Barrier island arcs along abandoned Mississippi River deltas

Marine Geology, 63 (1985) 197--233

197

Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

BARRIER ISLAND ARCS A L O N G A B A N D O N E D

M I S S I S S I P P I RIVER

DELTAS

SHEA PENLAND', JOHN R. SUTER' and RON BOYD2

' Louisiana Geological Survey, Coastal Geology Program, University Station, Box G, Baton Rouge, L A 70893 (U.S.A.) 2Centre for Marine Geology, Dalhousie University, Halifax, N.S. B3H 3J5 (Canada) (Accepted for publication May 17, 1984)

ABSTRACT

Penland, S., Suter, J.R. and Boyd, R., 1985. Barrier island arcs along abandoned Mississippi River deltas. In: G.F. Oertel and S.P. Leatherman (Editors), Barrier Islands. Mar. Geol., 63: 197--233. 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. INTRODUCTION S e q u e n t i a l episodes o f delta building f o l l o w e d by a b a n d o n m e n t have led to the development of five different barrier shorelines along the Holocene Mississippi delta plain. Transgression of these abandoned deltas generates a distinct evolutionary sequence of barrier shoreline facies from erosional headlands with paired flanking barrier islands, to transgressive barrier island a rc s , t o i n n e r s h e l f s h o a l s d e p e n d i n g o n age ( P e n l a n d e t al., 1 9 8 1 ; P e n l a n d 0025-3227/85/$03.30

© 1985 Elsevier Science Publishers B.V.

~98 and Boyd, 1981). Preservation of whole or part of any portion of these barrier shoreline sequences is controlled by: (1) translation of the shoreface profile: (2) age of the transgression; and (3) thickness of the transgressive sediment package (Fischer, 1961; Swift, 1976; Belknap and Kraft, 1981; Penland and Suter, 1983; Boyd and Penland, 1984). Relative sea-level rise controls the vertical translation of the shoreface. Sediment supply, inner shelf circulation, regional gradient and local wave--sediment relationships control the horizontal translation of the shoreface. Since age controls the time available for transgressive sedimentation, young barrier shorelines are thin and older barrier shorelines are thick. The thickness of barrier island sediments in relation to the base of shoreface erosion controls the potential for preservation of transgressive barrier shoreline facies on the inner shelf. This paper examines the stratigraphic record preserved in the retreat paths of transgressive barrier island arcs along the seaward periphery of the Mississippi River delta plain (Fig.l). The objectives are to: (1} examine the development of the Isles Dernieres and Chandeleur Islands; (2) describe the stratigraphic character of the shoreface and inner shelf associated with each barrier island arc; (3) d o c u m e n t the coastal facies preserved in each barrier island arc retreat path; and {4) explain the differences seen between the Isles Dernieres and Chandeleur Islands. The data base for the interpretations consists o f more than 500 km of shallow, high-resolution seismic profiles correlated with more than 100 vibracores and borings from the shoreface and inner shelf (Fig.2). A comparison of historical National Ocean Survey hydrographic surveys provided information on the depth of shoreface erosion and the rate of shoreface retreat.

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Fig.1. T w o Holocene barrier island arcs have developed along the Mississippi River delta plain. The youngest barrier island arc is Isles Dernieres associated with the Bayou Grand Caillou headland of the Early Lafourche delta abandoned 600--800 years B.P. The older, the Chandeleur Islands, is associated with the St. Bernard delta complex abandoned 1800-2000 yrs B.P.

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Delta complex chronology D e l t a - p l a i n m o r p h o l o g y is d o m i n a t e d b y a n e x t e n s i v e n e t w o r k o f d i s t r i b u t a r y c h a n n e l s t h a t radiate o u t f r o m t h e Mississippi alluvial valley near B a t o n Rouge and extend southward into the Gulf of Mexico (Fisk, 1944; Kolb and Van Lopik, 1958a; Frazier, 1967; Coleman, 1981). Each distributary network

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Fig.3. Since the end of the Holocene marine transgression, the Mississippi River has built a delta plain consisting of six major complexes which can be subdivided into at least sixteen individual lobes (modified from Frazier, 1967).

is separated by a series of connecting interdistributary lakes and ponds that increase in size and coalesce towards the coast, forming larger bays open to the Gulf (Morgan, 1967). The Mississippi River has built six major delta complexes over the last 7000 yrs (Frazier, 1967). These delta complexes can be divided into two distinct physiographic regions: abandoned deltas and active deltas. Four complexes, the Maringouin, Teche, St. Bernard and Lafourche are abandoned (Fig.3). In addition, the Plaquemines lobe of the Modern complex is abandoned. Approximately 80% of the delta plain is abandoned and fronted by a chain o f different age barrier shorelines (Penland et al., 1981; Penland and Boyd, 1981). Active delta building is restricted to the Balize lobe of the Modern complex and the Atchafalaya complex. BARRIER SHORELINE DEVELOPMENT

The geomorphic evolution and depositional history o f individual Mississippi delta barrier shorelines is best visualized within the framework of the threestage model depicting barrier shoreline generation following delta abandonment (Fig.4). Subsidence generates a relative sea.level rise, in which erosional shoreface retreat processes transform the once active delta into an erosional

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Fig.4. A three-stage m o d e l d e p i e t i n g t h e e v o l u t i o n o f barrier s h o r e l i n e s along a b a n d o n e d Mississippi River deltas. Delta a b a n d o n m e n t initiates barrier shoreline g e n e r a t i o n . Relative sea level rise a n d erosional s h o r e f a e e r e t r e a t drive s h o r e l i n e e v o l u t i o n . Barrier s h o r e l i n e s e x i s t as erosional h e a d l a n d s w i t h f l a n k i n g barrier islands (Stage 1), barrier island ares (Stage 2) or i n n e r shelf shoals (Stage 3) d e p e n d i n g o n age

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headland with paired flanking barrier islands, Stage 1 (Penland et al., 1981; Penland and Boyd, 1981). Long-term relative sea-level rise and erosional shoreface retreat leads to the detachment of the barrier shoreline from the mainland and the formation of a barrier island arc, Stage 2. Barrier island arcs have migrated landward past the position of their earlier Stage 1 shoreline and are separated from the mainland by a wide intradeltaic lagoon. In order to keep pace with relative sea-level rise barrier island arcs stack washover and flood tidal delta deposits. The final stage of barrier shoreline evolution occurs when relative sea-level rise and repeated storm impacts overcome the ability of the barrier island arc to maintain its subaerial integrity. The barrier island arc is submerged forming an inner shelf shoal, Stage 3. Shoreface retreat processes in Stage 3 continue to drive the inner shelf shoal landward across the subsiding inner shelf and to smooth the mainland shoreline. COASTAL PROCESSES

