Shell deposits and shell preservation in quaternary and tertiary estuarine sediments in Georgia, U.S.A.

Sedimentary Geology-Elsevier Publishing Company, Amsterdam-Printed in The Netherlands

SHELL DEPOSITS AND SHELL PRESERVATION IN QUATERNARY AND TERTIARY ESTUARINE SEDIMENTS IN GEORGIA, U.S.A. HARTMUT U. WIEDEMANN

University of Georgia Marine Institute, Sapelo Island, Ga. (U.S.A.) (Received October 27, 1970) (Resubmitted May 18, 1971)

ABSTRACT Wiedemann, H. U., 1972. Shell deposits and shell preservation in Quaternary and Tertiary estuarine sediments in Georgia, U.S.A. Sediment. Geol., 7:103 125. The present-day salt-marsh estuaries of Georgia are shallow-water bodies having expanses of intertidal realms as the result of a relatively large tidal range of 2 m. Three characteristic macro-invertebrate assemblages are distinguished in the intertidal realms: (1) an assemblage of vagrant benthos including numerous different clams and snails lives in and upon the sediments of the lower intertidal to shallow subtidal zone; (2) the oyster biocoenosis consists chiefly of sessile benthos associated with colonies of Crassostrea virginica in the lower intertidal zone; (3) the tidal-marsh assemblage is confined to grassy marshes of the higher intertidal zone; molluscs are represented by Littorina irrorata and

Modiolus demissus. Processes of sedimentation dominate over erosional processes in the estuaries. Detrital sand, silt and clay, together with organic debris and skeletal carbonates, contribute to the accumulations. Destruction of shell remains is promoted by marine borers, particularly Cliona, and by chemical leaching. Especially in tidal-marsh soils shells are subject to dissolution unless they are buried rapidly. In a marsh sediment calcitic concretions were found, which formed around severely leached shell remains. Where shells within a reduced sediment are leached, the local rise in pH often promotes precipitation of iron monosulfide, which eventually is converted into pyrite. Once included in finegrained sediments, however, shells have a good chance of becoming preserved, especially if they are thick and consist of calcite rather than aragonite. Oyster shells are thus particularly durable. Because of their local abundance, oysters yield sizable shell deposits, including reefs, reworked deposits, and cheniers. Reworked accumulations of considerable extent occur in intertidal and shallow subtidal realms, usually in the vicinity of major tidal inlets. The accumulations, which form lenticular bodies of varying lateral extent, are interbedded with detrital sediments. The matrix of the shell concentrates is generally muddy, but can also be sandy or conglomeratic. Oyster beds of Late Pleistocene and midTertiary ages in Georgia are similar to the recent ones.

PRESENT-DAY SALT-MARSH ESTUARIES OF THE GEORGIA COAST

General description Along the Georgia coast of eastern U.S.A. a special type of estuary, called a salt-marsh estuary (Redfield, 1967), is common. These Georgia salt-marsh estuaries contain features of both estuarine deltas and barrier-built lagoons, to-

104

H.U. WIEDEMANN

gether with their associated tidal flats and salt-marshes. The marshes are dissected by an intricate system of meandering tidal rivers, creeks and crevasses. The estuaries are shallow-water bodies, generally being much less than 15 m deep, and have wide intertidal expanses (Fig. 1). They form a 5-7 miles (10 km) wide strip behind the Georgia Sea Islands, and are connected with the open ocean through relatively few but deep, wide tidal inlets and sounds. Descriptions of this general area and its Late Quaternary history have been given by Hoyt et al. (1966). Salinities range between 20 and 30 parts per thousand. Studies on the hydrography of local saltmarsh estuarine watersheds have been published by Ragotzkie and Bryson (1955) and Ragotzkie (1959). According to them the local tidal range is 4.5-10.8 It., the mean range being 6.8 ft. (2 m) and the mean spring tide range 8 ft. (2.5 m). The tide cycles are semidiurnal. In salt-marsh estuaries processes of sediment accumulation dominate over erosional processes, thus leading to the formation of shoals, tidal flats, and tidal marshes, and to the steady deterioration of tidal channels other than the active delta distributaries. The complete record of a filled channel consists of a sequence

Fig. 1. Index map in lower right hand corner shows the study areas in the coastal plain of Georgia, U.S.A. The general map shows the extent of the salt-marsh estuaries in the vicinity of Sapelo Island: subtidal realms in black; intertidal realms dotted; land in white. U G a ~ University of Georgia Marine Institute; C = Cabretta Island; P H - Pumpkin Hammock.

SHELL DEPOSITS AND SHELL PRESERVATION IN ESTUARINE SEDIMENTS

105

of subtidal sediments overlain by an intertidal sequence, the grain size normally fining upwards. The thickness of the intertidal (eulittoral) deposits is limited by the local tidal range, if they are deposited during stable sea level. The intertidal deposits can be further subdivided into two parts : a lower part deposited on tidal flats, point bars, and channel banks below mean sea level (M.S.L.), and an upper veneer of tidal marsh sediments occupied by halophyte plants. The flora is dominated by the marsh grass, Spartina alterniflora, Salicornia, Distichlis, and Juncus are important locally. The tidal marsh essentially ranges from M.S.L. to mean high water. Tidal-marsh deposits may be called either sediments or soils, because their characteristics are quite ambivalent. They are usually fine grained, silt to clay-size material rich in organic detritus. Sandy marshes occur peripherally to islands and land margins. They originate almost invariably from washover and blowover sand sheets formed during storms. The intertidal grass marshes are designated low marshes to distinguish them from supratidal high marshes, which are notable for their greater diversity of plants, including shrubs and bushes. High marshes occur along most coasts of the world (Chapman, 1960), but on the Georgia coast are restricted to remote parts of the estuarine watersheds and to those areas that have been built up above the mean high tide level by deposition of washover and blowover sand.

