A new depositional model for the buried 4000 yr BP New Orleans barrier : implications for sea-level £uctuations and onshore transport from a nearshore shelf source Frank W. Stapor Jr. a , Gregory W. Stone b; a Department of Earth Sciences, Tennessee Technological University, P.O. Box 5062, Cookeville, TN 38505, USA b Coastal Studies Institute, and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA Received 25 March 2002; received in revised form 23 September 2003; accepted 13 October 2003 Abstract The Holocene New Orleans Barrier Complex, now buried by the St. Bernard delta of the Mississippi River, provides an excellent example of barrier deposition fed by a nearshore sediment source. This reworking and onshore transport was initiated by a sudden change in the shelf equilibrium profile caused by a sea-level fall about 4100 yr BP. Here we present a new model of barrier formation which does not invoke an Shepard-type Holocene sea-level curve nor the supply of sediment from a longshore source. The Holocene New Orleans Barrier Complex consists of fine- grained, locally cross-bedded, quartz sand that contains Ophiomorpha nodosa burrows and disarticulated mollusks, primarily marine, buried beneath up to 4 m of silty mud of the St. Bernard Lobe. This barrier island and shoal deposit overlies interbedded, fine-grained sand and mud containing marine mollusks, some articulated, that is interpreted to be a nearshore shelf deposit. Its deposition took place between 5500 and 4200 yr BP ( 14 C), based on individual dates on seven articulated and seven disarticulated shells. The barrier formation is effectively limited to a several-hundred- year window approximately 4000 yr BP by the 3800 yr BP Rangia sp. shells from the immediately overlying St. Bernard Lobe delta-plain deposits and the buried 3900^3500 yr BP Linsley archeological site, situated on a more gulfward distributary levee. In this paper we present a new depositional model on the New Orleans Barrier. The barrier complex contains an abundance of large mollusk shells that have been reworked to the extent that 14 individual shells yield a 2500-year range, 6000^3500 yr BP. An older, nearby source is required. The current model of a spit/shoal complex migrating westward from an eroding eastern Pleistocene headland probably cannot account for the deposition of large reworked shells given the effects of abrasion and selective size sorting over approximately 50 km of longshore transport. Furthermore, this model demands transport rates of millions of cubic meters per year for the present northern Gulf coast which are at least an order of magnitude higher than its highest known rates. We postulate a nearby shell and sand source that is subjacent and offshore rather than adjacent and littoral. We propose it to be the underlying nearshore shelf deposit that could be mobilized by a brief fall of sea level and carried landward. The barrier complex and the uppermost nearshore shelf deposit have markedly different net deposition rates. The upper 25 cm of the nearshore shelf deposit were deposited over 800 years, based on ages of articulated marine pelecypods. A 20-km-long, 3-km-wide and 4-m-thick segment of the barrier complex was deposited in less than 300 0025-3227 / 03 / $ ^ see front matter ȣ 2003 Elsevier B.V. All rights reserved. doi :10.1016/S0025-3227(03)00350-5 * Corresponding author. E-mail addresses: [email protected](F.W. Stapor Jr.), [email protected](G.W. Stone). Marine Geology 204 (2004) 215^234 R Available online at www.sciencedirect.com www.elsevier.com/locate/margeo
Transcript
A new depositional model for the buried 4000 yr BPNew Orleans barrier: implications for sea-level £uctuations
and onshore transport from a nearshore shelf source
Frank W. Stapor Jr. a, Gregory W. Stone b;�
a Department of Earth Sciences, Tennessee Technological University, P.O. Box 5062, Cookeville, TN 38505, USAb Coastal Studies Institute, and Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge,
LA 70803, USA
Received 25 March 2002; received in revised form 23 September 2003; accepted 13 October 2003
Abstract
The Holocene New Orleans Barrier Complex, now buried by the St. Bernard delta of the Mississippi River,provides an excellent example of barrier deposition fed by a nearshore sediment source. This reworking and onshoretransport was initiated by a sudden change in the shelf equilibrium profile caused by a sea-level fall about 4100 yr BP.Here we present a new model of barrier formation which does not invoke an Shepard-type Holocene sea-level curvenor the supply of sediment from a longshore source. The Holocene New Orleans Barrier Complex consists of fine-grained, locally cross-bedded, quartz sand that contains Ophiomorpha nodosa burrows and disarticulated mollusks,primarily marine, buried beneath up to 4 m of silty mud of the St. Bernard Lobe. This barrier island and shoal depositoverlies interbedded, fine-grained sand and mud containing marine mollusks, some articulated, that is interpreted tobe a nearshore shelf deposit. Its deposition took place between 5500 and 4200 yr BP (14C), based on individual dateson seven articulated and seven disarticulated shells. The barrier formation is effectively limited to a several-hundred-year window approximately 4000 yr BP by the 3800 yr BP Rangia sp. shells from the immediately overlying St.Bernard Lobe delta-plain deposits and the buried 3900^3500 yr BP Linsley archeological site, situated on a moregulfward distributary levee. In this paper we present a new depositional model on the New Orleans Barrier. Thebarrier complex contains an abundance of large mollusk shells that have been reworked to the extent that 14individual shells yield a 2500-year range, 6000^3500 yr BP. An older, nearby source is required. The current model ofa spit/shoal complex migrating westward from an eroding eastern Pleistocene headland probably cannot account forthe deposition of large reworked shells given the effects of abrasion and selective size sorting over approximately 50km of longshore transport. Furthermore, this model demands transport rates of millions of cubic meters per year forthe present northern Gulf coast which are at least an order of magnitude higher than its highest known rates. Wepostulate a nearby shell and sand source that is subjacent and offshore rather than adjacent and littoral. We proposeit to be the underlying nearshore shelf deposit that could be mobilized by a brief fall of sea level and carried landward.The barrier complex and the uppermost nearshore shelf deposit have markedly different net deposition rates. Theupper 25 cm of the nearshore shelf deposit were deposited over 800 years, based on ages of articulated marinepelecypods. A 20-km-long, 3-km-wide and 4-m-thick segment of the barrier complex was deposited in less than 300
0025-3227 / 03 / $ ^ see front matter A 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0025-3227(03)00350-5
years. A 10-cm-thick lag pavement of bryozoan- and oyster-encrusted mollusk shells that comprises the nearshoreshelf deposit beneath the northern edge of the barrier complex is evidence for an essentially zero net deposition rate.The current interpretation of a conformable, progradational relationship between these two units is rejected in favorof a disconformable contact. The hiatus across this disconformity must be less than the several-hundred-year durationof barrier complex deposition. The positioning of the shallower-water barrier complex disconformably over thedeeper-water nearshore shelf deposits indicates a sea-level fall.A 2003 Elsevier B.V. All rights reserved.
Since the exposition of Gilbert (1885), Johnson(1919), Evans (1942), Zenkovich (1967) andothers, the role of longshore versus onshore sedi-ment transport in barrier island formation hasbeen debated extensively (for a review see Wood-ro¡e, 2002). While considerable progress has beenmade on coupling coastal evolution to shorefacedynamics (Kraft, 1971; Swift, 1975; Thom, 1978;Cowell and Thom, 1994; Roy et al., 1994), due toits complexity, the exact role of cross-shore sedi-ment transport remains unclear when consideringmillennial time scales (Cowell et al., 1999). Thereis also an overwhelming predominance to invokelongshore versus onshore sand transport duringbarrier deposition (Saucier, 1963; Otvos, 1978;Davis, 1994).In this paper we present new radiometric dates
that emphasize the importance of an o¡shoresource in the initial phases of deposition of a Ho-locene barrier island now covered by the St. Ber-nard delta complex of the Mississippi River.These dates are used to estimate the depositionalages of the barrier complex and the underlyingunit. Also, the stratigraphic relationship betweenthese two units is reinterpreted from the perspec-tive of the sequence stratigraphy paradigm whichemphasizes sea-level £uctuations. These addition-al radiocarbon dates and the sequence strati-graphic reinterpretation of the barrier complex’sbasal contact are integrated into a new deposi-tional model of the New Orleans Barrier Complexthat requires not only a £uctuating Holocene sealevel but also a predominance of onshore sandtransport rather than longshore. These ¢ndingshave important implications for late Holocene
sea-level stands, trends and the importance ofsubjacent o¡shore sand sources.
