THE WORLD DELTA DATABASE

THE BALIZE-BIRDFOOT DELTA

Delta ID     # 25.

LOCATION LAT. 28°52’N, LONG. 88°14’W

LANDMASS DRAINED: NORTH AMERICA (USA)

CLIMATE HUMID SUBTROPICAL (Caf),

AIR TEMP RANGE 9.2°C TO 40.4°C,

TIDAL AMPLITUDE 0.50 M,

DISCHARGE WATER 172,000 M3/S; SEDIMENT 349X106 TONS/YR

DRAINAGE BASIN AREA 3.344X106 KM2

ID 7021040000117651, PATH 21 ROW 40

IMAGE ACQUIRED JUNE 25, 2001

Contributed  by Professor George F. Hart, LSU.

If the Gulf Basin is considered within a Plate Tectonic framework it is a Passive Margin or Trailing Edge Basin, and it clearly has some important and predictable characteristics of that class of basin. These characteristics include the coastal sedimentary wedges, the thermal history, and many of the large scale hydrological patterns. This predictability includes the type, distribution and abundance of the hydrocarbon resource: both the oil and gas, and the lignitic and sub-bituminous coals of northern Louisiana.

The origin of the Gulf Basin is associated with the origin of the Atlantic Ocean and probably formed during the Triassic Period when the northern continent domed and cracked. This split was probably three-rayed. Two of these rays continued to grow and fed by the sea-floor spreading mechanism, formed the Atlantic Ocean. Under this scenario the third ray became a failed rift and ceased to expand. However, down this rift an important event took place, starting in the Jurassic Period it became the Mississippi Embayment down which flowed the ancestral Mississippi River.

The sedimentary deposits of the Louisiana Coastal Plain have been of varying interest to scientists and engineers since the initial mapping of the deltaic region by Captain Talcott [1839] and have been intensely studied for over sixty years.  The facies architecture and chronostratigraphic framework of the Gulf Basin was established through a series of projects between 1930 and 1960 [Trowbridge, 1930; Russell & Howe, 1935; Howe et al, 1935; Russell, 1936; Russell & Russell, 1939; Fisk, 1944, 1947, 1948, 1952, 1960, 1961; Fisk et al, 1954; McIntire, 1958; Kolb & Van Lopik, 1958; Gould & McFarlan, 1959; Scruton, 1960]. Subsequent work revealed the details of the lithofacies and biofacies characteristics of the system and began with the work of Coleman and Gagliano who put the delta switching mechanism within a sedimentological framework [Coleman et al, 1964; Coleman & Gagliano, 1964; Frazier, 1967, 1974; Coleman & Wright, 1975; Coleman, 1966; Morgan & Shaver, 1970; Coleman et al, 1974; Hart, 1979; Coleman & Prior, 1980; Van Heerden & Roberts, 1980; Penland & Boyd, 1981; Wells & Kemp; 1981; Wells & Roberts, 1981; Bouma et al, 1985, 1986; Van Heerden & Roberts, 1988; Coleman, 1988; Coleman & Roberts, 1988 a, b]. The most recent phase of study has attempted to understand the coastal region in terms of the Sequence Stratigraphy paradigm [Suter, Berryhill & Penland, 1987; Penland, Boyd, and Suter, 1988; Coleman, 1988; Van Heerden & Roberts, 1988; Boyd, Suter & Penland, 1989; Penland et al, 1991; McBride, Penland & Mestayer, 1990;  Penland, 1991; Hart, 1991 a, b]. This provides a 3-dimensional spatial framework for understanding both the coastal geology and the coastal resources of Louisiana.

The Gulf Basin has received sediments from the Mississippi River drainage system since the late Jurassic Period [Worzel & Burke, 1978], producing a combined thickness of Mesozoic and Cenozoic sediments of over 15 km [Martin & Bouma, 1978, Bouma et al, 1978].  The main theme of the sedimentation is the interplay of Coastal Plain deposition characteristic of the Trailing Edge Basin, and the Deltaic deposition characteristic of the fluvial system that gorged from the failed rift. Sediment loading, salt and shale diapirism, and sea level fluctuations have interacted with the detrital progradation and produce vertical and lateral sequences that reflect not only the shifting depositional centers but also the phase of sea level change. Superimposed on this was the climatic effect: the region was tropical to subtropical during the whole of the period of the existence of the basin. One consequence of this was that on the larger scale carbonate sedimentation occurs to the east and west of the depositional centers of detrital sediments.

