Comparative Analysis of Ancient and Modern Carbonate Deposition
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The Delaware Basin of New Mexico and Texas is one of the most studied ancient shallow marine carbonate environments in the United States. Geologists from around the world visit the Permian Basin of West Texas and southern New Mexico to study the unique geology of this region. One of the ways that this ancient reef is analyzed is by looking at similar modern examples of carbonate platforms. One place to study carbonate depositional processes is the carbonate-producing Bahama-Florida region.
The Bahama Platform is one of a chain of sixteen carbonate platforms that stretches for 1500 km from the Little Bahama Bank off the Florida coast to the Navidad Bank near the Dominican Republic in the Caribbean Sea. The Great Bahama Bank is the largest of these platforms. The progradation of three smaller platforms formed the northwestern bank of the Great Bahama Bank. The major control of wave energy in this region comes from the northeasterly trade winds, which dominate wind patterns. Also, climate plays an important role in carbonate production because many important carbonate-producing organisms, such as corals and codiacean green algae, can only exist in the photic zone of warm tropical waters. However, to understand the similarities and variations between the ancient environment of the Delaware Basin and the modern environment of the Bahama Platform, comparisons of the geologic settings, tectonic settings and depositional environments will be made. The facies changes for these carbonate locations are basin, basin margin, slope, reef, immediate backreef, shoal, tidal flat, and evaporative lagoon. Each facies change will be defined, described and contrasted ancient to modern, specifically looking at the differences and similarities between the two.
The geologic history of carbonate sedimentation is important when comparing and contrasting two carbonate environments that not only differ in location but also are separated by 240 million years of geologic time. There are many factors that determine whether carbonates will be deposited, but the two overriding controls are geotectonics and climate. Geotectonics is important in carbonate deposition because lack of clastic input is an important factor for the formation of carbonates, especially those of modern coral reefs. Consequently, carbonates usually form along passive margin systems.
An intracratonic sea once extended over what is now West Texas and southern New Mexico between 260 MA and 248 MA. This Permian sea, spreading over 115,000 square miles, had passive margins and low clastic input that allowed the Basin to become a major carbonate producer. The carbonate platform of the Permian Basin extended out and over preexisting Pennsylvanian limestone. The full extent of the Permian Basin covers and includes the Marathon Basin, the Midland Basin, the Marfa Basin, the Diablo Platform, the Central Basin Platform, the Val Verde, and the Delaware Basins. Deep channels, such as the Hovey Channel, connected and intertwined themselves throughout the edges of these platforms. These carbonate platforms existed throughout the Leonardian, Guadalupian, and Ochoan ages of the Permian period. Deposition of the Capitan Reef Formation also occurred during the Guadalupian and is the focus of this comparison with the Bahama Platform. The Permian Basin, of which the Delaware Basin is part, has the majority of the subaqueous and littoral environments confined to continental shelf margins. However, the Delaware Basin received circulating oceanic water from the south but by the Ochoan age that source of oceanic circulation was closing up. This closure led to the end of the Delaware Basin. In fact, no such abundance of a good modern equivalent of intracratonic basins occurs in the present day.
The Bahama Platform is part of a carbonate depositional setting in which the Straits of Florida, Providence Channel, and Tongue of the Ocean help circulate the warm waters of the Gulf of Mexico with that of the Atlantic Ocean. At this location, the Gulf of Mexico is similar to the intracratonic sea of the Permian Basin in that they are both shallow marine environments. Low clastic input in both settings allows for carbonate growth. However, while the Delaware Basin was sheltered from the open ocean, the Bahama Platform faces the open ocean on one side. According to Tucker and Write (1990), Holocene carbonate sedimentation in the Bahama Platform has been accumulating at rate of 2.3 m/1000 yrs. Nevertheless, the statistics of preservation for ancient carbonates are not promising. The average of ancient carbonate deposition and preservation is between .04-.05 m/1000 yrs., and the Upper Permian strata of West Texas is no exception. At 800 m thick and approximately 240 million years old, its rate of carbonate deposition and preservation calculates to be about .05 m/1000 yrs. Still, carbonate sequences are shallow water sediments deposited close to sea-level, and their sedimentation rates are reflective of the amount of space in the basin itself. Additionally, organisms that live in a carbonate environment are soft-bodied invertebrates; and while these organisms may leave trace fossils, like burrows, no other record of them remains.
