Updip sequence development on a wave- and current-dominated, mixed carbonate-siliciclastic...

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Sedimentary Geology 18

Updip sequence development on a wave- and current-dominated,

mixed carbonate-siliciclastic continental shelf: Paleogene,

North Carolina, eastern U.S.A.

Jenny LaGesse, J. Fred Read *

Virginia Polytechnic Institute and State University, United States

Received 20 July 2004; received in revised form 30 September 2005; accepted 11 October 2005

Abstract

Cores, quarry exposures, and exploratory wells in the Paleogene sediments of the Albemarle Basin, North Carolina, span the

transition between the southern carbonate and northern siliciclastic provinces of the eastern U.S. continental shelf succession. This

swell-wave- and current-dominated shelf succession differs from the standard carbonate sequence-stratigraphic model because

available accommodation is limited by the depth of wave abrasion and scour. The study area is on the thin updip part of the shelf,

which includes the relatively positive Cape Fear Arch (Onslow Block) and the slowly subsiding (1.5 cm/ky) Albemarle Block to

the north.

The Paleogene supersequence set has a basal boundary that is a hardground on Cretaceous shoreface/shallow-shelf mollusc

facies. It is overlain by a thin Paleocene sequence of transgressive, deeper offshore, glauconitic fine sands to deep marine silt–shale.

Five regionally mappable, vertically stacked Eocene sequences (0–30 m thick), contain coastal sands, shoreface sandy-mollusc

rudstone, offshore bryozoan grainstone–packstone and subwave base fine wackestone–packstone and marl. Eocene sequences

commonly are bounded by hardgrounds overlain by thin local lowstand shallow-marine sands; they consist of a thin transgressive,

shallow-marine unit (commonly absent), overlain by an upward-shallowing highstand marine succession. On the arch, lowstand

and transgressive units may be condensed into lags. The lower Oligocene succession on the arch has a single marl to fine foram

sand-dominated sequence whereas downdip, two to three siliciclastic-rich sequences are developed, capped by nearshore sandy

molluscan facies. The upper Oligocene section is dominated by possibly three sequences composed of basal, thin sands grading up

into variably sandy mollusc rudstone.

Sequence development was influenced by differential movement of basement blocks (which influenced localization of fines in

lows), coupled with increasing third-order eustatic sea-level changes during later Paleogene global cooling. This was coupled with

swell-wave and current sweeping of the shelf that effectively decreased available accommodation by 20–30 m and generated the

distinctive hardgrounds at sequence boundaries, and variable development of lowstand and transgressive system tracts. The well-

developed highstand deposits reflect maximum accommodation allowing deposition of an upward-shallowing succession that

terminated at the depth of wave abrasion and current scour on the open shelf. The sequence-stratigraphic development contrasts

markedly with that from tropical shelves, which tend to be aggraded to sea level.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Paleogene; Sequence development; Nontropical wave-dominated; North Carolina

0037-0738/$ - s

doi:10.1016/j.se

* Correspondi

E-mail addr

4 (2006) 155–182

ee front matter D 2005 Elsevier B.V. All rights reserved.

dgeo.2005.10.004

ng author.

ess: [email protected] (J.F. Read).

J. LaGesse, J.F. Read / Sedimentary Geology 184 (2006) 155–182156

1. Introduction

Existing carbonate sequence-stratigraphic models

have mainly been based on tropical ramps and rimmed

shelves. However, more recently, attempts have been

made to define sequence-stratigraphic models for non-

tropical successions (cf. James, 1997). A major differ-

ence between tropical and temperate settings is the lack

of a reefal rim to the shelf, which is open to wave and

boundary current sweeping in these typically swell-

wave-dominated settings. Such open shelves are rarely

aggraded to sea level because of low sedimentation

rates and the depth of wave abrasion, which can extend

down to water depths of 20–60 m or more (Collins,

1988; James, 1997; Pekar and Kominz, 2001). This

limits available accommodation, which is essentially

the space between the sea floor and the depth of

wave abrasion, rather than sea level (Fig. 1; Osleger,

1991). Once the shelf reaches the wave abrasion zone,

it becomes a bypass surface (Pekar and Kominz, 2001),

which prevents deposition of thick units of shallow-

water facies on the shelf and inhibits shallowing to sea

level.

The Paleogene succession of North Carolina formed

in such a non-tropical open-shelf setting (Coffey and

Read, 2004). In order to understand sequence develop-

ment in this setting, three cores from the relatively thin,

updip part of the Paleogene of North Carolina (Figs. 2

and 3) were logged, and the facies were studied. The

cores were tied into quarry sections and previously

studied wells with sequence picks and age control

(Harris et al., 1986; Coffey and Read, 2004) to provide

high-resolution sequence-stratigraphic cross sections.

The study shows the profound influence that the

open-shelf setting, with its wave sweeping and bound-

ary current scour, had on sequence development, which

Fig. 1. Schematic diagram showing open-shelf wave climate. Accommodatio

30–60 m or more. The wave abrasion zone prevents sediment deposition o

also was influenced by significant third-order glacio-

eustatic sea-level changes from the Paleogene green-

house to icehouse transition, and differential movement

of basement blocks of the basin.

2. Methods

Three cores housed at the North Carolina Geolog-

ical Survey were used in this study, including the

Beaufort County core (BF-C-1-68), the Onslow Coun-

ty core (ON-C-1-94), and the USGS Kure Beach core

drilled during the summer of 2001. The cores were

logged to document colour, quartz grain size, compo-

sition (sand, shale and limestone), Dunham rock

groups, biotic composition, and sedimentary structures.

Representative samples of the lithologies were studied

using plastic-impregnated thin sections stained with

Dickson’s (1965) solution. Cores were incorporated

into regional cross sections based on cores logged by

Harris et al. (2000), published quarry data (Harris et

al., 1986), and in downdip areas, several oil and gas

exploratory wells that had been subjected to thin sec-

tioned cuttings analysis by Coffey (2000), as part of

his regional study. Published biostratigraphic and Sr

isotope data provided age control. Sequence bound-

aries, flooding surfaces, and systems tracts were picked

on the cross sections and traced to generate a regional

high-resolution framework.

3. Structural setting and shelf morphology

The study area is in the Albemarle Basin of coastal

North Carolina on the southern part of the Onslow

Block and the northern part of the Albemarle Block

(Fig. 2). The Onslow Block is bounded on the south-

west by the Cape Fear Arch, to the north by the Neuse

n is the space from the surface of the shelf to the wave abrasion depth,

n the shallow shelf. Modified from James (1997).

Fig. 2. Regional base map showing major structures, Paleogene isopachs (in meters) and location of core, quarry and well sections used. Location of

cross sections AAV, BBV and CCV is shown. Modified from Coffey (2000).

J. LaGesse, J.F. Read / Sedimentary Geology 184 (2006) 155–182 157

Hinge, which separates it from the Albemarle Block,

bounded to the north by the Norfolk Arch. Other Ce-

nozoic faults, such as those in the Grainger Wrench

Zone, include reverse–, wrench–, and strike–slip faults

that may have been caused by compressional stress

fields (Gardener, 1989; Gohn, 1988; McLaurin and

Harris, 2001).

Accommodation rates for the passive margin in

North Carolina are estimated at less than 1.7 cm/ky

(Coffey and Read, 2002). This was related to thermal

subsidence after Jurassic continental rifting, as well as

subsidence due to sediment loading in offshore basins

(Steckler and Watts, 1978; Popenoe, 1985).

Throughout much of the Cenozoic, the North Car-

olina shelf had a distinct depositional profile. This

consisted of an inner shelf, inner shelf break, deep

shelf (ancestral Blake Plateau), and continental slope

(Coffey and Read, 2004), a profile that also character-

ized the New Jersey margin (Miller et al., 1997).

4. Stratigraphic framework

The stratigraphic framework of the North Carolina

Paleogene (Fig. 3) has been documented by Thayer and

Textoris (1972), Baum et al. (1978), Ward et al. (1978),

Otte (1981), Popenoe (1985), Zullo and Harris (1987)

and Coffey (2000). The Paleogene sediments in the

Albemarle Basin are from 0 m to over 500 m thick,

thickening offshore (Figs. 2–4). The Paleocene Beau-

fort Formation disconformably overlies Upper Creta-

ceous units, the contact commonly having thick,

phosphatized hardgrounds (Baum et al., 1978). The

lower Paleocene (Danian) sequence consists of fine

quartz sands (Yaupon Beach Member) and a succession

of siliceous mudstones (Jericho Run Member), and an

upper Paleocene (Thanetian) sequence of glauconitic

foraminiferal sands and skeletal grainstones (Mosley

Creek Member; Harris and Baum, 1977; Harris and

Laws, 1994).

Fig. 3. Regional stratigraphic framework for the Paleogene of the North Carolina coastal plain (modified from Coffey, 2000). Paleogene global and

regional eustatic curves of Haq et al. (1988) with Oligocene curve of Kominz and Pekar (2001) are included. Biostratigraphic zonations and

radiometric time scale are from Berggren et al. (1995).


