ii

CAROLINA GEOLOGICAL SOCIETY

1996 OFFICERS

President: Ralph WilloughbyVice-President: Charles Gardner

Secretary-Treasurer: Duncan HeronBoard Members:

Alan DennisGeoff Feiss

Charles GardnerDuncan Heron

Ralph Willoughby

ALPHABETICAL LISTING OF FIELDTRIP LEADERS

AND AUTHORS OF GUIDEBOOK PAPERS

William J. Cleary Orrin H. Pilkey Department of Earth Sciences Department of Geology University of North Carolina at Wilmington Duke University Wilmington, NC 28403-3297 Durham, NC 2770-0228

William A. Dennis Stanley R. Riggs United States Army Corps of Engineers Department of Geology Wilmington District East Carolina University Wilmington, NC 28403 Greenville, NC 27858

James A. Dockal Stephen W. Snyder Department of Earth Sciences Department of MEAS University of North Carolina at Wilmington NC State University Wilmington, NC 28403-3297 Raleigh, NC 27695

W. Burleigh Harris E. Robert Thieler Department of Earth Sciences Department of Geology University of North Carolina at Wilmington Duke University Wilmington, NC 28403-3297 Durham, NC 2 7708-0228

Lynn A. Leonard Hugo Valverde Department of Earth Sciences Department of Geology University of North Carolina at Wilmington Duke University Wilmington, NC 28403-3297 Durham, NC 27708-0228

TABLE OF CONTENTS

Forward and Acknowledgements...................................................................................................... ............. iv

Overview of the Marine Paleogene, Neogene and Pleistocene DepositsBetween Cape Fear and Cape Lookout, North Carolina ............................................................... ............. 1

W. Burleigh Harris

The Coquinas of the Neuse Formation, New Hanover County, North Carolina........................... ............. 9 James A. Dockal

Shoreface Processes in Onlsow Bay................................................................................................... ............. 19 E. Robert Thieler

Morphology and Dynamics of Barrier and Headland Shorefacesin Onlsow Bay, North Carolina.......................................................................................................... ............. 29 Stanley R. Riggs, William J. Cleary, and Stephen W. Snyder

Inlet Induced Shoreline Changes: Cape Lookout - Cape Fear....................................................... ............. 41 William J. Cleary

Sedimentology and Depositional Processes in the Tidal Marshesof Southeastern North Carolina......................................................................................................... ............. 51 Lynn A. Leonard

Shoreline Stabilization in Onslow Bay.............................................................................................. ............. 59 Hugo Valverde and Orrin H. Pilkey

Fort Fisher Revetment Project........................................................................................................... ............. 65 William A. Dennis

Environmental Coastal Geology:Cape Lookout to Cape Fear, NC (Fieldtrip Guidebook)................................................................. ............. 73 William J. Cleary and Orrin H. Pilkey

Appendices........................................................................................................................................... ............. 109

iii

lastndivelines

First I

ld-ofd to

na

enter of

FOREWORD AND ACKNOWLEDGMENTS

The level of understanding of the North Carolina coastal systems has increased markedly over thedecade. The papers in this guidebook provide an overview of the environments, processes, underlying geology aman’s interaction with the coastal processes. The assemblage of papers provides the participant a comprehensaccount of ongoing research and perspectives on the environmental geology and processes that shape the shorewithin the Onslow Bay compartment.

I am grateful for the advice and counsel of all those who contributed to this guidebook and fieldtrip.would like to thank the authors of the papers. Peer reviews were instrumental in improving the content, focus andclarity of the papers. I would also like to thank the individuals who will contribute to the description of each fieldtripstop and the subsequent discussions.

Dave Blake and Duncan Heron offered valuable advice based on their experience with other CGS Fietrips. Richard Laws and Dave Blake assisted with the initial planning and logistics of the fieldtrip. Cathy Phillips the UNCW Print Shop offered valuable advice on the preparation of the guidebook. A debt of gratitude is extendea number of UNCW students: these include Kim Robinson, Doug Marcy and Tara Marden. Cathy Morris and DonCarlton provided administrative assistance.

I would also like to acknowledge the financial support provided by Dr. James F. Merritt of UNCW’s Cfor Marine Science Research and Dr. Jo Ann Seiple, Dean of the College of Arts and Sciences. Butch GoodsonJackson Beverage provided the refreshments for the opening reception. This guidebook is UNCW’s Center forMarine Science Research contribution #147.

Bill Cleary

UNCW

iv

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual Meeting

Pages 1-8

theof

si-d

y

neis

l-

osi-2, isge.ald

pe

r-owan

-

AN OVERVIEW OF THE MARINE TERTIARY AND QUATERNARY DEPOSITS BETWEEN CAPE FEAR AND CAPE LOOKOUT, NORTH CAROLINA

W. Burleigh HarrisDepartment of Earth Sciences

University of North Carolina at WilmingtonWilmington, NC 28403-3297

ABSTRACT

Tertiary and Quaternary marine sediments in the NorthCarolina Coastal Plain are distributed on two crustal blocks,the Onslow and the Albemarle. Differential uplift and sub-sidence of these blocks about the Neuse Hinge has controlledpatterns of relative coastal onlap, sediment distribution, andthe positions of late Tertiary and Quaternary scarps and ter-races.

Paleocene sediments (Beaufort Group) are restrictedprincipally to the southern part of the Onslow Block andnorth of the Neuse Hinge. Eocene sediments (Castle HayneLimestone and New Bern Formation) represent the mostwidely distributed early Tertiary unit in North Carolina, andare found over most of the Onslow Block. Oligocene sedi-ments (Trent, Belgrade and Silverdale Formations) arelocally distributed north of the New River, north of theNeuse Hinge and in Onslow Bay. The Miocene Pungo RiverFormation is mainly developed north and east of the WhiteOak River and Neuse Hinge, respectively. The PlioceneDuplin Formation represents the most extensive marineonlap during the late Tertiary, and occurs as outliers overmuch of the Onslow Block. The Pliocene Chowan River andBear Bluff Formations are developed north of the NeuseHinge and in the Cape Fear area, respectively. The lower andmiddle Pleistocene Waccamaw/James City Formations, andSocastee/Flanner Beach Formations are mainly developed onthe southern part of the Onslow Block and north of the WhiteOak River. ( Plio-Pleistocene distribution is related to forma-tion and ~ development of the Hanover-Surry Scarp, Bogue-Suffolk Scarp and the Alligator Bay Scarp.

INTRODUCTION

This paper presents a summary of the Tertiary and Qua-ternary marine stratigraphy and discusses the associatedscarps and terraces. In-depth discussions of North Carolinaearly Tertiary stratigraphy are presented by Harris and Zullo(1991), Harris et at. (1993), Harris and Laws (1994; inpress), Laws (1992), and Zullo and Harris (1987). Miocenestratigraphy for the area is discussed by Snyder et at. (1991 ).Pliocene and Pleistocene stratigraphy is discussed by Wardet at. (1991) and Soller and Mills (1991 ). Dockal (this vol-ume) discusses upper Pleistocene units on the Cape FearArch.

The area along the North Carolina-South Carolina stateline (approximate axis of the Cape Fear Arch) and the WhiteOak/Neuse Rivers (Neuse Hinge) is referred to as Onslow Block (Harris and Laws, in press). To the north the Neuse Hinge is the Albemarle Block. Differential upliftand subsidence of these blocks has controlled the stratalgeometries and patterns of relative coastal onlap on eachblock, the distribution of Coastal Plain units, and the potions of late Tertiary and Quaternary scarps and associateterraces.

TERTIARY

Tertiary units on the Onslow Block are assigned to theTejas Megacycle of Haq et at. (1987), and are represented bthe Paleocene Beaufort Group, the Eocene Castle HayneLimestone and New Bern Formation, and the OligoceTrent, Belgrade and Silverdale Formations [Fig. 1 ). In thpaper, the terminology of Baum et at. [1978) as modified byZullo and Harris (1987) and Harris and Laws (1994) is folowed.

Paleocene

The Paleocene Beaufort Group represents two deptional sequences (Fig. 1 ). The oldest sequence, the TA 1.represented by the Yaupon Beach Formation of Danian aThe youngest sequence, TA2.1 , is represented by the BHead Shoals Formation of Thanetian age.

Danian (Figure 2a)The Yaupon Beach Formation is recognized only on the

southern part of the Onslow Block on the axis of the CaFear Arch (Harris and Laws, 1994). It consists of olive greento gray, very fine to fine-grained slightly argillaceous biotubated quartz sand. A moderately to well preserved, ldiversity nannofossil assemblage including lower Danitaxa Cruciplacolithus prim us, C. tenuis, Ericsonia cava,Biscutum spp. and Neochiastozygus sp., and Cretaceous survivor species Placozygus sigmoides, Markalium in versusand Cyclogelosphaera reinhardtiiis present. This assem-blage, in the absence of Chiasmolithus danicus, correlates tothe lower Danian Cruciplacolithus tenuis Zone (NP2 or CP1b).

1

W. Burleigh Harris

edese

ate of

lesx-s-

t oft

Thanetian (Figure 2b)The Bald Head Shoals Formation is also restricted to the

southern part of the Onslow Block; however, Thanetian agedsediments are recognized at several localities in PenderCounty. The Bald Head Shoals Formation consists of almost7 m of sandy, molluscan-mold mudstone, wackestone topackstone, and contains very sparse calcareous nannofossilsand foraminifers, but abundant gastropods (turritelline) andpelecypods. Three mollusks that are age-diagnostic in theGulf Coastal Plain are identified; the gastropod Mesaliabiplicata Bowles, and the pelecypods Barbatia (Cucullae-arca) cuculloides and Acanthocardia (Schedocardia) tuom-eyi. Units that contain these mollusks in the Gulf CoastalPlain are considered Thanetian in age (Mancini and Tew,1988). The occurrence of the benthic foraminiferal speciesCibicides neelyi, Eponides lotus, Anomalinoides umbon-iferus, and Cibicidina sp. support a Thanetian age.

Eocene

Five Lutetian to Priabonian depositional sequencesoccur on the Onslow block from Brunswick to CarteretCounties. They are represented by the Castle Hayne Lime-stone and the New Bern Formation (Fig. 1).

Lutetian-Bartonian (Figure 3a)Three Lutetian and Bartonian depositional sequences

(TA3.3, TA3.4, and TA3.5/3.6) are recognized and assignto the Castle Hayne Limestone. Where indurated, thsequences consist dominantly of sandy, bryozoan and mol-luscan biomicrudite and biosparrudite, and locally phosphpebble conglomerate. Where unindurated, they consistbryozoan sand, which is often glauconitic.

