Biogenic sedimentary rocks



Coal seam in Pottsville Fm. - Curwensville, PA.

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:

In this lecture, we consider non-carbonate biogenic rocks only, leaving the complex topic of carbonates for later.



Diatomaceous earth outcrop - Santa Cruz Island, CA.

Cherts

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.

Silica geochemistry:

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.

This silica then preprecipitates as microcrystalline quartz - A.k.a. chert. Cherts can assume a variety of colors depending on impurities, from black flint to white novaculite.

We see chert in two forms:


Phosphorites


Phosphorite
Rocks containing >20% P2O5. Typically results in > 50% phosphate mineral content. (I.e. contain PO43- ion). The phosphate is derived from: Common phosphate minerals include:

Q: Why do we regard this as biogenic?

Because phosphorus is an essential component in organisms, including in:



Guano covers Mariela Islet, Galapagos Islands, Ecuador
Thus, living things tend to concentrate phosphate ions. They therefore concentrate it in the rock record in environments such as:

Carbonate rocks and mudrocks are typically hosts to phosphate nodules.

Coal


Coal seam from National Park Service
Formation:

Coal ranks:
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:

Temporal distribution: there were two major coal forming ages:

Global Distribution: Bituminous and anthracite coal tends to concentrate in regions that, during the Carboniferous, were: These conditions were obviously conducive to profuse plant growth, and coals have formed from the Carboniferous onward. But, why was the Carboniferous specifically such a good time for coal formation? Ironically, the fact that it was an ice-age seems to have helped. Many Carboniferous coal seams are associated with cyclothems, rhythmically repeating vertically stacked sedimentary sequences. In these, the rocks deposited on top of the coal represent increasingly deep marine environments. Apparently, the continental shelf was repeatedly exposed, allowing the growth of dense forests (right) in the tropics, then inundated by transgressions which quickly buried the plant material. Exactly what one would expect from eustatic sea level change during cycles of glacials and interglacials. One wonders what kind of coal deposits are being developed from the forests that covered regions like south Florida or the Sunda Shelf of southeast Asia during Quaternary glacial lowstands. Note: Not all coals formed this way!

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:



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:

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 - impervious layer that provides a seal and prevents further migration

Oil shale


Oil shale

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.