seaward-branching coastal-plain and piedmont incised-valley systems

Journal of Sedimentary Research, 2007, v. 77, 139–158 Research Article DOI: 10.2110/jsr.2007.016

SEAWARD-BRANCHING COASTAL-PLAIN AND PIEDMONT INCISED-VALLEY SYSTEMS THROUGH MULTIPLE SEA-LEVEL CYCLES: LATE QUATERNARY EXAMPLES FROM MOBILE BAY AND MISSISSIPPI SOUND, U.S.A. D. LAWRENCE GREENE, JR.,1 ANTONIO B. RODRIGUEZ,2

AND

JOHN B. ANDERSON3

1

ConocoPhillips, 600 N. Dairy Ashford, Houston, Texas, 77079, U.S.A. 2 University of North Carolina at Chapel Hill, Institute of Marine Sciences, 3431 Arendell St., Morehead City, North Carolina, 28557, U.S.A. 3 Rice University, Department of Earth Science, Box 1982, Houston, Texas, 77251-1982, U.S.A. e-mail: [email protected]

ABSTRACT: Most incised valleys become more organized seaward through tributaries merging with the main trunk valley. Late Quaternary incised valleys on the Mississippi and Alabama inner continental shelf branch seaward, although they do coalesce towards the shelf break where they feed shelf-edge deltas. To link fluvial systems with their associated previously mapped incised valleys on the shelf, and evaluate the contribution of small coastal-plain valley systems to the lowstand systems tract, high-resolution seismic data and cores were collected from Mobile Bay, eastern Mississippi Sound, and the Mobile bay-head delta. These data show four unconformity-bounded stacked units, and this study focuses on the upper two regionally mappable units. The upper two unconformities were sampled in core as exposure surfaces, and, on the basis of stratigraphic position, depth of incision, and 14C dates, are interpreted as sequence boundaries. The shallowest sequence boundary (A) formed in response to the Oxygen Isotope Stage 2 sea-level lowstand, while the underlying sequence boundary (B) formed in response to an earlier lowstand (Stage 6?). A map of Sequence Boundary B shows a network of seaward-branching valleys, 20–35 m deep and 0.6–7.7 km wide, that extend across Mobile Bay and Mississippi Sound. These valleys, which are bounded by welldeveloped terraces, are extensions of the piedmont Mobile fluvial system and the coastal-plain Fowl and La Batre fluvial systems. A map of the Stage 2 Sequence Boundary shows that all systems generally reoccupied their previous valley positions and are separated by interfluve plateaus. Valley fill between Sequence Boundary B and the Stage 2 Sequence Boundary is generally composed of a basal unit of alluvial sediments overlain by bay-head delta deposits; however, Stage 2 to 1 valley fill is composed entirely of central-basin sediments. The presence of bay-head delta deposits on the inner shelf indicates this depositional environment backstepped across the estuaries to modern positions. Backstepping likely occurred as the lowgradient paleovalleys were inundated, resulting in rapid rates of transgression. Fluvial gradients measured on Sequence Boundary B and the Stage 2 Sequence Boundary, below the modern Mobile bay-head delta plain and the estuaries, are very low (1.3–0.3 m/km). The Mobile bay-head delta and upper Mobile Bay are confined by steep topography, which opens basinward into the low-gradient shorelines surrounding lower Mobile Bay and Mississippi Sound. During falling sea level, channel branching likely occurred as the low-gradient fluvial systems extended basinward beyond the confined topography. These channels incised as sea level fell below the steeper shelf break during the lowstand. Stage 2 valley morphology is partially influenced by the position of the underlying Stage 6 valleys. Both coastal-plain and piedmont valley systems exhibit compound fill, indicating that differentiation between these types of incised valleys cannot be based on valley-fill architecture alone.

INTRODUCTION

The Mississippi and Alabama continental shelf is characterized by a complex network of paleovalleys that formed during Oxygen Isotope Stage 2 (last lowstand; Fig. 1; Kindinger et al. 1994; Roberts et al. 2004; Bartek et al. 2004). These valleys feed a 130 km stretch of shelf-margin deltas including the Lagniappe (Kindinger 1988; 1989a; 1989b; Kindinger et al. 1994; Roberts et al. 2004; Bartek et al. 2004) and four Mobile deltas (Fig. 1; Sager et al. 1999; Bartek et al. 2004; Bart and Anderson 2004). Although these valleys coalesce towards the shelf break, they branch seaward near the modern shoreline and on the inner shelf; this is different from other Gulf of Mexico incised-valley systems that become more

Copyright E 2007, SEPM (Society for Sedimentary Geology)

organized seaward (Fig. 1; Anderson et al. 2004). The Mobile incisedvalley system was interpreted as a source for each of the five shelf-edge deltas (Kindinger et al. 1994; Bartek et al. 2004; Fillon et al. 2004; Roberts et al. 2004). Kindinger et al. (1994) and Bartek et al. (2004) mapped incised valleys across the shelf, which link the eastern portion of the Lagniappe Delta, the Western Mobile Delta, and the Eastern Mobile Delta to the Mobile Oxygen Isotope Stage 2 valley system in Mobile Bay (Fig. 1). However, based on detailed chronostratigraphy of the Lagniappe delta and a regional seismic line collected along the shelf break, Roberts et al. (2004) showed that the Eastern and Western Mobile deltas predate the Oxygen Isotope Stage 2 lowstand by 2 or 3 cycles (Oxygen Isotope Stages 6 or 8). This calls into question

1527-1404/07/077-139/$03.00

140

D.L. GREENE, JR. ET AL.