Northern gulf coast process environment The northern Gulf coast is a microtidal storm-dominated environment (Hayes, 1979; Davies, 1980). Nearshore energy levels resulting from wind and wave processes are low except for the winter passage of extratropical cyclones and the summer occurrence of tropical cyclones. The wave height mode seaward of the Mississippi River delta plain is 1 m, wave periods are 5--6 s and the average deep-water wave power is only 1.8 X 103 W m -' (Boyd and Penland, 1984). On the average, modal wave conditions occur 4% of the time. Wright and Coleman (1972) indicated that over 99% of the deepwater wave power is dissipated before reaching the shoreline. Tides are diurnal with a mean range of 36 cm (U.S. Department of Commerce, 1983). The tidal regime in coastal Louisiana is complicated in fall and winter by extratropical cyclone passage which generates wind-driven tides in excess of normal astronomical conditions (Penland and Ritchie, 1979). In a typical year, frontal passages (10--25 per winter season) will elevate and depress mean sea level between 30 and 120 cm (Kemp et al., 1980; Boyd and Penland, 1981). The highest water levels along the shoreline are produced by tropical cyclones. In Louisiana tropical storms {wind greater than 63 km h -1 ) have a recurrence interval of 1.6 yrs, whereas the recurrence interval of hurricanes (winds greater than 118 km h -L) is 4.1 yrs (Neuman et al., 1978; Nummedal, 1982). On the Louisiana coast, hurricanes are capable of generating overwash elevations of 2--7 m above mean sea level (Boyd and Penland, 1981). Maximum storm surge levels are capable o f overwashing entire barrier shorelines and flooding the coast for m a n y kilometers inland.

Relative sea-level rise Relative sea-level rise drives the vertical translation of the erosional shoreface during the transgression of individual abandoned deltas. Tide gauge

203

measurements since 1913 from the Gulf coast of Florida (an area considered tectonically stable) indicate an eustatic sea-level rise ranging between 0.2 cm (Cedar Key) and 0.24 cm (Pensacola) per year (Harris, 1980). The most recent global sea-level rise s t u d y analyzed more than 700 tidal gauge stations and determined a lower sea-level rise rate of 0.12 cm per year (Gornitz et al., 1982). Relative sea-level rise reflecting subsidence along the Mississippi delta plain, as well as eustatic factors, ranges from 1.2 cm (Eugene Island and Grand Isle) to 4.3 cm (Shell Island) per year (Swanson and Thurlow, 1973). A comparison of the Harris (1980) and Gornitz et al. (1982) data sets suggests that eustatic factors account for only 5--10% of the relative sea-level rise and show that compactional subsidence is the d o m i n a n t mechanism driving sealevel transgressions across abandoned delta complexes. Present relative sea-level rise rates are estimated to range from 7.5 cm per century for old, shallow water delta complexes (Coleman and Smith, 1964) to 60 cm per century for intermediate age deltas (Kolb and Van Lopik, 1958a), to more than 400 cm per century for the presently active deep-water Balize delta (Swanson and Thurlow, 1973). An average relative sea-level rise rate for the last 7000 yrs was c o m p u t e d at 55 cm per century for the Mississippi River delta plain based on 155 C-14 dates from the abandoned Maringouin, Teche, St. Bernard, Lafourche and Plaquemines delta lobes (Fig.5; Penland and Boyd, in press). Years Before Present (x103) 0 0

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Erosional shore face retreat

Horizontal shoreface translation and generation of barrier shorelines are controlled by the process of erosional shoreface retreat. Repeated extratropical and tropical cyclones drive the landward retreat of the ravinement surface (Swift, 1968) across the surface of the abandoned delta. Rates of

204 barrier shoreline retreat range from less than 1 m yr -1 to over 15 m yr kl with extreme rates of over 50 m in years of high storm intensity (Morgan and Larimore, 1957; Penland and Boyd, 1981). During transgression, barrier shoreline development is controlled by the sediment dispersal pattern on the shoreface supplied by deposits being truncated and reworked. During storms, beach and nearshore bars are eroded and sand is transported seaward and stored lower on the shoreface and inner shelf (Komar, 1976; Short, 1979; Niedoroda et al., 1985, this volume}. Under fair-weather conditions a variable proportion of this material may again move onshore in the form of multiple nearshore bars and ridge and runnel systems (Fig.6}. The proportion of sediment returned to the beach following storm impact is dependent on the m a x i m u m depth from which waves can transport sediment landward under constructive fair-weather conditions, compared to the maxim u m depth which sediments can be transported seaward under erosive storm conditions. Offshore of the Mississippi River, wave refraction analyses, preand post-storm shelf sediment surveys, and shoreface and inner shelf current measurements show that significant sediment transport in non-storm conditions was restricted to the upper shoreface landward of the 5 m isobath (Krawiec, 1966; Murray, 1970, 1972; Penland and Boyd, 1982}. Under storm conditions, sediment can be offshore transported onto the inner shelf, well below the --10 m isobath. Therefore, fair-weather wave processes are not capable of returning all the material transported and deposited offshore following storm impact resulting in a net export of sediment from the barrier shoreline. Shoreface erosion controls sediment supply to the barrier shoreline (Fig.7). Fine-grained facies such as prodelta and interdistributary bay fill reworked by shoreface erosion supply very little coarse-grained material to the barrier shore. Erosion o f distributary and beach ridge plain facies are the major sand sources. Due to the inequality in shore normal transport, deposits truncated on the lower shoreface do not supply a significant quantity of material to the barrier shoreline under fair-weather wave conditions. Following delta abandonment, truncation of distributary and beach ridge plain facies supplies sufficient sand to build Stage 1 flanking barrier islands during transgression. Subsidence causes these sandy deposits to sink lower in the shoreface and decreases the sediment supply to the barrier shoreline. Longterm subsidence leads to the development of a barrier island arc detached from its sediment sources. A m o d e of sediment recycling is then initiated in which only a finite a m o u n t of sediment is available to maintain the barrier island arc against the effects of storms and sea-level rise. Repeated storm impacts drive sediment offshore and deplete the barrier island arc sand reservoir.

