Control of Upstream Variables on Incised-Valley Dimension - JSR, Mattheus Et Al, 2007

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Journal of Sedimentary Research, 2007, v. 77, 213–224

Research Article

DOI: 10.2110/jsr.2007.022

CONTROL OF UPSTREAM VARIABLES ON INCISED-VALLEY DIMENSION

CHRISTOPHER R. MATTHEUS,1 ANTONIO B. RODRIGUEZ,1 D. LAWRENCE GREENE JR.,2 ALEXANDER R. SIMMS,3 AND

JOHN B. ANDERSON4

1Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, North Carolina 28557, U.S.A. 2ConocoPhillips, 600 North Dairy Ashford, Houston, Texas 77079, U.S.A.

3Boone Pickens School of Geology, Oklahoma State University, Stillwater, Oklahoma 740784Department of Earth Sciences, Rice University, Box 1982, Houston, Texas 77251-1982, U.S.A.

e-mail: [email protected]

ABSTRACT: It is well documented that sea level fell during the Last Glacial Maximum, shifting graded-stream profiles out of equilibrium and causing rivers to incise into continental shelves. Although incised valleys have been heavily researched, theinterplay between upstream and downstream controls on incised-valley dimension are not well constrained. To address this lack

of understanding, we examined the cross-sectional dimension of nine incised valleys located across the northern Gulf of Mexicomargin and bounded by the sequence boundary associated with the last sea-level lowstand. These incised valleys aredistinguished by drainage basins that vary in size by three orders of magnitude, cover a margin that presently has a steepclimate gradient, and extend across a continental shelf that varies along strike in width and gradient.

Incision depths vary for valleys that have similar gradient profiles but different drainage-basin areas, suggesting significantcontrol of upstream variables on incised-valley morphology. Additionally, these data show a strong linear correlation betweendrainage-basin area and incised-valley cross-sectional area. This suggests applicability of the empirically derived relationshipbetween modern discharge and cross-sectional channel area to incised valleys when compared at the maximum highstandshoreline of the previous sequence. Incised-valley dimension adjusts over a longer period than the lowstand and is in equilibriumwith drainage-basin area, which is considered a proxy for long-term discharge. Although base-level fall promotes incision,upstream variables control incised-valley dimensions.

INTRODUCTION

The validity of applying the well-established empirically derivedrelationships that explain variations in channel dimensions to incised

valleys is unknown. This is important to address so that the principalcontrolling factors on the dimensions of incised valleys and theirassociated potential as hydrocarbon reservoirs or ground-water aquiferscan be established. Additionally, incised valleys often serve as conduitsthrough which sediment is delivered to the continental slope and riseduring lowstands. Better understanding the controls on incised-valleydimension may improve prediction of the size of their associated deep-water depositional environments.

Incised valleys are different from channels. Channels are the directresult of flowing water exerting an eroding force that exceeds theresistance of surrounding Earth materials (Schumm et al. 1984). Incised

valleys, in coastal settings, form through one or multiple cycles of sea-level fall and rise and are defined as fluvially eroded, elongate

topographic lows associated with a fall in base level and bounded belowby a regionally mappable sequence boundary (Zaitlin et al. 1994). Incisedvalleys are typically an order of magnitude larger than single channelforms (Schumm and Ethridge 1994) and can reach lengths in excess of hundreds of kilometers, widths of tens of kilometers, and depths to 100 m(e.g., Zaitlin et al. 1994; Morton et al. 1996; Anderson et al. 2004;Rodriguez et al. 2005; Gibling 2006). Controls on channel morphology

have been investigated from flume experiments and observational studies

of modern channel adjustments (Leopold and Maddock 1953; Schumm

1960; Schumm 1968; Knighton 1998). Controls on incised-valley depth

and width have been constrained for many individual ancient (Ardies et

al. 2002; Feldman et al. 2005) and late Pleistocene (Blum 1993; Blum et al.

1994; Talling 1998; Lericolais et al. 2001; Wellner and Bartek 2003)

valleys. However, Schumm and Ethridge (1994) found valley morphology

to be highly variable when comparing valleys of different age. To

accurately identify the chief factors that control incised-valley dimension

it is necessary to compare valleys of the same sequence at a similar

location with respect to the depositional shoreline break.

Fluvial processes are controlled by many interrelated parameters (see

Schumm 1977; Schumm and Brackenridge 1987; Orton and Reading

1993; Miall 1996; Blum and Tornqvist 2000). Discharge regime, including

stream power and regularity of the discharge, and sediment character-

istics (load and substrate) are the primary controls on channel dimension

(Schumm 1977; Knighton 1998). Studies of modern rivers show that the

cross-sectional area of channels grow with increasing discharge (Leopold

and Maddock 1953; Best 1987; Best and Ashworth 1997; Knighton 1998).

