ABSTRACT

Recent sediments in Mississippi Sound show interesting similarities to some ancient sediments in the Mississippi Embayment. The shallow mud bottoms of the Sound are being studied by the Mississippi Seafood Commission at the Gulf Coast Research Laboratory, Ocean Springs, Mississippi. The immediate purpose of the investigation is to determine how variations in mineral, chemical, and physical properties of the sediments affect the growth of oyster, shrimp, and fish.

In making this study three types of bottom were recognized: littoral, consisting mostly of sand; silty oyster bottom ranging from silt to fine sand; and muddy shrimp bottom consisting of clay or silty clay. All of these sediments contain the remains of brackish water, salt water, and even freshwater organisms, both megascopic and microscopic. The Sound acts as a faunal mixing bowl where salty waters of the open Gulf funneling between the barrier islands meet the fresh water from the mainland, especially during summer drouth.

Many ancient sediments show a similar mixing of fauna and sudden changes in lithology, especially the Upper Cretaceous Ripley and the less calcareous parts of the Selma on the outcrop in northeast Mississippi. There, oyster reefs, silty shales, argillaceous shales, and silty sands tongue in and out, just as their less consolidated equivalents do today on Mississippi's Gulf Coast. Cores and cuttings show the same confusing mixed fauna and sudden lateral and vertical lithologic changes down the dip in south Mississippi, in the more clastic beds of Eutaw (Eagleford), Selma (Taylor), and Ripley (Navarro) age.

We who have examined subsurface beds would like to see the outcrops of these beds, hence we take numerous field trips. But then, viewing the outcrops, we wish we could understand the manner in which these beds were laid down. In work with present-day sedimentation in Mississippi Sound answers have been found as to how some finer clastics are now being deposited. Likewise a few hints have been provided as to how some of the ancient finely clastic sediments were laid down in the Mississippi Embayment.

The occasion for this paper is an investigation of the shallow muddy bottom in the west part of Mississippi Sound. In Figure 1, the Sound is shown as a nearly landlocked body of brackish water between Mississippi's Gulf Coast on the north, the barrier islands on the south, and the delta of the Mississippi River on the west. The study was prompted by the need for cultivating oysters near shore where fresh water from the land dilutes the sea water. This dilution kills the saltwater predators and parasites, which in periods of low rainfall, move into the Sound with the influx of salt water.

End_Page 159------------------------

So, for the last several summers, the Mississippi Seafood Commission and the Gulf Coast Research Laboratory of Ocean Springs, Mississippi, formed a team of biologists, chemists, and geologists to examine some of the inshore bottoms. The object of the search is to find bottoms firm enough to support the old oyster shell to which the oyster larvae attach. In early 1953, four commercial sized oyster plantings were made, shown in Figure 2. Here muddy bottoms were found to be stiffened by recent additions of land-derived, angular quartz and chert, and fresh looking mica flakes. The success of these oyster plantings seems assured. A June 1954 visit showed many oysters up to three inches long and very few of the salt water enemies which had previously plagued the oyster beds.

The next several paragraphs explain the types of sediments now being deposited in the Sound, the sedimentary processes at work, and the relationships of the fauna which these sediments support. Here, only brief references are made to the probable consolidated equivalents of these sediments. Specific comparisons are reserved until later.

The first consideration is the petrographic and chemical constitution of the sediments now being deposited. Three types of bottom are recognized: beach, consisting mostly of sand; silty oyster bottom, ranging from silt to fine sand; and clay or silty clay bottom, too infirm to support many oysters but very adequate for shrimp. The shrimp and oyster bottoms are dark colored, even black. Microscopic study shows that all three types have nearly the same visible mineral composition; 95 percent quartz grains, 3 percent chert grains, and a remainder of mineral grains of muscovite, garnet, biotite, feldspar, apatite, magnetite, and zircon, in order of their relative abundance. These grains reveal marked differences in size and shape but the fragments in the beach and oyster bottom are the coarser.

Certain constituents of the oyster and shrimp bottoms were detected by chemical analyses; constituents which were not seen in petrographic work. Thus, aluminum is present as a water-soluble salt and iron is present as hydroxides and hydrosulphides. One of the hydrosulphides, occurring as amorphous black hydrotroilite, acts as the chief pigment. In contrast, the carbonaceous content is surprisingly low and contributes little to the darkness of these fine clastics. Consequently, it would be wise to use simple chemical tests for any dark fine clastic, unconsolidated or consolidated, in order to determine the nature of its pigment.

Regardless of whether the bottom is silty oyster bottom or still finer shrimp bottom, the uppermost sediment is comprised of water-logged organic debris and clay-sized mineral grains. Cores show that on oyster reefs this layer is rarely one inch in thickness, but above shrimp bottom the layer is up to 12 inches in thickness. In this stratum is found the greatest concentration of nitrogenous material, aluminum salts, and iron compounds. In quiet water this superficial water-logged material is flocculant, or even partly gelatinous. But the layer breaks up when bottoms are disturbed by wave action, by flooded streams, or by unusual tides, clouding the overlying waters. With the resumption of quiet water, the material once more settles to form a stratum composed of clay-sized quartz grains at its base, grading upward into the flocculant, largely organic matter. Tests show that changes in the settling velocity of these very fine sediments varies with changes in salinity. Thus cores of the upper one foot of sediment frequently show a series of such varve-like alternations, several to the inch. Similar but thinner alternating gradational layers can be seen in exposures and in cores of consolidated, fine, upper Cretaceous clastics.