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207 ISLES DERNIERES BARRIER ISLAND ARC

Development The Isles Dernieres formed in response to the Caillou headland abandonm e n t in the Early Lafourche delta. A b a n d o n m e n t occurred approximately 6 0 0 - 8 0 0 years B.P. (Frazier, 1967; Morgan, 1974). This symmetric barrier island arc is approximately 32 km in length (Fig.8). Typical barrier widths range from 1.5--2 km in the central island arc to 0.5--1 km in the downdrift flanks (Fig.9). The Isles Dernieres are fragmented into four smaller islands separated by tidal inlets. These inlets are 300--1200 m in width with depths ranging between 6 and 18 m. Inlet morphology varies from wave-dominated to tide-dominated depending on age and the size of the tidal prism. In the east-central portion of this barrier island arc the remnants of the Cheniere Caillou beach ridge plain can be recognized. The Cheniere Caillou beach ridge plain is associated with the progradation of the Caillou headland distributaries and is the result of interception of sand moving longshore towards the west (Penland

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Fig.9. A. The central Isles Dernieres consist o f t w o small a r c u a t e islands s e p a r a t e d by tidal inlets f l a n k e d by r e c u r r e d spits. B. T h e m o r p h o l o g y o f t h e central Isles Dernieres is low mangrove s w a m p dissected by tidal channels. Washover sheets w i t h m a n g r o v e s w a m p d e p o s i t s o u t c r o p p i n g o n t h e b e a c h f a c e are t h e c o m m o n barrier shoreline f o r m . C. Last Isle is t h e w e s t e r n m o s t p o r t i o n o f t h e Isles Dernieres. D. R e c u r v e d spit at t h e east e n d o f the Isles Dernieres. E p h m e r a l Wine Island shoal can be seen in t h e b a c k g r o u n d .

210

and Suter, 1983; Gerdes, in press). Cheniere Caillou consists of a series of partially submerged beach ridges which spread seaward on their western margin against the Caillou headland distributaries. The recent history of the Isles Dernieres is one of rapid barrier shoreline detachment, island fragmentation a n d land toss. The transition of the Isles Dernieres from an erosional headland with flanking barrier islands to a barrier island arc is illustrated by the historical map comparison in Fig.10. In 1853 Caillou Boca, Pelto Bay and Big Pelto Bay separated the Isles Dernieres from the adjacent mainland, with a separation of less than 500 m at the narrowest point. By 1978 these bays had coalesced and increased in size threefold to form the modern day Lake Pelto. The northern shore of Lake Pelto experienced greater land loss for this time period and retreated faster than the Gulf shoreline resulting in the detachment of the Isles Dernieres from the mainland by more than 7 km of open water. The Isles Dernieres have steadily decreased in size over time from 34.8 km 2 in 1887 to 10.2 km 2 in 1979, a rate of 0.25 km 2 yr -1 (Penland and Boyd, 1981). This steady reduction in the size of the Isles Dernieres is attributed to repeated storm impacts depleting the limited sand reservoir and to the loss of sands to tidal inlet and inner shelf sinks. The analysis of the historical evolution of the Isles Dernieres indicates that two modes of barrier island generation are responsible for their origin: (1) spit growth and breaching (Gilbert, 1885; Fisher, 1968); and (2) mainland

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211

detachment (McGee, 1890; Hoyt, 1967). A historic nautical chart from 1853 shows the Isles Dernieres consisted of a set of flanking spits downdrift of the Caillou headland (Fig.10). Progradation of these spits was initiated by deposition of sediments eroded from the central Caillou headland sand bodies. Historical changes during the last 125 yrs in the Caillou Headland and the Isles Dernieres show mainland detachment has become the dominant formative process responsible for the transition of this shoreline from an erosional headland with flanking barrier islands (Stage 1) to a barrier island arc (Stage 2). This transition was accbmplished through rapid backbarrier land loss and the faster retreat of the mainland shoreline. Vertical bar accretion (De Beaumont, 1845; Otvos, 1970~ is not considered a viable mode for the origin of the Isles Dernieres, although it appears to play a role in the maintenance of the beach and nearshore zones, particularly in the Wine Island shoal area. The subsurface of the Isles Dernieres consists of a complex set of interfingering distributary, interdistributary and beach ridge facies overlain by a sequence of lagoonal and barrier shoreline facies (Fig.l 1). A set of bifurcating ISLES DERNIERES

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213 distributaries associated with t he Caillou headland extends seaward under the east-central p o r t i o n of t he Isles Dernieres and interfingers with the Cheniere Caillou beach ridge plain. These distributary channels are 4--5 m in thickness and have eroded into an older underlying unit (Fig.12). Cheniere Caillou ranges from 5 to 6 m in thickness. A typical beach ridge sequence from the L a f o u r ch e delta c o m p l e x coarsens upwards t hrough lower and upper shoreface units capped by a thin washover-aeolian unit (Gerdes, in press). Cheniere Caillou is underlain by interdistributary bay fill which rests on an older ravinement surface. The t o p of this regressive beach ridge/shoreface sequence lies 2--3 m below mean sea level. In th e central Isles Dernieres the Caillou headland is overlain by a thin sequence of transgressive barrier/lagoon deposits (1--2 m). Downdrift {east and west), th e barrier island arc sand b o d y thickens to 4--5 m. The bulk of the sand is stored to t he west of the central headland in recurred spits and in ebb tidal deltas associated with the Coupe Colin tidal inlet and Last Isle barrier island.

Retreat path stratigraphy The stratigraphy of the Isles Dernieres retreat path was interpreted from m o r e th an 200 km of high-resolution seismic profile lines collected from the shoreface and inner shelf in D e c e m ber 1982 (Fig.2). High-resolution seismic profiles were correlated with nine deep borings ( + 2 0 m) from the New Orleans District, U.S. Army Corps o f Engineers and m o r e than 50 vibracores collected b y th e Coastal Geology Program of the Louisiana Geological Survey from the Isles Dernieres and offshore on t he shoreface and inner shelf. The slope o f the Isles Dernieres shoreface averages 1:350 and decreases to 1 : 1 2 0 0 on th e inner shelf. The shoreface base, the ravinement surface, lies at th e 4--5 m isobath in t h e eastern Isles Dernieres and at the 3--4 m isobath in the western portion. Variations in t he de pt h of t he ravinement surface are a fu n ctio n o f sediment supply and local h y d r o d y n a m i c conditions. The Isles Dernieres are oriented normal t o t he d o m i n a n t wave approach and receive an annual onshore deep-water wave energy flux of 3.60 X 103 W m -1 based on data f r o m th e New Orleans SSMO block (U.S. Naval Weather Service Comm an d , 1976). The differences in t he de pt h of t he ravinement surface alongshore may reflect t he differential wave-damping effects of Ship Shoal offshore. Ship Shoal provides m or e p r o t e c t i o n for the western Isles Dernieres t h a n for the eastern p o r t i o n o f t h e island arc. A comparison of historical hydrographic surveys from 1891 and 1934 indicates t h e shoreface erosion rate ranges bet w e en 5 and 15 m y r -1 (Fig.13). Shoreface erosion truncates t he upper 3 m o f th e western island arc sequence and the upper 4 m of t he eastern island arc sequence. Surficial sediment samples and the vibracores show that sand bodies exposed on th e shoreface and inner shelf include a regressive set of distributary and beach ridge plain/shoreface deposits of t he Caillou headland overlain by a thin sand sheet up to 50 cm thick (Fig.14A). From n o r t h t o south water