Since basin plains are flat and relatively deep, topography is smooth and relief is low. Any topographic irregularities are filled by turbidites. Basin plains are identified using the following criteria: 1) Composition, 2) basin restriction, 3) fill geometry, 4) depth, and 5) sediment supply. The deep basin setting of the Bahama region is similar to the Delaware Basin because of fine- grained lime mud, similar fossil content, like fusilinids, and debris flows. Also, the structure, the depth and slope of both the Delaware Basin and Bahama Region have similar characteristics.
The basin plane of the Delaware Basin is unique because of its mix of terrigionous material with carbonate material. The Basin is restricted and generally shallow. The basinal material of the Guadalupian Delaware Basin is derived from both the Cherry Canyon and the Bell Canyon Formations. According to Grover and Harris (1989), the Cherry Canyon Formation has a basinal siltstone unit containing fusilinids. In addition to the siltstone unit, the Manzanita Member of the Cherry Canyon Formation is an important limestone unit. This member occurs near the top of the unit and is significant in that it can be mapped throughout the Delaware Basin. The Manzanita Member contains Parafusulina and Raserella (fusilinids.)
The Bell Canyon Formation also includes basin margin and is up to 1100 ft. thick. Additionally, the Bell Canyon Formation is time equivalent to the Capitan Reef Formation. The Bell Canyon Formation is dominated by beds of siltstone and very fine sandstone. Four limestone members to the Bell Canyon Formation exist, and these are: The Lamar Limestone Member; the Hegler Limestone Member, which contains Polydiexodina, a species of fusilinid; the Pinery Limestone Member; and the Rader Limestone Member. All of these members are debris units. The Lamar Formation contains cherty limestone beds, interbedded with thick skeletal pelodial wackstone. Most grains are coarse-to-fine-sand-sized grains. Microfossils include: Foraminifera, spicuals, ostracodes, calcispheres, and shell fragments. Silisified braciopods, horn corals and bryozoans are also present. Lower beds of the Lamar are skeletal packstones with ripple marks, suggesting deposition by turbidity currents.
Comprised of mostly carbonate lime mud, the Bahama Platform basin differs from the Delaware Basin because of the lack of major clastics. Its lower slope is dominated by aprons of resedimented carbonates and periplatform oozes. The sediment supply is controlled by turbidity currents and slumps. The sediments of these two environments are similar to that of the Manzanita Member. The basinal sediments of the Bahamas are a lime mud with some skeletal debris (coccoliths, planktonic foraminifera). The lime mud and debris are transported in suspension by storms and tidal currents.
In deeper water the mud ooze is uncemented and bioturbated. Its carbonate material is carried down the slope and into the Bahama's basin through turbidity currents and slumping. The lithology of this material is lime mud with some pelagic skeletal debris. The presents of aragonite and high magnesium calcite leads to an increase in potential for sedimentation. The abundant presence of the mineral aragonite in modern material differs from that of the Permian Basin. The majority of present-day, aragonite-producing organisms did not evolve until well after the Permian extinction.
Like the Delaware Basin, the apron slope facies of the Bahamas consists of debris flow material, matrix-poor megabreccias, and thick proximal turbidites and periplatform oozes. Apron slopes are comparable to ramps, which are gentle sloping surfaces with a slope of less than one degree. Carbonate slope angles of active and ancient carbonate platforms range from a slope of about one degree to almost vertical. Slope profiles are highly variable, and the steep profiles of many carbonate slopes are due to the high shear strength of carbonate sediments, the presence of frame-building organisms, clear cementation of granular sediments, and the early lithification of carbonate muds. Several different slope facies models for the Bahamas exist, four of which are very relevant to ancient deep-water carbonates. According to Tucker and Wright (1990), the slope facies most like that in the Delaware Basin is the facies model that appears on the eastern side of Little Bahama Bank (off Abaco Island). This slope margin is very steep and similar to the slope of the reef talus occuring in the Delaware Basin. Also, the basins of the Bahamas are comparable to the Delaware Basin in that both are shallow, being about 800 m in depth. In contrast, the Straits of Florida has a depth of 4000 m.