2. The New Orleans Barrier Complex
A Holocene barrier complex lies buried 1^10 mbeneath the city of New Orleans, Louisiana. Thisfeature lies just south of and parallel to the south-ern shore of Lake Pontchartrain and extends fromthe western edge of the city nearly 35 km north-east to the Little Woods region (Fig. 1). It reachesthe surface to form the Oak and Pine Islandswhich are locally capped with aboriginal shellmiddens (Saucier, 1963; Shenkel, 1974). Corbeille(1962) ¢rst referred to this barrier complex as theNew Orleans Barrier, a designation continued byDeWindt (1974), Hollander and Dockery (1977),Otvos (1978), and Miller (1983); Saucier (1963)referred to it as the Pine Island Beach Trend, ausage followed by Shenkel (1974).This barrier sand was discovered through foun-
dation borings in the late 1940s (Treadwell, 1955).Highway borings, road construction, and canaldredging (Rowett, 1957; Kolb and Van Lopik,1958; Corbeille, 1962; Saucier, 1963) establishedits subsurface extent and provided gross samplesof the molluscan fauna. It was exposed in borrowpits dug for interstate highway ramp construction(DeWindt, 1974) where stratigraphically con-trolled samples of the molluscan fauna were taken(Otvos, 1978; Miller, 1983).The New Orleans Barrier Complex directly
overlies an older Holocene silty clay unit, withlocal ¢ne-sand interlaminations, that contains anopen marine, nearshore molluscan fauna (Hol-lander and Dockery, 1977; Miller, 1983); this is
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the unnamed bay-sound unit of Saucier (1963)and the Michoud Formation of Miller (1983)(Fig. 2). This oldest Holocene unit unconformablyoverlies poorly consolidated Pleistocene sedimentsthat are mantled with a cm- to m-thick paleosol(Saucier, 1963; Miller, 1983). The barrier complexis overlain conformably by interlaminated clay,silt, and ¢ne sand of the St. Bernard Lobe ofthe Mississippi Delta (Fig. 2).Previous radiocarbon dating of geologic depos-
its, shells (primarily disarticulated) and wood, andaboriginal middens, shells and charcoal has estab-lished a chronometric framework for the deposi-tion of the New Orleans Barrier Complex and itsassociated over- and underlying units. The follow-ing review will include only those geologic shelldates made on single-shell samples. Multiple-shellor composite samples have a potential to un-avoidably mix shells of di¡erent ages, especiallyif the shells are disarticulated, allochthonous
Fig. 1. Maps showing the location of (A) the New Orleans, LA, region in the northern Gulf of Mexico and (B) the HoloceneNew Orleans Barrier Complex beneath the city of New Orleans and the sites referred to in this paper. (C) Stratigraphic cross-sec-tion showing the relationship of the Linsley site to the New Orleans Barrier Complex, modi¢ed from Saucier (1963).
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clasts, resulting in an age that could primarilyre£ect the mixing rather than either the age ofthe deposit or even of the constituent shells.Corbeille (1962) reported a 3970 yr BP Dinocar-
dium robustum (marine mollusk) from the barriercomplex and a 2160 yr BP and a 2380 yr BPRangia cuneata (brackish mollusk) as well as a2220 yr BP root from the overlying St. BernardLobe deposits. Otvos (1978) found a 4930 yr BPStrombus sp. (marine gastropod), a 5780 yr BPDinocardium robustum, a 5020 yr BP Crassostreasp. (estuarine to shallow marine), and a 26 560 yrBP (Pleistocene?) Crassostrea sp. in the barriersand. In addition, he reported that the immedi-ately underlying, interbedded sand and clay unitcontained a 5020 yr BP Dinocardium robustum, a5740 yr BP Strombus sp., a 5040 yr BP Mercena-ria sp. (estuarine to shallow marine), and a 4850yr BP Crassostrea sp. Furthermore, the overlyingSt. Bernard Lobe deposits contained a 4050 yr BPand a 3860 yr BP Rangia cuneata as well as a 3290yr BP root (Otvos, 1978). Miller (1983) reported a4270 yr BP articulated Mercenaria campechiensisfrom the ‘¢rmground’ at the top of the inter-bedded sand and clay unit, his Michoud Forma-tion, immediately below the barrier sand. Peatrip-up clasts at the base of this interbedded sandand clay unit yielded ages of 7290 yr BP and 7200yr BP that date the marine transgression that£ooded the Pleistocene paleosol in this region,Miller (1983). These shell, wood, and peat datesindicate that the New Orleans Barrier Complexoriginated about 4300 yr BP with sand depositionoccurring over an interval of no more than 300years; deposition of the overlying St. BernardLobe deposits began by 4050 yr BP. Only the3970 yr BP Dinocardium robustum of Corbeille(1962) is contemporaneous with barrier complexdeposition. All of the other shells reported fromthe barrier sand are primarily clasts reworked
from older, nearby deposits, a situation identicalto that of the southwest Florida barrier islands(Stapor et al., 1991).Aboriginal middens are found on top of and
juxtaposed to the surface exposures of the NewOrleans Barrier Complex in the Oak and PineIslands region near the northeastern limit of thissand body. At Little Oak Island the humic siltmidden is separated from the underlying barriersand by a Rangia shell beach that contains intru-sive burials and 2740 yr BP charcoal (Shenkel,1974). A 2165 yr BP bone was found in the com-pacted £oor in the midden. A composite shellsample of the beach yielded an age of 3400 yrBP. If this is a true beach, as concluded by Shen-kel (1974) because of the lack of other culturalartifacts, this date is probably a mixing age thatcannot estimate either the age of the beach depos-it or that of the constituent shells. At Big OakIsland the Rangia shell midden is separated fromthe barrier sand by 2.5 m of gray clay the top ofwhich contained cultural material and 2470 yr BPcharcoal. The shell midden itself contained 2185yr BP charcoal and 2220 yr BP Rangia cuneatashells (Shenkel, 1974). These two middens are atleast 1000 years younger than the depositional ageof the New Orleans Barrier Complex as deter-mined by geologic deposits.The most important archeologic site relating to
the New Orleans Barrier Complex lies not on topof the sand body but rather southeast of it furthertoward the Gulf of Mexico. The Linsley site (Fig.1) was discovered during the excavation of theMississippi River^Gulf Outlet Channel in theearly 1960s and lies buried approximately 2.5^3.5 m below MSL (Gagliano and Saucier, 1963).This Rangia shell midden is located on a naturallevee adjacent to a distributary channel. This sitecontained 3540 yr BP, 3690 yr BP, and 3840 yrBP charcoal. In addition, immediately beneath the
Fig. 2. Stratigraphic columns describing the units that were exposed in the Lake Carmel (modi¢ed from Miller, 1983), BullardRoad (this paper and modi¢ed from Otvos, 1978), and Morrison Road (modi¢ed from Otvos, 1978) borrow pits. The correlationdatum is the contact between the Barrier Complex and the underlying interbedded sand and clay unit that contains marine mol-lusks. This contact is hypothesized to be a coeval surface of greatly reduced or non-deposition. S refers to a sharp, possibly ero-sional, contact; Sg to a contact that is locally sharp and gradational over several cm; G to an interface gradational over cm todecimeters (?). The Holocene New Orleans Barrier Complex of Otvos (1978) has been divided into a lower Nearshore Shelf De-posit and an upper Barrier Complex.
