Oolitic Texture Research Articles

Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania

ABSTRACTPedogenic calcretes are closely associated with Pliocene to Holocene wind‐worked deposits of volcanic ash in the Olduvai and Ndolanya Beds of northern Tanzania. The typical profile with calcrete consists of an unconsolidated sediment layer, an underlying laminar calcrete, and a lowermost massive calcrete. The laminar calcrete is a relatively pure limestone, whereas massive calcrete is aeolian tuff cemented and replaced by calcite. An Olduvai calcrete profile can develop to a mature stage in only a few thousand years. Carbonatite ash was the dominant source for most of the calcite in the calcretes.Replacement was a major process in formation of the massive calcretes, and oolitic textures have resulted from micrite replacing pelletoid clay coatings around sand grains. Phillipsite and possible other zeolites were extensively replaced in the massive calcretes. Replacement of clay by micrite in the Olduvai calcretes is accompanied by dissolution or leaching of phengitic illite and the formation of clay approaching the composition of halloysite or kaolinite. In the upper calcrete of the Ndolanya Beds, montmorillonite was altered to a kaolinite‐type mineral and to dioctahedral chlorite. Authigenic dolomite, zeolite, and dawsonite in the Olduvai calcretes probably received at least some of their components from replaced materials.

Origin and Lithification of the Pleistocene Carbonates of the Salum Area, Western Coastal Plain of Egypt

ABSTRACT Many geologists have attempted to explain the origin and lithification of the Pleistocene calcareous sediments of the western coastal plain of Egypt. The present study is an interpretation of the origin and lithification of the Pleistocene carbonates from the Salum area. The oldest deposit (Tyrrhenian limestone) consists of grains which lack any concentric structure, but instead have developed micritic calcite envelopes. They are interpreted as detrital grains of recycled carbonates. The younger deposits (Late Monasterian and Main Monasterian limestones) have a well developed oolitic texture probably formed in shallow, agitated marine waters. It is believed that the existing textures and mineralogy are the result of diagenesis which proceeded under subaerial conditions. Three stages of diagenesis are recognized, each exhibiting its own peculiar textural, mineralogical and geochemical characteristics. The decrease in strontium content as related to aragonite inversion to calcite is shown to be a reliable diagenetic guide.

Cambro-Ordovician Depositional Environments in Central East Greenland: ABSTRACT

The Cambro-Ordovician sedimentary succession in central East Greenland is considered to represent deposits formed during a slow, westward-advancing marine transgression. The sandstones, shales, limestones, and dolostones comprising the succession range in age from Early Cambrian through Middle Ordovician (Champlainian) and appear to reflect patterns of deposition comparable to modern shallow-marine shelf sediments. The roughly 3,000 m of Cambro-Ordovician strata lie with apparent conformity on Precambrian rocks of glacial and marine origin and are superposed unconformably by Devonian Old Red fluvial deposits. Tidal current-bedding structures in the sandstones exhibit a marked bimodality with vector means on the east and presumably downslope. Intraformational flat-pebble or flake conglomerates, stromatolitic algal structures, oolitic textures, and desiccation phenomena in the shale and carbonates also favor a shallow marine environment for these deposits. Measured sections at 2 widely spaced localities demonstrate a remarkable lithologic lateral continuity presumed attributable to deposition on a broad marine shelf with very low relief. End_of_Article - Last_Page 2507------------

Flint Kaolins in Non-Coal-Bearing Triassic of Sydney Basin, Australia: ABSTRACT

Claystones, similar in composition, texture, and structure to the flint kaolins of the Pennsylvanian of North America, the Westphalian of Europe, and the Permian coal measures of South Africa and Australia, form a persistent marker bed in the non-coal-bearing Triassic Narrabeen Group of the Sydney basin. End_Page 538------------------------------ The claystones are indurated and characteristically possess oolitic textures, although vermicular crystals and brecciated fragments are present in most samples. Well-crystallized kaolinite is the only clay mineral present and is associated invariably with anatase and locally with abundant (> 50 percent) boehmite or siderite. Quartz, either as discrete grains or as chalcedony, is rare or absent. The unit ranges in thickness from 1 to 6 ft and can be traced for more than 50 mi along the southeastern margin of the basin. However, extensive boring downdip has shown that it thins out abruptly in that direction. The underlying sediments consist of red claystones, known locally as the chocolate shales. Apart from well-crystallized kaolinite and anatase, these claystones contain appreciable quantities of hematite (15-25 percent), but quartz and clay minerals, other than kaolinite, are sparse or absent. The overlying sediments, however, are composed of quartz, illite, and degraded illite as well as kaolinite. The chocolate shales are believed to represent a transported laterite or lateritic bauxite and the flint kaolins are considered to be the reduced equivalent, formed by the advent of swampy conditions at the close of deposition of the sedimentary sequence. End_of_Article - Last_Page 539------------

Ferruginous Oolites and Pisolites

ABSTRACT A description of two important oolitic ironstones from central and northwestern Nigeria is used as the basis for a general discussion on oolitic texture in ironstones. The Nigerian ores are of late Senonian and Tertiary ages and their original oolitic texture has been extensively modified by the migration of iron during weathering. They are compared with oolitic and pisolitic laterites and with certain hitherto undescribed ferruginous pisolites in the basal Carboniferous Limestone of Skrinkle Haven, South Wales, Great Britain. During deposition, ferruginous oolites may be either hard and brittle or soft and easily deformed. The diversity of textures observed can be explained by this fact and by phenomena associated with the diffusion of iron through the rock after consolidation. The diffusion phenomena can produce what appear to be fractured oolites and other features apparently indicative of abrasion prior to deposition and therefore such features must be treated with caution in the interpretation of oolitic textures.

