Alluvial Soils

Abstract

Quartz cement as syntaxial overgrowths is one of the two most abundant cements in sandstones. The main factors that control the amount of quartz cement in sandstones are: framework composition; residence time in the “silica mobility window”; and fluid composition, flow volume and pathways. Thus, the type of sedimentary basin in which a sand was deposited strongly controls the cementation process. Sandstones of rift basins (arkoses) and collision-margin basins (litharenites) generally have only a few percent quartz cement; quartzarenites and other quartzose sandstones of intracratonic, foreland and passive-margin basins have the most quartz cement. Clay and other mineral coatings on detrital quartz grains and entrapment of hydrocarbons in pores retard or prevent cementation by quartz, whereas extremely permeable sands that serve as major fluid conduits tend to sequester the greatest amounts of quartz cement.In rapidly subsiding basins, like the Gulf Coast and North Sea basins, most quartz cement is precipitated by cooling, ascending formation water at burial depths of several kilometers where temperatures range from 60° to 100° C. Cementation proceeds over millions of years, often under changing fluid compositions and temperatures. Sandstones with more than 10% imported quartz cement pose special problems of fluid flux and silica transport. If silica is transported entirely as H4SiO4, convective recycling of formation water seems to be essential to explain the volume of cement present in most sandstones. Precipitation from single-cycle, upward-migrating formation water is adequate to provide the volume of cement only if significant volumes of silica are transported in unidentified complexes. Modeling suggests that quartz cementation of sandstones in intracratonic basins is effected by advecting meteoric water, although independent petrographic, isotopic or fluid inclusion data are lacking.Silica for quartz cement comes from both shale and sandstone beds within the depositional basin, including possibly deeply buried rocks undergoing low-grade metamorphism, but the relative importance of potential sources remains controversial and likely differs for different formations. The most likely important silica sources within unmetamorphosed shales include clay transformation (chiefly illitization of smectite), dissolution/pressure solution of detrital grains, and dissolution of opal skeletal grains; the most likely important sources of silica within unmetamorphosed sandstones include pressure solution of detrital quartz grains at grain contacts and at stylolites, feldspar alteration/dissolution, and perhaps carbonate replacement of silicate minerals and the margins of some quartz grains. Silica released by pressure solution in many sandstones post-dates the episode of cementation by quartz; thus, this silica must migrate and cement shallower sandstones in the basin or escape altogether. Some quartz-cemented sandstones are separated vertically from potential silica source beds by a kilometer or more, requiring silica transport over long distances.The similarity of diagenetic sequences in sandstones of different composition and ages apparently is the result of the normal temperature and time-dependent maturation of sediments, organic matter and pore fluids during burial in sedimentary basins. Silica that forms overgrowths is released by one or more diagenetic processes that apparently are controlled by temperature and time. Most cementation by quartz takes place when sandstone beds were in the silica mobility window specific to a particular sedimentary basin. Important secondary controls are introduced by compartmentalized domains produced by faults (e.g., North Sea) or overpressure boundaries (e.g., Gulf Coast Tertiary).Shallow meteoric water precipitates only small amounts of silica cement (generally less than 5% in most fluvial and colian sandstones), except in certain soils and at water tables in high-flux sand aquifers. Soil silcretes are chiefly cemented by opal and microcrystalline quartz, whereas water-table silcretes have abundant normal syntaxial quartz overgrowths. Silica for silcrete cements and replacements comes from quartz, silicate minerals, and locally volcanic glass, in alluvium and bedrock.