Although channel size is closely related to discharge (Lacey 1930;

Leopold and Maddock 1953; Knighton 1998; Miall and Jones 2003) and

hydrologic regime (Osterkamp 1980; Yu and Wolman, 1987; Knighton

1998), width/depth ratios are strongly linked to lithology (Schumm 1960;

Schumm 1977; Ferguson 1987; Friend 1993; Knighton 1998; Orton and

Reading 1993; Nanson et al. 2005; Gibling 2006).

Copyright E 2007, SEPM (Society for Sedimentary Geology) 1527-1404/07/077-213/$03.00



Studies of incised-valley geometry have outlined several key controllingfactors including rate and magnitude of base-level fall, gradient profiles,

climate, tectonics, substrate characteristics, and inherent fluvial processes

(Leopold et al. 1964; Schumm and Brackenridge 1987; Schumm 1993;Schumm and Ethridge 1994; Wood et al. 1993; Talling 1998; Posamentier

and Allen 1999; Blum and Tornqvist 2000; van Heijst and Postma 2001;

Ardies et al. 2002; Gibling 2006). The initial response of a river to base-

level fall below a break in slope from low gradient to high gradient is

vertical incision, with deeper incision associated with greater magnitude

and duration of the lowstand and/or greater difference between gradientsacross the break in slope (Schumm and Brackenridge 1987; Schumm

1993; Schumm and Ethridge 1994; Wood et al. 1993; Talling 1998;Posamentier and Allen 1999; van Heijst and Postma 2001; Wellner and

Bartek 2003; Simms et al. 2006). Slow rates of base-level fall result in

wider valleys than rapid rates because lateral channel migration and

associated slope adjustment processes operate over a longer time period

(Schumm and Ethridge 1994). Climate change, manifested as variations in

temperature, precipitation and flood frequency and/or magnitude, canresult in valley incision or accretion and varying aspect ratios (Schumm

and Brackenridge 1987; Gibling 2006); however, predicting the fluvial

response to any given direction of climate change is problematic

(Schumm and Ethridge 1994; Blum and Tornqvist 2000). Schumm and

Ethridge (1994) and Ardies et al. (2002) demonstrated that valley

dimension (width, depth, and cross-sectional area) may vary locally asa result of changes in substrate, tectonic influence (faulting or jointing),

valley bends, or the confluence of tributary junctions.

Although drainage-basin areas are linked to modern discharge, theirrole in defining incised-valley size and shape has not been constrained.

The relationship between incised-valley dimension and drainage-basinsize is examined here using nine incised valleys, which are bounded by

a sequence boundary associated with the Oxygen Isotope Stage 2

lowstand in sea level (Stage 2; Fig. 1). The systems are distinguished bydrainage basins that vary significantly in size and cover a margin that

presently has a steep climate gradient, and variable coastal-plain, shelf,

and slope widths and gradients.

STUDY AREA

The study area is located along the microtidal (, 1 m) northern Gulf

of Mexico margin, extending from south-central Texas to Alabama(Fig. 1). The margin is characterized by a steep E–W trending climate

gradient defined, in part, by an increase in precipitation from 50 cm/year

in central Texas to 150 cm/year in Alabama (Fig. 1; Thornthwaite 1948).Northern Gulf of Mexico (from Mustang Island, Texas to Morgan

Peninsula, Alabama) shelf gradients and widths vary across the margin

and range from 0.39 m/km for the 200-km-wide eastern Texas shelf to

0.76 m/km for the 85-km-wide central Texas shelf (Fig. 2; Table 1).Drainage systems examined include the Magnolia, Fish, Mobile–Tensaw,

Bayou La Batre, Calcasieu, Sabine, Trinity, Lavaca, and Nueces rivers.

These basins vary in size by three orders of magnitude, ranging fromsystems confined to the coastal plain that drain less than 100 km2 to

systems that originate in piedmont regions and drain over 100,000 km2.

FIG. 1.—Regional study area map of the northern Gulf of Mexico margin depicting examined drainage basins numbered from smallest to largest (1 5 Bayou LaBatre, 2 5 Magnolia River, 3 5 Fish River, 4 5 Lavaca River, 5 5 Calcasieu River, 6 5 Sabine River, 7 5 Nueces River, 8 5 Trinity River, 9 5 Mobile–TensawRiver), geologic units, the location of the Oxygen Isotope Stage 5e shoreline in Texas and Louisiana, isopleths (dashed lines) of mean annual precipitation in centimeters,shelf bathymetry, and map locations for Figures 3 and 4.

214 R. MATTHEUS ET AL. J S R



The systems primarily drain Cretaceous to Holocene deposits that

parallel the modern shoreline from east to west throughout the Gulf coastal plain (Fig. 1). The extent of the associated incised valleys was

previously mapped across the Gulf of Mexico shelf (Kindinger et al. 1994;

Thomas and Anderson 1994; Anderson et al. 2004; Bartek et al. 2004;

Eckles et al. 2004).

