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Comparison of Pleistocene and Holocene barrier island beach-to-offshore sequences, Georgia and northeast Florida coasts, U.S.A.
Sedimentary Geology, 34 (1983) 167 183 Elsevier Scientific Publishing Company, Amsterdam
167 Printed in The Netherlands
C O M P A R I S O N OF P L E I S T O C E N E A N D H O L O C E N E BARRIER ISLAND B E A C H - T O - O F F S H O R E S E Q U E N C E S , GEORGIA A N D N O R T H E A S T FLORIDA COASTS, U.S.A.
,lAMES D. HOWARD and RICHARD M. SCOTT
Skidawav Institute of Oceanography, P.O. Box 13687, Sar,annah, GA 31400 (U.S.A.) ('ities Sert~i('e Company, P.O. Box 1919, Midland, TX 79702 (U.S.A.) (Received Jul'~ 10. 1981: accepted November 11, 1982)
ABSTRACT ttoward, J.D. and Scott, R.M., 1983. Comparison of Pleistocene and Holocene barrier island beach-tooffshore sequences, Georgia and northeast Florida coast. U.S.A. Sediment. Geol.. 34:167 183. Well-exposed vertical sequences of Pleistocene shoreline deposits are rare on the coastal plain of the southeastern United States. An important exception is an exposure along the St. Mary's River on the Georgia--Florida state line. This outcrop contains an excellent depositional strike section of a prograding offshore, shoreface, foreshore and backshore sequence with well-preserved physical and biogenic sedimentary structures. Offshore sediments are composed of highly bioturbated, muddy fine sand. Distinct burrows and bedding become progressively more abundant upward through a transition zone into shoreface deposits. The shoreface is dominated by the trace fossil Ophiomorpha nodosa and physical sedimentary structures are poorly preserved. Foreshore sediments contain low-angle seaward dipping beds, high-angle landward dipping beds and ripple laminae. Heavy-mineral accumulations in the backshore accentuate bedding and biogenic structures such as ghost crab and insect burrows and bioturbation by amphipods. Direct correlation of most primary physical and biogenic sedimentary structures and textures can be made between the Pleistocene and Holocene beach-to-offshore facies assemblages. However, our studies of the Pleistocene indicate that the existing Holocene vertical sequence model for the Georgia coast needs to be modified to account for biogenically produced post-depositional effects.
INTRODUCTION
Comparison of modern and ancient coastal depositional facies is a major goal of sedimentary geology, but unfortunately few direct analogues exist. One exception to this is the Holocene and Pleistocene sequences of the Georgia and northeast Florida coasts of the U.S. Physical and biogenic sedimentary structures preserved in a few well-exposed Pleistocene outcrops are remarkably similar to those found in nearby modern coastal environments. In recent years numerous studies of modern shoreline depositional environments 0037-0738/83/$03.00
~-: 1983 Elsevier Science Publishers B.V.
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of the Georgia coast have been made (Frey and Howard. 1969. 1975; Howard. 1969. 1971; Howard and DOries, 1972; Howard and Reineck, 1972: Wunderlich, 1972: Howard and Frey, 1980). Of these, the studies by Wunderlich (1972) and Howard and Reineck (1972) developed a beach and nearshore model for the Georgia coast. More recently this model has been compared with similar but higher-energy facies of the California coast (Howard and Reineck, 1981). Our interpretation of the Pleistocene has greatly benefited from our Holocene studies and we have been impressed and surprised at the similarity between the ancient and modern. On the other hand, the Holocene model contains some ambiguities when considered in light of the Pleistocene record. Most of the Holocene examples cited in this paper are from the beach and shoreface of Sapelo Island, Georgia, a barrier island that lies midway on the Georgia coast. Pleistocene examples are from outcrops along the St. Marys River, on the Georgia and Florida border (Fig. 1). HOLOCENE ENVIRONMENTS
This is a mesotidal coast with average tide range of - 2 m and low wave energy; fair-weather wave heights averaging 0.25 m. Strongest winds on this coast are from
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the northeast and occur during fall and winter, but severe storms and hurricanes are rare. Light winds from the east and southeast occur in the spring and summer. Detailed discussion of Georgia's coastal hydrography can be found in Howard and Frey (1975). Beach and nearshore sediments are generally fine sand and muddy fine sand, respectively. Medium, and rarely coarse sands are present as lag deposits ill thin storm-reworked sediments. A description of physical and biogenic sedimentary structures on the modern Georgia shoreface was made by Howard and Reineck (1972) based on can cores and box cores. From the low-tide line to approximately 1 m of waterdepth, the sediments are parallel to subparallel laminated (Fig. 2a). From -- 1 to - 2 m of waterdepth the cores contained small-scale ripple laminae and practically no biogenic sedimentary structures (Fig. 2b). Below this (2-5 m waterdepth) in the transition zone arc laminated-to-burrowed beds (Fig. 2c; Howard and Reineck, 1972. 1981). Such beds have an erosional base overlain by parallel, rippled or hummocky beds that become bioturbated and burrowed upward. Seaward of the transition zone the sediments are totally bioturbated muddy sand (Fig. 2d) to a depth of - 1 0 m where relict or "'palimpsest" sediments of the shelf occur (Fig. 2e). Georgia coast beaches have low dipping foreshore slopes (less than 2 °) with a low-tide surface exposure of 120 to 150 m width (Fig. 3a). The foreshore grades into the backshore which has a width of - 3 0 m (Fig. 3b). The foreshore backshore boundary changes daily in response to conditions of wind and tide. Under high-tide a n d / o r strong onshore wind, the foreshore may extend to the base of the beach dunes. It is rarely possible to identify a berm on the beach surface but the backshore facies can be easily recognized in beach cores and trenches. On the modern beach, it is possible to distinguish between upper and lower foreshore. The lower, seaward, part is generally - 90 m wide and has a seaward dip of 2 ° and the upper, landward, part is typically - 30 m in width with a surface dip of 1°. In addition to the minor difference in dip, there is a marked difference in density of burrows of the shrimp Callianassa major. In the lower foreshore, burrow density is commonly 7-11 m 2 and in the upper foreshore 1--3 m e (Frey and Mayou, 1971). Ridge and runnel (Fig. 3c) systems are important morphologic features of the Georgia coast beaches and have been described by Wunderlich (1972). They are fair-weather features and during the infrequent storms on this coast, the ridge and runnel systems are destroyed and the foreshore surface becomes a seaward-dipping planar surface. Ridge and runnel systems form at the low-tide line and migrate up the beach over a period of 5 to 7 weeks. As this migration occurs, landward dipping beds are deposited, with dips as high as 35 ° . This inclination is in strong contrast to the typical foreshore laminae with dips of 1°-2 ° in a seaward direction. Another aspect of ridge and runnel migration is burial and preservation of ripple lamination in the runnel trough as the migrating foresets build landward (Fig. 3d). Backshore sediments (Fig. 3e), when studied in cores and trenches show marked
171
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Fig, 3. Holocene beach features: (a) aerial view of foreshore, backshore and dunes at low tide; tv, o ridge and runnel sets are present; (b) surface view of upper beach: upper foreshore to right and backshore 1o left; foreshore-backshore boundary is indicated by plant detritus; (c) foreshore ridge and runnel; observer is on the landward dipping face of the ridge; (d) wave ripples in the runnel: as the ridge migrates landward the ripple laminae are covered by high angle foresets; (e) backshore stratification showing numerous interruptions in layers, irregular contacts and light and hea~'-mineral layers: (f) X-ray radiograph print of subhorizontal, nearly parallel shelly laminae of the foreshore.
