The spatial and temporal distribution of diagenetic alterations has been constrained in relationship to depositional facies and sequence stratigraphy of the Upper Ordovician glaciogenic quartzarenite sandstones in the Murzuq Basin, SW Libya, which were deposited during the Haritanian glaciation when the basin was laying along the continental margin of Gondwana. Eogenetic alterations encountered include: (i) replacement of detrital silicates, mud matrix and pseudomatrix by kaolinite in paraglacial, tide-dominated deltaic, in foreshore to shoreface (highstand systems tract; HST) and in post-glacial, Gilbert-type deltaic (lowstand systems tract; LST) sandstones, particularly below the sequence boundaries (SB). Kaolinite formation is attributed to the influx of meteoric water during relative sea level fall and basinward shift of the shoreline. (ii) Cementation by calcite (δ18OVPDB = − 3.1‰ to + 1.1‰ and δ13CVPDB = + 1.7‰ to + 3.5‰) and Mg-rich siderite in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, in the glacial, tide-dominated estuarine (transgressive systems tract; TST) sandstones and in the post-glacial, shoreface TST sandstones is interpreted to have occurred from marine pore-waters. (iii) Cementation by Mg-poor siderite, which occurs in the post-glacial, Gilbert-type deltaic LST sandstones and in the paraglacial, tide-dominated deltaic and foreshore to shoreface HST sandstones, is interpreted to have occurred from meteoric waters during relative sea level fall and basinward shift of the shoreline. (iv) Pervasive cementation by iron oxides has occurred in the glacial, shoreface–offshore TST sandstones and post-glacial, shoreface TST sandstones immediately below the maximum flooding surfaces (MFS), which was presumably enhanced by prolonged residence time of the sediments under oxic diagenetic conditions at the seafloor. (v) Formation of grain-coating infiltrated clays mainly in the glacial, fluvial incised-valley LST sandstones and in the post-glacial, Gilbert-type deltaic LST sandstones as well as, less commonly, in the paraglacial, foreshore to shoreface HST sandstones and in the tide-dominated deltaic HST sandstones below the SBs.
Mesogenetic alterations include mainly the formation of abundant quartz overgrowths in the glacial, fluvial incised-valley LST sandstones, post-glacial, Gilbert-type deltaic LST sandstones and glacial, shoreface TST sandstones, in which early carbonate cements are lacking. Illite, chlorite and albitized feldspars, which occur in small amounts, are most common in the glacial, tide-dominated estuarine TST sandstones and paraglacial, shoreface HST sandstones. This study demonstrates that the spatial and temporal distribution of diagenetic alterations and their impact on reservoir-quality evolution in glacial, paraglacial and post-glacial sandstones can be better elucidated when linked to the depositional facies and sequence stratigraphic framework. 相似文献
This paper addresses research achievements during roughly the period 1991–1993 pertaining to the inner magnetosphere including the radiation belts, plasmasphere, and ring current region. It also addresses issues concerning the magnetic fields, electrical currents, and particle precipitation properties that relate to this region of the solar-terrestrial system. Recent analysis and modeling of magnetospheric substorms and geomagnetic storms affecting the inner edge of the plasma sheet are discussed and the regions from the geostationary orbit inward toward the Earth are examined in light of new observational and theoretical tools.Presented at the 7th Scientific Assembly International Association of Geomagnetism and Aeronomy Buenos Aires, Argentina 8–20 August 1993. 相似文献
Endeavour 42 is a structurally controlled Au deposit with similarities to adularia‐sericite deposits. It is the largest of four gold prospects discovered in the Late Ordovician Lake Cowal volcanic complex, adjacent to the Gilmore Fault Zone, in central New South Wales, Australia. The Lake Cowal volcanic complex consists of calc‐alkaline to shoshonitic volcanic rocks and related sedimentary rocks that were deposited in a relatively deep‐water environment. The volcanic and sedimentary rocks of the Lake Cowal volcanic complex were intruded by diorite and granodiorite. Low‐grade porphyry Cu (0.2–0.35% Cu) mineralisation is developed in parts of the granodiorite intrusion. The gold deposits are developed north of the porphyry Cu mineralisation and occur within a north‐south corridor adjacent to a north‐south‐oriented body of diorite. The Endeavour 42 deposit is hosted by three volcanic units and a diorite. The stratigraphic units at Endeavour 42, consistently strike 215° and dip 50°NW, and comprise an upper unit dominated by redeposited pyroclastic debris and a lower conglomerate unit with clasts of reworked volcanic rocks. Separating these units is a sequence of trachyandesite lava and hyaloclastite breccias. Laminated mudstone and siltstone throughout the sequence are indicative of a relatively deep‐water, below wave‐base, environment. Porphyritic dykes, which are typically associated with zones of faulting, cross‐cut both the volcano‐sedimentary sequence and the diorite. The major fault orientations are 290° and 340°, forming subparallel conjugate fault sets. Both sets of faults are mineralised, contain deformed porphyritic dykes and are associated with sericitic alteration. Endeavour 42 is a sulfide‐poor gold deposit with free native Au and Au associated with pyrite and sphalerite. Minor galena, pyrrhotite and chalcopyrite are also observed. Irregular pyrite veinlets and carbonate‐sulfide veinlets occur in the upper unit of re‐deposited pyroclastic debris. Auriferous veins are parallel‐sided dilatant veins with quartz‐sulfide‐carbonate‐adularia. These veins display a consistent strike of 305° and a dip of 35°SW. Alteration and mineralisation were influenced by host‐rock composition and rheology. A pervasive alteration assemblage of chlorite‐carbonate‐hematite‐epidote is developed throughout the Lake Cowal volcanic complex. This is overprinted by sericite‐silica‐carbonate alteration around fault zones and dykes, with patchy and pervasive alteration of this type developed in the lava sequence and upper volcani‐clastic unit, reflecting permeability and probable alteration zoning. In the lower clastic unit, the diorite and, in parts of the lava sequence, a chlorite‐carbonate‐pyrite assemblage partially overprints sericite‐silica alteration, suggesting an evolving fluid composition, changing physico‐chemical conditions or a different alteration fluid. Age dating of the intrusive phases and sericitic alteration associated with mineralisation at Endeavour 42 yields ages of 465.76 ± 1 and 438.6 ± 0.5 Ma, respectively, suggesting that mineralisation post‐dates the Lake Cowal intrusive event and is related to intrusion of magma during the 440 Ma mineralising event, an important period in the eastern Lachlan Fold Belt. 相似文献
A systematic petrographic and geochemical studies of 92 representative sandstone samples from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1 and E-D3 in the southern part of the Bredasdorp Basin was undertaken to classify the sandstones as well as unravel the main diagenetic processes and their time relations. Petrographic study shows that the sandstones are largely subarkosic arenite and arkosic litharenite, which have underwent series of diagenetic processes as a result burial, rifting and subsequent uplift. The main diagenetic processes that have affected the reservoir properties of the sandstones are cementation by authigenic clay, carbonate and silica, growth of authigenic glauconite, dissolution of minerals and load compaction. The major diagenetic processes reducing the porosity are calcite cementation in the subarkosic arenite, and compaction and quartz cementation in arkosic litharenite. On the other hand, the formation of secondary porosity due to the partial to complete dissolution of early calcite cement, feldspars and minor grain fracturing has improved the reservoir property of the sandstone to some extent. The clay minerals in the sandstones commonly acts as pore choking cement, which reduces porosity. In general, there is no particular diagenetic process that exclusively controls the type or form of porosity evolution in the sandstones. 相似文献