Page 310 - Carbonate Facies in Geologic History

Lime Mud-Sabkha Cycles 297

marine diagenesis. In areas of intense periodic dryness carbonate sediment of all
textures may develop fenestral fabric. Intertidal desiccation pans may develop
evaporites, dolomite, and iron carbonate crusts and blackened breccia form
around saline ponds.
In humid climates previously formed salina evaporites are commonly dis-
solved after subaerial exposure and form laterally extensive breccia. Intense rain-
fall results in microkarst and regoliths of red soil and collapse solution breccia in
deep fissures. In semiarid climates with seasonal precipitation caliche surfaces
may occur with breccia, pseudopeloids, vadose pisoids, reverse grading down-
ward to micritic fabric, and downward convex crinkly laminae. Bacterial action
may induce thorough micritization along these surfaces just as in marine hard
grounds. Caliche micrite crusts may form on rootlets, root hairs, and algal and
fungal tubules through continuous evaporation and transpiration.
Most of the above sedimentary features have been discussed in the glossary
section of Chapter III.

Summary

All the above cycles and many similar examples have the following characteristics
in common:
1. They generally occur on the parts of wide shelves marginal to basins. The
prominent grainstone member must result from seaward progradation of the
shelf margin environment (belt 6).
2. Well-developed oolite particles and coated, worn bioclastic grainstone domi-
nate. The well-formed ooids are, by analogy with Holocene sediments, indica-
tive of strong and regular tidal currents.
3. The cycle may grade upward and shelfward to restricted marine facies but in
places a hard ground surface of non-deposition directly overlies the grain-
stone~i.e., a surface oflag deposition over a long time-period.
4. When restricted marine facies do occur at the top of the cycle they tend to be
thick-bedded lagoonal muds with local hard grounds rather than thin-bedded
intertidal and sabkha deposits.
5. No reefs are prominent on the seaward margin of the shelves. Such oolite
cycles do not form behind a well-developed barrier reef which would restrict
too much the flow of tidal currents.
6. Crinoidallimestone in plac~s is important on the seaward side of the shelf. This
is particularly true in Mississippian and Jurassic strata.
7. The cycles commonly constitute part of a megasequence which shoals upward;
the cycles multiply and also become individually thinner and more restricted
marine in character higher in the section.

Lime Mud-Sabkha Cycles

A second type of upward shoaling cycle is composed mostly of micritic sediment
whose faunas and sedimentary structures show a progressive upward change
through restricted marine carbonates to laminated evaporites of sabkha origin. In