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differences from the adjacent foreshore (Fig. 3f). Sedimentary structures are variable and discontinuous. Whereas physical structures on the foreshore form in response to continuous tide, wave and current action, the backshore is affected intermittently by subaerial and subaqueous processes. Quartz sand grains in the backshore are winnowed and blown landward to form dunes or, less commonly, they are blown seaward, in either event the result is thinly laminated concentrations of heavy minerals (Woolsey et al., 1975). During very high tides the backshore is covered by marine waters, and ponds and temporary runnels form. Consequently, backshore bedding is highly variable with abundant wave and current ripples and occasional scour and fill features. Biogenic sedimentary structures in the backshore are distinctly different than those in the foreshore. Callianassa major burrows, so typical in the foreshore, are absent. In their place are insect burrows, the unlined burrows of the ghost crab Ocypode (Frey and Mayou, 1971)and abundant bioturbation formed by amphipods. PLEISTOCENE FACIES
Our main Pleistocene example is Bells Bluff, an east-west-trending cliff exposed on a meander of the Bells River, a tidal tributary of the St, Marys River (Fig. 1). This outcrop is approximately 700 m long and 10-12 m high. Quality and quantity of outcrop exposure varies from year to year in response to bluff erosion, slumping and growth of vegetation. Based on regional correlations, Hoyt and Hails (1974) considered the age of this outcrop to be - 110,000 yrs B.P., and they referred to the deposit as a prograding barrier sequence that formed when sealevel stood at - 8 m higher than present. They based this conclusion on the highest occurrence of Ophiomorphia nodosa, a trace fossil considered to be the fossil analogue of the burrow of the shrimp Callianassa (Frey et al., 1978). Erosion along Bells River has exposed a dip section of a prograding beach-to-offshore sequence. Present information indicates that the base of the exposure is a transgressive contact marked by coarse sands. In the Bells Bluff outcrop a complete vertical section can be recognized, including offshore through probable beach dunes; shoreface, upper and lower foreshore and backshore facies are best preserved (Fig. 4).
Bioturbated facies At the base of the Bells Bluff sequence, overlying a coarse sand layer, is a gray-green bioturbated muddy fine sand (Fig. 5a). No obvious physical sedimentary structures are seen but discontinuous, indistinct coarse sand layers can be recognized. These layers appear to be remnant storm-deposits that subsequently underwent intensive biogenic reworking. Bioturbation is 100%, although a few distinct burrows occur. We refer to these as Thalassinoides-type and polychaete-type burrows which are more abundant in the overlying unit. The upper contact of this facies is gradational and is based on the presence of increasing numbers of distinct burrows.
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Bioturbated and laminated facies This facies (Fig. 5b) is similar to the underlying b i o t u r b a t e d unit in that it is c o m p o s e d of m u d d y fine sand with m u d content decreasing upward. It differs from the u n d e r l y i n g facies, however, in that: (1) distinct burrows are very evident and are increasingly a b u n d a n t upward; and (2) it contains individual b i o t u r b a t e d , mud layers that are laterally continuous. At one of the better exposed sections at Bells Bluff this unit is divisible into three beds of sub-equal thickness ( - 50 cm). The lower and u p p e r units are c o m p o s e d of m u d d y fine sand and the m i d d l e unit of fine sand. Within this facies erosional c o n t a c t s can be recognized at intervals of 7 - 1 7 cm. These contacts are m a r k e d by
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Fig. 5. Bioturbated Pleistoceneunits: (a) bioturbated facies of gray-green muddy fine sand: measuring rod has 10-cm intervals: (b) bioturbated and laminated facies. This is a muddy fine sand that becomes more sandy upward. In addition to highly bioturbated characteristics of these sediments, some distinct burn~ws can be recognized. Remnant bedding is subtly expressed.
slightly burrowed m u d layers less than l cm thick and by truncated burrows. R e m n a n t flaser b e d d i n g also occurs at these contacts. Biogenic sedimentary structures are obvious and a b u n d a n t in this facies. Most c o m m o n is the Thalassinoides-type burrow. This structure lacks a constructed burrow-wall but is obvious in outcrop because it is filled with clean, white fine sand that contrasts sharply with the s u r r o u n d i n g sediments. The burrows are 8 18 mm in diameter. O n prepared vertical outcrop surfaces burrows of 2 0 - 3 0 cm length are c o m m o n and we have traced individual burrows vertically as much as 1 m. They are slightly inclined from vertical. Where the burrows b r a n c h or abruptly change orientation there is an enlargement of the burrow and b r a n c h i n g angles are consistently 120 °. Such features are so characteristic of decapod burrows (Frey and Howard, 1975: Frey et al., 1978) that there is little d o u b t as to their origin. Distinctly small Ophiomorpha burrows are also present in this facies. These knobby-walled burrows are vertical to inclined, have a diameter of 8 - 1 2 m m and are
175 generally 4
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Fig. 6. Burrowed and laminated facies: (a) weathered outcrop surface on which the resistant Ophiomorpha burrows stand out in relief; (b) expression of cut and trimmed outcrop surface. Ophiomorphaburrow walls are dark and burrow fillings are of light-hued sand. Burrowing is so dense that primary bedding characteristics have been destroyed. Marks on measuring rod are 10-cm intervals.
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Burrowed and laminated facies Recognition of this facies (Fig. 6) is based on the dense accumulation of the trace fossil Ophiomorpha nodosa. Sediment here is clean fine sand. Bedding is poorly preserved because of the density of burrows but when seen is composed of subparallel nearly horizontal lamina. Primary structures are further masked by what we assume to be post-depositional effects of fluctuating ground-water levels that have resulted in iron-stained irregular resistant layers. The base of the burrowed and laminated unit is easily recognized by the presence of large Ophiomorpha structures with diameters of 18-20 mm and lengths that in some places exceed 3 m and which extend upward into the overlying facies, Commonly, the base of this unit is marked by an erosional contact of muddy coarse sand and highly weathered shells that appear to be the surf clams Mulinia. Just above this erosional base are the horizontal branching terminations of Ophiomorpha. Within the burrowed and laminated facies subtle erosion surfaces can be recognized. These surfaces which separate beds of 30-35 cm in thickness are difficult to trace and are established by connecting the erosional tops of Ophiomorpha along the outcrop. Burrows other than Ophiomorpha may exist here but they are masked by the density and abundance of that one ichnospecies.
Laminated facies This is formed of clean fine sand with occasional laminae of coarse sand (Fig. 7). Laminations in the lower 50 cm are obscured by Ophiomorpha. However. the burrows here are noticeably less abundant than in the underlying facies (Fig. 7a). The upper two-thirds of the unit are characterized by well-preserved, low-angle parallel or subparallel laminae, cross-bedded sands and some ripple laminae. The low-angle laminae dip gently eastward at less than 5 ° , Higher-angle cross-beds dip westward at 15°-20 ° and grade into ripple laminae at the base of the foresets (Fig. 7b). Occasionally trough cross-beds of coarse sand or shell with eastward dips are observed. Numerous erosional surfaces occur throughout this facies and they are clearly shown by truncations of burrows. In the upper part of this facies heavymineral laminae are locally well preserved and some of these layers are disturbed by faint small-scale bioturbation. In this facies and in the underlying burrowed and laminated facies the burrows can be precisely identified as Ophiomorpha nodosa. Such burrows are well established as traces of the modern decapod Callianassa that is very characteristic of present day Georgia beaches (Weimer and Hoyt, 1964; Frey et al., 1978). This interpretation is further substantiated in the Pleistocene outcrops by the presence of fossil fecal pellets of the same size and morphology as those of Callianassa.
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Fig. 7. Laminated facies: (a) lower part of facies in which Ophiomorpha burrows, though present, are significantly fewer in number than in underlying facies: (b) upper part of laminated facies. Bedding includes parallel to subparallel laminae, cross-bedding that dips to the right (westward) and wave ripple laminae. In both photograph prints the measuring rod intervals are 10 cm,
Fig. 8. Laminated and bioturbated facies. Bedding is well preserved and emphasized by heavy mineral concentrations, the lowermost layers of which exhibit cryptobioturbation.