The oyster and shrimp bottom sediments of Mississippi Sound (Figure 1) are accumulating in shallow water at depths of 4 to 20 feet, where the tide has a normal daily range of about two feet. The salinity varies from nearly zero in creek and river mouths to 20

End_Page 160------------------------

Figure 1. Mississippi Sound and Environs

End_Page 161------------------------

Figure 2. 1953 Commercial Sized Oyster Plantings

End_Page 162------------------------

or 25 parts per thousand in the broader parts of the Sound. At times of high rainfall the Sound is nearly fresh. However, during low rainfall the salinity increases to that of sea water (35 parts per thousand) as the Gulf waters funnel in between the barrier islands.

This salinity range from fresh to salt water introduces the second consideration in a study of Mississippi Sound sediments, namely, the production of a mixed fauna, both microscopic and megascopic. Thus Phleger, in the April 1954 number of the A.A.P.G., pictures Mississippi Sound as a gigantic mixing bowl where Foraminifera of marsh, brackish water, and open Gulf facies meet. A part of one of his maps (reproduced here as Figure 3) shows marine Foraminifera in some places mingling with Sound facies and even estuarine facies, up to the very beaches. This observation should cause micropaleontologists to pause before beds are designated as marine on the basis of marine Foraminifera alone.

Similarly, the megascopic forms capable of fossilization are, in some places, all brackish water species; in others, mixed open Gulf and brackish water species; in a few places even mixed beach and open Gulf forms. To be specific, healthy oyster reefs are comprised of oysters and their common brackish water associates, only. But dying or decimated reefs show drilled oysters, the gastropods which are doing the drilling, and the boring sponge which weakens the shell. Dead reefs are not only distinguished by dead oysters but by their shapes.

The shape of the modern oyster reef (Figure 4) provides an interesting study, since the shape reflects differences in rate of growth due to changing salinities and the rapidity of burial. Drilling shows that the living, healthy reef in the open Sound (diagram A of Figure 4) is a broad but thin lens, slightly convex upward at the top but fairly convex downward at the bottom. This peculiar biconvexity is attributed to the year by year growth of individuals and accumulations of new oysters. As the centers grow slightly upward, the added weight pushes the older shells downward. Diagram B (Figure 4) shows the cross-section of a reef which is dying over a period of increasing salinity. The upper surface flattens, since there are few new additions to compensate for the continued sinking of the thickest part of the lens. One such dying reef, nearly one mile in diameter, is represented in Figure 2. The shell is being dredged to a depth of 35 feet and barged to New Orleans for road metal and concrete aggregate. Diagram C is a cross-section of a reef which has been dead for some years. With no new additions, the surface becomes concave upward as the weight of the shells continues to increase the convexity of the bottom. Interestingly, this surface concavity is also seen in shell middens left by historic and prehistoric Indians in the Mississippi Delta region. It requires no imagination to see that all three of these reef types could be preserved as fossil reefs. Students of the Cretaceous outcrops are already familiar with them.

Another observation is the confusing occurrence of oysters on firm oyster bottoms as unusually large but scattered individuals, in open channels and far from reefs. These hermits seem to owe their size to a lack of competition for food. However, the reason for their isolation has not yet been determined. Comparable huge and isolated Gryphea and Exogyra are common in many otherwise unfossiliferous silty upper Cretaceous shales croping out in northeast Mississippi.

Finally, the mixing of salt water dwellers and beach dwellers is noted where hermit crabs, living on the beach, take over unoccupied shells of the salt water gastropods Busycon and Thais. The shells of these salt water species were probably swept shoreward by storms or moved in on wind or tide immediately upon the death of the animal. In some places the more sandy upper Cretaceous units cropping out in Mississippi contain such a mixed beach and salt water fauna.

End_Page 163------------------------

The above description of sediments, sedimentary processes, and faunal relationships apply to present day sedimentation in Mississippi Sound. For convenience, the lithologic and faunal features are summarized on the left of the chart, Figure 5, in the order of their treatment. On the right is a stratigraphic column of upper Cretaceous beds on the outcrop in northeast Mississippi. The more clastic, less calcareous intervals are underlined, since they are the strata which are compared with the recent deposits described above. These clastic beds are in the Eutaw (Eagleford), Selma (Taylor), and Ripley (Navarro) sequences.

