11Introduction to carbonate sedimentology

On a cold day along the Niagara escarpment, you have to imagine a warm, Silurian carbonate sea, like the Grand Bahama Bank spread across much of North America.

22The majority of carbonate sediments are produced by organisms in shallow, warm, [normal] marine waters.

Oolite shoals [white, carbonate mud banks [darker] and reeflike structures [not shown]. Aerial view in the Caribbean [I am not sure where I wasmaybe over Cuba?]

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What about deep-water carbonates? [They are mostly produced by planktonic organisms in the photic zone and then fall to the deeps]

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In other words, carbonate production is basically organic, and readily influenced by water chemistry at low temperatures.

When conditions are right, carbonate accumulation can happen very fast. Thus, shallow-water carbonate producers can commonly build themselves out of existence [this would be more properly called ecological succession]

Another modern carbonate settingthe Great Barrier Reef. Why did James Lee Wilson liken carbonate sedimentation to a cadillac with a defective carburetor?

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Some of the classic sites of modern carbonate sedimentology studies are Florida Bay and the Bahamas. Some of the worlds nicest vacation spots

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The Bahames is a humid tropical setting. Why do carbonates do well in warm shallow waters, away from clastic influx? [lecture covers this]

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Small variations in water depths in shallow carbonate settings can generate facies differences. Dark green are deeper channels, white stripes are carbonate sand dunes [subaqueous], and tan to brown zones are carbonate mud with grasses and calcareous algae. What are other controls besides water depth on these different subenvironments?

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Although there are many controls on carbonate deposition, quite a few carbonates are redeposited by physical currents the bed forms in this view would be an example. What rock would you find if these dunes were cemented?

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Cross-bedded carbonate sands sand made of oolites, forams, and skeletal particles. This is the famous Salem or Indiana Limestone of Mississippian age. It is extensively quarried in Indiana and used as a dimension stone, especially in the eastern U.S. This is the doorway of my apartment building in NYC [photo from 1975].

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Heres another example of carbonate sand cross-bedding [labeled as cross-bedded oolitic grainstone photo from Eric Cheney], in a core. What factors must be considered when examining carbonates [even physically redeposited ones] that typically are not an issue in siliciclastics?

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These cross-bedded carbonate sands are eolian [wind-blown] in origin. In the Bahamas [this picture is from San Salvador], carbonate beach sands [Bahamas have ONLY carbonate sand] have been blown into eolian dunes, and then cemented. These are Pleistocene in age.

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12Oolitic shoals near a carbonate bank edge

In air photos, the clean oolitic and skeletal grains appear white. They tend to spill over onto a platform, near its edge. What about this photo tells you that physical circulation and transport is a control on oolite shoals?

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13Interior carbonate bankpelleted skeletal muds

Toward the interior of a bank, the environment is muddy, and the resulting sediment is a pelleted skeletal mud. Note the little white delta at the bankward end of a spillover channel. Darker patches are cloud shadows!

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Penicillus

Halimeda

Udotea

Clypeaster [an echinoid]

a coralline red alga

[Neo]goniolithon

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Porites,a coral

Calcareous green [codiacean] algae

These are some of the organisms that live in and end up making up the sediments in a pelleted skeletal mud. Upper row to the left are Halimeda, a coralline green algae. Below them is Penicillus, the shaving brush coralline green algae, and Udotea, another common calcareous green alga. These genera are among the most prolific carbonate mud producers on modern carbonate banks. How do you end up with mud out of these guys?

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Porites and turtle grass; goniolithon also present, and other organisms amongst the larger elements. Grasses [turtle grass] and coralline organisms, as well as gastropods and other creatures, are common in the interior of carbonate banks. Some forams and other encrusters attach to the grass blades.

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16Localized buildups of sediments themselves often alter the character of surrounding sedimentary environments.

This is a carbonate mud mound in Florida Bay. Its made of grasses, skeletals, and carbonate mud. Note people for scale. The mound is not solid, when you try to walk into/onto it, you sink [I sank] down into scratchy [shelly] carbonate mud and associated grasses and fauna.

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Yellow zones are mud mounds and other shallow but still wet zones.

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A mud mound will built up until it becomes shallow enough for mangroves to colonize [mangroves, though, are only Cenozoic]. These are red mangroves, the pioneers in Florida Bay. Characteristic are their prop roots.

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Black mangroves occupy drier ground but are still tolerant of salt water and common flooding. Characteristic are their pneumatophores [roots that turn around and come back out at the surface to breathe]

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Once the carbonate bank emerges to a subaerial environment, evidence of desiccation is also present.

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The black crinkly mats are [photosynthetic] cyanobacteria, sleeping until they are once again wetted, when they will become green.

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Here are Devonian laminated mudcracks, made of cyanobacterial laminae. Manlius Limestone, Bossardville Quarry, PA; my mom for scale!