natural levee material there was a 4040 yr BPpeat- and organic-rich clay (Gagliano and Sau-cier, 1963). The geographic location of the Linsleysite gulfward of the subsurface sand body makes acompelling argument that the New Orleans Bar-rier Complex can be no younger than about 4000yr BP.The existing geologic and archeologic radiocar-
bon dates indicate that the New Orleans BarrierComplex was most likely deposited between 4300and 4000 yr BP. The maximum estimate comingfrom the age of the underlying marine clay and
sand unit (Miller, 1983) and the minimum esti-mate from the age of the Linsley shell midden(Gagliano and Saucier, 1963) formed during theinitiation of St. Bernard Lobe delta-plain deposi-tion. Saucier (1963) interpreted this barrier com-plex to have been a spit attached to the Pleisto-cene mainland of the eastern shore of present-dayLake Pontchartrain that migrated 45 km or sowestward. Otvos (1978) replaced the attachedspit with a chain of shoals and islands fed bywestward-moving sand eroded from the Pleisto-cene mainland east of Lake Pontchartrain.
Fig. 3. (A) Computer-generated contour map of the depth to the Holocene/Pleistocene contact beneath the city of New Orleansbased on 1300 points (Roger Saucier, written communication). The superposed Holocene New Orleans Barrier Complex is shownin gray. (B) The ¢rst-order or planar trend surface de¢ned by these data accounts for 79% of the depth variation; it is gently in-clined to the SSW.
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3. Geomorphology
The New Orleans Barrier Complex is knownprimarily from subsurface borings and excava-tions. An extensive unpublished data set on thedepths to (a) the Pleistocene, 1300 points, and(b) the top of the Holocene barrier complex, 826points was obtained by the authors (Roger Sau-cier, personal communication). Depths were con-toured using the Rockware GRIDZO program.The Holocene/Pleistocene surface (Fig. 3A) hasmeter-scale relief, the northern part being lowerthan the southern. Contour lines have a promi-nent NE^SW orientation in the northern portion.A ¢rst-order or planar trend surface (Fig. 3B)accounts for 79% of the variance in the Pleisto-cene depth values; it is gently inclined to theSSW. The geographic location of the overlyingHolocene barrier complex does not appear to berelated to the con¢guration of this Pleistocenesurface. First, the barrier complex directly overliesseveral meters of Holocene mud and silty mudrather than the actual Pleistocene surface and,second, it is aligned NE^SW across both risesand falls in the Pleistocene surface (Fig. 3A).The top of the barrier complex is a segmented
ridge, 2.5^4 km wide, NE^SW oriented (Fig. 4).The westernmost segment, beneath New Orleansproper, is narrower, linear and has a steep, south-ern margin similar in geometry to the gulfwardedge of present-day gulf barriers. The wider east-ern portion of the ridge has rounded to lobatehighs, curvilinear valleys, and isolated, roundeddeeps. These topographic features are suggestiveof a submerged shoal rather than an emergentbarrier island. The northern ridge margin iswell-de¢ned only along the western two thirds ofthe westernmost segment. Here it is less steep thanthe southern margin but again similar in geometryto the lagoon shore of present-day gulf barriers,e.g. Santa Rosa Island along the Florida Panhan-dle to the east (Stone, 1991). The western portionof this sand ridge resembles present-day barrierislands. The eastern portion has shoal-like geom-etry.
4. Sedimentology
Corbeille (1962) reported that the HoloceneNew Orleans Barrier Complex north of the Mor-rison Road pit consists of well-sorted, ¢ne-grained
Fig. 4. Computer-generated contour map of the depth to the top of the Holocene New Orleans Barrier Complex based on 826points (Roger Saucier, written communication). The western third is linear with pronounced, planar margins with the steeper onefacing the Gulf of Mexico, a geometry resembling that of present gulf barrier islands. The remaining eastern portion is lobateand lacks clearly de¢ned margins, more closely resembling a shoal rather than a barrier island. Section AAP is shown in Fig. 1C.See Fig. 1 for designations of MR, LC, BR, LS, BO and LO.
(0.163^0.180 mm) quartz sand. Otvos (1978)found medium- to well-sorted, ¢ne-grained quartzsand in the Morrison Road pit with the upperunit being somewhat ¢ner (0.10^0.14 mm) thaneither the lower unit (0.14^0.23 mm), the BarrierComplex or the Nearshore shelf deposits (Fig. 2).Fine-grained sand (0.13^0.18 mm) makes up thebarrier complex in the Bullard Road pit (Otvos,1978).Ripple cross-lamination is the most common
sedimentary structure in the barrier complex.
Those near the top of the barrier complex haveclay drapes (Otvos, 1978). Decimeter-scale cross-bedding is rare; in one occurrence in the BullardRoad pit the cross-beds appear to be organizedinto tidal bundles (Fig. 5). Clay-walled Ophiomor-pha nodosa burrows are common (Fig. 5) andhave been reported from all of the pit exposures(DeWindt, 1974; Otvos, 1978; Miller, 1983). Thewidespread gray-mottled texture of the barriercomplex suggests that burrowing organisms areresponsible for the absence of physical structures.
5. Paleoecology
The Barrier Complex and the Nearshore ShelfDeposit contain an abundant, diverse, mostly ma-rine, molluscan fauna; Rowett (1957) identi¢ed 88species and Miller (1983) 130 species. Rowett(1957) compared the mollusk population fromthe New Orleans Barrier Complex with livingmollusk assemblages diagnostic of speci¢c mod-ern environments in the Mississippi Delta region(Parker, 1956). The Barrier complex contains 79%(30 of 38) of the shallow shelf species; 48% (16 of34) of the high-chlorinity bay; 52% (11 of 21) ofthe tidal inlet; 57% (4 of 7) of the pro-delta slope;14% (1 of 7) of the delta front; 67% (2 of 3) of themarsh; and 3% (1 of 31) of the deep shelf. Rowett(1957) concludes that the New Orleans BarrierComplex molluscan fauna is representative ofthe shallow shelf environment, de¢ned by Parker(1956) as extending from a barrier island shoreseaward to a depth of 13 fathoms (24 m). Noarticulated pelecypods have been reported fromthe Barrier complex. Articulated Mercenaria cam-pechiensis and Dosinia discus have been found inthe Nearshore Shelf Deposit (Miller, 1983; thispaper).An abundant, diverse assemblage of marine
mollusks occur in more distal portions of theunderlying unit, the nearshore Gulf and bay/sound deposits of Saucier (1963) and the MichoudFormation of Miller (1983). Hollander and Dock-ery (1977) reported 96 species of mollusks col-lected from Intracoastal Waterway spoil located5.5 km gulfward of the pit sampled by Rowett(1957). They concluded that this molluscan fauna
Fig. 5. The Holocene New Orleans Barrier Complex exposedin the Bullard Road pit. A marks a rare decimeter-scale,cross-bedded unit that appears to contain tidal bundles. Inthe vicinity of B there are numerous clay-walled Ophiomor-pha nodosa burrows. The majority of this exposure exhibitsno recognizable sedimentary structures, which, admittedly,are extremely di⁄cult to distinguish in clean, well-sorted, un-consolidated sand. However, the gray-mottled texture in theupper half of this exposure suggests that this absence couldpartly be the result of bioturbation by burrowing organisms.
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is representative of the shallow shelf and, further-more, that abundant Distortio clathrata and Pha-lium granulatum, and numerous Argopecten gib-bus, Noetia ponderosa, and Dosinia discusindicate a non-turbid, shallow, marine environ-ment. In addition, they reported rare articulatedAnadara brasiliana and Anadara ovalis.