Eocene sediments disconformably overlie Paleo-

cene units or Cretaceous strata (Fig. 4) and are bryo-

zoan-echinoderm limestones (Castle Hayne Limestone;

middle Eocene, Lutetian and Bartonian; Ward et al.,

Fig. 4. Cartoon showing the generalized supersequence stratigraphy of the P

this paper is located in the thin updip portion of the northwest–southeast c

Lowstand wedges shown by stippled pattern at foot of inner shelf break. D

1978; Harris et al., 1993). The overlying mollusc-rich

Eocene unit (mapped as New Bern Formation) is early

late Eocene (Priabonian, Baum et al., 1978) or latest

middle Eocene (Ward et al., 1978). Five Eocene de-

aleogene shelf, North Carolina (scale approximate). The study area for

ross section, as well as away from the cross section along the arch.

rawn from data in Coffey and Read (2004).

Table 1b

Water depths of inner, middle, mid-outer and outer neritic shel

environments are compiled from similar environments of the New

Jersey Oligocene, modern Australian shelf and North Carolina Eocene

Inner

neritic

Middle

neritic

Mid-outer

neritic

Outer

neritic

Jones, 1983 0–15 m 15–50 m – 50–100 m

Browning et al., 1997b 0–30 m 30–100 m – 100–200 m

Boreen et al., 1993 30–130 m 130–180 m

Pekar and

Kominz, 2001

0–40 m 10–110 m 30–130 m 70–145 m

James et al.,

2001

0–50 m 50–120 m – 120–160 m

Collins, 1988 0–60 m 60–100 m – 100–170 m


positional sequences were recognized by Zullo and

Harris (1987).

The lower Oligocene succession (Fig. 4) is mapped

as (Rupelian) Lower River Bend Formation and consists

of marl, fine sand, and sandy molluscan limestone. The

upper Oligocene (Chattian), upper River Bend Forma-

tion (Fig. 4) consists of silty–sandy molluscan lime-

stones (Ward et al., 1978; Harris et al., 2000). A single

depositional sequence was recognized in the lower Ol-

igocene succession and three sequences noted in the

upper Oligocene succession (Zullo and Harris, 1987).

The Oligocene units are unconformably overlain by

lower Miocene to Pliocene units (Baum et al., 1978).

Coffey and Read (2004), using thin-sectioned well

cuttings from 23 onshore wells tied into the offshore

seismic, provided a regional supersequence scale

framework (Fig. 4). However, in view of the limited

age control, third-order sequences although evident on

the cross sections were not tied to those defined in the

outcrop belt by Zullo and Harris (1987) and Harris and

Laws (1997).

5. Facies and depositional environments

The Paleogene facies and water depths summarized

in Tables 1a, 1b and 2 are modified from Coffey and

Read (2004) and supplemented by information from the

present study. The schematic facies profile (modified

from Coffey and Read, 2004) is shown in Fig. 5A and

water depths of facies are shown in Fig. 5B. The

estimated water depths of the facies were compiled

from the literature using modern or Tertiary analogues

for which the paleo-bathymetry had been determined.

The terms inner, middle and outer neritic are used here

loosely, because of the wide variation in the published

depth limits of these terms. Inner neritic maximum

depths are as little as 15 m (Jones, 1983) to as much

Table 1a

Estimated water depths for North Carolina Paleogene facies

Facies Location Depthsa Compiled depth

Quartz sand Inner neritic 0–15 m(2), b30 m(8), 10–30 m(1) 0–30 m

Fine glauconitic sand Inner neritic 20–50 m(8), 20–125 m(4), 25–75 m(9) 20–125 m

Sandy mollusk rudstone/grainstone/packstone Inner to middle neritic 30–85 m(9), 30–110 m(5,8), 35–50 m(6) 30–110 m

Silt–shale Middle to outer neritic 45–90 m(8), 60–185 m(4) 45–185 m

Mixed-skeletal grainstone/packstone Middle to outer neritic 30–90 m(5), 80–125(7), 120–200 m(6) 30–200 m

Fine foram sand Middle to outer neritic 30–70 m(8), 120–200 m(6), 145–185 m(1) 30–200 m

Bryozoan–echinoid grainstone/packstone Middle to outer neritic 60–210(7), 80–140 m(5), 90–170 m(3) 60–210 m

Skeletal fragment packstone/wackestone Outer neritic 80–200 m(7), 150–200 m(6), 170–230 m(3) 80–230 m

Marl Outer neritic to slope 200 to N300 m(6), 210 to N300 m(7) 200 to N360 m

Depths are compiled from similar environments of the New Jersey Oligocene and the modern Australian shelf.a From (1)Rao and Amini (1995), (2)Jones (1983), (3)Collins (1988), (4)Browning et al. (1997b), (5)James et al. (1997), (6)James et al. (1999)

(7)James et al. (2001), (8)Pekar and Kominz (2001), and (9)Pekar et al. (2003).

f

as 60 m (Collins, 1988), with an average maximum

depth of 40 m. Middle neritic maximum depths are as

little as 50 m (Jones, 1983) to as much as 130 m

(Boreen et al., 1993), with an average maximum of

120 m. Outer neritic maximum depths are as little as

100–200 m or more, with an average maximum depth

of 180 m. Distribution of the facies in the cross sections

and cores is shown in Figs. 6–10.

6. Sequence stratigraphy

6.1. Regional sequence framework

Five Paleogene supersequences occur in the North

Carolina Albemarle Basin and beneath the adjacent

continental shelf (Harris and Laws, 1997; Coffey and

Read, 2004). Supersequence 1 of Paleocene age is thin

to absent on the updip Onslow Block, but thickens

downdip and offshore to over 100 m. It is dominated

by deep-water marls. Supersequence 2 (early Eocene) is

absent in the study area. Supersequence 3 (middle

Eocene) is the most widespread unit. It is up to 200

m thick downdip and dominated by bryozoan lime-

stone, becoming more quartzose upward. These bryo-

,

Table 2

Mixed carbonate–siliciclastic facies descriptions and interpreted environments

Siliciclastic-prone facies Carbonate facies

Quartz sands Glauconitic sands Fine-grained foram

quartz sands

Silt–shale Sandy whole

mollusk rudstone/

grainstone/

packstone

Sandy, mollusk-

fragment

grainstone/

packstone and

sand-lean mollusk

fragment

grainstone/

packstone

Mixed skeletal

fragment

grainstone/

packstone

Bryozoan–echinoid

grainstone/

packstone

Fine skeletal

fragment packstone

wackestone/

mudstone

Marl/lime

wackestone to

mudstone

Hardgrounds

Stratigraphic

occurrence

Units 0.3 m or less;

may be reworked as

lithoclasts into

overlying limestone

units; occur at bases

of upward-

deepening and tops

of upward-

shallowing

successions

Units up to 2 m

thick in Paleocene,

Kure Beach core

10–30 m in thick in

Oligocene

Units up to 7 m

thick in Paleocene,

Kure Beach core;

overlie glauconitic

sand within

upward-deepening

unit of Beaufort

Formation

0.3–3 m thick units

in Cretaceous to

Oligocene;

associated with

quartz sand and

sandy mollusk

fragment

grainstone/

packstone

Units 1.5–3 m thick

in Cretaceous to

Oligocene;

commonly

interbedded with

sandy whole

mollusk beds

Units 0.3–6 m

thick; common to

dominant facies in

the middle Eocene

Units 0.3–4.6 m

thick, in Eocene;

interbedded with

mollusk fragment

and mixed skeletal

fragment

grainstone/

packstone and fine

skeletal fragment

wackestone/

mudstone

Units 0.3–3 m thick

in Eocene;

interbedded with

bryozoan–echinoid

beds

Units 0.3–10.7 m

thick in Eocene and

Oligocene

Glauconitic/

phosphatic bored

surfaces capping

carbonate and

carbonate-cemented

quartz sands

Color Gray-tan Brown-gray Gray-tan Brown-black White-gray Gray, tan-gray Gray, tan-gray Gray/tan-gray Gray, brown-gray Light gray, gray-tan Gray-black, green

Bedding/

sedimentary

structures

Structureless Structureless and

massively bedded,

rarely flaser bedded

Abundant burrows,

common mud

layers in sands

Massively bedded,

some burrows and

flaser bedding

Structureless to

faintly bedded

Massively bedded

or with rare

crossbeds; geopetal

fills in leached

moldic pores,

commonly capped

by hardgrounds

Massively bedded,

commonly capped

by hardgrounds

with bored surfaces

Massively bedded

with rare mud

layers

Massively bedded,

burrowed

Heavily burrowed Surfaces have

centimeter-size

borings and

phosphatic/

glauconitic crusts

J.LaGesse,

J.F.Read/Sedimentary

Geology184(2006)155–182

160

Depositional

texture and

constituents

Clean sands

composed of poorly

sorted subangular to

rounded fine to

medium and less

common coarse

quartz sand with

rare very coarse

skeletal fragments;

rare to common

very fine to medium

glauconite and

phosphate

Muddy sands

composed of well-

sorted subrounded

very fine to fine

quartz sand, rare

skeletal fragments,

very fine to

granule-sized

glauconite and fine

to medium

phosphate

Muddy fine sands

composed of

well-sorted very

fine to medium

quartz sand,

abundant benthic

and planktic

forams, rare to

common ostracods

and indeterminate

skeletal fragments,

rare delicate

bryozoans, and rare

very fine glauconite

and phosphate;

