Kansas Geological Survey, Subsurface Geology 12, p. 31-34
by
Lee C. Gerhard
Kansas Geological Survey
Modeling of carbonates requires detailed understanding of sediment-generation processes as well as sediment-distributive processes. Organic-generation and multiple-degradation processes operate to form carbonate-sediment bodies. This approach to modeling in reef carbonates was initiated with qualitative constructs of modem and ancient reef environments. Reefs have economic significance and are geographically distinct, although they derive from a complex environmental "mix" of processes.
Fortunately, there is a relatively good modem sedimentation-data base from which to work. Reefs are sensitive indicators of prevailing oceanographic conditions at the time of generation. The sequence of rock fabrics developed (architecture) constrains models. Processes and paragenesis of reef formation can be interpreted from rock architecture; once the sequence of processes has been interpreted, a detailed interpretation of sea-level history is possible.
Qualitative modeling indicates that architectural properties of reefs could provide a genetic classification, which in turn permits certain characteristics of the reef fabric to be predictable, including sediment sorting, early diagenesis, and relative effective porosity (Gerhard and Burke, 1986; Burke and Gerhard, 1987; Gerhard and Burke, in review). The qualitative model, still undergoing field testing in ancient reefs, is used as the basis from which to construct a quantitative model. Quantitative models may develop useful predictions of porosity and permeability. Initial attempts to develop polynomial descriptors of reef-sedimentation history and sea-level rise have been apparently successful. Work towards a more complete quantitative model of reef architecture continues; this paper summarizes progress to date.
It is our (Gerhard and Burke, in review) thesis that all three-dimensionally discrete bodies of organically derived carbonate rocks are "reefs," that they result from the interplay of definable major sedimentary processes, and that their depositional and diagenetic settings can be interpreted from observing their vertical sequence of fabrics. Reef fabrics and internal architecture evolve in response to changing rates of sea-level-rise. Availability of organisms within a specific reef setting and interruptions in this evolution may determine final architecture.
Part of the thesis is that the rates at which the processes of reef generation operate will determine the fabric of any reefs formed. As a corollary, fabrics of sediment (rocks) generated at a reef site can be interpreted with respect to the rates and balances of processes that operated.
We have deciphered a continuum of fabrics and architectural styles which are controlled more by sedimentary processes than by biotic evolution or community succession. Fabrics thus result from the interplay of organic growth, dynamic processes, and depositional topography.
It is a deliberate choice to use the term "reef" as a general term with appropriate architectural modifiers rather than "bank" or "bioherm" in order to stress the inherent continuum of fabrics and processes interpreted from reef studies.
Carbonate sediment is a product of growth of carbonate skeletons which are modified by biologic and mechanical degradation, providing sediment of varying size and shape. Sediment so generated is either incorporated into the reef or transported out of it. The balance between skeletal growth and degradation and transportation processes determines the reef-body fabric and architecture, and ultimately, the type and degree of diagenesis. The degree of transportation of detritus out of a reef system is a function of wave energy and depositional slope. Transportation of detritus from a reef system creates the opportunity for framework to develop and for syngenetic cementation. Lack of sediment removal provides smaller pore space and inhibits skeletal growth and syngenetic cementation. Production of carbonate sediment within an active reef system is so rapid that frameworks cannot develop unless the sediment is removed; frameworks can literally drown in their own detritus. In the opposite extreme, a hydromechanic pile of loose sediment will be the result of transportation or sweeping of carbonate sediment into a wave-induced bar-form.
Effects of the three major reef-construction processes, skeletal generation, degradation, and transportation, can be graphically represented on a ternary diagram (fig. 1). A corresponding reef-classification terminology, framework, biodetrital, and hydromechanical, is derived from the processes (fig. 2).
Figure 1--Ternary diagram showing the three major carbonate processes that determine reef architecture. Connecting plotted points representing individual samples can trace the evolution of process influence on the reel
Figure 2--Ternary diagram of three major reef classes based on architecture developed by processes shown in Fig. 1. There is a correspondence between points of the two ternary diagrams, that is, framework reefs are dominated by generation processes, biodetrital reefs by degradation, and hydromechanical reefs by transportation.
Eustasy is the remaining dynamic process that must be considered as controlling reef fabric. Neumann and Macintyre (1985) have interpreted part of this relationship in their "catch-up, keep-up, or give-up" view of succession. However, while addressing organism response to sea-level changes, they did not consider the sequence of fabrics which is coincident with a normal cycle. Normal cycles, characterized as sine curves, have rapidly changing rates of rise, which, in conjunction with relatively stable organism-growth rates, provide large variations in absolute water depth. We have not considered whether sea-level changes are true global eustasy or of local origin.
Three major processes appear to control the development of reefs. Generation of carbonate though biologic deposition provides the carbonate material from which the reef is constructed. Most of the carbonate is subjected to biologic and mechanical degradation, which reduces the size of primary particles and produces detritus. Finally, transportation of the carbonate (or lack thereof) determines the sorting and geometry of the reef-pile. The relative importance of each of these processes to an individual reef determines its architecture, easily displayed on a ternary diagram (fig. 1).
