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.