some interstitial

carbonate and clay

mud matrix

Well-sorted silt–

shale composed of

abundant silt to

very fine to fine

subrounded quartz

sand, rare to

common fine to

very fine sand size

mica, glauconite

and phosphate

Mud-lean to mud-

rich poorly sorted

rudstone,

grainstone, and

packstone

composed of

abundant to

common gravel-

sized whole

mollusks, common

to abundant

subangular fine to

coarse quartz,

common to rare

benthic forams, and

rare oyster and

pectin

Poorly sorted

grainstones and

muddy packstones

composed of

abundant to

common coarse-

grained, fragmented

mollusk, pectin,

oyster and

gastropod, common

silt–sand-sized

benthic forams,

common fine to

coarse subangular

to rounded quartz,

rare ostracods,

bryozoans,

echinoids, and

barnacles; rare

carbonate and

sandstone

lithoclasts, rare fine

to medium

glauconite and fine

phosphate

Mud-lean to mud-

rich, poorly sorted

grainstone and

packstone

composed of whole

and fragmented,

common mollusks,

bryozoans,

echinoids,

gastropods,

ostracods,

barnacles, pectins,

forams, rare to

common

crustaceans, rare

brachiopods and

oysters, and rare to

common very fine

to coarse,

subrounded quartz;

rare sandstone

lithoclasts and

clasts of

hardgrounds; rare

very fine to medium

glauconite and

phosphate

Mud-lean to mud-

rich poorly sorted

grainstone and

packstone

composed of

abundant to

common whole and

fragmented

bryozoans,

echinoids, forams,

common ostracods,

rare to common

crustaceans, rare

brachiopods,

mollusks and

oysters, rare very

fine to coarse

angular to rounded

quartz, rare

limestone and

phosphate

lithoclasts, and very

fine to fine

glauconite and

phosphate and a

lime mud matrix

Fine-grained

packstone,

wackestone, and

mudstone

composed of

common to

abundant

indeterminate

skeletal fragments,

common to

abundant bryozoan

and echinoid

fragments and

forams, rare

oysters, pectins and

barnacles, rare to

common very fine

to medium

subrounded quartz.

rare fine to medium

glauconite and

phosphate, rare

sandstone

lithoclasts and

phosphate clasts,

and a lime mud

matrix

Fine mud-rich

wackestone/lime

mudstone with

common to

abundant sand–silt-

size benthic and

planktic forams,

common to rare

bryozoan and

indeterminate

skeletal fragments,

common silt, to

very fine to medium

quartz sand; rare

clasts of phosphate

and lime mudstone,

rare very fine to

fine glauconite and

phosphate grains,

and common to rare

chert nodules

Multiply indurated

irregular to planar

surfaces, commonly

encrusted by

benthic forams and

bryozoans;

common fine to

medium glauconite

and phosphate;

cement commonly

cryptocrystalline

carbonate;

developed on

carbonates and

sandstones;

overlying beds may

contain reworked

hardground clasts

Marine cement Marine, bladed high

Mg-calcite cement

Marine, bladed high

Mg calcite cement

High Mg calcite

bladed cement

Some bladed high

Mg calcite cement

Interpreted

environment

Inner neritic

0–40 m; high-

energy, wave

reworked

Inner to outer

neritic 20–125 m;

glauconite formed

in quiet, deeper

water settings; may

be reworked into

shallow-water

facies during

transgression

Middle to outer

neritic 30–70 m in

New Jersey

Oligocene; 120–

200 in S. Australia

Middle to outer

neritic 45–185 m;

low to moderate

energy; fines

carried onto shelf

by muddy plumes

sourced from rivers

in flood, and

transported by

longshore drift

Inner to middle

neritic 30–110 m;

high to moderate

wave energy; may

have local

macroalgal or grass

cover to form

packstone

Inner to middle

neritic 30–110 m;

high to moderate

wave energy; mud-

free carbonates

beneath mobile

substrates;

packstones may be

associated with

macroalgal/grass

baffles

Middle to outer

neritic 30–210 m;

wave-swept sandy

and hardground

substrates;

lilthoclasts

reworked by wave

sweeping and

current incision

Middle to outer

neritic 60–210 m on

southern and

western Australian

shelves; rippled

sandy and rocky

substrates,

moderate energy

Outer neritic 120–

200 m; 100–150 m

on New jersey

Oligocene shelf;

80–230 m on

Australian shelves;

low energy; much

mud from updip

Outer neritic slope

200 to N360 m;

100–150 m on New

Jersey shelf; N200

m on Australian

shelf; low energy;

much mud

winnowed from

updip

Zone of wave

sweeping 40–60 m,

and Gulf stream

abrasion; middle

neritic to slope;

associated with

sediment bypass

surfaces and

swell-wave

sweeping

J.LaGesse,


Geology184(2006)155–182

161

Fig. 5. (A) Generalized carbonate facies distribution across the Paleogene shelf (modified from Coffey, 2000). (B) Compiled water depth of facies

(compiled from Rao and Amini, 1995; Jones, 1983; Collins, 1988; Browning et al., 1997b; James et al., 1997, 1999; Pekar and Kominz, 2001;

Pekar et al., 2003).


zoan limestones thicken into a major buildup on the

seaward edge of the Eocene inner shelf beneath Cape

Hatteras. Lower Oligocene Supersequence 4 is up to 50

m thick offshore, and has a thin basal marl, overlain by

silty, fine quartz foram sands and laterally equivalent

molluscan units. Upper Oligocene Supersequence 5 is

up to 50 m thick onshore and is dominated by mollus-

can facies; it thickens markedly offshore.

Fig. 6. Interpretive dip cross section A–A’ of Paleocene to Eocene succession. Section line runs parallel to Cape Fear Arch along Onslow Block (Fig. 1). Sections are thin and condensed and

thickness changes may reflect underlying structure. Nannoplankton dates from Worsley and Laws (1986). Published quarry data from Zullo and Harris (1987) and Harris et al. (1986).

J.LaGesse,


Geology184(2006)155–182

163

Fig. 7. Interpretive strike section B–B’ of Paleocene–Eocene sediments. Section location shown in Fig. 1. Lowstand and transgressive units are poorly developed, and highstand units dominate the

sequences. Hardgrounds are associated with both sequence boundaries and maximum flooding surfaces. Nannoplankton date from Bralower (personal communication, 2000). Well data sections 11,

12, 16, and 18 from Coffey (2000).

J.LaGesse,


Geology184(2006)155–182

164

Fig. 8. Lithologic log of USGS Kure Beach core. The basal sequence boundary on the Cretaceous Rocky Point is an unconformity/hardground.

Other sequence boundaries are marine hardgrounds; lowstand sands are thin (E3a) and may be reworked into pebble lags. Transgressive tracts are

present in sequences P1 and E3a, but in others are phosphatic lags. Hardgrounds mark some maximum flooding surfaces. Highstands are well-

developed upward-shallowing units. Age picks (stars) from L. Edwards (personal communication) and Harris and Laws (1997) (2).


6.2. Supersequence 1 (Paleocene)

The basal boundary of Supersequence 1 in the Kure

Beach core on the Cape Fear Arch is a phosphatic

hardground overlain by a thin (1 m) sandy mollusc

limestone (section 4, Fig. 6B). The sequence boundary

is also inferred to be present as a correlative conformity

in section 12 (Fig. 7), where it is overlain by lowstand

quartz sand to sandy mollusc limestone.

The Paleocene transgressive systems tract (Fig. 8)

deepens upward from fine glauconitic sand to the basal

1 m of a burrowed silt–shale in which the sand content

Fig. 9. Lithologic log of Onslow County core. Middle Eocene section contains at least four sequences, and two sequences occur in the Oligocene. Lowstand units are absent or very thin (e.g., thin

sand at base of E3a). Transgressive units occur in E1, E2, as a single parasequence in E3b, or as thin phosphatic lag (E3a and 01). Sequences are dominated by thick highstand units. In this downdip

core, hardgrounds are coincident with sequence boundaries or maximum flooding surfaces. However, some sequence boundaries are relatively conformable (E2, E3a).

J.LaGesse,


Geology184(2006)155–182

166

Fig. 10. Lithologic log of Beaufort County core. The middle Eocene section is composed of at least five depositional sequences. Lowstand deposits

are absent from Sequence E2 but occur as thin sands in E3a and b. In other sequences, they may have been reworked into pebble lags. Transgressive

units are relatively thin, while highstand deposits are thick. Parasequences may be present in Sequence E3b.


decreases upward. The maximum flooding surface is

picked at the base of the overlying laminated silt–shale.

Downdip, the fine glauconitic sand thickens to 10 m in

a local lobe (sections 1–4, Fig. 6).

The highstand systems tract in the Kure Beach core

consists of 8 m of dark gray laminated silt–shales that

grade up into a burrowed and cross-laminated silt–shale

in which the sand content increases upwards (Fig. 8).

Downdip, the succession grades into fine skeletal wack-

estone–packstone (sections 1 to 4, Fig. 6). On the

Albemarle Block, possible Paleocene highstand facies

include 7 m of fine skeletal wackestone–packstone

(section 16, Fig. 7).

6.2.1. Age of Paleocene sequences

Regionally, two Paleocene sequences are developed,

one lower Paleocene and the other upper Paleocene.

However, based on biostratigraphic ages in the study


area, only the lower sequence is developed (Lucy

Edwards, personal communication, 2003).

6.3. Supersequence 3 (Eocene)

Supersequence 3 (middle to late Eocene) strata un-

conformably overlie either Paleocene or Upper Creta-

ceous beds. Supersequence 3 contains five major

sequences, labeled Sequences 0, 1, 2, 3 and 4 by

Zullo and Harris (1987) and Harris et al. (1993).

