Petrography

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

Comparison of isotopic and petrographic provenance indicators in sediments from Tertiary continental basins of New Mexico

Abstract: Isotopic variations of Nd in sedimentary rocks provide a new tool for provenance determination. We have compared the isotopic technique with classic petrographic indicators of provenance in sediments from two Tertiary basins of the Rio Grande rift, north-central New Mexico. The siliciclastic sediments in these basins were derived from varying proportions of Precambrian basement and Oligocene volcanic rocks, which are isotopically and petrographically distinct. In the analyzed sediments, a good correspondence exists between petrographic and isotopic provenance indicators. The data also suggest that the isotopic tracer is more sensitive to minor detrital input from a second source than is the framework petrology. The similar Sm/Nd ratios for all analyzed sediments and the identical isot pic values of interlayered mudstone and sandstone indicate that no significant fractionation of Sm and Nd occurs during the formation of the sediments. Thus, the Nd model ages of the sediments preserve the crust-formation age of the source. The Sr isotopic and concentration data show trends generally similar to the Nd data, but they do not indicate the sedimentary provenance as clearly. This ambiguity is a consequence of strong Rb-Sr source heterogeneities and Rb-Sr fractionation during the sedimentary cycle.

Geochemistry of Rewalsar Lake sediment, Lesser Himalaya, India: implications for source-area weathering, provenance and tectonic setting

Abstract: Major, trace and rare earth element (REE) chemistry of fresh-water lake sediments, stituated in the middle Siwalik Formation (Pliocene), have been investigated. The log (Fe2O3/K2O) vs log (SiO2/Al2O3) plot of sediments discriminate the rock types of the catchment which are remarkably similar to those observed in thinsections. The Chemical Index of Alteration (CIA) between 75.2 to 78.1 and Chemical Index of Weathering (CIW) between 79.5 to 89.8 are higher than Post-Archean Australian Shale (PAAS), indicating moderate to high weathering at source area. The PAAS normalized REE patterns are similar and differentiate the major rock types on the basis of abundance of REEs in accordance to clay minerals, grain size, with the highest concentration in the finest grain sediments. It also indicates poor mixing of sediments. The consistency of REEs in sediments with those of rock types illustrate almost unaltered, unmodified transport of these from the provenance to the detritus deposited in the basin and also shows that the REEs are enriched in phyllosilicates. The major elements, Fe2O3, TiO2, MgO, K2O show strong positive correlation with Al2O3 and follow the trend of the latter, and REEs also indicate same characteristics, hence phyllosilicates are likely to be the promising host for REEs. The negative correlation of La, Yb and Y with Zr and positive correlation with REEs and Al2O3 do not indicate Zr control on REE distribution in the present case. The petrographic character of minerals in thinsection of rocks as well as of lake sediments and low Na2O/K2O ratio in the latter show short distance transport, textural immaturity and sorting of grains. The higher Zr/Th and Zr/Yb ratios in coarser sediments than in fine grain detritus are supporting evidence of sedimentary sorting. Chondrite normalized REE patterns and negative Eu anomaly of the lake sediments are similar to that of PAAS indicating that they may have been originally derived from differentiated silicic (felsic) and/or recycled sedimentary source and deposited in foreland basin. However, petrographic character, REEs and high field strength elements estimated on sediments, tends to support metamorphic source area for middle Siwalik detritus and is consistent to the observations made on rocks exposed in the catchment area, attesting to the rise of Himalaya in phases, exposing fresh rocks for erosion/weathering, lying northward in Lesser Himalayan domain. Plot of sediment chemistry of lakes on tectonic delineation diagrams show that Rewalsar detritus derived from middle Siwalik rocks are from passive margin setting. The discriminant function plots of sediment samples also indicate that they are derived from craton interior or a quartzose sedimentary orogenic terrain and deposited in a passive margin setting. The chemistry of Rewalsar sediments is thus a proxy for the middle Siwalik rocks deposited in the foreland basin and throw light on provenance, source area weathering, and tectonic setting of the source rock.

Provenance of Passive-Margin Sand (Southern Africa)

Abstract: This study investigates the petrographic, mineralogical, geochronological, and geochemical signatures of river sands across southern Africa. We single out the several factors that control sand generation, including weathering and recycling, and monitor the compositional changes caused by chemical and physical processes during fluvial transport from cratonic sources to passive-margin sinks. Passive-margin sands have two first-cycle sources. Quartz and feldspars with amphibole, epidote, garnet, staurolite, and kyanite are derived from crystalline basements exposed at the core of ancient orogens and cratonic blocks (dissected continental block provenance). Volcanic rock fragments, plagioclase, and clinopyroxene are derived from flood basalts erupted during the initial phases of rifting (volcanic rift provenance). First-cycle detritus mixes invariably with quartzose detritus recycled from ancient sedimentary successions (undissected continental block provenance) or recent siliciclastic deposits (e.g.,...

Using Magnetic Resonance Imaging and Petrographic Techniques to Understand the Textural Attributes and Porosity Distribution in Macaronichnus-Burrowed Sandstone

Abstract: Magnetic resonance images are paired with petrographic data to evaluate the textural characteristics of rocks dominated by Macaronichnus segregatis, a trace fossil that is commonly associated with rocks deposited in shallow, marginal marine sedimentary environments. MRI techniques used revealed the three-dimensional geometry of the trace fossil. Burrows are typically horizontal and in plan view range between straight, sinuous, meandering, and spiral geometries. Changes in burrow morphology may be related to population density and patchy resource distribution. The pairing of MRI and petrographic data helped map the distribution of porosity in the burrowed rock. Because MRI images represent complex composites of nuclear spin density and MR relaxation times, each of which is related to pore size, stronger MR signals must be calibrated to known porous zones by integrating petrographic data with MR data. The complex distribution of porosity and its relationship to the matrix show that this fabric represents a dual porosity-permeability system and may affect the resource (reservoir or aquifer) quality of similarly burrowed sedimentary rocks. Future research should elaborate upon the porosity-permeability model for this and similar fabrics.