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Geological History of Gray's Reef National Marine Sanctuary
Prepared by:
James L. Harding, Ph.D.
Vernon J. Henry, Jr., Ph.D.
Introduction
The presence of reefs, also called live bottoms and hard grounds, on the continental shelf of the southeastern United States has been known to commercial and sports fishermen for several decades (Strusaker, 1969; Barans and Burrell, 1976; Harris, 1978). However, little information has been available concerning their geological nature.
Increasing usage of the continental shelf waters by all interests led to the realization that at least one representative segment of these unique ecosystems should be afforded protection. Therefore, in 1981 an area of live bottom located 33 km east of Sapelo Island, Georgia was designated a National Marine Sanctuary. This area, known as Gray's Reef, is characterized by outcrops of calcarenite dissected into a series of northeast southwest trending ridges supporting abundant epifauna.
The first systematic investigation of the live bottom consisted of faunal collections initiated in 1960 by the late Milton B. Gray (Gray, 1961), biological curator for the University of Georgia Marine Institute on Sapelo Island, and for whom the feature was named. Except for a brief statement of the existence and possible origin of the reef by Henry and Hoyt (1968), it was not studied geologically until the work of Hunt (1974), followed by Woolsey (1977) who included Gray's Reef as part of an inner shelf stratigraphic study.
Gray's Reef was also included in a series of live bottom studies initiated in 1976 by the Bureau of Land Management (later the Minerals Management Service) in response to concerns of potential impacts on these features by petroleum exploration and production in the South Atlantic Outer Continental Shelf (OCS) region. The studies included selected live bottoms off the Carolinas, Georgia and northern Florida. In addition to mapping the regional occurrence and topographic (morphologic) expression of live bottoms, the studies included a biological characterization of the reef sites. A number of publications and reports resulted including those by Continental Shelf Associates, Inc. (1979), Henry and Giles (1980), Henry and others (1987), Van Dolah and others (1982, 1984a, 1987) and Henry (1983). The substrate of four selected live bottom sites (three off Charleston, SC and one off Brunswick, GA) was analyzed petrographically by Lemon (1979) and found to be mineralogically similar to the Gray's Reef substrate as reported by Hunt (1974). Little attention was given to the geologic nature or history of development of the live bottoms, however.
Following the designation of Gray's Reef as a National Marine Sanctuary in January 1981, the funded research has been primarily directed towards aspects with managerial objectives, including a hydrographic survey which utilized side-scan sonar and underwater CCTV to determine and characterize the morphology and community density of the live bottoms within and adjacent to the Sanctuary (Henry and Van Sant, 1982). A high resolution seismic sub-bottom profiler system was also employed to obtain data on the structural and stratigraphic aspects of the substrate.
Prior to this report, the most recent study of a geological nature was a comprehensive hydrographic and geophysical survey carried out in 1983 using the NOAA ship Whiting. This task, sponsored by the Sanctuary Programs Division, Office of Ocean and Coastal Resources Management, resulted in the development of a high resolution, two-dimensional side-scan SONAR mosaic of the seafloor within the Sanctuary, supplemented by transparent overlays depicting seabed sedimentary texture and substrate morphology (Henry, 1985).
Study Objective
The principal objective of this investigation, which began in 1987, was to establish the geological origin and developmental history of Gray's Reef from a synthesis of existing data and analysis of new data collected during the course of the study. It was anticipated that examination of lithology, texture, mineralogy, and faunal composition would yield useful information to interpret the formational history of the reef, starting with the environment in which the original sediments were deposited.
