Courier Forseh.-Inst. Senekenberg, 118: 241-251,4 Figs. Frankfurt a. M., 30. 4. 1990HGFEDCBA Thermal Maturation and Burial History from Conodont Colour Alteration Data, Holy Cross Mountains, Poland ZDZISLAW BELKA* Abstract The thermal and bu rial history of the Holy Cross Mountains is interpreted based on colour alteration indices (CAl) for conodonts from strata of Ordovician to Triassic age, Different paleoheat flow conditions are recognized at the end of the Variscan diastrophic cyc1e and during Late Cretaceous, The Paleozoic strata attained the present-day maturation levels before the uplifting in the Varis- can diastrophic cycle. The paleotemperatures estimated from conodont CAl values are tOGhigh in the Paleozoic strata and tOGlow in the Triassic to be accounted for by bu rial beneath the known stratigraphical overburden under contemporary observed heat flow regime (40-60 mWm -2). The paradox is interpreted as resulting from high heat flow associated with Variscan orogeny, with geot- hermal gradients of 60-70 o C/km, and much lower heat flow at the end of the Mesozoic, with geothermal gradients of about 10- 15°C/km. The maturation levels of the Paleozoic sequence reveal only a narrow zone south of the Holy Cross dislocation as poten- tial for further hydrocarbon exploration, while the Mesozoic strata around the Paleozoic core he above the liquid window but they may yield hydrocarbons received through migration. Introduction The Holy Cross Mountains oecupy relatively smali area in the central part of Poland (Fig. 1) where the Varisean substrate emerges from under the Mesozoic and Cainozoie eover. Because of location close to the East European Platform and far from the exposures of the Varisean Belt of Western Europe the area attracted special interest of ma- ny workers since the last century . The occurrence of many easily accessible outcrops offering usually well preserved and abundant macrofauna was also of great importance. As a result, several hundred papers concerning different aspects of paleontology and geology of the Holy Cross Mts have been published. The organie maturation studies, however, are lacking. The purpose of this study is to determine the thermal maturation and burial history of the Holy Cross Mts using the colour alteration of conodonts which are known to occur in many geological formations of this area. Conodonts are phosphatic mierofossils widely distributed in marine roeks of Cambrian through Triassic age. They are valuable stratigraphie tools particularly in those shelf areas, where earbonates predominated. These toothlike ele- ments evolved so rapidly that subdivisions and eorrelations of rock sueeessiorrs between different areas are possible at a degree of refinement greater than in those based on other fossil group. Conodonts constitute also a very impor- tant tool for paleotemperature analysis and hydrocarbon exploration. The method eomprehensivefy deseribed by Er- STElNet al. (1977) used the eolour of eonodont elements as a semiquantitative index CAl for assessing thermal matu- ration, The eolour of eonodonts is produeed by trace amounts of organie matter that oecur within the elements as thin laminae between thieker apatite layers. In their pioneering work, based on laboratory studies and field data from the Appalachian Basin, EpSTEINet al. (1977) showed that eolour alteration in conodonts is time and ternperaturedepen- dent. The sequential change in colour retleets the progressive and irreversible alteration of organie matter within the eonodont elements. The CAl values l-S have been experimentally ealibrated with temperature ranges and eorrelated with other indiees of organie metarnorphism. Thus, CAl l (pale yellow) eorresponds to 50-80°C, CAl 1.5 to 50- 90°C, CAl 2 (brown) to 60-140 o C, CAl 3 (dark brown) to I IO-200oC, CAl 4 (black) to 190-300 o C and CAl 5 to 300-350°C. The geologieal temperature ranges for CAl 6 to 8 have been recently established by REJEBIANet al. (1987). The highest CAl value of 8 corresponds to temperatures above 600°C, at whieh conodont elements begin slowly to disintegrate. *) Inxtitut und Muscum rur Geologie und Puluontologic , Univcrsitat Tubingcn, Sigwartstr. 10,0-7400 Tubingcn. 241