Incomplete manuscript
DETRITAL DEPOSITIONAL SEQUENCES
OF THE
MISSISSIPPI RIVER DELTAIC PLAIN
INTRODUCTION
The sedimentary deposits of the Louisiana detrital plain have been of varying interest to scientists and engineers since the initial mapping of the deltaic region by Captain Talcott [1839] and intensely studied for over sixty years. The facies architecture and chronostratigraphic framework 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]. Scruton presented the first conceptual framework for delta evolution from construction to destruction. Subsequent work principally centered at Louisiana State University revealed 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, largely spear-headed by the Louisiana Geological Survey, attempted to understand the detrital plain in terms of sequence stratigraphy [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]. Building upon this historic record the Louisiana detrital plain now can be viewed within a chronostratigraphic framework.
THE GULF OF MEXICO BASIN
The Gulf of Mexico 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]. Sediment loading, salt and shale diapirism, and sea level fluctuations have all interacted with detrital progradation to produce vertical and lateral deltaic sequences that reflect not only the shifting depo-center but also the phase of sea level change [text figure 1]. Over Coastal Louisiana, despite the changing global and local regressions and transgressions, overall sediment progradation, rapid shoreline advancement, and relative sea-level drop has occurred. 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 sediments of the northern Gulf of Mexico deposited during the latest depositional sequence [since late Wisconsinian] are primarily influenced by the last sea level raise and fall. Thus the events of interest commenced some 18,000 years ago [during lowstand] when sea level was 60-120 meters below present amsl [Bloom, 1983]. During periods of rising sea level and highstand condensed sections were formed that provide excellent chronostratigraphic markers covering large areas of the continental shelf and upper slope. Calcareous-rich deposits including hemipelagics and shell hashes also were deposited. During lowstand sediment thicknesses vary, with sections of expanded section [rapid sedimentation], coarse grained detritals, and well defined depositional trends. The essential deposition framework is shown in figure 2.
THE SEQUENCE STRATIGRAPHIC MODEL
Boyd et al [1989] outlined the present sequence stratigraphic framework for the Louisiana detrital plain. This view is a product of intense work during the last few years [Penland et al, 1988; Penland, 1991] and attempts to fit the sequences into the model illustrated in text figure 3.
Text figure 4 is a cross-section through the detrital plain showing the sequence stratigraphic relationships. The sequence starts with the Wisconsinian low stand systems tract consisting of the most recent lobe of the Mississippi Fan deposited during 10-12 ka and 22-25 ka by mass movement processes channeling through the incised Mississippi Canyon [Bouma et al, 1986; Mazzulo, 1986; Coleman et al 1983]. This sequence is over 400 meters thick and extend downslope over 500 km [Bouma et al., 1985]. Relative sea level was -130 meters [Berryhill, 1986] producing the shelf edge deltas off the exposed continental shelf [Berryhill & Suter, 1986]. An incised lowstand surface was formed at about 18 ka and is a Type 1 unconformity [Boyd et al, 1989] across the Pleistocene Prairie terrace. The characteristics of this unconformable surface were described by Fisk & McFarlan [1955].
Coastal onlap occurred as sea level rose between 18-9 ka to -20 meters when sedimentary infilling of the Mississippi Canyon with more than 600 meters of sediment took place [Coleman et al., 1983]. The river system expanded beyond the Canyon walls during the 9-3.5 ka period and developed shallow water, back-stepping, transgressive systems tract deltas [Outer Shoal, Maringouin, and Teche] on the middle and outer continental shelf. Accommodation space was occasionally filled as evidenced by the development of deltaic plains but in general the transgression was maintained and the back-stepping delta deposits were reworked [Frazier, 1967, 1974; Penland et al, 1988]. Boyd et al [1989] noted that this resulted in a "retrogradational parasequence set and defines a transgressive systems tract ..and ... the outer shelf region received little sediment supply after regression and constitutes a condensed section".