METHODOLOGY

Incised-valley dimensions vary in a downstream direction, making it

important to compare systems at the same respective location. Dimensions

were derived at or in close proximity to the modern bay-head delta for each

of the nine systems using single-channel high-resolution ‘‘chirp’’ and

‘‘boomer’’ seismic data and/or cores (Fig. 3). This common location ensures

that valley dimensions are compared and evaluated independently of wave

and tidal ravinement and respective drainage-basin areas are accuratelyquantified (Figs. 1, 3). The modern bay-head delta location is also close to

the Sangamon shoreline (named Ingleside Shoreline in Texas and Gulfport

Formation in Alabama) that parallels the modern shoreline across the

northern Gulf of Mexico (Figs. 1, 3; Bernard and LeBlanc 1965; Otvos

1991; Otvos and Howat 1992; Blum et al. 2003). The Sangamon shoreline is

the best location for comparison because this is approximately where

coastal-plain and shelf gradients intersected during the last interglacial

period (Oxygen Isotope Stage 5e;, 120 ka) and where a knickpoint would

have been created when the inner shelf initially became exposed.

Shelf gradients were calculated for the nine systems using digital

bathymetric data from the National Geophysical Data Center (Fig. 2;

Table 1). Paleogeographic maps that show positions of the drainage

systems along the shelf break during the last lowstand were used to

determine measurement locations (Kindinger et al. 1994; Thomas and

Anderson 1994; Anderson et al. 2004; Bartek et al. 2004; Eckles et al.

2004). Coastal-plain gradients, measured seaward of the first prominent

change in slope, were calculated from USGS digital elevation data

(Table 1). Oxygen Isotope Stage 5e coastal-plain and shelf gradients and

shelf-break depths are assumed to be similar to modern values.

Drainage basins were defined as catchment area landward of the

modern bay-head delta and measured using the GIS software package

ArcView GIS 3.2 and hydrologic data files for the Gulf coast states from

the 2000 US census database (Figs. 1, 3; Table 1). Structure maps of the

Stage 2 sequence boundary at estuarine locations (Table 1) for the Nueces

(Fig. 4), Trinity, Sabine, Calcasieu, Lavaca, Bayou La Batre (Fig. 3),

Magnolia (Fig. 3), Fish (Fig. 3), and Mobile–Tensaw (Fig. 3) incised

valleys demonstrate that the bay-head delta locations are not influencedby tributary junctions, valley bends and constrictions, or faults that could

locally influence valley dimensions (Best and Ashworth 1997; Ardies et al.

2002). Incised-valley cross-sectional area was calculated to the nearest

10 m2 from data oriented perpendicular to valley axis using the Stage 2

sequence boundary as the lower datum and the correlative interfluvial

area as the upper datum. Position of the upper datum relative to modernsea level varies slightly between systems (22.0 to +1.0 m) depending on

the location along dip with respect to the modern shoreline. Maximum

valley width and depth were measured to the nearest 1.0 m.

Incised-valley dimension for the Magnolia, Fish, La Batre, and

Mobile–Tensaw was measured from high-resolution ‘‘chirp’’ seismic

data, cores, and previously published platform boring descriptions from

the Alabama Department of Transportation (Fig. 3). Seismic data were

collected across the Magnolia, Fish, and La Batre valleys using anEdgeTech 216S sub-bottom profiler. The Nueces incised-valley dimen-sions were measured using high-resolution ‘‘boomer’’ seismic data

obtained from the USGS and cores (Fig. 4). Cores were collected using

a vibracoring system, a hydraulic rotary-drill rig mounted on a small

barge, and a truck-mounted GeoProbetm coring system. Two-way travel

time was converted to depth on the basis of a seismic velocity of 1500 m/s

(speed of sound in water). This velocity was validated by cores sampling

an exposure surface recognized by a sharp increase in oxidation, roots,clay rip-up clasts, and shear strength at depths corresponding to a regional

seismic surface characterized by erosional truncation below and onlap

above (Figs. 4–7). Shallow water depths at bay margins prevented seismic

data collection at the Nueces and Mobile–Tensaw Stage 2 interfluves. For

these systems, incised-valley flanks were projected linearly to the modern

floodplain (Figs. 4, 8). Error bars associated with the projections were

derived by measuring the difference between valley cross-sectional areas,calculated using end-member flank profiles (Fig. 8). Radiocarbon dating

(AMS) of shell and organic material sampled from the oldest trans-

gressive valley-fill sediments and below the valleys was performed by the

Woods Hole Oceanographic Institution (Table 2) to help verify the

timing of valley formation.