Intertidal faunal assemblages I n salt-marsh estuaries of the southeastern United States, which have relatively high salinities and large tidal ranges, the intertidal realms host a rich fauna of benthic macro-invertebrates that contribute significant quantities of shell material to the sediment. Three faunal assemblages may be distinguished: (1) The assemblage of sand and mud dwellers, a highly diverse fauna of more or less vagrant benthos, lives in and upon the shallow subtidal to lower intertidal sediment; molluscs are represented by numerous different clams and snails, Mercenaria mercenaria being the most characteristic member of the assemblage. (2) The oyster biocoenosis, associated with the reef-building estuarine oyster Crassostrea, consists chiefly of sessile benthos and is restricted to the lower intertidal zone. (3) The tidal marsh assemblage is confined to the grassy marshes of the higher intertidal zone. • Species diversity is greatest in the assemblage of sand and mud dwellers, which flourishes just below and above mean low water, preferably in muddy sand and sandy mud accumulating beyond the influence of reduced salinities associated with major river mouths. This assemblage is basically an impoverished foreshore and shoreface fauna, together with several species more typical of lagoonal and estuarine environments. A selected number of characteristic species were discussed by Frey and Howard (1969). In sandy deposits and firm muds of tidal rivers and creeks west and north of Sapelo Island (Fig. 1), where salinities rarely drop below

106

H . U . WIEDEMANN

20 parts per thousand, the following species are ubiquitous and contribute hard parts to the sediment; they are listed in order of decreasing relative abundance: Pelecypods Abra aequalis Say Mulinia lateralis Say Mercenaria mercenaria (Linn6) Tagelus plebeius Solander Cyrtopleur a costa ta (Linn6) Macoma constricta (Brugui6re) Macoma balthica Linn6 Mya arenaria Linn6 Ensis directus Conrad A nadara o valis (Brugui6re) Noetia ponderosa Say Glyeymeris americana (De France)

Gastropods llyanassa obsoleta (Say) Nassarius vibex (Say) Terebra dislocata Say Busycon canaliculata (Linn6) Busycon carica eliceans (Montfordl Busycon carica Gmelin Urosalpinx cinerea (Say) Odostomia impressa (Say) Polynices duplicatus (Say) Fasciolaria hunteria (Perry / Crepidula.lbrnicata ( Linne )

Commonly attached to shells lying on the sediment surface are barnacles (Balanus), bryozoans, and serpulid worms. Many shells are also bored by Cliona. Additional clams reported from the same environment by Frey and Howard (1969) include Tagelus divisus, Solen viridis and Mercenaria campechiensis. Several boring clams listed above also infest semi-consolidated sediments wherever they crop out in the shallow subtidal to lower intertidal zone: Crytopleura, Petricola, Tagelu,s and Mya, with Barnea truncata Say as an additional species there. The Crassostrea biocoenosis occupies the lower intertidal zone of the Georgia salt-marsh estuaries and may overlap partly with the assemblage of sand and mud dwellers described above. Oyster beds can support a large, diverse fauna, Wells (1961), who studied the ecology of oyster beds in the Newport River estuary in North Carolina, listed 303 species, most of which are associated intimately with the oyster biocoenosis. Under adverse conditions, such as lowered salinity or" extended subaerial exposure in the intertidal zone, species diversity decreases The intertidal patch reefs thus constitute a very limited assemblage; ribbed mussels (Brachiodontes, Modiotus) and barnacles (Balanus) are the most important contributors of shell material besides oysters. The only common oyster is Crassostrea virginica (Gmelin); Ostrea equestris Say is quite rare. Boring organisms are generally absent, except for perforating algae. Gastropod shells are found in reefs but are usually carried there by hermit crabs from the tidal flat and subtidal assemblage described above. Also very common are the crabs Rhithropanopeus harrisii (Gould), Uca pugnax (Smith) and U. pugilator (Bosc), the most durable parts of which are the tips of the claws. The oyster reefs develop on tidal mud and sand flats, creek banks, and on the floor of small tidal gullies and crevasses wherever firm objects have paved the ground for the oysters to settle, normally scattered shells, but also driftwood and

SHELL DEPOSITS AND SHELL PRESERVATION IN ESTUAR1NE SEDIMENTS

107

man-made constructions. Such reefs are illustrated in Galtsoff (1964, fig.365,367) and Frey and Howard (1969, plate I, fig. 4, 5). Well-developed reefs are somewhat rectangular in cross-section, having a rather flat top and steep sides rising 1-1.5 m above the mean low water level. Undisturbed colonies have a characteristic internal structure resulting from the general upward growth of tall oysters tightly spaced in the reef. The reef top may be covered with shell rubble, or may serve as a mud trap. Sediments so trapped are settled ultimately by Spartina grass and buried under salt-marsh soil, while reef accretion proceeds laterally. Below the mean low water level, development of reefs is effectively prevented by predators, particularly the carnivorous gastropods listed above. Abundant shells are also deposited on the subtidal talus slope. Having been broken off the reef by wave action, many oysters remain articulated and clumped together, but the talus lacks true reef type structure. Much of the shell material is affected by the boring sponge Cliona (Plate I, 5). The shell material scattered around the reefs aids in building up the channel floor and in paving the ground for the reef to spread laterally. Where the active reef is only 1 m thick, for example, the entire shell deposit is thus apt to be somewhat thicker than that, the lower part being composed of structureless rubble and the upper part of oysters in growth position. The reef structure hosts great numbers of barnacles and mussels. The small mussels Brachidontes exustus (Linn6) and B. recurvus (Rafinesque) prefer the lower one meter of the reef, whereas the large mussel Modiolus demissus (Dillwyn) occurs only above M.S.L. in the higher part of the reef and on the reef top, leading over to the tidal marsh assemblage, where it is one of the major faunal elements. The reason why barnacles are rare or absent in higher parts of oyster reefs but do flourish even up to the high water mark on pilings and stems of Spartina grass remains unclear. The tidal marsh assemblage is dominated by several species of burrowing crabs (Teal, 1962; Frey and Howard, 1969), whereas molluscs are represented essentially by two species, Modiolus demissus (Dillwyn) and Littorina irrorata Say. The small pulmonate snail Melampus bidentatus is rare. Littorina, which is unevenly distributed through the marsh, crawls about on the sediment surface and the grass. A count of shells in a place having an optimum population yielded 190 adult specimens per m 2, 178 living snails and 12 empty shells and fragments. Similar numbers were reported by Odum and Smalley (1959). Modiolus, which may be as much as 13 cm in size, is oriented vertically in the substrate and only its upper edge appears above the sediment surface. Its byssal threads are attached to other shells or to plant roots and rhizomes, and it typically occurs in clusters comprised of juvenile, mature, and dead specimens. According to Kuenzler (1961), the mean population density is 6.66 specimens per m 2 although 46'~, of the total population is concentrated in tall stands of Spartina at tidal-creek heads. Here an average of 52 individuals per m 2 was counted. These data do not include empty shells, isolated valves, or fragments, which also contribute to the shell content of the sediment. Mercenaria mercenaria and Crassostrea virginica may be found in low parts of the

108

H . U . WIEDEMANN

PLATE I

1. An average-size Crassostrea gigantissima from the Tertiary of Griffin Landing, Georgia (lefD ~> compared to a large-size modern Crassostrea virginica (right). 2. Sculpture prints of ribbed mussels on oyster shells from the Tertiary (left) and from a recent saltmarsh estuary in G e o r g i a . Crassostrea virginica grown on Modiolus demissus (right). 3. Casts of ribbed mussels from the Tertiary o f Griffin Landing, Ga. 4. Large barnacles (Balanus) on two oyster fragments from the Tertiary (left) and on a recent Crassostrea virginica (right). Note the superficial leaching of the Tertiary barnacles and some boring by Cliona in the oyster fragments. Same scale as 5. 5. Destruction of recent oyster shells by the boring sponge C/iona in the subtidal realm of a Georgi~ salt-marsh estuary.