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Laminated and bioturbated facies Clean fine sand and locally abundant heavy-mineral layers with cryptobioturbation and distinct burrows characterize this facies (Fig. 8). The upper and lower boundaries are poorly defined and gradational. Heavy-mineral layers are especially obvious in the laminated and bioturbated facies. Heavies make up individual, nearly horizontal sand layers in some places only a few grains thick and in other cases in beds as much as 50 cm thick with trough crossbedding. Generally, the quartz sands in this facies are white and the dark heavy-mineral layers stand out clearly and emphasize physical and biogenic sedimentary structures, Cryptobioturbation (Howard and Frey, 1975) is described as bioturbation in which biogenic activity, though extensive, is very small scale and has not masked the associated physical sedimentary structures. This type of bioturbation is known to be produced by amphipods, a small ( - 2 mm) crustacean (Howard and Elders. 1970; Howard, 1971). Also present in this facies are J-shaped ghost crab burrows (Frey and Mayou, 1971) and insect burrows (Frey and Howard, 1969).
Mottled facies "Mottled" as a facies designation is used with some reluctance because this term has been widely applied in a loose way and is at best vague. However. the uppermost part of Pleistocene sequence has been made indistinct by modern plant roots and to some extent present-day burrows of insects and mammals. Texture is mostly clean fine sand and appears to have been extensively leached by ground water. Physical sedimentary structures are very rare but remnant laminae of high-angle (200-30 ° ) cross-beds, marked by heavy minerals, locally occur. Throughout most of this facies, however, numerous trenches have failed to expose any biogenic sedimentary structures other than those associated with modern plants. The upper contact of this unit is a highly irregular and generally poorly exposed. The maximum vertical extent at the Bells Bluff outcrop is 12 m. Unconformably overlying the mottled facies are active Holocene dune deposits. PLE1STOCENE-HOLOCENE FACIES COMPARISONS
In this section we discuss similarities and differences we have found between the Holocene Sapelo Island beach-to-offshore sequences and the Bells Bluff Pleistocene outcrop. Obviously, the specter of circular reasoning always lurks in the eroded recesses of the outcrops when making such comparisons and indeed it is our previous studies of modern coastal Georgia that have guided our interpretation. However, we have also frequently used the Pleistocene outcrops to aid in our understanding of Holocene.
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OfJshore-- bioturbated facies In the modern Georgia coastal setting the offshore facies consists of muddy fine sands that lie between - 5 and - 1 0 m waterdepth. Beyond 10 m of depth the modern shelf consists of palimpsest (relict) sands (Howard and Reineck, 1972, 1981 ). In the Pleistocene outcrop the base of the sequence is a coarse sand that we presently suspect represents a marine transgression or storm deposit. In both the Holocene and Pleistocene examples this facies is a highly bioturbated muddy fine sand and no significant differences exist between the two. So greatly reworked are these sediments that even X-ray radiographs fail to reveal greater detail than that seen on the outcrop. This facies lies below average wave base and even most storm waves would probably not reach this depth. Long periods of little or no storm activity allows time for macrobenthic organisms to rework the sediment and destroy physical structures. The result is a dominance of biogenic features and bioturbation in this facies. The few recognizable burrows that occur in this facies mostly extend down from the overlying facies.
Transition-- bioturbated and laminated facies In this facies of the Pleistocene, bioturbation continues to be expressed in the muddy fine sand but superposed on this are three distinct burrow types and remnant mud layers. Here we see the beginning of the interplay of physical and biogenic sedimentary structures as expressed by mud layers associated with erosional contacts that truncate burrows. This is the equivalent of the laminated-to-burrowed sequence widely recognized in modern and ancient nearshore shelf sediments (Howard, 1972; Goldring and Bridges, 1973; Howard and Reineck, 1981). In the Bells Bluff Pleistocene exposure the biogenic effects significantly dominate the physical sedimentary structures. This was surprising to us because in the box cores from Sapelo Island the parallel laminated-to-burrowed beds were better developed and occasionally even segments of apparent hummocky bedding were present. Such is not the case in the Pleistocene section, although we could recognize beds of 7 17 cm thickness with readily identifiable erosional boundaries. The absence of physical sedimentary structures possibly reflects the infrequent occurrence of major storms and the ability of burrowing organisms to nearly keep pace with sedimentation. Polychaete-type burrows and the presence of small forms of Ophiomorpha found in the Pleistocene outcrop certainly have their counterpart in the Holocene sediments of the Georgia coast. However, we did not recognize a modern equivalent of the Thalassinoides-type burrow in our studies of the modern coast. It is interesting to speculate on whether these burrows may be formed by the same organism (Callianassa) that formed the Ophiomorpha nodosa structures of the overlying facies. The only apparent difference between the two is that the Thalassinoides-type burrows lack the well-developed knobby wall structures; this may be a function of the higher mud content in sediments of this facies.
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Shoreface--burrowed and laminated facies Physical and biogenic sedimentary structures of the shoreface have received considerable attention through studies of modern coastal sediments in recent years (Clifton et al., 1971; Howard and Reineck, 1972, 1981; Davidson-Arnott and Greenwood, 1976). All studies of modern shoreface deposits point out that this environment is almost continuously subjected to the direct effects of waves and currents and that it contains the best developed and most varied suite of physical sedimentary structures in the beach-to-offshore sequence. Preservable biogenic sedimentary structures are everywhere rare to non-existent in modern shoreface sediments. Based on our Sapelo Island Holocene model we expected the Pleistocene shoreface to be a unit approximately 2 m thick and dominated by ripple laminae in the lower half and parallel laminae in the upper half with little or no significant biogenic record. To our surprise we found instead a unit totally burrowed by the trace fossil Ophiomorpha nodosa and a nearly complete absence of physical sedimentary structures. The reason for this incongruity lies in the nature of burrowing organisms and the depositional setting of this unit. Callianassa major burrows are impossible to exhume from modern foreshore beach sediments on the Georgia coast because the burrows extend well below the beach surface; however, we now know (Frey et al., 1978) that the burrows extend to as much as 5 m depth. Hence, in a prograding sequence long Ophiomorpha burrows made in the intertidal foreshore will penetrate completely through that facies and through the underlying shoreface. In addition, burrow density in the foreshore beach, which typically is 7-11 burrows m : at any one time (Frey and Mayou, 1971), is greatly increased by "overprinting". As the shoreline progrades and with time many more burrows are constructed and preserved than would exist as living burrows at any moment. What results is "wall-towall" burrows and destruction of bedding. Additionally, periods of non-progradation (associated with a variation in the rate at which sediment is supplied to the beach environment) or minor transgressions, allow subsequent generations of burrows to be superposed and preserved. Thus, in this case, a facies that is developed exclusively by physical processes is actually characterized in the stratigraphic record by biogenic structures. The remnant lamination that does exist is very difficult to distinguish but can be faintly seen between burrows or can be subtly established by tracing horizons of truncated burrows.
Foreshore-- laminated facies This facies of the Pleistocene exposure has the best preserved physical sedimentary structures. The trace fossil Ophiomorpha is still present in the lower part of this facies but they are less dense than in the underlying facies. Physical sedimentary structures include subparallel, eastward (seaward) dipping
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lamina with dips less than 5 ° , and higher angle 15 ° 20 ° landward dips and ripple laminae. This agrees well with features that we find on the modern Georgia beaches. The higher angle landward dips correlate with the structures found on the migrating edge of ridge and runnel structures and, like their modern counterparts, the cross-beds grade into ripples at their base as described by Wunderlich (1972) for the Sapelo Island beach. We had not expected to find ridge and runnel systems so well preserved because they are fair-weather features which are destroyed during storms. This suggests to us that progradation at least in some cases proceeded under day-to-day conditions of longshore drift rather than only occurring in response to buildup of the foreshore beach during storms. Preservation of these features may also be indicative of fairly rapid progradation related to periodic high influx of sediments. Bedding in this facies is poor to well preserved and laminae are most easily seen in the upper part of the facies where heavy minerals accentuate bedding.
