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GEOLOGY OF THE NÓGRÁD—CSERHÁT AREA by G éza Hámor FOREWORD The N Hungarian Miocene is a member of particular importance and beauty of the Later Tertiary in the Carpathian Basin and the adjacent regions. Palaeogeographically, it is a link connecting the W and E European facies areas including the Vienna Basin, the W Carpathians, the Transylvanian Basin and the Gethic Depression in the Central Paratethys. The most beautiful, diversified and intriguing element of this link is the Nógrád—Cserhát area. It has been promoted to the rank of a classic type area as a result of nearly two centuries of busy efforts by outstanding geologist representatives of five generations. It is here that the most complete Miocene sequence in this country is found; that a school of thought of generations of geologists, paleontologists, stratigraphers, volcanologists and miners engaged in the study of Tertiary formations has evolved; that has been the subject and scene of many decades of debate on Miocene stratigraphy now entitled to be regarded as classic. This area has never been subjected to fashionable research trends, to effortless stylistic exercises, as the magnitude of the geological past, breathtaking and majestic, has obliged everybody studying this area to full devotion and to painstaking efforts for achieving even an infinitesimal result. Nevertheless—and maybe for this very reason—research institutes, mines, university departments and laboratories have been sending their growing offspring to repeated assaults which has cost for many almost a lifetime (several decades in most cases) of devotion to this noble struggle. I have joined this campaign as a volunteer. The facilities I have been provided with, including a profound preliminary training, a “heavy artillery” of high-standard equipment, fellow-fighters devoted to their profession and fanatically attached to their task (both veteran flag-bearers and recruits) and the calming and safe hinterland, my family, are owed to the past 30 years of peace. To continue the metaphor: the geological record of the study area has withstood six assaults, many tracts of its ramparts having remained unbroken. Summarizing the results of my work, I hope to have succeeded in reconnoitering some points heretofore unknown and in creating, by means of enveloping-marches (taking possession of an extended neighbourhood), a more advantageous strategic position for the solution of forthcoming tasks. It is in this spirit that I dedicate my work to all who have helped me. The Author 28 217 INTRODUCTION After ten years of work the large-scale mapping and mineral prospecting program launched by the Hungarian Geological Institute in 1958 reached the stage of the first syntheses and of the formulation of further objectives. In order to ensure the continuity of publication of the monographs on the Mátra and Tokaj Mountains now in the stage of completion and to concentrate efforts for the solution of N Hungarian research tasks being formulated in an increasingly distinct form, academician J. F ülöp then director of the Institute, entrusted me, in 1967, with organizing a N Hungarian Department within the Division of Geological Mapping. The Department had as its task to finish work in the Mátra and Tokaj Mountains, to carry out the geological mapping of the Nógrád—Cserhát area on a scale of 1:25 000 and to carry out the close geological survey of the Börzsöny ore exploration area. This work I am going to presents the results of the geological study of the Nógrád—Cserhát area only. The economic geological importance of the area and its extended neighbourhood was pointed out as early as 100 years ago by F. H a u e r (1868, p. 77) when the first geological field-surveys carried out by the Geologische Reichsanstalt of Vienna were being finished: “The area occupied by the later Tertiary formation is larger than that covered by the earlier one. The study of that formation just commenced will add to our knowledge and widen our outlook achieved during our studies in the Vienna Basin, one of the embayments of the Tertiary sea of what is now Hungary. In fact, this study will be of practical value since the later Tertiary beds include a great deal of browncoal and lignite the exploration and exploitation of which will certainly go on increasing, as industry grows and forest holdings dwindle... Already during the preparation of outline maps, it was attempted to divide thrachytoids into several rock groups and to find out the conditions responsible for their ore mineralization, for it is to one of these groupings, the zöldegy(greenish)-trachytes mainly, that the wealth of noble metal ores in the Carpathian realm is owed...” In the light of experience gained since the formulation quoted above which is still pertinent today, the motives for studying the area in question have been summarized as follows: 1. In