SHELL DEPOSITS AND SHELL PRESERVATION IN ESTUARINE SEDIMENTS

109

tidal marsh, particularly at the fringes, and also in poorly drained areas behind the natural levees of large creeks. Here the oysters commonly settle on Modiolus clusters but do not form reefs.

Recent intertidal shell deposits The fauna of sandy and muddy low intertidal to shallow subtidal deposits is too sparse to form sizable shell aggregates. Yet the infauna of clams and snails in undisturbed sediment is well suited for in-situ preservation during episodes of rapid sediment accumulation. Surface samples and cores from recent shallow estuarine sediments generally contain some characteristic representatives of this fauna. llyanassa, Mulinia, and Abra are particularly numerous and are usually well preserved. The bulk of all salt-marsh estuarine shell deposits is furnished by Crassostrea. Two kinds of deposits are common: buried reefs, and concentrations of reworked shell material. Gradations between these two are common too, because oyster spat prefer to settle on shell accumulations below M.S.L. The fecundity of oysters enhances rapid accumulation of enormous quantities of shells. Oyster colonies are best developed and most abundant along the margins of major sounds and tidal inlets, such as Doboy and Sapelo Sounds (Fig.2, 3), and also along wide tidal channels. In all narrow tidal creeks oyster colonies are insignificant, but are

land

tidal •

flats

concretions

oyster found

reefs

cheniers

road

............. d i k e

Fig.2. Shoals, tidal flats and marshes with associated oyster colonies and shelly to sandy cheniers or beach ridges adjacent to Doboy Sound inlet. S . E . C . ~ South End Creek; R . M . ~ Reynolds' Marsh (diked 1953) where fossiliferous concretions were found; U G a = University of Georgia Marine Institute.

| 10

H . U . WIEDEMANN

....

land

:

tidal flat

oyster r e e f

chenier

shell dep.

Fig.3, Shoals, tidal flats and marshes with associated oyster colonies, subsurface shell deposits. and sandy to shelly cheniers or beach ridges in Blackbeard Marsh adjacent to Sapelo Sound intel a, b: sites of cross-sections Fig.4 a and b.

more conspicuous in small gullies and crevasses that flush and drain the vast tidal marshes. This peculiar distribution presumably may be attributed to a number of different circumstances, one of which is the provision of nutrients. Other limiting factors may be the general lack of firm substrates on the muddy banks of small creeks and the common slumping of the fine-grained deposits, which endangers any oyster colony that does get established here. According to Ragotzkie (1959), slope failure of the steep creek banks is triggered by heavy rains and may be seasonally controlled. My impression is that the burrowing activity of crabs greatly promotes slope failure and might well be the primary cause. At the margins of' large channels and sounds, by comparison, wave action not only provides nutrients and oxygen but also prevents silt accumulation on the reef and promotes development of gentle slopes and extensive tidal flats for the oysters to settle on (Fig.4). Reefs in these exposed positions are more vulnerable to storm waves~ but the positive result is that shell debris scattered during rare storm surges prepares the ground for spat to set during normal times, thus enabling reefs to spread laterally over bare tidal flats. Indeed, very exposed colonies consist almost entirely of rubbly shell ridges (Fig. 4, section b). Formed by the surf these ridges are asymmetrical in cross section. The gentler slope faces the open waters of the channel or sound and is occupied by isolated clumps of living oysters (Fig.5). Commonly the shell debris is wedged tightly in vertical position, thus forming a very coherent pavement under the impact of strong waves. This kind of wedging has also been observed on

SHELL DEPOSITS AND SHELL PRESERVATION IN ESTUARINE SEDIMENTS

111

Section e (No vertical exaggeration) NE

SW

B,ackbeard m.h.t. - . . . . . . . . . Cree M&L . . . . . . .

ldllllll

'2

~.---.-

• 1 0

I'~--" (n ~'.

÷

m

NE

Section b (Vertical exaggeration x5)

Sapelo Sound . . ,

Shell rages

scarp .

SW

shell,~chenier ~::,xvJ vJ x.,~,

mllllll i I LI,mm, '' ~-~¢~-'~

. . . . . . . . . . . . . . . . . . . . . . . . . .

m

[*23 |*

......

:!~i~:~!~i~!~!!!!!~``~!!~:::~[~!~i~]~::~i~!i!i~i~i~!!~::i:::::!i]~i!i!:!~!!i~!~!!!i!i!i~i!!i~i~iO ~i!i~i~)i~. V'~'Y"

....

Borrichia

oysters

ll~l,I . . . . . Spartina

Salicornia

mlm mud

~

m

shell d e p o s i t s

Fig.4. Cross-section a, through the bank of Blackbeard Creek, shows subsurface shell accumulations and tidal-marsh soils, also encountered in a core taken nearby. For section locations see Fig.3. Cross-section b, through the bank of Sapelo Sound, shows subtidal m u d deposits, intertidal shell accumulations and marsh soils, overlain by a shell chenier with associated washover fan on its landward side. Note the erosional scarp in the m a r s h soil. Density distribution of living oysters and zonation of halophyte plants are indicated, the varying height of S p a r t i n a grass is symbolized, m . h . t , mean high tide; M . S . L . -- mean sea level; m . l . t , mean low tide.

sand

lOm

~

shell deposit hying o y s t e r s

d 68 =nnel

rnorker

~and

living oysters Dshell

deposit

sand with shells I*-sandy

mud lOre

Fig.5. M a p and cross-section of a lower intertidal sand bar topped by an asymmetric shell ridge at channel marker No. 168 in the Intracoastal Waterway west of Sapelo Island. C o n t o u r interval 1 ft. (30 cm).

