About This Item
Share This Item
Comparative study of Permian and modern caliche, laterite, bauxite, and beach rock have shown that vadose cement, regardless of composition, can be distinguished from cementation below the water table or sediment-water interface.
Phreatic and connate cementation which occurs beneath the water table or sediment-water interface is more coarsely crystalline than vadose cement and is interstice-filling rather than grain-coating. Early carbonate cement precipitated from water-filled voids at shallow depths consists of sparry calcite and may push apart and replace quartz and feldspar to produce "floating" textures. Silica cement occurs as void-filling overgrowths on quartz grains. After deep burial uncemented sand grains become tightly packed, the depositional texture is modified, and late aqueous cement occupies smaller interstices.
The following characteristics and stages of intensity of vadose cementation can be distinguished.
1. Initial stage:
The original depositional texture is lost as precipitation of fine-grained evaporitic films of carbonate, iron oxide, or aluminum hydroxide forces the grains apart so that films separate grains at former contact points. Sedimentary structures such as beach laminae and current ripple cross-bedding still may be preserved in this stage.
2. Intermediate stage:
"Floating" textures are produced as grains become pushed apart by evaporite cement and by replacement of quartz and feldspar by carbonate. Coalescence of evaporite films around clusters of grains forms small pisolitic concretions. Sedimentary structures become obliterated or greatly obscured.
3. Late stage:
Extreme floating textures and large composite pisolites form by addition of more cement and replacement of quartz and feldspar. Brecciated caliche anticlines (also known as teepee structures) are formed by expansion resulting from the addition of large volumes of carbonate cement.
Extreme caution must be employed in interpreting the genetic significance of rocks classified as sandy micrite, wackestone, and boundstone or biolithite. Transformation of quartzose sand into sandy carbonate, consisting of up to 90% fine-grained carbonate (micrite) which commonly is pisolitic (strongly resembling algal boundstone), is well documented by numerous examples. Similarly, accumulations of ooliths, clasts, skeletal fragments, pellets, and calcarenaceous material may be transformed into rocks which could be identified as oomicrite, intramicrite, biomicrite, pelmicrite, wackestone, and boundstone or biolithite (pisolitic) by vadose processes (calichefication).
Many so-called oolitic iron ores represent vadose concentrations of iron oxide and hydroxide.
End_of_Article - Last_Page 208------------