Chemical Sedimentary rocks

Outcrop du jour

Chemical sedimentary rocks include:

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+ is soluble in water, but is rapidly oxidized to Fe3+ in the presence of oxygen. Fe3+ is insoluble. It flocculates and settles out. Thus:

The deposition of iron-rich rocks is facilitated by the presence of O2.


Banded iron formation
Precambrian banded iron formations (BIFs):

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:

The exact mechanism of deposition is enigmatic, but seems related to scavenging of ferrous iron by free oxygen in the oceans. Note: The iron constituted an oxygen sink, binding oxygen into oceanic sediment until they were saturated at ~1.8 ga.


Bar River Formation red-bed sample
Phanerozoic ironstones: After oceanic oxygen sinks were saturated, O2 could begin to accumulate in the atmosphere, allowing the rise of iron oxide minerals like hematite and goethite in terrestrial environments (starting in early Proterozoic, right). Major iron mineral deposits are shallow marine, in which these minerals form ooids (sand-sized concretions) around a mineral nucleus.

Evaporites


Layered sylvite and halite - Permian near Carlsbad, NM
Rocks made of minerals that precipitate from hypersaline solutions. If we evaporate sea water, we see a regular sequence on the precipitation of minerals, from least to most soluble:

Observed in three types of environment:


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
Volcanogenic: Often called "volcaniclastic." Rocks formed from fragments of volcanic material behaving like clasts. E.G.: Welded tuff (right - Note distinct lapilli) or volcanic breccia.


Cataclastic breccia
Cataclastic: Breccias formed by the grinding of rock in fault planes.


Solution collapse breccia
Collapse or solution breccias: Breccias formed through the collapse of cavities. Often from the solution of soluble materials (calcite, anhydrite, etc.)


Impact breccia
Impact or fallback breccias: Rock pulverized by meteorite impacts and deposited as crater ejecta. Rare but interesting when you find it.