The final phase was the development of the deltas formed during the highstand system tract [St Bernard, Lafourche, Plaquamines-Balize, and the Atachafalaya]. These progradational deltaic parasequences average 10-50 meters thick [Frazier, 1967; Penland et al., 1988] and extend some 200 km down slope. The highstand saw maximum flooding stage forming the coastline at the entrance to the Mississippi Alluvial Valley [the Teche Shoreline]. The progradational deltaic parasequences of the St Bernard [4.6-1.8 ka], Lafourche [3.5-0.4 ka], Balize [1.0 ka], and Atachafalaya [initiated circa 1952] expanded over 150 km to the south-east [Frazier, 1967]. The modern depositional environments are part of the transgressive and highstand systems tracts [text-figure 3]. Sea level is believed to have drop a couple of meters during this progradational phase.
Standstill occurred during the two phases of transgressive systems tract development resulting in a more extensive revinement surface than is normally present at the top of an abandoned delta lobe. This allowed Penland et al, 19XX, to identify an early and a late Holocene deltaic plain sequence existing over central coastal Louisiana. Effectively the depo-center was switching further to the east.
Coleman [1988] most recently summarized the overall pattern of deltaic plain development. Depo-center switching takes place about every 1,500 years. Each delta covers an average area of 30,000 sq km and has an average thickness of some 35 meters. The Mississippi River has formed six major delta complexes and at least 18 individual deltas during the last 7,000 years [Frazier, 1967]. In noting that the shifting sites of sedimentation result in overlapping and laterally displaced deltaic sequences extending 400 km along depositional strike and 200 km along depositional dip, with sediment thickness ranging from only a few tens of meters to a maximum of 200m Coleman made a very pertinent observation relevant to sequence stratigraphy. This is that the size, scale and temporal relationship of the various facies make it difficult, if not impossible, to use biostratigraphic or radiometric age dating techniques that can resolve such differences in the subsurface. Essentially, this helps to define the resolution of sequence stratigraphy at the basinal level. Stratigraphic resolution is 10,000 to 20,000 years for the late Pleistocene and 40,000 to 50,000 years for the early Pleistocene [Williams & Trainor, 1986]. Resolution within such limits must be based upon detailed facies correlation's.
The Balize Delta has already finished it's major progradational stage and the Mississippi River has begun to switch to form the Atachafalaya Delta. The Corp of Engineers has attempted to control the switching. The extensive interdistributary 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 will 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 subdelta [six of which exist for the present Balize Delta]. The smaller splay 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 subdelta, West Bay subdelta, and Garden Island Bay subdelta are well understood subdeltas.
The low density fresh water [1.0 g/cc] overrides the higher density sea water [1.028 g/cc] and forms a visible offshore plume [Wright & Coleman, 1974]. Gravity and hydraulic sorting results in the classical coarse to fine grain size further down-plume from the river mouths [figure ..]. Coleman [1988] notes 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 continental slope this has led to unstable conditions and gravity induced mass movement [on slopes less than 2o. 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.
THE HIGHSTAND SYSTEMS TRACT DELTAIC PARASEQUENCES
Currently relative sea level raise for the Balize Delta is more than 400 cm per century [Swanson & Thurlow, 1973]; and, for the highstand systems tract deltaic plain an average of 55 cm per century over the last 7,000 years [Penland & Boyd, 1986]. The highstand system tract consists of four progradational deltaic parasequences.
1. The Atachafalaya Deltaic Complex [initiated circa 1950].
2. The Plaquamine-Balize Deltaic Complex [1.0-present ka].
3. The Lafourche Deltaic Complex [3.5-0.4 ka].
4. The Saint Bernard Deltaic Complex [4.6-1.8 ka].
The present physiography of the detrital plain shows that each deltaic parasequence is in a different stage of development. In classical geomorphological terminology the Atachafalaya is in the stage of Youth, the Balize is in the stage of Maturity, the Lafourche in Old Age and the St. Bernard in Late Old Age. In litho-facies terms the Atachafalaya is forming argillaceous - arenaceous delta front - distributary mouth bar sediments, prograding onto bay deposits; the Plaquamine-Balize provide a full spectrum of main sequence fluvio-deltaic facies; the present-day Lafourche is a bay-marsh-swamp organic rich argillaceous facies; and, the St. Bernard is a deteriorating marsh dominated by reworking of delta front arenaceous deposits.