Backshore--laminated and bioturbated facies It is not possible to establish a precise boundary between foreshore and backshore in the Pleistocene outcrop. The main criteria used to recognize this facies are: (1) absence of ridge and runnel structures; (2) concentrations of heavy-mineral laminae: (3) greater variability of physical sedimentary structures; and (4) presence of ghost crab and insect burrows and amphipod bioturbation. Ghost crab burrows, though not abundant are well preserved in the heavy-mineral layers. Ghost crabs build a very characteristic J-shaped, unlined burrow and are characteristic of the backshore. Associated with these are simple vertical burrows of 2 5 mm diameter and up to 20 cm in length. Our interpretation of these as insect burrows is based on the association of insect burrows with ghost crab burrows in modern beach sediments. Cryptobioturbation, the biogenic activity of amphipods, is an especially good backshore indicator when associated with heavy-mineral layers (Howard, 1971) and such structures are extremely abundant in these Pleistocene deposits. The presence of amphipod cryptobioturbation in backshore sediments is another biogenic anomaly. Amphipods do not normally live in the backshore because they are filter feeders and backshore sands are usually dry. In the adjacent foreshore, however, amphipods are extremely abundant; sometimes in densities measured in thousands of individuals m -2 (Howard and Elders, 1970). In spite of this the bioturbation is seldom well preserved in the foreshore because of constant erosion and deposition of sand by waves and swash action. During storms and unusually high tides, however, amphipods are trapped in backshore sands. When the high waters recede, the small crustaceans are unable to return to the foreshore. It is likely that the amphipods follow the slowly receding "water table" and continue to disturb the sediments to depths of - 1 m. So here again, as in the example of Ophiomorpha
182
burrows in the Pleistocene shoreface, we find the dominance of trace fossils in sediments of a facies which is not typically inhabited by the associated organism.
Mottled facies In the upper 3 to 4 m of the Pleistocene outcrop primary sedimentary structures of any description are infrequent. Occasionally heavy-mineral laminae are found with dips in excess of 20 ° . Unfortunately, the clean sands in this facies have been highly leached of minerals and significantly disturbed by roots of trees and by the burrows of modern insects. We interpret this facies as that of beach dunes based on the rare remnant heavy-mineral cross laminae and the position of this unit in the stratigraphic sequence. CONCLUSION
Comparison of Holocene and Pleistocene sediments of the Georgia coast show the strong similarities that exist between deposits separated by over 100,000 yrs in time. Observed physical and biogenic sedimentary structures and the thickness of facies within the vertical sequence indicates that Pleistocene sediments accumulated in a analogous depositional setting in response to similar sedimentary processes and energy conditions. Comparison with the Pleistocene, however, points out that some incorrect assumptions can be made if the model is based wholly on a reconstruction of a vertical sequence from a modern example. Most serious of these misinterpretations are related to the effects of biogenic activity. ACKNOWLEDGEMENTS
Numerous colleagues have visited the Pleistocene outcrop with us and their observations and insight have helped to shape our interpretation. Of these many people we wish to especially thank Richard Moiola and Robert W. Frey. We very much appreciate the valuable manuscript criticism given to us by Robert W. Frey. Part of our field work was greatly assisted by Mike Boyles. We thank Lynn O'Malley for drafting and photo printing. This study was supported by the Oceanographic Section of the National Science Foundation, N.S.F. Grant GA-39999X. REFERENCES Clifton, H.E., Hunter, R.E. and Phillips, R.L., 1971, Depositional structures and processes in the non-barred high energy nearshore. J. Sediment. Petrol., 41:651-670. Davidson-Arnott, R.G.D. and Greenwood, B., 1976. Facies relationships on a barred coast, Kouchibouguac Bay, New Brunswick, Canada. In: R.A. Davis, Jr. and R.L Ethington (Editors), Beach and Nearshore Sedimentation. Soc. Econ. Paleontol. Mineral., Spec. Publ., 24: 149-168.
183
f:rey, R.W. and Howard, J.D., 1969. A profile of biogenic sedimentary structures in a Holocene barrier island-salt marsh complex, Georgia. Gulf Coast Assoc. Geol. Soc. Trans., 19: 427-444. Frey, R.W. and Howard, J.D., 1975. Endobenthic adaptations of juvenile thalassinidean shrimp. Geol. Soc. Denmark Bull., 24: 283-297. Frey, R.W. and Mayou, T.V., 1971. Decapod burrows in Holocene barrier island beaches and washover fans, Georgia. Senckenbergiana Mar., 3:53 77. Erey, R.W., Howard, J.D. and Pryor, W.A., 1978. Ophiomorpha: its morphologic, taxonomic, and environmental significance. Palaeogeogr., Palaeoclimatol., Palaeoecol., 23:199 229. Goldring, R. and Bridges, P., 1973. Sublittoral sheet sandstones. J. Sediment. Petrol., 43:736 747. Hertweck, G., 1972. Georgia coastal region, Sapelo Island, U.S.A.: sedimentology and biology. V. Distribution and environmental significance of lebensspuren and in-situ skeletal remains. Senckenbergiana Mar., 4: 125-167. lloward, J.D., 1969. Radiographic examination of variations in barrier island facies: Sapelo lsland, Georgia. Gulf Coast Assoc. Geol. Sot., Trans., 19:217 232. Howard, J.D., 1971. Amphipod bioturbate textures in recent and Pleistocene beach sediments. In: J.D. Howard, J.W. Valentine and J.E. Warme (Editors), Recent Advances in Paleoecology and lchnology. Am. Geol. Inst.. Short Course Lecture Notes, pp. 213-223. Howard, J.D.. 1972. Trace fossils as criteria for recognizing shorelines in stratigraphic record. In: .I.K. Rigby and W.K. Hamblin (Editors), Recognition of Ancient Sedimentary Environments. Soc. Econ. Paleontol. Mineral., Spec. Publ.. 16: 215-225. Howard, J,D. and D6rjes, J., 1972. Animal-sediment relationships in two beach-related tidal flats: Sapelo lsland, Georgia. J. Sediment Petrol., 42:608 623. Howard, J.D. and Elders, C.A., 1970. Burrowing patterns of haustoriid amphipods from Sapelo Island, Georgia. In: T.P. Crimes and J.C. Harper (Editors). Trace Fossils. Geol. J., Spec. Issue, 3: 243-262. Howard, J.D. and Frey, R.W., 1975. Estuaries of the Georgia coast, U.S.A.: sedimentology and biology. 1. Introduction. Senckenbergiana Mar., 7: 1-31. Howard, J.D. and Frey, R.W., 1980. Holocene depositional environments of the Georgia coast and continental shelf. In: J.D. Howard, C.B. DePratter and R.W. Frey (Editors), Excursions in Southeastern G e o l o g y - - t h e Archaeology-Geology of the Georgia Coast, Atlanta. Geol. Surv. Guidebook. 20: 66 134. Howard, J.D., and Reineck, H.-E., 1972. Georgia coastal region, Sapelo Island, U.S.A.: sedimentology and biology. IV. Physical and biogenic sedimentary structures of the nearshore shelf. Senckenbergiana Mar., 4: 81-123. Howard, J.D. and Reineck, H.-E., 1981. Depositional facies of a "high-energy'" beach-to-offshore sequence and their comparison with a "low-energy" sequence. Bull. Am. Assoc. Pet, Geol., 65: 807-830. Hoyt, J.H. and Hails, J.R.. 1974. Pleistocene stratigraphy of southeastern Georgia. In: R.Q. Oaks and J. Dunbar (Editors), Post-Miocene Stratigraphy: Central and Southern Atlantic Coastal Plain. Utah State Univ. Press, Logan, Utah, pp. 191-205. Weimer, R.J. and Hoyt, J.H., 1964. Burrows of Callianassa major Say, geologic indicators of littoral and shallow neritic environments. J. Paleontol., 38: 761-767. Woolsey, J.R., Henry, V.J. and Hunt, J.L., 1975. Backshore heavy-mineral concentration on Sapelo Island, Georgia. J. Sediment. Petrol., 45: 280-284. Wunderlich, F., 1972. Georgia coastal region, Sapelo Island, U.S.A.: sedimentology and biology. Ill. Beach dynamics and beach development. Senkcenbergiana Mar., 4: 47-79.
