Paleo-karst controls reservoirs in the fourth member of the Sinian Dengying formation in the Sichuan Basin. It is crucial to characterize the paleo-karst zone architecture to understand better the ...distribution of reservoirs controlled by the multi-stage ancient water tables. We divide vertical paleo-karst zones using cores and formation micro-scanner images based on the groundwater movement and development of pores and fractures. Then we analyze positions and variations of paleo-karst water tables according to the characteristics of different paleo-karst zones. Besides, we illustrate the distribution of underflow subzones and the development of reservoirs in each subzone. This paper reveals the relationship between paleo-karst landform relative elevation, paleo-karst zone thickness and reservoir thickness. Thus, we build the paleo-karst evolutionary pattern in the study area. Research suggests that vadose, underflow, and slow flow zones develop in the intended interval. We identify four underflow subzones in response to four paleo-karst water tables. Compared with the vadose zone, the underflow zone is thicker and holds more reservoirs, mainly distributed in the upper part of the single underflow subzone. Higher paleo-karst landform can possess a thicker vadose zone and thinner underflow zone. Besides, an excellent positive linear correlation exists between reservoir thickness and paleo-karst zone thickness. The thickness of the subzone constrains the development of reservoirs in it. The second and third underflow subzones are the most favorable intervals, owning more reservoirs.
Mississippi Valley-type (MVT) Pb-Zn deposits serve as the world’s major supply of Pb-Zn resources. However, the age constraint of MVT Pb-Zn deposits has long been a big challenge, due to the lack of ...minerals that are unequivocally related to ore deposition and that can be used for radioisotopic dating. Here we show sporopollens can provide useful chronological information on the Changdong MVT Pb-Zn deposit in the Simao basin, Sanjiang belt, West China. The Pb-Zn ores in the Changdong deposit are hosted by internal sediments in paleo-karst caves of meteoric origin. Sphalerite and galena occur as replacements of carbonate minerals and void infillings in the internal sediments. The relations suggest that the Pb-Zn mineralization occurred after the deposition of the internal sediments. A palynological assemblage mainly composed of angiosperm pollen dominated by
Castanea, Quercus
, and
Carya
and fern spores dominated by Polypodiaceae,
Pteris
, and Athyriaceae was identified. These pollen and spores place the ore-hosting internal sediments and the Changdong paleo-karst at early to middle Oligocene. Consequently, the Changdong Pb-Zn deposit must have formed after the early Oligocene (∼34 Ma). These age constraints, together with the geological characteristics, indicate that the Changdong Pb-Zn deposit is a paleo-karst-controlled MVT deposit related to fold-thrust systems in the Sanjiang belt. The Changdong deposit is similar to other MVT Pb-Zn deposits in the northern part of the Sanjiang belt, making it possible to extend this Pb-Zn belt 500 km further to the South. Results presented here highlights the potential of sporopollens in dating the age of MVT deposits related to paleo-karst formation in young orogenic belts.
The Kisanga iron caps in Southern Shaba (Zaïre), situated in the Kakontwe Limestone, result from the filling of an Upper Proterozoïc paléokarst. These ancient karstic traces have been related to a ...local and temporary emergence in the Likasi district caused by the Kipushi rift structuration during the early Lower Kundelungu.
Les gisements de fer de Kisanga (Shaba méridional, Zaïre), sont situés dans le Calcaire de Kakontwe (Kundelungu inférieur) et correspondent au remplissage d'un paléokarst du Protérozoïque supérieur. Ces anciennes traces karstiques ont été rapportées à une émersion locale et temporaire de la région de Likasi causée par la structuration du rift de Kipushi au début du Kundelungu inférieur.
Les minéralisations Pb-Zn de type Mississippi Valley du district de Touissit-Bou Beker (Maroc Nord oriental) se présentent sous forme de remplissage d'un réseau interconnecté de paléo-karsts ...hydrothermaux et de brèches d'effondrement. Les études géologiques montrent que les dolomies encaissantes ont subi les effets de trois phases de dissolution, dont une phase majeure reliée à la circulation des fluides minéralisateurs. Les structures de dissolution qui en résultent se distinguent les unes des autres aussi bien par leur distribution spatiale que par la nature de leurs produits de remplissage.
Les paléo-dolines relevant de la première phase de dissolution présentent des shales de teinte vert noirâtre interprétés comme des paléosols. En revanche, les structures issues de la phase de dissolution majeure montrent des produits de remplissage constitués de dolomite en selle, de sulfures et de pyrobitumes, reliés à la circulation des hydrothermaux (saumures bassinales) libérés au cours de l 'enfouissement profond. Enfin, la troisième phase de dissolution a conduit à l 'enrichissement du minerai primaire et au développement des karsts verticaux modernes (mérokarsts) et aux zones de collapse.
Mississippi Valley-type lead-zinc orebodies at Touissit-Bou Beker district of northeastern Morocco are hosted by paleokarst features in a Middle Jurassic (Aalenobajocian) carbonate platform. The ore occurs mainly as open-space fillings of karsts and collapse breccias. Geological investigations coupled to core sample examinations show that the carbonate platform were subjected to three main stages of dissolution and brecciation ; one of which (stage II) is related to the Pb-Zn mineralization. The resulting dissolution structures are distinguished from each other on the basis of their spatial distribution and the nature of their infilling materials.
The spatial distribution of sinkholes, whose formation are related to stage I, are interpreted to have been formed by the downward circulation of meteoric waters during the exposure of the carbonate platform at the end of the Aaleno-Bajocian time. The created voids are commonly filled with green to black shale interpreted as paleo soils. In contrast, the stage II structures, commonly fdled with saddle dolomite, sulphides, and pyrobitumen, form a paleokarst solution network. The observations suggest that these structures are linked to the ore-forming hydrothermal fluids that circulated during deep burial. Later recent kartification events have produced the vertical modern continental karsts (merokarsts) and collapse-zones that enriched the primary ore.
The karstification started at the beginning of the Tertiary and the process is still going on today . During the Eocene, modifications brought about under the tropical climate resulted in ...siderolithic deposits (siliceous sands , ferruginous soils) which can be found in some fossile karsts. In the Oligocene, active tectonics modified the eocene surface. Erosion during the Miocene levelled the Jura mountains into a peneplain even though the climate remained tropical. Towards the end of the Miocene, present-day structure and landforms were produced when the main folding, subjected at the same time to powerful erosion, occurred. During the Upper Pleistocene the climate became cooler and wetter. At least two glacial periods have been recognized in the Pleistocene. Present-day karst landforms and most of the caves can be considered as being shaped during the Plio-Quaternary. The karst fillings of the Quaternary provide evidence of the extension of the Wiirm and Riss (glaciers)
La karstification jurassienne débute dès la fin du Crétacé et le début du Tertiaire et se poursuit jusqu'à nos jours. A l'Eocène, les reliefs sont modestes et l'altération sous climat tropical engendre des dépôts sidérolithiques (sables siliceux, altérites ferrugineuses) que l'on retrouve dans des karsts fossiles (ex : Salève). A l'Oligocène, une tectonique active reprend la surface d'aplanissement chimique éocène. Au Miocène, l'érosion et l'altération aplanissent à nouveau le Jura en une pénéplaine alors que le climat demeure tropical. La tectonique fini-miocène est responsable des structures et reliefs actuels, et s'accompagne d'une puissante érosion. Au Plio-Quaternaire se mettent en place le modelé karstique actuel et la quasi totalité des cavités. Les remplissages karstiques quaternaires renseignent sur l'extension des glaciers du Wurm et du Riss.