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Rip-up clasts [carbonate mud intraclasts] are common in carbonate facies, because carbonates commonly form in shallow to intertidal/supratidal environments, and storms rip up the mud-cracked clasts and redeposit them. Ordovician Cool Creek Fm., Oklahoma.

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What happens when cyanobacterial mats get buried?

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Here is a Silurian limestone made predominantly of cyanobacterial-mat laminae. The crinkly/bumpy nature of the laminae is a strong clue of their origin.

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Because cyanobacteria are photosynthic, they thrive on bumps. Because they are sticky, they trap [carbonate or other] mud, and the bumps grow upward. We call these laminated forms stromatolites.

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Here are some Devonian examples, Manlius Limestone, New York Statefairly subtle bumps.

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There are the most famous modern examples, in Shark Bay, western Australia.

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Ordovician digitate [finger-like] stromatolites; Cool Creek Formation, Oklahoma.

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More examples, Cool Creek Formation, Oklahoma.

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Some higher-relief stromatolites, Cool Creek Formation, Ordovician, Oklahoma.

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Fenestral[birdseye]fabric

Soback to cyanobacterial-mat lamination. What are all the holes in this rock? [Silurian, Iowa]

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When the organic mats decay, they generate gas, and the sediments are sticky, so the gas gets stuck in pockets. They generate different shapes. In carbonates, this is called fenestral, or birdseye fabric. [fenestrae means windows in latin]. This is a peanut butter jar I turned upside down. Note the evidence of which way is upthe large void half-filled with peanut oil [such a structure in a carbonate would be called a geopetal structure because it tells you which way the earths center was]

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Oncolite[algal biscuit]Actually, mobile cyanobacterial agglomeration

Here is some fenetral fabric in the Ordovician McLish Formation [Oklahoma]. Grain is this photo include pellets, other carbonate mud clasts, and oncolites [cyanobacterial-laminated clasts that rolled aroundmobile stromatolitesalso called algal biscuits].

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35Look for skeletals, non-skeletals, fenestral fabric, other porosity types, cements good example of sparry calcite

Another outcrop of McLish Limestone with fenetral fabric, various carbonate mud clasts, and some skeletal elements, esp. large bivalve shells. Do you see any evidence of shelter porosity? Try to identify different particles and fabric elements.

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fenestral fabric, possibly moldic porosity, some cement lining of pores

carbonate mudintraclasts

erosion or dissolution

This core was labeled limestone offshore shelf facies by the original author [photo by Eric Cheney, but not the interpretation]. HOWEVER, mud intraclasts and fenestral fabric are characteristic of the intertidal to supratidal zone. Soa misinterpretation we can now correct.

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37Carbonates are commonly [contemporaneously] cemented on the sea floor

Carbonates are subject to contemporaneous cementation, either in the subaqueous or subaerial zone. This is Holocene beachrock [recently cemented carbonate sand] on San Salvador Island in the Bahamas.

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This surface is an exposed [in a quarry] Cretaceous hardground a surface cemented at that time. It was shallow subaqueous at or shortly after the hardground formed.

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How do we know that? The hardground is encrusted with oysters, and shows also boring structures [holes made where creatures bored into the hard rock].

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Ordovician paleo-hardgroundsOrdovician paleo-hardgrounds

The dark, squiggly lines here are Ordovician hardgrounds, exposed in a quarry in Wisconsin. They are pretty subtleI wouldnt have noticed them, but a fellow grad student was doing his Ph.D. on them!

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Localized buildups of sediments themselves often alter the character of surrounding sedimentary environments.

Because carbonates are produced in shallow water and build upward, they are commonly subjected to exposure [due to small sea level fluctuations], resulting in intensive dissolution, cementation and recrystallization possibly many generations of such diagenesis.

Because carbonate-producers thrive in shallow environments, they can build themselves right up to sea level [and then get occupied by mangroves and other organisms such as tourists]. This is Crane Key in Florida Bay.

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Map showing location of Crane Key. Brown areas are always above water [except in hurricanes]

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Exposed carbonate are subject to weathering and dissoluation, of course. [this is Bahia Honda, in the Florida Keys]

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Heres a fossil [Quaternary] weathered surface on San Salvador, Bahamas. It is mini-karstic in nature. Hammer at extreme left.

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Heres another paleo-soil-karst surface in the Quaternary of San Salvador, Bahamas. [the reddish horizon]

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46Sessile, benthonic organisms who need to live in shallow, agitated waters must have strategies for survival

Back to the underwater world. Lets talk about how organisms manage to thrive in shallow, agitated water, which is where carbonate-generating organisms [especially those who are sessile, i.e., non-moving, attached], must live to thrive.

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Dott and Batten

Why do these organisms live where they do? [lecture material]

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Barrier reefs, at the outer edge of the platform, are a great place to live if you can take the agitation.