167 Printed in The Netherlands
C O M P A R I S O N OF P L E I S T O C E N E A N D H O L O C E N E BARRIER ISLAND B E A C H - T O - O F F S H O R E S E Q U E N C E S , GEORGIA A N D N O R T H E A S T FLORIDA COASTS, U.S.A.
,lAMES D. HOWARD and RICHARD M. SCOTT
Skidawav Institute of Oceanography, P.O. Box 13687, Sar,annah, GA 31400 (U.S.A.) ('ities Sert~i('e Company, P.O. Box 1919, Midland, TX 79702 (U.S.A.) (Received Jul'~ 10. 1981: accepted November 11, 1982)
ABSTRACT ttoward, J.D. and Scott, R.M., 1983. Comparison of Pleistocene and Holocene barrier island beach-tooffshore sequences, Georgia and northeast Florida coast. U.S.A. Sediment. Geol.. 34:167 183. Well-exposed vertical sequences of Pleistocene shoreline deposits are rare on the coastal plain of the southeastern United States. An important exception is an exposure along the St. Mary's River on the Georgia--Florida state line. This outcrop contains an excellent depositional strike section of a prograding offshore, shoreface, foreshore and backshore sequence with well-preserved physical and biogenic sedimentary structures. Offshore sediments are composed of highly bioturbated, muddy fine sand. Distinct burrows and bedding become progressively more abundant upward through a transition zone into shoreface deposits. The shoreface is dominated by the trace fossil Ophiomorpha nodosa and physical sedimentary structures are poorly preserved. Foreshore sediments contain low-angle seaward dipping beds, high-angle landward dipping beds and ripple laminae. Heavy-mineral accumulations in the backshore accentuate bedding and biogenic structures such as ghost crab and insect burrows and bioturbation by amphipods. Direct correlation of most primary physical and biogenic sedimentary structures and textures can be made between the Pleistocene and Holocene beach-to-offshore facies assemblages. However, our studies of the Pleistocene indicate that the existing Holocene vertical sequence model for the Georgia coast needs to be modified to account for biogenically produced post-depositional effects.
INTRODUCTION
Comparison of modern and ancient coastal depositional facies is a major goal of sedimentary geology, but unfortunately few direct analogues exist. One exception to this is the Holocene and Pleistocene sequences of the Georgia and northeast Florida coasts of the U.S. Physical and biogenic sedimentary structures preserved in a few well-exposed Pleistocene outcrops are remarkably similar to those found in nearby modern coastal environments. In recent years numerous studies of modern shoreline depositional environments 0037-0738/83/$03.00
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of the Georgia coast have been made (Frey and Howard. 1969. 1975; Howard. 1969. 1971; Howard and DOries, 1972; Howard and Reineck, 1972: Wunderlich, 1972: Howard and Frey, 1980). Of these, the studies by Wunderlich (1972) and Howard and Reineck (1972) developed a beach and nearshore model for the Georgia coast. More recently this model has been compared with similar but higher-energy facies of the California coast (Howard and Reineck, 1981). Our interpretation of the Pleistocene has greatly benefited from our Holocene studies and we have been impressed and surprised at the similarity between the ancient and modern. On the other hand, the Holocene model contains some ambiguities when considered in light of the Pleistocene record. Most of the Holocene examples cited in this paper are from the beach and shoreface of Sapelo Island, Georgia, a barrier island that lies midway on the Georgia coast. Pleistocene examples are from outcrops along the St. Marys River, on the Georgia and Florida border (Fig. 1). HOLOCENE ENVIRONMENTS
This is a mesotidal coast with average tide range of - 2 m and low wave energy; fair-weather wave heights averaging 0.25 m. Strongest winds on this coast are from
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the northeast and occur during fall and winter, but severe storms and hurricanes are rare. Light winds from the east and southeast occur in the spring and summer. Detailed discussion of Georgia's coastal hydrography can be found in Howard and Frey (1975). Beach and nearshore sediments are generally fine sand and muddy fine sand, respectively. Medium, and rarely coarse sands are present as lag deposits ill thin storm-reworked sediments. A description of physical and biogenic sedimentary structures on the modern Georgia shoreface was made by Howard and Reineck (1972) based on can cores and box cores. From the low-tide line to approximately 1 m of waterdepth, the sediments are parallel to subparallel laminated (Fig. 2a). From -- 1 to - 2 m of waterdepth the cores contained small-scale ripple laminae and practically no biogenic sedimentary structures (Fig. 2b). Below this (2-5 m waterdepth) in the transition zone arc laminated-to-burrowed beds (Fig. 2c; Howard and Reineck, 1972. 1981). Such beds have an erosional base overlain by parallel, rippled or hummocky beds that become bioturbated and burrowed upward. Seaward of the transition zone the sediments are totally bioturbated muddy sand (Fig. 2d) to a depth of - 1 0 m where relict or "'palimpsest" sediments of the shelf occur (Fig. 2e). Georgia coast beaches have low dipping foreshore slopes (less than 2 °) with a low-tide surface exposure of 120 to 150 m width (Fig. 3a). The foreshore grades into the backshore which has a width of - 3 0 m (Fig. 3b). The foreshore backshore boundary changes daily in response to conditions of wind and tide. Under high-tide a n d / o r strong onshore wind, the foreshore may extend to the base of the beach dunes. It is rarely possible to identify a berm on the beach surface but the backshore facies can be easily recognized in beach cores and trenches. On the modern beach, it is possible to distinguish between upper and lower foreshore. The lower, seaward, part is generally - 90 m wide and has a seaward dip of 2 ° and the upper, landward, part is typically - 30 m in width with a surface dip of 1°. In addition to the minor difference in dip, there is a marked difference in density of burrows of the shrimp Callianassa major. In the lower foreshore, burrow density is commonly 7-11 m 2 and in the upper foreshore 1--3 m e (Frey and Mayou, 1971). Ridge and runnel (Fig. 3c) systems are important morphologic features of the Georgia coast beaches and have been described by Wunderlich (1972). They are fair-weather features and during the infrequent storms on this coast, the ridge and runnel systems are destroyed and the foreshore surface becomes a seaward-dipping planar surface. Ridge and runnel systems form at the low-tide line and migrate up the beach over a period of 5 to 7 weeks. As this migration occurs, landward dipping beds are deposited, with dips as high as 35 ° . This inclination is in strong contrast to the typical foreshore laminae with dips of 1°-2 ° in a seaward direction. Another aspect of ridge and runnel migration is burial and preservation of ripple lamination in the runnel trough as the migrating foresets build landward (Fig. 3d). Backshore sediments (Fig. 3e), when studied in cores and trenches show marked
171
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Fig, 3. Holocene beach features: (a) aerial view of foreshore, backshore and dunes at low tide; tv, o ridge and runnel sets are present; (b) surface view of upper beach: upper foreshore to right and backshore 1o left; foreshore-backshore boundary is indicated by plant detritus; (c) foreshore ridge and runnel; observer is on the landward dipping face of the ridge; (d) wave ripples in the runnel: as the ridge migrates landward the ripple laminae are covered by high angle foresets; (e) backshore stratification showing numerous interruptions in layers, irregular contacts and light and hea~'-mineral layers: (f) X-ray radiograph print of subhorizontal, nearly parallel shelly laminae of the foreshore.