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Fig.13. A c o m p a r i s o n o f historical b a t h y m e t r i c surveys o n t h e s h o r e f a c e a n d i n n e r s h e l f o f t h e Isles Dernieres f r o m 1 8 9 2 t o 1 9 3 4 s h o w s t h e p a t t e r n o f s h o r e f a c e erosion, T h e base o f erosion, t h e r a v i n e m e n t surface, ranges f r o m t h e 4--5 m i s o b a t h in t h e west a n d decreases t o 3--4 m in t h e east. R a t e s o f e r o s i o n r a n g e b e t w e e n 10 a n d 3 0 m y r -i ( s o u r c e : N a t i o n a l O c e a n Survey).

depth increases from 4 to 5 m, which is the zone of transition between the lower shoreface and inner shelf. Figure 14A shows distributary channeling eroded into the underlying beach ridge/shoreface, delta front and prodetta deposits associated with the Caillou headland. The Cheniere Caillou beach ridge plain sand b o d y pinches out seaward. Channels: exposed 'on t h e shoreface represent the seaward extension o f the distributaries identified in Fig.12. Transgressive shoreface and inner shelf deposits include barrier shoreline sands, backbarrier marsh and bay deposits, ebb-tidal deltas and an inner shelf

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Fig.14. A. A line drawing of a high-resolution seismic profile (dip section) offshore o f Cheniere Caillou in the east-central island arc. Profile shows distributary channeling eroded into underlying beach ridge, prodelta and delta front deposits. B. A line drawing o f a h i g h - r e s o l u t i o n seismic profile (strike section) across the western margin of the ebb-tidal delta associated with Wine Island Pass (see Fig.2A, p r o f i l e s B

g

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216

sand sheet. The present barrier island arc and backbarrier deposits pinch out on the upper shoreface and overlie Cheniere Caillou and its associated distributaries in the east-central island arc. The barrier shoreline sands increase m thickness to 5 m east and west away from Cheniere Caillou towards Wine Island and Last Isle, respectively (Fig.11). Ebb-tidal deltas constitute significant sand units which extend onto the inner shelf. These ebb-tidal deltas are the major sediment sinks from the Isles Dernieres. Figure 14B shows a seismic reflection profile across the western flank of the Wine Island Pass ebb-tidal delta. Several main channels of the tidal inlets exceed 10 m in depth. Tidal inlet migration both shore-parallel and shore normal can produce localized sand filled channel scars thicker than the ebb-tidal delta shown in Fig.14B. Shoreface and inner shelf sediment surveys (Krawiec, 1966; Frazier, 1967) and the seismic reflection profile in Fig.14A show a thin sand sheet spreading offshore. The Caillou headland and the Isles Dernieres barrier island arc represent early stages of transgression. Shoreface erosion has truncated the upper 3--4 m of this regressive--transgressive sequence; however, sufficient time has not elapsed to allow the development of a significant stratigraphic record preserved on the inner shelf. Once the ravinement surface has truncated and bypassed the position of the 1853 Isles Dernieres shoreline, the basal portions of the barrier island arc flanks and tidal inlet sand bodies should be preserved in the retreat path of the Isles Dernieres, which t o d a y is only marked by a thin discontinuous sand sheet. C H A N D E L E U R B A R R I E R ISLAND ARC

Development The oldest transgressive barrier island arc found in the Mississippi River delta plain is the Chandeleur Islands (Fig.15). The asymmetric shape of the Chandeleur Islands is due to an oblique orientation to the d o m i n a n t southeast wave approach, which leads to the preferential transport of sediment northward. The island arc is more than 75 km long and island widths range from 200 m to over 2500 m (Penland and Boyd, in press; Kahn, 1980). In the northern portion of the island arc, large washover channels and fans separated by h u m m o c k y dune fields dominate barrier island morphology (Fig.16). The beaches and foreshore are wide; multiple bars occur in the surf zone reflecting an abundance o f sediment. Towards the south, island widths narrow, dune heights decrease and washover channels and fans give way to discontinuous washover terraces and sheets. Further south, the island arc fragments into a series of small ephemeral inlets separated by tidal inlets. Chandeleur Sound is shallow (3--4 m) and separates the Chandeleur Island arc from the retreating mainland shoreline by more than 25 km of open water. For the last 100 yrs, the Chandeleur Islands have retreated landward

217

DELTAPLAIN ENVIRONMENTS ~

Marsh BARRIER ENVIRONMENTS

~

SubaerialBarrierSands

~

SubaqueousBarrier~nds

~

RecurredSpit Tidal InletChannel

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km

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Fig.15. The geomorphology and depositional environments of the Chandeleur Islands. Boyd, 1981). Retreat rates along the Gulf shoreline are greater t han 15 m yr -1 in the southern island arc, decreasing n o r t h w a r d t o less t han 5 m yr -1 (Fig.17). The Chandeleur Islands have experienced an average land loss rate o f 0.08 km 2 y r -1 since 1869. Periods o f high hurricane f r e q u e n c y correspond t o periods o f high land loss. During periods of low hurricane frequency, constructive fair-weather processes lead t o island recovery followed by an increase in area. However, t he rate of recovery is not sufficient to maintain the Chandeleur Islands against t he f r e q u e n c y of hurricane impact due t o the diminishing sediment supply. As a result these islands experience a net longterm land loss. T he mainland shoreline retreat rates exceed those o f the Gulf shoreline o f th e Chandeleur Islands indicating t hat the d e t a c h m e n t process o f Stage 2 barrier shorelines continues. The recent patterns of hurricane destruction followed by re-emergence seen in t he sout hern Chandeleur Islands, particularly Errol Island and Grand Gosier Island, represent the transition from a Stage 2 barrier shoreline, t he barrier island arc, into a Stage 3 barrier shoreline, an inner shelf shoal (Penland et al., 1981; Penland and Boyd, 1982). R e p e a t e d hurricane impacts causing subaerial barrier shoreline destruction followed by re-emergence have resulted in a net loss of subaerial barrier island arc area and a net gain in subaqueous shoal area. This process will ultimately lead t o destruction o f t he Chandeleur Islands.