JSR

JSR

SEAWARD BRANCHING OF COASTAL-PLAIN AND PIEDMONT INCISED-VALLEY SYSTEMS

the nature and timing of the feeder systems mapped on the shelf and bay. Previous studies of the Stage 2 Mobile valley differ in regard to valley location and depth in Mobile Bay. Mars et al. (1992), Davies and Hummel (1994), and Hummel and Parker (1995a) mapped the valley as 13–15 m deep, extending from the modern bay-head delta to 15 km north of Morgan Peninsula. Here, it bifurcates into a western valley that continues through the modern tidal inlet and an eastern valley that extends below Morgan Peninsula (Fig. 1; Mars et al. 1992; Davies and Hummel 1994; Hummel and Parker 1995a). Kindinger et al. (1994) mapped the valley as 25–45 m deep, extending along eastern Mobile Bay from south of the modern bay-head delta to 15 km north of Morgan Peninsula. At this location, it bifurcates into a western valley that extends through the modern tidal inlet and an eastern valley that extends below Morgan Peninsula (Fig. 1; Kindinger et al. 1994). The eastern valley bifurcates offshore of Morgan Peninsula, where it is difficult to trace (Kindinger et al. 1994). For the most part, these studies were based on the same seismic and vibracore data sets, indicating that there is significant difficulty with imaging the stratigraphy of Mobile Bay. Incised valleys commonly result from fluvial downcutting in response to a base-level fall and fill during the subsequent transgression with a variety of facies that range from nonmarine through estuarine to open-marine (Nichols 1991; Allen and Posamentier 1991, 1993; Boyd and Honig 1992; Shanley and McCabe 1994; Dalrymple et al. 1992; 1994; Thomas and Anderson 1994; Zaitlin et al. 1994; Buck et al. 1999). Valley-fill stratigraphy may be affected by numerous parameters, including changes in shelf gradient, valley morphology, sea level, climate, and hydrodynamic processes. Zaitlin et al. (1994) classified incisedvalley systems as simple, if the valley filled completely during one lowstand–transgressive–highstand sequence, or compound, if the valley persisted through multiple cycles of incision and fill. A compound incised-valley fill is characterized by multiple internal sequence boundaries in addition to the main basal sequence boundary, whereas a simple incised-valley fill is characterized by only one basal sequence boundary (Zaitlin et al. 1994). Incised-valley systems that extend into mountainous regions (piedmont incised-valley systems) are thought to more commonly exhibit compound valley fills than fluvial systems that are confined to the coastal plain (coastal-plain incised-valley systems; Zaitlin et al. 1994). This is because piedmont incised-valley systems are usually associated with underlying structural features that promote existence through multiple relative sea-level cycles (Zaitlin et al. 1994). The Mobile is a piedmont incised-valley system; however, Kindinger et al. (1994) and Dalrymple et al. (1994) characterized its fill as simple. This study examines the nature and timing of valley incision and fill in Mobile Bay and eastern Mississippi Sound to better evaluate the source of sediment to shelf-margin deltas. Small-scale coastal-plain valley systems, including Bayou La Batre and Fowl River, are examined and compared with the larger Mobile piedmont valley system to evaluate whether their respective valley-fill facies architecture is truly simple and compound, as predicted by Zaitlin et al. (1994).

141

STUDY AREA

Estuaries The Mobile Bay Estuary, which includes Bon Secour Bay in the southeast, is shallow (average depth of 3.5 m), approximately 1,000 km2 in area, and elongate in a north–south orientation (Fig. 2). The estuary is protected from the Gulf of Mexico by Dauphin Island and Morgan Peninsula, is 46 km long from the western end of Morgan Peninsula to the modern bay-head delta, and is 32 km wide at its widest point 6 km north of the Main Pass tidal inlet. Approximately 85% of the Mobile River water discharge passes through the Main Pass Inlet to the Gulf of Mexico, while Pass aux Herons, which connects Mobile Bay to Mississippi Sound, transmits the remainder into the Sound (Ryan 1969). Discharge from coastal streams and rivers results in variations in surface salinity from 0 to 30% (McBride et al. 1991; Hummel and Parker 1995b; Hummel 1996). The western and southeastern shorelines of the estuary contain small pocket bays, low-lying topography (0.5–5 m elevations extending 1–3 km inland), and tidal marshes, whereas the northeastern shoreline is marked by steep topography (7–10 m elevations less than 0.25 km inland) (Fig. 2). The northern shoreline comprises the modern Mobile bay-head delta, with distributary channels, interdistributary bays, and extensive marshlands. The eastern portion of Mississippi Sound includes Heron, Fowl River, Portersville, and Grand bays, has an average depth of 4 m, and is approximately 375 km2 in area. This part of the estuary is located along the Mississippi–Alabama coast and is protected from the Gulf of Mexico by Dauphin Island (Fig. 2). Eastern Mississippi Sound extends 31 km from the eastern end of Dauphin Island to the Grande Batture Headlands and reaches its maximum width of 19 km from the western end of Dauphin Island to the northern shoreline of Grand Bay. Petit Bois Pass connects eastern Mississippi Sound to the Gulf of Mexico. The northern shoreline of the estuary comprises a series of pocket bays, islands, and tidal marshes and is marked by low-lying topography (0–5 m elevation 4– 6 km inland). In general, turbidity is greater in the northern part of the estuary, which is largely attributed to sediment-laden water from Mobile Bay entering the sound through Pass aux Herons (McPherson 1970). The estuaries along the Alabama coast are microtidal with a diurnal tidal range of 0.35–0.80 m (Knowles 1989; Hummel 1996). Wave intensity along coastal Alabama is low to moderate, except for cold fronts and tropical storms that impact the area on average 2.2 times per year (Neumann et al. 1981). Average wave height along the coast is 0.30 m (Jensen 1983) with periods ranging from 3 to 8 seconds (Upshaw et al. 1966). Offshore wave propagation is predominantly from the southeast, corresponding to the prevailing wind direction, which produces a 0.5 to 1.5 m/s net westward-flowing longshore current (Boone 1973; Wilson 1984). Fluvial Systems The piedmont Mobile–Tensaw river system, with a drainage area of approximately 133,500 km2, discharges into northern Mobile Bay forming an extensive (465 km2) bay-head delta (Fig. 2; Gastaldo 1989). The delta is constrained by steep topography on the west and east

