John R. Suter

Transgressive depositional systems of the Mississippi Delta plain; a model for barrier shoreline and shelf sand development

ABSTRACT Depositional sequences generated in the Mississippi River delta plain consist of a regressive and a transgressive component. The transgressive component has been considerably less studied but accounts for the majority of the surface area on the lower Mississippi River delta plain and up to 50 percent of the total sequence thickness in shallow-water deltas. The development and preservation of transgressive depositional systems in abandoned delta complexes follows the process of transgressive submergence in which the horizontal component of reworking occurs during shoreface retreat, combined with a vertical component of submergence acting to preserve the sequence. The evolution of transgressive depositional systems in each of the abandoned Holocene Mississippi River delta complex s can be summarized in a three-stage model beginning with stage 1, an erosional headland and flanking barriers. In this stage, regressive sand deposits contained within abandoned deltaic headlands are reworked by the eroding shoreface and dispersed longshore into contiguous flanking barriers enclosing restricted interdistributary bays. Submergence of the delta plain during relative sea-level rise generates an intradeltaic lagoon separating the former stage 1 sand body from the shoreline, forming stage 2, a transgressive barrier island arc. The landward-migrating barrier island arc is unable to keep pace with relative sea-level rise and the retreating mainland shoreline, resulting in submergence and the formation of stage 3, an inner-shelf shoal. Following submergence the former barrier island arc sand body continues to be reworked into a marine sand body on the inner continental shelf during stage 3. This sequence of coastal evolution provides direct evidence of barrier island formation, with each stage producing a distinctive stratigraphic signature. The current sea-level-rise models of shoreface retreat and in-place drowning developed for the U.S. Atlantic continental shelf do not adequately explain either the morphology or the stratigraphy of transgressive Mississippi River delta sand bodies. Current models of Mississippi deltaic stratigraphy emphasize the deep-water, artificially maintained Balize delta, which differs considerably from the shallow-water, shelf-phase delta complexes that are the primary depositional constituents of he Holocene Mississippi River delta plain.

Late Quaternary Shelf-Margin Deltas, Northwest Gulf of Mexico

Interpretations of 35,000 km (21,900 mi) of single-channel, high-resolution, seismic profiles traversing the continental shelf and upper continental slope of the northwest Gulf of Mexico indicate the existence of five late Wisconsinan shelf margin deltas, including the Rio Grande and Mississippi deltas. The deltas were recognized by geomorphic pattern, high-angle clinoform seismic reflections, and association with buried river systems. Isopach patterns show that the deltas range in size up to 5,000 km2 (1,900 mi2) and reach thicknesses of over 180 m (590 ft). The deposits are elongate parallel with depositional strike, indicating subsidence of the shelf margin as a whole. Internal reflection patterns show the deltas to be fluvially dominated. Multilo ate structure resulted from both short-term eustatic sea level fluctuations and delta switching. The late Quaternary shelf-margin deltas provide models for analogous deposits in the ancient record. They are primary indicators of the position of ancient shelf margins, and are important for predicting sand occurrence in that environment as well as farther downslope. As exploration moves to the shelf edge and beyond, instability hazards posed by late Wisconsinan deltas, as well as older deposits, must be understood and dealt with.

Sequence Stratigraphic Distribution of Coaly Rocks: Fundamental Controls and Paralic Examples

Significant volumes of terrigenous organic matter can be preserved to form coals only when and where the overall increase in accommodation approximately equals the production rate of peat. Accommodation is a function of subsidence and base level. For mires, base level is very specifically the groundwater table. In paralic settings, the groundwater table is strongly controlled by sea level and the precipitation/evaporation ratio. Peat accumulates over a range of rates, but always with a definite maximum rate set by original organic productivity and space available below depositional base level (groundwater table). Below a threshold accommodation rate (nonzero), no continuous peats accumulate, due to falling or low groundwater table, sedimentary bypass, and extensive erosion by fluvial channels. This is typical of upper highstand, lowstand fan, and basal lowstand-wedge systems tracts. Higher accommodation rates provide relatively stable conditions with rising groundwater tables. Mires initiate and ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received September 7, 1995; revised manuscript received November 25, 1996; final acceptance May 5, 1997. 2Exxon Production Research Company, 3120 Buffalo Speedway, Houston, Texas 77096. 3Exxon Production Research Company, 3120 Buffalo Speedway, Houston, Texas 77096. Present address: Conoco, Inc., P.O. Box 2197, Houston, Texas 77252. We benefited from the input and assistance of many people. Of special assistance were the teams involved in collaborative studies with Esso Australia and Esso Malaysia: P. Moore, M. Sloan, J. Emmett, B. Burns, A. Partridge, S. Creaney, Hanif Hussein, R. Hill, R. Lovell, and M. Feeley. We also thank the Rock Springs team: R. Beauboeuf, P. McLaughlin, W. Devlin, A. Carroll, Y. Y. Chen, G. Grabowski, Jr., K. Miskell-Gerhardt, M. Farley, R. Webster, and J. Schwalbach. Group members D. Curry and J. Yeakel were always helpful. We also enjoyed and profited from many discussions of these concepts with our colleagues outside Exxon: C. Diessel, R. Boyd, K. Shanley, B. Zaitlin, P. McCabe, M. Hendricks, A. Cohen, and M. Kirschbaum. We thank the reviewers of company reports, whose careful comments on several generations of this work improved it: S. Creaney, M. Feeley, J. Van Wagoner, F. Wehr, J. Yeakel, and A. Young. F. Weber and J. Zullig provided extensive management support. K. Linke translated our sketches into the fine figures herein. We value all the help.

