Granitic Rock Fragments Research Articles

A Petrographic Reconnaissance of Sand Size Sediment Upper St. Francis River, Southeastern Missouri

ABSTRACT The St. Francis River of southeastern Missouri along its first 31.5 mi has an average gradient of 9.2 ft./mi in a region of hilly terrain with maximum relief of 1000 ft. The drainage area encompasses approximately 870 mi2 in an area of temperate climate with a mean annual rainfall of 40 in. Coarse and fine-grained Precambrian granitic rocks and isolated patches of Paleozoic sedimentary rocks each contribute unique detritus to the fluvial environment. Petrographic analysis of the coarse, medium, and fine sand size fractions of main channel and tributary samples reveal the respective roles of dilution, abrasion, and selective sorting in altering sediment composition. Dilution of the main channel from a rhyolitic source terrain in the southern portion of the study area tends t mask the effects of abrasion and selective sorting. Downstream decreases in relative percentages of monocrystalline quartz, polycrystalline quartz, and granite rock fragments, however, can be attributed, in part, to abrasion and/or selective sorting. Granite rock fragments show the least amount of physical durability, being almost totally absent from the size range examined at the southern end of the study area.

Source and dispersal of reservoir sands in El Morgan field, gulf of Suez, Egypt

More than 1,500 samples of cuttings from 17 wells in El Morgan Field were studied for grainsize distribution, degree of roundness, and feldspar content. The sands belong to the Kareem Formation of the Miocene and could be described as arkose or subarkose with feldspar content ranging from 5–40%. Percentage maps clearly indicate a source area to the south and southwest of the field. The presence of high feldspar content and the angularity of the very coarse quartz and feldspar grains, in addition to the presence of granitic rock fragments, indicate a plutonic source for the sands. The source is believed to be the nearby Zeit Range. Minor contributions from a Nubian sandstone source from both the southwest and northeast directions may have taken place at certain intervals. The lack of feldspar alteration is attributed to rapid erosion as a result of the high relief attained by large-scale block faulting in pre-Miocene and Early Miocene times and to conditions of extreme airdity. The bottom topography of the Miocene Sea controlled the movement of water currents and determined the size, shape, and distribution of the sands.

Sedimentology of Archean Turbidites at Yellowknife, Northwest Territories

The Burwash Formation is a major formation of the Archean Yellowknife Supergroup at Yellowknife. It consists of about 15 000 ft (4572 m) of interbedded graywackes and mudstones and shows many of the features characteristic of turbidites. Analysis of the internal sedimentary structures and paleocurrent data on the sediments indicate that the sediments were derived from the west, possibly from an area now occupied by an extensive granitic terrain, and accumulated in depositional fan valleys on a submarine fan complex near the margin of a large Archean sedimentary basin that lies to the east of Yellowknife. The high proportion of volcanic rock fragments, particularly silicic–volcanic lithic clasts, along with abundant quartz and feldspar and the minor but ubiquitous granitic rock fragments in the graywackes, indicates a mixed silicic–volcanic and granitic provenance. The modal and chemical composition, and volumetric abundance, of these sediments denotes the presence, and considerable extent, of sialic crust prior to the deposition of the Burwash Formation.

Provenance Model Studies: Piedmont Rivers to Atlantic Deep-Sea Floor: ABSTRACT

Petrographic analyses of 500 impregnated sands from the piedmont rivers, coastal plain, beach, shelf, and deep sea off the southeastern United States indicate that they have differences in composition which reflect changes in depositional history. Piedmont rivers transport mainly nonpolycrystalline quartz (20-80%), polycrystalline quartz (5-50%), microcline (2-15%), orthoclase (0-10%), plagioclase (0-4%), and minor amounts of reworked red sediments. Coastal-plain sands are more quartzose (more End_Page 841------------------------------ than 90%) and commonly show corrosion indicating postdepositional solution. Rock fragments in these sands consist mainly of reworked Tertiary limestone and sandstone. Near Cape Hatteras, sands of the shelf and upper slope consist mainly of quartz and are characteristically higher in feldspar (10-15%) than the shelf sands on the south. The feldspar is dominated by orthoclase with minor amounts of plagioclase and microcline. This content is in contrast to the high microcline content of the piedmont, coastal-plain rivers, and southern shelf sands but is similar to the mineralogy of the sands from the Hatteras abyssal plain. The relatively plentiful microcline and granitic rock fragments on the shelf between Cape Hatteras and Cape Romain suggest that these surficial sands are the result of reworking of older piedmont-coastal plain sands during late Pleistocene. Farther south of Cape Romain the terrigenous fractions have a piedmont signature but are m ch diluted with relict-residual carbonates and oolitic, algal, and micritic limestone debris. End_of_Article - Last_Page 842------------

THE MINERALOGY AND PETROLOGY OF THE PERMIAN SANDSTONES AT GIDGEALPA, SOUTH AUSTRALIA

Detailed mineralogical and petrographic examinations were carried out on the gas-bearing Permian sandstones at Gidgealpa, South Australia, on behalf of Delhi Australian Petroleum Ltd. The investigation was carried out to obtain data on the possible formation damage during drilling. The rocks consist dominantly of clay-cemented protoquartzites and contain sedimentary and rare (?)granitic rock fragments in the upper members, and acid volcanics fragments in the lower members. The matrix consists of kaolin, illite and quartz. Some of the kaolin is authigenic and consists of vermicular kaolinite. Silicification of the rocks is a common feature, and its effect on the recovery of petroliferous constituents is considered significant. Treatment of the clays with artesian water, saline solution and drilling mud filtrate revealed no modification to the crystal structures, but flocculation of the clays with salt solution was evident. The significance of the laboratory characteristics of these samples to their behaviour in situ is discussed.

Concentration changes of pore solutions during compaction of clay sediments

ABSTRACT Mineral, chemical, and textural compositions of classical graywackes from the Harz Mountains (Germany) and of classical arkoses from the Auvergne (France) are discussed. The type graywackes cannot be defined by the presence of a matrix; nor does the presence of granitic rock fragments or the absence of other rock fragments characterize the type arkoses. Both rock types may be distinguished only by their total mineral and chemical compositions.

Platy xenoliths from Komagatake Volcano, Hokkaido

Mt. Komagatake which stands in the southwestern part of Hokkaido, Japan, is a strato-volcano formed by repeated eruptions of andesitic lava and pyroclastic materials. Many platy xenoliths stretched by the magma are found in a lava flow cropping out near Sawaradake, the northern wall of the horseshoe-shaped explosion crater situated in the apical part of this volcano. Such xenoliths are composed of granitic and sedimentary rock fragments, showing some evidences of having been effected by Komagatake magma to such an extent that their original textures scarcely remain. Besides a fairly large quantity of fused glasses, newly recrystallized minerals such as cordierite, sillimanite, andalusite, quartz, plagioclase, etc., are found to occur in several xenoliths which originated from sedimentary rocks. Some considerations on contamination of Komagatake magma are also offered on the basis of microscopic observation on contact parts between xenoliths and the lava.