r FIG. 1.— Composite paleogeographic map of the last sea-level lowstand (oxygen isotope stage 2) showing a complex network of fluvial systems on the northeastern Gulf of Mexico continental shelf and their associated shelf-edge deltas. Position of the Mobile Incised Valley (MV; Kindinger et al. 1994; Hummel and Parker 1995b; Mars et al. 1992) in Mobile Bay, the Pearl Incised Valley (PV; Kindinger 1988; Kindinger et al. 1994), the Pascagoula Incised Valley (PIV; Kindinger 1988; Kindinger et al. 1994), the West Mobile Valley (WMV; Bartek et al. 2004), and the East Mobile Valley (EMV; Bartek et al. 2004) on the continental shelf, and the East Mobile Deltas (EMD-S, Sager et al. 1999; EMD, Bart and Anderson 2004; Bartek et al. 2004), the West Mobile Delta (WMD-S, Sager et al. 1999; WMD, Bartek et al. 2004), and the Lagniappe Delta (Kindinger 1988, 1989a, 1989b; Bartek et al. 2004; Roberts et al. 2004) at the shelf break are indicated.

142


JSR

FIG. 2.—Map showing the locations of seismic lines and cores collected and incorporated into this study. Inset digital elevation model shows that the Mobile bay-head delta and Upper Mobile Bay are confined by steep topography, which opens basinward into the low-gradient shorelines surrounding lower Mobile Bay and Mississippi Sound. Data shown in subsequent figures are indicated by dark-gray lines. The locations of Grand Batture Headlands (GBH), Grand Bay (GB), Portersville Bay (PB), Fowl River Bay (FRB), Mon Louis Island (MLI), Heron Bay (HB), Pass aux Herons (PAH), and Point Clear (PC) are also indicated.

(Fig. 2). The location of the lower Mobile–Tensaw river system and associated bay-head delta may be, in part, controlled by the interpreted Mobile Graben (Smith 1988). The drainage basin is characterized by anastomosed channels, extensive marshlands and low-lying topography (0–5 m) within the delta plain, extensive floodplains (2–20 km in width) and meander scars in lower and central Alabama, and numerous faultbounded synclines and anticlines within the Appalachian Mountains of northern Alabama and Tennessee. Average water discharge for the Mobile–Tensaw River is approximately 1,788 m3/s (Hummel and Parker 1995b). The Fowl and Bayou La Batre coastal-plain fluvial systems flow into Eastern Mississippi Sound. The Fowl River drainage basin covers

approximately 197 km2 (Fig. 1), and the river meanders 28 km from its headwaters in central Mobile County, flowing southward into Fowl River Bay, a pocket bay located along the north shore of Mississippi Sound (Fig. 2). Average water discharge is 1 m3/s. The drainage basin is characterized by marshland, low-lying areas (0–5 m), unpaired terraces, and well-developed floodplains (averaging 0.5–1.2 km in width). The Bayou La Batre drainage basin covers approximately 75 km2 (Fig. 1), and the river meanders 12 km from its headwaters in southern Mobile County southward through the Grand Bay Swamp before flowing into eastern Mississippi Sound. Average water discharge is 0.4 m3/s. The drainage basin is characterized by small-scale floodplains (averaging only a few hundred meters to 1.0 km in width), marsh, and swampland areas.

JSR


143

TABLE 1.— Radiocarbon ages and type of material sampled from the cores.

Lab code

Sample name

Material

Conventional 14C age: yr BP; 1s

Calibrated 14C age: cal. yr BP; 1s

OS-44717 OS-44718 OS-44719 OS-44720 OS-44722 AA35692 AA35693 OS-41067 OS-36428 OS-36435 OS-41176 OS-44826

Mississippi Sound, MS-04-1 (776 cm) Mississippi Sound, MS-04-1 (866 cm) Mississippi Sound, MS-04-1 (1052 cm) Mississippi Sound, MS-04-4 (591 cm) Mississippi Sound, MS-04-5 (1258 cm) Mobile Bay, MB-98-4 (637 cm) Mobile Bay, MB-98-4 (697 cm) Mobile Bay Bay, MB-03-04 (1514 cm) Mobile Bay-head Delta, MD-02-1 (1726 cm) Mobile Bay-head Delta, MD-02-1 (1809 cm) Bon Secour Bay, MB-03-05 (1368 cm) Bon Secour Bay, MB-03-05 (1412 cm)

Probythenella louisianae Wood Wood Peat Wood Crassostrea virginica Peat Peat Wood Wood Corbula contracta* wood

4030 6 30 . 48000 . 48000 5470 6 50 38400 6 330 4980 6 55 . 47000 8270 6 60 8710 6 35 8770 6 50 7680 6 40 8060 6 40

3981–4092 — — 6210–6308 — 5278–5416 — 9196–9318 9595–9697 9687–9892 8069–8190 8976–9029

Samples were processed at the NSF AMS Radiocarbon laboratory of the Woods Hole Oceanographic Institution’s National Ocean Sciences Accelerator Mass Spectrometry Facility and Beta Analytic and calibrated using Stuiver and Reimer (1993), Hughen et al. (2004), and Reimer et al. (2004). Articulated bivalves dated are indicated by an asterisk. Sample depths (reported parenthetically) are relative to sea level. METHODS