Incised-valley dimension of the Mobile, Sabine, Trinity, Lavaca, and

Calcasieu were taken from published data and interpretations. May

(1976) delineated the Mobile Incised Valley from boring descriptionscollected by the Alabama Department of Transportation’s Bureau of

Materials and Tests, collected in association with interstate construction

FIG. 2.—Gradient profiles extending from thelower coastal plain to the continental slope forthe studied fluvial systems. Profiles are hinged atthe shoreline for easy comparison. The La Batre(1), Magnolia (2), and Fish (3) profiles, seawardof the shoreline, are the same as the Mobile– Tensaw (9). See Table 1 foradditional information.

CONTROLS ON INCISED-VALLEY DIMENSION 215J S R



across the Mobile Delta. Before using the cross section to measure valley

dimensions, the descriptions of May (1976) were verified by four

vibracores and six rotary-drill cores collected throughout the delta andnorthern Mobile Bay (Figs. 3, 7B). Published maps of the Stage 2

sequence boundary were used to measure incised-valley dimensions for

the Sabine, Trinity, Lavaca, and Calcasieu (Table 1; Fig. 8). The large

contour interval of the Lavaca Stage 2 map (26 m) prohibited precise

delineation of the valley flanks, which explains the large margin of

measurement error (Fig. 8).

RESULTS AND INTERPRETATION

It is essential that the position of the Stage 2 sequence boundary be

verified at valley-dimension measuring locations for the comparisons

between incised valleys to be meaningful. A seismic surface, characterizedby erosional truncation below and onlap above, was recognized in strike-

oriented profiles collected just bayward of the La Batre, Magnolia, Fish,

Mobile, and Nueces bay-head deltas (Figs. 4, 5). The surface was mapped

regionally throughout the estuaries (Figs. 3, 4), was sampled with cores(Fig. 6), and correlates with the previously mapped Stage 2 sequence

boundary on the northern Gulf of Mexico shelf (Anderson et al. 2004;

Simms et al. in press).

La Batre Valley

Core MS-04-4, collected in Mississippi Sound adjacent to the La Batrevalley, sampled olive-gray clay with a basal peat layer directly above

mottled dark-yellowish-orange to medium-gray stiff oxidized clay to

sandy clay containing plant fragments (Fig. 6B). The contact between the

peat and the oxidized clay is sharp and is interpreted as marking the

transition from subaerial exposure to estuarine conditions. The peat wasradiocarbon dated at 5470 6 50 yr BP, which approximates the timing

of inundation (Table 2; Tornqvist et al. 2004). The contact between the

peat and oxidized clay was traced regionally throughout MississippiSound and Mobile Bay with seismic data. This surface defines the La

Batre and Mobile valleys and is interpreted as the Stage 2 sequence

boundary. The cross-sectional area of the La Batre Stage 2 Valley was

measured from seismic section A–A9 as 6,000 m

2

(Figs. 5A, 8; Table 1).

Fish and Magnolia Valleys

A strike-oriented core transect across the modern Fish River bay-head

delta (Fig. 6A), located 0.75 km landward of seismic transect C–C9

(Fig. 5C), sampled medium-gray to dark-greenish-gray clay overlying

fine- to coarse-grained, subrounded, well-sorted, dark-yellowish-orangeto yellowish-gray, oxidized sand (Fig. 6C). Radiocarbon dates above the

contact were all Holocene in age, while dates below the contact were

radiocarbon dead or Pleistocene in age (Fig. 7A; Table 2). A dip-orientedseismic line (Fig. 5D) collected up the middle of Weeks Bay into the Fish

River imaged a high-amplitude reflection at the same depth as the

contact. This seismic line was used to tie cross section E–E9 (Fig. 7A) with

the seismic grid, and shows that the high-amplitude reflection defines the

base of the Fish and Magnolia valleys (Fig. 5B, C). The contact isinterpreted as the Stage 2 sequence boundary on the basis of the dates

(Fig. 7A; Table 2), evidence of subaerial exposure (Fig. 6C), andcorrelation with a regional seismic surface characterized by incision

(Fig. 5). The cross-sectional areas of the Magnolia and Fish Stage 2

valleys, measured from seismic sections B–B9 and C–C9, are 2,200 m2 and

5,750 m2, respectively (Figs. 5B, C, 8; Table 1).

Mobile Valley

Cores collected in the Mobile bay-head delta front (MD-02-1; Fig. 7B)

and northern Mobile Bay (MB-03-1; Fig. 6A) sampled organic-rich olive-

T A B L E

1 . —

D i m e n s i o n s o f e x a m i n e d d r a i n a g e s y s t e m s a n d a s s o c i a t e d

i n c i s e d v a l l e y s .

N o .