218 The Chandeteur Islands are retreating landward over a thick sequence of lagoon deposits overlying t he subsiding St. Bernard delta complex (Fig.18), The major distributary headlands which were once the major barrier shoreline sand sources now lie on the lower shoreface and inner shelf and extend

219

Fig.16. A. Washover c h a n n e l s a n d fans s e p a r a t e d b y h u m m o c k y d u n e field d o m i n a t e t h e m o r p h o l o g y o f t h e s e d i m e n t - a b u n d a n t n o r t h e r n C h a n d e l e u r Islands. B. Beaches in t h e s o u t h e r n C h a n d e l e u r Islands are low w a s h o v e r sheets c o m p o s e d of shell a n d sand. C. In t h e G r a n d Gosier Island region t h e island arc f r a g m e n t s into a d i s c o n t i n u o u s c h a i n of small islets s e p a r a t e d b y tidal inlets. D. A view t o w a r d s t h e s o u t h w e s t o f B r e t o n Island a n d associated shoal region in t h e s o u t h e r n p o r t i o n of t h e C h a n d e l e u r b a r r i e r island arc.

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Fig.17. Shoreline changes observed for the Chandeleur Islands between 1870 and 1978 (source: National Ocean Survey).

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seaward under th e central and southern Chandeleur Islands. Here, the barrier island arc is thin and discontinuous and overlies three major distributaries. Towards the north, the Chandeleur Islands sand b o d y is thicker and m ore c o n t i n u o u s and overlies a sequence of lagoonal deposits which also increases in thickness n o r t hw a r d (2--4 m). Maximum lagoonal thickness occurs in the n o r t h e r n part o f the island arc where t he sequence is 5--7 m thick. The base o f the Chandeleur Island transgressive sediment package averages 8--10 m NORTH

SOUTH

M ISSISSIPPI SOUND

CHANDELEUR ISLAND

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Fig.18. A stratigraphic strike section of the Chandeleur Islands showing the facies relationships of the Chandeleur Islands and St. Bernard delta complex (modified from Frazier et al., 1978}. Strike section corresponds to section F in the inset.

222

below mean sea level (Fig.18). At tidal inlets, deep isolated sand filled scars can develop due to channel migration. In areas where recurved spits build into deep-water, thick sand bodies develop with the basal portions lying below the advancing ravinement surface.

Retreat path stratigraphy The stratigraphy of the Chandeleur barrier island arc retreat path was determined by more than 250 km of high-resolution seismic profile lines collected on the shoreface and inner shelf in August, 1981 (Fig.2). The slope of the Chandeleur Islands shoreface is 1:300, decreasing to 1:1200 on the

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Fig.19. A c o m p a r i s o n o f historical b a t h y m e t r i c surveys on the shoreface inner shelf o f the Chandeleur Islands from 1 8 8 5 and 1 9 2 4 illustrates the pattern o f s h o r e f a c e erosion. The base o f shoreface erosion, the r a v i n e m e n t surface, ranges from 10 m in t h e s o u t h t o 4 m in the north. Rates o f erosion range b e t w e e n 1 0 and 3 1 m yr -~ (source: National Ocean S urvey ).

223

inner shelf. The Chandeleur Islands shoreface experiences an annual onshore (southeast) deep-water wave energy flux of 5.17 × 10 ~ W m -~ based on data f r o m the Pensacola SSMO block (U.S. Naval Weather Service Com m and, 1976). The base of shoreface erosion, the ravinement surface, ranges from 10 m in the south to 4 m in the north. A comparison of historical hydrographic surveys from 1885 and 1924 indicates the rate of shoreface retreat ranges between 10 and 31 m yr -~ (Fig.19). The greatest shoreface retreat rates are associated with the deepest areas of shoreface. This relationship is a f u n ctio n of the orientation of the Chandeleur Islands to t he dom i nant southeast wave approach. Major regressive distributary sand bodies are exposed on the lower shoreface and inner shelf in t he southern Chandeleur Islands (Frazier, 1967; Kolb and Van Lopik, 1958a, b). Seaward of Grand Gosier and Breton Island dist r i b u t a r y sand bodies lie between the 8 and 12 m isobaths m o r e than 2--3 km f r o m shore, well below t he base of fair-weather constructive wave processes and the effective zone of onshore sediment transport to t he barrier shoreline. These regressive sand bodies are no longer a viable sand source for the Chandeleur Islands because of the seaward inequality in shore normal sedim e n t dispersal on the shoreface. Modern and recent transgressive deposits can be delineated on the shoreface and inner shelf o f the Chandeleur Islands. These deposits include barrier island sands, lagoonal deposits, tidal inlets and recurved spit complexes. In MSL

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Fig.2l. A shore-normal highcesolution seismic profile (dip section) on the shorefaee in the central Chandeleur Islands correlated w i t h a

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Fig.22. A shore-parallel (north to south) high-resolution seismic profile (strike section) showing downdrift (northward) dipping beds in the spit at the northern end of the Chandeleur Islands. This spit overlies a truncated sequence of distributary channeling associated with an earlier St. Bernard delta lobe (see Fig.2B, profile C).

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Fig.23. A shore-normal high-resolution seismic profile (dip section) across the seaward margin of the Chandeleur Island retreat path. This profile shows an inner shelf sand sheet c o m p o s e d of seaward-dipping reflectors (see Fig.2B, profile D).

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227

the southern Chandeleur Islands, the upper portion of the barrier-lagoon sequence is being truncated except where tidal inlet channels extend below the advancing ravinement surface. The majority of the Southern Chandeleur Islands consists of a subaqueous shoal complex of flood-tidal delta and washover deposits separated by small ephemeral islands (Figs.16 and 20). In the central island arc, the entire barrier-lagoon sediment package lies above the ravinement surface (--10 m isobath) and is reworked by shoreface erosion (Fig.21). The barrier island arc consists of a sequence of lagoonal, washover and flood-tidal delta deposits capped by aeolian sands (Van Heerden et al., in press). In the northern Chandeleur Islands only the upper portion of the large migrating spit complex ( > 1 0 m thick) is being truncated by shoreface erosion (Fig.22). The remainder of the barrier island--lagoon sequence lies below the advancing ravinement surface and m a y eventually be preserved on the inner shelf. On the inner shelf, a broad sand sheet marks the Chandeleur Island retreat path (Fig.23). The internal structure of the sand body consists of gently seaward dipping beds reflecting offshore sediment transport during storm impact. Recent transgressive deposits preserved on the inner shelf include truncated barrier islands and tidal inlet sequences marking former Stage 1 shorelines representing earlier stages in the evolution of the Chandeleur Islands. Figure 24 shows the basal portions of a truncated barrier Air

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Fig.24. A shore-parallel (north to south) high-resolution seismic profile (strike section) showing a truncated barrier island preserved on the inner shelf (see Fig.2B, profile E).