Priabonian (Figure 3b)Two younger sequences are recognized and assigned to

the Castle Hayne Limestone. The older (TA4.1 ) straddthe Bartonian- Priabonian boundary with the surface of maimum flooding approximating the stage boundary. Trangressive deposits are interpreted below this surface to beLutetian/Bartonian; highstand deposits above this surface areinterpreted to be Priabonian in age. The high stand parthe sequence is the thickest and best developed, therefore, i

Figure 1. Generalized early Tertiary lithostratigraphy andsequence stratigraphy on the Onslow Block, North Carolina,modified from Harris et al. (1993) and Harris and Laws (1994,in press).

Figure 2. A. Isopach of Danian sediments on the OnslowBlock between the axis of the Cape Fear Arch and the NeuseHinge (modified from Harris and Laws, in press). B. Isopachof Thanetian sediments on the Onslow Block between the axisof Cape Fear Arch and the Neuse Hinge (modified from Har-ris and Laws, in press).

2

OVERVIEW OF THE MARINE TERTIARY AND QUATERNARY DEPOSITS BETWEEN CAPE FEAR AND CAPE LOOKOUT, NC

ale by

the itio-le,on-is-

is included in this section for discussion. The youngersequence (TA4.2 or TA4.3) is exclusively Priabonian in ageand has a more restricted spatial distribution. Both sequencesconsist predominantly of. bryozoan, sponge and molluscanbiomicrite and biomicrudite except along the northern part ofthe Onslow Block. Along the Neuse Hinge, the New BernFormation that represents either the TA4.2 or TA4.3sequence consists of sandy pelecypod-mold biosparite andbiosparrudite.

Oligocene/Early Miocene

One Rupelian, several? Chattian and one Aquitaniansequence are recognized between Brunswick and CarteretCounties (Fig. 1). The Rupelian sequence (TA4.4) is repre-

sented by the Trent Formation of Baum et al. (1978), theChattian sequences (TB1.1-1.4) by the Belgrade/SilverdFormations, and the Aquitanian part of sequence TB 1 .4the Crassostrea channel deposits of Baum et al. (1978) andZullo and Harris (1987).

Rupelian (Figure 4a)The Trent Formation is confined to the area between

New and Neuse Rivers. In the vicinity of the Neuse Hingeconsists of three ascending lithofacies; sandy echinoid bsparite, sandy pelecypod-mold biomicrudite and barnacpelecypod-mold biosparrudite. To the south near Jacksville it consists of sandy foraminiferal silt and silty clay. Thsequence is assigned to the TA4.4 cycle based on the occurrence of the barnacle Lophobalanus kellumi and the pectinid

Figure 3. A. Isopach of Lutetian and Bartonian sediments onthe Onslow Block between the axis of the Cape Fear Arch andthe Neuse Hinge. Note that outliers occur on the Onslow Block(modified from Harris and Laws, in press). B. Isopach of Pria-bonian sediments on the Onslow Block between the axis of theCape Fear Arch and the Neuse Hinge (modified from Harrisand Laws, in press).

Figure 4. A. Isopach of Reupelian sediments on the OnslowBlock between the axis of the Cape Fear Arch and the NeuseHinge (modified from Harris and Laws, in press). B. Isopach ofChattian sediments on the Onslow Block between the axis ofthe Cape Fear Arch and the Neuse Hinge (modified from Har-ris and Laws, in press).

3

W. Burleigh Harris

ari-d by),ue

hena-di-rle

r

is

Chlamys trentensis (Zullo and Harris, 1987), mollusks thathave early Vicksburgian (Rupelian) affinities (Rossbach andCarter, 1991), foraminifers indicative of the Globergerinaampliapertura Zone (P19/20) (Zarra, 1989), and calcareousnannofossils indicative of zones NP21-22 (Worsley andTurco, 1979).

Chattian-Aquitanian (Figure 4bThe Chattian Belgrade and Silverdale Formations are

restricted to quarries and core holes from about the NewRiver in Onslow County northward through eastern Jones,Craven and Carteret Counties. Chattian sequences are alsowell developed in Onslow Bay (Snyder et al., 1991). TheAquitanian Crassostrea channel deposits are only foundwithin a few kilometers north and south of the White OakRiver. The Belgrade Formation consists of about 8 m ofsandy, pelecypod-mold biomicrudite with minor interbeds ofquartz sand. The Silverdale Formation consists of about 3 mof mollusk- rich quartz sand, which is occasionally lithifiedand moldic. It occurs downdip (eastward) of the BelgradeFormation and is considered equivalent in age. Calcareousnannofossils (Laws and Worsley, 1986; Laws, 1992; Parkerand Laws, 1991), planktonic Foraminifera (Zarra, 1989), andmegafauna indicate that the Belgrade and Silverdale Forma-tions span planktonic foraminiferal zones P21 and P22(Zullo and Harris, 1987). The Belgrade and Silverdale For-mations were suggested by Zullo and Harris (1987) to repre-sent four depositional sequences ranging in age fromChattian to Aquitanian (TB1. 1-lower part of 1.4). TheAquitanian Crassostrea channel deposits were interpreted byZullo and Harris (1987) to represent the highstand of theTB1.4 sequence.

Miocene

Miocene sediments onlap the emerged Coastal Plainalong a north-south line that approximates the White OakRiver and are referred to the Pungo River Formation (Snyderet al., 1991) (Figs. 5 and 6). The Pungo River Formation isbest developed on the Albemarle Block and in Onslow Bay.Based on seismic analysis, Miocene sediments are inter-preted to represent three unconformity bounded packagesidentified as the Frying Pan, Onslow Bay and Bogue BanksSequences. Lithofacies of the Frying Pan Sequence includemuddy, quartzitic phosphatic sand; organic-rich, phosphaticmud; and molluscan-barnacle shell gravels interbedded withquartz sand or foraminiferal quartz sand (Riggs and Mallette,1990). The Onslow Bay Sequence consists of calcareousmuds and biogenic sands and gravels with varying amountsof siliciclastic sand and chert (Riggs and Mallette, 1990).The Bogue Banks Sequence consists mainly of siliciclasticmuds and sands; the sands usually contain minor phosphateand the muds usually contain abundant silt-sized dolomite(Riggs and Mallette, 1990). Based on study of planktonic

foraminifers, calcareous nannofossils, diatoms and radiolans, the three Miocene depositional sequences were dateSnyder et al. (1991) as Burdigalian (Frying Pan SequenceLanghian (Onslow Bay Sequence), and Serravallian (BogBanks Sequence).

Pliocene

Pliocene units in North Carolina are referred to as tDuplin/Yorktown Formations and the Bear Bluff/ ChowaRiver Formations (Figs. 5, 7a and 7b). The Yorktown Formtion is usually used for lower and lower upper Pliocene sements that occur north of the Neuse Hinge on the AlbemaBlock (Ward et al., 1991). The Duplin Formation is used foage equivalent sediments that occur south of the NeuseHinge on the Onslow Block. The Chowan River Formation

Figure 5. Late Tertiary and Quaternary lithostratigraphy ofthe Onslow Block.

Figure 6. Distribution of Miocene sediments on the OnslowBlock (modified from Brown et al. 1974, and Snyder et al.,1991)

4

OVERVIEW OF THE MARINE TERTIARY AND QUATERNARY DEPOSITS BETWEEN CAPE FEAR AND CAPE LOOKOUT, NC

dja-or-ffme-axi-

then.

also used for latest Pliocene sediments that occur on theAlbemarle Block, and the Bear Bluff for age equivalent sedi-ments on the Onslow Block. The Duplin Formation consistsof sand, sandy and silty clay, and very shelly sand commonlyoverlying a basal phosphate pebble conglomerate. North ofthe Neuse Hinge the Rushmere and Mogarts Beach Membersof the Yorktown Formation (=Duplin Formation) are contin-uous; however, to south on the Onslow Block, the Duplin ispreserved as outliers. The thickest section of the Duplin For-mation also occurs to the south where almost 5 m are foundin Bladen County (Ward et al., 1991). However, most outli-ers on the Onslow Block contain less than 2 m of the DuplinFormation.

The upper Pliocene Chowan River Formation is onlyused in North Carolina north of the Neuse Hinge; to thesouth, the Bear Bluff Formation of DuBar et al. (1974) is

recognized. The Bear Bluff Formation is known mainly fromthe area south of the Cape Fear River, and may occur acent to the Intracoastal Waterway below the Waccamaw Fmation on the central Onslow Block. The Bear BluFormation consists of calcareous sandstone, sandy listone, subarkosic sand, and calcareous silt and has a mmum observed thickness that exceeds 33 m (DuBar et al.,1974).

QUATERNARY

Pleistocene {Figures 8a and 8b)

Pleistocene geology along the seaward side of Onslow Block south of the Neuse River is poorly know

Figure 7. A. Distribution of the Pliocene Duplin Formation andequivalents (TB3.6 Sequence) on the Onslow Block. Sequencedesignation is after Zullo and Harris (1992). B. Distribution ofthe Pliocene Chowan River/Bear Bluff Formations (TB3.8Sequence) on the Onslow Block (modified from Ward et al.,1991). Sequence designation is after Zullo and Harris (1992).

Figure 8. A. Distribution of lower Pleistocene Waccamaw/James City Formations (TB3.9 Sequence) on the Onslow Block(modified from Owens, 1989; Ward et al., 1991). Sequence des-ignation is after Zullo and Harris (1992). B. Distribution of themiddle Pleistocene Socastee/Flanner Beach Formations on theOnslow Block (modified from Mixon and Pilkey, 1976; Owens,1989; Ward et al., 1991).

5

W. Burleigh Harris

ess

versto

ch,

y

d

-

fy

ofrd

arehisape

ape

Mixon and Pilkey (1976) mapped the geology of the CapeLookout area (Carteret-Craven Counties), Owens (1989)mapped the Florence, South Carolina, and North Carolina,10 x 20 Quadrangle (Brunswick and western New HanoverCounties), and Dockal (this volume) is currently examiningthe area of southern New Hanover and Brunswick Counties.However, no detailed geologic mapping of Pleistocene unitshas been completed on the Onslow Block south of NewRiver. The following discussion is mainly of those areasmarking the southern and northern parts of the OnslowBlock.