6. Stratigraphy
The New Orleans Barrier Complex lies in themiddle of the Holocene deposits exposed in bor-row pits in this region (Fig. 2). The underlyingsilty mud that contains marine mollusks uncon-formably overlies Pleistocene deposits. Miller(1983) reported a rooted paleosol developed ontop of the Pleistocene in the Lake Carmel pit.The basal Holocene silty mud is the nearshoreGulf and bay/sound deposits of Saucier (1963);the bay deposits of Frazier (1967); the o¡shore-lower shoreface deposits of Hollander and Dock-ery (1977); and the Michoud Formation of Miller
(1983). The overlying St. Bernard Lobe delta-plain silty mud is locally rooted and containsshells of the brackish/estuarine mollusk Rangiacuneata. The contact between the units is sharpand abrupt on the pit walls (Fig. 6), however,detailed examination reveals that locally thereare cm-thick zones of lenticular- to £aser-bedded¢ne-grained sand and silty mud (Fig. 7).The Holocene New Orleans Barrier Complex of
previous workers is divided into two distinctstratigraphic units in this paper: (a) NearshoreShelf Deposit, the lower ¢ne-grained sand con-taining abundant marine mollusks, including ar-ticulated pelecypods, with scattered lenses and in-terbeds of mud and silty clay that increase inoccurrence downward, and (b) Barrier Complex,the upper ¢ne-grained sand containing abundantOphiomorpha nodosa burrows as well as marinemollusks, that contains ripple cross-laminationand cross-bedding, and has only a minor amountof interbedded mud. The contact between them iseither sharp or gradational over only several cen-timeters (Fig. 2).
Fig. 6. The sharp, abrupt contact between the underlying New Orleans Barrier Complex Sand (A) and the overlying St. BernardLobe silty mud (B) in the Bullard Road pit. The thin, planar, parallel bedding of the delta-plain mud is in marked contrast tothe massive-appearing barrier complex sand. Mollusk shells are locally scattered along the sand exposure.
The geomorphology, the clean quartz sandcomposition, and the presence of abundant ma-rine shells were compelling arguments that ledearly workers to interpret a barrier island originfor the sand ridge buried beneath New Orleans,including Rowett (1957), Corbeille (1962) andSaucier (1963). The discovery of Ophiomorpha no-dosa burrows and the absence of any eolian dunebedding in the excavated pits allowed for a moregeneral barrier/bar/shoal complex interpretation(Otvos, 1973, 1978). We ¢nd these argumentsequally satisfactory and agree with the more in-clusive interpretation of a barrier complex thatincludes subtidal bars and shoals.The authors emphasize that the entire sand
mass, as described by workers from Rowett(1957) through Otvos (1978), is probably not abarrier complex. The lower sand unit, the Near-shore Shelf Deposit in Fig. 2, contains articulatedpelecypods and interbedded mud and is better in-terpreted to represent a lower shoreface and/or
open-shelf environment. The contact betweenthese two units is sharp or gradational only overseveral centimeters. Miller (1983) reported a ‘¢rm-ground’ pavement of mollusk shells separatingthem in the Lake Carmel pit. These contact rela-tionships question the conformable, progradation-al or shallowing-upward relationship betweenthem envisaged by Otvos (1978) and Miller(1983).
8. Age of the New Orleans Barrier Complex
Eighteen shells collected from the NearshoreShelf Deposit and the Barrier Complex exposedin the Bullard Road pit were radiocarbon-datedto constrain the age of these deposits. Dates weredetermined on relatively large samples, s 10 g,and fresh specimens free from encrusting materialand borings. Analyses were done by Beta Analyticof Coral Gables, Florida (Table 1). The 18 datesaugment the existing 11 single-shell dates (onefrom Corbeille (1962), seven from Otvos (1978),
Fig. 7. Detailed view of the contact between the New Orleans Barrier Complex (A) and the St. Bernard Lobe delta-plain siltymud (B) in the Bullard Road pit. C marks a 5^8-cm-thick unit of mostly lenticular-bedded ¢ne sand and silty mud. The whitematerial shown at D is an encrusting, secondary mineral, probably gypsum, formed during the weathering of the newly exposedmud.
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one from Miller (1983), and two from Shenkel(written communication)). Only dates based onsingle-shell samples have been considered in thisage determination. Multiple-shell samples havepotential to mix shells of di¡erent ages, especiallyif the shells are disarticulated, allochthonousclasts, that may well result in an age which mayre£ect the mixing rather than either the age of thedeposit or even of the constituent shells. Thesemarine mollusk dates have not been corrected be-cause the mollusk isotope fraction and the near-shore ocean water e¡ect, the two major correc-tions that should be applied, are essentiallyequal in magnitude and opposite in sign. Further-more, all radiocarbon ages discussed in this paperare conventional, uncorrected dates; however,calibrated calendar years BP are shown in Table1 as the respective 1c ranges.The New Orleans Barrier Complex was depos-
ited over a 200^300-year interval about 4000 yrBP (Fig. 8). This age estimate is constrained by(a) the 4270 yr BP age of the youngest shell in theunderlying Nearshore Shelf Deposit, an articu-
lated Mercenaria campechiensis from the LakeCarmel pit (Miller, 1983), and (b) the 4040 yrBP age of peat immediately beneath the gulfwardLinsley site (Fig. 1C) buried in the overlying St.Bernard Lobe delta-plain deposits (Gagliano andSaucier, 1963). Otvos (1978) reported dates of4050 yr BP and 3860 yr BP on two Rangia cunea-ta shells collected from the St. Bernard Lobe de-posits in the Morrison Road pit. The youngestshell dates from the Barrier Complex itself aretoo young by 300^400 years. These young shellswere collected during this study and presentlythere is no reasonable explanation for their agediscrepancy. Because the St. Bernard Lobe siltyclays blanket the borrow pit exposure these youngshells could not have been naturally reworkedinto the Barrier Complex after its deposition;they most likely represent an undetected, minorrecrystallization that added younger carbon, pos-sibly from groundwater. This radiometric anom-aly does not invalidate the 4000 yr BP deposition-al age for the Barrier Complex since it isconstrained by the stratigraphic superposition of
Table 1Radiocarbon ages of mollusk shells collected from the Barrier Complex Sand and the Nearshore Shelf Deposit in the BullardRoad pit
Mollusk species Beta Analytic sample number Uncorrected 14C age Calibrated 14C age(years BP) (years BP) 1c range
the St. Bernard Lobe and the gulfward location ofthe Linsley site.The 2500-year range in radiocarbon ages of the
15 disarticulated mollusks (Fig. 8) analyzed fromthe New Orleans Barrier Complex makes a com-pelling argument that older, nearby shell-bearingdeposits were being eroded prior to their deposi-tion in the Barrier Complex. This situation is sim-ilar to that reported by Stapor et al. (1991) for theHolocene barrier islands of southwest Floridathat consist of multiple beach-ridge sets boundedby erosional truncations. The material comingfrom older, nearby deposits consisted of bothshells and, perhaps more importantly, quartzsand. Because shells cannot experience extensive
transport without showing abrasion and selectivesize sorting, a potential subjacent source may be amore reasonable candidate than an adjacent one.Deposition of the underlying Nearshore Shelf
Deposit ceased about 4300 yr BP, the age of itsyoungest mollusk, an articulated Mercenaria cam-pechiensis from the Lake Carmel pit (Miller,1983). This unit had a range of ages, about 1500years (Fig. 8). Seven of the 14 mollusk shells arearticulated pelecypods, most likely to be in place.Six articulated and three disarticulated specimenswere collected from the uppermost 20^25 cm ofthe Nearshore Shelf Deposit in the Bullard Roadpit (Fig. 8). The 800-year range of these six articu-lated pelecypods strongly suggests a low net de-
Fig. 8. Radiocarbon dates made on single shells, roots, and peat collected from the Nearshore Shelf (SHELF) sand, the New Or-leans Barrier Complex (BARRIER) sand, and the St. Bernard Lobe (DELTA PLAIN) deposits. The time axis is in conventionalradiocarbon years BP with calibrated calendar years BP (approximate) below (from appendix IX in Roberts, 1998). Charcoaldates are from aboriginal middens located within the St. Bernard Lobe (Linsley site) or superjacent to the barrier complex (OakIsland sites). The barrier complex formed during a 200^300-year interval about 4000 yr BP, the vertical gray band. This intervalis constrained by the youngest date in the underlying nearshore shelf deposits and the oldest date from the overlying St. BernardLobe delta-plain deposits, especially the buried Linsley site which is 2^3 km gulfward of the New Orleans Barrier Complex.There is a problem with the two youngest dates from the barrier complex itself ; they are too young by 300^400 years. There isno explanation for this discrepancy. All ages are in uncorrected radiocarbon years. Note the pronounced reworking of oldershells into the Barrier Complex. Furthermore, note the almost 800-year range in age of articulated pelecypods, arguably in placeand not reworked, from the uppermost decimeter or two of the nearshore shelf sand in the Bullard Road pit, those with the sssuperscript.