112

H.U. WIEDEMANN

shell ridges along the coast of Essex (Greensmith and Tucker, 1969, fig.5). The steeper slope functions as the avalanche slope and progrades under surf action. The shell material, a thanatocoenosis, consists predominantly of oysters but typically contains mixtures of faunal elements from all three intertidal assemblages. The sturdy shells of Mercenaria, llyanassa, and Busvcon are particularly common. In places where neither sizable reefs nor shell ridges develop, oysters may still grow in scattered clumps that are uprooted and transported easily by strong surf Many of the bivalves remain articulated during the transport, but their accumulations do not resemble a reef. The resulting sediment consists of more or less sandy mud with oysters and other shells admixed. In summary, the shell content of sediments in protected parts of salt-marsh estuarine watersheds is insignificant and is confined to lenses, pockets, and small but well-structured reefs enclosed by generally muddy sediments. Widespread shell deposits form almost exclusively in the intertidal to shallow subtidal environment adjacent to major sounds. During the process of soundward progradation of the marsh facies, the shell concentrates become sandwiched between subtidat estuarine sediments below and salt-marsh soils above (Fig.4). In the Blackbeard Marsh south of Sapelo Sound (Fig.3) the sah-marsh soils in the area of half a square mile adjacent to the open sound are underlain b? more or less continuous layers and lenses of shell concentrates. The subsurface accumulations were mapped by means of a 4 m long steel probing rod. The top of the deposits was encountred at 0-3.5 m below the marsh surface, often at a depth corresponding to the present sea level, or even lower. The deposits were every~ where at least half a meter and usually one to several meters thick. Two cores were taken (see Fig.3), but only one was long enough to penetrate the entire sequence of marsh mud and shell concentrates overlying muddy sand with a characteristic infauna of clams (Table I). The faunal analysis of the core suggests that the upper 3.8 m of sediment were deposited intertidally, although this range exceeds the present mean tidal range which is 2 m (Fig.4, a). The excess thickness points to a synsedimentary rise in sea level or compaction of the underlying sediments or both. Carbon-14 dates were obtained on oyster shells from two sections of this core. The bottom of the subtidal shell rubble, at about 4.15 m depth (Sample no. UGa93), dated 1640 i 150 years B.P. An articulated oyster from the bottom of the structured oyster reef facies, at 2.80 m depth (Sample no. UGa-95), dated 1720 90 years B.P. The matrix of the shell concentrate is more or less sandy mud, fining upwards, whereas the shell material itself contributes only 10~,; of the sediment volume. Since the mud consists of approximately 70% by volume, of water, the shell content has to increase considerably during compaction of the sediment.

Recent supratidal shell deposits Beach ridges, cheniers, and washover fans composed chiefly of oyster shells form at the margins of major sounds, particularly near the inlets, where storm

SHELL

DEPOSITS

AND

SHELL

PRESERVATION

IN

•=

~'gu!doqvUO!lD

ESTUARINE

1 13

SEDIMENTS

~

P

.~

.~

P

E

o°

:

:

:

::

.:

•:

o:

::

.:

•

oo

~uvozoa:t~ s'nuvlv ff .~',*lvp q m D nt. utol~opo

xu!dlw'O~l~ s,n/.~t~s,s,oN

i~s'~'vuv, fll DU!JOII!7 mqv ,¢

~a ~.

n?u!lnl~ <

.s'nla~vz ©

"~

o!avua,~aal4~

~a

z

:

.s'nlo!pol4I

:

•

<

.~

•:

s'aluop!qav.tff

o:

.:

,:

•

.I .k

z z

°°.~ls'O

.......

t~9,tlgogs'~.tD

•

<

g

.

• • oo

%........:

,-4

.

.

OoO • •

,4

• :o:

114

H . U . WIEDEMANN

surges are most effective (Fig.2, 3). Where the sounds are bordered by tidal marshes, the shell material is piled up on the marsh surface to form narrow ridges and low washover fans. The shells are fragmented and waterworn where wave energy is relatively high. Tight vertical wedging of shell debris is as common here as on intertidal shell ridges. Actively forming ridges are barren, having steep slopes of as much as 40' on both the landward and the seaward sides. They are elevated rarely more than 2 m above the marsh surface (Fig.4, b). Associated washover fans are characterized by a lower profile and a lobate or tongue-like shape having a gentle slope on the soundward side and a steep avalanche slope encroaching upon the marsh. On the fan surface the shells may be either wedged vertically or imbricated and aligned with the prevailing current direction. The shell material includes faunal elements from all three intertidal assemblages derived from reworked fine grained intertidal to shallow subtidal sediments. Sand is admixed where available, and near tidal inlets the shell ridges merge into sandy beach ridges. Georgia salt-marsh cheniers are small features compared to the cheniers of southwestern Louisiana (Russel and Howe, 1935), but are similar to shell ridges described from New Zealand (Chapman and Ronaldson, 1958) and England (Greensmith and Tucker, 1969). Some cheniers are sufficiently large to be mapped and designated as " h a m m o c k s " on charts of the Coast and Geodetic Survey. Older cheniers and washover fans found in the marsh interior IFig.2, 31 are overgrown by a high marsh flora, often including terrestrial bushes and trees. These ridges may now be situated as much as half a mile away from the presem sound margins, although they once formed right next to a sound or large channel. They thus testify to the former positions of sound limits and to the steady migration of tidal fiat and marsh facies outward into the sounds. The distinct shape of the ridges and the thorough fragmentation, imbrication and wedging of shells on their slopes are generally sufficient to distinguish such deposits from Indian kitchen mkldens, which also occur in this area. In addition to oysters, the heavy shells of Mercenaria and Busvcon carica eliceans are commonly preserved. Preservation of skeletal material Two general conclusions can be drawn concerning the preservation of skeletal carbonate in salt-marsh estuarine sediments: (1) the higher above M . S . L shells are deposited, the more subject they are to terrestrial weathering processes that dissolve calcium carbonate; and (2) the longer shells remain at or near the sediment surface, the more likely they will be destroyed. Because the sedimentation rate is generally slight at high-tide levels and increases toward lower levels, where shells can then be buried more quickly, the above relationships work in conjunction. High marsh soils thus tend to be void of skeletal carbonates, although chitinous remains of crabs may be preserved (Stevenson and Emery, 1958). Also carbonates in supratidai cheniers, overgrown with trees and shrubs, are subject to chemical weatherin~ and cannot persist for lone.