Relation of sequence stratigraphy to modern sedimentary environments

One method of testing the concept of sequence stratigraphy is to compare it to Quaternary sediments in which chronology, stratigraphic relations, and facies geometry are more clearly understood than in older rocks. Rapid deposition rates during Quaternary glacial-eustatic cycles in large deltaic depocenters generate sequences comparable to those in the ancient stratigraphic record. In the northern Gulf of Mexico, the late Wisconsinan-Holocene Mississippi River has deposited a Type 1 sequence that includes lowstand, transgressive, and high-stand systems tracts. Characteristics of modern Mississippi River sedimentary environments support the methodology used in sequence analysis, but the short time taken for sequence generation here raises important questions about sequence time scales, correlation, and driving mechanisms.

The geomorphology of the Mississippi River chenier plain

Abstract The chenier plain of the Mississippi River is a shore-parallel zone of alternating transgressive clastic ridges separated by progradational mudflats. The term chenier is derived from the cajun term chene for oak, the tree species that colonizes the crests of the higher ridges. The Mississippi River chenier plain stretches 200 km from Sabine Pass, Texas, to Southwest Point, Louisiana and ranges between 20 and 30 km wide, with elevations of 2–6 m. The timing and the process of formation could be re-evaluated in the light of new chronostratigraphic findings in the Mississippi River delta plain. The stratigraphic relationship between the Teche and Lafourche delta complexes and Ship Shoal offshore indicates that these delta complexes belong to different delta plains that developed at different sealevels. It appears that the Teche delta complex is associated with the late Holocene delta plain which developed 7000 to 3000 yrs B.P. when sealevel stood 5–6 m lower than present. A regional transgression occurred between approximately 3000 BP and 2500 yrs B.P., leading to the transgressive submergence of the late Holocene delta plain, producing the regional Teche shoreline. The timing of this transgression conforms to the age of the most landward ridge in the chenier plain, the Little Chenier-Little Pecan Island trend, which dates at about 2500 yrs B.P. This ridge trend was originally interpreted as representing the Teche delta complex switching event with the landward Holocene/Pleistocene contact representing the high stand shoreline. The implication of this new interpretation is that the Little Chenier-Little Pecan Island trend represents the high stand shoreline, a continuation of the Teche shoreline separating the late Holocene and Recent delta plains, and that the Holocene/Pleistocene contact represents the leading edge of the marshes transgressing onto the Prairie Terrace. Significant mudflat progradation seems to require a westerly position of the Mississippi River, but the numerous different forms and ages of cheniers do not correspond well to the timing of major delta complex switching. Progradation of the chenier plain appears to be associated with building of the Recent delta plain and not the Teche complex of the late Holocene delta plain. The occurrence of individual ridges appears to be primarily tied to delta lobe switching within the Lafourche complex and variations in sediment supply from local rivers. The recent development of the Atchafalaya delta complex to the west is the closest position of an active distributary to the chenier plain since sealevel stabilization; a new episode of rapid mudflat progradation is thus taking place.

Barrier island arcs along abandoned Mississippi River deltas

Abstract Generation of transgressive barrier island arcs along the Mississippi River delta plain and preservation of barrier shoreline facies in their retreat paths on the inner shelf is controlled by: (1) shoreface translation; (2) age of the transgression; and (3) the thickness of the barrier island arc sediment package. Barrier island arcs experience an average relative sea level rise of 0.50–1.00 cm yr−1 and shoreface retreat rates range from 5–15 m yr−1. Young barrier island arc sediment packages (Isles Dernieres) are thin and have experienced limited landward retreat of the shoreface. Older barrier island arcs (Chandeleur Islands) are thicker and have experienced significant landward movement of the shoreface because of the greater time available for retreat. If the transgressed barrier shoreline sediment package lies above the advancing ravinement surface, the entire sequence is truncated. A thin reworked sand sheet marks the shoreface retreat path. The base of the transgressive sediment package can lie below the ravinement surface in older barrier shorelines. In this setting, the superstructure of the barrier shoreline is truncated, leaving the basal portion of the transgressive sequence preserved on the inner shelf. A variety of transgressive stratigraphic sequences from sand sheets to truncated barrier islands to sand-filled tidal inlet scars have been identified by high resolution seismic profiling across the shoreface retreat paths of Mississippi delta barrier island arcs. One of these examples, the Isles Dernieres, represents a recently detached barrier island arc in the early stages of transgression. An older example, the Chandeleur Islands, represents a barrier island arc experiencing long-term shoreface retreat. This paper describes the stratigraphic character and preserved transgressive facies for the Isles Dernieres and Chandeleur Islands.

