Geochemistry and Crystal Structure of Recrystallized Dolomites

Shallow marine to peritidal carbonates of the Triassic Csukma Formation in the Mecsek Mts. and Villany Hills of SW Hungary are made up of dolomites, limestones, and dolomitic limestones that show evidence of a complex diagenetic history. Integration of petrographic, conventional stable oxygen and carbon isotope, clumped isotope, and strontium isotope data with the paleogeography, paleoclimate, and burial history of the region revealed four major diagenetic stages in the Mecsek Mts. Stage 1: Reflux dolomitization of the peritidal carbonates during the Middle Triassic. Stage 2: Recrystallization of the reflux dolomites in intermediate burial setting. Stage 3: High-temperature, fault-controlled dolomitization and renewed recrystallization of the reflux dolomite during the Early Cretaceous by seawater drawn down and circulated through rift-related faults in a deep burial setting. Stage 4: Saddle dolomite cementation related to tectonic expulsion of basinal fluids during the Late Cretaceous and Cenozoic. Due to striking differences in the burial history of the Mecsek Mts. and Villany Hills, evidence of diagenetic phases resulting from deep burial and rift-related faulting, such as high-temperature dolomite recrystallization and considerable saddle dolomite cementation, are missing in the Villany Hills. Here dolomitization likely occurred penecontemporaneously via geothermal convection of normal to slightly modified seawater in a near-surface to shallow burial setting followed by partial recrystallization of the dolomites in an intermediate burial setting with low water to rock ratios.Geochemical data and unit cell parameters obtained by Rietveld refinement of conventional and synchrotron X-ray and neutron diffraction data collected on recrystallized dolomites of the Csukma Formation suggest that recrystallization with high water to rock ratio versus recrystallization in low water to rock ratio may be reflected in the unit cell dimensions of recrystallized dolomites. Recrystallization processes with low water to rock ratio result in a noticeable trend between crystal size and unit cell parameters that cannot be explained by Ostwald ripening. In samples recrystallized with high water to rock ratios compositionally indistinguishable overgrowth cement on the matrix dolomite may have a significant effect on the overall unit cell dimensions determined from powder diffraction analyses. This study is the first to apply combined synchrotron X-ray diffraction and neutron diffraction to investigate crystal structural changes in sedimentary dolomites due to recrystallization in various diagenetic environments

A Hosszúhetényi Mészmárga Formáció szerves anyagának vizsgálata

Munkám során a Pécsváradtól északra (Kelet-Mecsek) található, felhagyott kőfejtőből begyűjtött minták vizsgálatát végeztem (1. ábra). A kőfejtő az alsó-jura korú Hosszúhetényi Mészmárga Formációt (HMF) tárja fel. A kőzetet a későbbi tektonikai mozgások felgyűrték és a kőzet anyagát repedések szabdalták fel, mely repedéseket kalcit ásványok töltöttek ki. A repedéskitöltő kalcit erekből begyűjtött minták vizsgálata során bebizonyosodott, hogy az erek anyagában fluidumzárványok formájában olaj csapdázódott (LUKOCZKI, 2009), amely azt bizonyítja, hogy a repedésrendszerben egykor olaj migrációja zajlott (MUNZ, 2001)

Genesis of Upper Triassic peritidal dolomites in the Transdanubian Range, Hungary

In the Late Triassic, a 2–3-km-thick platform carbonate succession formed along the passive margin of the Tethys Ocean. Certain parts of the succession were affected by pervasive dolomitization whereas other parts are only partially dolomitized or non-dolomitized. In the Transdanubian Range, Hungary, the Upper Triassic platform carbonates are extensively distributed and numerous data are available for the space and time relations of the dolomitized and non-dolomitized units. This geological setting provides a unique opportunity for the study of paleogeographical and diagenetic controls of dolomitization of the whole platform complex. This paper presents the characteristic features of the dolomite types of the dolomite-bearing formations and lithofacies types, with a view to interpret the dolomite-forming processes and to determine the main controlling factors of the dolomite genesis. Petrographic features and stable isotope characteristics of the studied successions suggest the predominance of penecontemporaneous and early diagenetic dolomite genesis. Study of the transitional interval between the pervasively dolomitized and the non-dolomitized sequences revealed the general presence of microcrystalline dolomite in the peritidal microbial deposits and the characteristics of partial dolomitization both in the peritidal and subtidal facies. In the peritidal facies, microbially induced Ca–Mg carbonate precipitation is inferred, which was probably complemented by penecontemporaneous mimetic dolomitization of precursor carbonates due to evaporative pumping or seepage influx. Dolomitization of the subtidal facies took place via reflux of slightly evaporated seawater. Dolomitization of the previously deposited carbonate mud commenced during subsequent subaerial exposure but the process of early diagenetic dolomitization may have continued during later exposure events. Recurring subaerial exposure is one of the factors that determine the areal extent of the early dolomitization of the platform carbonates. However, climatic conditions were also crucial. Although the sealevel-controlled, unconformity-bound cyclic facies pattern did not change significantly in the internal platform belt during the nearly 20-My-long time range, a drier climate favored dolomite formation while increasing humidity led to a gradual decreasing intensity of early dolomitization

Sideritic—kaolinitic and green clay layers in the Mecsek Mountains (SW Hungary): Indicators of Middle Triassic volcanism—Myth or reality?