S y s t e m

D r a i n a g e - b a s i n

a r e a ( k m

2 )

I n c i s e d - v a l l e y c r o s s -

s e c t i o n a l a r e a ( m

2 )

I n c i s e d - v a l l e y

w i d t h ( m )

I n c i s e d - v a l l e y

d e p t h ( m )

S h e l f - b r e a k

d e p t h ( m )

S h e l f g r a d i e n t

( m / k m )

L o w e r c o a s t a l - p l a i n

g r a d i e n t ( m / k m )

R e f e r e n c e s f o r m a p o f S t a g e 2 s e q u e n c e

b o u n d a

r y a t e s t u a r i n e l o c a t i o n

1

L a B a t r e

7 5

6 0 0 0

1 0 6 5

1 1

2 5 0

0 . 6 3

2 . 3 5

G r e e n e 2 0 0 6

; G r e e n e e t a l . 2 0 0 7

2

M a g n o l i a

1 1 1

2 2 0 0

5 2 2

7

2 5 0

0 . 6 3

1 . 6 3

D u r a n 2 0 0 6

3

F i s h

4 2 7

5 7 5 0

8 6 2

1 2

2 5 0

0 . 6 3

1 . 4 9

D u r a n 2 0 0 6

4

L a v a c a

6

7 7 8

3 7 8 0 0

5 5 2 3

1 8

2 6 5

0 . 7 6

0 . 5 4

W i l k i n s o n a n d B y r n e 1 9 7 7

5

C a l c a s i e u

1 0

3 3 9

3 2 0 0 0

2 7 8 1

2 4

2 8 0

0 . 4 0

0 . 2 5

N i c h o l e t a l . 1 9 9 6

6

S a b i n e

2 9

6 1 6

4 4 4 0 0

4 2 3 5

1 3

2 7 0

0 . 3 9

0 . 1 8

A n d e r s o n e t

a l . 1 9 9 1 ; M o r t o n e t a l . 1 9 9 6

7

N u e c e s

4 9

0 0 2

4 4 0 0 0

3 7 1 0

2 2

2 1 0 5

1 . 2 0

0 . 2 4

S i m m s 2 0 0 6

8

T r i n i t y

5 3

5 7 3

9 7 5 0 0

1 6 5 9 4

2 2

2 6 0

0 . 4 4

0 . 2 0

S m y t h 1 9 9 1 ; R o d r i g u e z e t a l . 2 0 0 5

9

M o b i l e – T e n s a w

1 3 3

5 5 2

2 0 1 0 0 0

1 4 3 0 4

1 9

2 5 0

0 . 6 3

0 . 1 3

K i n d i n g e r e t a l . 1 9 9 4 ; G r e e n e 2 0 0 6

C o a s t a l - p l a i n g r a d i e n t s , s h e l f g r a d i e n t s , a n d s h e l f - b r e a k d e p t h s w e r e c a l c u l a t e d f r o m d i g i t a l b a t h y m e t r i c ( N a t i o n a l G e o p h y s i c a l D a t

a C e n t e r ) a n d e l e v a t i o n ( U S G S ) d a t a .




gray clay to clayey sand with wood fragments and Rangia cuneata shellsabove a mottled greenish-gray to dark yellowish-orange stiff oxidized clay(Fig. 6A; Metcalf and Rodriguez 2003). The contact between the two

units is sharp and erosional as evidenced by clay rip-up clasts (Fig. 6A)and corresponds with the base of the Mobile Valley as interpreted by May(1976). The contact is interpreted as the Stage 2 sequence boundary on the

basis of evidence of subaerial exposure, Holocene dates obtained abovethe contact (Table 2) and seismic data collected in northern Mobile Baythat shows the contact to be a regional erosional surface. The cross-sectional area of the Mobile Stage 2 Valley was measured from core

transect F–F9 as 201,000 m2 (Figs. 7B, 8; Table 1).

Nueces Valley

Core NB02-04, collected from the center of Nueces Bay (Fig. 6D),sampled a gray sandy mud with organic material and juvenile Rangia flexuosa, Mulinia lateralis, and sparse oyster (Crassostrea virginica orOstrea equestris) shells, above a stiff gray mottled mud with calcareous

nodules. The sediment stiffness and presence of calcareous nodules belowthe contact is evidence of subaerial exposure (Fig. 6D). The contactbetween the two units is sharp and corresponds with the base of the

Nueces paleovalley as mapped with seismic data (Fig. 4). A Balanus sp.

plate sampled above the contact at 216.25 m below sea level in coreNB02-04 is 8430 6 45 yr BP (Table 2), and the contact is interpreted as

the Stage 2 sequence boundary. Core NB03-01, collected over seismicprofile G–G9 (Fig. 4), did not sample below the base of the valley, but anarticulated R. flexuosa and N. concentrica sampled directly above the baseof the valley are 7700 6 70 and 7900 6 100 yr BP, respectively

(Table 2). The cross-sectional area of the Nueces Stage 2 Valley wasmeasured from seismic section G–G9 as 97,500 m2 (Figs. 4, 8; Table 1).