It is important at this point to state that the modern Gray's Reef is not an example of a tropical carbonate reef such as those found in south Florida, the Bahamas and the Caribbean. Such reefs are organically derived with little or no input to the marine environment from outside sources such as rivers. The reef structure is largely built by the in situ biochemical secretion of calcium carbonate by indigenous plants and animals. On the other hand, the modern Gray's Reef is considered to be a "temperate" reef, or live bottom, in which a pre-existing, submerged rock or semi-consolidated surface provides a suitable substrate for colonization by epibenthic organisms, such as sponges and soft corals. Hard grounds are simply outcrops that exhibit little or no epibenthos, probably because of being very recently exposed . The term "live bottom" and "hard ground" are often used interchangeably.
Discussion
It should be understood that the following discussion covers the original depositional post-lithification settings of the various lithologies which constitute the present substrate of Gray's Reef and not the positive bathymetric features and modern epifauna and flora which constitute the "reef", per se.
Although Hunt (1974) proffered several conclusions as to the formation of the substrate rocks, an update is necessary, as our understanding of carbonate rocks formation has greatly increased during the years since his work.
Several basic concepts were widely accepted at the time that Hunt conducted his research. Principal among these was the idea that the presence of lime mud (micrite) was indicative of a low energy environment. However, recent investigations by Dill and Steinen (1988) show that laminated carbonate mud beds are currently being deposited in inter-island channels in the Bahamas, where tidal currents approach 3 knots which is, of course, a high energy environment. Such disclosures mandate caution when using lithologic textures as indicators of depositional parameters. Additional caution is warranted by the work of Reid, et al.(1990), who state that microcrystalline carbonate, or carbonate mud, commonly forms as an internal precipitate below the sediment-water interface, and as such does not represent ooze deposited in quiet waters.
The problems that these two investigations pose for the sedimentologist are further aggravated by the work of Boardman and Carney (1991) who state that lime-mud accumulations may be due to suspension by storms with subsequent transport to the depositional site. The origin of the mud could be due to (l) skeletal breakdown in a marine lagoon setting, or (2) formed in part by inorganic precipitation.
Although settling of suspensoids according to Stoke's law make storm-related h and deposition difficult to explain, this and the other concepts presented above essentially invalidate much of the classification criteria for carbonate rocks which have been commonly accepted to date. Not only are the "energy" interpretations of limestones in n, but, the term micrite has lost some of its identity through misuse. Folk (1959) introduced the term "micrite" to refer to matrix which characterizes low-energy carbonate deposits, the term being a contraction of "microcrystalline calcite". The term "matrix" has long been used to signify a mechanically deposited material between particles, distinct from precipitated cement (Friedman and Sanders, 1978). As thus interpreted, the loosely-termed "micrite cement" used in the literature is a contradiction, as physically deposited interparticle material cannot be both micrite and cement. Because of the present confusion in terminology, combined with the well-documented occurrences of precipitated cryptocrystalline cement which can easily be mistaken for micrite (physically deposited matrix), caution should be exercised with regard to interpreting the depositional history of the Gray's Reef substrate based on the occurrence of micrite.
Hunt (1974) devoted several pages to the discussion of mode of dolomitization of the Gray's Reef substrate. The subject of dolomite and dolomitization has been discussed and argued in the geologic literature for decades and is continuing. The dolomitization problem is complex and involves a large number of parameters such as; thermodynamics, hydrology, isotopic chemistry, mineralogy, fabric, texture, etc.. When one adds the dimensions of time and temperature, the complexity is multiplied many fold.
Hardie (1987) presents a critique of the two principal modes of dolomitization favored at present: (I) the hypersaline brine model for those dolomites associated with evaporites and (2) the brackish-water "mixing zone" model for those not obviously associated with evaporites. During the period of Hunt's research, the concept of evaporite brine reflux as envisioned by Adams and Rhodes (1960) and later expanded upon by numerous authors (Zanger, 1972; Randazzo and Hickey, 1978; Ward and Halley, 1985) was widely accepted. This mode of dolomite formation, which involves hypersaline brines in supratidal environments is currently in question due to several basic weaknesses, including the fact that in none of the known coastal mixing zones currently extant in limestone or carbonate sediments has dolomite replacement been noted, and (2) in a number of dolomites which had been interpreted as being supratidal origin, dolomite had precipitated without dissolution of the calcitic substrate.
