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Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 23 (1973), Pages 266-277

Depositional Systems of the Alabama, Mississippi, and Western Florida Coastal Zone (1)

Peter A. Boone (2)

ABSTRACT

The northeastern Gulf of Mexico, from the Mississippi River to DeSoto Canyon, is a complex of interrelated dynamic depositional systems. Fluvial-deltaic, estuarine, barrier-island and marine-shelf systems characterize this part of the Gulf. The Pearl, Pascagoula, and Mobile fluvial-deltaic systems are major sources of sediment to the area. This complex is similar to that of the Texas coastal zone, but specific facies, geometry, and spatial relations differ.

Recognition of these aspects of the Alabama, Mississippi, and western Florida coastal-zone depositional systems is an important consideration in planning and developing a petroleum exploration program.

INTRODUCTION

The coastal zone of Alabama, Mississippi, and western Florida consists of a natural association of interrelated, dynamic, depositional systems extending from the Chandeleur Islands to DeSoto Canyon (Fig. 1). It is characterized by a series of barrier islands west of Mobile Bay separated from the mainland by Mississippi Sound; a series of barrier spits and islands east of Mobile Bay; a number of estuaries; and Pleistocene and Holocene age deltas of the Pearl, Pascagoula, and Mobile Rivers. The mainland bordering the estuaries is fringed with marsh, marshy islands and sand beaches, and indented by numerous tidal streams.

This paper summarizes, interprets, and unifies the available data related to the coastal zone in terms of natural depositional systems (Hayes and Scott, 1964; Fisher and others, 1972). As such, this paper draws heavily on published sources, supplemented by observations of the author during the past two years.

Physiographically, the coastal zone is divisible into two southward sloping plains of differing elevations. The lower plain corresponds to Cooke's (1939) Coastal Lowlands physiographic subdivision and ranges from sea level to about 30 feet (9 m) in elevation. The essentially flat to gently undulating, locally swampy Coastal Lowlands are underlain by alluvial, deltaic, estuarine, and coastal deposits of Pleistocene and Holocene age. They extend along the coast and merge with the fluvial-deltaic plains of the streams in the area. The Coastal Lowlands are indented by many tidal streams and fringed by tidal marsh.

The upper plain corresponds to the Southern Pine Hills physiographic subdivision of Fenneman (1938) and ranges in elevation from about 30 feet (9 m) in the south to about 400 feet (120 m) in the north. It is a moderately dissected plain underlain by Miocene-Pliocene-Pleistocene age estuarine and fluvial-deltaic sediments. The nearly level interfluves have numerous shallow saucer-like depressions scattered over them, which hold water the greater part of the year. The two plains are separated by a gentle escarpment with relief up to 80 feet (24 m).

The coastal area has a humid, warm-temperate to subtropical climate, although occasional subfreezing temperatures do occur. Seasonal temperature variations average 52°F (11°C) in January to 82°F (27°C) in July. Rainfall at Mobile averages up to 65.0 inches (165 cm) per year, making this one of the wettest sections in the United States. Precipitation is rather evenly distributed with only slight concentration in the summer months. Winds are predominantly southerly and southeasterly with northerly and northeasterly winds during the winter months. During the winter the winds are somewhat variable and erratic. The directional path includes winds from the south-southeast with an average velocity of 10.8 mph (17.3 km/hr) and from the north at a velocity averaging in excess of 13 mph (20km/hr). Winds are generally southeasterly in the spring and summer with an a average velocity of slightly over 9 mph (14.5 km/hr). Hurricanes strike the coastal area on the average of about once in seven years (South Alabama Regional Planning Commission, 1968).

Large streams that influence the area are the Mississippi, Pearl, Pascagoula, and Mobile Rivers. Between the Pearl and Pascagoula are several smaller streams that flow southeast and then turn to parallel the shore and enter Mississippi Sound through estuaries and bays. East of Mobile Bay, several smaller rivers empty into Perdido, Pensacola, and Choctawhatchee Bays and thence into the Gulf. Most of these rivers are influenced by tides for a few miles upstream and are fringed with marsh. West of the area lie the extensive marshes and tidal drainage systems along the eastern margins of the Mississippi Riber delta.

Long-range planning and management of the coastal zone require knowledge of the nature and distribution of natural environments; the physical, chemical, and biologic processes active; and the rate of change caused by these activities. Knowledge of modern depositional systems facilitates recognition of similar systems in the geologic record that contain a large percentage of the world's

FOOTNOTE (1) Approved for publication by the State Geologist.

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economic mineral deposits. Recognition of the type of depositional system in which a mineral deposit occurs is important for planning the most economical and environmentally safe program for extracting the resource.