The Waccamaw/James City Formations are used forearly Pleistocene sediments of similar lithology that occur onthe southern and northern parts of the Onslow Block, respec-tively (Figs. 5 and 8a). The Waccamaw Formation occursover most of the area south of the Cape Fear River, particu-larly in low areas developed on older units (i.e., the PeedeeFormation), and north of the Cape Fear River in small pitsand dredge spoils just west of the Intracoastal Waterway. Ithas also been identified in Burnt Mill Creek in New HanoverCounty and probably occurs at other lower elevation locali-ties that are associated with the margins of the OnslowBlock. The Waccamaw Formation consists of poorly to mod-erately well sorted fossiliferous fine to coarse sand whichgrades upward into unfossiliferous sediments (Owens,1989). A local thin conglomerate of phosphate and quartzpebbles occurs at the base of the unit. Sediments that containtypical Waccamaw fossils range in thickness to almost 7 m(DuBar et al., 1974) in Brunswick County.

The James City Formation of DuBar and Solliday(1963) is recognized along the Neuse River below NewBern. It extends to the north onto the Albemarle Blockalmost to Virginia, and to the south to the New River (Black-welder, 1981). Several small pits west of the IntracoastalWaterway between the New and Cape Fear Rivers indicatethat the unit extends to the southeast eventually becomingthe Waccamaw Formation (Ward et al., 1991). AlthoughWard et al. (1991) indicate that the Waccamaw/James CityFormations extend west of the Hanover Scarp, I know of nolower Pleistocene marine sediments on the central OnslowBlock north of the Cape Fear River. The James City Forma-tion is an unconsolidated shelly argillaceous sand and sandyclay (DuBar and Solliday, 1963). Although the unit is con-sidered to be early Pleistocene in age (Ward et al., 1991),Campbell (1993) suggested that it is late Pliocene based onoxygen isotopes.

Numerous lithostratigraphic names have been applied tomiddle and upper Pleistocene units between the Cape Fearand Neuse Rivers (Fig. 5). Middle Pleistocene units arereferred to the Socastee/ Canepatch and Flanner Beach For-mations (Soller and Mills, 1991). The Socastee Formation,the major coastal Pleistocene unit in the Cape Fear region,consists of basal coarse sand, fine gravel and reworked shellsto 1 m in thickness, and interbedded sand and clay. The sand

and clay are commonly peaty and contain upright tree trunks(Owens, 1989). The Socastee ranges up to 5 m in thickn(DuBar et al., 1974) in the northern coastal area of SouthCarolina. Its extent between the Cape Fear and Neuse Riis unknown; however, if present, it is probably restricted the seaward edge of the Onslow Block (Fig. 8b). The Cane-patch Formation, named for exposures in the Myrtle BeaSouth Carolina area by DuBar (1971), was restricted to onesubsurface locality along the Intracoastal Waterway bOwens (1989). Therefore, the name is not used in this paper.The Socastee Formation is middle Pleistocene in age baseon isotopic dates (McCartan et al., 1982).

In the Neuse River area, the Socastee Formation is correlated to the Flanner Beach Formation of DuBar and Solli-day (1963). The Flanner Beach Formation consists ounconsolidated clay, sandy clay, argillaceous sand, and peatsand and clay, which reach almost 12 m in thickness; mollus-can fossils are common in the lower part (DuBar and Solli-day, 1963). Although the Flanner Beach Formation wasrestricted to exclude some of the originally defined partsthe unit by Mixon and Pilkey (1976), the unit occurs seawaof north-trending elements of the Suffolk Scarp. The FlannerBeach Formation is also considered to be middle Pleistocenein age; its distribution is shown in Figure 8b.

Late Pleistocene units are poorly described and assigned numerous lithostratigraphic names. Dockal (tvolume) discusses upper Pleistocene stratigraphy in the CFear region.

SCARPS AND PLAINS ON THE ONSLOW BLOCK

Several scarps and associated terraces (plains) are rec-ognized on the Onslow Block between Cape Fear and C

Figure 9. Relation of scarps and terraces to major structuralfeatures associated with the Onslow Block (modified from Zulloand Harris, 1979; and Harris and Laws, in press).

6

OVERVIEW OF THE MARINE TERTIARY AND QUATERNARY DEPOSITS BETWEEN CAPE FEAR AND CAPE LOOKOUT, NC

Lookout (Fig. 9). Zullo and Harris (1979) recognized threescarps that formed the seaward borders of tilted plains in thearea: the Hanover Scarp, the Bogue-Suffolk Scarp, and theAlligator Bay Scarp. The Hanover Scarp originated at aninterpreted cape in central New Hanover County north of theCape Fear River. To the south, Zullo and Harris (1979) sug-gested that the scarp paralleled the north side of the CapeFear River for several kilometers eventually becoming theSurry Scarp 80 km inland of the coastal margin. AlthoughFlint (1940) recognized the Surry Scarp inland on theOnslow Block, Zullo and Harris traced the Hanover Scarpnortheastward to just south of the New River where it turnedabruptly to the north eventually merging inland along theNeuse Hinge with the Surry Scarp. Soller and Mills {1991)followed the identification and location of the Surry Scarp asmapped by Flint {1940), and did not recognize the HanoverScarp. The plain delimited on the Onslow Block by theOrangeburg Scarp and the Hanover-Surry Scarp is identifiedas the Duplin Plain {Zullo and Harris, 1979). Sediments ofDuplin age represent the youngest marine formation under-lying the area. Zullo and Harris {1979) indicated that DuplinPlain was at an elevation of more than 12 m in central NewHanover County and over a distance of 60 km graduallyincreased to about 21 m on the west side of the New River.

The Bogue-Suffolk Scarp is located seaward of theHanover Scarp and essentially delimits the modern mainlandcoast on the Onslow Block. Mixon and Pilkey {1976)mapped the Bogue Scarp north of the New River, and indi-cated that in central Carteret County, it abruptly turned northand became part of elements associated with the SuffolkScarp. The plain delimited by the Hanover Scarp and theBogue-Suffolk Scarp is called the Waccamaw/CanepatchPlain {Zullo and Harris, 1979) and ranges in elevation fromabout 7.5 m in central New Hanover County to over 10.5 mjust north of the New River. Waccamaw and James City For-mation sediments represent the youngest marine sedimentsunderlying the plain. Zullo and Harris {1979) also proposedthe Alligator Bay Scarp for a linear feature that occurred sea-ward of the Bogue Scarp between Spicer and Alligator Bays,Onslow County. The plain bounded by Bogue Scarp and theAlligator Bay Scarp rose from sea level 12 km south of NewRiver to about 4.5 mat New River and was designated theSocastee Plain. North of New River Inlet, the Alligator BayScarp may merge with the Bogue Scarp, forming the westernlimit of the Core Creek Sand.

ACKNOWLEDGMENTS

I thank the University of North Carolina at Wilmington,and the Center for Marine Science Research for providingpartial support for this work. Appreciation is also expressedto William J. Cleary for the invitation to submit a paper forthe guidebook, and James A. Dockal for his careful review ofthe paper. This is CMSR contribution #143.

REFERENCESBaum, G.R., Harris, W. B., and Zullo, V.A., 1978, Stratigraphic

revision of exposed middle Eocene to lower Miocene forma-tions of North Carolina: Southeastern Geology, v. 20, p. 1-19.

Blackwelder, B. W., 1981, Stratigraphy of the upper Pliocene andlower Pleistocene marine and estuarine deposits of northeasternNorth Carolina and Virginia: U.S. Geological Survey Bulletin1502-B, 16 p.

Campbell, L. D., 1993, Pliocene mollusks from the Yorktown andChowan River Formations in Virginia: Virginia Division ofMineral Resources, Publication 127, 259 p.

DuBar, J.R., 1971, Neogene stratigraphy of the lower Coastal Plainof the Carolinas: Atlantic Coastal Plain Association, 12thAnnual Field Conference, Myrtle Beach, SC, 128 p.

DuBar, J.R., and Solliday, J.R., 1963, Stratigraphy of the Neogenedeposits, lower Neuse Estuary, North Carolina: SoutheasternGeology, v. 4, p. 213-233.

DuBar, J.R., Johnson, H.S., Thorn, B., and Hatchell, W.O., 1974,Neogene stratigraphy and morphology, south flank of the CapeFear Arch, North and South Carolina; in R.Q. Oaks and J.R.DuBar, eds., Post-Miocene stratigraphy, central and southernAtlantic Coastal Plain: Utah State University Press, Logan,Utah, p. 139-173.

Ferenczi, I., 1959, Structural control of the North Carolina CoastalPlain: Southeastern Geology, v. 1, p. 105-116.

Flint, R.F., 1940, Pleistocene features of the Atlantic Coastal Plain:American Journal of Science, v. 238, p. 757-787.

Gibson, T.C., 1983, Stratigraphy of Miocene through lower Pleis-tocene strata of the United States Central Atlantic CoastalPlain; in C.E. Ray, ed., Geology and Paleontology of the LeeCreek Mine, North Carolina: Smithsonian Contribution to Pale-obiology 53, p. 35-80.

Haq, B.U., Hardenbol, J., and Vail, P.R., 1987, Chronology of fluc-tuating sea levels since the Triassic: Science, v. 235, p. 1156-1167.

Harris, W.B. and Zullo, V.A., 1991, Eocene and Oligocene geologyof the outer Coastal Plain; in J.W. Horton and V.A. Zullo, eds.,The Geology of the Carolinas: University of Tennessee Press,Knoxville, Tennessee, p. 251-262.

Harris, W.B. and Laws, R.A., 1994, Paleogene sediments on theaxis of the Cape Fear Arch, Long Bay, North Carolina: South-eastern Geology, v. 34, p. 185-199.

Harris, W.B. and Laws, R.A., in press, Paleogene Stratigraphy andsea-level history of the North Carolina Coastal Plain: Globalcoastal onlap and tectonics: Sedimentary Geology and Evolu-tion of the Atlantic Coastal Plain -Sedimentology, Stratigraphyand Hydrogeology, Special Volume, Elsevier.

Harris, W.B., Zullo, V.A. and Baum, G.R., 1979, Tectonic effects onCretaceous, Paleogene, and early Neogene sedimentation,North Carolina; in G.R. Baum, W.B. Harris and V.A. Zullo,eds., Structural and Stratigraphic Framework for the CoastalPlain of North Carolina: Carolina Geological Society and theAtlantic Coastal Plain Association, Field Trip Guidebook,Wrightsville Beach., North Carolina, p. 17-29.

Harris, W.B., Zullo, V.A., and Laws, R.A., 1993, Sequence stratig-raphy of the onshore Palaeogene, southeastern Atlantic CoastalPlain, USA; in H.W. Posamentier, C.P. Summerhayes, B.U.Haq and G.P. Allen, eds., Sequence Stratigraphy and Facies

7

W. Burleigh Harris

Associations: Special Publication of the International Associa-tion of Sedimentologists, v. 18, p. 537-561.