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position rate in addition to possible local rework-ing. The Nearshore Shelf Deposit in the LakeCarmel pit occurs only locally and consists of aMercenaria campechiensis shell pavement en-crusted with bryozoans and oysters (Miller,1983). Radiometric and paleoecologic evidence in-dicates that between 5000 and 4300 yr BP thenearshore shelf in this region had low and declin-ing net deposition rates. By 4300 yr BP the regionmay well have been essentially ‘starved’ of sedi-ment. The Nearshore Shelf Deposit is a very likelyolder, nearby source for both the shells and muchof the quartz sand in the immediately overlyingNew Orleans Barrier Complex.
9. Discussion: origin of the New Orleans BarrierComplex
9.1. Sediment transport and source
Previous workers have considered the origin ofthe New Orleans Barrier Complex from the as-sumptions of (a) longshore transport with thequartz sand being eroded from a Pleistoceneheadland, and (b) a continually, but not uni-formly, rising, non-£uctuating, Holocene sea levelasymptotically approaching its present-day posi-tion, the Shepard (1963, 1964)-type curve. Saucier(1963) interpreted this barrier to have been origi-nally a spit attached to the Pleistocene mainlandof the eastern shore of Lake Pontchartrain thatmigrated about 45 km westward to New Orleans.Otvos (1978) modi¢ed the spit into a shoal-bar-island combination but still used westward-mov-ing sand eroded from the same Pleistocene main-land (Fig. 9). The Pleistocene deposits along theeastern shore of Lake Pontchartrain are an ob-vious quartz sand source, given that the alterna-tive is the Teche Lobe of the Mississippi Deltalocated approximately 90 km to the west (Fig.10). However, not only is the survival of largeshells over such a transport distance certain, butalso there are no reported Holocene deposits oldenough to serve as a source for 6000^5000 yr BPshells. The required average net migration ratewould have approximated 200 m/yr, a rate con-siderably higher than observed historically at oth-
er spit termini along the northern Gulf (cf. inletmigration rates along the Florida, Alabama andMississippi coasts reviewed in Stone, 1991). Aconservative estimate of the net longshore trans-port rate would be almost 3 million m3/yr, using asand mass 45 km long, 3.5 km wide, and 4.5 mthick that was deposited over 250 years. This ratewould be an order of magnitude greater than anypresently occurring along the northern Gulf coast.The highest known net longshore sediment trans-port rates, approximately 125 000 m3/yr, are forless than 5-km portions of Santa Rosa Island(Stone et al., 1992). Maximum modern-day net
Fig. 9. The westward-migrating shoal-bar-island model forthe origin of the New Orleans Barrier Complex, modi¢edfrom Otvos (1978). Panel 1: the New Orleans Barrier Com-plex forms from sand transported west from an erodingPleistocene headland on the eastern shore of present-dayLake Pontchartrain. Panel 2: the eastward-prograding St.Bernard Lobe buries the New Orleans Barrier Complex. Pan-el 3: the more seaward Hancock-Sauvage Barrier Complexforms by westward transport of sand eroded from the Pleis-tocene headland on the eastern shore of Lake Pontchartrain.
transport rates range between 60 and 100 000 m3/yr along Alabama/Mississippi/Louisiana barriers(Stone et al., 1992; Ellis, 1998; Stone and Stapor,1996; Cipriani and Stone, 2001; Stone andZhang, 2001a,b). Although the littoral transportmodel is intuitively attractive, it demands a netmigration rate and a net longshore transportrate entirely too large for both the wave climateand shelf morphology of the northern Gulf. These£aws, if not fatal, are serious enough to warrant amuch closer look at the previously mentioned fun-damental assumptions.Since longshore transport cannot supply the
needed sand transport rate, then other sourcesand transport directions must be considered. An-other source for the ¢ne-grained quartz sand isthe adjacent nearshore shelf. Shoals located nomore than several tens of kilometers o¡shore ofthe eroding remnants of the Maringouin andTeche Lobes have been interpreted by Penlandet al. (1988) as late-stage remnants created duringthe long-term transgression of more extensive del-ta lobes. These putative late-stage remnants arenot reasonable choices for this shelf source eventhough their sand masses contain burrows andmarine shells. They disconformably overlie lagoo-
Fig. 10. (A) The New Orleans Barrier Complex (NOBC) is located almost 90 km east of the coeval Teche delta lobe and nearly130 km east of the older Maringouin/Sale-Cypremort delta lobe. The silty mud deposits that underlie the barrier complex aretypically no more than 5 m thick, a further indication of the relative distance to these major sediment sources. Sediment compac-tion, therefore, probably played only a minor role in reducing the e¡ects of sea-level £uctuations, especially the falls. (B) Sea-levelcurve for the past 5000 years based on geologic and archeologic data collected along the Florida Gulf coast (Stapor et al., 1991;Walker et al., 1995). The dashed-line extension depicts the hypothesized location of Nearshore Shelf Deposits (Shelf) and theNew Orleans Barrier Complex.
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nal, shallower-water deposits, rather than openmarine, deeper-water deposits as does the NewOrleans Barrier Complex (Hollander and Dock-ery, 1977). There is no known older MississippiDelta lobe in this eastern region. Although thisremnant-shoal model emphasizes onshore trans-port across a gently sloping shelf from an o¡shoresand source, its required overall transgressive set-ting is not indicated by the deposits exposed inthe Morrison Road, Bullard Road, and Lake Car-mel borrow pits.Shore-perpendicular transport that leads to a
long-term net change in the beach sand masshas typically been considered from a beach ero-sion perspective. Storm-generated bottom cur-rents, the coastal jet of Swift and Niedoroda(1985), move material o¡shore. Some fraction re-turns to the beach during fair-weather conditions,resulting in a net loss of sand from the beach.This o¡shore transport is also commonly put inthe context of an ever rising sea level acting tocontinually deepen the shelf and cause its pro¢leto need increasing amounts of coast-supplied sandto remain in equilibrium, i.e. the Bruun e¡ect(Bruun, 1962, 1988). However, the shelf can alsoserve as a sand source with net onshore transport.This situation has occurred historically (100years) along portions of the inner shelf o¡ theNorthwest Florida/Alabama coast (Stone et al.,1992; Stone and Stapor, 1996; Taylor and Stone,1996), and has occurred episodically over the past3000 years in part of southwest Florida (Stapor etal., 1991). Onshore transport over similar timescales has been documented along beaches aroundthe world (see reviews in Cowell et al., 1999;Woodro¡e, 2002).
