Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.

Petrology of some eclogites and metagabbros of the Oetztal Alps, Tirol, Austria

Abstract: Petrographic, chemical and mineralogical data are presented on the Oetztal eclogites and their co-existing minerals. The available evidence indicates that they constitute the metamorphic derivates of an original gabbroic rock, the plagioclase and clinopyroxene of which reacted to form the garnet, omphacite and kyanite components of the eclogites. According to the available subsolidus experimental data these reactions are believed to have taken place in a 6–10 kb pressure range at about 550°–750° C.

Sandstone Diagenesis, Reservoir Potential, and Sequence Stratigraphy of the Eocene Tyee Basin, Oregon

Abstract: Sandstone petrography and diagenetic analysis within a sequence stratigraphic framework provides a better understanding of the reservoir characteristics in the Eocene Tyee basin, an accretionary and forearc sequence, southern Oregon Coast Range. Detailed comparison of relative abundance of major detrital framework grains documents a marked difference of sandstone composition in each depositional sequence. Such a difference is mainly due to an abrupt change in provenance from a local Klamath Mountains metasedimentary source to a more distant extrabasinal Idaho Batholith-Clarno volcanic arc source. Furthermore, the composition of framework grains varies systematically from the lowstand systems tract to the highstand systems tract within a depositional sequence. This suggests that the patterns of sedimentation and sandstone composition can be affected by relative changes in sea level and tectonic uplift in the source area. In addition, the Eocene Tyee basin sandstones exhibit a down-section distribution of authigenic minerals, consisting of early-formed zeolites and late-stage quartz as well as a change in the abundance of smectite to mixed-layer chlorite/smectite with increasing burial depth. The down-section distribution of authigenic minerals is also causally linked to the compositional variation of detrital framework grains in each depositional sequence with increasing burial temperature. Much primary porosity has been filled with these authigenic minerals, thus diminishing the permeability of potential reservoir rocks. Secondary porosities and permeabilities of reservoir quality (averaging 10.80%; 2.76 md), however, are present locally in some highstand delta-front sandstone facies in the southern part of the basin as well as in lowstand turbidite sandstones in the deeper part of the basin to the north. The development of these reservoir-quality sandstones within the Eocene Tyee basin sequence is due to a complex burial diagenesis, which is directly related to temporal and spatial variations in original detrital mineralogy, in sedimentation pattern, and in burial temperature in the basin.

Metamorphic evolution of the Briançonnais units along the ECORS-CROP profile (Western Alps): new data on metasedimentary rocks

Abstract: Brianconnais units are squeezed between two Mesozoic eclogitic belts (Piemont-Ligurian ocean and Valaisan ocean) along the ECORS-CROP seismic line in the Italian-French Western Alps (France, Italy). The metamorphic evolution of this area plays a key role for understanding the evolution of the Western Alps and is discussed on the basis of detailed petrographic investigations carried out on weathered sediments issued from the erosion of the Hercynian belt, especially on lower Permian to Mesozoic sediments. In the Zone Houillere, as well in the Permo-Triassic cover of the Brianconnais basement, the index metamorphic mineral assemblage is mainly composed of white micas with varying chemical composition, chloritoid and garnet. This same assemblage occurs within different lithologies (metaarkose, metapelite, metasandstone). Consequently, equilibrium phase diagrams were computed for different whole rock compositions using DOMINO software. The results of the P-T investigations clearly show that each unit underwent a different sequence of metamorphic reactions. An increase in metamorphic grade from greenschist facies conditions in the Northwest (Zone Houillere) to the transition between blueschist and eclogite facies conditions in the Southeast (Internal Brianconnais) is observed. A major discontinuity in metamorphic grade is located at the contact between Zone Houillere and Ruitor unit, as documented by a pressure gap of ~ 7 kbar. In general, the observed metamorphic field gradient is inverted and is interpreted to represent different depths of burial during subduction, which correlates with the paleogeographic position of the different units.

Geochemistry of Early Carbonate Cements in the Cardium Formation, Central Alberta

Abstract: Authigenic minerals and diagenetic textures in the marine conglomerates and sandstones of the Cretaceous Cardium Formation in the northeast Pembina area preserve a complex sequence of diagenetic events. Textural relationships observed in thin section and under the scanning electron microscope were used to determine the relative timing of diagenetic events in these marine sediments. Paragenetic sequences are similar for the conglomerate, sandstone, and siderite nodules in the enclosing shales. Extensive cementation by siderite and calcite occurred early in the diagenetic history, before any significant compaction. The geometry and distribution of lithofacies in the Cardium may have influenced the diagenesis and internal stratigraphy of the conglomerate. Overlying shale matrix conglomerate may have trapped upward-migrating fluids increased in buoyancy by dissolved CO2 produced by decay of organic matter in the enclosing shales. These fluids could have infiltrated the upper portion of open matrix conglomerate, causing cementation. Petrographic evidence shows alternating precipitation of siderite and pyrite, implying fluctuating activities of dissolved carbonate and hydrogen sulfide, probably caused by bacterially mediated processes during early diagenesis. Early sequences of siderite and pyrite precipitation can be related to zones of iron reduction, sulfate reduction, and, possibly, decarboxylation during diagenesis. Carbon and oxygen isotopic data indicate a systematic change in the isotopic compositions of calcite and siderite from -25 to -3013C and 018O, for cements early in the paragenetic sequence, to 0 to -513C and -15%18O for cements which are interpreted to occur later in the paragenetic sequence. This observation is consiste t with the influence of meteoric water on later stages of cement deposition in the Cardium, a unit considered to be marine and deposited well offshore on a shallow marine shelf. The intrusion of meteoric water far offshore may be related to changes in sea level. Many variables must be accounted for, but it is feasible, using the estimated regional dip of the Cardium at the time of deposition (0.005°), for a 1-m drop in sea level to push the freshwater--seawater interface seaward on the order of 100 km.