Valley Characteristics

In addition to cross-sectional area, maximum incised-valley depth andwidth were measured for the valleys discussed above, and these data were

supplemented with measurements from data interpreted previously for

FIG. 3.—Map of coastal Alabama showing the position of the Gulfport Formation (Oxygen Isotope Stage 5e shoreline), the examined coastal-plain drainage basins of Bayou La Batre, the Fish River, and the Magnolia River, seismic and lithologic data examined, and the locations of key seismic and lithologic transects used to delineateincised-valley dimensions shown in Figures 5, 6, and 7.




the Sabine (Anderson et al. 1991; Morton et al. 1996), Trinity (Smyth

1991; Rodriguez et al. 2005), Lavaca (Wilkinson and Byrne 1977), and

Calcasieu (Nichol et al. 1996) Stage 2 valleys (Fig. 8; Table 1). Valley

cross-sectional areas vary from 2,200 m2 for the Magnolia System to

201,000 m2 for the Mobile–Tensaw System (Fig. 8; Table 1). The

drainage basins are defined by comparable drainage-network character-

istics, exhibiting well-defined dendritic drainage patterns with the

exception of the northeastern wing of the Mobile system, which ischaracterized by fault-bounded drainage. Drainage densities are similar

for all systems; however, drainage-basin sizes range from 75 km2 for

Bayou La Batre to 133,552 km2 for the Mobile–Tensaw (Table 1).

DISCUSSION

Incised-valley depth is influenced by the magnitude and duration of the

sea-level lowstand (Schumm 1993; van Heijst and Postma 2001) and the

difference between slopes of the exposed sea floor and the fluvial profile

(Wood et al. 1993; Talling 1998; Posamentier and Allen 1999; Wellner

and Bartek 2003; Blum and Tornqvist 2000). Differences in dimensions

between the incised valleys cannot be attributed to variations in the

magnitude and duration of the Stage 2 eustatic sea-level event across

the Gulf of Mexico margin. However, along-strike variability in shelf

morphology, particularly the differences in the depth of the shelf break

(250 m offshore Alabama to 2105 m offshore southern Texas), should

impact the duration of the lowstand and associated knickpoint migration.

Talling (1998) predicted increased incision depth with increased margin

convexity from comparing end-member type margins (e.g., Tenryu River,Japan vs. Colorado River, Texas). Coastal-plain gradients for the studiednorthern Gulf of Mexico systems vary from 0.13 to 2.35 m/km and are

approximately an order of magnitude less than their associated shelf gradients (0.39 to 1.20 m/km; Table 1). Thalweg depths among the ninestudied incised valleys vary between 7 and 24 m (Fig. 8). Incision depthcorrelates neither with the ratio of coastal-plain to shelf gradient nor with

the depth of the shelf break (Fig. 9A). The adjacent Sabine and Trinitysystems, for example, are characterized by similar coastal-plain gradients(, 0.2 m/km), shelf gradients (0.4 m/km), and shelf-break depths (70–

60 m), but rather than downcutting to similar depths during the LastGlacial Maximum, they incised 13 m and 22 m at the bay-head deltalocation, respectively (Fig. 8). Although no clear correlation betweenincision depths and the ratio of coastal-plain to shelf gradients was

recognized, this could be the result of insufficient variability in slopesacross the northern Gulf of Mexico for a relationship to emerge.

Varying depths of fluvial incision could also be attributed to upstream

variables. For example, the large degree of variability between incisiondepths for the Sabine and Trinity rivers could be linked to varying degreesof erosion generated by these systems due to differences in flood regimes.This would imply that the Sabine did not progress as far towards an

equilibrium profile and incise as deeply into the coastal prism as theTrinity. Furthermore, differences in long-term sediment supply across themargin control highstand-wedge dimension, which should influence

incision depth. This variation is largely accounted for in the measuredratio of coastal-plain to shelf gradients, under the assumption that the

FIG. 4.—Structure map of the Stage 2Sequence Boundary in Nueces Bay (top). Inter-preted ‘‘boomer’’ seismic profile G–G9 collectedperpendicular to the Nueces Incised Valley closeto the modern bay-head delta shows the posi-tions of the seafloor and the Oxygen IsotopeStage 2 Sequence Boundary (bottom). Dashedlines indicate projection of the sequence bound-ary up the valley flanks to the interfluve whereseismic data collection was not possible. Thearea used to calculate incised-valley dimensionsis indicated by the cross-hatch pattern. Valuesfor valley dimensions are listed in Table 1.