6.3.1. Sequence E0

Sequence E0 has patchy distribution and typically

occurs as isolated, single facies for which no systems

tract can be identified. The basal boundary of Sequence

0 is only observed updip along the arch in quarry

sections 8 and 9, where Sequence 0 units disconform-

ably overlie Cretaceous strata (Fig. 6). No lowstand

systems tract is observed.

Sequence E0 is localized in erosional depressions

along the arch and consists of 14 m of a bryozoan–

echinoid grainstone–packstone that thins rapidly to 0 m

within 8 km (sections 8, 9, Fig. 6). Downdip in well

section 16, Sequence 0 is 6 m of bryozoan–echinoid

grainstone–packstone (Fig. 7). Its systems tracts are not

identified.

6.3.2. Sequence E1

On the Cape Fear Arch (Fig. 6), Sequence 1 is 0–8 m

thick. On the strike line, boundaries between Sequences

E1 and E2 cannot be determined, and together, the

sequences thicken to an aggregate thickness of 37 m

(Fig. 7).

The base of Sequence E1 is a hardground as over the

local high on the arch between sections 5 to 7 (Fig. 6)

or a sharp contact. Downdip in sections 11, 12 and 18,

the sequence boundary is placed beneath a lowstand

quartz sand (Fig. 7).

On the arch (Fig. 6), sequence E1 has a patchy

distribution. The transgressive systems tract consists

of a mixed skeletal grainstone–packstone (section 8)

grading laterally above a local high (sections 7 and 6)

into sandy mollusc grainstone–packstone. These are

overlain by local bryozoan grainstone–packstone (sec-

tion 6). Downdip, the transgressive tract is a bryozoan

grainstone–packstone.

The highstand systems tract for Sequence E1 on the

arch is thin to absent except updip at section 10 (Fig. 6),

where it is a local 6-m succession of marl to fine

skeletal wackestone–packstone with an NP 15 age

pick (Worsley and Laws, 1986). Farther south (sections

5 and 6, Fig. 6), the fine wackestone reappears as a thin

veneer and phosphatic lag deposits. Highstand wack-

estones are traceable as a sheet (a few metres thick)

throughout much of the basin (sections 11 to 18, Fig.

7). These shallow up into late highstand mixed skeletal

grainstone–packstone and bryozoan–echinoid grain-

stone–packstone (sections 11 to 18, Fig. 7). The flood-

ing unit is split at section 16 by a bryozoan limestone

(Fig. 7).

6.3.3. Sequence E2

The Sequence E2 boundary on the arch is a regional

phosphatic hardground (sections 5 to 9, Fig. 6) overlain

on the local high (at sections 6 and 7) by a thin (1 m)

sandy mollusc limestone. Downdip the sequence

boundary is a hardground in the Beaufort core (section

17, Figs. 7 and 10), but is difficult to trace elsewhere.

The Sequence E2 transgressive systems tract on the

arch may be a phosphate pebble lag (sections 9, 8, 5,

Fig. 6). On the strike line (sections 13 to 18, Fig. 7), the

transgressive units of Sequence E2 cannot be defined,

but may be within the 7-m mixed skeletal grainstone–

packstone and bryozoan–echinoid grainstone–pack-

stone units beneath a major flooding unit.

The Sequence E2 highstand systems tract on the arch

is a relatively widespread bryozoan grainstone–pack-

stone that grades up to a sandy mollusc limestone at

section 9 (Fig. 6) and laterally grades into sandy mol-

lusc limestone at section 4 (Figs. 6 and 8). Downdip,

the highstand systems tract contains a shallowing-up-

ward succession consisting of a semi-regional fine

wackestone and local marl, overlain by a complex

facies mosaic of shallower water, bryozoan–echinoid

grainstone–packstone and sandy molluscan limestone

(sections 13 to 18, Figs. 7, 9 and 10).

6.3.4. Sequence E3a

Sequence E3 was mapped by Zullo and Harris

(1987) as a single sequence with a marine flooding

surface in the middle of the section, but Baum (1986)

considered that the hardground surface formed a se-

quence boundary subdividing the sequence into two

smaller sequences. We have subdivided sequence E3

into two minor sequences, E3a and b, because at some

localities, quartz sand and/or sandy limestones appar-

ently occur in the position of the hardground separating

the two units. However, such a subdivision is tentative

and needs better time control for correlation between

sections. Sequence E3 is 0–12 m thick along the Cape

Fear Arch (Fig. 6), thickening to 47 m basinward on the

Albemarle Block (Fig. 7).

Along the Cape Fear Arch, the basal boundary of

Sequence E3a is a hardground on the local high at


sections 5 to 7 (Fig. 6). In the Kure Beach core, the

Sequence E3a basal boundary is placed beneath a thin

(less than 1 m), local sand with sandstone rip-up clasts,

capped by a hardground (section 4, Figs. 6 and 8). This

sand above the sequence boundary is traceable downdip

as a thin laterally extensive unit (sections 11 to 18, Fig.

7), as in the Onslow and Beaufort cores (Figs. 9 and

10). In the Beaufort core (Fig. 10), the lowstand sand

also is capped by a hardground.

The transgressive systems tract of Sequence E3a on

the arch is only seen at section 4 (Figs. 6 and 8) where it

is a local, thin (1-m) bryozoan–echinoid grainstone–

packstone unit (with sandstone rip-up clasts), capped by

a hardground. Basinward, transgressive units are only

developed at sections 11 and 12 (Fig. 7) as a 3-m-thick

sandy mollusc limestone and may be present in the

Onslow core (section 13, Figs. 7 and 9) as a very thin

phosphate pebble lag. Elsewhere along strike, the trans-

gressive systems tract cannot be differentiated from the

highstand except in the Beaufort core (section 17, Figs.

7 and 10), where a transgressive sandy mollusc lime-

stone occurs beneath highstand fine skeletal wackes-

tone–packstone.

On the arch, the sequence E3a highstand forms a

shallowing-upward succession at sections 4 and 5 (Fig.

6), consisting of fine skeletal wackestone up into bryo-

zoan and mixed skeletal carbonates. In most sections

downdip (section 11 to 17, Fig. 7), the sequence 3a

highstand also forms a shallowing-upward succession

of fine skeletal wackestone–packstone to bryozoan–

echinoid grainstone–packstone locally capped by

sandy mollusc limestone or mixed skeletal limestone.

6.3.5. Sequence E3b

Along the arch, the Sequence E3b basal boundary is

a hardground at sections 5, 6 and 7 (Fig. 6). In the

highly thinned sections 1 to 3 (Fig. 6), Sequence E3b

cannot be differentiated from Sequence E3a. Downdip

(Fig. 7), the E3b sequence boundary is placed at the top

of upward-shallowing succession of sequence E3a (sec-

tions 5, 11, 12, 13) or beneath thin sands (sections 16

and 17).

On the arch, the transgressive tract is poorly devel-

oped or absent (Fig. 6). Downdip (Fig. 7), the trans-

gressive tract can be recognized at sections 12 and 13,

where it is a shallowing-upward parasequence of fine

skeletal wackestone/marl grading up into bryozoan and

mixed skeletal limestone; a thin sand caps this parase-

quence at section 17. Elsewhere, the transgressive tract

is dominated by a single lithology (sections 11, 16, 17).

The Sequence E3b highstand systems tract along the

arch forms a shallowing-upward succession only at

section 5 (Fig. 6) where it consists of fine skeletal

wackestone–packstone (NP 18 age pick; Worsley and

Laws, 1986) that shallows up to a bryozoan–echinoid

grainstone packstone. The systems tracts are not sepa-

rable in the thin bryozoan limestones in sections 6 and 7

(Fig. 6).

Downdip (Fig. 7), shallowing-upward highstand

units include marl and fine skeletal wackestone–pack-

stone up into bryozoan limestone and sandy mollusc

limestone (sections 5, 12, 13, 17). In the Beaufort core

(section 17, Figs. 7 and 10), the highstand consists of two

shallowing-upward parasequences, the lower one com-

posed of a very thin (less than 1 m) unit of marl to fine

skeletal wackestone–packstone to sandy mollusc lime-

stone and the upper one of very thin fine skeletal wack-

estone–packstone capped by sandy mollusc limestone.

6.3.6. Sequence E4

Sequence E4 has patchy distribution along the Cape

Fear Arch and is locally capped by Oligocene, Pliocene

or Pleistocene units. It commonly is a single facies,

making systems tract identification difficult. The Se-

quence 4 boundary is only evident along the arch at

section 5 (Fig. 6), where it is a hardground.

Downdip, the Sequence E4 boundary is a hard-

ground as in section 15 (Fig. 7). Elsewhere (sections

11 to 18, Fig. 7), the sequence boundary was placed

above shallowing-upward units of the underlying se-

quence E3b. The Sequence E4 transgressive systems

tract was not recognizable along the arch. Downdip

(sections 17, 18, Figs. 7 and 10), the transgressive

tract is a thin locally developed bryozoan–echinoid

grainstone–packstone.

On the arch, the Sequence E4 highstand systems

tract appears to be present only at section 5 (Fig. 6)

as a localized 4-m-thick fine skeletal wackestone–pack-

stone that pinches out laterally. Downdip at sections 12,

16, 17 (Fig. 7), the Sequence E4 highstand is an up-

ward-shallowing succession of fine skeletal wackestone

to bryozoan or mixed skeletal carbonate. Elsewhere, it

consists of a single lithology.