9.2. Holocene sea-level history
Regional sea-level history is a major factor de-termining if the shelf is a sediment source or sink.A continually rising sea level should indeed resultin the shelf being a sink, whereas a £uctuating sealevel could well allow the shelf to serve as a sedi-ment source during and immediately after sea-lev-el falls. There are two markedly di¡erent sea-levelcurves or paradigms used to describe the middle-and late-Holocene sea-level recovery along the
northern Gulf coast from its Pleistocene low.The Shepard-type curve, widely accepted in NorthAmerica, describes a continually, but not uni-formly, rising sea level that asymptoticallyapproaches its present-day position. The Shepard-type Scholl and Stuiver (1967) and Scholl etal. (1969) curves, widely accepted for the Gulfof Mexico, were developed from subsurfacecoastal peat deposits in southwest Florida; theirdata points have been smoothed into a best ¢tcurve that removes temporal and spatial varia-bility.The Fairbridge (1961, 1976)-type curve de-
scribes a series of £uctuations, which have ex-ceeded present-day level by perhaps up to severalmeters, and that decrease in both amplitude andduration toward the present-day position. TheFairbridge (1961, 1976) curve is a compilationof worldwide data from surface as well as subsur-face deposits of both geologic and archeologicmaterials. It is not ‘smoothed’ but rather maxi-mizes temporal and spatial variability. In NorthAmerica this curve type is somewhat controversialand much less widely accepted. Perhaps the bestdocumented curves of this type, characterized by£uctuations that closely resemble those of Fair-bridge (1961, 1976), have been constructed forthe Atlantic coast of France (Ters, 1987), Sweden(Morner, 1969, 1976), China (Zhao and Zhang,1985), and the Gulf of Mexico coast of Florida(Walker et al., 1995).Compaction of Holocene clays within the Mis-
sissippi Delta region should be re£ected in aneven more rapidly rising local sea level. A tide-gauge station located in the lower delta measuredan historic sea-level rise of 9.2Q 0.75 mm/yr,much higher than the 2.3 Q 0.35 mm/yr for tec-tonically ‘stable’ Pensacola, Florida (Hicks,1973; Emery and Aubrey, 1991). Lowrie andFairbridge (1991) and Lowrie and Hamiter(1995) interpreted a very low regional subsidencefrom seismic sections traversing the MississippiDelta region and considered this tide-gauge datato represent a very local compaction. Any e¡ectsof clay compaction, however, should be minor inthe region of the New Orleans Barrier Complexbecause the coeval delta lobe, the Teche, is lo-cated at least 90 km to the west ; the oldest known
middle Holocene lobe, the Maringouin/Sale-Cy-premort, is a further 50 km west (Fig. 10A).Geologic data (Stapor et al., 1991) and archeo-
logic data (Walker et al., 1995) from the FloridaGulf coast can be used to constrain both theamplitude and the duration of sea-level £uc-tuations over the past 3000 years (Fig. 10B).Archeologic data from southwest Florida (Russo,1991) indicate that there were intervals priorto 3000 yr BP when sea levels were at least equalto present day. Stapor (1975) reported a wave-cut scarp in the Pensacola and the Apalach-icola region of the Florida Panhandle at 1^2 mabove MSL that has 3500^3000 yr BP aboriginalmiddens located on its seaward terrace. Thisfeature is incised into Pleistocene coastal sand de-posits.
9.3. Holocene stratigraphic relationships
Saucier (1963), Otvos (1978), and Miller (1983)have interpreted the sequence of Holocene unitspenetrated by borings or exposed in the MorrisonRoad, Bullard Road, and Lake Carmel borrowpits to be conformable. This sequence recordsthe progradation and/or aggradation of siltymud into the basal nearshore Gulf deposits, suc-ceeded by the westward progradation of the bar-rier complex out into this muddy nearshore re-gion, and ends with the eastward progradationof the St. Bernard Lobe. They did not di¡erenti-ate the Nearshore Shelf Deposit from the BarrierComplex. A vastly di¡erent net deposition rate isindicated for the uppermost Nearshore Shelf De-posit, 25 cm over 800 years, versus that of theBarrier Complex, with its 20-km-long mass westof the Linsley site which would have had to havebeen deposited in 200^300 years. This paradoxseriously questions a conformable, progradationalrelationship between these two units. The ‘¢rm-ground’ pavement of bryozoan- and oyster-en-crusted pelecypod shells that makes up the Near-shore Shelf Deposit in the Lake Carmel pit(Miller, 1983) strongly suggests a net depositionrate reduced essentially to zero. This ‘¢rmground’pavement is analogous to the ‘hardground’ non-depositional surfaces found in limestone deposits.Consequently, the present authors conclude that
the uppermost Nearshore Shelf Deposit representsa sediment-starved region and not part of an ac-tively depositing and hence prograding system.We therefore conclude that the Barrier Complex,as de¢ned in this paper, disconformably overliesthe Nearshore Shelf Deposit.A disconformable rather than progradational
placement of the shallower-water Barrier Com-plex Sand, containing remnant cross-beds andabundant Ophiomorpha nodosa burrows, on topof the deeper-water Nearshore Shelf Deposit, in-terbedded sand and silty mud containing articu-lated pelecypods, indicates an o¡shore and hencedownward movement of a proximal environment.This disconformable relationship of a more prox-imal facies on top of a more distal facies shouldnot be confused with the situation at transgressivebarriers where just the reverse occurs. This in-ferred shelfward movement was abrupt, becausethe hiatus across this proposed disconformity isless than several hundred years, essentially withinthe precision of radiocarbon dating. It is postu-lated to be a sea-level fall that occurred about4100 yr BP, the magnitude of which may havebeen up to 3 m (Fig. 10B), an estimate con-strained by the di¡erence in elevation of the Flor-ida Panhandle scarp and the low beach ridges thatcomprise the oldest parts of St. Vincent and Sa-nibel Islands, Florida Gulf coast barriers. In thesequence stratigraphy paradigm it could be con-sidered a very small-scale downward shift (VanWagoner et al., 1990), or perhaps a forced regres-sion (Posamentier et al., 1992), both of which arecaused by a sea-level fall. Lowrie and Fairbridge(1991) and Lowrie and Hamiter (1995) interpretedseveral such £uctuations in their seismic sectionsof the Mississippi Delta region.This postulated sea-level fall only introduced a
higher level of energy into the nearshore shelf ; nopart of any existing barrier was moved o¡shore.Further, the shelf pro¢le had to adjust to thislower sea-level position and could now serve asa sediment source. Where the sea£oor consisted ofthe Nearshore Shelf Deposit, this pro¢le readjust-ment resulted in its erosion, subsequently fol-lowed by onshore transport of the reworkedsand and shells to construct the Barrier Complex(Fig. 11).
