Reefs Through Time & Space
Steven Schafersman (Department of Geology, Miami University, Oxford, Ohio, USA)
Dry Dredgers meeting (Cincinnati, Ohio, USA)
23 October 1998
Many ancient reefs were not just composed of corals or weren’t composed of corals at all.
Schafersman’s advisor at Rice University was Wilson of “Wilson’s carbonate facies model”.
Reefs are usually called organic/carbonate buildups.
Geologists usually called “reefs” structures that are mounds or buildups in a stratigraphic sense - with some topographic height. “Reef” has many different meanings: an obstacle to navigation was the original meaning. “Reef” in an ecologic sense means an organic buildup.
Many terms are available for specifying types of buildups - “banks” vs. “buildups” vs. “reef” vs. “bioherm”, etc. Buildups are a mix of loose skeletal material & lime mud.
Hydrodynamic rearrangement is an important component to the structure of reefs/buildups. For the Phanerozoic, the reef play is the same, but the characters are different through time. Reefs seem to always be in ~ the same place, and are always are ~mound-shaped.
For the Devonian, stromatoporoids are important, but in the late Paleozoic, get forams being important. Always had algae as an important component. Modern corals are reef builders only from the Mesozoic onward. In the Paleozoic, byrozoans, brachiopods, sponges, and echinoderms are important builders of reefs. Had tabulate corals making reefs in the Paleozoic.
Components of buildups have changed through time. Today, we’ll focus on Paleozoic & Mesozoic reefs.
The best Cenozoic reefs in the world are not in America.
Where do reefs form? Buildups occur in shallow marine areas. Reefs occur especially on the shelf edge (barrier reef). Also get reefs behind shelf edge (lagoonal), also just below (in front of) the shelf edge, on the ramp. Why do buildups always occur here? The world’s oceans have a prevailing current pattern. Get counterclockwise gyres in the southern hemisphere & counterclockwise gyres in the northern hemisphere. In the past, current patterns had to be the same - caused by prevailing winds. These are surface currents we’re talking about. Get tropical & temperate & polar cells of wind updrafts & downdrafts. Wind directions are controlled by the Coriolis Effect. So, near the equator, have easterly winds (a designation of wind origination direction or upwind direction) & get gyres going westerward along the equator.
Wind direction & ocean current connection: Eckman Spiral - ocean surface currents are not parallel to the wind direction due to the Coriolis Effect. The ocean surface current is 45˚ to the surface wind direction. Downward in the water column, the ocean current direction due to surface winds is at increasing angles to the surface wind direction, but the component due to those winds becomes less and less. The net current direction of the ocean water is 90˚ to the surface wind direction.
Example: the California Current (wind current) extends clockwise from the Alaskan coast, southward along the Canadian and American west coasts. The ocean current direction will be 90˚ to this wind direction. Because the prevailing wind gyres are ~ parallel to the coasts, the water current direction will be ~ directly offshore (perpendicular to coasts).
Relationship of Eckman Effect - water moved away from the coast - deep water replaces the water moved out; the deep water is nutrient rich. Shallow water is nutrient poor - all the plankton live there & consume all the nutrients. Upwelling deep, nutrient-rich waters are thus occurring near continental coasts - get areas of high productivity along the shelf edges. This is why buildups are there. Reefs are rainforests of the ocean - because ther’s lots of nutrients always coming up. This pattern breaks down only during ENSOs (El Niños - El Niño Southern Oscillations). Otherwise, this is always going on.
Why are buildups in shallow water? This is due to the relationship between reef builders and zooxanthellae (photosynthetic) - needs to have sunlight - symbiosis. Zooxanthellae are dinoflagellates [JSJ: not universally true]. Such symbiosis was probably around throughout the Phanerozoic. Reef builder-zooxanthellae relationships occurs in hermatypic corals only (reef builders).
Get large fusulinids making buildups in the Permian. It has been inferred that they were symbiotic. Symbiosis relationships are good for buildups - animals eat algae & zooxanthellae take CO2 waste.
There are some large modern forams the size of nickels.
Cambrian buildups - not spectacular or very big - archaeocyathans.
Ordovician - Laurentia was on the equator. Had mountains (Taconics) to the south (subduction complex formed by Baltica approaching). This wasn’t the perfect area for buildups. Had epicontinental seas behind the Taconics. A ramp came down northward from the Taconics. Lots of carbonate production on the ramp. Get Middle Ordovician buildups in New York of tabulates, brachiopods, stromatoporoids, rugosans, byrozoans, algae. Upper Ordovician - in the Bardstown area, west of Lexington, Kentucky, had a platform setting (the Cincinnati Arch had some expression even then), with a shelf edge to the southwest with upwelling occurring. Get good corals in the Richmondian Stage - usually relatively small, lens-shaped tabulate corals, which make good buildups when tons of them occur together. Schafersman has made an extensive collection from the Bardstown, Kentucky area - no new species, but a good fauna.