6.3.7. Ages of Eocene sequences

The five Eocene sequences are dated in terms of

nannoplankton (NP) zones (Figs. 3, 6 and 7).

Sequences 0 and 1 are NP 15 age; Sequence 2 is NP

16 age; Sequence 3a spans NP 16 and NP 17; Sequence

3b is NP 18 age; and Sequence 4 spans NP 19 and 20

(Zullo and Harris, 1987; Harris et al., 1993; Harris and

Laws, 1994). This implies that Sequence 3b and 4 are

late Eocene. However, Ward et al. (1978) included all

these sequences in the middle Eocene (Fig. 3).


6.4. Supersequences 4 and 5 (Oligocene)

Defining third-order sequences in the Oligocene

succession is difficult given the mixed data set of

core, well and quarry exposures, and the poor time

control on individual units.

Supersequence 4 contains up to three sequences (O1

to O3) that are recognizable in offshore seismic profiles

(Snyder et al., 1994) and in cuttings from a few onshore

wells (Coffey, 2000), but in general, they are difficult to

identify onshore. Supersequence 5 contains at least

three sequences (O4 to O6) that can be recognized

from offshore seismic data (Snyder, 1982) and in

some wells (Coffey, 2000).

Supersequence 5 ranges in thickness from 2 m in

the south at section 4 (Fig. 11), slightly thickening to

8 m in section 14, and then undergoes rapid thicken-

ing into the basin (sections 14A, 14B, Fig. 11). It then

thins to a thin veneer updip to the north (sections 14C,

15, Fig. 11).

6.4.1. Sequence O1

Given the 87/86Sr age ranges in section 14 (Harris et

al., 2000; Fig. 11) and the absence of any breaks within

the Kure Beach core (section 4, Fig. 11), the southern

part of the study area is interpreted as a single sequence

informally labeled O1. It is approximately 15 m in the

Kure Beach core to 30 m in section 14 (Fig. 11).

Sequence O1 thins to 13 m in section 14A (Fig. 11),

beyond which it cannot be traced with any confidence.

It may continue updip into sections 14B and 14C, but

there is no age control.

The basal sequence boundary of Sequence O1 is a

hardground in core sections. No lowstand systems tract

or transgressive systems tract can be recognized, and

the maximum flooding surface is coincident with the

sequence boundary. The highstand systems tract con-

sists of a lower unit of marl 5–10 m thick, overlain by

14–30 m of fine foram sand.

6.4.2. Sequence O2a

Given the age control in section 14B and the sand

bodies in section 14A (Fig. 11), it is inferred that

Sequence O1 is overlain by Sequences O2a and O2b

downdip. Updip to the north in section 14C (Fig. 11),

the O2 silty sand may pass into undifferentiated shell

beds given the 87/86Sr age picks in the Belgrade Quarry.

The basal sequence boundary of O2a in section 14A

(Fig. 11) is placed beneath a local lowstand to early

transgressive coarse sand body. The sand unit is over-

lain by 10 m of transgressive/highstand fine foram

sand.

6.4.3. Sequence O2b

The sequence boundary for Sequence O2b in section

14A (Fig. 11) is placed beneath a local lowstand to

early transgressive coarse sandy zone. This sand reap-

pears to the north in section 14C (Fig. 11) and is dated

at 30 Ma (Harris et al., 2000). The highstand systems

tract consists of an upward-shallowing succession of

10–20 m of fine foram sand overlain by 15 m of sandy

mollusc limestone that thins to the northeast to 2 m in

section 14C (Fig. 11). Sequence O2b is preserved only

in the basin, as it appears to pinch out to the southwest

and northeast.

6.4.4. Sequence O3

A third locally preserved sequence tentatively la-

beled O3 is recognized in section 14C (Fig. 11). It is

bracketed by 87/86Sr age dates of 27 Ma above and 29

Ma below (Denison et al., 1993). The O3 sequence

boundary is placed beneath the local lowstand to

early transgressive sand, and the highstand systems

tract consists of 2 m of sandy mollusc limestone (sec-

tion 14C, Fig. 11).

6.4.5. Sequences O4, O5, O6

The Supersequence 5 boundary on the Onslow

Block is placed at the base of either lowstand to early

transgressive local coarse sands (sections 12, 14A, Fig.

11) or where these are absent, at the base of sandy

mollusc limestone units that are locally marked by

phosphatized pebbles (sections 13, 14C, Fig. 11). In

the basin at section 14A (Fig. 11), there is a weak

suggestion that three sequences may be developed

(Sequences O4, O5, O6) based on slight increases in

sand in the well cuttings at the presumed bases of the

sequences. The remainders of the sequences are com-

prised of sandy mollusc limestones. Sequence O4

appears to extend out of the basin to the north into

section 14C and 15 (Fig. 11) based on 87/86Sr ages at

14C (Denison et al., 1993). On the Onslow Block,

although only one sequence appears to be developed,

it is unclear if it is Sequence O4 or O5. The top of

Supersequence 5 is placed above lithified sandy mol-

lusc limestones and beneath unlithified Miocene and

younger sandy beds.

6.4.6. Age of Oligocene sequences

Ages shown in Fig. 11 include 87/86Sr dates (Deni-

son et al., 1993; Harris et al., 2000). Harris et al. (2000)

gave a lower Oligocene (Rupelian) age to the Lower

River Bend units (Sequences O1–O3 of this paper; Fig.

11). The Upper River Bend was considered upper

Oligocene (Chattian) to early Miocene (Aquitanian)

Fig. 11. Interpretive Oligocene cross section C–C from the Onslow Block to the Albemarle Block, location shown in Fig. 1. Oligocene sequences unconformably overlie Eocene carbonates.

Succession is broadly upward-shallowing, with deeper water marl overlain by thick fine foram sands, overlain by sandy mollusc carbonates. The two supersequences are lower Oligocene (Rupelian)

and upper Oligocene (Chattian). Note that updip, there are only one or two sequences, whereas downdip, there are several possible sequences. Interpretation constrained by biostratigraphic ages and87/86Sr dating (Ward et al., 1978; Denison et al., 1993; Harris et al., 2000). The marked thickening in sections 14a and b reflects their more seaward position on the shelf (Fig. 2).

J.LaGesse,


Geology184(2006)155–182

171


(Harris et al., 2000); thus, Sequences O4–O6 of this

paper are late Oligocene.

7. Discussion and interpretation

7.1. Controls

7.1.1. Subsidence rates

Tectonics were extremely important in influencing

thickness, facies and intensity of wave- and current-

scour because of their effect on shelf topography. Sub-

sidence rates for onshore Paleogene units average 1.4

cm/ky (Coffey and Read, 2002), but in the study area,

probably were below 0.5 cm/ky based on preserved

thicknesses. Subsidence rates elsewhere along the east

coast, especially in more offshore areas, were higher

(up to 4 cm/ky) (Steckler and Watts, 1978). Abrupt

changes in sediment thickness in onshore sections

(e.g., Fig. 6) is due to numerous faults evident on

seismic data with localized thickenings, facies changes

and fining into small graben-like depressions (Baum,

1977; McLaurin and Harris, 2001). Areas of low sub-

sidence rates generally were sites of shallow-water

sedimentation and local preservation of thin deeper

water units, while widespread deeper water units were

only preserved in more rapidly subsiding areas. These

tectonically induced lows acted as sediment traps

allowing finer grained facies to accumulate in region-

ally high-energy settings. The Cape Fear Arch resisted

subsidence and thus was the site of deposition of thin

successions. Numerous hardgrounds here attest to

boundary current sweeping as the ancestral Gulf Stream

traversed the arch. Thus, tectonics played an important

role in controlling thickness and facies developed.

7.1.2. Eustasy

Eustatic sea-level changes have been cited as the

major influence on the timing of supersequence and

sequence development in the Paleogene of North Car-

olina and New Jersey (Harris et al., 1993; Harris and

Laws, 1997; Miller et al., 1998). This is indicated by

the correspondence of unconformities and maximum

flooding surfaces in New Jersey and along the Atlantic

margin to the Haq et al. (1988) sea-level record

(Browning et al., 1997a; Miller et al., 1997). Although

we lack sufficient age control to correlate individual

third-order sequences in North Carolina with eustatic

events elsewhere, there is a reasonable correspondence

between most of the sequences of this study and those

defined by Miller et al. (1998) from New Jersey.

Paleocene sea-level rise of at least 100 m, evidenced

by near-shore Upper Cretaceous facies overlain by

deep-water marls, is associated with the transition into

global greenhouse climate following Late Cretaceous

cooling (Coffey and Read, 2004). Delta18O values of

Foraminifers of New Jersey indicate that greenhouse

conditions continued into the early Eocene (Miller et

al., 1987), but show a cooling trend in the middle to late

Eocene into a period of transitional, moderate ampli-

tude eustasy or bdoubthouseQ (Miller et al., 1991) be-

fore passing into the icehouse of the Oligocene. In the

study area, widespread erosion at the Eocene–Oligo-

cene boundary and the transition from Eocene subtrop-

ical bryozoan limestone facies to siliciclastic-dominated

facies deposited in the Oligocene icehouse (Figs. 6, 7

and 11) reflect significant climate cooling and subse-

quent progressively lower relative sea levels (Coffey

and Read, 2004).