End-member reef classes which correspond to the stated major processes can be graphically displayed on a similar ternary diagram (fig. 2). The three end-member reef classes are framework (generation process is dominant), biodetrital (degradation process is dominant), and hydromechanical (transportation process is dominant).
Characteristics of the framework-reef setting and resultant reef architecture are moderately steep depositional slope, high mechanical energy, framework development, and syngenetic cementation. Porosity tends to be occluded by early cementation and relatively uniform primary mineral assemblage (mostly aragonite in modem reefs).
Characteristics of the biodetrital-reef setting and resultant reef architecture are moderate depositional slope and moderate mechanical energy; the resulting reef is a biodetrital sediment-laden mass and has little or no framework and virtually no cementation. Porosity tends to be well-developed in fossil examples because the sediment-mineral assemblage is composed of well-mixed calcite, mg-calcite, and aragonite with differing susceptibility to dissolution and replacement. Early diagenetic-porosity development appears to be common through subaerial exposure or exposure to freshwater wedges in a submarine setting.
Characteristics of the hydromechanical-reef setting and resultant reef architecture are very low depositional slope, variable but frequently low mechanical energy, hydromechanical accumulation of reef grains, and little or no syngenetic cementation. Algal and other skeletal grains appear to be partly autochthonous to the accumulations. High flotation potential of the algal plates common in hydromechanical reefs obviates the need for high mechanical energy during reef construction. Little seismic evidence exists for current-bedded internal structure, likely because there is constant turnover by burrowing infauna as well as autochthonous particle contribution. Porosity may be high due to the susceptibility of aragonitic algal plates to early diagenetic cementation and dissolution; open spaces may be early cemented with calcite or aragonite and muds in the reef matrix appear to be less permeable than algal plate masses and thus less susceptible to dissolution or replacement.
From many field observations, and from the above discussion, as reefs of any framework percentage grow, a certain amount of detritus is generated and accumulates within the reef. If this detritus is not removed from the reef, it will to some degree inhibit organic skeletal growth. If detrital generation is high and transportation is low, the accumulation of detritus (bioclasts) will be high and win provide a significant or major part of the reef mass in comparison with the amount of interlocking framework. In the extreme, the reef will develop as a three-dimensionally discrete body of organic detritus (hydromechanical or biodetrital reefs).
In contrast, if the reef setting is such that waves and currents winnow the degradation products (biodetritus) from the reef, then the reef will be characterized by organic skeletons and open space, encouraging the establishment of well-developed framework. From this, one can infer that fast-growing reef organisms in a sediment-sweeping setting will most likely produce framework (framework reefs).
Early diagenesis of reefs and other shallow-water carbonate deposits is commonly characterized by submarine cementation, as in the example of ancient framework reefs where adequate open space and water flushing occurred. For most other reefs, cessation of sea-level rise or elevation of the reef and exposure to meteoric waters provide a significant early diagenetic paragenesis that can influence the reef's potential for future petroleum trapping.
From this qualitative model, compared to sea-level-change rates, derives the basis for quantitative modeling (fig. 3). As modem reef-growth rates are compared to modem sea-level-change rates (Adey and Burke, 1976), a numerical basis for predicting architecture evolves (fig. 4). Current work is focusing on the development of polynomial equations to describe modem sea-level changes and comparative sedimentation (reef-growth) rates. The relationship of reef-growth rates and species diversity to water depth has been contrasted to abundance of detritus. Future work will develop these concepts towards prediction of reef occurrence and porosity based upon geophysical log-interpreted sea-level changes.
Figure 3--Schematic diagram showing the relationships of sea-level change to absolute water depth, detritus generation, sedimentation, and reef architecture.
Figure 4--Application of the Cross and Lessenger (unpubushed) one-draensional stratigraphic model to a reef sequence.
Adey, W. H., and Burke, R. B., 1976, Holocene bioherms (algal ridges and bank-barrier reefs) of the eastern Caribbean: Geological Society of America, Bulletin, v. 87, p. 95-109
Adey, W. H., and Burke, R. B., 1977, Holocene bioherms of lesser Antilles--geologic control of development: American Association of Petroleum Geologists, Studies in Geology, no. 4, p. 67-81
Burke, R. B., and Gerhard, L. C., 1987, Reefs, bioherms, and banks--a semantic and genetic continuum (abs.): American Association of Petroleum Geologists, Bulletin, v. 71, p. 535
Cross, T. A., and Lessenger, M. A., unpublished, One-dimensional stratigraphic model for the Macintosh computer: Colorado School of Mines
Gerhard, L. C., and Burke, R. B., 1986, Reefs, bioherms, and banks--a semantic and genetic continuum (abs.): 12th International Sedimentological Congress, Canberra, Australia, Abstracts, p. 118
Gerhard, L. C., and Burke, R. B., in review, Reefs, bioherms, and banks--a semantic and genetic continuum: American Association of Petroleum Geologists, Bulletin
Neumann, A. C., and Macintyre, I., 1985, Reef response to sea-level rise--keep-up, catch-up or give-up: Fifth International Coral Reef Congress, Proceedings, v. 3, p. 105-109
Kansas Geological Survey
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