228

Air

MSL 0 '

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c~

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TIDAL INLET SCAR

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Fig.25. A shore-parallel (north to south) high-resolution seismic profile (strike section) showing a truncated tidal inlet scar preserved on the inner shelf (see Fig.2B, profile F).

island lying more than 6 km offshore of Monkey Bayou. This barrier island sequence consists o f a migrating spit complex with southerly dipping foresets. North of the truncated barrier island is a large tidal inlet scar produced by a southerly migrating channel (Fig.25). This scar is more than 10 m thick and approximately 500 m wide. The close proximity and geometry of these sand bodies suggest they are part of an ancestral Stage 1 flanking barrier island system associated with one of the major abandoned St. Bernard delta lobes n o w lying on the lower shoreface and inner shelf. CONCLUSIONS

(1) Barrier island arc development within the Mississippi River delta plain is initiated by an episode o f delta building followed by abandonment. Compactional subsidence generates a sea level transgression. Shoreface retreat processes transform the abandoned delta into an erosional headland supplying sediment for barrier shoreline formation. Natural levee, distributary m o u t h bar, beach ridge and shoreface deposits are the primary sand sources. (2) Spit breaching (Gilbert, 1885; Fisher, 1 9 6 8 ) f o l l o w e d by mainland detachment (McGee, 1890; Hoyt, 1967) are recognized as the primary modes of barrier island origin in the Mississippi River delta plain. Barrier island arc formation is a two-step process.

229

Step 1. Barrier island arcs evolve from flanking barrier islands supplied with sediment from a recently abandoned delta headland (Stage 1; Fig.4). Shoreface retreat processes erode distributary sand bodies with the reworked sand first accumulating on the flanks of the headland in the form of recurved spits. Ongoing downdrift spit building and repeated storm impacts lead to spit breaching and flanking barrier island formation. Step 2. Long-term subsidence leads to the submergence of the delta plain and formation of a large intradeltaic lagoon which separates the erosional headland barrier shoreline (Stage 1) from the mainland. Mainland detachment occurs by the coalescence of backbarrier lakes and bays and by the faster retreat of the mainland shoreline than the barrier shoreline resulting in d e t a c h m e n t and barrier island arc formation (Stage 2; Fig.4). Long-term shoreface retreat leads to the reworking and truncation of the erosional headland and flanking barrier island sequence dominated by recurved spit and tidal inlet deposits into a barrier island arc sequence dominated by washover and flood tidal delta deposits, reflecting a transition from a tide-dominated barrier shoreline to a wave-dominated barrier shoreline. (3) The barrier sediment budget is controlled by the balance between the volume of sediment supplied by erosional shoreface retreat processes and the volume of sediment loss to inner shelf, washover, recurved spit and tidal inlet sinks. At abandonment, shoreline sediment sources lie on the upper shoreface and the volume of sediment supplied is greater than the volume of sediment loss to littoral sinks. Subsidence causes the sediment sources to move lower on the shoreface and onto the inner shelf, resulting in a diminishing sediment supply. Eventually the eroding barrier headland shoreline evolves into a barrier island arc where the volume of sediment loss to littoral sinks is greater than the volume of sediment supplied by erosional shoreface retreat. Sedim e n t dispersal consists of the recycling and gradual degradation of the barrier sand reservoir. Land loss ensues and leads to the erosion of the subaerial barrier island arc and the generation of an inner shelf shoal. (4) Preservation of barrier shoreline facies on the inner shelf is controlled by the relationship between the geometry of the transgressive sequence and the base of shoreface erosion. Relative sea-level rise, sediment supply, shoreface retreat processes and age control the stratigraphic character of the transgressive sediment package. Young barrier island arcs are thin due to limited relative sea-level rise and sedimentation. In contrast, older barrier island arcs are thicker because of the greater time available for sedimentation. When the transgressive sequence lies above the base of shoreface erosion, the entire unit is eroded and a thin reworked sand sheet is preserved in the retreat path. For thick barrier island arcs, basal portions of the transgressive sequence m a y lie below the base o f shoreface erosion. In this setting, when the barrier island arc is truncated, the basal portions of the barrier platform and a reworked sand sheet are preserved on the inner shelf. (5) The a b a n d o n m e n t and reworking of the Caillou delta headland supplied sands for the generation of the Isles Dernieres. Recent shoreline history illustrates barrier island arc development by mainland detachment and tidal inlet

230 development by island fragmentation. The 1853 Isles Dernieres configuration represents a Stage 1 erosional headland barrier shoreline dominated by recurved spits. By 1978, mainland detachment had occurred and the shoreface had eroded more than 250 m; however, sufficient time had not elapsed for the truncation and possible preservation of portions of the 1853 shoreline. Because of the limited time available for sedimentation and shoreface retreat, only a thin sand sheet marked the Isles Dernieres retreat path. Continued sedimentation and retreat will lead to the development of a more complex transgressive stratigraphic record on the inner shelf. Barrier shoreline sequences with the greatest preservation potential include the tidal inlet deposits and the recurved spit deposits at each end of the barrier island arc. A generalized stratigraphic sequence preserved for the Isles Dernieres--Caillou delta headland would lie u n c o n f o r m a b l y on a ravinement surface. The regressive unit would coarsen upward through a sequence of prodelta, delta front, distributary and beach ridge shoreface deposits capped by a thin transgressive sand sheet. The total Isles Dernieres--Caillou headland sequence is 5--7 m thick with a transgressive unit 10--50 cm thick. (6) Transgression of at least three major distributary headlands in the eastern St. Bernard delta supplied sands for the development of the Chandeleur Islands. Recent shoreline history shows continued mainland detachment, land loss and erosional shoreface retreat. The Chandeleur Islands transgressive unit is 5--15 m thick and has experienced more than 10 km of erosional shoreface retreat. Seaward of this barrier island arc, truncated barrier islands and tidal inlet scars mark the retreat paths of ancestral flanking barrier islands on the inner shelf. The identification of barrier shoreline facies preserved in the retreat path indicates that the Chandeleur Islands originally formed at a more seaward position on the inner shelf and have migrated to their present position. A broad sand sheet 1--3 m thick and 10--20 km wide marks the Chandeleur Island retreat path. The size of this sand sheet reflects the effect of long-term storm impacts and extensive erosional shoreface retreat. Barrier shoreline sequences lying on the shoreface with the greatest preservation potential include the basal portions of the recurved spit in the northern Chandeleur Islands and tidal inlet deposits in the southern Chandeleur Islands. A generalized stratigraphic sequence for the Chandeleur Islands--St. Bernard delta sequence would lie unconformably on a ravinement surface at the base of the Holocene section and coarsen upward through prodelta, delta front and distributary deposits, capped by a thick transgressive sand unit. This sand unit coarsens upward with lagoon, tidal channel, tidal delta and recurved spit deposits overlain by a sand sheet. The regressive component is 15--25 m thick and the transgressive c o m p o n e n t is 2--10 m thick. (7) Depth differences in the ravinement surfaces between the Isles Dernieres and Chandeleur Islands are a function of the wave approach and energy. The Chandeleur Islands are oriented oblique to the dominant southeast wave approach and as a result experience greater shoreface erosion in the southern portion of the island arc. The Isles Dernieres are oriented normal to the