172
differences from the adjacent foreshore (Fig. 3f). Sedimentary structures are variable and discontinuous. Whereas physical structures on the foreshore form in response to continuous tide, wave and current action, the backshore is affected intermittently by subaerial and subaqueous processes. Quartz sand grains in the backshore are winnowed and blown landward to form dunes or, less commonly, they are blown seaward, in either event the result is thinly laminated concentrations of heavy minerals (Woolsey et al., 1975). During very high tides the backshore is covered by marine waters, and ponds and temporary runnels form. Consequently, backshore bedding is highly variable with abundant wave and current ripples and occasional scour and fill features. Biogenic sedimentary structures in the backshore are distinctly different than those in the foreshore. Callianassa major burrows, so typical in the foreshore, are absent. In their place are insect burrows, the unlined burrows of the ghost crab Ocypode (Frey and Mayou, 1971)and abundant bioturbation formed by amphipods. PLEISTOCENE FACIES
Our main Pleistocene example is Bells Bluff, an east-west-trending cliff exposed on a meander of the Bells River, a tidal tributary of the St, Marys River (Fig. 1). This outcrop is approximately 700 m long and 10-12 m high. Quality and quantity of outcrop exposure varies from year to year in response to bluff erosion, slumping and growth of vegetation. Based on regional correlations, Hoyt and Hails (1974) considered the age of this outcrop to be - 110,000 yrs B.P., and they referred to the deposit as a prograding barrier sequence that formed when sealevel stood at - 8 m higher than present. They based this conclusion on the highest occurrence of Ophiomorphia nodosa, a trace fossil considered to be the fossil analogue of the burrow of the shrimp Callianassa (Frey et al., 1978). Erosion along Bells River has exposed a dip section of a prograding beach-to-offshore sequence. Present information indicates that the base of the exposure is a transgressive contact marked by coarse sands. In the Bells Bluff outcrop a complete vertical section can be recognized, including offshore through probable beach dunes; shoreface, upper and lower foreshore and backshore facies are best preserved (Fig. 4).
Bioturbated facies At the base of the Bells Bluff sequence, overlying a coarse sand layer, is a gray-green bioturbated muddy fine sand (Fig. 5a). No obvious physical sedimentary structures are seen but discontinuous, indistinct coarse sand layers can be recognized. These layers appear to be remnant storm-deposits that subsequently underwent intensive biogenic reworking. Bioturbation is 100%, although a few distinct burrows occur. We refer to these as Thalassinoides-type and polychaete-type burrows which are more abundant in the overlying unit. The upper contact of this facies is gradational and is based on the presence of increasing numbers of distinct burrows.
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Bioturbated and laminated facies This facies (Fig. 5b) is similar to the underlying b i o t u r b a t e d unit in that it is c o m p o s e d of m u d d y fine sand with m u d content decreasing upward. It differs from the u n d e r l y i n g facies, however, in that: (1) distinct burrows are very evident and are increasingly a b u n d a n t upward; and (2) it contains individual b i o t u r b a t e d , mud layers that are laterally continuous. At one of the better exposed sections at Bells Bluff this unit is divisible into three beds of sub-equal thickness ( - 50 cm). The lower and u p p e r units are c o m p o s e d of m u d d y fine sand and the m i d d l e unit of fine sand. Within this facies erosional c o n t a c t s can be recognized at intervals of 7 - 1 7 cm. These contacts are m a r k e d by
174
Fig. 5. Bioturbated Pleistoceneunits: (a) bioturbated facies of gray-green muddy fine sand: measuring rod has 10-cm intervals: (b) bioturbated and laminated facies. This is a muddy fine sand that becomes more sandy upward. In addition to highly bioturbated characteristics of these sediments, some distinct burn~ws can be recognized. Remnant bedding is subtly expressed.
slightly burrowed m u d layers less than l cm thick and by truncated burrows. R e m n a n t flaser b e d d i n g also occurs at these contacts. Biogenic sedimentary structures are obvious and a b u n d a n t in this facies. Most c o m m o n is the Thalassinoides-type burrow. This structure lacks a constructed burrow-wall but is obvious in outcrop because it is filled with clean, white fine sand that contrasts sharply with the s u r r o u n d i n g sediments. The burrows are 8 18 mm in diameter. O n prepared vertical outcrop surfaces burrows of 2 0 - 3 0 cm length are c o m m o n and we have traced individual burrows vertically as much as 1 m. They are slightly inclined from vertical. Where the burrows b r a n c h or abruptly change orientation there is an enlargement of the burrow and b r a n c h i n g angles are consistently 120 °. Such features are so characteristic of decapod burrows (Frey and Howard, 1975: Frey et al., 1978) that there is little d o u b t as to their origin. Distinctly small Ophiomorpha burrows are also present in this facies. These knobby-walled burrows are vertical to inclined, have a diameter of 8 - 1 2 m m and are
175 generally 4
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Fig. 6. Burrowed and laminated facies: (a) weathered outcrop surface on which the resistant Ophiomorpha burrows stand out in relief; (b) expression of cut and trimmed outcrop surface. Ophiomorphaburrow walls are dark and burrow fillings are of light-hued sand. Burrowing is so dense that primary bedding characteristics have been destroyed. Marks on measuring rod are 10-cm intervals.
176
Burrowed and laminated facies Recognition of this facies (Fig. 6) is based on the dense accumulation of the trace fossil Ophiomorpha nodosa. Sediment here is clean fine sand. Bedding is poorly preserved because of the density of burrows but when seen is composed of subparallel nearly horizontal lamina. Primary structures are further masked by what we assume to be post-depositional effects of fluctuating ground-water levels that have resulted in iron-stained irregular resistant layers. The base of the burrowed and laminated unit is easily recognized by the presence of large Ophiomorpha structures with diameters of 18-20 mm and lengths that in some places exceed 3 m and which extend upward into the overlying facies, Commonly, the base of this unit is marked by an erosional contact of muddy coarse sand and highly weathered shells that appear to be the surf clams Mulinia. Just above this erosional base are the horizontal branching terminations of Ophiomorpha. Within the burrowed and laminated facies subtle erosion surfaces can be recognized. These surfaces which separate beds of 30-35 cm in thickness are difficult to trace and are established by connecting the erosional tops of Ophiomorpha along the outcrop. Burrows other than Ophiomorpha may exist here but they are masked by the density and abundance of that one ichnospecies.
Laminated facies This is formed of clean fine sand with occasional laminae of coarse sand (Fig. 7). Laminations in the lower 50 cm are obscured by Ophiomorpha. However. the burrows here are noticeably less abundant than in the underlying facies (Fig. 7a). The upper two-thirds of the unit are characterized by well-preserved, low-angle parallel or subparallel laminae, cross-bedded sands and some ripple laminae. The low-angle laminae dip gently eastward at less than 5 ° , Higher-angle cross-beds dip westward at 15°-20 ° and grade into ripple laminae at the base of the foresets (Fig. 7b). Occasionally trough cross-beds of coarse sand or shell with eastward dips are observed. Numerous erosional surfaces occur throughout this facies and they are clearly shown by truncations of burrows. In the upper part of this facies heavymineral laminae are locally well preserved and some of these layers are disturbed by faint small-scale bioturbation. In this facies and in the underlying burrowed and laminated facies the burrows can be precisely identified as Ophiomorpha nodosa. Such burrows are well established as traces of the modern decapod Callianassa that is very characteristic of present day Georgia beaches (Weimer and Hoyt, 1964; Frey et al., 1978). This interpretation is further substantiated in the Pleistocene outcrops by the presence of fossil fecal pellets of the same size and morphology as those of Callianassa.
177
Fig. 7. Laminated facies: (a) lower part of facies in which Ophiomorpha burrows, though present, are significantly fewer in number than in underlying facies: (b) upper part of laminated facies. Bedding includes parallel to subparallel laminae, cross-bedding that dips to the right (westward) and wave ripple laminae. In both photograph prints the measuring rod intervals are 10 cm,
Fig. 8. Laminated and bioturbated facies. Bedding is well preserved and emphasized by heavy mineral concentrations, the lowermost layers of which exhibit cryptobioturbation.