DEPOSITIONAL SYSTEMS

A depositional system is a large-scale three-dimensional unit "characterized by a distinctive suite of natural environments in which certain geologic processes result in unique sedimentary deposits" (Fisher and others, 1972, p. 11). Eleven principal depositional systems are recognized in the Alabama, Mississippi, and western Florida coastal zone (Fig. 1). They are the Mobile Bay estuarine system, the Mississippi Sound estuarine system, the Mississippi Sound barrier island system, the western Florida barrier-spit-and-island system, the Mississippi-Alabama shelf system, the smaller estuatine systems of Perdido, Pensacola, and Choctawhatchee Bays, and the fluvial-deltaic systems of the Pearl, Pascagoula, and Mobile Rivers.

Fluvial-deltaic systems of the Pearl, Pascagoula, and Mobile Rivers introduce most of the sediment of the area, but are of passing interest at this time. The Mississippi River delta system forms the western boundary of the area.

Mobile Bay Estuarine System

Mobile Bay is 32 miles (51.5 km) long from Mobile Point to the mouth of the Mobile River (Fig. 3). It has an average width of 11 miles (17.7 km) in the northern part and widens to a maximum width of 23 miles (37 km) in the south. It has a surface area of 413 square miles (1,058 sq km) with 142.4 miles (229.3 km) of shoreline (Crance, 1971). Mobile Bay is

FIGURE 1. Modern depositional systems of Alabama, Mississippi, and western Florida.

FIGURE 2. Explanation of symbols used in Figures 3 through 9. Illustrating techniques after Fisher and others, 1972.

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open to the Gulf of Mexico through the tidal pass between Dauphin Island and Mobile Point and to Mississippi Sound through Pass aux Herons. A navigation channel 400 feet (120 m) wide and 40 feet (12 m) deep traverses the length of the bay.

Mobile Bay has an average depth of 9.7 feet (2.9 m) (Crance, 1971). It is essentially flat-bottomed, sloping gently toward the Gulf. Depths range from 10 to 12 feet (3 to 3.6 m) over most of the flat-bottomed part of the bay. A narrow shelf with water depths as much as 6 feet (1.8 m) occurs around the periphery of the bay. Between the peripheral shelf and flat bottom, there is a slope with gradients ranging from 8 to 30 feet per mile (1.6 to 5.6 m/km) (Ryan, 1969). Water depths less than 6 feet (1.8 m) occur on both sides of the ship channel in the northern part of the bay and on the south side of the Intracoastal Waterway in the southern part of the bay. Spoil from construction of the Ship Channel and shoaling in the lower bay have produced two large shallow depressions in the bay bottom (May, in press).

In the northern part of the bay, the bathymetry is complicated by the progradation of distributaries of the Mobile delta in the estuary. The tidal inlet between Mobile Point and Dauphin Island is scoured to depths between 54 and 58 feet (16.2 and 17.4 m). Seaward of the tidal inlet is a large tidal delta with water depths of less than 18 feet (5.4 m). One large island and several shoal areas exist on the seaward margins of the tidal delta.

The western and northern shores of Mobile Bay are 3 to 10 feet (0.9 to 3 m) above sea level; whereas, the eastern shore in the northern part of the bay is characterized by sandy clay bluffs as much as 100 feet (30 m) above sea level. Tidal marsh occupies 9.7 square miles (15.6 sq km) mostly along the northern shore of the bay and along the small streams entering the bay south of the Mobile River (Crance, 1971).

Mobile Bay is the terminus of a major fluvial system draining an area of approximately 44,000 square miles (113,960 sq km). The system has a mean annual discharge of 58,636 cfs (1,659 m3/sec) (Crance, 1971) with a daily range from 539,000 cfs (15,254 m3/sec) to 4,310 cfs (122 m3-/sec) (Peirce, 1966). Several small streams south of the Mobile River system discharge an undetermined, but insignificant, amount of fresh water to Mobile Bay. The average daily discharge from the Mobile River system is 506.6 x 106 cubic feet (14.3 x 106 m3) with a maximum of 4,657 x 106 cubic feet (131.8 x 106 m3) and a minimum of 37.2 x 106 cubic feet (1.0 x 106 m3).

Mobile Bay has diurnal tides that vary from 1.2 feet (0.4 m) at Mobile Point to 1.5 feet (0.4 m) at the mouth of the Mobile River and average 1.4 feet (0.4 m). The tidal cycle exhibits seasonal fluctuations with mean low water in the winter varying from 1.0 to 0.5 feet (0.3 to 0.15 m) below that of the summer. A tidal prism of 14,618 x 106 cubic feet (414.1 x 106 m3) was calculated by McPhearson (1970) for the bay.

Changes in meterological conditions (prevailing winds and hurricanes) cause pronounced changes in the tidal cycle of estuaries having a diurnal tide and small tidal range. Northerly winds tend to depress water levels; whereas, southeasterly winds raise water levels above predicted levels. Hurricanes may have a profound influence by markedly raising (10.8 ft, 3.2 m, in 1916) or lowering (10.5 ft, 3.1 m, in 1926) water levels (Ryan, 1969).