Facies relationships and systems tracts in the late Holocene Mississippi Delta Plain

ABSTRACT Facies relationships in abandoned Holocene Mississippi Delta complexes are characteristic of both retrogradational transgressive systems tracts (TST) and progradational highstand systems tracts (HST). In the Barataria interlobe basin, delta-plain facies of the early Holocene Maringouin/Teche delta complex (TST), which accumulated from 7500 to 6000 yr BP, are overlain by a lagoonal facies 1-2 m thick (MFS) that accumulated during the maximum flooding event from 6000 to 3500 yr BP. Wave reworking transformed the distributary sands of retrogradational delta complexes into stratigraphically backstepping shoreline sand bodies. The most landward of these shorelines, the Teche shoreline, overlies the MFS and is, by definition, the shoreline of maximum transgression (SMT) (Penland et al. 1987 , 1987b; Kosters 1989). Relatively thick peats of high organic content, dating from 2400 to 1100 yr BP, are located immediately landward of this shoreline. Younger delta lobes, rapidly prograding since 1100 yr BP, have shifted the coastline seaward of the Teche shoreline, and form the first progradational HST parasequence. Thin, organic-poor salt marsh sediments are accumulating within this parasequence landward of the present shoreline. Rising relative sea level provides increased accommodation space while fresh water may be held within the delta plain, creating conditions of both groundwater and nutrients favorable to accumulation of high-quality organic facies of this type. In a subsequent progradational setting, stable relative sea level results in less accommodation space landward of the shoreline, while fresh water and nutrients are discharged into the Gulf of Mexico, forcing formation of brackish and salt marsh environments, unfavorable to accumulation of high-quality organic facies. These hypotheses may help explain the variability of some littoral high-quality coals vs. carbonaceous shales in the rock record.

Barrier Island Erosion and Protection in Louisiana: A Coastal Geomorphological Perspective

ABSTRACT Louisiana has the highest rates of coastal erosion and land loss in the United States. Rates of coastal land loss exceed 50 mi2/yr. Lousianas barrier islands, whose presence creates and maintains an extensive estuarine system and protects the salt marshes from the wave energy of the open Gulf of Mexico are rapidly vanishing, decreasing in area and migrating landward at rates up to 65ft/yr. Between 1898 and 1978, Louisianas barrier islands decreased in area by 41 percent, shrinking from 37 mi2 to 22 mi2. The life-expectancy of individual barrier islands systems ranges between 30 years for the Isles Dernieres and 225 years for the Chandeleur Islands. Disappearance of the barrier islands will result in destruction of the barrier-built estuaries and accelerated salt marsh deterioration. Such destruction will severely impact the fishery, fur, and waterfowl industries, valued at an estimated

LOW-PROFILE BARRIER ISLAND OVERWASH AND BREACHING IN THE GULF OF MEXICO

1 billion per year, whose harvests depend on the habitat provided by these fragile estuaries. Understanding the coastal geomorphological processes, both natural and human-induced, that control barrier island erosion, estuary deterioration, and salt march loss in the Mississippi River delta plain is essential in evaluating the performance of the various coastal protection methods currently envisioned or being employed. Previous attempts at coastal preservation and restoration have shown that an integrated approach that enhances natural processes, rather than combatting them, is the most effective. Highways are built with regularly scheduled maintenance programs and this same concept should be applied to the coastal zone. Preservation and restoration of our coastal environments requires a dynamic landscape maintenance program of regularly scheduled beach nourishment, barrier restoration, shoreface nourishment, vegetation, and coastal modification projects.

Late Quaternary Shelf Margin Deltas, Northwest Gulf of Mexico: ABSTRACT

This report will update the coastal zone practitioner on the National Flood Insurance Program (NFIP) as it affects the implementation of manmade changes along the coastline. It is our intent to place in proper perspective this fast-changing and often difficult to interpret national program. Readers will achieve an overall understanding of the NFIP on the coast, and will be in a position to apply the programs requirements in their efforts. We will begin with a history of the application of the NFIP to the coastal zone. The history of the problems encountered will lead into current regulations, methodologies, and the changes the Federal Emergency Management Agency plans for the future.The spatial variability of the nearshore wave field is examined in terms of the coherence functions found between five closely spaced wave gages moored off the North Carolina coast in 17 meters depth. Coherence was found to rapidly decrease as the separation distance increased, particularly in the along-crest direction. This effect is expressed as nondimensional coherence contours which can be used to provide an estimate of the wave coherence expected between two spatial positions.Prediction of depositional patterns in estuaries is one of the primary concerns to coastal engineers planning major hydraulic works. For a well-mixed estuary where suspended load is the dominant transport mode, we propose to use the divergence of the distribution of the net suspended load to predict the depositional patterns. The method is applied to Hangzhou Bay, and the results agree well qualitatively with measured results while quantitatively they are also of the right order of magnitude.