The sediments of the northern Gulf Basin deposited during the latest depositional sequence [since the late Wisconsinian] are primarily influenced by the last sea level raise and fall. Again we can predict some important and predictable characteristics of this class of sequence [Text figure 3]. From the system science viewpoint these are first order effects on the coastline and influence both the current coastal sedimentary patterns and the future physiographic changes.

During periods of lowstand sediment thicknesses vary, with occasional rapid sedimentation causing expanded sequences, deposition of coarse grained detrital sediments, and recognition of well defined depositional trends [Text figure 4]. During raising sea level transgressive sequence tracts are formed, and condensed sections are initiated offshore [Text figure 5]. During the highstand the condensed sections are emphasized and provide excellent time-markers covering large areas of the continental slope and outer shelf [Text figure 6].

The Lowstand System Tract includes part of the more recent lobes of the Mississippi Fan, deposited during 25 - 22 ka by mass movement processes channelling through the incised Mississippi Canyon [Coleman et al., 1983; Bouma et al., 1986; Mazzulo, 1986]. Sea level was 60 - 130 meters below present amsl [Bloom, 1983]. The low relative sea level resulted in shelf edge deltas off the exposed continental shelf [Berry & Suter, 1986].

An incised lowstand erosional surface was formed at about 18 ka as a Type I unconformity across the Pleistocene Prairie Terrace [Boyd et al., 1989]. The characteristics of the surface were described by Fisk & McFarlan [1955].

Coastal onlap occurred as sea level rose between 18 - 9 ka to - 20 meters., with further mass movement feeding the Mississippi Fan during the period 12 - 10 ka. The Fan sequence is over 400 meters thick and extends downslope over 500 km [Bouma et al., 1985].

The Transgression System Tract also resulted in the sedimentary infilling of the Mississippi Canyon, where more than 600 meters of deposition took place [Coleman et al., 1983]. The river system expanded then expanded beyond the canyon walls during 9 - 3.5 ka and developed shallow water, back-stepping, deltas on the outer and middle continental shelf [the Outer Shoal, the Maringouin, and the Teche deltaic systems]. Accommodation space was occasionally filled as evidenced by the development of deltaic plains but in general the transgression was maintained and the back-stepping deltaic deposits were reworked [Frazier, 1967, 1974; Penland et al., 1988]. Boyd et al. [1989] noted that although the process resulted in a “retrogradational parasequence set and defines a transgressive systems tract ... the outer shelf region received little sediment supply after regression and constitutes a condensed section”.

Apparent standstill occurred during the transgression due to switching of the depositional center further to the east. This resulted in the development of an extensive revinement surface at the top of the Outer Shoal Delta lobe and allowed Penland et al. [1988] to recognize an early and a late Holocene deltaic plain sequence over central coastal Louisiana.

The final phase was the development of the progradational deltaic sequences formed during the highstand system tract.: the St. Bernard [4 - 1.8 ka], the Lafourche [3.5 - 0.4 ka], the Plaquamine-Balize [1 ka], and the Atchafalaya [initiated circa 1952]. These deltaic parasequences average 10 - 50 meters thick [Frazier, 1967; Penland et al., 1988] and as laterally coalescing wedges extended some 200 km downslope to the south east.

The highstand contains the maximum flooding stage, that formed the coastline at the entrance to the Mississippi Alluvial Valley [the Teche Shoreline]. Sea level is believed to have dropped a couple of meters during the progradational phase. Over the past 7,000 years the average relative sea level raise is estimated as 55 cm per century [Penland & Boyd, 1986]. Currently relative sea level raise for the Balize Delta is averaging more than 400 cm per century [Swanson & Thurlow, 1973].