17~
Laminated and bioturbated facies Clean fine sand and locally abundant heavy-mineral layers with cryptobioturbation and distinct burrows characterize this facies (Fig. 8). The upper and lower boundaries are poorly defined and gradational. Heavy-mineral layers are especially obvious in the laminated and bioturbated facies. Heavies make up individual, nearly horizontal sand layers in some places only a few grains thick and in other cases in beds as much as 50 cm thick with trough crossbedding. Generally, the quartz sands in this facies are white and the dark heavy-mineral layers stand out clearly and emphasize physical and biogenic sedimentary structures, Cryptobioturbation (Howard and Frey, 1975) is described as bioturbation in which biogenic activity, though extensive, is very small scale and has not masked the associated physical sedimentary structures. This type of bioturbation is known to be produced by amphipods, a small ( - 2 mm) crustacean (Howard and Elders. 1970; Howard, 1971). Also present in this facies are J-shaped ghost crab burrows (Frey and Mayou, 1971) and insect burrows (Frey and Howard, 1969).
Mottled facies "Mottled" as a facies designation is used with some reluctance because this term has been widely applied in a loose way and is at best vague. However. the uppermost part of Pleistocene sequence has been made indistinct by modern plant roots and to some extent present-day burrows of insects and mammals. Texture is mostly clean fine sand and appears to have been extensively leached by ground water. Physical sedimentary structures are very rare but remnant laminae of high-angle (200-30 ° ) cross-beds, marked by heavy minerals, locally occur. Throughout most of this facies, however, numerous trenches have failed to expose any biogenic sedimentary structures other than those associated with modern plants. The upper contact of this unit is a highly irregular and generally poorly exposed. The maximum vertical extent at the Bells Bluff outcrop is 12 m. Unconformably overlying the mottled facies are active Holocene dune deposits. PLE1STOCENE-HOLOCENE FACIES COMPARISONS
In this section we discuss similarities and differences we have found between the Holocene Sapelo Island beach-to-offshore sequences and the Bells Bluff Pleistocene outcrop. Obviously, the specter of circular reasoning always lurks in the eroded recesses of the outcrops when making such comparisons and indeed it is our previous studies of modern coastal Georgia that have guided our interpretation. However, we have also frequently used the Pleistocene outcrops to aid in our understanding of Holocene.
179
OfJshore-- bioturbated facies In the modern Georgia coastal setting the offshore facies consists of muddy fine sands that lie between - 5 and - 1 0 m waterdepth. Beyond 10 m of depth the modern shelf consists of palimpsest (relict) sands (Howard and Reineck, 1972, 1981 ). In the Pleistocene outcrop the base of the sequence is a coarse sand that we presently suspect represents a marine transgression or storm deposit. In both the Holocene and Pleistocene examples this facies is a highly bioturbated muddy fine sand and no significant differences exist between the two. So greatly reworked are these sediments that even X-ray radiographs fail to reveal greater detail than that seen on the outcrop. This facies lies below average wave base and even most storm waves would probably not reach this depth. Long periods of little or no storm activity allows time for macrobenthic organisms to rework the sediment and destroy physical structures. The result is a dominance of biogenic features and bioturbation in this facies. The few recognizable burrows that occur in this facies mostly extend down from the overlying facies.
Transition-- bioturbated and laminated facies In this facies of the Pleistocene, bioturbation continues to be expressed in the muddy fine sand but superposed on this are three distinct burrow types and remnant mud layers. Here we see the beginning of the interplay of physical and biogenic sedimentary structures as expressed by mud layers associated with erosional contacts that truncate burrows. This is the equivalent of the laminated-to-burrowed sequence widely recognized in modern and ancient nearshore shelf sediments (Howard, 1972; Goldring and Bridges, 1973; Howard and Reineck, 1981). In the Bells Bluff Pleistocene exposure the biogenic effects significantly dominate the physical sedimentary structures. This was surprising to us because in the box cores from Sapelo Island the parallel laminated-to-burrowed beds were better developed and occasionally even segments of apparent hummocky bedding were present. Such is not the case in the Pleistocene section, although we could recognize beds of 7 17 cm thickness with readily identifiable erosional boundaries. The absence of physical sedimentary structures possibly reflects the infrequent occurrence of major storms and the ability of burrowing organisms to nearly keep pace with sedimentation. Polychaete-type burrows and the presence of small forms of Ophiomorpha found in the Pleistocene outcrop certainly have their counterpart in the Holocene sediments of the Georgia coast. However, we did not recognize a modern equivalent of the Thalassinoides-type burrow in our studies of the modern coast. It is interesting to speculate on whether these burrows may be formed by the same organism (Callianassa) that formed the Ophiomorpha nodosa structures of the overlying facies. The only apparent difference between the two is that the Thalassinoides-type burrows lack the well-developed knobby wall structures; this may be a function of the higher mud content in sediments of this facies.
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Shoreface--burrowed and laminated facies Physical and biogenic sedimentary structures of the shoreface have received considerable attention through studies of modern coastal sediments in recent years (Clifton et al., 1971; Howard and Reineck, 1972, 1981; Davidson-Arnott and Greenwood, 1976). All studies of modern shoreface deposits point out that this environment is almost continuously subjected to the direct effects of waves and currents and that it contains the best developed and most varied suite of physical sedimentary structures in the beach-to-offshore sequence. Preservable biogenic sedimentary structures are everywhere rare to non-existent in modern shoreface sediments. Based on our Sapelo Island Holocene model we expected the Pleistocene shoreface to be a unit approximately 2 m thick and dominated by ripple laminae in the lower half and parallel laminae in the upper half with little or no significant biogenic record. To our surprise we found instead a unit totally burrowed by the trace fossil Ophiomorpha nodosa and a nearly complete absence of physical sedimentary structures. The reason for this incongruity lies in the nature of burrowing organisms and the depositional setting of this unit. Callianassa major burrows are impossible to exhume from modern foreshore beach sediments on the Georgia coast because the burrows extend well below the beach surface; however, we now know (Frey et al., 1978) that the burrows extend to as much as 5 m depth. Hence, in a prograding sequence long Ophiomorpha burrows made in the intertidal foreshore will penetrate completely through that facies and through the underlying shoreface. In addition, burrow density in the foreshore beach, which typically is 7-11 burrows m : at any one time (Frey and Mayou, 1971), is greatly increased by "overprinting". As the shoreline progrades and with time many more burrows are constructed and preserved than would exist as living burrows at any moment. What results is "wall-towall" burrows and destruction of bedding. Additionally, periods of non-progradation (associated with a variation in the rate at which sediment is supplied to the beach environment) or minor transgressions, allow subsequent generations of burrows to be superposed and preserved. Thus, in this case, a facies that is developed exclusively by physical processes is actually characterized in the stratigraphic record by biogenic structures. The remnant lamination that does exist is very difficult to distinguish but can be faintly seen between burrows or can be subtly established by tracing horizons of truncated burrows.
Foreshore-- laminated facies This facies of the Pleistocene exposure has the best preserved physical sedimentary structures. The trace fossil Ophiomorpha is still present in the lower part of this facies but they are less dense than in the underlying facies. Physical sedimentary structures include subparallel, eastward (seaward) dipping
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lamina with dips less than 5 ° , and higher angle 15 ° 20 ° landward dips and ripple laminae. This agrees well with features that we find on the modern Georgia beaches. The higher angle landward dips correlate with the structures found on the migrating edge of ridge and runnel structures and, like their modern counterparts, the cross-beds grade into ripples at their base as described by Wunderlich (1972) for the Sapelo Island beach. We had not expected to find ridge and runnel systems so well preserved because they are fair-weather features which are destroyed during storms. This suggests to us that progradation at least in some cases proceeded under day-to-day conditions of longshore drift rather than only occurring in response to buildup of the foreshore beach during storms. Preservation of these features may also be indicative of fairly rapid progradation related to periodic high influx of sediments. Bedding in this facies is poor to well preserved and laminae are most easily seen in the upper part of the facies where heavy minerals accentuate bedding.