7.1.3. Wave climate and ancestral Gulf Stream

The open-shelf environment of the Paleogene Atlan-

tic margin was subject to intense wave sweeping gen-

erated by storms in the broaring forties,Q as it was

positioned north of 308 latitude (Scotese, 1997); swellsalso would have been generated from tropical storms to

the southwest. Swell waves created a zone of abrasion

that caused continuous scouring of the shallow shelf, as

on the present southwestern Australian shelf (Collins,

1988; James et al., 1997; Osleger, 1991). However,

because of the less energetic setting of the Paleogene

Atlantic margin, the inner, middle and outer neritic sub-

environments of the New Jersey to North Carolina

margin (Jones, 1983; Browning et al., 1997a; Pekar

and Kominz, 2001; Pekar et al., 2003; Tables 1a and

1b) likely occurred at shallower depths than their mod-

ern southern Australian counterparts (Collins, 1988;

James et al., 1997, 1999, 2001). This was because the

Paleogene Atlantic Ocean was shallow to the north, and

not as wide as now, limiting the distances that the swell

waves traveled prior to reaching the shelf and subject to

less intense wind and storm systems (Scotese, 1991;

Coffey and Read, 2004). The Albemarle Embayment

also would have been somewhat protected from swell

and storm waves by the bordering Norfolk and Cape

Fear arches, and at least for the Eocene, by the offshore

Hatteras buildup (Coffey and Read, 2004).

The ancestral Gulf Stream current began its circu-

lation across the Blake Plateau at least by the late

Paleocene and the early Eocene, migrating onto the

shelf during highstands of sea level, in depths as

shallow as 200 m (Pinet and Popenoe, 1985, Popenoe

et al., 1987) and perhaps even shallower over the

arches. Gulf Stream erosion produced gullies, pits

and bscour bandsQ in Paleogene strata across the


Blake Plateau seaward of the study area (Pinet and

Popenoe, 1985; Popenoe et al., 1987), and possibly

hardground surfaces in outer neritic facies in the study

area. The southwestern coast of Australia experiences

similar effects from the warm Leeuwin Current (James

et al., 1994, 1999). The Gulf Stream within the study

area formed hardgrounds in outer neritic facies (Figs.

9 and 10), and ripped-up, reworked limestone clasts

within middle to outer neritic facies (Fig. 8). This was

especially pronounced where the current passed across

the shallow Cape Fear Arch to form local phosphatic

lag gravels.

7.1.4. Paleocene supersequence 1 controls

Upper Cretaceous sediments probably were exposed

on the arch, perhaps when the latest Cretaceous glaci-

ation (Barrera et al., 1987) lowered sea level. Super-

sequence 1 deposition was initiated when relative sea

level rose over 100 m to flood the shelf in the latest

Maastrichtian (Haq et al., 1988; Frakes et al., 1994).

This caused deposition under a wet–temperate to sub-

tropical climate (Nystrom et al., 1991) of an upward-

deepening succession of lower Paleocene deeper water

clastics grading basinward into carbonates in the down-

dip portion of the arch.

Possible differential uplift of the Onslow Block

along with the smaller late Paleocene sea-level rise

(Haq et al., 1988) prevented accumulation (or preser-

vation) of a subsequent late Paleocene succession in the

study area and may have allowed erosion of any earlier

deposited, updip shallow-water lower Paleocene units

from the region.

The area also appears to have remained above sea

level (or above wave-abrasion depth) throughout the

early Eocene, in part due to regional uplift, during

which Supersequence 2 accumulated in the basin down-

dip from the study area (Harris and Laws, 1997; Coffey

and Read, 2004).

7.1.5. Eocene supersequence 3 controls

Widespread deposition of bryozoan carbonates of

Supersequence 3 was initiated by major sea-level rise

in the middle Eocene, during NP 14 and 15 time, aided

by subsidence of the Onslow and Albemarle Blocks that

shifted the updip depositional limit 175–200 km updip

of the earlier depositional edge (Harris and Laws, 1997).

At least five sea-level cycles in the middle to late Eocene

generated the sequences under warm, marginally sub-

tropical conditions (Harris et al., 1993; Coffey and

Read, 2004). Estimates of sea-level rises (uncorrected

for loading) were made using the minimum water depths

of the shallow-water facies, subtracted from those of the

overlying deeper water facies. These mainly ranged

from 35 to 55 m, but could be as little as 20 m to as

much as 80 m. Maximum rises were in Sequences 3a

and b. Crude estimates of sea-level falls involved in

sequence development (not corrected for unloading)

were made using minimum depths of facies in up-

ward-shallowing sequences, minus shallowing due to

sediment-upbuilding. Estimates of fall magnitudes are

15–40 m, with perhaps up to 70 m of fall associated with

the basal boundary of Sequence 3a. These estimates of

Eocene sea-level changes are not compatible with

greenhouse, ice-free conditions, but suggest limited ice

sheets compatible with the bdoubthouseQ of Miller et al.

(1991) or btransitional eustasyQ of Read (1995), betweengreenhouse and ice-house.

Low subsidence rates over the arch and corres-

ponding low accommodation generated thin sequences

and made these areas susceptible to erosion by marine

currents, leaving Sequences E0 and E1 as erosional

remnants or phosphate lags. The sequences double or

triple in thickness downdip in the study area into the

more rapidly subsiding basin. Late middle Eocene cool-

ing, aridification, and relatively prolonged sea-level fall

(Miller et al., 1987) caused regional sand influx at the

base of Sequence 3. Subsequent flooding possibly as-

sociated with late middle Eocene warming (McGowran

et al., 1997) was followed by a latest middle Eocene

sea-level fall of 20 m (Miller et al., 1998) to 100 m

(Haq et al., 1988), and deposition of the basal sand of

Sequence 3b. If Sequence 3b is late Eocene age (Harris

et al., 1993), and the 3a–3b boundary is indeed a

sequence boundary and not a maximum flooding sur-

face (Zullo and Harris, 1987), then it marks the end–

middle Eocene sea level fall (TA 3.5 of Haq et al.,

1988). Globally, late Eocene eustatic sea-level rises did

not reach as far onto the shelves as in the middle

Eocene (Harris and Laws, 1997).

7.1.6. Oligocene supersequences 4 and 5 controls

The basal boundary of Supersequence 4 formed

during major global cooling in the late Eocene, culmi-

nating in onset of Oligocene icehouse and sea-level

lowstand (Miller et al., 1997; Coffey and Read,

2004). The cooler climate, increased aridity and overall

lower Oligocene sea levels promoted siliciclastic depo-

sition in the area. Sea-level lowstand was followed by

at least 50 m (Miller et al., 1998) to 100 m sea level rise

(Haq et al., 1988) that drowned the shelf, depositing

widespread basal Oligocene marls; these values are

compatible with the facies succession. Accommodation

during drowning was aided by space created by con-

tinued subsidence during prior emergence, coupled


with subsequent water loading. Oligocene Sequence 1

appears to have filled in much of the accommodation

on the arch with fine foram sands. The remaining

accommodation in the basin was filled by three or

more early Oligocene sequences that ultimately shal-

lowed into sandy shell beds. Sea level changes were in

excess of 50 m (Kominz and Pekar, 2001). However,

low accommodation in the study area allowed only

shallow flooding of the shelf following deposition of

Sequence O1. Sequences O2 and 03 did not extend

onto the arch (Fig. 11), according to the 87/86Sr age

constraints of Harris et al. (2000), due to low accom-

modation here.

Middle Oligocene global cooling and sea-level fall is

marked by a regional hiatus on the arch between lower

Oligocene (Rupelian) and upper Oligocene (Chattian)

units, around 28.5 Ma (Berggren et al., 1995). The

subsequent sea level rises of 50 m or so (Kominz and

Pekar, 2001) flooded the shelf, but there was little

accommodation remaining over the arch where a single,

thin upper Oligocene sequence dominated by mollus-

can limestones was deposited. Downdip, however,

greater subsidence allowed three or more shallow-

water shell bed-dominated sequences to accumulate,

possibly in response to late Oligocene glacio-eustasy

(Kominz and Pekar, 2001). Although these late Oligo-

cene sea-level changes may have been 50–60 m

(Kominz and Pekar, 2001), the slowly subsiding shelf

in the study area remained shallow and probably was

never flooded to more than a few tens of metres,

evidenced by the development of only inner neritic

sand and molluscan limestones (Fig. 11).

7.2. Systems tract development

7.2.1. Sequence boundaries and hardgrounds

In the study area, two-thirds of sequences have

recognizable sequence boundaries, and of these, almost

half are hardgrounds (Fig. 12A). In an individual sec-

tion, it is difficult to separate sequence bounding hard-

grounds from hardgrounds beneath maximum flooding

surfaces. For example, the sequence E3a–3b boundary

was interpreted as a maximum flooding surface (Zullo

and Harris, 1987) or a sequence boundary (Baum,

1986, and this paper). However, regional tracing of

such surfaces allows them to be differentiated. The

sequence boundaries will, at least locally, overlie up-

ward-shallowing highstand deposits and locally have

closely associated sandy facies indicating a major

basinward shift in facies.

The coincidence of sequence boundary and hard-

ground apparently is related to the wave-swept charac-

ter of the shelf surface during low sea level (Fig. 13).

During lowered sea-level phases, unfilled accommoda-

tion from the previous highstand kept the inner shelf

within the zone of abrasion in a few tens of metres of

water (Fig. 13). Continuous wave-sweeping and low

sedimentation rates generated the sequence boundary

and hardgrounds (Collins, 1988; Tucker and Wright,

1990, p. 329; Boreen et al., 1993; Riggs et al., 1998).