Marine Seismic Data Approximately 260 and 240 km of high-resolution seismic data were collected from Mobile Bay and eastern Mississippi Sound, respectively (Fig. 2) using an EdgeTech SB-216S chirp system that emitted a 2– 15 kHz pulse at 0.25 second intervals. Data were collected in digital form with Edge Tech MIDAS and Codatm DA 50 acquisition systems. A velocity of 1500 m/s was used to convert the two-way travel time to depth. This velocity was verified by cores that sampled changes in lithology at the same depths as seismic facies transitions. Data collected using the MIDAS system were processed by applying a time-varying gain of 4 db per 10 ms. Data collected using the Codatm system were processed by applying a time-varying gain of 2.5 db per 10 ms and a 250 Hz highpass filter. These data were incorporated with approximately 250 km of high-resolution single-channel (200–400 J) boomer data published in Kindinger et al. (1994). After the positions of the paleovalleys were constrained, seismic lines were collected up the axis of each valley to examine flooding surfaces and associated depositional changes in greater detail. Lithologic Data Seventeen cores were obtained from Mobile Bay, the Mobile bay-head delta, Mississippi Sound, and along Cedar Point Peninsula to ground truth seismic facies to lithology and to provide shell, plant, and wood material for environmental analysis and radiocarbon dating (Fig. 2). Four rotary drill cores (MS-04-1, MS-04-2, MS-04-4, and MS-04-5) were collected from the northern and southern portions of the Sound. Seven vibracores (MB-98-1 through MB-98-4, MD-02-7 through MD-02-9) and nine rotary drill cores (MD-02-2, MD-02-3, MB-03-1 through MB-03-5, MB-04-1, and MB-04-2) were taken throughout the northern, central, and lower portions of Mobile Bay. One rotary drill core (MD-02-1) was taken in Chacaloochee Bay, an interdistributary bay of the Mobile Delta. The drill cores were collected from a small barge using a hydraulic rotary drill that allows nearly continuous sampling to approximately 35 m. During drilling, cuttings are removed by circulating water through the drill stem. Cores are obtained by lowering a push-coring device through the drill casing, allowing the extraction of an undisturbed interval of core from below the level of the drill bit. An additional core (MS-04-3) was collected on land along Cedar Point Peninsula using a truck-mounted GeoProbetm coring system. Core lengths ranged from 2.6 to 18.1 m. Radiocarbon dating (AMS) of shell and organic material was performed by the Woods Hole Oceanographic Institution and Beta

Analytic (Table 1) and dates were calibrated using CALIB 5.0.2 software (Stuiver and Reimer 1993). To ensure that dated material was preserved in situ, whenever possible individual wood fragments and articulated bivalves were chosen for dating instead of bulk peat or unpaired valves. Species from well-defined macrofossil assemblages indicative of the depositional environment were preferentially chosen for dating. RESULTS AND INTERPRETATIONS

Regional Seismic Units and Bounding Surfaces Across Mobile Bay and Mississippi Sound, four regional unconformity-bounded stacked seismic units were recognized (Unconformity D is the deepest and Unconformity A is the shallowest; Fig. 3). The regional unconformities are characterized by high-amplitude reflections that exhibit 7–35 m of relief, truncate underlying reflections, and define broad (0.6–17.9 km wide) valleys. Only the uppermost two unconformitybounded units were mapped because in Mississippi Sound Unconformity B completely removed entire sections of the lower seismic units and throughout much of Mobile Bay seismic attenuation completely obscures the lower seismic units. Unconformity B was mapped throughout Mobile Bay and Mississippi Sound and defines broad (0.8–7.7 km wide) valleys that extend updip to the modern Bayou La Batre, Dog River, Fish–Magnolia River, Fowl River, and Mobile–Tensaw River systems (Fig. 4). Valley thalwegs are 20–35 m deep (Figs. 3, 4), and broad, flat interfluvial areas, found at 215 m and 210 m (relative to modern sea level), separate the different fluvial systems. Core MS-04-5 penetrated Unconformity B and sampled clay rip-up clasts and plant roots at a sharp contact separating a tannish gray to grayish blue-green clayey sand above from a bluish green bioturbated clay below, at the depth of the seismic surface (Fig. 5). Unconformity A also defines multiple broad (0.6–5.4 km wide) valleys that extend to the modern La Batre, Fowl, Dog, Mobile–Tensaw, and Fish–Magnolia river systems (Fig. 6). The main trunk valleys are generally in the same location as those mapped on Unconformity B. Valley thalwegs are 10–25 m deep, and a broad, flat interfluvial area at 210 m separates the East and West Mobile valleys while flat interfluvial areas at 25 m separate the Fowl River Valley from the Mississippi Sound extension of the Mobile Valley (Fig. 6). Unconformity A was sampled by ten cores, and is a sharp contact separating a dark greenish-gray to medium gray sandy clay above, from a stiff, oxidized medium gray, dark greenish-gray to yellow-orange clay below (Fig. 5). The stiff, oxidized nature of the clay below the contact is indicative of subaerial exposure. Based on the presence of incision, evidence of subaerial exposure (roots

144


JSR

FIG. 3.—Seismic profile from northern Mississippi Sound showing the four unconformities imaged in the study area and the Fowl Paleovalley delineated on Sequence Boundaries (SB) A and B. See Figure 2 for profile location.

and oxidation), and regional extent, unconformities A and B are interpreted as sequence boundaries. In interfluvial locations, sequence boundaries A and B are sometimes amalgamated but generally are separated by approximately 5 m. Mobile Incised-Valley System Sequence Boundary B.—The Mobile Valley is 10.6 km wide at the bayhead delta (Fig. 7) and bifurcates seaward into east and west valleys (Fig. 4). The start of bifurcation is resolved in the core transect at the bayhead delta, where distinct east and west channels are recognized at sequence boundary B (Fig. 7; May 1976). The East and West Mobile valleys also bifurcate seaward and become wider and deeper towards the south (Fig. 4). In the northern and middle part of the bay, the East Mobile Valley is approximately 1.4–3.9 km wide and 25–33 m deep (Fig. 4). In the southern part of the bay, the valley bifurcates around a small paleo-high into two valleys that are 3.4–3.2 km wide and , 35 m deep (Figs. 4, 8). The Fish–Magnolia Valley merges with the East Mobile Valley along the southern margin (Fig. 4). In the northern and middle part of the bay, the West Mobile Valley is approximately 0.6–3.5 km wide and 26 m deep (Figs. 4, 9). The Dog River Valley merges with the western valley along the northern margin of the bay (Fig. 4). In the southern part of the bay, the valley bifurcates into a valley, 3.2 km wide and 20–25 m deep that extends into Mississippi Sound (Figs. 4, 10) and a valley, 4.0 km wide and 25–30 m deep, that extends offshore at the location of the Main Pass inlet. The Mississippi Sound extension of the Mobile Valley branches into smaller channels that extend seaward under eastern Dauphin Island (Fig. 4). Interfluvial plateaus form a natural divide between the East and West Mobile valleys in the center of the Mobile estuary and between the