Silurian - famous for its so-called reefs. Lots in the Michigan Basin - many of the Silurian reefs are oil reservoirs in the subsurface. Get Michigan Basin outcropping reefs in Indiana, Illinois, Wisconsin. These are not like our reefs - rugosans, tabulates, sponges, bryozoans - no scleractinians. Facies are rather different (different components in the facies). The Thornton Reef (south of Chicago) is very famous - can see an ecologic zonation. Why do reefs have such porosity and permeability as seen in the subsurface oil reservoirs & in the surface outcrops? First of all, these reefs have lots of primary porosity because of the nature of buildups, originally. Also, when sea level falls, leaching in the vadose zone (exposed to air) causes great porosity to form. The Georgetown Reef (Indiana), on the Wabash River, has stromatoporoids, tabulates, rugosans, etc. This was never a cemented framework - a buildup of productive organisms. Some cementation occurred after death - but no living cemented framework. Probably didn’t grow up to sea level.
Devonian reefs - Canada & Australia have the best examples. Main critters are stromatoporoids, tabulates, rugosans, and sponges. In the subsurface of Canada & Australia, Devonian reefs produce lots of hydrocarbons (note Calgary, Alberta, Canada - an oil town over subsurface Devonian reefs).
Mississippian buildups - Waulsortian Mounds - mudmounds. These were originally defined in Ireland, Belgium, and Great Britain. Get lime mud with a surrounding ring of crinoidal debris. This results in limestone surrounding mud mounds. In America, we get them too - first noticed in the New Mexico Sacramento Mountains - they have fossils, but it is still overall a mud mound. Typical explanation for Waulsortian Mounds/mudmounds - crinoids with attached fenestrates trapped lime mud by baffling - mud builds up. Lime mud production was by algae, probably. Weak currents transported mud, but collected around the crinoids with encrusted fenestrate byrozoans. Lake Cumberland area, south-central Kentucky... Waulsortian facies are Lower Mississippian (Osagean Stage). They are above the Devonian Chattanooga Shale. They are the same age as the Waulsortian mounds of England & New Mexico, but are slightly different. Interpretation of Lake Cumberland mounds - in a ramp setting, not on a shelf edge proper. They occur in deeper water (not extending upward to sea level, but are in sunlight, certainly. Have wackestone (crinoidal) buildups over a core of mud (not lime mud, as in Waulsortian mounds). The mud core is a greenish, terrigenous mud, with fenestrates, crinoids, etc. Mounds are beautifully exposed along the lake. Wackestone is thin atop the green shale core & is thicker laterally. What caused the green shale mounds, originally? Tilted beds were original - not tilted afterward. They are the original depositional angles. Dave Meyer & colleagues recognized 4 types of facies. Crinoidal limestones are over green shale mounds that were already there. Beautiful crinoid fossils - stems & heads.
Pennsylvanian buildups - Texas & New Mexico have lots of these. Wherever you have basins, you have edges with buildups. Famous Horseshoe Reef in Texas. Large fusulinid forams are very important in these buildups. Very visible algae. Sponges and brachiopods are main builders. Some of the reef-building brachiopods are flattened for light penetration for their symbiotic zooxanthellae. Many Pennsylvanian reefs are loaded with oil. The Paradox Basin in the Four Corners area has buildups of organisms other than corals - fusulinids, brachiopods, algae. Most of the porosity in these Pennsylvanian oil-bearing reefs is from subaerial exposure.
Permian reefs - West Texas is famous for its Permian reefs, in the Permian Basin. Carlsbad Caverns in New Mexico is developed in this. The Guadalupe Mountains (National Park) show these - has a well studied reef. El Capitan is a famous peak there where the reef core is - has a beautiful exposure of a Permian reef. Builders: sponges, forams, algae, bryos., brachiopods, a few Late Paleozoic tabulate corals. Highest energy facies are fusulinid grainstones. At the end of the Permian - lots of reef builders die out. The new Mesozoic builders: scleractinian corals, probably evolved from a previously soft-bodied cnidarian. In the Mesozoic, scleractinians were part of the reef component. In the Cenozoic, scleractinians are the reef component.
Triassic buildups - occur in the Bavarian Alps, in southern Germany, where beautiful Triassic reefs are found with sponges, lots of algae, corals, and various forams.
Jurassic buildups - lots occur in Europe. Components of Jurassic reefs: sponges, scleractinian corals, algae, forams. Very much in the Paleozoic model - not real frameworks - but getting closer. Getting closer to cemented frameworks, rather than buildups of high productivity.
Cretaceous buildups - Dominant organic builders were pelecypods - were only secondary organisms before the rudists appear on the scene. Rudists built extremely impressive reefs. Enormous rudist barrier reef occur along the eastern and southern coast of North America, which outcrops nicely in Texas & Mexico. A little oil occurs in these in southern USA. The Cretaceous of Texas is fantastic for fossil collecting. The Early and mid-Cretaceous have reefs of certain rudists (Toucasia & Monopleura). Later, caprinid & radiolitid rudists dominate. Other organisms occurred in Cretaceous reefs, too, but were minor components. Rudists first appear in the latest Jurassic, then exploded in the Cretaceous. Radiolitids dominated in the Late Cretaceous. All rudists are gone at the K-T extinction. When rudists went, it allowed scleractinian reefs to dominate in the Tertiary & Quaternary. Rudists were thick-shelled, and had zooxanthellae. Enormous Cretaceous rudist reefs occur along the eastern & southern margins of the USA.
Today - a few non-scleractinian coral reefs occur. These include oyster reefs and worm tube reefs, but these are generally uncommon. “Lithoherms” - deep buildups, sometimes associated with ahermatypic corals - probably associated with methane seeps.