It should be stated here (primarily for those uninitiated in the lore of the dolomite problem) that the underlying reason for interest in modes of dolomite deposition involving either mixing zones, supratidal hypersaline fluid percolation or variations thereof is due to the widely-held belief that dolomite cannot form directly from seawater because of kinetic relationships. In any regard, all new concepts (and there are bound to be some more in the future) should be treated as working hypotheses and should be examined and tested in detail.
In light of the more recent hypotheses of dolomitization which have been formulated since 1974, where does the dolomite in the rocks of the Gray's Reef substrate fit? More importantly, do these newer concepts negate the conclusions of Hunt (1974)? Not necessarily, as it is probable that the Gray's Reef dolomite represents a composite origin, i.e. two or more dolomitization events - first by shallow, hypersaline waters, followed in turn by recrystallization by meteoric waters and then possible dissolution. The latter processes, if selective, could be responsible for some of the more visible porosity noted in the rock samples. The recrystallization by meteoric waters could have occurred in the vadose zone during a period or periods of subaerial exposure. The recrystallization may have involved a solid-solution exchange from aragonite (the stable form of calcium carbonate in marine waters) to calcite (stable in fresh water and subsurface brine); or, it may have been a change from magnesium calcite (stable in marine waters) to dolomite. Common diagenesis in fresh, mixed and subsurface waters involves the alteration of aragonite and high Mg calcite and dolomite. When this takes place, the resultant crystallographic fabric consists of rhombs and micrite, as is the case in the Gray's Reef rocks.
The lithologic characteristics of the Gray's Reef rocks closely resemble those reported by Coniglio, et al. (1988) in Miocene carbonates from the Gulf of Suez. It is interesting to note that these workers report that the last precipitates are vadose calcite cement and ferric oxide/hydroxide crusts. The sparry calcite in the Gray's Reef rocks could represent vadose cement and iron-stained crusts were noted on some of the substrate rocks collected to date. Also, it is possible that the bleached rocks specimens collected from the Gray's Reef substrate represent calcrete or caliche formed in the vadose zone during subaerial exposure similar to that reported by Shinn, et al. (1990) in the Florida Keys. Such surface crusts would be localized in the intertidal and supratidal zones, so that following inundation and associated perturbations, their occurrence would be rare relative to other lithologic types, as is the case with the substrate rocks of Gray's Reef.
Age of Gray's Reef Substrate
With reference to a recent revision of Neogene lithostratigraphy of the Georgia Coastal Plain by Huddlestun (1988), the Gray's Reef substrate appears to be stratigraphically and lithologically equivalent to the Raysor-equivalent shelly sand originally referred to as the Sapelo facies of the Duplin formation by Woolsey (1977). According to Huddlestun, the Raysor-equivalent shelly sand consists of shells, calcerous material and quartz sand that was deposited in an inner to midshelf environment. In projecting the depth of the unit from -18m MSL beneath Sapelo Island (Woolsey 1977) seaward to the Sanctuary, the unit would outcrop at approximately -22m MSL. This depth corresponds to the average depth of the live-bottom substrate.
Based on the planktonic foraminifera identified within the Raysor-equivalent shelly sand, Huddlestun places the age of this unit in early late Pliocene or between two and three million years before the Present. During this time, at least several sea level fluctuations exposed the substrate to subaerial processes associated with lithification, erosion, solution and diagenesis.
Diagenesis
Diagenesis in rocks is defined as changes that take place
after lithification. The principal diagenetic effects of
marine waters are micritization and neomorphic grain growth.
Evidence of both were noted in the Gray's Reef thin
sections. Freshwater diagenesis occurs at the surface or
during shallow burial (Harris, et al., 1985) in both the
vadose and phreatic zones, and during periods of subaerial
exposure, affected the rocks now comprising Gray's Reef
substrate.