SHELL DEPOSITS A N D SHELL P R E S E R V A T I O N 1N ESTUARINE SEDIMENTS

c5

~5

~ r-, o r.,

÷

,v ¢q

~5 .d

©

a~

z -._

¢-i

¢,i

.< tr~ Z Z ©

v

Z

0

z o D

U

r, .<

t~

=

~

o

<

z

z

©

1 15

116 P L A T E II

H. U. W I E D E M A N N

SHELL DEPOSITS AND SHELL PRESERVATION IN ESTUARINE SEDIMENTS

117

Low marsh soils are interlaced with plant rhizomes and roots, contain decaying organic matter, and are exposed regularly to tidal flooding as well as rain water (Table II). The average of sixty in-situ pH measurements in the top 50 cm of tidal marsh and mud flat sediments is 7.3 (maximum 8.4, minimum 6.2), indicating the general absence of strongly acidic soils in the intertidal zone. Nevertheless, if one considers a pH of around 8 as equilibrium pH for calcium carbonate, the pH in intertidal sediments is too low for carbonates to be stable. For shells exposed at the sediment surface, biochemical attack by algae is an additional factor. On the other hand, skeletal carbonates often show a surprising resistance to leaching that is attributed to the protective conchiolin basis of the shell fabric (Kennedy and Hall, 1967). Although the marsh periwinkle Littorina is abundant, one rarely finds empty shells at the marsh surface that are not severely leached, having holes in the whorls and the spire destroyed (Plate II, 1), Comparison of living and dead Littorina collected from the surface of the Spartina marsh at two sites in the South End Creek Marsh (Fig.2), where the water-soaked surface mud had a pH of 6.85 and 6.2 respectively, yielded living to dead ratios of 3,4/1 and 15/1, reflecting the fast rate of dissolution of empty shells deposited at the surface. A number of well-preserved, empty shells were inserted physically into the top 3 cm of soil (the root zone) at these two sites. After one year the shells were recovered, and all of them showed evidence of severe leaching. Mussels living in this surface soil protect their shells with a strong periostracum, and the living to dead ratios observed were more favorable for them: 1/3.6 and 1/2, respectively. Nonetheless, even the valves of living mussels exhibit signs of severe leaching wherever the periostracum is damaged, most commonly at the umbones and on the postumbonal ridge, but also at the upper edge of the shell that is exposed to top soil, surface waters, and algal attack (Plate II, 2). Marsh mussels can often be distinguished from mussels that grow in the open framework of oyster reefs and do not show this characteristic pattern of etch marks. P L A T E I1 1. Shells of Littorina irrorata, gathered from the surface of a tidal marsh at Sapelo Island, show progressive destruction by leaching in contact with soil and surface waters. Scale on left. 2. Valves of Modiolus demissus from tidal-marsh soils at Sapelo Island exhibit characteristic damages of the dark periostracum and progressive destruction by leaching in contact with soil waters. Note that m u c h of the leaching takes place while the mussel is still alive and able to grow by accretion at the upper margin. Same scale as 3. 3. (a~[). Concretionary calcite formed around and within subfossil mussel shells from Reynolds" Marsh at Sapelo Island (text Fig.2). Mussel valves are cemented together (a). Littorina shells are attached to mussel valves (b, c). The concretionary material may show shrinkage cracks (c, d) and is generally associated with severely leached shell remains (b, e) in contact with rootlets of saltmarsh plants (d, e, f ) . Scale on left. 4. (a~[). Concretionary calcite around and within subfossil shells of Littorina and one Mulinia (f) from the same site as the mussels above. Both sides of a fossiliferous concretion are shown in d. Strong leaching of the shells is c o m m o n . Scale on left.

118

H.U. W1EDEMANN

All shell material incorporated into muddy estuarine sediments experiences a certain amount of leaching, thus weakening the shell fabric. Even seemingly well preserved shells recovered from cores or outcrops of older Holocene and Late Pleistocene sediments are generally brittle. Piston coring through thick layers is therefore possible. The shells of small snails, particularly Littorina, may be found in a peculiar pasty condition. X-ray diffraction analysis of this carbonate paste shows that the aragonite is not converted into calcite and that the process is disintegration rather than recrystallization of the shell fabric. During compaction of muddy sediments the weakened shells are apt to fracture and collapse, thus resembling flattened fossils known from numerous ancient shales. Frequently the carbonate shells become coated or impregnated with iron sulfide, precipitated first as monosulfide (FeS) that converts to pyrite (FeS2) during early diagenesis to give the shell a bluish-gray coloration. Shells of Littorina and Modiolus, also vertebrate bones and teeth that I buried in the upper half meter of different tidalmarsh soils lbr one year were patchily coated and impregnated by black iron monosulfide upon recovery. Since iron monosulfide is soluble in acids but not i1~ neutral or alkaline solutions, this process is probably related directly to the observed leaching of shells mentioned above and the resulting local rise in pH ol the interstital water. These observations shed light on the pyritization of subfossil shell material recovered from estuarine muds in Hynes Bay, Texas, by Shepard and Moore (1955, p.1555).

Conditions of fossilization of tidal-marsh molluscs A large number of shells of Littorina and Modiolus were buried in the top half meter of different tidal marsh soils in the South End Creek Marsh (Fig.2), to be excavated after one year. Observations made on the condition of shells arc summarized in Table IlI, together with analyses of pH, chlorosity, alkalinity, and dissolved calcium for a few samples of soil water drained from the soils at the time of excavation. In the table is also noted whether naturally occurring fossil shells have been found at the excavation site. The chemical data may be compared with the equivalent parameters of surface waters collected in the same area (Table If). As indicated by this study, calcareous shells have little chance of escaping dissolution in the well-drained soils of overbanks and natural levees near tidal creeks, represented by site no.l, (Table III). Here the soil-water composition approximates that of local sea water, except for the detrimentally low pH. In the poorly drained muds of the marsh interior (represented by sites no. 2, 3), chances of preservation are better, provided the shells are buried quickly and deeply in fine grained sediment. Once enclosed by the relatively impermeable mud, shells are leached very slowly. The surrounding interstitial water is swiftly saturated with respect to calcium carbonate, assumes a pH above 7, attains a high alkalinity, and thereby becomes less reactive. Such conditions would account for the local occurrence of subfossil marsh faunas in tidal marsh soils, as encountered at site no. 2.

SHELL DEPOSITS AND SHELL PRESERVATION IN ESTUARINE SEDIMENTS

I 19

TABLE III PRESERVATION OF SHELLS OF

Modiolus

AND

Zittorina AFTER (Fig.2) A N D

OF T I D A L M A R S H SOILS N E A R S O U T H E N D C R E E K

ONE YEAR OF B U R I A L IN T H E TOP H A L F METER C H E M I C A L DATA ON SOIL W A T E R AT T H E DAY

OF EXCAVATION 1

Location numbers 1

2

3

4

Site description

overbank near tidal creek, well drained

marsh interior, poorly drained

marsh interior, poorly drained

at margin of island, mean high water level, sand

Plants

tall Spartina

dwarf Spartina

Juncus and dwarf Spartina

Salicornia and Batis

Fossil shells at depth

absent

present

absent

absent

Effect on buried shells

severe etching

little or no etching

little or no etching

slight etching

Soil water pH

6.3

7,05 7.35

7.3-7.4

6.4

C l - (chlorosity)

