Chemical and biogenic rocks
I. Biogenic sedimentary rocks:
Rocks that form as a result of biologic processes - i.e. rocks made of organismal remains. These can be unaltered, or diagenetically altered to varying degrees:
- Most calcium carbonates (limestone and dolostone)
- Silicates (chert),
- Phosphatic rocks,
- Plant remains (coal)
- Marine organics (oil shale and petroleum)
In this lecture, we consider non-carbonate biogenic rocks only, leaving the complex topic of carbonates for later.
Usually formed through the dissolution and reprecipitation of silica oozes. Recall that the most common sources of such oozes are diatoms and radiolarians. Such oozes typically are concentrated in the sediments of the abyssal plains, where few other clasts are deposited.
Q: Crystalline silica (quartz, chert) is fairly insoluble in water of pH<11. Where, then, does the dissolved silica that the critters use to make their skeletons come from?
A: It is originates with the dissolution of amorphous silica - glassy silica with no large-scale crystalline structure. This is produced copiously by the hydrolysis of feldspars.
In the Phanerozoic world, virtually all dissolved silica in the ocean is immediately grabbed by diatoms and radiolarians to produce their opaline skeletons. Note: Opal is hydrated amorphous silica. Thus, it, too, can dissolve readily when the skeletons are deposited as oozes.
We see chert in two forms:
- Bedded (primary) cherts: Individual layers or laminae that form where the oozes were deposited. These can be:
- Fossiliferous: Still contain some obvious fossils of the skeletons making up the ooze
- Nonfossiliferous: Lacking conspicuous skeletal remains (although these are usually observed through technical means.)
- Nodular (replacement) cherts: Spherical or ovoidal silica masses that form in a host rock (usually shallow-marine carbonate) from reprecipitation of dissolved silica that has migrated from its source. Dissolved silica replaces preexisting structures such as fecal pellets. Indeed, sedimentary structures in the host rock are often visible in the nodule.
Rocks containing >20% P2O5. Typically results in > 50% phosphate mineral content. (I.e. contain PO43- ion). The phosphate is derived from:
- Hydrothermal veins
- dissolution of phosphate minerals like fluorapatite in igneous and metamorphic rocks.
- Fluorapatite (Ca5(PO4)3F) - the most common overall
- Hydroxyapatite (Ca5(PO4)3OH - the mineral component of vertebrate bones and teeth.
Q: Why do we regard this as biogenic?
Because phosphorus is an essential component in organisms, including in:
- Nucleic acids, RNA and DNA
- The phospholipids of cell membranes
- Adenosine triphosphate (ATP) and Adenosine diphosphate (ATP), the energy currency of living cells.
Thus, living things tend to concentrate phosphate ions. They therefore concentrate it in the rock record in environments such as:
- Concentrations of nodular phosphorites on the outer margins of continental shelves caused by blooms of biological productivity when upwelling deep ocean water enters the photic zone, yielding algal blooms that cause anoxia and mass mortality. (E.G, Red tides of dinoflagelates.)
- Bone beds: Vertebrate bone concentrated by hydraulic action.
- Phosphatization: Mobilization of phosphate rich fluids leached from fecal matter (guano) or carcasses are concentrated and reprecipitated.
Carbonate rocks and mudrocks are typically hosts to phosphate nodules.
- Plants (mostly continental) grow so profusely that when they die they are quickly buried and sealed off from decomposers and oxygen. This can be from burial by later plants or by sediments.
- Buried plant material is exposed to heat and pressure, driving off volatile and soluble components leaving residual carbon.
Carbonized plant remains span a continuum from simple compressed plant material to graphite, which is pure carbon. Grades of transformation correspond to everything from simple diagenesis to high-grade metamorphism:
- Peat: Porous compressed plant material in which parts can be distinguished. ~50% C.
- Lignite: Soft brownish ~70% C.
- Greater depth of burial and temperature ==> sub-bituminous - bituminous coal. Flat black and soft. ~85% C.
- Ultimately anthracite or hard coal. 95% C. Typically found only in folded rocks that have undergone regional metamorphism.
Temporal distribution: there were two major coal forming ages:
- Carboniferous -> most bituminous and anthracite
- Cretaceous. -> Lignite and subbituminous.
Global Distribution: Bituminous and anthracite coal tends to concentrate in regions that, during the Carboniferous, were:
Petroleum and natural gas
The deep oceans receive a constant rain of sapropel - giblets of organic material raining from the upper layers. Researchers with submersibles refer to this as "marine snow." (Right). The incorporation of this material in oceanic sediment is the source of petroleum. Tends to occur in areas of great biological productivity, rapid deposition in anoxic conditions:
- Continental Shelves
- Some river deltas
- Inland seas
Buried, heated, and compressed, organic material is transformed into kerogen - a mixture of hydrocarbons - (chain or ring-like molecules consisting of carbon and hydrogen). Further heating yields petroleum or natural gas (primarily methane CH4), depending on the temperature and duration of heating:
- Oil window: 60 - 160 deg. C
- Gas window: 150 - 200 deg. C
Note, petroleum tends to originate in relatively non-porous mudstones - source rocks- and migrate, under pressure, to porous and permeable reservoir rocks. Ancient reefs, with their extensive pore space, are especially effective reservoir rocks, although any porous and permeable rock will do. Migration occurs due to lower density of oil and natural gas. (Oil floats on water, the other ubiquitous pore fluid.)
Petroleum traps: If petroleum reaches the surface, its volatile ingredients evaporate, leaving tar. To be useful, it must be captured underground. This is facilitated by traps formed by impermeable seal rocks. impervious layer that provides a seal and prevents further migration
Migration occurs due to lower density of oil and natural gas Trap Ð impervious layer that provides a seal and prevents further migration
- Stratigraphic traps: Petroleum pools up beneath an impermeable seal stratum.
- Structural traps:
- Fault trap: Displacement along a fault creates a pocket in which petroleum is trapped.
- Anticlinal trap: Petroleum pools up beneath the axis of an anticline in a seal rock.
- Salt structure trap: Rising bodies of salt deform adjacent impermeable rocks, creating traps.
- Fault trap: Displacement along a fault creates a pocket in which petroleum is trapped.
What would happen if buried organic material never matured into petroleum and migrated away? We get oil shale. Shale (usually) rich in organic material (5-50% hydrocarbons - kerogen or petroleum). These can be burned directly or hydrocarbons can be extracted. Oil shales typically seem to originate in disaeobic basins (including lakes), often showing varves - seasonal laminations. Of local interest, the Devonian Marcellus shale that formed in deep waters of the Appalachian Basin, a foreland basin that received sediment from the Acadian Mts. during the Acadian Orogeny. Now the focus of the lucrative and controversial process of hydrolic fracturing.
Chemical sedimentary rocks
- 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.
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:
- 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.
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.
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:
- 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.
- 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.
Nonepiclastic sedimentary rocks:
Epiplastic means formed from the weathering of preexisting rocks. Other sedimentary rocks that are not formed from weathering and lithification, biochemical production, or chemical precipitation:
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: Breccias formed by the grinding of rock in fault planes.
Collapse or solution breccias: Breccias formed through the collapse of cavities. Often from the solution of soluble materials (calcite, anhydrite, etc.)
Impact or fallback breccias: Rock pulverized by meteorite impacts and deposited as crater ejecta. Rare but interesting when you find it.