Mississippi Sound Mobile Valley and the Fowl River Valley in eastern Mississippi Sound (Fig. 4). These plateaus at 215 and 210 m depth vary in width from 1.6 to 6.9 km and from 0.55 to 6.3 km, respectively. Small plateaus at 215 m and 210 m depth (both 0.3–0.55 km wide) also serve as a natural divide between the West Mobile Valley and the Mississippi Sound Mobile Valley. Terraces at 215 m exist along the landward-facing flanks of the valleys in Mobile Bay (adjacent to the modern shoreline) and are 0.5–3.0 km wide (Fig. 4). The valley fill (Seismic Unit B) is composed of a basal acoustically transparent, chaotic to semi-parallel seismic facies (Figs. 8, 9B, 10). This facies was sampled only in core MS-04-5, collected from the center of the Mississippi Sound Mobile Valley (Fig. 10), and is a tannish gray to grayish blue-green clayey sand (Fig. 5). Although , 1 m of this facies was sampled, it is interpreted as alluvial on the basis of its stratigraphic position, seismic character, and lithology. Alluvial sediments are overlain by acoustically laminated to inclined (shingled) reflections (Figs. 8, 10, 11). The shingled facies has reflections inclined both towards the valley flanks and down the valley axis and is interpreted to represent lateral accretion and progradation. This facies was sampled in cores MS-04-5 and MB-03-05 and is a medium gray to dark greenish-gray bioturbated clay with numerous wood fragments, shell hash, and sand-filled subhorizontal Ophiomorpha and Thalassinoides burrows (Fig. 5). This upper valley-fill facies is interpreted as bay-head delta. The contact between the alluvial and bay-head delta facies shows small-scale channels, and this surface is interpreted as a bay-head delta diastem (Fig. 9B; Zaitlin et al. 1994). Sequence Boundary A.—The Mobile Valley is 10.6 km wide at the modern bay-head delta and bifurcates seaward into East and West Mobile valleys (Figs. 6, 7). The morphology of Sequence Boundary A at

JSR


145

FIG. 4.—Structure map of Sequence Boundary B in Mobile Bay and eastern Mississippi Sound showing that valleys branch seaward in the area. Contour interval is 5 m and depths are below sea level.

the bay-head delta core transect generally mimics Sequence Boundary B (Fig. 7; May 1976). The East Mobile Valley is 1.3–5.4 km wide and 15– 20 m deep in northern and middle Mobile Bay, and it deepens to 25 m towards the south (Fig. 6). The Fish–Magnolia Valley, which extends through Weeks Bay (Duran 2006), merges with the eastern valley across a broad flat terrace (Figs. 6, 8). The East Mobile Valley extends seaward below the Holocene accretionary-spit portion of Morgan Peninsula (Fig. 6; Blum et al. 2003; Rodriguez and Meyer 2006). The West Mobile Valley trifurcates seaward and deepens toward the south (Fig. 6). In the northern and middle part of the bay, the valley is 1.4–3.0 km wide with a maximum depth of 20 m and the Dog River Valley merges with it (Figs. 6, 9A). In the southern part of Mobile Bay, the valley deepens to 24 m and trifurcates (Fig. 6). The eastern two branches, which are about 2.5 km wide and 23 m deep, extend seaward through the modern Main Pass tidal inlet and the western tip of Morgan Peninsula (Fig. 9B). The western branch of the West Mobile Valley extends into Mississippi Sound. This Mississippi Sound extension of the West Mobile Valley diverges around a small 25 m paleo-high (Figs. 6, 10) and extends seaward below the Holocene portion of Dauphin Island. At Dauphin Island, the valley thalweg reaches a depth of 210 m (Fig. 6; Otvos 2004). The Mobile Valley is bounded by unpaired terraces at 214, 210, and, 25 m. A large (12.6 km wide) 214 m terrace is located along the East Mobile Valley flank in the southern part of the estuary (Figs. 6, 8). Broad interfluve plateaus in central and northern Mobile Bay at 210 m vary in width from 0.3 to 6.1 km wide and terraces and paleo-highs in Mississippi

Sound at 25 m vary in width from 0.8 to 2.9 km. Cedar Point Peninsula is located above a 25 m terrace or paleo-high, which may have functioned as a nucleus for sediment accumulation (Fig. 6). Additional terraces and paleo-highs found at 25 m form a natural drainage divide separating the Mississippi Sound branch of the Western Mobile Valley from the Fowl River Valley (Fig. 6). The valley fill (Seismic Unit A) is composed of acoustically transparent to laminated seismic facies sampled in 9 cores as a 10–20-m-thick package of homogeneous to bioturbated dark greenish-gray to medium-gray clay to sandy clay with whole oysters, shell hash, and sand-filled subhorizontal Ophiomorpha, Chondrites, and Thalassinoides burrows (Figs. 5, 8, 9, 10). Cores from the upper bay and bay margins that sampled this unit contain wood fragments. This unit is interpreted as central basin, which is consistent with previous studies by Kindinger et al. (1994) and Bartek et al. (2004). Peat was sampled at the base of the unit in cores MD-02-1, MB-03-4, and MS-04-4 directly above Sequence Boundary A. An individual plant fragment sampled from a peat at 215.14 m directly above Sequence Boundary A in core MB-03-4 from the West Mobile Valley was dated at 9,260 6 60 cal. yr BP, and a wood fragment sampled at 214.12 m directly above Sequence Boundary A in core MB-03-5 from the East Mobile Valley was dated at 9,005 6 25 cal. yr BP. The dates indicate central basin conditions were present at these locations sometime after 9,000 cal. yr BP. Core MD-02-1 was collected at the modern bay-head delta front and sampled 8.2 m of organic-rich olive-gray bioturbated medium- to finegrained massive sand to thinly laminated clayey sand (Metcalf and