Average current velocities at Mobile Point are 1.4 knots (70 cm/sec) at flood tide and 1.5 knots (75 cm/sec) at ebb tide (Fig. 3). At Pas aux Herons current velocities are 1.3 knots (65 cm/sec) for both ebb and flood tide. Maximum currents predicted at Mobile Point are 3.3 knots (165 cm/sec) (Ryan, 1969).

Circulation patterns in Mobile Bay are controlled primarily by tide, river discharge, configuration of the bay and Coriolis force. Tidal currents enter the bay from the Gulf and are deflected in an east-northeast direction during flood tide. Fresh water from the Mobile River system tends to flow down the west side of the bay at reduced velocities. During ebb tide the flow of the entire bay tends to be southward toward the passes to the Gulf and Mississippi Sound. Approximately 75 percent of the total volume of water entering or leaving Mobile Bay passes through the tidal inlet between Dauphin Island and Mobile Point peninsula and the remaining 25 percent passes through Pass aux Herons (Austin, 1954 in Ryan, 1969).

Mobile Bay is moderately stratified in terms of salinity (McPhearson, 1970; May, in press) and well mixed thermally (May, in press). Bay water temperature generally ranges from about 50°F (10°C) in January to about 88°F (31°C) in August. The annual average temperature is approximately 72°F (22°C); there is about 1 degree difference between top and bottom. Bay salinities are generally low from January to May and range from 15 parts per thousand (150/00) in the southern part of the bay

FIGURE 3. Circulation, waves, sediment transport, and other physical processes, Mobile Bay estuarine system. Symbols explained in Figure 2.

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to less than five parts per thousand (50/00) in the northern part. During severe upstate floods, such as occurred in 1961, salinities may be depressed far below these values almost uniformly throughout the bay. Salinities increase during June to November and range from 30 parts per thousand (300/00) in the southern part to ten parts per thousand (10°/00) in the northern part of the Bay. A saltwater wedge exists in the Mobile ship channel and Mobile River during most of the year (Bault, 1972).

McPhearson (1970) classified Mobile Bay as a moderately stratified estuary, but during periods of low water discharge mixing may be more thorough and stratification less distinct than for times of high flow. Mobile Bay, using the "mixing index" of Schubel (1971b), can be classified as a partially mixed estuary (Type B of Pritchard and Carter, 1971) during average and high-flow conditions and as a vertically homogeneous (Type C) estuary during low-flow periods. The "mixing index" is defined as the volume of fresh water entering an estuary during a half-tidal period divided by the volume of water of the tidal prism. It should be emphasized that this classification should be used only in a general way because in a large estuary one part may differ from another under the same conditions of wind and river discharge.

Mobile Bay is the site of a possible graben (Copeland, 1968). The alluvial deltaic plain north of the estuary coincides nicely with the subsurface trace of the graben, but it is difficult to fit Mobile Bay itself into the graben. At least part of the difficulty results from lack of data to delineate the graben accurately.

Mobile Bay and the alluvial-deltaic plain are underlain by Pleistocene and Holocene fluvial, estuarine and coastal sediments that are in turn underlain by claystone and sandstone of Miocene age. The elevated high plains on both sides of the estuary and alluvial-deltaic plain are capped by friable sandstone, conglomeratic sandstone, conglomerate, and lenticular white to variegated clays of the Citronelle Formation.

Sediments in the northern part of Mobile Bay consist of prodelta silt, clayey silt and delta-front sand and silty sand (Fig. 4). Sediments in the southern part of the bay consist of estuarine silty clay and clay. Bay-margin sands and clayey sands occur around the periphery of the bay. Locally, the accumulation of oyster shells is significant. Holocene sediment thicknesses range from about 15 to 20 feet (4.5 to 6 m) in the western part of the bay to about 40 feet (12 m) in the eastern part of the bay. Sediments are up to 125 feet (37.5 m) thick in the ancient Alabama River valley near the mouth of the Bay (K. R. McLain, personal communication).

The suspended sediment load of the Mobile River system reaching Mobile delta and bay has been estimated by Ryan (1969) as averaging 4.7 million tons (4.3 million metric tons) per year and ranging from 2.1 million tons to 8.3 million tons (1.9 million metric tons to 7.5 million metric tons) per year. No quantitative data of bed load transported by the Mobile River system is available. Of the 4.7 million tons per year introduced, an estimated 1.4 million tons (1.3 million metric tons) per year, or 30 percent of the total introduced, passes through the estuarine system to the Gulf.