THE ATCHAFALAYA DELTAIC COMPLEX
The Atchafalaya delta is a modern Bay Head Delta [Van Heerden & Roberts, 1988], which became a subaqueous delta in 1952 [Morgan et al., 1953; Shlemon, 1972] and developed subaerial expression in 1973. At it's present stage of development it is comparable with a subdelta of the Balize Delta. Current activity involves distributary channel elongation and bifurcation and channel abandonment with associated lobe fusion. Small crevassing in the form of narrow overbank channels supply sediment to the interior of the sediment lobes formed between second order channels. The growth sequence for the Atchafalaya Delta are shown in text figure X-449.
Depositional environments
The present depositional environments are similar to those present in a subdelta and include distributary mouth bars, distal bars, distributary channels, levees, and algal flats. An oyster reef forms the delta front.
Lithofacies
The distribution of lithofacies are shown in Text figures X-446 and X-447.
Biofacies
Shell hash layers and back-bar algal flats are obvious major biofacies. Detailed biofacies analysis is lacking on this delta.
Vertical succession
Vibro-core studies from 5 lines are the basis of subsurface information on the delta. The basal sediments are bluish-gray clay deposited as bay sediments. The initial prodelta deposits are a brownish-gray clay [up to 1.5 meters thick] with oyster shells hash layers [3-8 cm thick]. The upper prodelta clays are parallel laminated similar to those occurring off the front of the modern Mississippi River.
THE BALIZE DELTAIC COMPLEX
Depositional environments
Lithofacies
Biofacies
Vertical succession
THE LAFOURCHE DELTAIC COMPLEX
Depositional environments
The Bayou Petit Caillou subdelta was responsible for the sedimentation in the Isle Derniers region. The isles formed with the abandonment of this part of the Lafourche delta forming a barrier about 32 km long, and 0.5 - 2.0 km wide. The chain consists of four small islands separated by tidal inlets [text-figure X-210]. Distributary channels [4-5 meter thick] underlie the island are associated with interdistributary and beach facies overlain by the barrier [up to 5 meters thick] and lagoonal facies [1-2 meters thick].
The main Lafourche Delta was formed over the Caminada-Moreau coastal area. Since the abandonment of the delta shore-face erosion has moved sand to the flanking barrier islands [Grande Isle to the east and Timbalier Isle to the west. Large tidal inlets separate the islands at Barataria Pass, Caminada Pass, Little Pass Timbalier, and Cat Island Pass. Lagoons are formed by Barataria Bay, Caminada Bay and Timbalier Bay. Ebb-tidal delta off the passes a up to 6 km long and 8 km wide [Penland et al., 1988].
Lithofacies
Biofacies
Vertical succession
THE SAINT BERNARD DELTAIC COMPLEX
Depositional environments
The largest barrier island system of the detrital plain is associated with the reworking of the distributary channel and mouth bar sands of the Saint Bernard deltaic complex. The chain is over 45 miles [75 km] long and the individual islands up to 1.5 miles wide. They have been moving landward for at least the last 100 years [Penland et al., 1985], retreating over a thick [up to 7 meters] sequence of lagoonal deposits. Because of the dominant south-east wave energy the sediments migrate northward. Smaller islands are to the south [12-15 feet of sand] and larger to the north [15-30 feet of sand]. Submergence is causing a diminishing sediment supply and, especially to the south, the destruction of the barrier islands to leave remnant inner-shelf shoals. The original source of the sand are buried distributary channels and river mouth bars and lie on the lower shoreface and inner shelf and extend seaward under the thin and discontinuous central and southern Chandeleur Islands. Distributary channels form a veneer 50 feet thick and 1,300 feet wide [Suter et al., 1988]. and extend up to 8 miles seaward of the central barrier islands. A broad sand sheet consisting of gently offshore dipping beds occurs under the ravinement surface.
Tidal channels, probably cut by hurricanes, are important associated environments. Offshore of the barrier islands are [assumed] submerged beach-ridge environments, about 20 feet thick and extending several miles offshore, identified as high angle clinoform reflectors.
The thickness of barrier island sediments relative to the location of the revinement surface determines whether or not the sands will be preserved. If the transgressed barrier shoreline sediment package lies above the advancing revinement surface, the entire sequence is truncated [Penland, Suter, and Boyd, 1985]. The mechanism of formation of the Barrier Island Chains from the deterioration of a delta was outlined by Penland and Boyd, 1981; and Suter et al, 1985 and is depicted in text-figures X-201 and X-206].
Lithofacies
Biofacies
Vertical succession