146


JSR

FIG. 5.— Core photographs showing Holocene central basin (cores MB-03-4, 14.48–14.57 m; MB-03-5, 13.91–14.08 m; and MS-04-5, 8.32–8.52 m), bay-head delta front (core MD-02-1, 8.11–8.34 m), bay-head delta plain (core MD-02-1, 9.15–9.34 m), and marsh (core MB-03-4, 15.06–15.14 m), Pleistocene bay-head delta (cores MB03-4, 15.14–15.21; MB-03-5, 14.08–14.28 m; and MS-04-5, 12.75–13.17 m) and alluvial (core MS-04-5, 19.15–19.36 m), and sequence boundaries A (cores MB-03-5 and MB-03-4) and B (core MS-04-5). All depths are relative to sea level. See Figure 2 for core locations.

JSR


147

FIG. 6.—Structure map of Sequence Boundary A in Mobile Bay and eastern Mississippi Sound showing broad interfluve plateaus. Contour interval is 5 m and depths are below sea level. Areas that are relatively flat are indicated with patterns.

FIG. 7.— Cross-section A–A9 across the Mobile bay-head delta along the Interstate-10 bridge based on borings, which penetrated . 35 m, collected by the State of Alabama Highway Department (B-15 to B-28) and core MD-02-1 (Modified from May 1976). Generalized lithologic descriptions are after May (1976). At this landward location, sequence boundaries (SB) A and B define a single broad Mobile–Tensaw Paleovalley. See Figures 2, 4, and 6 for location.

148


JSR

JSR


Rodriguez 2003; Figs. 5, 7). Whole Rangia cuneata shells, shell and wood fragments, and numerous mud- and sand-filled subhorizontal Ophiomorpha, Chondrites, and Thalassinoides burrows were also recognized (Fig. 5; Metcalf and Rodriguez 2003). This unit is interpreted as delta front and overlies a 9.3-m-thick package of organic-rich dark-gray to olive-gray bioturbated clay to sandy clay with whole Rangia cuneata shells, shell and wood fragments, and sand-filled Ophiomorpha, Chondrites, and Thalassinoides burrows interpreted as delta plain (Metcalf and Rodriguez 2003). The facies succession indicates modern bay-head delta aggradation and retrogradation. A radiocarbon date from a piece of wood sampled from the core directly above Sequence Boundary A at 218.09 m is 9,780 6 90 cal. yr BP, which marks the initiation of deltaic sedimentation in the area (Metcalf and Rodriguez 2003). Fowl and La Batre Incised-Valley Systems Sequence Boundary B.—The Fowl River Valley is 1.0 km wide and 20– 28 m deep, and bifurcates seaward (Figs. 3, 4, 11), while the La Batre Valley branches into an intricate 7.7-km-wide network of valleys just seaward of the northern Mississippi Sound shoreline (Figs. 4, 12). La Batre trunk valleys vary in width from 1.3 to 2.2 km, smaller valleys vary in width from 0.3 to 1.1 km, and all are 20–33 m deep (Figs. 4, 12). The Fowl and La Batre valleys extend seaward under western Dauphin Island and Petit Bois Pass (Fig. 4). Valleys are bounded by stepped interfluve plateaus. The plateaus are at 215 and 210 m, and the 210 m plateaus contain terraces at 215 m. The terraces and plateaus range between 0.6 and 5.5 km in width. These features form drainage divides between the Fowl and La Batre paleofluvial systems in the west and between the Fowl and Mississippi Sound extension of the Mobile Valley in the east. The valley fill (Seismic Unit B) is composed of a basal acoustically transparent to chaotic seismic facies that has semi-parallel reflections in places (Figs. 3, 11). This unit was not sampled, but on the basis of its similarity to the Mobile basal valley-fill seismic unit it is interpreted as alluvial. This unit is capped by sets of semi-parallel inclined reflections (inclined both towards the valley flanks and down the valley axis) that are interpreted to represent lateral accretion and progradation (Fig. 11), or an acoustically transparent facies may be present (Fig. 12). Core MS-04-1 sampled the acoustically transparent facies in the La Batre valley as a medium gray to dark greenish-gray bioturbated, clay to sandy clay with numerous wood fragments, tree roots, shell hash, and sand-filled subhorizontal Thalassinoides burrows. Based on the lithologic and seismic data, this unit is interpreted as a bay-head delta. Seismic-section transects oriented up the axis of the valleys show that the Fowl valley fill is composed of continuous alluvial sediments below bay-head-delta sediments whereas the La Batre valley fill is composed solely of bay-headdelta sediments (Fig. 13). Wood fragments taken from the upper portion of the bay-head-delta unit in core MS-04-1 were radiocarbon dated at . 48,000 yr (radiocarbon dead), making the timing of valley fill problematic. Sequence Boundary A.—The Fowl Valley is 0.9 km wide and 10–11 m deep in northern Mississippi Sound; however, narrower tributary channels that are beyond mapping resolution incise to depths of 16– 17 m (Figs. 3, 11). This valley flows across a broad low-gradient plain in