231 d o m i n a n t wave a p p r o a c h a n d e x p e r i e n c e g r e a t e r s h o r e f a c e erosion in t h e c e n t r a l island arc. T h e d i f f e r e n c e s seen in t h e d e p t h o f e r o s i o n are a f u n c t i o n o f t h e wave climate. T h e o n s h o r e d e e p - w a t e r w a v e e n e r g y f l u x is g r e a t e r f o r t h e C h a n d e l e u r Islands t h a n t h e Isles Dernieres, t h e r e f o r e t h e d e p t h o f erosion is greater. (8) T h e d i f f e r e n c e s in t h e d e p o s i t i o n a l h i s t o r y a n d stratigraphic r e c o r d o f t h e Isles Dernieres a n d t h e C h a n d e l e u r Islands are a f u n c t i o n o f t i m e and s e d i m e n t s u p p l y . T h e C h a n d e l e u r Islands are larger b e c a u s e o f a g r e a t e r sedim e n t s u p p l y f r o m t h e r e w o r k i n g o f larger m u l t i p l e sand-rich h e a d l a n d s versus t h e r e w o r k i n g o f t h e single small s a n d - d e f i c i e n t h e a d l a n d a s s o c i a t e d w i t h t h e Isles Dernieres. T h e C h a n d e l e u r Islands are t h i c k e r and t h e stratigraphic r e c o r d o f its r e t r e a t p a t h is m o r e c o m p l e x t h a n t h e Isles D e r n i e r e s b e c a u s e o f t h e g r e a t e r t i m e available f o r s e d i m e n t a t i o n a n d s h o r e f a c e t r a n s l a t i o n . ACKNOWLEDGEMENTS T h e Coastal G e o l o g y P r o g r a m o f t h e L o u i s i a n a G e o l o g i c a l S u r v e y ( L G S ) is s u p p o r t e d b y t h e Coastal P r o t e c t i o n T r u s t F u n d ( A c t 41) a d m i n i s t e r e d through the Department of Natural Resources. The authors acknowledge the c o o p e r a t i o n o f t h e U.S. G e o l o g i c a l S u r v e y , Marine G e o l o g y B r a n c h , C o r p u s Christi, T e x a s , p a r t i c u l a r l y Mr. H e n r y Berryhill, in s u p p o r t o f t h e seismic cruises. S u p p l e m e n t a l v i b r a c o r e d a t a f r o m t h e Isles D e r n i e r e s was p r o v i d e d b y Kevin Neese ( L G S ) . L o u i s i a n a Universities Marine C o n s o r t i o n ( L U M C O N ) p r o v i d e d t h e R / V " R . J . R u s s e l l " a n d logistical s u p p o r t . Individuals involved with various c o m p o n e n t s o f this p r o g r a m include L e e Black, Wilton D e l u a n e a n d Steve Rabalais o f L U M C O N , A d a m S h a m b a m , Bo T y e , K a r e n Westphal a n d J u d y A r d o i n o f LGS. REFERENCES Belknap, D.F. and Kraft, J.C., 1981. Preservation potential of transgressive coastal lithosomes on the U.S. Atlantic shelf. In: C.A. Nittrouer (Editor), Sedimentary Dynamics of Continental Shelves. Elsevier, Amsterdam, pp.429--442. Boyd, R., 1980. Sediment dispersal on the New South Wales continental shelf. Proc. 17th Int. Coast. Eng. Conf., pp.1364--1381. Boyd, R. and Penland, S., 1981. Washover of deltaic barriers on the Louisiana coast. Trans. Gulf Coast Assoc. Geol. Soc., 31: 243--248. Boyd, R. and Penland, S., 1984. Shoreface translation and the Holocene stratigraphic record: Examples from Nova Scotia, Mississippi Delta and eastern Australia. In: B. Greenwood and R.A. Davis, Jr. (Editors), Hydrodynamics and Sedimentation in Wave-Dominated Coastal Environments. Mar. Geol., 60: 391--412. Coleman, J.M., 1981. Deltas -- Processes of Deposition and Models for Exploration. Burgess, Minneapolis, Minn., 124 pp. Coleman, J.M. and Smith, W.G., 1964. Late Recent rise of sea level. Geol. Soc. Am. Bull., 75 : 833--840. Davies, J.L., 1980. Geographical Variation in Coastal Development, Longman, London, 2nd ed., p.212. De Beaumont, E., 1845. Leqons de G~ologie Practique. Reproduced in Schwartz, M.L. (Editor), 1973. Barrier Islands. Benchmark Pap. in Geol., Vol. 9, Dowden, Hutchison and Ross, Stroudsburg, Pa., pp. 5--43.