Laws, R.A., 1992, Correlation of Cenozoic continental marinedeposits in North and South Carolina to standard calcareousnannofossil and diatom zonations; in V.A. Zullo, W.B. Harrisand V. Price, eds, Savannah River Region: Transition Betweenthe Gulf and Atlantic Coastal Plains. Proceedings of the SecondBald Head Island Conference on Coastal Plains Geology, Uni-versity of North Carolina at Wilmington, Wilmington, p. 110-116.

Laws, R.A. and Worsley, T.R. 1986, Onshore/offshore Oligocenecalcareous nannofossils from southeastern North Carolina:Geological Society of America, Abstracts with Programs, v. 18,p. 251.

Mancini, E.A. and Tew, B.H., 1988, Paleogene stratigraphy andbiostratigraphy of southern Alabama: Field Trip Guidebook forthe GCAGS- GCS/SEPM, 38th Annual Convention, NewOrleans, Louisiana, 63 p.

McCartan, L., Owens, J.P., Blackwelder, B.W., Szabo, B.J.,Belknap, D.F., Kriausakul, N., Mitterer, R.M., and Wehmiller,J.F., 1982, Comparison of amino acid racemization geochro-nometry with lithostratigraphy, biostratigraphy, uranium-seriescoral dating, and magnetostratigraphy in the Atlantic CoastalPlain of the southeastern United States: Quaternary Research, v.18, p. 337-359.

McCartan, L., Lemon, E.M., and Weems, R.E, 1984, Geologic mapof the area between Charleston and Orangeburg, South Caro-lina: U.S. Geological Survey Miscellaneous InvestigationSeries Map 1-1472.

Mixon, R.B. and Pilkey, O.H., 1976, Reconnaissance geology ofthe submerged and emerged Coastal Plain Province, CapeLookout area, North Carolina: U.S. Professional Paper 859, 45p.

Owens, J.P., 1989, Geologic map of the Cape Fear region, Florence10 x 20 Quadrangle and northern half of the Georgetown 10 x20 Quadrangle, North Carolina and South Carolina: U.S. Geo-logical Survey Miscellaneous Investigation Series Map 1-1948-A.

Parker, W. and Laws, R.A., 1991, Calcareous nannoplankton bios-tratigraphy of the exposed and subsurface Oligocene and lowerMiocene strata in southeastern North Carolina: GeologicalSociety of America, Abstracts with Program, v. 23, p.113.

Riggs, S.R. and Mallette, P.M., 1990, Patterns of phosphate deposi-tion and lithofacies relationships within the Miocene PungoRiver Formation, North Carolina continental margin; in W.Burnett and S.R. Riggs, eds., Phosphates of the world, v. 3:Cambridge University Press, Cambridge, p. 424- 445.

Rossbach, T.J. and Carter, J.G., 1991, Molluscan biostratigraphy ofthe Lower River Bend Formation at the Martin Marietta Quarry,New Bern, North Carolina: Journal of Paleontology, v. 65, p.80- 118.

Soller, D.R. and Mills, H.H., 1991, Surficial geology and geomor-phology; in J.W. Horton and V.A. Zullo, eds., The Geology ofthe Carolinas: University of Tennessee Press, Knoxville, Ten-nessee, p. 290- 308.

Snyder, S.W. and Riggs, S.R., 1993, Geological overview of LeeCreek Mine and vicinity, North Carolina Coastal Plain: TheCompass, Earth Science Journal of Sigma Gamma Epsilon, v.

70, p. 13-35.Snyder, S.W., Snyder, S.W., Riggs, S.R., and Hine, A.C., 1991,

Sequence stratigraphy of Miocene deposits, North Carolinacontinental margin; in J.W. Horton and V.A. Zullo, eds., TheGeology of the Carolinas: University of Tennessee Press,Knoxville, Tennessee, p. 263-273.

Ward, L.W., Bailey, R.H., and Carter, J.G., 1991, Pliocene and earlyPleistocene stratigraphy, depositional history, and molluscanpaleobiogeography of the Coastal Plain; in J.W. Horton andV.A. Zullo, eds., The Geology of the Carolinas: University ofTennessee Press, Knoxville, Tennessee, p. 274-289.

Worsley, T.R. and Turco, K, 1979, Calcareous nannofossils fromthe lower Tertiary of North Carolina; in G.R. Baum, W.B. Har-ris, and V.A. Zullo, eds" Structural and stratigraphic frameworkfor the Coastal Plain of North Carolina: Carolina GeologicalSociety, 1979 Field Trip Guidebook, p.65-72.

Zarra, L. 1989, Sequence stratigraphy and foraminiferal biostratig-raphy for selected wells in the Albemarle Embayment, NorthCarolina: Open- file Report, North Carolina Geological Survey,Department of Environment, Health and Natural Resources,No.89-5, 48 p.

Zullo, V.A. and Harris, W.B., 1979, Plio-Pleistocene crustal warp-ing in the outer Coastal Plain of North Carolina; in G.R. Baum,W.B. Harris and V.A. Zullo, eds., Structural and StratigraphicFramework for the Coastal Plain of North Carolina: CarolinaGeological Society and the Atlantic Coastal Plain Association,Field Trip Guidebook, Wrightsville Beach, North Carolina, p.31-40.

Zullo, V.A. and Harris, W.B., 1987, Sequence stratigraphy, bios-tratigraphy and correlation of Eocene through lower Miocenestrata in North Carolina; in C.A. Ross and D. Haman, eds.,Timing and Depositional History of Eustatic Sequences: Con-straints on Seismic Stratigraphy: Cushman Foundation for For-aminiferal Research, Special Publication 24, p. 197-214.

Zullo, V.A. and Harris, W.B., 1992, Sequence stratigraphy ofmarine Pliocene and lower Pleistocene deposits in southwesternFlorida; preliminary assessment; in T. M. Scott and W. D. All-mon, eds., The Plio-Pleistocene stratigraphy and paleontologyof southern Florida: Florida Geological Survey, Special Publi-cation 36, p. 27-40.

8

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual Meeting

Pages 9 - 18

naows

of.ineina.pei-of

use

THE COQUINAS OF THE NEUSE FORMATION, NEW HANOVER COUNTY, NORTH CAROLINA

James A. DockalDepartment of Earth Sciences

University of North Carolina at WilmingtonWilmington, NC 28403-3297

ABSTRACT

The coquinas of the Neuse Formation in southern NewHanover County, North Carolina, represent only a small por-tion of a depositional suite, which formed in the high-energyenvironment of a Late Pleistocene shoreface at a time corre-sponding to oxygen isotope stage 3 or 75 to 55 ka BP. Thefauna associated with the coquina indicates climatic condi-tions that are i indistinguishable from the present climate.The coquinas are the product of post-depositional diagenesisof carbonate shell bearing shoreface sands where dissolution,cementation, and calcification of aragonite occurred at ornear the paleo-water table. A later dissolution episode of thecarbonate fraction of the coquina and associated strata byoxygenated meteoric waters resulted in the formation of anunlithified, generally reddish, non-fossiliferous sand whichgenerally blankets the area. I

INTRODUCTION

The coquina found on the beach in the area of FortFisher in New Hanover County, North Carolina, representsone of the very few naturally occurring rock out croppings inthe Coastal Plain Province of the Carolinas. The coquina isnot laterally extensive nor is it of significant thickness, but itoccurs as sporadic isolated patches in a north to south arcu-ate band over an area roughly 15 km long by 2 km wide (Fig-ure 1}. This paper presents a synopsis of the stratigraphicnomenclature applied to the coquina and then presents adetailed petrologic description and interpretation of the con-ditions of deposition and diagenesis of the coquina.

The published record of coquinas in southeastern NorthCarolina and especially New Hanover County is sparsealthough the area has been visited frequently by geoscientistsfor over 200 years. The first published record is in Stephen-son's (1912} description of the Pamlico Formation under"Detailed Sections" where he notes the presence of coquinarock at "Old Fort Fisher" and at a site “one mile southeast ofCarolina Beach wharf." Stephenson's report, however, onlyprovides a brief list of some of the fauna collected by a Dr.Vaughan and provides a photograph of the outcrop on thebeach at Fort Fisher. U. S. Army engineers, during the courseof a beach erosion study in 1931 made 14 "wash borings" atthe Fort Fisher site (House Document 204, 72 ND Congress,1 st Session). These provide some insight into the lateralvariability of the strata though the records lack detailed litho-

logic descriptions. Richards (1936), in describing the fauof the Pamlico Formation, mentioned the exposures at SnCut and Fort Fisher and provided a comprehensive list of thefauna. Wells (1944) provided the first detailed descriptionthe Pleistocene strata in the Carolina Beach-Fort Fisher areaHe divided the strata into five units: Galveston Sand, PSand, Castalia Sand, Kure Sand, and Cape Fear CoquThis is apparently the first in print usage of the term .'CaFear Coquina." Fallow and Wheeler (1969) in their defintion of the Neuse Formation noted several locations coquina in the Carolina Beach area. They designated thecoquina as representing the .'Coquina facies" of the Ne

Figure 1. Map of the study area showing the occurrences ofcoquina both on shore and off shore in southern New HanoverCounty, North Carolina. A-A’, B-B’, and C-C’ indicate thelocations of profiles illustrated on Figure 7.

9

James A. Dockal

-sr.re

ini-

r

Formation and they designated the section on the north sideof Snows Cut as a reference section for the proposed NeuseFormation. Fallow (1973) later considered the coquina to bethe "High-energy Facies" of the Neuse Formation. Moore-field (1978 was the first to recognize and describe sub-merged coquina outcrops in the area. He noted that thesubmerged outcrops, though identical to those on the landsurface, were encrusted with algae, barnacles, colonies of thebivalve Mytilus, serpulid worms, bryozoans and the coralAstrangia. Moorefield reported that these submerged out-crops were undergoing biological erosion especially by therock- boring bivalve Lithophaga. The U. S. Army Corps ofEngineers (1982), during the course of a design study forbeach stabilization, made a number of split spoon borings atthe Fort Fisher Historic site. Some of these penetrated to theunderlying Castle Hayne Limestone (Eocene). The recordsof these borings in combination with the 1931 records pro-vide a good view of the lateral variance in lithology over avery limited geographic area. Prosser (1993) attempted toascertain the age of the coquina by using the Uranium seriesmethod on Mercenaria shells collected at Snows Cut.Dockal (1992, 1995b) applied the radiocarbon method toshells of Donax variabilis, Nassarius obsoleta, and Crassos-trea virginica, which were also collected at Snows Cut. Weh-miller and others (1988) and Wehmiller and others (1995)applied the amino acid racemization to specimens of Merce-naria from the coquina.