149

central and southern Mississippi Sound where the valley branches into smaller channels that are below mapping resolution (Figs. 6, 12). In northern Mississippi Sound, the La Batre Valley is 1.12 km wide and 15 m deep, and extends seaward beneath Petit Bois Pass (Fig. 6). The valley fill of the Fowl and Bayou La Batre systems (Seismic Unit A) is composed of an acoustically transparent seismic facies with laminated to wavy parallel reflections in places. This seismic facies was sampled in cores MS-04-1 and MS-04-4 as a bioturbated dark greenishgray to medium gray clay to clayey sand with wood fragments, whole shells, shell hash, and sand-filled subhorizontal Ophiomorpha, Chondrites, and Thalassinoides burrows. This unit is exposed at the bay floor and on the basis of the seismic and lithologic data is interpreted as central basin. Seismic-section transects (Fig. 13) and cores show that the paleovalleys are filled continuously with central-basin sediments; no other unit exists above Sequence Boundary A. Radiocarbon dating of a Probythenella louisianae shell found at 27.76 m in core MS-04-1 indicates that central basin conditions have existed since at least 4,124 6 25 yr cal. BP. DISCUSSION

Age of Sequence Boundaries All dates above Sequence Boundary A are Holocene (, 10 ka) whereas dates obtained below are Pleistocene (. 20 ka), indicating that Sequence Boundary A formed in response to the Oxygen Isotope Stage 2 lowstand in sea level (Table 1) approximately 22,000–17,000 years ago (Bard et al. 1990). This is consistent with previous work in the area by Kramer (1990), McBride et al. (1991), Mars et al. (1992), Davies and Hummel (1994), and Hummel and Parker (1995a; 1995b). The timing of Sequence Boundary B could not be constrained by radiocarbon dating. A Nuculana acuta shell sampled at 212.54 m in core MS-04-5, from the center of the Mississippi Sound Mobile Valley below Sequence Boundary A, was radiocarbon dated at 38,400 yr BP. Two other dates from wood fragments in core MS-04-1, sampled from the center of the La Batre Valley at the same stratigraphic level, came back radiocarbon dead (Table 1). Grootes (1983) showed that modern contamination of 1% modern carbon would produce an age date of 37 ka from an isotopically dead sample. The reported 38,400 yr BP date is likely the result of contamination and is disregarded. The Oxygen Isotope Stage 4 lowstand (, 80 m below sea level) was shallower and of shorter duration than the Stage 2 lowstand (, 120 m below sea level; Chappell et al. 1996). Anderson et al. (2004) did not recognize significant fluvial incision during Stage 4 along the northern Gulf of Mexico shelf. Because the valley incisions mapped on Sequence Boundary B are overall wider and deeper (Figs. 3, 11, 12) than those mapped on the Stage 2 Sequence boundary, it is unlikely that they are associated with Stage 4. It is more likely that valleys mapped on Sequence Boundary B formed during the Oxygen Isotope Stage 6 lowstand when sea-level was , 2140 m, 155,000–136,000 years ago (Shackleton 1987), although it is possible that Sequence Boundary B formed earlier. The placement of the Oxygen Isotope Stage 2 Sequence Boundary on uppermost Unconformity A is distinct from previous work conducted on the Alabama shelf and Mobile Bay. Based on its deeper incision depth, Kindinger et al. (1994) placed the Stage 2 Sequence Boundary on Unconformity B and interpreted Unconformity A as a transgressive surface separating early from late fluvial deposition. Developing

r FIG. 8.—Seismic chirp profile shows the 214 m terrace on Sequence Boundary A that was penetrated by core MB-03-5 (see Fig. 5A). Seismic boomer profile shows the East Mobile Valley delineated by Sequence Boundary B (SB-B; B). Accretionary bay-head delta deposits are truncated above by the 214 m terrace on Sequence Boundary A (SB-A). See Figures 2, 4, and 6 for profile locations.

150


JSR

FIG. 9.—Seismic chirp profiles across the West Mobile Valley in A) central and B) southern Mobile Bay. Core MB-03-4 penetrated Sequence Boundary A (SB-A) at about 215 m (see Fig. 5). The West Mobile Valley occupied this location through multiple sea-level cycles. Figures 2, 4, and 6 show profile locations.

JSR


151

FIG. 10.— Seismic chirp profile across the Mississippi Sound Mobile Valley in southeast Mississippi Sound showing truncation of bay-head delta reflections by Sequence Boundary A (SB-A). Core MS-04-5 penetrated sequence boundaries A and B (see Fig. 5). Figures 2, 4, and 6 show the profile location.

a chronostratigraphy from the vibracores examined by Kindinger et al. (1994) is difficult; these cores penetrated through the Stage 2 Sequence Boundary only in interfluve areas, near the modern bay shorelines, where Unconformities A and B are amalgamated. Valley-Fill Architecture The Mobile and Fowl valley-fill successions, between Sequence Boundary B and the Stage 2 Sequence Boundary, are generally composed of alluvial deposits overlain by bay-head delta deposits (Figs. 10, 11). The absence of alluvial facies in the La Batre valley could indicate either that these sediments were removed by downcutting associated with the bayhead delta diastem or that these sediments were never deposited (Fig. 12). Pleistocene bay-head delta sediments cover the 215 m and 210 m terraces to a level of approximately 25 m in Mississippi Sound (Figs. 3, 11), indicating that deltaic sedimentation completely filled the valleys. Central-basin sediments were not recognized above the bay-head delta. These sediments could have been deposited but then were removed during the subsequent Stage 2 incision, or central-basin sediments were never deposited at modern estuarine locations. Bay-head delta deposits are absent above the Stage 2 Sequence Boundary in Mobile Bay and Mississippi Sound; however, they were recognized offshore in the Mobile Paleovalley on the Alabama shelf (Cabote 1998; Bartek et al. 2004). The absence of this facies at modern estuarine locations and the presence of modern bay-head deltas associated with the Mobile, Fowl, and La Batre rivers indicate that these environments backstepped across the modern estuaries. Based on radiocarbon dates above the Stage 2 Sequence Boundary, these backstepping events occurred before 9,780 6 80 cal. yr BP in Mobile Bay (the oldest deltaic sediments at the modern bay-head delta) and 6,223 6 25 cal. yr BP in Mississippi Sound (the age of bay sediments directly above the Stage 2 Sequence Boundary). Possible causal mechanisms for bay-head delta backstepping include an increase in the rate of sea-level rise, a decrease in sediment supply, and