Over Coastal Louisiana, despite the changing global and local regressions and transgressions, there has been an overall sediment progradation and rapid shoreline movement seaward into the Gulf Basin since it’s origin.  The average progradation during the Cenozoic was 5-6 km per million years [Coleman et al, 1989]. During the Quaternary alone some 3,600 meters of sediments accumulated on the shelf, and 3,000 meters in the deep basin [Mississippi Fan]. The physiographic regions observed today have probably been around since the Mesozoic Era, in one form or another.

The modern depositional environments are part of the highstand systems tract. The High Stand System Tract shows switching takes place about every 1,500 years.  Each of these deltaic complexes, formed by a river switch, covers an average area of 30,000 sq km and has an average thickness of some 35 meters. The Balize delta is one of four major delta complexes presently existing over coastal Louisiana.  Within this complex at least 12 individual sub-deltas have formed during the last 4,000 years.

The Mississippi River, the largest river system in North America, drains an area of 3,226,300 km²; this broad drainage area lies between the Appalachian Mountains (east), the Rocky Mountains (west), and the pre-Cambrian Shield of Canada (north). The river rises in the foothills of the Rocky Mountains and the northwestern portion of the drainage basin lies within the Mississippian and Cretaceous sediments of the Williston, Powder River and Denver Basins and flows southward for a distance of 6,211 km and enters the Gulf of Mexico. The density of the tributary network in the basin is relatively dense (Figure 63), the average drainage density being 0.19 km stream length per 500 sq km. Average elevation in the drainage basin is 659 m, with a maximum of 2,980 m and a minimum of 30 m. Average relief in the drainage basin is quite high, averaging some 915 m.

Average annual rainfall in the basin is 688 mm with a maximum of 1,532 mm and a minimum of 169 mm. [25-g02]. The rainy season lasts from May through August when the rainfall rarely falls below 80 mm. The driest month is January, with an average rainfall of only 34 mm.

The alluvial valley is extremely well defined all along its course and has a length of 870 km. Meandering is the most common type of channel process, but some braided stretches occur in the upper valley. The lower alluvial valley is characterized by an abundance of abandoned meander belt, each belt marking a former course of the river. Within each meander belt are well-developed abandoned meander loops and oxbow lakes. Separating the meander belts are broad wetlands composed of water tolerant trees and are referred to as backswamps. Only fine-grained suspended sediments are deposited in these regions and organic accumulations are common. The major geologic work on the alluvial valley was conducted by the H. N. Fisk of the U.S. Army Corps of Engineers (Fisk, 1944). This paper is a classic study of processes and sedimentation in alluvial valleys.

The average discharge of the river at the delta apex is approximately 17,704 m³/sec, with a maximum and minimum of 28,161 and 9,579 m³/sec, respectively. [25-g01]. Sediment discharge has been estimated to be about 2.4 billion kg annually. The sediment load brought down by the river consists primarily of clay, silt, and fine sand (approximately 70 percent of the load).

During the past 7000 years, the sites of maximum deltaic sedimentation (delta lobes) have shifted and occupied various positions (Kolb and Van Lopik, 1966). The currently active delta lobe is the Birdfoot or Balize delta (25-i02). An older abandoned lobe, the St. Bernard delta lobe is located north of the active lobe, while a younger abandoned lobe, the Lafourche delta lobe flanks the modern delta to the west (25-i01).

In Recent times, the seaward progradation and lateral switching of the deltas has led to the construction of a broad coastal or deltaic plain that has an area of 28,568 km², of which 23,900 km² is subaerial.

In the abandoned St. Bernard delta, inactive for approximately 3,000 years, the geomorphic forms displayed result from transgressive processes or subsidence and marine inundation. After the delta stopped receiving sediments, seaward progradation ceased, and subsidence and wave and current reworking processes became dominant. Along the seaward margins of the delta, the elongate distributaries are reworked by wave processes, concentrating small barrier islands at the tips of the channel mouths. In the interior of the delta, the marsh surface, no longer replenished by overbank sedimentation, begins to break up and small bays begin to form. Increased salt-water encroachment changes the marsh from a freshwater environment to a saline marsh. The salt marsh cannot keep pace with subsidence, and the former delta surface gradually subsides below sea level, forming a broad marine sound. During this period of time, wave reworking and alongshore sediment transport have resulted in the formation of an elongated offshore barrier island chain (25-i03). In the protected back-barrier sound, where biological processes abound, currents rework the shell-rich sound sediments into broad coquina or shell banks.