The Tombigbee sand, the upper part of the Eutaw, is chiefly silty fine sand, gray-green in color, glauconitic, and sericitic. It is best studied at the classic locality, Plymouth Bluff on the Tombigbee River, near Columbus in Lowndes County, Mississippi. Here the upper 35 feet is exposed at low river level. The outcrop is often visited because it contains gigantic, isolated oysters (Exogyra, Gryphea, and Inoceramus), large isolated ammonites, and an intermixture of brackish water and marine Foraminifera. Compared to modern sedimentation in Mississippi Sound, this interval is chiefly Type D of the chart, where there are relatively few gigantic Crustacea and a mixed microfauna (Type B), probably produced by the meeting of waters of different salinities. However, a few more resistant beds in the Tombigbee form ledges containing abundant casts of Inoceramus. These pelecypods constitute poorly developed reefs (Type C) one foot to three feet in thickness. They probably represent reef building during a brief time when the optimum brackish water conditions were maintained.

On the outcrop, in the superjacent Selma chalk series, there are a few intervals of dark, slightly calcareous, finely clastic materials in a thick series of chalks and marls. These beds vary from the gray silty Coffee and Tupelo sands, through the dark silty shales in the Mooreville chalk, to nearly black silty clay shales at the top of the Selma unit. In the more sandy beds of Lee County there is a confusing mixture of shore type crabs and marine, brackish, and beach type molluscs, presumably mingled on a shoreline, such as Type E of the chart. In contrast, the dark silty Mooreville shales exhibit isolated large oysters (Exogyra and Gryphea) and large ammonites (Mortoniceras), again an example of Type D deposition. However, the nearly black, silty, clay shales at the top of the Selma at the Pontotoc-Lee Counties line are devoid of fossils except for rare but large hermit oysters, a near perfect example of what is believed to be the ancient counterpart of modern shrimp bottom. These clay shales grade upward into silty shales showing varve-like alternations comparable to the varve-like deposition shown as Type A in the chart.

The fine clastics in the next overlying Ripley (Navarro) strata abound in examples of all of the above types of sedimentation. But the most impressive sights are all three forms of oyster reefs, as shown in Type C. Some reefs on the outcrop in Pontotoc County are composed exclusively of Exogyra. These are all biconvex to some degree. Many measure one hundred or more feet across and are 5 to 10 feet in maximum thickness. They were apparently healthy and growing when buried, for the larger valves are still in position of life and there is no evidence of destruction by salt water enemies. Hence it is believed that these reefs were quickly buried by some rapid subsidence or incursion by the sea. The plano-convex reef shape of Figure 5 is also noted in Pontotoc County. Such reefs were evidently dying at the time of burial as evidenced by drilled valves, gastropod shells, and Cliona, the encrusting sponge. The concavo-convex shaped reefs are also present, with or without evidence of saltwater incursion. Those without additions of marine fauna but with weathered surfaces are assumed to have died due to isolation by uplift or retreat of the sea, and therefore are similar to the midden shell dumps described above. In contrast, those concavo-convex reefs whose shells have been perforated and have additions of marine fossils were evidently long dead before burial.

End_Page 164------------------------

Figure 3. Mixing of Shore, Sound and Gulf Foraminifera in Mississippi Sound

End_Page 165------------------------

Figure 4. Cross-sections of Oyster Reefs in Mississippi Sound

End_Page 166------------------------

Figure 5. Chart Lithologic and Faunal Features Seen in Present Day Mississippi Sound Sediments and in Ancient Cretaceous Beds

End_Page 167------------------------

Of course the recognition of shrimp type and oyster type bottom in subsurface beds is more difficult than on the outcrop. Nevertheless there is one downdip unit in which these types are easily seen. This is the Tuscaloosa "marine shale" of upper Cretaceous age, an interval 300 to 400 feet in thickness. In south Mississippi, a cautious well-sitter will recognize this zone and may start coring it to assure coring into the frequently petroliferous "massive" or "transitional" Tuscaloosa sands immediately below. Those of us who have examined well cuttings from this shale interval or have cored its lower part have no doubt been surprised that an interval which is dominantly shale on an electric log should contain so many individual large fossils and even beds of fossils. The abundance of life is probably due to deposition of the oyster bottom type, such as Type D, alternating with shrimp type bottom, with the occasional development of oyster reefs (Type C). In a few south Mississippi oil fields some of these reefs in the "marine shale" seem to have such great lateral extent that they can be correlated from well to well, for a distance of several wells. Reefs of this size are common in Mississippi Sound today.

It is evident from the above similarities in stratification and faunal relationships that many of the sedimentary processes at work today in Mississippi Sound were also at work at intervals in upper Cretaceous time, in at least the Mississippi portion of the Mississippi Embayment. Some of the similarities in these clastic sediments are probably due to changing salinities, changing shorelines, temporary embayments, and tidal or storm action. Other depositional features in the ancient Cretaceous clastics do not seem to have a counterpart in recent deposition. However, it is apparent that at various times broad muddy bottoms bordered the shores of the upper Cretaceous sea, at least in the Mississippi part of the Mississippi Embayment, just as broad muddy bottoms characterize Mississippi Sound today.

End_of_Record - Last_Page 168-------