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Fischer, A.G., 1961. Stratigraphic record of transgressing seas in light of sedimentation m the Atlantic coast of New Jersey. Bull. Am. Assoc. Pet. Geol., 45: 1656--1666. Fisher, J.J., 1968. Barrier island formation discussion. Geol. Soc. Am. Bull., 79: 1421-1426. Fisk, H.N., 1944. Geologic investigation of the alluvial valley of the lower Mississippi. U.S. Army Corps Eng., Mississippi River Comm., Vicksburg, Miss., p.78. Frazier, D.E., 1967. Recent deltaic deposits of the Mississippi River: their development and chronology. Trans. Gulf Coast Assoc. Geol. Soc., 27: 287--315. Frazier, D.E., Osanik, A. and Elsik, W.C., 1978. Environments of peat accumulation -coastal Louisiana. In: W.R. Icaiser (Editor), Gulf Coast Lignite Conference: Geology, Utilization, and Environmenta] Aspects. Univ. Tex. Bur. Econ. Geol. Rep. Invest., 90: 5--20. Gerdes, R.G., in press. The Caminada--Moreau beach ridge plain. In: S. Penland and R. Boyd (Editors), Transgressive Depositional Environments of the Mississippi River Delta Plain. La Geol. Surv., Guideb. Ser. Gilbert, G.K., 1885. The topographic features of lake shores. U.S. Geol. Surv. 5th Annu. Rep., pp.87--88. Gornitz, V., Lebedeff, S. and Hansen, J., 1982. Global sea-level trend in the past century. Science, 215: 1611--1614. Harris, D.L., 1980. Tides and tidal datums in the United States. U.S. Army Corps Eng., Coast. Eng. Res. Center, Spec. Rep., 7, p.382. Hayes, M.O., 1979. Barrier island morphology as a function of tidal and wave regime, In: S.P. Leatherman (Editor), Barrier Islands: F r o m the Gulf of St. Lawrence to the Gulf of Mexico. Academic Press, New York, N.Y., pp.1--28. Hoyt, H.H., 1967. Barrier island formation. Geol. Soc. Am. Bull., 78: 1125--35. Kahn, J.H., 1980. The role of hurricanes in the long-term degradation of a barrier island chain. M.S. Thesis, Louisiana State Univ., 120 pp. (unpublished). Kemp, G.P., Wells, J.T. and Van Heerden, I.Ll., 1980. Frontal passages affect delta development in Louisiana. Coast. Oceanogr. Climatol. News, 3: 4--5. Kolb, C.R. and Van Lopik, J.R., 1958a. Geology of the Mississippi River Deltaic Plain, Southeast Louisiana. U.S. A r m y Corps Eng. Waterw. Exp. Stn, Vicksburg, Miss., Technical Report 3-483, 120 p. Kolb, C.R. and Van Lopik, J.R., 1958b. Geological Investigation of theMississippi River Gulf Outlet Channel. U.S. Army Corps Eng. Waterw. Exp. Stn, Vicksburg, Miss., Technical Report 3-259, 120 p. Komar, P.D., 1976. Beach Processes and Sedimentation. Prentice-Hall, Englewood Cliffs, N.J., 429 pp. Krawiec, W., 1966. Recent sediments of the Louisiana inner continental shelf. Ph.D. Dissertation, Rice Univ., Houston, Tex., 50 p. (unpublished). Mariner, H.A., 1952. Changes in sea level determined from tide observations. 2nd Conf. Coast. Eng. Univ. Calif. Counc. Wave Res., Berkeley, pp.62--67. McGee, W.J., 1890. Encroachments of the sea. In: L.S. Metcalf (Editor), The Forum. Forum, New York, N.Y., Vol. 9, pp. 437--449. Morgan, J.P., 1967. Ephemeral estuaries of the deltaic environment. In: G.H. Lauff (Editor), Estuaries. Am. Assoc. Adv. Sci., Monogr., pp.115--120. Morgan, J.P., 1974. Recent geological history of the Timhalier Bay area and adjacent continental shelf. Mus. Geosci., Louisiana State Univ., Melanges 9, p.17. Morgan, J.P. and Larimore, P.B., 1957. Changes in the Louisiana shoreline. Trans. Gulf Coast Assoc. Geol. Soc., 7: 303--310. Murray, S.P., 1970. Bottom currents near the coast during hurricane Camille. J. Geophys. Res., 75(24): 4579--4582. Murray, S.P., 1972. Observations on wind, tidal, and density driven currents in the vicinity of the Mississippi River delta. In: D.J.P. Swift, D.B. Duane and O.H. Pilkey (Editors), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson and Ross, Stroudsberg, Pa., pp.127--1421 -

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Neuman, C.J., Cry, G.W., Caso, E.L. and Jarvinen, B.R., 1978. Tropical Cyclones of the North Atlantic Ocean, 1871--1977. National Climatic Center, Asheville, N.C., p. 170. Niedoroda, A.W., Swift, D.J.P., Figueiredo, A.G. and Freeland, G.L., 1985. Barrier island evolution, middle Atlantic shelf, U.S.A. Part II: Evidence from the shelf floor. In: G.F. Oertel and S.P. Leatherman (Editors), Barrier Islands. Mar. Geol., 6 3 : 3 6 3 - - 3 9 6 (this volume). Nummedal, D., 1982. Hurricane landfalls along the N.W. Gulf coast. In: D. Nummedal (Editor), Sedimentary Processes and Environments along the Louisiana--Texas coast. Earth Enterprises, pp.63--78. Otvos, Jr., E.G., 1970. Development and migration of barrier islands, northern Gulf of Mexico. Geol. Soc. Am. Bull., 8: 241--246. Penland, S. and Boyd, R., 1981. Shoreline changes on the Louisiana barrier coast. IEEE, Oceans, 81: 209--219. Penland, S. and Boyd, R., 1982. Assessment of geological and human factors responsible for Louisiana coastal barrier erosion. In: D.F. Boesch (Editor), Conference on Coastal Erosion and Wetland Modification in Louisiana: Causes, Consequences, and Options. U.S. Fish Wildl. Serv., FWS/OBS-82159: 14--38. Penland, S. and Boyd, R., in press. Mississippi delta barrier shoreline development. In: S. Penland and R. Boyd (Editors), Transgressive Depositional Environments of the Mississippi River Delta Plain. La Geol. Surv., Guideb. Ser. Penland, S. and Ritchie, W., 1979. Short-term morphological changes along the Caminada-Moreau coast, Louisiana. Trans. Gulf Coast Assoc. Geol. Soc., 29: 342--346. Penland, S. and Suter, J.R., 1983. Transgressive coastal facies preserved in barrier island arc retreat paths in the Mississippi River Delta Plain. Trans. Gulf Coast Assoc. Geol. Soc., 33: 367--382. Penland, S., Boyd, R., Nummedal, D. and Roberts, H., 1981. Deltaic barrier development on the Louisiana coast. Trans. Gulf Coast Assoc. Geol. Soc., 3 1 : 4 7 1 - - 4 7 6 . Short, A.D., 1979. Three dimensional beach-stage model. J. Geol., 87: 553--571. Swanson, R.L. and Thurlow, C.I., 1973. Recent subsidence rates along the Texas and Louisiana coasts as determined from tide measurements. J. Geophys. Res., 78: 2665-2671. Swift, D.J.P., 1968. Coastal erosion and transgressive stratigraphy. J. Geol., 76: 445--456. Swift, D.J.P., 1976. Coastal sedimentation. In: D.J. Stanley and D.J.P. Swift (Editors), Marine Sediment Transport and Environmental Management. Wiley, New York, N.Y., pp.255--310. U.S. Department of Commerce, 1983. Tide Tables, East Coast of North and South America. National Oceanic and Atmospheric Administrators, National Ocean Survey, Rockville, Md. U.S. Naval Weather Service Command, 1976. Climate of the Coastal Zone, U.S. East Coast. National Climatic Center, Asheville, N.C. Van Heerden, I.L1., Penland, S. and Boyd, R., in press. A transgressive stratigraphic sequence from the central Chandeleur Islands, Louisiana. In: S. Penland and R. Boyd (Editors), Transgressive Depositional Environments of the Mississippi River Delta Plain. La Geol. Surv., Guideb. Ser. Wright, L.D. and Coleman, J.M., 1972. River delta morphology: wave climate and the role of the subaqueous profile. Science, 176: 282--284.