Long term sediment accumulation in Mobile Bay has been estimated as being 1.7 feet (0.5 meters) per century (Ryan, 1969). Carbon-14 dates of oyster shells indicate that the rate of sedimentation has been 0.1 to 0.5 feet (3 to 15 cm) per century over the past 5 to 6 thousand years. Present rates are considerably higher than in the past and are probably still accelerating. This is due, at least in part, to progradation of the delta toward the mouth of the bay, shifting the locus of deposition "down-bay" and increasing sedimentation rates in the process.

Mississippi Sound Estuarine System

The Mississippi Sound estuarine system is a bar-built (Schubel, 1971a) estuary approximately 85 miles (137 km) long and 7 to 15 miles (11 to 24 km) wide, extending from the mouth of the Pearl River to Mobile Bay (Fig. 5). Its southern limit is the Mississippi Sound barrier-island system. It is bounded on the north by tidal salt marshes of the mainland.

The sound is shallow, generally less than 10 feet (3 m) in the northern part, deepening to an average of 15 to 20 feet (4.5 to 6 m) in the southern part. Greater depths occur behind barrier islands and in tidal passes.

The Pascaguola and Pearl River fluvial systems are the major contributors of fresh water to the estuary. The Pascagoula has an average annual discharge of 15,200 cubic feet per second (430 m3/sec) and the Pearl 12,900 cubic feet per second (365 m3/sec). Several smaller streams enter the estuary between the Pascagoula and Pearl Rivers so that the total average annual discharge to the estuarine system is 31,200 cubic feet per second (883 m3/sec) from a drainage area of 19,700 square miles (51,000 sq km) (Wilson and Iseri, 1967).

FIGURE 4. Generalized sediment distribution, Mobile Bay estuarine system. Symbols explained in Figure 2.

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Diurnal tides of less than 2 feet (0.6 m) characterize the Sound (Upshaw and others, 1966). As in Mobile Bay, pronounced changes in the tidal cycle may be brought about by changing meteorological conditions such as prolonged high winds and hurricanes.

Current patterns are varied, being influenced by both the tidal currents of the passes and the discharge of streams (Fig. 5). However, there is a slow longshore current that moves westward at about 0.8 knot (40 cm/sec). This current is sufficient to cause a gradual westward drift of sand-size sediments (Foxworth and others, 1962).

Salinities are variable, ranging from 0 to 30 parts per thousand (0 to 30°/00) as compared to open Gulf salinities of 30 to 40 parts per thousand (30 to 40°/00). Data are not available on salinities for all Mississippi Sound, but those available for the eastern part (McPhearson, 1970; Bault, 1972) indicate moderate salinity stratification with seasonal and annual variation of intensity. Salt-water invades the estuary through the tidal channels, locally increasing salinities in their vicinity. Faunal evidence indicates that salinity was once lower in the eastern part of the sound and that widening of the tidal pass between Petit Bois and Dauphin Islands and construction of the causeway connecting Dauphin Island with the mainland have acted to enhance the flow of Gulf waters into this part of the sound and restrict the flow of fresher water from Mobile Bay (Lamb, 1972; May, 1971).

Sediments in Mississippi Sound (Fig. 6) consist of estuarine silt and clay in much of the central part and bay-margin sands around the periphery (Upshaw and others, 1966). The estuarine facies is characterized by variable lithology, general lack of stratification, abundance of mottles (Bioturbation), and irregular pods of differing lithology (Curray and Moore, 1963; Rainwater, 1964). Bay-margin sands are quartzose with 1 to 2 percent heavy minerals (Foxworth and others, 1962). Medium and coarse sand generally occurs along the mainland beaches west of Pascagoula whereas fine sand, silt, and clay occur east of Pascagoula (Upshaw and others, 1966). Medium to coarse sand occurs along barrier-island beaches facing the sound (Upshaw and others, 1966; Weide, 1968).

Mississippi Sound Barrier-Island System

The Mississippi Sound barrier-island system is part of a chain of barrier islands and spits that extend from Cat Island, Mississippi, to Choctawhatchee Bay, Florida. Cat, Ship, Horn, Petit Bois, and Dauphin Islands constitute this system (Fig. 5). The islands are generally less than a mile (0.6 km) wide. Dunes average 10 to 20 feet (3 to 6 m) in elevation with a maximum of 40 feet (12 m) on the eastern end of Dauphin Island. A ridge of sand between Cat and Ship Islands and between Ship and Horn Islands (the only areas of available data) led Curray and Moore (1963) "to assume that the present islands represent the top of a formerly more continuous barrier island."

The islands consist of a broad, well-developed beach, backed by dunes on the Gulf side. Beach and intermittent marsh, backed by dunes occur on the mainland side of the islands. The interior of the islands are either broad low sand flats, 1 to 2 feet (0.3 to 0.6 m) above sea level, with marshes and shallow lakes or vegetated beach ridges, 5 to 15 feet (1.5 to 4.5 m) above sea level. Some of the lakes are intermittently connected with Mississippi Sound or the Gulf of Mexico (Ludwick, 1964).