Similar in structure to the Bahama Platform, the Delaware Basin has a well-developed slope apron of resedimented carbonates. Its reef talus occurs in clingforms having a maximum dip of 30 degrees. However, the Delaware Basin's clingforms were large-scale and extended out onto the basin floor, lying about 700 m below the shelf-margin reefs (Tucker and Wright, 1990). Furthermore, many of the slump blocks, slides and turbidites can be correlated for hundreds of miles into the Basin.
The character of reef types has generally been determined by four basic, but interrelated, processes: Construction, destruction, sedimentation, and cementation (Reading, 1996). The emphasis of each of these individual characteristics changes during the evolution of a single reef. Reefs are comprised of frame-builders, sediment contributors, bafflers, binders, and precipitators. The various types of contributors differ not only in the type of environments but also the times at which both reefs have existed. For the two environments being compared, the most striking differences occur in the frame-builders. The differences in the frame-builders of the Delaware Basin and the Bahama Platform have been caused by the evolution of organisms, such as algae, and the world-wide decline of sponges.
Reef builders come in many forms, and all that is required is of the organism is that it be able to encrust and live on a hard substrate. Sponges in the Paleozoic were such opportunists because many genera of sponges were able to secrete strong calcareous skeletons in response to environmental opportunities. Throughout time, Archaeocyathids (sponges) have contributed in various ways to reef formation.
The unique design and formation of the ancient reef structure of the Delaware Basin fit perfectly into its unique environment. The depth of the Capitan Reef Formation, below sea level, has changed laterally and vertically though time. The reef is composed of a sponge-Archaeolithoporella boundstone. Almost all reef outcrops contain several growth forms of ramose and fenestellid bryozoans. Research don in the 1980's on the Capitan Reef suggests that several features of the reef are analogous to those of modern reefs (Harris 1989). These features are the: Presence of topographic relief; the presence of encrusting organisms and an organic framework; abundant inorganic cements to produce an inorganic framework; and extensive backreef, reef, and forereef skeletal debris. One major difference between modern reefs, like the Bahama Platform, and the Capitan Reef is the water depth at which the reefs formed. Based on the dip and the angle of repose of the slope where the reef was forming, Yurewicz estimated that the approximate depth of the reef was 100 to 140 ft (1977).
Many of the organisms in the Capitan reef have the potential to be frame-builders. Much of the rock in the massive upper Capitan reef consists of laminated limestone. The irregular thin layers in the upper Capitan limestone are thought to be a product of Archaeolithoporella, a red algae that has strong binding potential. Green algae, like Eugonophyllum and Mizzia, have grown in leaflike and cylindrical forms, and along with the sponges serve as a baffler. The fauna in the Capitan Reef are mainly filter feeders, including crinoids and bryozoans.
Extinction is an event that occurs to all taxa eventually. Just as new taxa have arisen through time, most genera and species are only briefly present in geologic time. The Permian extinction reduced the number of marine invertebrate families by 57% and up to 95% of all species disappeared. The decline of coralline sponges, and especially the stromatoporiois in the Mesozoic, is correlated with the rise of the reef-building scleractinian corals.
Corals and algae spread out taxonomically during the Mesozoic and Cenozoic, until corals became the dominant reef-building organisms. In zooxanthellate corals the endodermal cells are replete with symbiotic algae (zooxanthellae). A symbiotic relationship occurs in which corals benifit by using the organic carbon produced by the zooxanthellae, and the polyp uses the algae for food production. The corals are able to grow almost three times faster because of the extra oxygen produced by the algae in the sybiotic environment. These algae exist in enormous numbers, and they are essential to the metabolism of the coral's supply of nutrients and oxygen. Contrasted to the Guadalupian sponges, the corals are confined to the shallow water within the photic zone because of their need for photosynthesis to survive. The reefs in the Bahamas form along windward margins that allow for the central parts of the Bahama Platform to have quiet waters. The highest parts of the reef zone are dominated by encrusting organisms, like coralline algae because these organisms are physically able to withstand wave action. In low-energy zones the hydrozoans Millepora and corals, such as Acropora palmate, may occur.