R

FIG. 5.—Uninterpreted and interpreted strike-oriented chirp seismic profiles for the A) La Batre, B) Magnolia, and C) Fish incised valleys collected close to the modernbay-head delta showing the position of the seafloor and the Oxygen Isotope Stage 2 Sequence Boundary. D) A dip-oriented seismic profile collected up the Fish River wasused to tie cross section E–E9 with the seismic grid (Figs. 3, 7). The cross-hatch pattern indicates the area used to calculate incised-valley dimensions. Valley depth, width,and cross-sectional area are listed in Table 1. The locations of the seismic profiles are shown in Figure 3.




modern reflects the previous highstand. The assumption should be valid,

considering that the Alabama valleys had similar histories over at leastthe last two cycles and sequence boundaries amalgamate at the interfluves

(Greene et al. 2007). Additionally, elevations of the interfluves (the upperdatum) with respect to sea level vary by only 3 m between valleys.

Comparison of incised-valley dimensions to drainage-basin size

suggests a significant control of upstream variables on incised-valleymorphology. There is a distinct linear relationship between drainage-

basin size and incised-valley cross-sectional area and width (Fig. 9B, C).This is analogous to the empirically derived relationship between

discharge and cross-sectional channel dimension (Leopold and Mad-

dock 1953; Leopold et al. 1964; Schumm 1977; Yu and Wolman 1987;

Knighton 1998). Discharge is a reflection of drainage-network

characteristics (drainage density, area, and lithology) and climate

(precipitation, flood frequency and magnitude, and vegetation). Climateexhibits significant control on incised-valley morphology by influencing

incision and accretion (Schumm and Ethridge 1994; Blum and Tornqvist

2000) and regulating vegetation, which can have an effect on incised-valley width by governing bank stability (Schumm and Brackenridge

1987). The studied systems drain comparable sedimentary deposits, have

comparable drainage densities, are characterized by similar fluvial

profiles (smooth, low gradient, concave upward), were influenced by the

FIG. 6.—Core photographs of the Oxygen Isotope Stage 2 Sequence Boundary from: A) upper Mobile Bay (MB 03-1); B) Mississippi Sound (MS 04-4); C) the FishRiver Delta (WB-04-6); and D) Nueces Bay (NB-02-04) showing evidence of subaerial exposure. The cores were used to help verify seismic data interpretations. SeeFigures 3 and 4 for core locations.




same rate and magnitude of sea-level fall, and were examined at similar

locations deemed not to be influenced by local perturbations (valleybends, constrictions, or faults). These factors, along with the non-

relationships between incision depth and ratio of coastal-plain to shelf gradients and shelf-break depth (Fig. 9A), suggest that inland controls,

which dictate discharge (climate, drainage-basin characteristics), are thedominant variables controlling incised-valley dimensions across the

study area.

Drainage basins of similar size located along the same margin but indifferent climates should have different discharge regimes and rates of

morphologic change. Climate proxies suggest that the northern Gulf of Mexico margin was characterized by a more uniform climate regime

during the Last Glacial Maximum (LGM) than today. Stalagmites fromthe Edwards Plateau of central Texas exhibit an increase in growth rate

during the LGM, indicating a regionally cooler and wetter climate(Musgrove et al. 2001). Vegetation maps compiled for the Gulf during the

LGM show a broad distribution of temperate woodland species east of the Mississippi River (Adams and Faure 1997), a region presently

characterized by steep gradients of climate and vegetation (Koch et al.

2004; Anderson et al. 2004). The strong correlation between drainage-basin size and incised-valley width and cross-sectional area either support

these studies that suggest that the northern Gulf of Mexico margin wascharacterized by a more uniform climate regime during the LGM than

today, and/or indicate that variations in drainage-basin size overwhelm

climate change as a control on incised-valley dimensions.

Incised-valley dimensions near the bay-head delta were influenced over

a longer duration than the LGM. This downplays the importance of

a uniform climate on valley dimensions during this relatively short time

period. The bounding surface that defines the studied valleys is mapped

throughout the Gulf and is constrained by numerous radiocarbon ages. It

is clear that the surface upon which measurements of valley dimensions

are based formed in response to falling sea level, which culminated in

a lowstand at the LGM. However, at this up-dip location the surface is

diachronous. For example, the interfluve, which is the upper datum used

for measurement, and the upper valley flanks were sculpted over a longer

period than the base of the valley (Strong and Paola 2006). Drainage-

basin size likely reflects a combination of all inland controls on valley

geomorphology and can be considered a proxy for long-term discharge.

Incised-valley cross-sectional area and width develops over a longerperiod than the duration of the lowstand, which in part explains the

strong correlation with drainage-basin size (Fig. 9B).