There shoreface of the islands slopes abruptly to depths of about 20 feet (6 m). Within 1,000 feet (300 m) of the shoreline three offshore bars generally parallel the coastline. The mainland shoreface of the islands descends abruptly to the floor or Mississippi Sound, where water depths greater than 20 feet (6 m) occur locally.

Tidal passes between islands vary considerably in depth and width. The maximum depth of any of the passes, 58 feet (17.4 m), is between Dauphin Island and Mobile Point. The widest pass, 6.5 miles (10.4 km), is Dog Keys Pass between Ship and Horn Islands. Tidal deltas, normally of limited extent, form seaward of some of the passes. The tidal delta at the mouth of Mobile Bay is as much as 8 miles (13 km) wide at its base and extends seaward some 5 miles (8 km). Pelican Island is an emergent part of this tidal delta.

Prevailing southerly and southeasterly winds generate waves that produce a westward-flowing longshore current on the Gulf side of the barrier islands (Fig. 5). Longshore currents 1 to 2.5 knots (50 to 125 cm/sec), increasing to 2.5

FIGURE 5. Circulation, waves, sediment transport, and other physical processes, Mississippi Sound estuarine system and Mississippi Sound barrier-island system. Symbols explained in Figure 2.

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to 5 knots (125 to 250 cm/sec) with the incoming tide, occur along the Mississippi Sound barrier island system. Tidal currents of 1 to 2.5 knots (50 to 125 cm/sec) on the flood tide and 3.5 to 7 knots (175 to 350 cm/sec) on ebb tide occur in the tidal passes. Ebb tides produce a 1.7- to 3.5-knot (85 to 175 cm/sec) westward-flowing current along the mainland side of the islands (Foxworth and others, 1962).

Longshore drift is the primary factor in transporting sediment to the islands (Kwon, 1969) although it has never been quantitatively measured for the Mississippi Sound barrier-island system. However, erosion of the eastern ends of the islands composing the system and accretion of sediment to the western ends indicates that considerable drift does occur. The rate of accretion is greater than the rate of erosion so that the islands lengthen and migrate westward with time (Ludwick, 1964; May, 1971; U.S. Army Corps Eng., 1971). The rate of migration decreases from east to west indicating a westward decrease in longshore drift.

Dauphin Island is primarily being elongated by the accretion of sediment to its western end. Accretion has extended the western end of the island about 4 miles (6.4 km) in the last 100 years (May, 1971). Erosion is active on the eastern end, but has not caused significant westward migration of that part of the island. Marsh deposits and tree stumps exposed in the surf zone indicate significant erosion on the Gulf side of the island.

The tidal pass east of Dauphin Island is nearly stable, as shown by old charts. Sand transported by the westward longshore drift by-passes the tidal inlet by way of the tidal delta (Kwon, 1969). In the past several small islands have been present on the delta. However, Pelican Island is the only one occurring today.

Erosion and accretion have resulted in Petit Bois Island migrating 8.5 miles (13.7 km) westward since 1851. During the same period, the western end of Dauphin Island has advanced 4.7 miles (7.6 km). This has resulted in the tidal pass between the two islands widening from 1.7 miles (2.8 km) in 1851 to 5 miles (8 km) at present. Early maps and reports indicate that the two islands were connected well into the eighteenth century (May, 1971).

The westward migration of Horn Island is at a slower rate than that of Petit Bois. The westward end of Horn Island has extended nearly 3 miles (4.8 km) while the eastern end has eroded about 2.25 miles (3.6 km) (U.S. Army Corps Engineers, 1971).

Ship Island has undergone numerous changes, but generally is migrating to the south and west. Fort Massachusetts was completed on the western end of the island in 1859. At that time it was about 400 feet (120 m) from the Gulf, the west end of the island, and Mississippi Sound. Today the fort is awash in Mississippi Sound, 1,500 feet (450 m) from the Gulf, and 3,800 feet (1,140 m) from the tip of the island (U.S. Army Corps Engineers, 1971).

Cat Island, the westernmost island of the Mississippi Sound barrier system, is much more protected and stable. Only minor erosion has occurred on its northern and southern tips (U.S. Army Corps Engineers, 1971).

The barrier-island facies consists of well-sorted, medium-grained, mature quartzose sand containing practically no feldspar (less than 3 percent) and having a heavy mineral suite rich in staurolite and kyanite (Hsu, 1960). The average width of the facies is 2.5 miles (4 km). The thickness varies, but is usually less than 40 feet (12 m) (Ludwick, 1964). A maximum thickness of 60 to 65 feet (18 to 19.5 m) occurs at the eastern end of Dauphin Island.

A model for barrier islands was developed by Bernard and others (1962) from studies of Galveston Island, Texas. No such comprehensive study has been carried out on the Mississippi Sound barrier island system. However, by combining the work of Upshaw and others (1966), Foxworth and others (1962), Ludwick (1964), Pryor (1973), Weidie (1968), Ryan (1969), Rainwater (1963), Curray and Moore (1963), Priddy and others (1955), and reconnaissance studies made by the author, some comparisons can be made..

