Quartz cement of meteoric origin in silcrete and nonsilcrete sandstones, Lower Carboniferous, western Sinai, Egypt

Publisher Summary This chapter discusses diagenesis of sandstones. The presence of authigenic silicate minerals have been reported to occur in sandstones, that such occurrences can no longer be regarded as accidents of mineral formation. Common appearance of secondary feldspars and micas, which is normally recognized as a representative of high-temperature minerals, suggests that certain progression has occurred in the development of such minerals. It can be demonstrated that as conditions suitable to their stability were attained, they were formed even at low temperatures at the expense of other minerals no longer stable. Among sandstones, those minerals which appear to be most sensitive to changes in conditions are silica and clay minerals. Silica alters readily into other forms, dissolves, and reprecipitates repeatedly throughout the history of certain sandstones. Clay minerals appear to be exceedingly stable in certain sandstones, whereas in others, they can be recognized to have altered to mica or perhaps feldspar. The thesis that has been presented in the chapter is discussed to regard such authigenesis as not accidental, but rather as part of an orderly sequence of events that lead to the formation of a rock that has crossed the boundary of metamorphism.

Chapter 2 Diagenesis of Sandstones

Abstract Pennsylvanian–Early Permian carbonate sedimentation in east-central British Columbia records a complex history of changing environments influenced by evolving palaeogeography and climate. Newly recognized tectonically controlled features affected the distribution and variability of carbonate associations, providing new interpretations for this portion of the NW coast of Pangea. Both a heterozoan (cool-water) and photozoan (warm-water) association were identified on either side of a palaeogeographical high. Cool-water carbonates were located outboard or to the west of this high, an area influenced by upwelling waters. Inboard of this high, a warm, protected sea developed at about 20°N palaeolatitude during the Asselian and Sakmarian. This configuration and palaeolatitude is similar to that of Baja California, Mexico and the Sea of Cortéz, providing a good modern analogue for these deposits where warm-water carbonates grow at latitudes otherwise dominated by cool-water deposits. The warm sea provided a place for a photozoan association to develop during the Early Permian when the low-latitude NW coast of Pangea was dominated by cool-water carbonates.

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Variability of Pennsylvanian-Permian Carbonate Associations and Implications for NW Pangea Palaeogeography, East-Central British Columbia, Canada

Length-slow chalcedony was recently discovered in the Mesozoic Mino Terrane, Central Japan. Lutecite and quartzine, two varieties of length-slow chalcedony, occur in exotic limestone, chert, and dolostone blocks as secondary minerals replacing carbonates or as vein minerals. Clastic fragments of length-slow chalcedony were also found in Mesozoic turbidite sandstone in this terrane. Indigenous rock-formations carrying length-slow chalcedony have not been recognized in Japan. Length-slow chalcedony is generally considered to form under geologic environments different from those for length-fast chalcedony, that is, length-slow chalcedony tends to form in sedimentary rocks deposited on cratonic margins in arid, evaporitic environments. Accordingly, it is likely to consider that the Mino Terrane is composed of tectonic blocks and detritus from cratonic regions containing much length-slow chalcedony formed in evaporitic environments. Recent geologic, paleomagnetic, and paleontologic syntheses of the tectonic evolution of the Japanese Islands have revealed the allochthonous nature of the Mino Terrane. I suggest that length-slow chalcedony can be used as a tracer mineral to identify the provenance of the allochthonous Mino Terrane.

Length-Slow Chalcedony in Sedimentary Rocks of the Mesozoic Allochthonous Terrane in Central Japan and Its Use for Tectonic Synthesis

Publisher Summary This chapter discusses the diagenesis of sandstones and compaction. The raw material of diagenesis consists of organic and inorganie sediment of allochthonous and/or autochthonous origin, interstitial fluids, and other components subsequently formed or introduced into the system. In general, it is possible to subdivide the components that interact during the diagenetic processes into the following: (1) diagenetic–endogenic; (2) diagenetic–exogenic, this is further divided as (1) supergenic–exogenic and (2) hypogenic–exogenic. In most cases, diagenesis derives its raw material from both endogenic (within the sediments) and exogenic–supergenic (outside source—from above) sources. One or the other may prevail. Under unusual conditions, however, a volcanic—that is, exogenic-hypogeni—source may supply components for diagenesis without a marked increase in temperature. This would be particularly true for siliceous material introduced into a geosyncline (or other depocenter); during the diagenesis of sediments, quartzose arenites can be converted to orthoquartzites, carbonates can become siliceous, and fossils can be replaced prior to dolomitization. Although compaction is a part of diagenesis, if compaction is taken as the “center of consideration”, then diagenesis could be subdivided into pre-, syn-, and postcompaction stages. This may seem as an artificial division; however, in detailed work on, for example, the relationship among fluid movements during diagenesis and their influence on the origin of various types of chemical cements including metalliferous ores, such an approach can prove to be useful.

Chapter 3 Diagenesis of Sandstones and Compaction

Summary

The Ranger Canyon and Mowitch Formations are the youngest Permian strata in the Rocky Mountain Front Ranges. They constitute a thin, widespread and clearly defined stratigraphic entity covering approximately 60,000 sq. miles; they occur below Triassic beds and above a major, regional, intra-Permian unconformity, initiated by a transgression which started in the northwest. Beds of the above entity are therefore diachronous. The erosion surface is overlain by polymict, phosphatic conglomerates containing mature lag gravels and immature breccio-conglomerates and corrosion breccias.



Rock types are either sandstones or silty and sandy cherts, phosphorites and occasionally dolomites. Clastic components are dominantly mature quartz (some pseudomorphs gypsum), phosphate, some feldspar, negligible clay, and stable detrital minerals, necessitating: (1) many cycled, clastic sedimentary rock; (2) local gypsum; and (3) phosphate producing, distributive provinces.



Secondary features include primary cementation by quartz, chalcedony, dolomite, sulphate and phosphate, with replacement mainly by chalcedony. Contraction fractures are cemented by quartz (after anhydrite and fluorite), calcite and barite. Accessory authigenic minerals are hematite, sulphates (pseudomorphed by quartz), fluorite, apatite and dolomite. Diagenesis therefore occurred within a chemically active environment.



Minerals present indicate: (1) phosphate producing and (2) evaporite producing environments, the components for which may be obtained from sea water by: (a) a biogeochemical phosphate-fixation cycle; and (b) concentration of brines by interstitial refluxion.



A Recent environmental parallel occurs along the coast of Baja, California. Phosphate produced off-shore contributes to quartz-rich sediments prograding seawards; evaporitic conditions produce minimum carbonate and maximum sulphate precipitation.



The sequence of events envisaged for the Permian rocks started with phosphate corrosion of bed-rock and cementation of lag gravels; some phosphate was contributed to shoreline, with quartz and gypsum sands, prograding over the basal conglomerate. Sand cementation by carbonate occurred in shoreline and intertidal zones, and by sulphate in the supratidal zones. Concentration of brines by refluxion, and local silica enrichment, facilitated silica precipitation and replacement. Penecontemporaneous silicification of Late Permian sediments was therefore instrumental in their ultimate preservation.

The diagenesis and depositional environment of the permian ranger canyon and mowitch formations, ishbel group, from the southern canadian rocky mountains

J. E. RAPSON-McGUGAN

Papers

The diagenesis and depositional environment of the permian ranger canyon and mowitch formations, ishbel group, from the southern canadian rocky mountains