Quinn (1991) noted that the Plio-Pleistocene limestones at Eniwetok Atoll in the South Pacific underwent extensive meteoric diagenesis characterized by the pervasive calcitization of aragonite and extensive dissolution marked by well-developed moldic porosity. The latter condition is prevelant in the thin sections from Gray's Reef. The lack of abundant sparry calcite may be due to its removal by meteoric water, undersaturated with respect to calcium carbonate, percolating into and through the vadose zone during subaerial exposures.
Age of Modern Gray's Reef
According to numerous citations in the literature, the
present global rise in sea level began approximately 20,000
years ago, moving across the exposed continental shelves at
a rate of 1m/century until approximately 6000 years ago, at
which time the rate of rise slowed to approximately
0.3m/century to the present day. Hindcasting these dates
relative to the present average water depth at Gray's Reef
(-22m MSL), the exposed substrate began to be inundated some
7000 years before the Present. Thus, after several cycles of
submergence and emergence, with the latest period of
exposure lasting 40,000 years, the substrate was once again
covered by the ocean and once again became a live
bottom.
Conclusions
The substrate lithologies which comprise Gray's Reef are a mixture of calcareous dolomitic sandstone, as identified in this investigation, and dolomitized sandy biomicrites as identified by Hunt (1974). The depositional environments and processes in which these rocks were formed and by which they were subsequently altered remain conjectural.
The origin of the dolomite is diagenetic or secondary, as opposed to being a primary precipitate. Whether or not it resulted from evaporitic reflux from hypersaline brines in a restricted body of water, as concluded by Hunt (1974) or resulted from magnesium-rich solutes in a non-evaporitic environment is impossible to tell from the data examined to date. Whatever the origins of the solutions, it can safely be stated that they probably entered the substrate material via ground-water percolation at least penecontemporaneously with lower stands of sea level, and possibly during times of complete subaerial exposure during the Pleistocene. The degree of lithification exhibited by the substrate material during such times of exposure is unknown, but it must have been fairly mature to have allowed selective percolation to occur.
Hunt (1974) used the state of fragmentation of the fossils to postulate a shallow marine depositional environment for the substrate rocks. While current knowledge is still insufficient to either confirm or negate this conclusion, an alternate origin deserves some consideration. The fragmentation of the fossil remains implies a high-energy environment, with surf zone and wave-base activities, yet the micrite (lime mud), given the classic interpretation, is indicative of quiescent conditions. This apparent paradox could be the result of extreme bioturbation, but extensive burrowing was not identified in either hand specimens or thin sections.
An alternative mode of deposition should be considered: a dunal origin. This reasoning is based on the textural nature of the quartz and feldspar grains common in the substrate rocks, all of which are angular to subangular, indicative of an eolian or subaerial dunal environment. Also, cross-bedding was noted in the larger hand speciments. The quartz-rich dunes could have been subjected to some form of marine encroachment, with shallow water biogenic material added to the mixture, and subsequently, the admixture could have undergone the normal processes of lithification, i.e. compaction, cementation and consolidation. The diagenetic alterations apparent in the thin sections, such as digestion of the micrite and introduction of dolomite were then possibly initiated following lithification.
Although the origin of the present-day "reef", i.e. the bathymetric expressions of sub-parallel ridges that constitute the "broken-bottom" area known as Gray's Reef, is somewhat conjectural, the differential, erosional undercutting of consolidated rocks by wave action during the Flanderian (Holocene) Transgression, as postulated by Hunt (1974), remains a plausible scenario.
An associated plausibility would be that karsification took place over portions of the exposed substrates during one or more periods of sub-aerial exposure resulting from the glacio-eustatic sea level fluctuations. The linearity of the present-day ridges and swales could be the result of solution corridors common in karst areas. Such ridges would also be subject to undercutting and the subsequent formation of blocks at the base of the ledges.
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