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Spur-and-groove structure is a passive response of corals to the wave action near the edge of a platform. Why does this help the coral withstand breaking wave action?

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Strategies for the high wave-active zone include fast growth, as well as strong framework structure. Acropora palmata is very successful in modern barrier reefs.

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This is somewhat more delicate Acropora palmata more delicate forms may break more easily but grow faster. So they can recover from breakage.

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Those pictures were from the modern reefs off the Florida Keys [marked in red at the lower right]. The Florida Keys themselves are at their core a Pleisteocene barrier reef.

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Here is an exposure of the Key Largo Limestone, showing the coral-reef framework structure [wed call this a coral framestone].

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Behind the barrier reef are patch reefs; they can include more sensitive creatures.

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The dark zones just offshore here, on the more protected side of San Salvador Island, Bahamas, are patch reefs.

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In patch reefs, you can see more delicate coral forms the most prominent here is Acropora cervicornis.

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And other things like Millepora, a stinging coralline organism. Ouch. Note the brain coral. Brain coral is very strong, but its risky for brain coral to live near the reef edge, because it can fall off.

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In the modern world, corals are the big reef builders, but in geologic history, many other organisms have made wave-resistant, reef-like structures. These include, e.g., the rudistid bivalves of the Cretaceous.

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The mounding in the right side of this outcrop is a rudistid bioherm, Cretaceous of Texas [note bluebells in left foreground].

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Here are some rudistids [and bluebells] up close.

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61Be flexible, if you can

Another strategy for surviving in shallow agitated water [though not so good for breaking waves, necessarily] is flexibility, which is practiced, e.g., by the octocorals, or sea fans and sea whips [e.g.]

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Its also practiced by coralline red algae, which youll see in lab and you can find in almost any rocky tide pool setting on the coast of Washington or Oregon [and elsewhere].

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Here is a fossil bioherm from the Pennsylvanian in Kansas.

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The primary skeletal element in this bioherm is phylloid or potato-chip [coralline] algae. These potato chips were leaves on a stem. How did such a delicate form survive in shallow, agitated water?

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Many organisms take advantage of baffling living together in dense patches, hence increasing the roughness [z-nought] and thereby decreasing the boundary shear stress on their bed

If you live in storm country, would you rather be out on the plains, or deep in the forest?

Back in Florida Bay, a possible modern analogue [though without potato chip algae] the mud mounds which comprise skeletals, sea grass and mud. Why does sediment accumulate here, rather than just getting washed away?

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Heres a view of some of the sea grasses and coralline forms found in the Florida Bay region, but this photo is actually from the seaward side of the Keys, rather than the baysideit was too muddy, and we were too traumatized by our slog, to take photos in the mud mound. We sank into it beyond our knees, and it [the mud mound] was scratchy with skeletal and grassy elements in the mud.

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67Attach yourself to others, or to something stable

Another strategy for survival in agitated waters [well, this is at low tide, in Clallam Bay]. Attachment, and grouping by mussels [or oysters, e.g.]. Why does this work?

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Here is a fossil oyster bank, Cretaceous walnut shell bed Edwards Limstone, Texas. What strategy did these oysters have? [can you tell from the outcrop? Or have you seen modern oyster banks?

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These are fossil oysters attached to a fossil hardground, also of Cretaceous age.

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Keep a low profile

These coralline red algae are encrusted on rocks. Pretty good strategy for the Washington Pacific coast with very strong breaking waves.

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[more] words about carbonate constituents

SkeletalsLime mudNon skeletals

peloidsoolitespisolitesintraclastscoated grains, micritized skeletals, grapestone

Porosity [see handout]primarysecondary

Cement [any pore-filling CaCO3]

See lecture and lab materials

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A few thin sectionsThis is a colonial creaturedepending on the scale, it could be a coral or a bryozoan. The skeletal part is clearer, the interskeletal voids are filled with carbonate mud.

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Quartz grains are gray, carbonates are yellow-tan-brown. Carbonate-coated grains, and broken coatings. Cement between grains is also carbonate.

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Globogerinid forams in carbonate mud. Former porosity inside forams is filled with carbonate cement.

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Modern globogerinid foram, alive and dead.

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Some skeletal elements in mostly carbonate mud. Probably a pelagic limestone.

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Carbonate mud commonly ends up as various sizes of pelletsyeah, mostly fecal pellets, though crabs may make pellets just to keep the mud from being muddy

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Another carbonate grain typepisolites [Permian, West Texas].

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More pisolites. I dont have a picture of oolites, because they are too small for my camera.

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Another fabric element of carbonatesporosityso many different types, intraskeletal, interparticle, moldic [from solution], shelter, [see handout]. These are molds of evaporites, Permian, West Texas.

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Porosity [in this case, it was mostly interparticle] commonly gets filled with cement.