Backreef facies in ancient rimmed-shelf sequences are commonly cyclic and shallow upward. Back-reef facies also contain intratidal and subtidal zones, and the water table may interact with the sediment that has been deposited. In the Delaware Basin the back-reef environment contains the Seven Rivers, Yates, Tansil and Salado Formations. The Seven Rivers Formation is directly above the Capitan Reef. Seven Rivers is comprised of interbedded gypsum and dolomite beds. The Yates Formation is above the Seven Rivers Formation and is comprised of sandstone and siltstone beds. The Yates contains a reef/outer-shelf transition facies with fenestral laminated breccia that is evidence of subaerial exposure. Reefward the Formation contains carbonate stromatalites. The Tansil is above the Yates Formation and is comprised of dolomites and dolomitic limestone. The Tansil Formation is comprised of pelloid-algal-skeletal packstones and large subvertical pelloid filled burrows that are succeeded by mollusk-algal pelloid packstones. The Salado and Castile Formations are comprised of the evaporates of the Ochoan and cap the Guadalupian Limestone, marking the end of the Delaware Basin.
In the Bahamas the tidal range is very low, and the wind-wave action is weak. Parts of the tidal flats, such as ponds and channels are permanently subaqueous; whereas, others have subarial exposure. Like the Delaware Basin, the Bahama Platform also has lagoons. The lagoons are surrounded by tidal flats and mangrove swamps. The lagoons are both fresh and saltwater lagoons and may have interconnecting blue holes, or caverns. As in the Delaware Basin, the Bahama platform's, tidal deltas have formed at the ends of the tidal passes and islands.
The major differences between the Delaware Basin and the Bahama Platform are the reef builders, the depths at which they occur, and their unique evolutions. In addition to the basin type, the open marine environment of the Bahamas and the intercontinental sea environment of the Delaware Basin. However, despite these differences, the manner in which carbonate material has been deposited in both the environments is very similar. Therefor, carful observation of the modern Bahama Platform can provide answers as to how these depositional processes occur.
Bebout , Don G. and Kerans, Charles., Guide to the Permian Reef Geology Trail, McKittrick Canyon, Guadalupe Mountains National Park, West Texas. Bureau of Economic Geology; University of Texas at Austin 1993.
Curran, H. Allen and White, Brian., "Terrestrial and Shallow Marine Geology of the Bahamas and Bermuda". Special Paper: Geological Society of America, 1995.
Matthews, Robert K., Dynamic Stratigraphy: An Introduction to Sedimentation and Stratigraphy. Prentice-Hall Inc., 1984
Hardie, Lawrence A., Sedimentation on the Modern Carbonate Tidal Flats of Northwest Andros Island, Bahamas. Johns Hopkins University Studies in Geology; no 22, 1977.
Harris, Paul M. and Grover, George A. Subsurface and Outcrop Examination of the Capitan Shelf Margin, Northern Delaware Basin. SEPM Core Workshop No. 13, April 23, 1989.
Harris, Paul M., "The Enigma of Prograding Steep, High relief Carbonate platform Margins." Abstract with Programs: Geological Society of America, November 2001, Vol. 33, Issue 6, pp. 409
Reading, H. G., Sedimentary Environments: Processes, Facies and Stratigraphy Blackwell Science Ltd, 1996.
Rohr, David M., "The Cibolo Reef Block Revisited" Abstracts with Programs: Geological Society of America, Vol. 34, Issue 3, pp. 34, March 2002.
Scholle, Peter A., A Color Illustrated Guide To Carbonate Rock Constituents, Textures, Cements and Porosities. The American Association of Petroleum Geologists, 1978.
Tucker, Maurice E. and Wright, Paul V., Carbonate Sedimentology. Blackwell Science Ltd., 1990.
Yurewicz, D. A., "The origin of the massive facies of the lower and middle Capitan Limestone (Permian), Guadalupe Mountains, New Mexico and West Texas" in Hileman, M. E. and Mazzullo, S. J. (Eds.) Upper Guadalupian Facies, Permian Reef Complex, Guadalupe Mountians, New Mexico and Texas: Reef Complex, Gudalupe Mountians, New Mexico and Texas: Field Conf. Guidbook, v. 1 Permian Basin Sec. SEPM, Pub. 77-16, pp. 45-92, 1977. .
Website(s): http://www.geoinfo.nmt.edu/staff/scholle/guadalupe.html http://www.science.ubc.ca/~eoswr/slidesets/guad/slidefiles/
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