spite of some excellently studied details, the degree of understanding of the study area is low. Since the general small-scale surveys by the Viennese staff (1858-1866) and the detailed mapping carried out by J. N o s z k y Sr between 1906 and 1935 and the subsequent publication of the maps scaled to 1:75 000, no regular serial geological mapping has been done. 2. It is in the study area that the most complete Oligocène—Miocene sequence of Hungary is developed. Accordingly, it likely to provide clues to a lot of basic problems linked with the Tertiary stratigraphy of this country. To prove this, let us quote that 32%, of the 83 Miocene stratigraphic units listed in the second edition of the volume “Hongrie” of Lexique Stratigraphique International have been described from this area! 3. The study area lies above a NE—SW oriented Oligocène—Miocene depression. Judging by the oil- and natural gas shows found on the limbs at (Fedémes and Orszentmiklós), the more than 3000 m thick sedimentary sequences, the analogy of the oil traces encountered in the northern foothills of the Mátra Mts and the hydrocar­ bon deposits of Eger, the depression is promising for oil- and gas prospecting. The lateral and vertical changes in facies and the generation and reservoir-trapping characteristics of the hydrocarbons are known to be inter­ related. Therefore the prospective area as well as the Darnó Lineament forming its southern boundary and the relatively elevated Balassagyarmat—Szécsény zone bounding it in the north must be studied geologically and structurally in the reconnaissance stage of exploration. 4. Situated in the western front of the Mátra Mts and the southern one of the Cserhát, the Zagyva Graben area is prospective for nonmentallic mineral deposits in the long run. The particular palaeogeographic setting and geohistory has brought about a differential alteration of a great variety of volcanic rocks, their selective genesis and accumulation (bentonites, trasses, various kinds of raw materials for construction). 5. Regardless of the restrictions imposed on browncoal mining between 1966 and 1972, the delimitation of the prospective areas had to be completed. With a view to the available literature data and to the analogy with 28* 219 the deposits in the Mátra—Bükk foreland, preparations had to be made for clearing the prospects of lignite exploration in the S Cserhát. 6. Preparations for métallogénie studies to be performed in the Börzsöny Mts required the exploration of the sedimentary basement and the extended neighbourhood. 7. Last but not least, the time was ripe to collect and reinterpret all the available information prior to the withdrawal of mining. Remarkably enough, J. N o sz k y S r , as a response to the economic recession of that time, referred to the study area, already in 1912 (p. 67), as follows: "... it is to be feared that, once having lost its economic importance, it may scientifically relapse into the state of "terrae incognitae" in spite of the fact that every possibility existed (and still exists) for exploiting the acquired information to the benefit of the geological sciences.” Accordingly, the investigation of the Nógrád—Cserhát area has had the following aims: — to improve the ideas on the geological and structural setting of the study area by discovering new information, relationships and regularities and by synthesizing them; — to enhance the availability of mineral raw materials to the nation by forecasts backed by up-to-date and complex research methods; — to direct attention to the prospects for exploring local mineral deposits, with particular emphasis on the situation caused, at that time, by a decline in browncoal mining and on the problems of long-term development of this particular region of the country as accentuated by their ever growing significance. Although seemingly satisfactory at the outset, the geographical frame involved has been burst and pushed apart by the objectives just motivated. It cost me painstaking work to get to recognize that which J. N o s z k y S r (1912a, p. 90) had discovered half a century before: “Generalizations usually change, when a larger area is taken into consideration. This is how earlier generalizations have become partial essays valid to only a very restricted portion of their original scope.” To avoid this trap, I have extended the detailed studies to the Börzsöny area and eventually, by changing both scale and method, to the western and then the eastern parts of N Hungary as well. Thus it will be no self-justification, but rather a means concomitant of any approach to natural sciences, if—following the teaching of my great master academician E. V a d á s z —I present a detail, the title-giving area, fitted into a “larger entity". 220 GEOGRAPHIC SITUATION The Nógrád—Cserhát area lies in the middle part of Hungary’s northern border (Fig.

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