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10. New depositional model
We propose that the New Orleans BarrierComplex was constructed from sand transporteddirectly onshore from the adjacent nearshore shelf(Fig. 11). A sea-level fall forced a readjustment of
the shelf’s equilibrium pro¢le so that it couldserve as a sediment source rather than sink. Shellabrasion and selective size sorting were minimizedbecause these reworked materials traveled onlykilometers from their source and thus shells ofdi¡erent ages were incorporated into the BarrierComplex. This sea-level fall occurred about 4100yr BP and may be coeval with the Bahama Emer-gence of Fairbridge (1961) and Post Glacial Re-gression Maximum PR5 of Morner (1969, 1976).The simultaneity of late Nearshore Shelf, BarrierComplex, and early St. Bernard delta lobe depo-sition indicated by the overlapping radiocarbondates (Fig. 8) is only an apparent relationship,whereas stratigraphic superposition provides theactual sequence of events. However, this apparentsimultaneity demands that during a 200-or-so-year interval, essentially the precision limit ofthe radiocarbon technique applied to these sam-ples, sea level fell. During those two centuries theBarrier Complex was constructed, and was subse-quently buried by the prograding St. Bernard del-ta lobe. This supports the direct relationship be-tween delta-lobe switching and sea-level fallsuggested by Lowrie and Fairbridge (1991) andLowrie and Hamiter (1995). We argue that thismodel (a) better accounts for the vastly di¡erentdeposition rates of the Nearshore Shelf Depositversus the Barrier Complex and (b) does not re-quire an order of magnitude increase in the long-shore transport rate, two serious £aws in the west-
Fig. 11. Proposed evolutionary model of the Holocene NewOrleans Barrier Complex with sand supplied primarily by di-rect onshore transport from an o¡shore shelf source. Thenearshore shelf became a sediment source during and imme-diately after a hypothesized sea-level fall at about 4100 yrBP. This model greatly minimizes the contribution of long-shore transport from easterly Pleistocene headlands. Lafay-ette, LA (L); Baton Rouge, LA (BR); New Orleans, LA,and the western terminus of the New Orleans Barrier Com-plex (NO); and Gulfport, MS (G) are located as referencepoints. Observe that during deposition of the Maringouinand Teche delta lobes the future site of the New OrleansBarrier Complex was 50^100 km to the east and part of theshallow, nearshore shelf. The hypothesized sea-level fall, de-position of the New Orleans Barrier Complex, and its subse-quent burial by the prograding St. Bernard delta lobe alltook place during an interval of no more than 200 or soyears.
ward-migrating barrier model of Saucier (1963)and Otvos (1978).
11. Signi¢cance
This new depositional model for the New Or-leans Barrier Complex is yet one more example ofthe potential signi¢cance of onshore transportfrom a shelf source as a method to create Holo-cene barrier deposits. This speci¢c example is per-haps even more important because of its locationadjacent to a major Holocene delta and a Pleisto-cene headland, either of which, this new modelproposes, contributed minor, if any, material tobarrier deposition. Additionally, the mode of sedi-ment deposition and barrier formation contrastsmarkedly with the formative mechanisms associ-ated with delta abandonment on the Mississippidelta plain. The former mechanism also contrastswith comparable low tidal range and wave energydeltaic systems where the longshore supply of sedi-ment is important (e.g. Ebro Delta (Sanchez-Ar-cilla et al., 1998) and the Po Delta (Cencini,1998)). Furthermore, because a sea-level fall isone mechanism that can cause the shelf to switchfrom being a sink to serving as a sediment source,a £uctuating or Fairbridge type of sea-level curvebecomes a better paradigm in which to considerthe deposition of non-deltaic, strand plains andbarriers composed of multiple sets of coastal units.
Acknowledgements
A grant from the Tennessee Technological Uni-versity Faculty Research Fund and LouisianaState University O⁄ce of Research provided theradiocarbon dates. The writers acknowledge theuse of subsurface data of depths to Pleistoceneand to the top of the New Orleans Barrier Com-plex supplied by the late Dr. Roger Saucier. Dr.Ervin Otvos, Gulf Coast Research Laboratory,served as a sounding board during the early stagesof this investigation. Thanks are owed to Drs.Rhodes C. Fairbridge and John Dingler, and ananonymous reviewer for their critical and con-structive reviews.
References
Bruun, P., 1962. Sea-level rise as a cause of coastal erosion.ASCE J. Waterways Harbors Div. 88, 117^130.
Bruun, P., 1988. The Bruun Rule of erosion by sea-level rise.J. Coast. Res. 4, 627^648.
Cencini, C., 1998. Physical processes and human activities inthe evolution of the Po delta, Italy. In: Stone, G.W., Don-ley, J.C. (Eds.), The World Deltas Symposium: A Tribute toJames Plummer Morgan (1919^1995). J. Coast. Res. 14,774^793.
Cipriani, L.E., Stone, G.W., 2001. Net longshore sedimenttransport and textural changes in beach sediments alongthe southwest Alabama and Mississippi barrier islands,USA. J. Coast. Res. 17, 443^458.
Cowell, P.J., Thom, B.G., 1994. Morphodynamics of coastalevolution. In: Carter, R.W.G., Woodro¡e, C.D. (Eds.),Coastal Evolution, Late Quaternary Shoreline Morphody-namics. Cambridge University Press, Cambridge, pp. 33^86.
Cowell, P.J., Hanslow, D.J., Meleo, J.F., 1999. The shoreface.In: Short, A.D. (Ed.), Handbook of Beach and ShorefaceMorphodynamics. John Wiley and Sons. Chichester, pp. 37^71.
Davis, R.A., 1994. Barrier island systems ^ A geologic over-view. In: Davis, R.A. (Ed.), Geology of Holocene BarrierIsland Systems. Springer Verlag, New York, pp. 1^46.
DeWindt, J.T., 1974. Callianassid burrows as indicators ofsubsurface beach trend, Mississippi River delta plain.J. Sediment. Petrol. 44, 1136^1139.
Ellis, Jr., R.J., 1998. Computer Simulated Net Longshore Sedi-ment Transport and Inferences from Beach Sediment Gran-ulometry: Chandeleur Island, Louisiana. MS Thesis, Loui-siana State University, Baton Rouge, LA, 134 pp.
Emery, K.O., Aubrey, D.G., 1991. Sea Levels, Land Levels,and Tide Gauges, Springer-Verlag, New York, 226 pp.
Evans, O.F., 1942. The origin of spits, bars and related struc-tures. J. Geol. 50, 846^865.
Fairbridge, R.W., 1961. Eustatic change in sea level. In: Ah-rens, L.S., Press, F., Raukawa, K., Runcorn, S.K. (Eds.),Physics and Chemistry of the Earth, vol. 4, Pergamon, NewYork, pp. 99^185.
Frazier, D.E., 1967. Recent deltaic deposits of the MississippiRiver: their development and chronology. Trans. GulfCoast Assoc. Geol. Soc. 17, 287^315.
Gilbert, G.K., 1885. The topographic features of lake shores.U.S. Geol. Surv. Annu. Rep. 5, 75^123.
Hicks, S.D., 1973. Trends and variability of yearly mean sealevels 1893^1971, NOAA Tech. Mem. NOS 12, 13 pp.
Hollander, E.E., Dockery, D.T., III, 1977. Molluscan assem-blages of the mid-Holocene New Orleans Barrier Trend.Compass Sigma Gamma Epsilon 55, 1^29.
MARGO 3432 4-2-04
F.W. Stapor Jr., G.W. Stone /Marine Geology 204 (2004) 215^234232
Johnson, D.W., 1919. Shore Processes and Shoreline Develop-ment. Wiley, New York, 608 pp.
Kolb, C.R., Van Lopik, J.R. (1958) Geology of the MississippiRiver deltaic plain, southeastern Louisiana. U.S. Army En-gineer Waterways Experiment Station Technical Report No.3-483, Vicksburg, MS, vol. 1, 80 pp.
Kraft, J.C., 1971. Sedimentary facies patterns and geologicalhistory of holocene marine transgression. Geol. Soc. Am.Bull. 82, 2131^2158.
Lowrie, A., Fairbridge, R.W., 1991. Role of eustasy in Holo-cene Mississippi delta-lobe switching, 12th Annual GulfCoast Section of SEPM Research Conference, Abstracts,pp. 111^115.
Lowrie, A., Hamiter, R., 1995. Fifth and sixth order eustaticevents during Holocene (fourth order) high stand in£uencingMississippi delta-lobe switching. J. Coast. Res. 17 (SpecialIssue), 225^229.
Miller, W., III, 1983. Stratigraphy of newly exposed Quater-nary sediments, eastern Orleans Parrish, Louisiana. TulaneStud. Geol. Paleontol. 17, 85^104.