16.10 g/1

19.17 g/1

17.65 g/l

25.00 g/l

Alkalinity

3.24 mequiv./1

30.44 mequiv./1

8.65 mequiv./I

5.55 mequiv./l

Specific alkalinity

0.0002

0.0016

0.0005

0.0(102

Ca 2+

0.387 g/l

0.506 g/l

0.460 g/1

0.590 g/l

Ca/C! ratio

0.0240

0.0264

0.0260

0.0236

1Concentrations are given per 1 l at 25'C

A more effective way of fossilizing the tidal-marsh assemblage, however, is through the sudden burial of a marsh surface with its infauna under a sheet of washover sand. When one digs through sand sheets, which are associated with modern barrier beaches east of Sapelo Island (Fig.2) and unconformably overlie older marsh soils, one invariably encounters at the contact indigenous populations of mussels, generally well preserved and in living position. Such a marsh surface, once covered up by washover sands and dunes is now exhumed on the foreshore of Cabretta Island (Fig.l). The complete sequence of fine-grained deposits there is as much as 3 m thick, topped by a veneer of sandy peat developed from the dense root systems of high marsh plants. The peat is underlain by firm gray mud containing Spartina stems and rhizomes and an exhumed mussel population. The mussels in this in-situ assemblage occur in clusters and many valves exhibit the typical

120

H.U. WIEDEMANN

etch patterns of marsh mussels described above. Shells from this deposit (Sample no. UGa-101), dated by 14C, yielded an age o f 1025 -~ 300 years B.P. In a partb cularly dense population twenty-five large individuals were counted per m 2, A large number of Littorina shells were also gathered. At somewhat deeper levels in the intertidal zone this assemblage mingles with lenses, pockets, and small bio.herms of Crassostrea, which also contain crab claws and numerous shells of lh,anassa. By their periostracum, these snail shells are better protected against leaching than Littorina and are thus more abundant and generally better preserved. Washed out by the surf, llyanassa shells are commonly reocupied by living hermit crabs.

Formation o/'calcareous concretions Numerous contemporary carbonate concretions were found in association with an indigenous tidal-marsh assemblage buried under a 40 cm thick sand sheet in Reynolds" Marsh near the University of Georgia Marine Institute (Fig.2). Formed in the mud, the concretionary calcite was found cemented to shells and shell aggregates of Modiolus and Littorina. It was noted that only those shells that were severely leached in intimate contact with decaying roots exhibited concretion formation (Plate II, 3, 4), The in-situ pH of the sediment was 7.1 at the time of excavation. Because of the complex history of Reynolds' Marsh, the conditions under which the concretion formation took place can hardly be reconstructed. The mussel population was buried under the sand fan before the marsh was diked in 1953. Sediments were then exposed to freshwater until 1963. when the area was flooded again by sea water and reverted to a tidal marsh. Basically, the process appears to have been: (1) solution of skeletal aragonite; (2! diffusion of the dissolved solids; and (3) precipitation of calcite in the interstices of the mud near the sources of calcium and carbonate. Little evidence indicates that intermediate stages, as suggested by Berner (1968), were involved in the calcite formation. A case for calcium soap formation could possibly be made where the calcite precipitated preferentially in the interior of articulated Modiolus valves around the points where once adductor muscle and tissues were attached. L A T E P L E I S T O C E N E E S T U A R I N E S H E L L DEPOSITS

Shell deposits, especially of Crassostrea virginica, are widespread in Pleistocene deposits of the coastal plain as well as on the continental shelf of eastern and southern North America. The shells are often used for the absolute dating of Late Pleistocene and Hotocene sea levels and shore lines (Milliman and Emery, 1968). Three Late Pleistocene examples will be given for the Georgia coast, A shell bed crops out at Pumpkin Hammock (Fig.l) on the bank of Duplin River west of Sapelo Island that has been dated at more than 37,200 years B.P. (Hoyt et al., 1966). The bed is exposed in the lower intertidal zone and extends laterally about 250 m. It is about 1 m thick, and consists of firm gray mud having about 10~;,

SHELL DEPOSITS AND SHELL PRESERVATION 1N ESTUARINE SEDIMENTS

] 21

oyster shells by volume. The shells are well preserved but brittle; the red stain of the muscle scars is retained frequently. Reef structures are recognizable in the upper half meter of the shell concentrate, formed by articulated oysters little bored by Cliona. This bed attests to a Late Pleistocene sea level that happened to coincide with the present level. At shallow depth the oyster bed is underlain by sandy mud containing only scattered oyster shells and a faunal assemblage characteristic for the low water level-i.e., some Clione borings in shells, numerous llyanassa, barnacles, crab claws, Odostomia, and Abra. In addition Tagelus pleheius and Cvrtopleura costata were found articulated in living position. During the Sangamon Interglacial, when sea level stood 7.5 m higher than that along the present Georgia coast~ a system of barrier islands and salt-marsh estuaries existed there quite similar to present-day ones (Hoyt and Hails, 196/). The fluviatile, aeolian, estuarine, and marine deposits of that period are included in the Pamlico Formation and contain many rich fossil deposits (Richards, 1965). A drill core (no. G-3) taken by Thomas Logan 13,3 miles west of Brunswick (Fig. 1) at an elevation of 4.5 m above sea level, penetrated through 2 m of clayey sand and gray clay--presumably former salt-marsh sediments--overlying shell concentrates (Logan, 1968 ; Logan and Henry, 1968). No shell remains other than Crassostrea virginica were recognized. Caliche-type carbonatic material indicated progressed weathering, With the kind permission of Mr. Logan I have re-examined the core. The shell concentrates consist of two beds, the upper from 2 3.35 m depth and the lower from 4.9-5.8 m depth. The layers are separated by gray clay containing scattered oysters. The sequence within the lower layer commences with a basal conglomerate made up of shells and shell hash in a sandy matrix. Rounded pebbles up to 3 cm in diameter are admixed, consisting of quartz, sandstone, and limestone. This conglomerate grades upward into a shell concentrate having a clayey matrix. All the shell material is extensively bored by Cliona. The layer is interpreted as a subtidal channel lag deposit. The upper shell bed is composed of reworked shells in a clayey matrix. Since the shells are much less bored by Cliona, this layer might represent an intertidal deposit. The Pamlico Formation crops out in several bluffs along the St. Marys River (Fig.l). On the Florida side of the river, at Reids Bluff, a lens of gray clay as much as 5 m thick is included in the generally sandy sediments of this formation. The common occurence of the characteristic burrows of Callianassa mqior Say (Weimer and Hoyt, 1964) in the sands below and above the clay deposit suggests accumulation of the clay near sea level. The top of the lens is more or less horizontal and lies about 6 m above present mean sea level. The sand below the clay deposit is topped by a dark brown zone interpreted as the B-horizon of a paleopodzol that was trangressed by the rising sea. Included in the clay deposit are several lenticular bodies of shell concentrates in a clayey matrix. Individual lenses are as much as 2 m thick. They contain less than 15)o shell material by volume, almost all being Crassostrea virginica. The bulk of the shell material has been

