Department of Geochemistry of the Geological Institute. The “magical boundary”, defined by Acad. Strashimir Dimitrov and followed by the majority of Bulgarian geologists, that “the South Bulgarian granites are pre-Mesozoic in age, since fragments of them are present in the Permian–Triassic conglomerates of the Lozen Mountain” has been overcome, mainly due to the results of geochemical studies. Also, the absence of Archean and Proterozoic metamorphism in the Rhodope Massif has been revealed by U-Th-Pb and Pb/Pb radiogeochronological investigations. The analysis of available mineralogical, geochemical and radiogeochronological data, supported by the ages of the migmatites from Ardino area (63–32 Ma), allowed to assume that both migmatized gneisses and the South Bulgarian granites of the Rila and Rhodope Mountains, as well as the volcanic rocks and associated intrusions and ore mineralizations, were a product of a single and prolonged stage of the Alpine activation of the Rhodope crystalline complex that started during the Cretaceous.

Magnetostratigraphy, calpionellid and calcareous nannofossil biostratigraphy of the upper Tithonian and lower Berriasian of section Barlya in the West Balkan Mts are herein presented. The studied interval comprises the magnetozones from M21r to M17r, with a large interval of partial remagnetization. It has been directly correlated with a continuous succession of the calpionellid Chitinoidella, Praetintinnopsella, Crassicollaria and Calpionella zones, and with the nannofossil zones from NJT 15b to NK-1. The accumulated evidence in the last twenty years on the magnetostratigraphy, calpionellid and calcareous nannofossil biostratigraphy and events across the J/K boundary in the European Tethys has provoked the authors to plot the main micropalaeontological events against magnetostratigraphic column in order to assess the extent of diachronism of these events. Data from Slovakia, Poland, Italy, Austria, Hungary, France, Spain and Bulgaria has shown specific behaviour of different calpionellid and nannofossil events with respect to the column of the magnetic polarity chrons, which have been considered as ‘isochronous’ or at least less diachronous than the microfossil events. Thus, some rather consistent events have appeared, such as the first occurrences (FOs) of calpionellids Calpionella grandalpina, Crassicollaria brevis, Calpionella minuta and Remaniella ferasini, and the FOs of calcareous nannofossils Nannoconus globulus minor, Hexalithus geometricus, Nannoconus wintereri, Nannoconus steinmannii minor and Nannoconus kamptneri minor. The J/K boundary, as widely accepted, has been traced in section Barlya at the base of the Calpionella alpina Subzone.

The archaeological site of Nova Nadezhda is located on a T1 terrace on the right bank of the Maritsa River and on the left bank of its right tributary, the Dermen Dere stream. Comparison of the grain-size and mineral composition of sediments from the site and natural deposits from the Maritsa and the Dermen Dere suggests that the stream recurrently flooded the eastern part of the site in the later prehistory (6^th and 5^th mill. BC) and later shifted its course to the east. Terrace sediments and later incisions of the Maritsa and the Dermen Dere are obvious in 2D resistivity images.

The area around Topola Village is affected by landslide processes, which are the major factor in the formation of the modern relief. The terrains affected by the landslides are mainly composed of sediments of the Topola Formation. In the engineering-geological studies carried out in the region, main attention was paid to the geotechnical parameters of aragonite sediments and calcareous clays of the Topola Formation. The purpose of this article is to define the geotechnical parameters of the micritic limestones from this formation, which have a major stabilizing role on the lands affected by landslide processes. Тhe mean values of bulk density, compressive strength and tensile strength of these rocks were determined. The quadratic deviation, the coefficient of variation, and the calculation value of the individual parameters, with confidence at 90%, 95% and 99%, respectively, were also calculated.

A major issue in groundwater and surface water management and monitoring is transboundary groundwater flow between neighbouring countries. The subject of our study is the assessment of the groundwater flow across the state border between Bulgaria and Greece, which is characterized by complex natural features. We present a study on the factors that control the groundwater flow and formation, such as lithological composition, geological structure, topographic features, surface water network, etc. It was determined that, in most sections along the state border, the probability of transboundary groundwater flow is rather small, due to the position of the surface and groundwater divides. In the areas where the state border follows a ridge with fissured groundwater system, there are no prerequisites for transboundary flow. Groundwater flow is likely to occur only in the marble-dominated karst areas, as well as in the Quaternary or Neogene–Quaternary aquifers in river valleys that host porous groundwater bodies. The probability of transboundary groundwater flow could increase in response to certain human activities.

Gathering and studying a rich collection of fossil corals in Bulgaria led to the definition of various new taxa, but highlighted the gaps in our knowledge of fossil Scleractinia. I needed to learn from living corals. An actuopaleontological project with massive sampling of all phenotypes following coral life in depth in Cuba (1970–1973) showed extraordinary scleractinian variability and led to “heresy” regarding the conventional Caribbean taxonomy. Subsequent fundamental new knowledge challenged Scleractinia taxonomy and justified the results of the investigation of the Cuban corals, through continuing contributions to: holistic approach to scleractinian species; integrative character of coral taxonomy; new paleobiological perspectives and tasks for study; and the evolutionary benefits of hybridization for reef survival. Continuing research can be supported by: an 80% preserved collection; a scientific audiovisual documentary; a monograph in Russian, French, and soon Spanish; and a substantially and rarely preserved coral life and reef ecosystem.

The area of Kamchiya River’s inflow into the Black Sea has an important role not only as a natural landmark, but also as a leading factor for the national economy. It has been subjected for many years to oil and gas exploration, as well as to extraction of deep, highly mineralized waters containing chemical elements of industrial value. There are currently only few of the artesian wells related to these activities in the area. Outflowing water has very high mineralization (over 50 g/l). It is chlorine-sodium type and has extremely high concentrations of some components. They have formed small salt ponds around the wells, whose water and salt balance are controlled by the evaporated and infiltrated in depth water quantities. The abrupt change in the thermodynamic conditions in the porous part of the wells favours the processes of sedimentation and deposition of a substance that partially refreshes the leaking surface water and limits the possibilities for infiltration of highly mineralized waters. Salt infiltration causes enhanced local salinization of the soils and fresh groundwater accumulation in the present-day sediments of the Kamchiya River. An assessment of both the dimensions and extent of salinization in the area of the deep wells is made by applying hydrochemical and geoelectrical survey.

In the present study, we apply a combination of Sr-isotope whole-rock analyses and LA-ICP-MS U-Pb zircon dating to understand the magmatic evolution of the igneous rocks in the Ruen Zone, western Bulgaria, and bracket the time of their formation. This approach reveals a general interval of magmatic activity from 35.57 ± 0.27 Ма (Pishtilski volcanics) to 28.63 ± 0.58 Ма (cross-cutting dykes in the Osogovo Mountain). The volcanic rocks from the Prekolnitsa Graben are formed with significant (50%) crustal contamination/assimilation. Three major sources of crustal rocks can be outlined, with Triassic, Ordovician and Cambrian age, respectively. For the subvolcanic to plutonic magmatic rocks of the Osogovo Mountain, we suggest maturation and zircon crystallization over 2–4 Ma. The magma was generated dominantly in the continental crust and evolved through mainly fractional crystallization (FC) in the upper crustal chamber, whereas the evolution of the Osogovo dykes requires additional involvement of AFC (Assimilation and Fractional Crystallization) and contamination processes.