NOMENCLATURE REVIEW

The fossiliferous sands of Pleistocene age in NewHanover County and adjacent offshore areas are informallyreferred to today as the "Cape Fear Coquina," a term firstused by Wells (1944) for the coquinas between Snows Cutand Fort Fisher. This term can not be used as a formal lithos-tratigraphic name because the name Cape Fear is already inuse for Cretaceous strata located in the coastal plain of theCarolinas; the Cape Fear Formation.

The earliest workers within the southeastern Atlanticseaboard region applied the term Columbia Formation andlater Columbia Group to all the Quaternary strata. Bothterms have not been applied in the region for decades andwere never used in reference to the strata encompassed bythis study outside of Stephenson's (1912) usage of PamlicoFormation which was considered at that time to be a subdivi-sion of the Columbia Group. Stephenson (1912) and laterRichards (1936) definitely referred to the strata considered inthis report as belonging to the Pamlico Formation; howeverlater workers did not make use of the Pamlico name in alithostratigraphic sense (Figure 2). Du Bar and Solliday(1963) argued not to use the term Pamlico Formation partlybecause the type area was a terrace plane, a geomorphic fea-ture and therefore the unit did not represent a true lithostrati-graphic unit. Du Bar and Solliday (1963) proposed the

Fanner Beach Formation to replace the concept of the Pamlico as a lithostratigraphic unit. The name Fanner Beach hanever been applied directly to the coquinas of the Cape FeaFallaw and Wheeler (1969) objected to usage of the FanneBeach Formation because the name included an assemblagof "distinct lithologic units" or units of terrestrial origin andthose of clearly marine character. Fallaw and Wheeler (1969)proposed the name Neuse Formation which was by deftion to encompass just the "marine fossiliferous Pleistocenedeposits in North Carolina." This they divided into foufacies: "Fine-grained sand facies", Very fine-grained sand

Figure 2. Lithostratigraphic nomenclature that has beenapplied in the literature to the coquina and associated strata insouthern New Hanover County.

10

COQUINAS OF THE NEUSE FORMATION

et)res

by1)

al

ic",4)va-ofpon lefthelerrn beow

facies", Sand-silt-clay facies", and "Coquina facies." Theoutcrop on the north side of Snows Cut was designated as areference section for their Neuse Formation. Du Bar and oth-ers (1974) proposed the name Socastee Formation for agroup of related strata near Myrtle Beach, South Carolina.They indicate that the Socastee is found within the Wilming-ton, North Carolina area but did not specify where. Thedescription of the Socastee is very similar to that of the stratareported here and to that of the Neuse Formation. It isbelieved here that the Socastee Formation is in synonymywith the Neuse Formation, differing only in the state inwhich each are found. The name Neuse Formation thereforehas priority over the name Socastee Formation when beingapplied to North Carolina strata.

Owens (1989) applied the name Wando Formation tothe sands at Snows Cut and Waccamaw Formation to thecoquina. Use of the term Wando is valid for the surficialunlithified sands which overlie the coquina; but applicationof the term Waccamaw to the coquina is wrong. The Wacca-

maw Formation occurs at a depth of -10 meters (-35 febelow mean sea level and below the coquina exposuthroughout the area as indicated by drilling at Fort Fisherthe U. S. Army Corps of Engineers (1982). Zarra (199referred to the coquina as the "Fort Fisher Coquina" butmade no attempt to formerly define the name. Dock(1995b) applied the informal name "Cape Fear Coquina" anddivided it into three subdivisions or lithofacies of diagenetorigin; "shell hash lithofacies", sandy limestone lithofaciesand "Kure sand." The later being equivalent to Wells (194use of the term "Kure Sand" and the other two being equilent to his "Cape Fear Coquina." Dockal's incorporation the "Kure sand" into the Cape Fear Coquina was based uthe interpretation that it represented the insoluble residueafter the leaching of the carbonate shells and cements of tcoquina; a view also suggested by Fallaw and Whee(1969). It is recommended here that the name Cape FeaCoquina be suppressed and that name Neuse Formatioapplied both to the coquinas as originally intended by Fall

11

James A. Dockal

12

Figure 3. Stratigraphic section from the North Side of Snows Cut. Unit 1 is below sea level and is inferred from drilling data.Mean low is roughly at the base of Unit 2.

COQUINAS OF THE NEUSE FORMATION

ly to

to ofarys

r

e andass.

dis-er

ternannesdi-

ce ahetere.ld

is-olu-

rainsi-

r-d

teds

n-

bygehell

tingni-

ut-the

tifi-lec-

and Wheeler (1969) and also to the strata in the area that aredepositionally related to the coquina as described below.

DESCRIPTION OF THE COQUINAS AND RELATED BEDS

The best exposure of the coquina and associated strata isto be found on the north bank of Snows Cut west of the US421 bridge (Figure 1) (Site 24 of Carter and others, 1988).The coquina at Snows Cut ranges from a medium to verycoarsely grained fossiliferous sand to an arenaceous fossilif-erous limestone; the siliciclastic fraction ranges from 32% to54% of the mass (Table 1 ). The dominant grains are suban-gular to rounded monocrystalline quartz and well roundedmolluscan shell fragments. These are weakly to moderatelycemented with blocky calcite. Molds after various aragoniticbioclasts are abundant but aragonitic shells and shell frag-ments are still in abundance as well as shells of calcitizedaragonite. Grain size, though moderately to well sortedthroughout, is not constant (Table 1). There are coarser zoneswhich contain whole molluscan shells, especially Merce-naria, Busycon, Crassostrea, and Rangia. These zones areup to 0.5 meters thick and traceable across the whole out-crop. Shells are generally concave down and imbricate sug-gesting a southerly transport direction. Trough cross-stratification, though weakly marked, also indicates a south-erly transport direction. The coquina at the west end of thenorth bank of Snows Cut has a maximum thickness of 1 .5meters and pinches rapidly eastward and westward. Those onthe south bank are over 2 meters thick but generally present apoorer exposure.

The coquina at Snows Cut is overlain by an unlithifiedarenaceous shell hash or marl (Figure 3). Dominant grainsare like that of the coquina; monocrystalline quartz androunded shell fragments with the siliciclastic fraction repre-senting 34% to 46% of the mass (Table 1 ). Grain size distri-butions of the siliciclastic fraction are comparable to those ofthe coquina (Table 1 ). Scattered throughout are well-pre-served molluscan shells, but none are in growth position,none are articulated, and most but not all exhibit somedegree of abrasion (Dockal 1995b). Like the coquina there isa weak sense of imbrication and cross-stratification. Cementis absent except for a minor amount of pendant cementwithin some of the shelter pores and meniscus calcite cementnear the base of the arenaceous shell hash.

The contact with the underlying coquina is a rapid gra-dation in the degree and type of cementation. Dockal(1995b) interpreted this to have resulted from diagenesisnear the paleo-water table. The shell hash proper with itspendent cements lying within the vadose lone, the coquinawith its blocky calcite cement lying within the phreatic lone,and the boundary area with the meniscus cement representthe capillary fringe just above the water table.

Overlying the shell hash and the coquina where the shell

hash is absent is a medium to coarse grained, moderatepoorly sorted, near symmetrical to coarse skewed, platykur-tic to leptokurtic sand (Table 1 }. The sand is noninduratedmoderately indurated with ferruginous cement. The colorthe sand ranges from a pale yellow to black. Sedimentstructures are generally absent but one can observe zonewhere larger grains, granules and pebbles occur within thestill poorly sorted sand. These when tracked laterally passinto the coarser shell bearing zones of the shell hash ocoquina. On the north side of Snows Cut and about halfwaybetween the main coquina outcrop and the US 421 bridglens of finer and better sorted sand occurs within this salayer (Table 1, samples SCN-40, 51, 55}. This lense hweakly marked trough cross-stratification and some burrowThe boundary between these two sand types is very intinct suggesting that the finer sand my have been a lowenergy phase with less carbonate bioclasts. At the easend of Snows Cut several dark brown to black horizons cbe seen within the sand layer. These appear to be only zoof enrichment of iron oxides and have no relation to a sementary structure or to fossil soil horizons.

The contact between the sand and the underlying shellhash or coquina is a very distinct and undulatory surfawhere the overlying sand appears to literally interfinger invertical sense with the underlying shell hash or coquina. Tsand forms cylindrical bodies 0.2 to 0.4 meters in diamewhich project into the underlying material a meter or morDockal (1995b} described these as "fingers" and Moorefie(1978} referred to them as "potholes." They represent ageochemical front along which carbonated material dsolved. The sand, which sits above this surface, is the insble residue left over after dissolution of all the carbonatefrom the shell hash or coquina. The sand has the same gtypes and grain size distribution as that of the insoluble redues from the shell hash and coquina (Table 1 }.

At the top of this sand layer is a well-marked nearly hoizontal lying paleosol. Overlying this is finer grained santhat is moderately to well sorted. This sand is directly relato some of the Holocene dunes in the immediate area such aSugar Loaf within Carolina Beach State Park just to thesouth of Snows Cut. Overlying this are spoils from the costruction of the canal.

What underlies the sand and where present, the coquinaat Snows Cut is not as well understood. Recent drilling the U.. S. Army Corps of Engineers just east of the bridrevealed 610 7 meters of gray green sands with abraded sdebris and bands of shells and greenish gray silty sand sitbelow the ferruginous sand (Ben Lackey, personal commucation 1996). This material appears to be little different oside of color from the main sand noted above and unlithified shell hash.