Upper shoreface-beach deposits consist of well-sorted, medium-grained, quartzose, well-laminated sand. Laminae are planar to trough, low angled (less than 10° on Ship Island to 12° to 13° on the eastern end of Dauphin Island), and dip gulfward. Burrowing is scarce.

Separation of middle and lower shoreface is not possible with available data. However, grain size decreases with depth and the sediments become more poorly sorted.

FIGURE 6. Generalized sediment distribution, Mississippi Sound estuarine system. Symbols explained in Figure 2.

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Sediments capping the island consist of cross-laminated dune sand and bioturbated marsh deposits.

The contact of barrier-island sands with Mississippi Sound sediments is generally well defined, with a transition zone less than a quarter mile wide. The contact between lower shoreface and offshore sediments occurs in water depths of less than 60 feet (18 m) and is gradational.

Striking occurrences of heavy minerals occur along the beaches of the Gulf and mainland sides of the barrier system. In these areas, dark laminae, rich in heavy minerals, are intercalated with lighter layers rich in quartz. The heavies are concentrated in the silt-to-finesand fraction in thin laminae that may contain as much as 30 to 95 percent heavy minerals. The sands average 2 to 6 percent heavies, but locally may average as much as 60 percent (Foxworth and others, 1962).

A suite of 26 different species of heavy minerals is common in these sands. This suite, rich in metamorphic minerals, is characterized by the dominance of staurolite and kyanite (Foxworth and others, 1962). Goldstein (1942) classified this suite as the East Gulf petrologic province, with the source being the southern Appalachian region.

Foxworth and others (1962) considered the transportation and concentration of heavies in laminae on the beaches to be the result of several processes:longshore currents and swell waves move heavies shoreward; wind waves assisted by high tides deposit them on beaches; and storm waves, rain runoff, and dry winds concentrate the heavy minerals once they are on the beaches.

Western Florida Barrier-Spit-and-Island System

The western Florida barrier-spit-and-island system extends from Mobile Bay to Choctawhatchee Bay (Fig. 7). It consists of Mobile Point peninsula, Perdido Key peninsula, and Santa Rosa Island.

Mobile Point peninsula is a large spit attached to the mainland on the east and extending westward, forming part of the southern terminus of Mobile Bay. The western part of the peninsula consists of broad well-developed beaches backed by lines of discontinuous dunes that reach a height of 20 feet (6 m). Several large lagoons and marsh areas lie between the Gulf beaches and the mainland in the eastern part of the peninsula. Several sets of intersecting dune ridges indicate a complex depositional history for this spit.

Perdido Key is a narrow peninsula connected at about its midpoint to the mainland. Dunes are as much as 20 feet (6 m) high in the central part and decrease in height and frequency toward each end (U.S. Army Corps Engineers, 1971).

Santa Rosa Island is 52 miles (84 km) long and less than 1 mile (1.6 km) wide over most of its length. It consists of a broad beach backed by dunes that average 16 feet (4.8 m) in height (U.S. Army Corps Engineers, 1971). Dunes reach a maximum height of 40 feet (12 m) on the western end of the island. It is separated from the mainland by a continuous lagoon that tapers from about 2 miles (3.2 km) wide at its western end to less than 0.25 mile (0.4 km) at its eastern end.

The system has a steep shoreface, the base of which is at a depth of about 18 feet (5.4 m) off Mobile Point and increases to a depth of 50 to 60 feet (15 to 18 m) at the eastern end of Santa Rosa Island. Along the coast from Mobile Point to Pensacola Bay the 30-foot (9 m) contour outlines a topographic bulge toward the Gulf. East of this coast, along Santa Rosa Island, the 30-foot (9 m) contour continues along the barrier-island shoreface in a smooth line.

Tidal passes between the islands and spits range from a depth of about 6 feet (1.8 m) at the mouth of Choctawhatchee Bay (Goldsmith, 1966) to a depth of 30 to 40 feet (9 to 12 m) at the mouth of Pensacola Bay (Horvath, 1968 in Folger, 1972). Bathymetric data indicates tidal deltas are not significant seaward of these passes. However, an older tidal delta may be present west of the tidal pass at the mouth of Choctawhatchee Bay.

Prevailing south and southeast winds generate waves

FIGURE 7. Circulation, waves, sediment transport, and other physical processes, Perdido, Pensacola, and Chocawhatchee estuarine systems and the western Florida barrier-spit-and-island system. Symbols explained in Figure 2.

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that produce a westward-flowing longshore current along the Gulf side of these barriers (Fig. 7). Longshore drift of 130,000 cubic yards (99,398 m3) per year has been reported at the mouth of Perdido and Pensacola Bays (U.S. Army Corps Engineers, 1971).