As waves abraded the shelf, the sediment stayed near

the sediment-water interface beneath thin, patchy, mi-

grating sediment units, and constant wave and current

agitation provided a continuous source of CaCO3 that

promoted micritic cementation (Tucker and Wright,

1990, p. 325). This formed the indurated, bored and

abraded hardgrounds in core sections. Their phosphatic

and glauconitic composition reflects the low sedimen-

tation rates associated with hardground formation

(McRae, 1972; Moran, 1989).

There is little evidence for exposure of the shelf

during Paleogene sequence boundary development in

the study area, except for the Cretaceous–Tertiary con-

tact, below which there is a negative shift in y13C and

y18O isotope values in the heavily cemented molluscan

limestone (Baum and Vail, 1988). Although Moran

(1989) suggested that hardground formation required

exposure, this is difficult to prove given the relative

scarcity of undoubted subaerial fabrics beneath se-

quence boundaries, and the subsequent marine diage-

netic overprint.

In middle Eocene updip sections, most sequences

have identifiable sequence boundaries that are hard-

grounds (Fig. 12B). By comparison, just over half of

downdip sequences have recognizable sequence bound-

aries and only a few of these are hardgrounds (Fig.

12C). The higher number of recognizable sequence

boundaries and coincident hardgrounds in updip sec-

tions indicate that the further landward sections expe-

rienced intense wave sweeping during lowstand or

transgression, preventing deposition of significant

transgressive deposits. The low number of coincident

sequence boundaries and hardgrounds in downdip sec-

tions may be due to underestimation because of poor

resolution of well cuttings data (the hardgrounds being

best recognized in quarry or core sections).

Although sequences are difficult to trace in the

Oligocene, two-thirds have recognizable sequence

boundaries, and about one-third are also hardgrounds,

the remainder underlie quartz sands (Fig. 12E). As in

the middle Eocene, the number of sequence boundaries

developed as hardgrounds in the Oligocene may have

been underestimated in sections with only well cuttings

data.

Fig. 12. (A–E) Histogram diagrams showing percentages of sequence boundaries, maximum flooding surfaces and systems tracts present in the

Paleogene sequences.


7.2.2. Lowstand systems tract

Lowstand wedges (Fig. 4) are only developed at the

supersequence scale in the Paleogene of North Caro-

lina. They are evident in offshore seismic sections at the

base of the lower Paleocene, lower Eocene, lower

Oligocene, and upper Oligocene sections (Coffey and

Read, 2004). These wedges lie offshore from the study

area and onlap the inner shelf margin updip and down-

lap onto the inner shelf slope and deep shelf downdip.

In the present study area, possible lowstand deposits are

siliciclastic-prone shelf margin sheets and wedges lo-

cated on the downdip part of the inner shelf. These

Fig. 13. Schematic diagram showing systems tract development on a swell-wave-dominated shelf during a sea-level cycle and uniform subsidence.

During low sea level, the sediment surface is in the zone of wave sweeping and abrasion which causes hardground formation at the sequence

boundary. There is little accommodation space for sandy lowstand units to form. During sea-level rise, open-shelf transgressive units may be

deposited under low sedimentation rates. After maximum flooding, accommodation favors deposition of a highstand upward-shallowing succession.

Sea-level fall causes the shelf to reenter the zone of wave sweeping, preventing sediments building to sea level. Note that the sequence only partly

fills available accommodation space.


lowstand shelf margin sheets and wedges are developed

in 33% of Paleogene sequences, suggesting that sedi-

ment was rarely deposited or preserved on the hard-

ground-dominated surfaces within the zone of wave

sweeping during lowered sea level (Fig. 13).

In the Paleocene in the study area, the only evidence

of lowstand (to early transgressive) deposits is a thin,

quartz sand in one well (section 12, Fig. 7). In the

middle Eocene, updip sections had lowstand deposits

in only one sequence of a single section (Fig. 8), but

downdip sections had lowstand sands in 40% of

sequences. Similarly, Oligocene sequences also have

thin lowstand sands in at least 40% of sequences, but

the data are poor.

These thin sands suggest that during lowered sea

levels, local thin veneers and sand waves formed on

the sediment-starved, swell-wave-swept shelf, similar

to modern Australian bshaved shelvesQ where there is

negligible net accumulation of quartz sands on the

inner shelf (James et al., 1994, 2001). Some of these

thin sand units that had become calcite-cemented (ei-

ther in the intertidal zone or on the shallow shelf)

were reworked during transgression into sandstone

lithoclast lags (section 4, Fig. 6; section 17, Fig. 7).

If these sands are the distal edge of shoreface sands,

then transgression may have stranded them on the

shelf, shutting off sediment supply, allowing them to

cement, and then be bioeroded and physically

reworked by swell waves.

7.2.3. Transgressive systems tracts

Transgressive systems tracts are recognizable in only

about 33% of the Paleogene sequences. About 50% of

the sequences have undifferentiated transgressive and

highstand systems tracts and lack a distinct maximum

flooding surface (Fig. 12A).

The only Paleocene transgressive systems tract in

the study area is a locally developed deepening-up

succession of a thin, nearshore shelly limestone to

marine shelf glauconitic sandstone, with the maximum

flooding surface at the base of laminated, deeper water

shales at Kure Beach (Fig. 8). This succession reflects

increasing accommodation relative to siliciclastic sedi-

mentation rates.

In the middle Eocene sections, the transgressive

systems tract is absent from 80% of updip sections

and 50% of downdip sections. The transgressive sys-

tems tract is recognized in only 20% of sequences

updip (Fig. 12D), where it is either a thin veneer of

more offshore facies on the underlying sequence

boundary or lowstand unit (sections 6, 7, Fig. 6), or a

phosphatic pebble lag (sections 5, 8, 9, Fig. 6). The

poor development or lack of a transgressive systems

tract was probably due to wave/current sweeping pre-

venting deposition of sediments during transgression

(Fig. 13). Some of these transgressive sediments

could have been reworked and winnowed by wave

and boundary currents to form phosphatic gravel lags

or a condensed zone.


Fifty percent of the downdip Eocene sections ap-

pear to have a recognizable transgressive systems

tract. This reflects greater accommodation downdip

associated with the higher subsidence, so that with

eustatic rise, a transgressive unit was able to accumu-

late as wave-sweeping progressively decreased as the

shelf deepened. Some transgressive tracts in downdip

sections are an upwards-shallowing parasequence

capped by the maximum flooding surface; these are

not able to be traced regionally, due to the quality of

the well data (sections 12, 13, 17, Fig. 7). Such

transgressive tract parasequences may be caused by

fourth-order sea-level fluctuations superimposed on

the third-order sea cycle, or they may be due to

local, high sedimentation rates exceeding accommoda-

tion. In sequences dominated by a single shallow-

water lithology which lack a deeper water unit, a

maximum flooding surface cannot be recognized.

Thus, the transgressive and highstand system tracts

cannot be separated. This is the case in about 50%

of updip middle Eocene sections and 33% of downdip

sections (Fig. 12B, C).

7.2.4. Maximum flooding surface

A maximum flooding surface is recognizable in

about 50% of Paleogene sequences (Fig. 12A). The

maximum flooding surface of Supersequence 1 is in

the lower Paleocene of the study area, with the upper

Paleocene missing at least from the arch. This greater

flooding in the lower Paleocene compared to the upper

Paleocene is compatible with the Haq et al. (1988)

chart. However, the regional distribution of the Paleo-

cene throughout the basin suggests syndepositional

differential uplift of the Onslow Block and subsidence

of the Albemarle Block that caused the upper Paleocene

to onlap farther than the lower Paleocene units (Harris

and Laws, 1997; Coffey and Read, 2004).

The maximum flooding surface of the middle to late

Eocene(?) Supersequence 3 appears to be in Sequence 1

(NP 15 age; Harris et al., 1986; Zullo and Harris, 1987)

beneath locally preserved marls updip (section 10, Fig.

6), and regional deeper water fine skeletal wackestone–

packstones in the basin (Fig. 7). This is compatible with

the regional subsurface data of Coffey and Read (2004)

showing downlap onto a maximum flooding surface in

roughly this stratigraphic position adjacent to the Hat-

teras buildup outside the study area.

In the third-order sequences, a maximum flooding

surface is recognizable in 50% of updip middle Eo-

cene sequences and in 66% of downdip sequences

(Fig. 12B, C). The coincidence of the maximum

flooding surface with a hardground at the sequence

boundary (sections 4 to 6, 9, Fig. 6), or with phos-

phatic pebble lags, reflects wave sweeping during

lowstand and transgression that inhibited sediment

deposition. Limited core data downdip suggest that

some maximum flooding surfaces on transgressive

tracts are hardgrounds (Figs. 7, 9 and 10) and mark

a period of non-deposition prior to highstand aggra-

dation. Scour may have been caused by ancestral Gulf

Stream currents and gyres which migrated onto the

shelf during high sea levels (Popenoe et al., 1987).

The maximum flooding surface for Oligocene

Supersequence 4 is at the base of sequence O1

(Lower River Bend Formation; Fig. 11) and formed

during the major 50 to 100 m of sea-level rise (Kominz

et al., 1998; Haq et al., 1988) that incipiently drowned

the shelf (Coffey and Read, 2004). However, third-

order maximum flooding surfaces were difficult to

recognize for Oligocene units because of limited ac-

commodation and little facies differentiation.