The coquina and associated beds at Fort Fisher (Figure1) are very similar to those of Snows Cut. The cross stracation of the coquina is easier to see but this is more a ref

13

James A. Dockal

and-

eit isina,low

gytialtandply

blete-ish

n-thevi-

dsbles-

tarylbe

eed ag-

a-

t)edara.,

ethe

d

tion of outcrop type and not a true lithologic difference withthat of Snows Cut. The cross- stratification at Fort Fisher hasbeen described in detail by Fallow and Wheeler (1969), Fal-low (1973), and Mansfield (1978). Mansfield (1978) foundthree sets of directions indicating paleocurrent flows ofS65°W, S23°E and S22°W. As at Snows Cut the coquina isnot laterally extensive. In comparison between the 1931 bor-ings, the 1982 borings, and the present outcrop it appearsthat the main body of coquina was to the east of the presentbeach and has since 1931 largely eroded away. The 1931study indicated that the coquina was as much as "9 feetthick" in places and extended continuously from boring toboring north to south (Figure 4). U.S. Army Corps of Engi-neers (1982) reports that "The coquina is irregular in thick-ness and elevation" and that "it is also discontinuous " Theshell hash observed overlying the coquina at Snows Cut hasnot been observed in recent times at Fort Fisher however the1931 study indicates a similar lithology being present. Theferrugenous sand that dominated the Snows Cut outcrop ispresent as well as the paleosol at the top of the sand. Well's(1944) description of this area notes several sand and associ-ated paleosols. However these appear now to be no differentthat the color banding noted at Snows Cut in the main sandlayer. Underlying the coquina is a plastic greenish- gray siltwhich contains sand filled burrows. The 1982 Corps of Engi-neers borings indicate 10 meters (32 feet) of gray to greensand similar to that found at Snows Cut underlie the coquina

and associated ferruginous sand. The change from the fer-rugenous dark brown to buff sands to the gray green stype "is irregular with elevation from boring to boring varying as much as 5 feet and ranging from zero to -10 m.s.l."U.S. Army Corps of Engineers (1982).

The coquina occurs at other scattered localities through-out the area from just north of Snows Cut to Fort Fisher andadjacent off shore waters (Figure 1 ). Lithology of thcoquina is always the same and in the on shore areas always associated with the ferruginous sand. The coquthe shell hash, and the surrounding sands, which lie bethe paleosol, are all considered here to be of the same origi-nal depositional layer. There present difference in litholobeing due entirely to post-depositional diagenesis. The inisediment was probably much like the shell hash seen aSnows Cut or the finer sand noted also at Snows Cut from the Corps of Engineers borings. The coquina is simcemented shell hash where the cementation formed at or nearthe paleo-watertable. The ferrugenous sand is the insoluresidue left from the later dissolution of all carbonate marial from the shell hash, coquina, or finer sand. The reddcoloration represents the zone of oxidation of the sedimentswhich is probably modern or Holocene. This passes dowward to the greenish gray colored sediments found in borings which are situated in a reducing geochemical enronment.

Lithoclasts found in the coquina, shell hash, and sanare the same. These range in size from granules to coband vary in lithology ranging from metamorphic rock fragments derived from Piedmont source areas to sedimenrock fragments of Coastal Plain sources. Most are welrounded and half have a discoidal shape which would characteristic of a pebble that has been within the surf envi-ronment for some time. Of the sedimentary rock fragmentsmost probably were derived from sandstone in the Pee DFormation (Cretaceous) which would have been exposefew tens of kilometers northwest of the area near Wilminton. One lithoclast contained the oyster Conradostrealawrencei which would have been derived from the Waccmaw Formation (Pliocene/ Pleistocene). The Waccamawoccurs in the subsurface in the area about 11 meters (35 feebelow the top of the coquina and probably was exposwithin a few kilometers northwest of the area. Of particulinterests are lithoclasts of coquina found within the coquinThese range up to 15 cm in diameter, are somewhat roundedspherical, and identical in all aspects of lithology to the mainbody of coquina. This indicates that the area has or had morthan one body of coquina and further points out that coquina and associated beds contain material reworked fromolder beds.

Mollusks which are commonly found in the coquina anshell hash include: Anadara brasiliana, Crassostrea virgin-ica, Dionocardium robustum, Donax variabilis, Mercenariamercenaria, Rangia cuneata, Tagelus plebius, Busycon con-

Figure 4. Wash boring logs of Line “A” from the beach face atFort Fisher Historic Site. (Modified from House DocumentNo. 204, 72 nd. Congress, 1 st. Session, Plate VII).

14

COQUINAS OF THE NEUSE FORMATION

age or-ent ofrs

dshk bycid

25s,peth-

un-e

eratan)utf aCut92,ar-een

fluxoutlynowsent

nd 55

ticernudef

r

trarium, and Nassarius obsoletus. Other identified mollusksare listed in Figure 5 and Richards (1936), Fallaw (1973),and (Dockal 1995b). The foraminifera fauna is dominated byOuinqueloculina and Hanzawaia (Fallaw, 1973) The twocorals present are, Septastraea crassa which is quite com-mon though always greatly abraded and Siderastrea radianswhich is rare but always well preserved, lacking abrasion,and encrusting other bioclasts. The barnacle Balanus impro-visus has been described from the strata (Zullo and Miller,1986). Flat echinoid fragments are common and an occa-sional whole specimens of the sand dollar, Millita sp., arefound. Small fish teeth and crab claws are readily found inthe finer fractions and shell fillings. Mammalian bones andteeth are infrequently encountered. These include mastodon(Richards, 1936), bison (Wilmington Star News, February4,1995), camel and deer.

AGE OF THE COQUINAS AND RELATED STRATA

The age of the coquina has generally been considered tobe Late Pleistocene (Fallaw and Wheeler, 1969). Thecoquina contains Anadara brasiliana (Lamarck), a molluskswhich is characteristic of Blackwelder's (1981) upper Pleis-tocene and Holocene Yongesian Substage of the LongianMolluscan Stage. The present near sea level topographicposition of these deposits suggest deposition in associationwith a sea level high stand above the altitude of the present

sea level but below that of the high stand associated with thenearby 7-meter (25 foot) terrace. The Yongesian Substdesignation for the coquina, its geomorphic position eastseaward of what is possibly the Suffolk Terrace, and the elevation of the strata relative to sea level suggests assignmto Stage 5 of the oxygen isotope base sea level curveChappel and Shackleton (1986). Wehmiller and othe(1988), applying amino acid racemization, found a mean D/L Leu value of 0.52 from two specimens of Mercenaria sp.which were collected at Snows Cut. This value corresponto amino zone Illd of Wehmiller and others (1988) whicapparently has an age in excess of 220 ka.. Recent worWehmiller and others (1995) notes the additional amino aanalysis of 16 specimens of Mercenaria sp. from Snows Cut.The AI I values of these "cluster into two apparent amino-zones with mean values of approximately 0.46+/-0.0(n=4) and 0.34+/-0.025 (n= 12)" (Wehmiller and other1995, p.331). The lower ratio corresponds to oxygen isotostage 5 the higher ration to stages 7 to 9 (Wehmiller and oers, 1995, figure 2). Dockal (1995b) argued that the abdance of fossils reworked from older units and the presencof purple colored Mercenaria, which elsewhere in the regionhave lower A/l values than 0.34, would imply that even lowA/l values should be obtainable from the Snows Cut strand therefore they should represent an age younger thastage 5. Uranium series dating conducted by Prosser (1993on Mercenaria sp. shells from the shell hash at Snows Cindicate an age of 62 ka BP. Radiocarbon evaluation ovariety of shells from the same site and stratum at Snows found an apparent age of 24 to 29 ka BP (Dockal 191995b). However, as Dockal (1995a) pointed out radiocbon assays giving this range of apparent age may have baffected by an extreme enhancement of the cosmic ray and thus may represent an age closer to 60 ka BP. Withadditional and improved geochronologic work it is probabsafest to assume that the coquina and related strata at SCut and Fort Fisher were deposited before the most recglaciation, isotope stage 2, yet sometime after isotope stage5. This would best correspond to a slight warming period asea level high stand belonging to isotope stage 3 or 75 toka BP.

ENVIRONMENT OF DEPOSITION

The molluscan fauna suggest deposition under climaconditions similar to that found today along the southeastAtlantic seaboard between 34 and 36 degrees north latit(Figure 5). Fallaw (1973) considered the molluscan fauna othe Neuse Formation as a whole to indicate climate condi-tions similar to those of today but with slightly higher watetemperatures than present. Part of the evidence of this wasthe presence of Cardita floridana, Pyramidella crenulata,and Cantharus cancellarius. The present northern limit ofthe range of the former is Florida and the later two is South

Figure 5. Modern latitude ranges for the molluscan fauna fromthe Neuse Formation of Snows Cut, New Hanover County,North Carolina (34°× 3’14” N; 77°× 54’23” W).

15

James A. Dockal

nt

aytaw of

8)in a andave

iesre-ent

-wout

).

Figure 6. Diagrammatic stratigraphic profiles. I. corresponds to the Holocene or modern (Isotopes Stage 1) beach and slat marshsediments. III. (shaded) corresponds to the coquina and associated strata (Isotope Stage 3). V? corresponds to earlier strata (IsotopeStage 5 and older). All three profiles trend normal the present beach face. A-A’ is located at the top of the area covered by Figure 1.B-B’ passes through the center of Snows Cut. C-C’ passes directly through Fort Fisher. Surface profile taken from 7.5 minute topo-graphic maps; subsurface control is very minimal. Please note vertical exaggeration is approximately 100 times.

Carolina. All three of these are, however, absent from thecoquinas of New Hanover County and were only reported byFallow (1973) from the Neuse estuary exposures of theNeuse Formation. If these coquinas are truly coeval with theNeuse estuary exposures then the lack of these taxa could beexplained as a result of differing environmental niches. Onthe other hand if they are not coeval then that leaves open thepossibility that water temperature during deposition of thecoquina was not necessarily warmer than present.

The fauna, based upon the ecological ranges of theirmodern counterparts, represents a mixture of taxa from sev-eral environments: salt marsh, beach surf zone or intertidal,and shallow shelf (water depth less that 10 meters). The pres-ence of a coral which is effectively in growth position andarticulated sand dollars indicates shallow subtidal waterdepth and normal marine salinity (see Heckel, 1972). Fur-thermore the corals and to a lessor extent the barnacles implyclear water. The dominant foram, Quinqueloculina, is arobust thick wall form that would be expected to thrive in ahigh-energy environment (Fallaw 1973). The presence of the

clam Rangia and the numerous lithoclasts of both Piedmoand nearby sedimentary rock units suggests close proximityto a fluvial system. The terrestrial mammalian remains malso have been transported via a river or they could represenfauna living close to the sediment depositional site. Fall(1973) considered the environment of deposition to be onehigh energy possibly a shoal or tidal inlet. Moorefield (197suggested that "the coquina may have been deposited migrating Cape Fear River mouth, which is essentially"mega-inlet'." Dockal (1995b) envisioned the coquina ashell hash exposed on the north side of Snows Cut to hresulted from a single storm event where storm debris weredeposited above the level of high tide. None of these studhave taken into consideration the bigger picture. The boholes in the area indicate at least 1 p meters of sedimassociated with this package. Furthermore they suggest multiple depositional events of both a high energy and loenergy nature. These strata extend north to south for ab15 kilometers forming an arcuate shaped package (Figure 1In cross-sectional profile (Figure 6) they closely resemble

16

COQUINAS OF THE NEUSE FORMATION

sdylec-

the modern and ancient barrier systems in the region differ-ing only in their rather limited lateral extent and lack of asso-ciated back barrier salt marsh sediments. The interpretationof environment of deposition favored here is one of shore-face sediments deposited from the level of the subaeriallyexposed beach to below the level of low tide, where deposi-tion occurred within a largely sediment starved basin. As sealevel rose the shoreface environment shifted laterally land-ward by storm wave action eroding sediment from the foot ofthe shoreface and redepositing it at the top, on the subaerialbeach. The deposited material was a mixture of old reworkedsediment from previous beach face deposits and what fluvialsediment that had accumulated after the last high stand Theresultant fossil assemblage consisted of varying amounts ofin situ forms such as Donax variabilis, proximal indigenousforms such as Crassostrea virginica and Nassarius obsole-tus, distal indigenous forms like Aequipectin gibbus and Sid-erastrea radians, exotic forms like the terrestrial mammalianbones, and remanie forms reworked from older strata as evi-denced by the amino acid racemization results of Wehmillerand others (1995). The modern equivalent of this is the beachdeposits found just south of Fort Fisher. There the sedimenton the beach contains an abundance of modern and fossilshells, blocks of coquina, and debris that has recently arrivedfrom the Cape Fear River.