Morner, N.-A., 1969. The Late Quaternary history of the Kat-tegatt Sea and the Swedish West Coast. Deglaciation, shore-level displacement, chronology, isostasy and eustasy, Sver.Geol. Unders. C 640, 487 pp.
Morner, N.-A., 1976. Eustatic changes during the last 8,000years in view of ratiocarbon calibration and new informa-tion from the Kattegatt region and other northwesternEuropean areas. Palaeogeogr. Palaeoclimatol. Palaeoecol.19, 63^85.
Otvos, Jr., E.G., 1973. Geology of the Mississippi-AlabamaCoastal Area and Nearshore Zone, New Orleans GeologicalSociety, New Orleans, LA, 67 pp.
Otvos, E.G., Jr., 1978. New Orleans-South Hancock Holocenebarrier trends and origins of Lake Pontchartrain. Trans.Gulf Coast Assoc. Geol. Soc. 28, 337^355.
Parker, R.H., 1956. Macro-invertebrate assemblages as indica-tors of sedimentary environments in East Mississippi Deltaregion. Bull. Am. Assoc. Pet. Geol. 40, 295^376.
Penland, S., Boyd, R., Suter, J.R., 1988. Transgressive depo-sitional systems of the Mississippi delta plain: a model forbarrier shoreline and shelf sand development. J. Sediment.Petrol. 58, 932^949.
Posamentier, H.W., Allen, G.P., James, D.P., Tesson, M.,1992. Forced regressions in a sequence stratigraphic frame-work: concepts, examples, and exploration signi¢cance. Am.Assoc. Pet. Geol. Bull. 76, 1687^1709.
Roberts, N., 1998. The Holocene: An Environmental History,2nd edn. Blackwell, New York, 253 pp.
Russo, M., 1991. Archaic Sedentism on the Florida Coast: A
Case Study from Horr’s Island. Unpublished Ph.D. Disser-tation, Department of Anthropology, University of Florida,Gainesville, FL.
Sanchez-Arcilla, A., Jimenez, J.A., Valdemoro, H., 1998. TheEbro Delta: Morphodynamics and vulnerability. In: Stone,G.W., Donley, J.C. (Eds.), The World Deltas Symposium:A Tribute to James Plummer Morgan (1919^1995). J. Coast.Res. 14, 754^773.
Saucier, R.T., 1963. Recent geomorphic history of the Pont-chartrain Basin, Louisiana State Univ. Stud. Coast. Stud.Ser. 9, 114 pp.
Scholl, D.W., Stuiver, M., 1967. Recent Florida submergence:a comparison with adjacent coasts and other eustatic data.Geol. Soc. Am. Bull. 78, 437^454.
Scholl, D.W., Craighead, F.C., Sr., Stuiver, M., 1969. Floridasubmergence curve revised: its relation to coastal sedimen-tation rates. Science 163, 562^564.
Shenkel, J.R., 1974. Big Oak and Little Oak Islands: excava-tions and interpretations. Bull. Louisiana Archeol. Soc. 1,37^65.
Shepard, F.P., 1963. Thirty-¢ve thousand years of sea level.In: Essays in Marine Geology in Honor of K.O. Emery.University of Southern California Press, Los Angeles, CA,pp. 1^10.
Shepard, F.P., 1964. Sea level changes in the past 6000 years:possible archeological signi¢cance. Science 143, 574^576.
Stapor, F.W., Jr., Mathews, T.D., Lindfors-Kearns, F.E.,1991. Barrier-island progradation and Holocene sea-level history in southwest Florida. J. Coast. Res. 7, 815^838.
Stone, G.W., 1991. Di¡erential Sediment Supply and the Cel-lular Nature of Coastal Northwest Florida and SoutheastAlabama during the Late Holocene. Ph.D. Dissertation,University of Maryland, College Park, MD, 376 pp.
Stone, G.W., Stapor, F.W., Jr., May, J.P., Morgan, J.P., 1992.Multiple sediment sources and a cellular, non-integrated,longshore drift system northwest Florida and southeast Ala-bama coast, USA. Mar. Geol. 105, 141^154.
Stone, G.W., Jr., Stapor, F.W., 1996. A nearshore sedimenttransport model for the northeast Gulf of Mexico coast,USA. J. Coast. Res. 12, 786^792.
Stone, G.W., Zhang, X., 2001a. A Longshore Sediment Trans-port Model for the Timbalier Islands, Louisiana. TechnicalReport, Coastal Studies Institute, Louisiana State Univer-sity, Baton Rouge, LA, 26 pp.
Stone, G.W., Zhang, X., 2001b. A Longshore Sediment Trans-port Model for the Isles Dernieres, Louisiana. TechnicalReport, Coastal Studies Institute, Louisiana State Univer-sity, Baton Rouge, LA, 26 pp.
Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S.,Hughen, K.A., Kromer, B., McCormac, G., van der Plicht,J., Spurk, M., 1998. INTCAL98 radiocarbon age calibra-tion, 24,000^0 cal BP. Radiocarbon 40, 1041^1083.
Swift, D.J.P., 1975. Barrier island genesis: Evidence from the
Central Atlantic Shelf, Eastern USA. Sediment. Geol. 14,1^43.
Swift, D.J.P., Niedoroda, A.J., 1985. Fluid and sediment dy-namics on continental shelfs. In: Tillman, R.W., Swift,D.J.P., Walker, R.G. (Eds.), Shelf Sands and SandstoneReservoirs. Society of Economic Paleontologists and Miner-alogists, Short Course 13, Notes, pp. 47^133.
Talma, A.S., Vogel, J.C., 1993. A simpli¢ed approach to cal-ibrating 14C dates. Radiocarbon 35, 317^322.
Taylor, M., Stone, G.W., 1996. Beach-ridges: A review.J. Coast. Res. 12, 612^621.
Ters, M., 1987. Variations in Holocene sea level on the FrenchAtlantic coast and their climatic signi¢cance. In: Rampino,M.R., Sanders, J.E., Newman, W.S., Konigsson, L.K.(Eds.), Climate: History, Periodicity, and Predictability.Van Nostrand Reinhold, New York, pp. 204^237.
Thom, B.G., 1978. Coastal sand deposition in southeast Aus-tralia during the Holocene. In: Davies, J.L., Williams,M.A.F. (Eds.), Landform Evolution in Australia. AustralianUniversity National Press, Canberra, pp. 197^214.
Treadwell, R.C., 1955. Sedimentology and Ecology in South-east Coastal Louisiana. Louisiana State University Techni-cal Report 6 (ONR), Baton Rouge, LA, 78 pp.
Van Wagoner, J.C., Mitchum, R.M., Campion, K.M., Rahma-nian, V.D., 1990. Siliciclastic sequence stratigraphy in welllogs, cores and outcrops: concepts for high resolution cor-relation of time and facies, Am. Assoc. Pet. Geol. MethodsExplor. Ser. 7, 55 pp.
Walker, K.J., Stapor, F.W., Jr., Marquardt, W.H., 1995. Ar-cheologic evidence for a 1750^1450 BP higher-than-presentsea level along Florida’s Gulf coast. J. Coast. Res. 17 (Spec.Issue), 205^218.
Woodro¡e, C.D., 2002. Coasts, Form, Process and Evolution.Cambridge University Press, Cambridge, 623 pp.
Zenkovich, V.P., 1967. Processes of Coastal Development.Oliver and Boyd, Edinburgh, 783 pp.
Zhao, X.T., Zhang, J.W., 1985. Basic characteristics of theHolocene sea level changes along the coastal areas in China.In: Liu, T.S. (Ed.), Quaternary Geology and Environmentof China. China Ocean Press, Beijing, pp. 210^217.
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