Backshore--laminated and bioturbated facies It is not possible to establish a precise boundary between foreshore and backshore in the Pleistocene outcrop. The main criteria used to recognize this facies are: (1) absence of ridge and runnel structures; (2) concentrations of heavy-mineral laminae: (3) greater variability of physical sedimentary structures; and (4) presence of ghost crab and insect burrows and amphipod bioturbation. Ghost crab burrows, though not abundant are well preserved in the heavy-mineral layers. Ghost crabs build a very characteristic J-shaped, unlined burrow and are characteristic of the backshore. Associated with these are simple vertical burrows of 2 5 mm diameter and up to 20 cm in length. Our interpretation of these as insect burrows is based on the association of insect burrows with ghost crab burrows in modern beach sediments. Cryptobioturbation, the biogenic activity of amphipods, is an especially good backshore indicator when associated with heavy-mineral layers (Howard, 1971) and such structures are extremely abundant in these Pleistocene deposits. The presence of amphipod cryptobioturbation in backshore sediments is another biogenic anomaly. Amphipods do not normally live in the backshore because they are filter feeders and backshore sands are usually dry. In the adjacent foreshore, however, amphipods are extremely abundant; sometimes in densities measured in thousands of individuals m -2 (Howard and Elders, 1970). In spite of this the bioturbation is seldom well preserved in the foreshore because of constant erosion and deposition of sand by waves and swash action. During storms and unusually high tides, however, amphipods are trapped in backshore sands. When the high waters recede, the small crustaceans are unable to return to the foreshore. It is likely that the amphipods follow the slowly receding "water table" and continue to disturb the sediments to depths of - 1 m. So here again, as in the example of Ophiomorpha
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burrows in the Pleistocene shoreface, we find the dominance of trace fossils in sediments of a facies which is not typically inhabited by the associated organism.
Mottled facies In the upper 3 to 4 m of the Pleistocene outcrop primary sedimentary structures of any description are infrequent. Occasionally heavy-mineral laminae are found with dips in excess of 20 ° . Unfortunately, the clean sands in this facies have been highly leached of minerals and significantly disturbed by roots of trees and by the burrows of modern insects. We interpret this facies as that of beach dunes based on the rare remnant heavy-mineral cross laminae and the position of this unit in the stratigraphic sequence. CONCLUSION
Comparison of Holocene and Pleistocene sediments of the Georgia coast show the strong similarities that exist between deposits separated by over 100,000 yrs in time. Observed physical and biogenic sedimentary structures and the thickness of facies within the vertical sequence indicates that Pleistocene sediments accumulated in a analogous depositional setting in response to similar sedimentary processes and energy conditions. Comparison with the Pleistocene, however, points out that some incorrect assumptions can be made if the model is based wholly on a reconstruction of a vertical sequence from a modern example. Most serious of these misinterpretations are related to the effects of biogenic activity. ACKNOWLEDGEMENTS
Numerous colleagues have visited the Pleistocene outcrop with us and their observations and insight have helped to shape our interpretation. Of these many people we wish to especially thank Richard Moiola and Robert W. Frey. We very much appreciate the valuable manuscript criticism given to us by Robert W. Frey. Part of our field work was greatly assisted by Mike Boyles. We thank Lynn O'Malley for drafting and photo printing. This study was supported by the Oceanographic Section of the National Science Foundation, N.S.F. Grant GA-39999X. REFERENCES Clifton, H.E., Hunter, R.E. and Phillips, R.L., 1971, Depositional structures and processes in the non-barred high energy nearshore. J. Sediment. Petrol., 41:651-670. Davidson-Arnott, R.G.D. and Greenwood, B., 1976. Facies relationships on a barred coast, Kouchibouguac Bay, New Brunswick, Canada. In: R.A. Davis, Jr. and R.L Ethington (Editors), Beach and Nearshore Sedimentation. Soc. Econ. Paleontol. Mineral., Spec. Publ., 24: 149-168.
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f:rey, R.W. and Howard, J.D., 1969. A profile of biogenic sedimentary structures in a Holocene barrier island-salt marsh complex, Georgia. Gulf Coast Assoc. Geol. Soc. Trans., 19: 427-444. Frey, R.W. and Howard, J.D., 1975. Endobenthic adaptations of juvenile thalassinidean shrimp. Geol. Soc. Denmark Bull., 24: 283-297. Frey, R.W. and Mayou, T.V., 1971. Decapod burrows in Holocene barrier island beaches and washover fans, Georgia. Senckenbergiana Mar., 3:53 77. Erey, R.W., Howard, J.D. and Pryor, W.A., 1978. Ophiomorpha: its morphologic, taxonomic, and environmental significance. Palaeogeogr., Palaeoclimatol., Palaeoecol., 23:199 229. Goldring, R. and Bridges, P., 1973. Sublittoral sheet sandstones. J. Sediment. Petrol., 43:736 747. Hertweck, G., 1972. Georgia coastal region, Sapelo Island, U.S.A.: sedimentology and biology. V. Distribution and environmental significance of lebensspuren and in-situ skeletal remains. Senckenbergiana Mar., 4: 125-167. lloward, J.D., 1969. Radiographic examination of variations in barrier island facies: Sapelo lsland, Georgia. Gulf Coast Assoc. Geol. Sot., Trans., 19:217 232. Howard, J.D., 1971. Amphipod bioturbate textures in recent and Pleistocene beach sediments. In: J.D. Howard, J.W. Valentine and J.E. Warme (Editors), Recent Advances in Paleoecology and lchnology. Am. Geol. Inst.. Short Course Lecture Notes, pp. 213-223. Howard, J.D.. 1972. Trace fossils as criteria for recognizing shorelines in stratigraphic record. In: .I.K. Rigby and W.K. Hamblin (Editors), Recognition of Ancient Sedimentary Environments. Soc. Econ. Paleontol. Mineral., Spec. Publ.. 16: 215-225. Howard, J,D. and D6rjes, J., 1972. Animal-sediment relationships in two beach-related tidal flats: Sapelo lsland, Georgia. J. Sediment Petrol., 42:608 623. Howard, J.D. and Elders, C.A., 1970. Burrowing patterns of haustoriid amphipods from Sapelo Island, Georgia. In: T.P. Crimes and J.C. Harper (Editors). Trace Fossils. Geol. J., Spec. Issue, 3: 243-262. Howard, J.D. and Frey, R.W., 1975. Estuaries of the Georgia coast, U.S.A.: sedimentology and biology. 1. Introduction. Senckenbergiana Mar., 7: 1-31. Howard, J.D. and Frey, R.W., 1980. Holocene depositional environments of the Georgia coast and continental shelf. In: J.D. Howard, C.B. DePratter and R.W. Frey (Editors), Excursions in Southeastern G e o l o g y - - t h e Archaeology-Geology of the Georgia Coast, Atlanta. Geol. Surv. Guidebook. 20: 66 134. Howard, J.D., and Reineck, H.-E., 1972. Georgia coastal region, Sapelo Island, U.S.A.: sedimentology and biology. IV. Physical and biogenic sedimentary structures of the nearshore shelf. Senckenbergiana Mar., 4: 81-123. Howard, J.D. and Reineck, H.-E., 1981. Depositional facies of a "high-energy'" beach-to-offshore sequence and their comparison with a "low-energy" sequence. Bull. Am. Assoc. Pet, Geol., 65: 807-830. Hoyt, J.H. and Hails, J.R.. 1974. Pleistocene stratigraphy of southeastern Georgia. In: R.Q. Oaks and J. Dunbar (Editors), Post-Miocene Stratigraphy: Central and Southern Atlantic Coastal Plain. Utah State Univ. Press, Logan, Utah, pp. 191-205. Weimer, R.J. and Hoyt, J.H., 1964. Burrows of Callianassa major Say, geologic indicators of littoral and shallow neritic environments. J. Paleontol., 38: 761-767. Woolsey, J.R., Henry, V.J. and Hunt, J.L., 1975. Backshore heavy-mineral concentration on Sapelo Island, Georgia. J. Sediment. Petrol., 45: 280-284. Wunderlich, F., 1972. Georgia coastal region, Sapelo Island, U.S.A.: sedimentology and biology. Ill. Beach dynamics and beach development. Senkcenbergiana Mar., 4: 47-79.