In the younger Lafourche delta, the transgressive processes did not have as much time to operate; it has been abandoned for less than 1,000 years. Wave and current reworking of the ends of the distributaries resulted in formation of coastal barriers still connected to the mainland. The delta plain has been undergoing subsidence, opening up small bays of brackish water not yet the size of those in the St. Bernard delta. The abandoned distributary channels in both of these relict deltas are well displayed, along with freshwater lakes. Land loss is quite high, amounting to some 100 km² per year during the past decade over the entire delta plain. Although a rising sea level has contributed to this loss, subsidence, wave reworking, and modification by man, causing salt-water intrusion, has been responsible for a high percentage of this total land loss. On the landward side of the bays and sounds, ragged marsh remnants indicate continuing encroachment of the marine waters.

The modern Birdfoot or Balize delta of the Mississippi River is the youngest of the Recent delta lobes; it commenced its seaward progradation some 600 to 800 years ago (Fisk and McFarlan, 1955). This newest delta has prograded over a relatively thick sequence of prodelta clays and, as a result of differential sediment loading, has built a relatively thick but laterally restricted deltaic sequence. In contrast, the older Recent deltas, mostly built over shallow bay and shelf deposits, are laterally widespread and relatively thin. The main channel of the river is almost 2 km wide, is 30 to 40 m deep, and displays relatively well-developed natural levees. At image top, the natural levees are up to 1 km wide and have heights of 3 to 4 m. Along the active distributaries in the lower delta, the natural levees narrow considerably, to widths less than 100 m, and display heights generally less than 0.5 m.

The channels of actively prograding distributaries in the delta display bifurcated patterns both upstream and near their mouths (25-i10). This type of pattern normally is associated with extremely low offshore slopes and low wave energy. Situated between the channels are interdistributary bays displaying a variety of sizes and shapes. These bays are usually extremely shallow (generally less than a few meters) and contain brackish to normal marine water during periods of low flooding and fresh water during periods of high flooding. Sedimentation rates are relatively low. The bays receive sediment only during periods of overbank flow associated with floods.

Breaches (crevasses) in the distributary levees result in the formation of subdeltas in the bays. Ultimately, these result in large, complex bay fills or subdeltas of the delta distributary. The smaller breaches are referred to as overbank splays and are active for only short periods, generally less than 10 years (Coleman and Prior, 1980). The bay fills comprise the most common geomorphic landforms in the active delta. Each bay fill forms initially as a break in a nearby major distributary natural levee during flood stage, gradually increases in flow through successive floods, reaches a peak of maximum deposition, wanes, and becomes inactive. Owing to continued subsidence, the marsh surface of the bay fill is inundated by marine waters, reverting to a bay environment and thus completing its sedimentary cycle. The mass of sediment resulting from this crevassing process can vary in thickness from 5 to 20 m, covers an area of 150 to 300 km², and requires 100 to 200 years to complete a cycle. West Bay formed by a break in the levee in 1838, Cubits Gap in 1862, Garden Island Bay in 1872, and Baptiste Collette in 1874. Today, these bay fills are undergoing rapid land loss, with destruction of marshes and swamps, as a result of subsidence and compaction. New bay fills are no longer being formed, due to diking by man to maintain flow in the major navigation channels. Dredged canals are present throughout the image and are constructed as access canals for subsurface hydrocarbon exploration and production. The circular patterns represent well sites associated with buried salt diapiric intrusions [25-i10].