The barrier facies, like that of the Mississippi Sound barrier-island system, consists of well-sorted, medium-grained, mature quartz sand containing practically no feldspar (less than 3 percent) and having a heavy mineral suite rich in staurolite and kyanite (Hsu, 1960). No data are available on the thickness of the sands.

No comprehensive studies detailing the barrier facies and subfacies have been reported. However, a study by Pryor (1973) reports that the upper shoreface-beach sediments on Santa Rosa Island are indistinctly laminated, with dips up to 35°. This dip is considerably higher than those reported for both the Mississippi Sound barrier-island system and the barrier islands of the Texas coast. Further study is required to determine how these islands, if indeed they do, fit the barrier island model of Bernard and others (1962).

Other Estuarine Systems

East of Mobile Bay are several smaller, irregularly shaped estuarine systems. From west to east, they are: the Perdido Bay estuarine system, the Pensacola Bay estuarine system, and the Choctawhatchee Bay estuarine system. They are similar to the Mobile Bay estuarine system, but smaller and have less fresh-water inflow. Sufficient data, however, were not available to permit comprehensive treatment.

Figure 7 and 8 summarize the generalized and, in part, inferred physical processes and sediment distribution in these estuaries.

Mississippi-Alabama Shelf System

The Mississippi-Alabama shelf is a triangular area, extending from the Mississippi-River delta to DeSoto Canyon (Fig. 1). The shelf is about 80 miles (128 km) wide in the west and narrows to about 35 miles (56 km) in the east. The shelf is an extensive, almost flat, plain bounded on the landward side by the relatively steep but narrow shoreface of the Mississippi Sound barrier-island and western Florida barrier-island-and-spit systems. The break in slope between shoreface and shelf occurs at a depth of about 20 feet (6 m) along the Mississippi Sound barrier-island system and as far west as Pensacola Bay. East of Pensacola Bay the break increases to a depth of about 30 feet (9 m) at the western end of Santa Rosa Island, and to about 60 feet (18 m) at the eastern end of the island.

The shoreface has a gradient of up to 50 to 60 feet per mile (9.4 to 11.2 m/km). The shelf has a gradient of 3.2 feet per mile (0.6 m/km) off Dauphin Island and 8.5 feet per mile (1.6 m/km) off Pensacola Bay. At a depth of approximately 180 feet (54 m) the slope increases to about 31 feet per mile (6 m/km) (Upshaw and others, 1966).

The surface of the shelf is relatively smooth in the west, but becomes highly irregular east of Mobile Point. Ridge and valley topography with relief up to 30 feet (9 m) accounts for the irregularity (Hyne and Goodell, 1967; Parker, 1968; Kwon, 1969). These are relict features of subaerial erosion of lower stands of the sea during the Pleistocene. Linear lows, off the mouths of several of the rivers, cross the shelf and probably represent the partly filled valleys these streams occupied during lower stands of the sea.

Wave action intensity on the Mississippi-Alabama shelf is low to moderate, with wave periods ranging from 3 to 8 seconds and wave height rarely over 3 feet (0.9 m). Such waves affect the bottom only in the nearshore zones; however, the longshore transport of sediment by currents produced by these waves is of major importance. The intense wave action associated with hurricanes is an important factor in the reworking of sediments on the shelf, but little horizontal displacement takes place in offshore areas. Near the edge of the shelf, sediments are stirred about once every five years; the rest of the shelf is affected about once every two years (Upshaw and others, 1966).

The major water movement on the shelf consists of slow westward drift across the central continental shelf turning southward opposite Mobile Bay, and slow eastward drift seaward of the continental shelf margin (Leipper, 1954).

FIGURE 8. Generalized sediment distribution, Perdido, Pensacola, and Choctawhatchee estuarine systems. Symbols explained in Figure 2.

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Longshore drift is westward.

Sediments of the Mississippi-Alabama shelf occur as six well-defined facies; two of which are relict Pleistocene sediments, whereas the remainder are of Holocene age (Fig. 9). The relict Pleistocene facies are the Alabama-Mississippi sand facies and the Alabama-Mississippi reef and inter-reef facies. The Holocene facies are the St. Bernard prodelta facies, the Chandeleur sand facies, the Mississippi prodelta facies, and a nearshore fine-grained facies. The discussion of shelf facies is taken from Ludwick (1964) unless otherwise noted.

The Mississippi-Alabama sand facies covers most of the western part of the shelf area. It consists predominantly of well-sorted fine-grained, "clean" quartz sand. Shelly sands occur locally. This facies occurs in an area of very slow deposition or slow erosion where sands deposited during a lower stand of the sea are being reworked by marine processes but not buried by normal shelf deposits. However, as surface layer of silt and clay ranging from 0.04 to 1.2 inches (0.1 to 3 cm) is present over much of the area off Choctawhatchee Bay, Florida (Hyne and Goodell, 1967).