CONCLUSIONS

A distinct linear relationship between drainage-basin size and incised-

valley cross-sectional area and width emerged when nine Stage 2 incised

valleys were compared along the northern Gulf of Mexico margin. These

correlations suggest that the empirically-derived relationship between

channel size and discharge is applicable to incised valleys when compared

at a common location. Since the level of variability across the northern

FIG. 7.—Cross sections based on cores acrossthe modern A) Fish and B) Mobile bay-headdeltas used to verify the position of the OxygenIsotope Stage 2 Sequence Boundary (Fig. 6).Light-gray shading indicates undifferentiatedHolocene valley-fill sediments. Pre-Holocenesediments are dark gray. White boxes representthe distribution of platform-boring descriptions.Black boxes indicate lithologic data from col-lected rotary-drill cores (F–F9) and GeoprobeTM

cores (E–E9). Locations are shown in Figure 3.




FIG. 8.—Incised-valley cross sections at uniform scale. The elevation of the interfluve is 0 m on each cross section. Valleys are numbered in order of their drainage-basin size with 1 being the smallest and 9 the largest (see Fig. 1). Light-gray areas indicate projection of the sequence boundary to respective interfluves. Calculated errormargins are based on estimated minimum and maximum values associated with these projections as determined from hypothetical end-member flank profiles.

TABLE 2.— Radiocarbon ages and type of material sampled from the cores.

Lab code Sample name Material Conventional 14C age: yr BP; 1s

OS-46675 Mississippi Sound, MS-04-4 (591 cm) Peat 5470 6 50

OS-42768 Mobile Bay, MB-03-01 (1095 cm) Wood 6220 6

35OS-37369 Mobile bay-head delta, MD-02-1 (1726 cm) Wood 8710 6 35OS-37362 Mobile bay-head delta, MD-02-1 (1809 cm) Wood 8770 6 50OS-47774 Weeks Bay, WB-04-2 (1427 cm) Wood 20500 6 100OS-46677 Weeks Bay, WB-04-2 (1645 cm) Wood . 48000OS-46678 Weeks Bay, WB-04-3 (625 cm) Wood . 48000OS-46679 Weeks Bay, WB-04-7 (674 cm) Wood 4620 6 25OS-46680 Weeks Bay, WB-04-7 (982 cm) Wood 12150 6 45OS-42917 Nuece s Bay, NB-02-04 (1625 cm) Balanus sp. 8430 6 45OS-41704 Nuece s Bay, NB-03-01 (1068 cm) R. flexuosa* 7700 6 70OS-41705 Nuece s Bay, NB-03-01 (1153 cm) N. concentrica* 7900 6 100

Samples were processed at the NSF AMS Radiocarbon Laboratory of the Woods Hole Oceanographic Institution’s National Ocean Sciences Accelerator MassSpectrometry Facility. Articulated bivalves dated are indicated by an asterisk. Sample depths (reported parenthetically) are relative to sea level.




Gulf of Mexico margin is not unique, comparison of incised-valleys alongdifferent margins should yield insight into the size of respective drainagebasins and perhaps the size of associated basin-floor fans. The degree of lateral variability across a margin necessary to degrade the linearrelationships is unknown, but it is important to constrain. Measurements

of incised-valley dimensions should be taken close to the maximumhighstand shoreline of the previous sequence, and local effects on

morphology need to be taken into account. The absence of a robust

correlation between incised-valley depth and difference between fluvialand shelf slopes may be due to insufficient variability in gradients and sealevel along the margin and/or upstream variables.

The processes that sculpt an incised valley operate over a longer periodthan the duration of the lowstand. This enables incised-valley dimensionsto equilibrate with the size of its drainage basin, which likely reflects thespectrum of inland controls on valley geomorphology. The strong

correlation between drainage-basin size and incised-valley cross-sectionalarea and width can be applied only when different valleys across a marginare compared at a common location. The relationship is not necessarilyapplicable for an incised valley measured at multiple locations along dip.Although the sea-level fall that exposed a steeper shelf (downstreamvariables) promoted valley incision, incised-valley dimensions aredominantly controlled by landward variables.

ACKNOWLEDGMENTS

This research was supported by the National Science Foundation GrantEAR-0107650 and the American Chemical Society Petroleum Research FundGrant ACS-PRF 36694-G8. Valuable field assistance was provided by JoeLambert, Jesse Maddox, and Patrick Taha. We thank Jack Kindinger forproviding copies of USGS seismic data collected with the help of Mike Blumin Nueces Bay. The manuscript was improved by comments on an earlierversion by Ron Steel, Robert Dalrymple, Brian Zaitlin, and TorbjornTornqvist. Journal reviewers Martin Gibling, Andres Aslan, and JohnHolbrook provided insightful and conscientious reviews, for which we aregrateful.

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Received 31 March 2006; accepted 10 October 2006.


Control of Upstream Variables on Incised-Valley Dimension - JSR, Mattheus Et Al, 2007

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