CONCLUSIONS

The term "Cape Fear Coquina" should be suppressedand in its place the name Neuse Formation should be usedboth for the coquina and the associated arenaceous shell hashand carbonate free sands. The coquinas represent a post dep-ositional diagenetic product of a shell bearing sand depositedin a shoreface environment. Initial diagenesis took place ator near an ancient water table where dissolution of aragoniticbioclasts gave rise to calcite spar cements and the formationof the coquina. The ferrugenous sands which were in the pastwere referred to as the "Kure Sand" also represent a productof the diagenesis of both the coquinas and the associatedunlithified sands which were the precursors to the coquina.Here. diagenesis took place during subaerial exposure whereoxygenated meteoric waters both dissolved calcium carbon-ate and oxidized what iron bearing heavy minerals werepresent, resulting in a ferrugenous stained insoluble residue.The exposed coquinas and ferrugenous residue sands repre-sent only the present surficial expression of a much thickersediment package of limited lateral extent. These weredeposited during a minor sea level high stand whichoccurred after isotope stage 5 but before stage 2, a period oftime dating from approximately 75 ka to 55 ka beforepresent or isotope stage 3.

ACKNOWLEDGMENTS

I am grateful to William J. Cleary and William B. Harrisfor their reviews of the manuscript, to Victor Zullo for hiintroduction to the coquina and its fauna, and to JoDuMond, who as a high school student assisted in the coltion and identification of the fossils from Snows Cut.

REFERENCESBlackwelder, B.W., 1981 Late Cenozoic stages and Molluscan

sones of the U.S. Middle Atlantic Coastal Plain. Journal ofPaleontology, v. 55, pt. II of II, supplement to no.5; Paleonto-logical Society, Memoir 12, 34 p.

Carter, J. G., Gallagher, P. E., Valone, R. E., and Rossbach, T. J.,1988, Fossil Collecting in North Carolina: North CarolinaDepartment of Environment, Health, and Natural Resources,Division of Land Resources, Geological Survey Section Bulle-tin 89.

Chappel, J. and Shackleton, N.J., 1986 Oxygen isotopes and sealevel. Nature v. 324, p. 137-140.

Dockal, J. A., 1992 Radiocarbon dating of late Pleistocene marinedeposits, New Hanover County, North Carolina. GeologicalSociety of America Abstracts With Programs, v. 24, No.2, p.12.

Dockal, J. A., 1995a Evaluation of an apparent Late Pleistocene(25-40 ka BP) sea level high stand: An artifact of a greatlyenhanced cosmic ray flux of -60 ka BP. Journal of CoastalResearch, v. 11, No.3, p. 623-636.

Dockal, J. A. 1995b Documentation and evaluation of radiocarbondates from the "Cape Fear Coquina" (Late Pleistocene) ofSnows Cut, New Hanover County, North Carolina. Southeast-ern Geology, v. 35, No.4, p. 169-186.

Du Bar, J.R., Johnson, H.S., Jr., Thorn, B, and Hatchell, W.O., 1974Neogene stratigraphy and morphology, south flank of the CapeFear Arch, North and South Carolina. In: Oaks, R.Q. and DuBar, J.R. (editors) Post-Miocene stratigraphy centra and south-ern Atlantic Coastal Plain. Utah State University Press, Logan,Utah, p. 139-173.

Du Bar, J.R. and Solliday, J.R. 1963 Stratigraphy of the Neogenedeposits, lower Neuse estuary, North Carolina. SoutheasternGeology, v. 4, p. 213-233.

Fallaw, W. 1973 Depositional environments of marine Pleistocenedeposits in southeastern North Carolina. Geological Society ofAmerica Bulletin, v. 84, no.1, p. 257-268

Fallaw, W. and Wheeler, W.H., 1969, Marine fossiliferous Pleis-tocene deposits in southeastern North Carolina. SoutheasternGeology, v. 10, no. 1, p. 35-54.

Folk, R. L., 1980, Petrology of Sedimentary Rocks: Hemphill Pub-lishing Co., Austin, Texas, 182 p.

Heckel, P.H. 1972. Recognition of Ancient Shallow Marine Envi-ronments. In: Rigby & Hamblin (editors) Recognition ofAncient Sedimentary Environments. SEPM Special PublicationNo.16, p. 226-286.

Mixon, R.B., 1986 Depositional environments and paleogeographyof the intergalcial Flanner Beach Formation, Cape Lookoutarea, North Carolina. Geological Society of America Centen-nial Field Guide-Southeastern Section, p. 315-320.

Mixon, R.B. and Pilkey, O.H., 1976 Reconnaissance geology of the

17

James A. Dockal

submerged and emerged Coastal Plain Province, Cape Lookoutarea, North Carolina. U.S. Geological Survey ProfessionalPaper 859, 45 p.

Moorefield, T P., 1978, Geologic processes and history of the FortFisher coastal area, North Carolina, Unpublished Masters the-sis, East Carolina University, Greenville NC, 100 p.

Owens, J. Po, 1989, Geologic map of the Cape Fear region, Flo-rence 10 X 20 Quadrangle and northern half of the Georgetown10 X 20 Quadrangle, North Carolina and South Carolina, U.S.Geological Survey, Miscellaneous Investigations Map 1-1948-A.

Prosser, J. F., 1993, Apparent uranium-series dates for mollusksfrom Snow's Cut, North Carolina: Implications for Late Pleis-tocene chronology, sea- level, and tectonics along the CoastalPlain of Southeastern North Carolina, Unpublished masters the-sis, University of North Carolina, Chapel Hill, NC. 44 p.

Richards, H. G., 1936, Fauna of the Pleistocene Pamlico Formationof the Southeastern Atlantic Coastal Plain. Bulletin GeologicalSociety of America, v. 47, p. 1611-1656.

Stephenson, L. W., 1912, The Quaternary formations, in Clark,W.B., Miller, B.L., Stephenson, L.W., Johnson, B.L., andParker, H.N., The Coastal Plain of North Carolina. North Caro-lina Geological and Economic Survey, v. 3, p. 266-290.

U.S. Army Corps of Engineers, 1982, Fort Fisher North CarolinaGeneral Design Memorandum Phase II Design Memorandum 2Project Design. U.S. Army Corps of Engineers, WilmingtonDistrict.

Wehmiller, J.F., Belknap, D.F., Boutin, B.S., Mirecki, J. E. Rahaim,S.D., and York, L.L., 1988 A review of the aminostratigraphyof Quaternary mollusks from the United States Atlantic CoastalPlain sites. Geological society of America Special Paper 227,p.69-110.

Wehmiller, J.F., York, L.L., and Bart, M.L., 1995 Amino acid race-mization geochronology of reworked Quaternary mollusks onU.S. Atlantic coast beaches: implications for chronostrati-graphic, taphonomy, and coastal sediment transport. MarineGeology, v. 124, p. 303-337.

Wells, B. W., 1944, Origin and development of the lower Cape FearPeninsula: Elisha Mitchell Science Society Journal, v. 60, no.2,129-134.

Zarra, L., 1991, Subsurface stratigraphic framework for Cenozoicstrata in Brunswick and New Hanover Counties, North Caro-lina, North Carolina Geological Survey Information Circular27.

Zullo, v. A. and Miller, W, Ill, 1986, Barnacles (Cirripedia: Bal-anidae) from the lower Pleistocene James City Formation,North Carolina coastal plain, with the description of anew spe-cies of Balanus Da Costa: Proceedings Biological Society ofWashington, v. 99(4), 717-730.

18

CAROLINA GEOLOGICAL SOCIETYGuidebook for 1996 Annual Meeting

Pages 19 - 27

rier

fis

t

geo-

astron-

ofand

astal,

vi-

-tifyons.

;dd to

ngg of

pro-

-

SHOREFACE PROCESSES IN ONSLOW BAY

E. Robert ThielerDuke University, Dept. of Geology

Program for the Study of Developed ShorelinesDurham, NC 27708-0228

THE SHOREFACE ENVIRONMENT

The shoreface is the interface between the continentalshelf and the subaerial coastal plain. The shoreface canbehave as a source, barrier, filter, or conduit for the bi-direc-tional exchange of materials between the land and the sea.The shoreface of barrier islands is the generally concaveupward surface extending from the surf zone to the pointwhere the slope becomes the same as the very gentle slope ofthe inner and central continental shelf. By this definition, thebase of the shoreface off southeastern North Carolina islocated at 10-12 m water depth (Figure 1).

Oceanographic and geologic processes in this environ-ment determine how a shoreline will respond to storms, tosea-level rise and to human-induced changes in sand supplyover time scales from hours to years to millennia. Under-standing shoreface processes is also critical to understandingthe behavior of replenished beaches, which provide manybeachfront communities with storm protection, recreationareas, and an important tourism resource. Sediment transportacross the shoreface is a key factor affecting 1) short- andlong-term fluctuations of beach and surf zone sand storage(Wright et al., 1985) ; 2) the morphology and stratigraphy ofthe shoreface (Niedoroda et al., 1985); and 3) the nature of

the inner shelf sand sheet (Swift, 1976). On retreating barisland coasts, the shoreface is also a major source of newsediment to the coastal system, via the erosion and release opreviously deposited lagoonal and fluvial sediments. Thprocess, termed "shoreface bypassing" by Swift (1976), is allthe more important on the southeastern U.S. Atlantic coasdue to the absence of a modern fluvial contribution. Curray(1969) suggested that the present is a unique moment in logic time with regard to shoreface evolution. Specifically,the relative stillstand in sea-level along most of the U.S. ECoast since about 4500 BP has allowed shoreface enviments to mature and steepen as they seek an equilibriumform. If true, this process would minimize the amount sediment available to beaches from the continental shelf, perhaps increase the rate of shoreface retreat.