122

H . U . WIEDEMANN

reworked, much is fragmented and bored by Cliona, some is water-worn. Reel'type structures have not been recognized. Only a few clams were found articulated and in living position. Minor faunal elements include Mercenaria mercenaria, Modiolus demissus, Cyrtopleura costata, Tagelus plebeius, Ilyanassa obsoleta, Littorina irrorata, Busycon carica, Diodora cayenensis, and rare ark shells. EOCENE-OLIGOCENE ESTUARINE SHELL DEPOSITS

When the sea stood high on the coastal plain during the Tertiary, oyster beds formed in the area of Burke County, Georgia (Fig. 1). According to Herrick and Counts (1968), the age of the deposits is Eocene (top of McBean Formation) or Lower Oligocene (Cooper Marl equivalent). The oyster beds crop out at blufl~ above the Savannah River about 20 miles inland from the present coast, at altitudes between 100 and 200 ft. (30-60 m). Descriptions of the outcrops and some typical stratigraphic profiles were presented by Veatch and Stephenson (1911, pp.243-251). The shell concentrates consist primarily of Crassostrea gigantissima (Finch), redescribed by Howe (1937). The largest reported specimen exceeded 50 cm in length, and oysters 30 cm long are quite common; the recent C. virginica does not reach such a size (Plate I, 1). The shells are embedded in light gray, more or less sandy clays. Individual oyster beds are as much as 9 m thick, as for example at Shell Bluff. Here the shells are usually disarticulated, and many are bored by Cliona and pholadids. Minor faunal elements are barnacles, ribbed mussels, and bryozoans. Kilbourne (1969) also noticed abundant echinoid spines. Some microfossils from the bed were described by Herrick (1960). At Griffin Boat Landing an oyster bed 7 m thick is well exposed on the river bank. It is generally divided into two layers by an irregular clay layer (Veatch and Stephenson, 191 l). Coarse quartz sand containing mica flakes, shell debris, and shark teeth forms lenses within the lower bed. The foraminiferal fauna from the shell layers was described by Herrick (1964). Oysters are abundant and well preserved, even fragile valves of juvenile specimens, and many are articulated Individual clumps are common, but do not form coherent reef structures. Large barnacles are found attached to shells, and some shells are bored by Cliona and pholadids. The valves of the boring clams are frequently preserved in their borings. In the lower shell layer ribbed mussels are also conspicuous, although their shells are invariably dissolved and only molds in the sediment are found or occasional sculpture prints on oyster valves, which were once attached to mussel shells. The mussels are often articulated also. Judging from the best preserved molds they belong to the genera Modiolus or Brachidontes, or both. Specimens as much as 6 cm in length, having strong postumbonal ridges, resemble closely in size and shape the modern Atlantic Ribbed Mussel, Modiolus demissus (Plate I, 3). The oyster beds at Griffin Landing thus exhibit the typical Crassostrea assemblage. They constitute an autochthonous oyster bottom showing little reworking of the

SHELL DEPOSITSAND SHELLPRESERVATIONIN ESTUARINESEDIMENTS

123

articulated shell material in the lower layer, although the upper layer is in part reworked just like the oyster bed at Shell Bluff. These shell deposits are interpreted as shallow estuarine sediments. It is possible that the Savannah River had been established already in Eocene-Oligocene times and that the area of Burke County was a river mouth estuary. Kilbourne (1969) reached the same conclusion, based upon his study of the microfauna of the shell beds from both locations. He identified Elphidium texanum, Nonion laeve, Cibicides pseudoungerianus, C. sp., Valvulineria jacksonensis, and Buliminella elegantissima as the principal species, and compared this assemblage with a modern assemblage described by Miller (1953) from the Mason Inlet estuary in North Carolina. CONCLUSION Holocene and Late Pleistocene oyster deposits in Georgia compare well with deposits reported from estuaries along the Gulf Coast (Norris, 1953; Puffer and Emerson, 1953 ; Ladd et al., 1957; Parker, 1960; Shepard and Moore, 1960), from Chesapeake Bay (R. B. Biggs, Univ. of Maryland, personal communication) and from Massachusetts (Burbanck et al., 1956). Shell concentrates composed chiefly of Crassostrea are indicative of shallow estuarine environments, since oysters of this genus flourish only in coastal lagoons, sheltered bays, and river mouths having estuarine hydrographic regimes, i.e., reduced salinity or strong tidal influence, or both (Hopkins, 1957; Galtsoff, 1964). Only one exception of local peculiarity has become known (Wells and Gray, 1960) and does not affect this general concept. Crassostrea deposits contained in Quaternary and Tertiary sediments can thus aid in environmental interpretation. ACKNOWLEDGEMENTS This study was supported by National Science Foundation Grant GA-4619. i am indebted to the personnel of the University of Georgia Marine Institute for much practical help. Thanks to Leonard Bahr and Robert W. Frey for critically reading the manuscript and aiding with many suggestions. Three absolute dates on shell samples were kindly provided by Betty.Lee Brandau and Darwin Chapman, Geochronology Laboratory of the University of Georgia. This is contribution number 209 from the University of Georgia Marine Institute, Sapelo Island, Ga. 31327. REFERENCES Berner, R. A., 1968. Calcium carbonate concretions formed by the decompositionof organic matter. Science, 159:195-197. Burbanck, W. D., Pierce, M. E. and Whiteley, G. C., 1956. A study of the bottom fauna of Rand's Harbor, Massachusetts : an applicationof the ecotoneconcept. EcoLMonogr.,26 ;213 243.