Sideritic—kaolinitic and green clay layers were previously reported from the Mecsek Mountains (SW Hungary) as indicators of Tethyan volcanism in the otherwise germanotype Middle Triassic succession. The aim of the present study is to provide a review and a critical re-evaluation of the previously published data on both the sideritic—kaolinitic layers (the so-called “Mánfa Siderite”) and the green clay layers. New results of mineralogical investigation of the green clay layers are also presented. The Middle Triassic volcanic origin of the “Mánfa Siderite” cannot be confirmed. In addition to a possible volcanic contribution, the sideritic—kaolinitic layers were probably formed in a freshwater swamp under humid, tropical climatic conditions, whereby weathering in an organic-rich, acidic environment led to the formation of “underclays” and siderite in the coal-bearing formations of Late Triassic to Early Jurassic age. These layers were probably tectonically placed over Middle Triassic carbonates. The illitic green clay layers intercalated in the Middle Triassic dolostone may represent terrigenous deposits, and the illite mineralogy probably is the result of burial diagenesis of detrital clays

Dolomitization of shallow-water, mixed silicilastic-carbonate sequences: The Lower Triassic ramp succession of the Transdanubian Range, Hungary

The Lower Triassic succession of the Transdanubian Range in Hungary comprises limestones, dolomite, marl, sandstones and siltstones, deposited in tidal flat, lagoon and ooid shoal environments on the marginal ramp of the western Neotethys. Seven cores were chosen for petrographic and stable isotope investigations aiming to reconstruct the paragenetic sequence with special regard to the dolomitization and hydrothermal events. Five lithotypes were differentiated: (i) dolomite, (ii) sandy, silty, clayey dolomite, and dolomarl, (iii), dolomitic siltstone and sandstone, (iv) dolomitic limestone, and (v) limestone. In these lithotypes, three types of dolomites are present: non-ferroan replacive, ferroan replacive, and ferroan cement. Fabric retentive and fabric destructive non-ferroan replacive dolomitization are interpreted to have occurred by seepage reflux. Supporting evidence includes the presence gypsum and anhydrite in the Lower Triassic beds. Stable isotope values of the ferroan dolomite (δ18O of −10.7 to −4.2‰ and δ13C of −4.8 to 4.7‰) suggest dolomitization by fluids of relatively high temperature. The similar stable isotope values (δ18O of −9.3 to −5.8‰ and δ13C of −1.9 to 2.5‰) of the non-ferroan dolomite phase suggest that the reflux dolomite was overprinted by this second dolomitization event. Traces of exotic minerals, such as barite, chalcopyrite, galena and sulphosalts were found as fillings of vugs and fractures in the dolomite-cemented sandstone. The metals could have been sourced from the underlying Permian red sandstone beds. The heterogeneous sediment composition had profound impact on the diagenesis of these sedimentary successions

Indication of hydrocarbon migration in the Western Mecsek Mountains evidenced by fluid inclusion chemostratigraphy

Primary and secondary hydrocarbon-bearing fluid inclusion (HCFI) assemblages occur in the Middle Triassic Lapis Limestone in the Szuadó Valley of the Western Mecsek Mts. The primary HCFIs were trapped in saddle dolomite crystals, and the secondary HCFIs were enclosed in calcite neospar and fracture-filling calcite. Solid bitumen is also present along fractures. The volatile compounds liberated from fluid inclusions are characterized by non-hydrocarbon and hydrocarbon species. The fluorescent properties of HCFIs, the occurrence of the solid bitumen, as well as the composition of inclusion oils indicate the migration of light oils through the Lapis Limestone. Petrographic observations suggest a prolonged oil charge event, which resulted in HCFIs beeing trapped under evolving diagenetic conditions

Earth MRI Western Kentucky Fluorspar District 3D Geological Modeling

The Illinois–Kentucky Fluorspar District (IKFD) three-dimensional (3D) geological data model plays a critical role in the EarthMRI IKFD project, which aims to utilize the vast and detailed geologic maps and datasets that the Kentucky and Illinois state geological surveys possess to develop a Geographic Information System (GIS)-based 3D geological framework model for the district. The model serves as a framework in which to view the subsurface geological and geochemical data, and to better comprehend the structural and other geologic processes acting on the region as they relate to the locations of critical minerals. The development of the model will also lead to creating new methodologies and discerning best practices for modeling different types of data for viewing in a web-service environment.https://uknowledge.uky.edu/kgs_data/1013/thumbnail.jp