The Mississippi-Alabama reef and interreef facies occurs along shelf edge. The reef facies consists of a zone of pinnacle reefs approximately 1 mile (1.6 km) wide. The distance between pinnacles is 10 to 20 miles (16 to 32 km). The average relief of the pinnacles is 30 feet (9 m) with the highest being 54 feet (16 m). The reef facies consist of well-cemented "carbonate sand" (20 percent terrigenous sand, 70 percent carbonate sand, 10 percent silt and clay) and some unconsolidated "carbonate sand." The interreef parts of the facies consist of an unconsolidated sand-silt-clay mixture (30 percent terrigenous sand, 20 percent carbonate sand, and 50 percent silt and clay). Calcareous algae are the most common constituents of the pinnacle rock (Ludwick and Walton, 1957).

The St. Bernard prodelta facies occurs in a broad arc in the western part of the shelf area. Homogeneous silty clay constitutes 95 percent of the sediment. To the east the deposit overlaps the Mississippi-Alabama sand facies. The contrast between the two deposits is distinct, one being a silty clay, the other a sand. Between the two deposits is a transitional zone in which bottom sediment is a mixture of both facies. This zone varies in width, but averages about 7 miles (11 km) with a maximum width of 10 miles (16 km). There is no indication that the width of the zone is related to water depth, since narrow widths are found in both shallow water and deep water. The transition zone exists because of lateral and vertical mixing of the two facies by storm waves, currents, and burrowing organisms.

The Chandeleur sand facies is coincident with the Chandeleur Islands, which flank the Mississippi River delta. The facies consists chiefly of fine-grained, well-sorted quartzose sand. The Chandeleur sand facies is up to 30 feet (9 m) thick (Lankford and Shepard, 1960) and overlies the St. Bernard prodelta facies.

The Mississippi prodelta facies consists of clay and silt introduced to the extreme southwestern part of the shelf by the modern birdfoot delta of the Mississippi River. Sediments of this facies overlie sediments of the Chandeleur and St. Bernard facies.

Immediately south of the Mississippi Sound barrier-island system is a nearshore fine-grained facies similar in lithology to that of Mobile Bay and Mississippi Sound. Sand, muddy sand, sandy mud, and mud occur in water depths less than 60 feet (18 m) in a zone about 7 miles wide

FIGURE 9. Facies distribution, Mississippi-Alabama Shelf. After Ludwick, 1964. Symbols explained in Figure 2.

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(11 km). Tidal flushing of the estuaries moves turbid waters seaward where the suspended silt and clay are deposited to form this facies.

COMPARISON WITH TEXAS BARRIER COAST

The depositional systems of the Alabama, Mississippi, and western Florida coastal zone are generally similar to those of the better-known barrier coast of Texas. The types of systems present and their relation one to the other are similar. However, the scale and geometry differ; Mississippi Sound separating the barrier islands from the mainland by up to 15 miles (24 km) is a good example. The islands of the Texas coast are separated from the mainland by about 4 miles (6.5 km). Other differences, greater yearly rainfall, generally greater discharge from rivers, and lower expenditures of wave energy on the shoreface (Price, 1954b) impart differences to the sedimentary deposits of these systems.

The islands of the Mississippi Sound and western Florida barrier systems do not have the extensive washover fans like those developed on the islands of the Texas coast. The Texas barriers are separated from the mainland by very shallow bodies of water usually less than 8 feet (2.4 m) deep with broad areas that are much shallower (Rusnak, 1960; Shepard and others, 1960; Fisher and others, 1972). The islands of Mississippi Sound and western Florida are separated from the mainland by deeper bodies of water, as much as 20 feet (6 m) deep, which do not have extensive shallow margins. These deep waters adjacent to the islands preclude the development of extensive washover fans.

Little specific data on the primary geologic properties of the sediments deposited in these systems are available. However, dips in the upper shoreface-beach sediments show significant differences. Upper shoreface-beach deposits have laminae that dip 12° to 13° on Dauphin Island and 30° on Santa Rosa Island. This is significantly greater than the 6° to 8° for Galveston Island.

Texturally and compositionally, sediments from similar environments are analogous. However, quartz beach sands are generally medium grained in Alabama, Mississippi, and western Florida; whereas, they are generally fine grained along the Texas coast. Heavy minerals of Alabama, Mississippi, and western Florida belong to the Eastern Gulf petrologic province; whereas, those of Texas belong to the Western Gulf petrologic province. Kyanite and staurolite are the most abundant heavy minerals in the Eastern Gulf petrologic province; whereas, epidote and hornblende are most abundant in the Western Gulf petrologic province.

Where data are available, general and specific differences between the two areas do occur. Further investigations are required to determine just how extensive these differences are.


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