124

H.U. WIEDEMANN

Chapman, V. J., 1960. Salt Marshes and Salt Deserts of the World. Leonard Hill, London, 392 pp. Chapman, V. J. and Ronaldson, J. W., 1958 The mangrove and salt-marsh flats of the Auckland Isthmus. N.Z. Dep. Sei. Ind. Res., Bull. 125:79 pp. Frey, R. W. and Howard, J. D., 1969. A profile ofbiogenic sedimentary structures in a Holocene barrier island-salt-marsh complex, Georgia. Tram. Gulf Coast Assoc. Geol. Soc., 19:427--444. Galtsoff, P. S., 1964. The American oyster. Crassostrea virginiea Gmelin. U.S. Dep. Inter. Fish. Ball., 64:480 pp. Greensmith, J. T. and Tucker, E. V., 1969. The origin of Holocene shell deposits in the chenier plain facies of Essex (Great Britain), Mar. Geol., 7:403425. Herrick, S. M., 1960. Some small Foraminifera from Shell Bluff, Georgia. Bull. Am. Paleontol., 41 117 130. Herrick, S. M., 1964. Upper Eocene smaller Foraminifera from Shell Bluff and Griffin Landings, Burke County, Georgia. U.S. Geol. Surv., Prof. Pap., 501-C:64-65. Herrick, S. M. and Counts, H. B., 1968. Late Tertiary Stratigraphy of Eastern Georgia. Guidebook 3rd Annual Field Trip (Oct. 1968j. Georgia Geol. Soc., Atlanta, Ga., 88 pp. Hopkins, S. H.. 1957. Oysters (annoted bibliographyl. Geol. Soc. Am., Mere. 67, 1:1129 1133 Howe, H. V., 1937. Large oysters from the Gulf Coast Tertiary. J. Paleontol., 11:355 366. Hoyt, J. H. and Hails. J. R., 1967. Pleistocene shoreline sediments in coastal Georgia: deposition and modification Science. 155:1541 1542. Hoyt, J. H., Henry, V. J. and Howard. J. D.. 1966. Pleistocene and Holocene Sediments. Sapelo Island. Georgia, and I/ieinitv. Geo/. Soe Am.. Southeast. Sect.. Guidebook Field Trip I (Apr. 1966 ~. Univ. Georgia Mar. Inst. Publ.. Sapelo Is.. Ga.. 27 pp Hoyt, J. H., Weimer, R. J. and Henry V.J.. 1963. Late Pleistocene and Recent sedimentation, Central Georgia Coast. U.S.A In: L. M. J. U. van Straaten (Editor), Deltaic and Shallow .Marine Deposits-Developments in Sedimentology, 1. Elsevier. Amsterdam, pp.170 176. Kennedy, W. J. and Hall. A.. 1967. The influence oforgamc matter on the preservation o['aragonite H~ fossils. Proc. Geol. Sou. Lond.. 1643:253 255. Kilbourne, R. T., 1969. Pa&oecoh*gy qf the Upper Eocene Outcrops at Shell Bluff and Gr(t]~n Landing.~, Burke County, Genrzht. Thesis. Uni~ Georgia. Athens. Ga., 20 pp., unpublished Kocl}, F F., 1956. The specific alkalinity Deep Sea Res.. 3:279 288. Kuenzlcr. E. J., 1961 Structure and energy flow of a mussel population in a Georgia salt marsh Limnol. Oceanogr., 6:191 204. Ladd, H. S., Hedgpeth. J. W. and Post. R.. 1957. Environments and facies of existing bays on the Cen tral Texas coast. Geol. So~. Am. Mere.. 67 ~2j:599 639. Land, L. S. and Hoyt. J. H.. 1966 Sedimentation in a meandering estuary. Sedimentology, 6 : 191 207 Logan, T. F., 1968. Pleistocene Stratigraphy in Glvnn and Mclntosh Counties, Georgia. Thesis, Univ. Georgia, Athens. Ga.. 103 pp., unpublished. Logan. T. F. and Henry, V. J.. 1968. Subsurface Pleistocene sediments and stratigraphy of the Central Georgia Coast. Abstr. Geol. Soc. Am.. Ann. Meet. Southeast. Sect.. Program, 53. Miller, D. N., 1953. Ecological stud 3 of Foraminifera of Mason Inlet, North Carolina. Contr. ('ushmau Found. Foram. Res. 4:41 43. Milliman. J. D. and Emery, K. D.. 1968. Sea levels during the past 35,000 years. Science, 162:1121 1123. Norris, R. M., 1953. Buried oyster reefs in some Texas Bays. J. Paleontol., 27:569 576. Odum. E. P. and Smalley, A. E., 1959. Comparison of population energy flow of a herbivorous and a deposit-feeding invertebrate in a salt-marsh ecosystem. Proe. Nat. Aead. Sci.. 45:617 622 Parker. R. H., 1960. Ecology and distributional patlerns of marine macroinvertebrates, northern (.iulf of Mexico. In : F. P. Shepard (Editor), Recent Sediments Northwest Gul! of Mexico. Am, Assoc Petrol. Geol., Tulsa, Okla., pp. 302- 337. Puffer, E. L. and Emerson, W. K., I953. The molluscan community of the oyster-reef biotope on the central Texas coast : J. Paleontol., 27:537-544. Ragotzkie, R. A., 1959. Drainage patterns in salt marshes. Proe. Salt Marsh Con.['., Mar In~t. Univ. Georgia, 1958:22 28. Ragotzkie, R. A. and Bryson, R. A., 1955. Hydrography of the Duplin River, Sapelo Island. Georgia Bull. Mar. Sei. Gulf Caribbean. 5:297 314.

SHELL DEPOSITS AND SHELL PRESERVATION IN ESTUARINE SEDIMENTS

| 25

Redfield, A. C., 1967. Ontogeny of a salt marsh estuary. In: G. H. Lauff (Editor), Estuaries-Am. Assoc. Adv. Sci., Publ. 83:108 114. Richards, H. G., 1965. Pleistocene stratigraphy of the Atlantic Coastal Plain. In: H. E. Wright and D. G. Frey (Editors), The Quaternary of the United States. Princeton Univ. Press, Princeton, N.J.,pp.129 133. Riley, J. P. and Tongudai, M., 1967. The major cation/chlorinity ratios in sea water. Chem. Geol., 2:263 269. Russel, R. J. and Howe, H. V. W., 1935. Cheniers of southwestern Louisiana. Geogr. Rev., 25:449461. Shepard, F. P. and Moore, D. G., 1955. Central Texas coast sedimentation: characteristics of sedimentary environment, recent history, and diagenesis. A m Assoc. Petrol. Geol., Bull., 39:1463 1593. Shepard, F. P. and Moore, D. G., 1960. Bays of Central Texas coast. In: F. P. Shepard (Editor), Recent Sediments Northwest Gulf of Mexico. Am. Assoc. Petrol. Geol., Tulsa, Okla., pp. 117 152. Stevenson, R. E. and Emery, K. O., 1958. Marshlands at Newport Bay, California. Allan Hancock Found., Occas. Pap., 20:109 pp. Teal, J. M., 1962. Energy flow in a salt-marsh ecosystem of Georgia. Ecology, 43:614~624. Veatch, O. and Stephenson, L. W., 1911. Geology of the coastal plain of Georgia. Ga. Geol. Surv., Bull. 26:466 pp. 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. Wells, H. W., 1961. The fauna of oyster beds, with special reference to the salinity factor. Eco/. Monogr.. 31:239-266. Wells, H. W. and Gray, I. E., 1960. Some oceanic sub-tidal oyster populations. Nautilus, 73 : 139 147.