Petrography

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

Field geometry, petrography and geochemistry of a dolomitization front (Late Jurassic, central Lebanon)

Abstract: This contribution describes the field geometry, petrography and geochemistry of a well-exposed dolomitization front in Upper Jurassic carbonates, and attempts to highlight the sedimentological, structural and relative sea-level controls on multiphase dolomitization and related diagenetic events. The data presented reflect the superposition of various diagenetic phases which have resulted in a single dolostone body, whose dimensions are well defined in the field. Local microbial intraclastic dolomites of Late Tithonian age accumulated in a hypersaline lagoon during relative sea-level fall. These pre-date beige hydrothermal dolostones (51 to 55 mol% CaCO3; δ18O: −9·3 to −4·0‰ V-PDB; δ13C: −1·5 to +2·1‰ V-PDB; 87Sr/86Sr: 0·70742; matrix porosity: ≈6%; Klinkenberg permeability: ≈0·5 mD), whose dolomitizing fluid circulated along faults and invaded the nearby facies. First, the burrows were dolomitized, then the bulk rocks, resulting in the investigated 'tongue'-shaped dolomite body. Upon Late Jurassic–Early Cretaceous uplift, near-surface water percolated through – and altered – the underlying beige dolostones. This event was followed by a ferroan dolomite cement phase, which occurred during further burial. This contribution, featuring a well-defined geometric pattern of a dolomitization front with a large petrographic and geochemical data set, may also serve as a case study illustrating the complexity of superimposed diagenetic processes which have to be taken into account in modelling exercises of multiphase hydrothermal dolomitization.

Jurassic volcanic rocks in northeastern Mexico: A possible remnant of a Cordilleran magmatic arc

Abstract: Pre-Oxfordian Mesozoic subaerial volcanogenic rocks occur in a band extending northwest from Ciudad Victoria, Tamaulipas, to Santa Maria del Oro, Durango. These strata include Nazas, Rodeo, and Caopas Formations in Durango, Coahuila, and Zacatecas; La Boca Formation and its underlying volcanic basement at Canon de Huizachal, Tamaulipas; and volcanic units below La Joya Formation at Real de Catorce and Charcas, San Luis Potosi. Rocks at these localities have similar lithologies, stratigraphic positions, and paleontologic and isotopic ages. Field mapping in the Caopas-Pico de Teyra area, northern Zacatecas, and ancillary research provide insight into the nature of this suite. At least 3 km of abundant airfall tuff, tuffaceous siltstone, and uncommon ashflow tuff are present near Pico de Teyra; this sequence appears to belong to a more distal facies than the flows, breccias, and laharic conglomerates of the Nazas and Rodeo Formations exposed 25 km to the north. Porphyritic rhyolite (Caopas Formation) occurs within these volcanogenic rocks as a fault-bounded block and is interpreted as a cogenetic, subvolcanic pluton. A relatively undeformed portion of the Caopas has yielded a zircon U-Pb age of 158 ± 4 Ma. Petrographic and limited chemical data from these formations show that calc-alkaline andesite, dacite, and rhyolite are the most common compositions. The volcanogenic rocks in northeastern Mexico are south of the inferred trace of the Mojave-Sonora megashear. Their large volume, their lithologic and chemical characteristics, and their age suggest that these rocks may be a component of the Jurassic arc of western North America that was translated southeastward along the megashear.

Lithologic Controls on the Development of Solution Porosity in Carbonate Aquifers

Abstract: Movement of ground water in maturely karsted limestone aquifers is commonly through solution conduits of considerable size. The object of this study was to determine the lithologic controls on the distribution of conduits within a heterogeneous sequence of carbonate rocks. The volume of all accessible caves more than 100 feet in length in the Nittany Valley area of central Pennsylvania was measured. Most caves are entirely within limestones, cave development in dolomite is extremely rare. Within the limestone sequence the bulk of the cave volume is concentrated in a few members. Chemical and petrographic analyses of the carbonate rocks were associated with cave volume by bar graphs and components analyses. The more cavernous limestones seem to be those with low amounts of dolomite, clay, and other impurities, and a high micrite grain fraction. Coarse limestones and dolomites are less cavernous.

Reflections on the interpretation of heavy mineral analyses

Abstract: The study of heavy minerals, an important field of endeavor for the sedimentary petrographer prior to 1940, has declined in recent years. This decline, caused primarily by the disappointing results of heavy mineral correlation, is regrettable because in regional stratigraphic and paleogeographic research the method is capable of contributing important information concerning the location and character of source areas and the distribution patterns of sediments. The following factors can modify the heavy mineral composition and limit the interpretation of the assemblage in terms of source area conditions: (1) weathering, (2) abrasion, (3) selective sorting, and (4) post-depositional solution. This paper attempts to show that (3) and (4) are of minor importance. In basins with a low rate f sediment supply and under conditions of intense weathering and reworking the others can strongly modify the composition of the heavy mineral assemblage. In basins of rapid deposition deriving their sediment in general from source areas with active erosion, the influence of all factors is negligible and the heavy mineral assemblage directly reflects the petrography of the source area. An example illustrates the type of problem for which heavy mineral analysis is of value. Problems of sediment source and distribution in the Mississippi valley and the Gulf Coastal Plain in the Cenzoic are a field where heavy mineral analysis is considered rewarding.