Chemical Sedimentary rocks
Chemical sedimentary rocks include:
- Iron-rich rocks
Form from dissolved constituents in water (fresh and salt.)
Iron-rich sedimentary rocks:
Rocks with iron content >15%. Recall two main oxidation states of iron:
- Fe2+ (ferrous)
- Fe3+ (ferric)
The deposition of iron-rich rocks is facilitated by the presence of O2.
Banded iron formation
During the early Archean, O2 was present only in trace quantities, so ferrous iron could exist in solution in the oceans. Indeed, because the contemporary atmosphere was rich in CO2, fresh and ocean waters were probably more acidic, facilitating the release of iron through weathering.
Beginning ~3.0 ga, photosynthesizing cyanobacteria began releasing O2 into the oceans. At the same time, we begin to see BIFs - finely interbedded cherts and iron-rich mudrocks. These took the form of:
- Algoma-type BIFs: Archean - Relatively thin ribbons of banded iron that may have formed adjacent to particularly rich hydrothermal sources of dissolved iron.
- Superior-type BIFs: Proterozoic - Thick broad deposits of shallow-marine BIFs.
Bar River Formation red-bed sample
Layered sylvite and halite - Permian near Carlsbad, NM
- Calcite and aragonite CaCO3
- Gypsum and anhydrite CaSO4
- Halite NaCl (right - white)
- Sylvite KCl (right - pink)
- "Bittern salts" Various soluble trace substances, mostly bitter tasting borates and nitrates).
Observed in three types of environment:
- Playas: Ephemeral lake beds in hot arid regions.
Castile Formation - Delaware Basin near Carlsbad, NM
- Deep oceanic basins: Under unusual circumstances, peripheral ocean basins can be isolated from general ocean waters. This happened to the Mediterranean during the Miocene, and to the Delaware Basin of west Texas during the Permian. This can result in the deposition of extensive thick evaporites (including the varved Castile Formation of the Delaware Basin).
- Sabkha: Flat shorelines in arid regions (like the Persian Gulf - right) experience evaporite deposition through the evaporation of saline groundwater. The result is a complex mixture of:
- shoreline sediments
- clastic sediment from the land.
Playa - western NV
Nonepiclastic sedimentary rocks:
Epiplastic means formed from the weathering of preexisting rocks. Other "orphan" rocks that are not formed from weathering and lithification, biochemical production, or chemical precipitation, but which display some sedimentary aspects:
Welded tuff, Chiricahua National Monument, AZ
Solution collapse breccia