Dolomitization, Silicification and Calcitization Patterns in Cambro-Ordovician Oolites from Northwest Scotland

ABSTRACT Diagenetic replacement phenomena involving dolomite, silica, and calcite are demonstrated from oolitic horizons in the Cambro-Ordovician Durness carbonates of northwest Scotland. The oolitic texture of the rocks has exerted a control over the patterns of diagenesis and facilitates the interpretation of the diagenetic history. A possible five-stage diagenetic sequence is proposed to account for the observed relationships of the recrystallization and authigenic phenomena. A dilemma regarding a void filling interpretation of drusy calcite textures is demonstrated and arguments favoring the use of the term calcitization rather than dedolomitization are offered.

Etude stratigraphique et micropaleontologique d'une serie jurassique de l'le de Minorque, Baleares (Espagne)

Abstract The Jurassic of Minorca is known from a composite section described from outcrops on the Cape of Fornells and on the Atalaya de Fornells peninsula. The Liassic is primarily a microcrystalline dolomite; the middle Jurassic, containing Orbitammina (Dictyoconus) cayeuxi (Lucas), is composed of dolomites containing large dolomite crystals, and always displaying oolitic texture; the upper Jurassic consists of limestone and microcrystalline dolomites. Comparison of the Orbitolinidae genera, Meyendorffina and Coskinolina, does not reveal significant differences between the two, therefore justifying the reduction of Meyendorffina to the rank of subgenus in the genus Coskinolina. Coskinolina (Meyendorffina) bizonorum, n. sp., C. (M.) minoricensis n. sp., and C. (M.) sp. 1 are described and figured. The beds containing Meyendorffina, which lie between the middle and upper Liassic brachiopod marls and the upper Jurassic limestones with Valvulinella sp., are assigned to the Dogger, possibly the upper part, on the basis of 'Dictyoconus' cayeuxi (also figured).

Origin of oolites and the oolitic texture in rocks

Research Article| January 01, 1914 Origin of oolites and the oolitic texture in rocks THOMAS CLACHAR BROWN THOMAS CLACHAR BROWN 2Introduced by Florence Bascom. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1914) 25 (1): 745–780. https://doi.org/10.1130/GSAB-25-745 Article history received: 31 Dec 1913 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation THOMAS CLACHAR BROWN; Origin of oolites and the oolitic texture in rocks. GSA Bulletin 1914;; 25 (1): 745–780. doi: https://doi.org/10.1130/GSAB-25-745 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Introduction and historical SketchThe study of oolites and oolitic texture has no claim to novelty. This peculiar texture early attracted the attention of scientists, and the microscope was used to elucidate the minute structure of these rocks nearly two centuries before the petrographic microscope came to be recognized as a necessary adjunct of a petrographic laboratory. As early as 1664 Hooke described in his volume on “Micrography”3 an oolite under the name of “Kettering-stone.” His description runs thus:“This stone which is brought from Kettering in Northamptonshire, and digg’d out of a quarry, as 1 am inform’d, has a grain altogether admirable, nor have I ever seen or heard of any other stone that has the like. It is made up of an innumerable company of small bodies, not all of the same cize or shape, but for the most part, not much differing from a globular from, nor . . . This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Formation of Oolite

If a thin section of oolite be mounted in Canada balsam and examined under a microscope, it is seen to be made up of a number of granules associated with fragmental remains of calcareous organisms, and the interstitial spaces are filled in with calcite. Looked at with a 2-inch object-glass these oolitic granules show, for the most part, a series of concentric strata or laminæ of calcium carbonate around a nucleus. There are also other secondary structures, the principal of which are dark striæ and patches, the former placed more or less at right angles to the nucleus. At the Bath meeting of the British Association in 1888 Prof. H. G. Seeley read a paper ‘On the Origin of Oolitic Texture in Limestone Rocks.’ In this communication the author stated that he ‘believed that oolitic texture might originate in many ways……’ He also drew attention to the ‘close resemblance of the internodal grains of the nullipores to grains of oolite as furnishing a further explanation of oolitic texture. These grains show a concentric structure as well as a radiated tubular structure, which would favour the recrystallization such as commonly occurs.’ Generally speaking, up to the year 1889 the concentric layers were supposed to be chemically deposited calcium carbonate. Thus Dr. Sorby, F.R.S., in his most able and memorable address to the Geological Society in 1879, describes the formation of normal Jurassic oolitic grains in the following words : They are ‘ composed of more or less well-defined layers of small