Immediately seaward of the actively prograding distributaries are the turbid river-mouth effluent plumes (Wright and Coleman, 1971). Deceleration of a turbid plume as it spreads laterally allows the coarser sediment being transported to be deposited, forming the distributary mouth-bar and delta-front environments. The finer grained sediments remain suspended and spread laterally over broad distances, forming a turbid plume  that fronts the entire offshore delta; as these fine-grained sediments are deposited, they form the prodelta platform as the distributaries build seaward at rates of 100 to 150 m per year. Wave energy is relatively low offshore of the Mississippi River delta and 25-g03 illustrates, by month, the average wave power along the coast. Wave power is highest during the low discharge months and this often results in excessive coastal erosion.

Wetland loss in Louisiana is extremely rapid, probably the highest in the world (Walker, et al, 1987). Since the 1930s, 2,800 sq km of wetlands have been converted to open water. This loss is primarily the result of the high subsidence rate that is taking place in the delta region. Rising sea level, dredging, and conversion of wetlands for agricultural and industrial uses have also played a major role in this wetland loss. Because of the size of the delta, satellite images of only the active delta were acquired in 1983 and 1995 and were utilized to detect changes in open water and industrial modification of the delta wetlands in the active Birdfoot delta.

25-i05 illustrates the changes in open water and land gain in this twelve year period. The delta plain area in the image illustrated in Plate 24A is 1,904 sq km (470,489 acres). In the 1983 image, 365,830 acres of interior open water or 78% of the image contained open water. By 1995, the total interior water was 428,086 acres or an increase of 62,256 acres [25-i07]. Thus in a twelve year period, the open water increased by some 85percent. Most of the land loss occurred in the numerous bay fills, especially in the West Bay area (South of the Mississippi River channel). In isolated regions, new marsh was created and a total of 20,824 acres of new marsh was created in this twelve year period.

Much of the wetlands of the active delta have been modified by man and converted into industrial use [25-i04]. In 1983, some 35,778 acres had been converted from wetlands to industrial use [25-i09] and in 1995, the total modification had increased by 27,640 acres [25-i08] for a total of 63,418 acres. Thus in a twelve year period, a total of 90,464 acres of wetlands had been converted to open water or into industrial use, while the gain in new marsh only amounted to 20,824 acres [25-i06]. Thus there was a net loss of 69,640 acres of wetlands and the average annual rate of wetland loss is 5,803 acres/year.

Contributed  by Professor George F. Hart, LSU.

The Balize Delta has already finished it's major progradational stage and the Mississippi River has begun to switch to form the Atchafalaya Delta. The extensive inter-distributary bays that exist between the main distributary channels are important sites of deltaic deposition. Initially receiving fine grained argillaceous material by overbank flooding these regions form extensive mud-flats if the process is uninterrupted. However, in most cases the levees are cut by crevasses and a splay deposit results.  On a large scale this results in the formation of a sub-delta [six of which exist for the present Balize Delta].  The smaller splays are rarely active for more than 15 years, by which time they have filled-in the local bay area and the crevasse is finally choked-off.  Cubits Gap sub-delta, West Bay sub-delta, and Garden Island Bay sub-delta are well understood sub-deltas. Coleman [1988] noted the distributary mouth bar at South West Pass has prograded 17 km in 200 years producing a sand body some 8-10 km wide, 17 km long, and in excess of 80 m thick.  The finer grained offshore deposits have high sedimentation rates and in parts of the continental shelf [from as shallow as 5 meters], and, on the continental slope this has led to unstable conditions and gravity induced mass movement [on slopes less than 2 degrees.  In recent years the whole of the delta front has been mapped from side-scan sonar and high resolution geophysical techniques and it is now apparent that the whole area is scarred by mass movement indicating that the most dynamic part of the delta is actually the subaqueous portion.

Over Coastal Louisiana, despite the changing global and local regressions and transgressions, there has been an overall sediment progradation and rapid shoreline movement seaward into the Gulf Basin since it’s origin.  The average progradation during the Cenozoic was 5-6 km per million years [Coleman et al, 1989]. During the Quaternary alone some 3,600 meters of sediments accumulated on the shelf, and 3,000 meters in the deep basin [Mississippi Fan]. The physiographic regions observed today have probably been around since the Mesozoic Era, in one form or another.