The shoreface is one of the most complex and leunderstood coastal environments (Wright, 1987; Nummed1991). Geologists, oceanographers and engineers are onlyjust beginning to understand that nearly all shoreface enronments are different, where processes and controls vary inimportance both spatially and temporally (Niedoroda et al.,1985; Kraft et al., 1987; Wright et al., 1991). The shorefaceis also the interface that couples the beach to the shelf. Theory and empirical observations have done much to idenshoreface sediment transport rates under various conditiPresently, however, we can neither identify nor predict thenet transport of material on the shoreface (Wright, 1987Pilkey, 1993; Nittrouer and Wright, 1994). An applieunderstanding of shoreface processes is also neededesign coastal engineering projects, as well as to evaluateand improve models used in coastal engineering to predictthe behavior of beaches. On a millennial time scale, sedi-mentation on coastal plain shelves during a time of risisea-level such as the Holocene is driven by the bypassinsediment onto the shelf via the shoreface; fluvial sedimentsare trapped in the estuarine system. Swift (1976) describedthis process as "shoreface bypassing." This mechanism vides the primary source of new sediment to the shelf as theravinement surface bevels previously deposited coastal plainmaterial. Thus, shoreface bypassing regulates sediment sup

Figure 1. The shoreface is defined as the region between thesurf zone and the inner continental shelf. Off southeasternNorth Carolina, the base of the shoreface is located between 10-12 m water depth. (Modified after Wright et al., 1991).

19

E. Robert Thieler

re-

r-

ntseo-eirrlinfacey)

re-

on6).

ofreent)

ed

re-edi-

rned

ply, and in effect the rate and character of shelf sedimenta-tion.

The shoreface maturation process postulated by Curray(1969) seems to have peaked in Onslow Bay. The regressivebarrier islands (e.g., Shackleford Banks, Bogue Banks andBear Islands) are no longer prograding seaward; they appearto have accumulated all of the available inner shelf sand.This process of "inner shelf sweeping up" appears to havebeen very efficient; much of the inner shelf shows little or noevidence of modem sedimentation (Hine and Snyder, 1985).The transgressive islands (e.g., Topsail Island, WrightsvilleBeach and Masonboro Island) exhibit much the same shore-face characteristics, although they have substantially lesssediment volume. Nearly all of the sediment in the TopsailIsland barrier system, for example, is contained within thebody of the island landward of the beach; the shoreface sedi-ment volume is very small.

Over shorter time frames (e.g., individual storm eventsto several weeks), a variety of processes operate on theshoreface and inner shelf (Figure 2). These processes createhigh bottom stresses that mobilize sediment. As described byGrant and Madsen (1979), the bed agitation required for sed-iment resuspension is furnished primarily by gravity waves,but sediment exchange is accomplished by quasi- steady-state mean flows. It is generally recognized that along-shelfflows predominate over across-shelf flows, but that across-shelf transport gradients are relatively high (Nummedal,

1991; Nittrouer and Wright, 1994). Several types of shoface and inner shelf currents have been recognized, includ-ing: 1) storm- driven pressure gradient currents (e.g. wind-induced upwelling and downwelling currents); 2) tidal curents; 3) storm surge ebb currents; and 4) turbidity currents.Ekman (1905) predicted the presence of inner shelf curreand Sverdrup, Johnson and Fleming (1942) noted the thretical basis for currents related to pressure fields in thclassic textbook. Shi and Larsen (1984) and Dean and Pe(1986) suggested that cross-shore transport on the shorecould also be affected by forced long-period (infragravitwaves associated with groupy incident waves.

Our contemporary understanding of short-term shoface and inner shelf processes is derived from both sedimenttransport modeling and field measurements. Early work bottom boundary layer models was done by Jonsson {196Bijker {1967), and Sternberg {1972). Today, a number different models {see review by Dyer and Soulsby. 1988) aused to examine boundary layer structure and sedimtransport. In the past few years the Grant and Madsen {1979and Glenn and Grant {1987) models for combined wave andcurrent flow have come into wide use {Cacchione et al.,1994; Nittrouer and Wright. 1994). These models couplwith field measurements of the bottom boundary layer, haveidentified a number of important factors that influence shoface sedimentary processes. The rates and directions of sment transport on the shoreface and inner shelf are gove

20

Figure 2. Schematic diagram of the major interacting components of the shallow-water bottom boundary layer. (Modified afterWright et al., 1989).

SHOREFACE PROCESSES IN ONSLOW BAY

d )

y,

- the

p-

di-e-h (ats

do-m of

vior

]),

srch-

s-ster

rast

leat,m- its-eo-

th)hore-thun

primarily by wave-current interactions {Madsen and Grant,1976); the micromorphology (ripple geometry, biologicroughness. etc.) and sedimentary characteristics of the sea-bed {Nielsen, 1979; Wright, 1993); and the geologic frame-work of the shoreface {Pilkey et al. 1993; Riggs et al..1995). Figure 2 illustrates the combined effects of these fac-tors in determining the nature of the bottom boundary layerand the resultant bed stresses and sediment transport.

A number of field studies have documented that signifi-cant suspended and bed load transport occurs frequently onthe shoreface and inner shelf. These include Sternberg andLarsen {1975); Gadd et al. {1978); Lavelle et al. {1978);Cacchione and Drake {1982); Vincent et al. {1982); Wibergand Smith {1983); Cacchione et al. {1987; 1994); andWright et al.. {1986; 1991; 1994) among many others. Thesestudies, however, capture only a brief moment in the large-scale evolution of the shoreface. A decade-scale view ofshoreface processes and evolution is currently lacking. Thisis particularly true in engineering studies of coastal pro-cesses, which typically require a decade- scale understand-ing of shoreface evolution.

Engineering models used to predict shoreline evolutionand to design replenished beaches usually assume I that theshoreface has an equilibrium shape related I to wave climateand surficial sediment grain size: {Dean. 1977; Zeidler.1982). As applied to the design of replenished beaches, theprofile of equilibrium is considered to be the stable configu-ration that a beach will try to achieve under the influence ofincident waves {Dean, 1983). Maintenance of the profile ofequilibrium during shoreline retreat is also central to the con-cept of Bruun Rule response to sea-level rise (Bruun, 1962).The equilibrium profile equation was first proposed byBruun (1954) for the Danish North Sea coast, and has theform

h = Ayn (1)where h is water depth, y is the distance offshore from

the shoreline, n is a variable shape parameter and A is a scal-ing parameter. Bruun (1962) used this equation to develop asimple model for coastal evolution, in which the shorefaceprofile responds to sea-level rise by moving landward andupward such that the profile shape remains constant down toa depth of no wave influence (beyond which little sedimentis supposedly transported). This simple relationship was oneof the first models of shoreface transgression, preceding themore "classic" geologic conceptualizations of Curray (1969)and Swift (1976).

The Bruun Rule was, and still is, a good concept. It isnot a good quantitative model. The concept, as originallyconceived by Bruun, provided a strong conceptual basis forfurther thought about the nature of shoreface evolution. Sub-sequent work, however, sought to verify its basic principles.For example, Dean (1977) used a least squares approach tofit the data of Hayden et al. (1975) to an equation of the formshown in (1), where n=0.67. Infixing the value of n, Dean

(1977) left the sediment scaling parameter, A, as the onlyindependent variable in the equation. Dean (1987) relateAto sediment fall velocity by transforming Moore's (1982sediment grain size data to the equation

A = 0.067 wO.44 (2)where w is the sediment fall velocity in cm S-1. Essentiallthis relationship implies that any shoreface profile can bedescribed solely on the basis of the grain size present.

The profile of equilibrium concept makes several fundamental assumptions about the nature of the shoreface andprocesses acting on it (Dean, 1977; 1991; cf. Pilkey et al.,1993). Pilkey et al. (1993) argued that several basic assumtions of the shoreface profile of equilibrium concept are notmet in most field settings. The assumptions include: 1) sement movement is driven solely by diffusion due to wavenergy gradients across the shoreface; 2) closure deptseaward limit of significant net sediment movement) exisand can be quantified; 3) the shoreface is sand-rich, anunderlying geologic framework does not influence the prfile shape; and 4) the profile described by the equilibriuprofile equation (Dean, 1977) provides an approximationthe real shoreface shape useful for coastal engineeringprojects.

The shoreface profile of equilibrium is a fundamentalprinciple behind most analytical and numerical models ofshoreline change used to predict large-scale coastal beha(e.g., Hanson and Kraus, 1989 [the GENESIS model]) and todesign replenished beaches (e.g., Hansen and Lillycrop,1988; Larson and Kraus, 1989 [the SBEACH modelincluding those used on beaches in Onslow Bay. There hasbeen no systematic field verification of the physical basis forthe equilibrium profile equation (Kraft et al., 1987; Wright etal., 1991; Pilkey et al., 1993). The concept, however, habeen accepted as valid and useful by many coastal reseaers, and is used to predict coastal evolution in a variety ofcoastal settings (e.g., Rosen, 1978).

The Bruun Rule effectively states that shoreface slope ithe only control of shoreline retreat and that for a given sealevel rise, beaches with gentle shorefaces will recede fathan those with steep shorefaces. In typical applications,retreat rates are based on the slope of the shoreface rathethan the slope of the migration surface. As a result, on ECoast shorefaces the Bruun Rule usually predicts a sea-leverise to shoreline retreat ratio of 1: 200. However, the retractually occurs across the surface of the lower coastal plainthe slope of which in southeastern North Carolina, for exaple, averages about 1: 2000. The Rule is also flawed inassumptions concerning areal restriction of sediment movement on shorefaces, and in its lack of consideration for glogic control of shoreface slope. In actual use, theassumption of the depth of no wave motion (closure dephas decreased to between 4 and 8 m on East Coast sfaces, in contrast to Bruun's original 18 to 20 m depassumption. There is no basis in reality for using the Bru

21

E. Robert Thieler

ac-ride

end-sl-

arend.

hermal the

utour

our

nge

nete

ied

-

chner

eys byly asstsnta-

leftvestics

ur-allf

Rule, as it is currently being used,