7.2.5. Highstand systems tracts

Highstand systems tracts are recognized in about

50% of the sequences, and they comprise the bulk of

these sequences (Fig. 12A).

For the locally developed Paleocene deposits on the

arch, the highstand unit does not shallow up out of the

deep-water facies of the maximum flooding unit (sec-

tions 1–4), suggesting that deposition of Paleocene

shallow-water facies during the late highstand may

have been inhibited by wave-sweeping or removed by

subsequent erosion, as indicated by incision on the

Paleocene downdip (Fig. 6). Coffey and Read (2004)

showed that farther basinward, the thick Paleocene

supersequence highstand is a complex upward-shallow-

ing succession, reflecting increased accommodation

downdip, and this is also suggested by the downdip

Paleocene (?) units in the study area.

Most Eocene sequences in the study area show well-

developed highstands that make up the bulk of the

sequences. Many consist of an upward-shallowing suc-

cession from deeper water muddy carbonates up into

more shallow, grainy facies. This accommodation shift

reflects some shallowing due to aggradation (typically a

few metres to 15 m per sequence), coupled with sea-

level fall, with deposition continuing up to the depth of

vigorous wave sweeping (a few tens of metres). In more

downdip or more rapidly subsiding areas, this shallow-

ing caused deposition of bryozoan limestone, mixed

skeletal limestone, or in more updip areas, mollusc

facies. A few highstand units that lack deeper water

facies are dominated by a single shallow-water facies,

which filled the small amount of available accommo-


dation. The few parasequences that are developed in the

highstand are only mappable locally. This may be due

to low accommodation or to higher frequency sea-level

oscillations either being absent or of low amplitude

such that they were too small to affect the surface of

the shelf at depths of tens of metres.

7.2.6. Need for more age constraints

Since the cross sections in this paper connect with

the regional cross sections in Coffey (2000) and Coffey

and Read (2004), the study raises questions regarding

the ages of subsurface units. Coffey (2000) showed that

the subsurface Eocene carbonates adjoining the study

area were mainly middle Eocene and that there was

little upper Eocene developed. However, assuming the

nannoplankton picks of Worsley and Laws (1986) and

Harris et al. (1993) are correct, then there is a signifi-

cant subsurface upper Eocene section adjoining the

study area. Given that this is a time of considerable

climatic instability in transition into icehouse condi-

tions, it would be of great value to acquire more com-

plete subsurface age constraints.

8. Differences between the Paleogene ramp and the

tropical ramp sequence model

The sequence development of the updip portion of

wave- and current-swept ramps such as the Paleogene

example (Fig. 14) differs from that of classical, tropical

ramps (Handford and Loucks, 1993; James, 1997; Cof-

fey and Read, 2004) in the following features.

Fig. 14. Schematic sequence-stratigraphic model for the Paleogene ramp

aggradation to sea level, resulting in the distinctive sequence development.

Sequence boundaries on tropical ramps are com-

monly subaerial surfaces on aggraded peritidal facies,

which contrast with the dominance of hardground-

sequence boundaries developed on open-marine car-

bonates or cemented shelly sands of this study (cf.

James, 1997). The difference largely relates to wave

and current sweeping tending to inhibit the Paleogene

ramp from shallowing to sea level (cf. Pomar, 2001).

Sediment bypassing, non-deposition and cementation

occurred once the ramp shallowed up to the zone of

wave abrasion and current sweeping. This decreased

sedimentation and the consequent, more deeply sub-

merged aspect of the shelf prevented the bulk of the

shelf becoming exposed during sea-level fall. Hard-

grounds on tropical ramps are commonly associated

with drowning (maximum flooding) rather than se-

quence boundaries; because during deepening, the

sediment supply is shut off, allowing cementation

and marine diagenesis to form hardgrounds, as well

as putting the sediment surface into a deeper water

setting favorable to phosphatization due to upwelling.

Differentiating between hardgrounds marking se-

quence boundaries, hardgrounds caused by superim-

position onto a subaerial sequence boundary, and

hardgrounds formed during maximum flooding,

depends largely on the stratigraphic context of the

surface. In the context of the whole Paleogene shelf,

and not just the updip part discussed in this paper,

hardgrounds may form on any or all of the facies or

systems tracts in response to changing sea level and

migration of belts of wave- and current sweeping, with

, North Carolina. Wave and current sweeping of the shelf inhibits


preferential preservation of the indurated hardgrounds

(Coffey and Read, 2004).

Transgressive systems tracts are commonly present

on updip parts of tropical ramps. They are much less

common on the Paleogene ramp, where the intense

wave and current sweeping in near-shore areas or on

arches, tended to inhibit deposition until sufficient ac-

commodation had developed during the highstand (cf.

James, 1997).

Grainstones on most tropical ramps are commonly

restricted to a relatively shallow depth range, typically

less than 15 m within fair-weather wave base. Much of

this material is produced by phototrophic organisms

within the well lighted waters. In contrast, grainy

facies may extend over wide areas to water depths of

many tens of metres on the Paleogene ramp. This is

because depth of swell wave reworking generates

grainy sheets over wide areas to many tens of meters

depth, and because many of the grain-producing

organisms belong to heterozoan assemblages of

bryozoans and mollusks that do not require light (cf.

Collins, 1988; James, 1997). Such heterozoan assem-

blages also are not capable of generating shelf edge

reefs, thus leaving the ramp open to wave and current

sweeping.

Few parasequences are developed within sequences

on the Paleogene ramp. In contrast, on tropical ramps

that formed under greenhouse or transitional conditions,

parasequences are more numerous. Again, this likely

reflects the dominance of sediment bypassing due to

wave and current sweeping, tending to inhibit rapid

upward-shallowing to sea level during high-frequency

sea-level changes. Thus, this tends to keep the sediment

surface well below the zone of active sea-level fluctua-

tions, decreasing the likelihood that a small change in

sea level will be preserved as an upward-shallowing

parasequence in the shelf stratigraphy.

9. Conclusions

The Paleogene of southern North Carolina was se-

lected for the study of sequence development of open-

shelf carbonates in a low-accommodation, swell-wave

setting using cores that were then tied into previously

studied quarries (Ward et al., 1978; Harris et al., 1986)

and exploratory wells from the regional study of Coffey

(2000) and Coffey and Read (2004).

A positive arch to the south (Cape Fear Arch) and

a slowly subsiding basinal block to the north (Albe-

marle Block) influenced relative sea-level changes and

accommodation space on the shelf. Local tectonic

depressions allowed fine carbonates to accumulate in

a region of relatively high-energy facies. The Cape

Fear Arch also interacted with the Gulf Stream, result-

ing in phosphatic erosional lags on some sequence

boundaries.

In the study area, Paleocene sequences were domi-

nated by thin, sandy molluscan facies, glauconitic

sands, and offshore silt–shales and marls. Middle Eo-

cene sequences were characterized by molluscan lime-

stone, bryozoan–echinoid grainstone–packstone, and

deep-water fine skeletal wackestone–packstone and

marl. Middle Eocene sequences generally were better

developed downdip than along the updip Cape Fear

Arch. Early Oligocene sequences are dominated by

localized sand units, deep-water foram sands and

marls. Late Oligocene sequences are typically mollus-

can limestones with local sandy units.

Rather than subaerial surfaces, most sequence

boundaries in the Paleogene study area are marine

hardgrounds that formed as sediment-starved wave-

abrasion surfaces during lowered sea levels. Lowstand

sands are thin and patchily developed on the shelf.

Transgressive units also are commonly thin, upward-

deepening and variably developed and are even con-

densed into phosphate lags, due to sediment-starvation,

wave-sweeping, sediment-bypassing, and backstepping

of sediment sources. Rare transgressive units are a

single upward-shallowing parasequence overlain by a

maximum flooding surface. Highstand units make up

the bulk of sequences and consist of a single shallow-

ing-up unit that generally lacks parasequences. Sea

level rise and flooding of the shelf formed relatively

regional, deeper water carbonates above the maximum

flooding surface in areas of regional subsidence. How-

ever, on the arch, these deeper water units commonly

are discontinuous and confined to local areas (gra-

bens?) with higher subsidence. Highstand tracts shal-

lowed up to inner shelf depths, but there is little

evidence that coastal shoreface units ever filled the

remaining accommodation, due to low sedimentation

rates on the shelf, wave sweeping and sediment

bypassing.

The wave- and current-swept shelf setting and

Paleogene transitional global climate with its moder-

ate-amplitude eustasy generated distinctive sequences

in the study area. These are characterized by hard-

ground-dominated sequence boundaries, limited low-

stand and transgressive systems tract development

and dominance of highstands marked by unfilled

accommodation. These contrast markedly with many

tropical shelves with their multiple parasequences,

inboard peritidal cycles and well-developed subaerial

sequence boundaries.


Acknowledgments

We thank Bill Hoffman, Kathleen Farrell and Laura

DeMoe of the North Carolina Geological Survey, and

Rob Weems, Laurel Bybell, Jean Self-Trail, and Lucy

Edwards of the U.S. Geological Survey for access to

the Kure Beach core, and Bill Harris and Buck Ward for

field discussions. Bill Harris, as well as heading the

North Carolina Earth Sciences Coalition, also provided

many helpful comments on the manuscript as did Brian

Coffey who also gave access to his data sets. Tim

Bralower provided information on ages. Support was

received from the North Carolina Graduate Student

Research Fund. We thank Ellen Mathena for editing

and word processing.

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