variations in antecedent topography. The rate of sea-level rise during the Holocene was not increasing (Bard et al. 1996; To¨rnqvist et al. 2004; Otvos 2005), making this an unlikely mechanism for backstepping. Although a decrease in sediment supply cannot be ruled out, the low gradients of the Stage 2 valleys, 0.3 m/km for the Mobile, Fowl, and La Batre, must have played a large role in bay-head delta backstepping. When sea level reached , 220 m, the low-gradient Mobile valley was rapidly inundated. This forced the bay-head delta to backstep landward and accrete at its modern location as opposed to gradually transgressing across the estuary. Mobile bay-head delta deposition began at its modern position around 9.7 ka at 218 m depth (based on core MD-02-1), and this age and depth is close to sea level (Otvos 2004). This implies that the inundation of the low-gradient fluvial systems resulted in the facies transitions. Given the higher elevation of the low-gradient Stage 2 Sequence Boundary in Mississippi Sound (around 210 m), backstepping of the Fowl and La Batre bay-head deltas must have occurred later, between 8.0 and 7.0 ka judging from the sea-level curves of Otvos (2004) and To¨rnqvist et al. (2004). Inundation of low-gradient antecedent topography has been recognized elsewhere as an important mechanism for backstepping (Heap and Nichol 1997; Amorosi et al. 2005; Rodriguez et al. 2005). As defined by Zaitlin et al. (1994), incised valleys are classified as simple if the valley filled completely during one sea-level cycle, or compound if the valley persisted through multiple cycles. Coastal-plain valleys are predicted to have simple fill, and piedmont valleys are expected to have compound fill. This is thought to be the case because the location of piedmont systems is strongly controlled by underlying structural features (Zaitlin et al. 1994). As expected, the Mobile piedmont system shows a compound fill with two distinct sequences. However, the coastal-plain Fowl and La Batre systems also persisted through the same two sea-level cycles and show compound fill. It is likely that these coastalplain systems generally reoccupied the same valleys because of higher compaction rates and greater erodibility of the younger valley-fill sediments versus interfluve areas. It may be difficult to distinguish

152


JSR

FIG. 11.—Seismic chirp profile across the Fowl Valley in northern Mississippi Sound showing two branches of the Fowl Valley delineated by Sequence Boundary B (SB-B). The eastern branch was reoccupied by the Fowl river during the subsequent lowstand. Figures 2, 4, and 6 show the profile location.

between a coastal-plain valley (Fowl Valley; Fig. 3) versus a piedmont valley (Mobile Valley; Fig. 8) on the basis of valley-fill architecture alone. Incised-Valley Branching Maps of Sequence Boundary B and the Stage 2 Sequence Boundary (Figs. 4, 6) show an increase in fluvial complexity seaward through valley branching. This is different from other coeval Gulf of Mexico valley systems that become more organized seaward through tributaries merging with the main trunk valley (Thomas and Anderson 1989; Nichol et al.

1996; Abdulah et al. 2004; Bartek et al 2004). Valleys examined along the Alabama coast are similar to seaward-branching valleys recognized in the Upper Campanian Sego Sandstone Member of the Mancos Shale in the Book Cliffs, Utah; however, the mechanism for valley formation has been the focus of much debate (Van Wagoner et al. 1990; Willis and Gabel 2003; Wood 2004; Wood and Yoshida 2004). Seaward branching of the Mobile, Fowl, and La Batre valleys was likely caused by a decrease in gradient and a transition from restricted to unconfined topography at the location of the estuaries. Absolute timing of branching cannot be constrained; however, all valleys mapped in

JSR


153

FIG. 12.—Seismic chirp profile across the eastern branch of the La Batre Valley in southern Mississippi Sound delineated by Sequence Boundary B (SB-B). Sequence Boundary A (SB-A) defines only small channels in this area. Figures 2, 4, and 6 show the profile location.

Mobile Bay and Mississippi Sound are confined to the same regional bounding surfaces (sequence boundaries) and are not distributaries. Sequence Boundary B and the Stage 2 Sequence Boundary have very low gradients in interfluve areas, as demonstrated by the flat interfluve plateaus that divide valley branches in the estuaries. Average fluvial gradients of the modern Mobile (0.8 m/km), Fowl (1.25 m/km), and La Batre (2.25 m/km) rivers decrease at their associated bay-head deltas (Fig. 14) and are all greater than gradients measured down the axis of their associated paleovalleys, mapped in the vicinity of the estuaries on Sequence Boundary B (0.69, 0.55, and 1.33 m/km, respectively) and the Stage 2 Sequence Boundary (0.27 m/km, 0.28 m/km, and 0.31 m/km, respectively). Fluvial gradients during the highstands that preceded formation of the sequence boundaries mapped here cannot be determined from the data set; however, using the modern as an analog, it is probable that gradients decreased near the northern edge of the bay-head deltas. Wescott (1993), Wood et al. (1993), and Ritchie et al. (2004) recognized that changes in channel pattern (sinuosity, geometry, and complexity) can occur as a fluvial system crosses a break in slope. Alluvial aggradation and valley branching observed here likely occurred during the falling limb of the sea-level curve in response to fluvial systems extending across newly exposed and lower-gradient areas. While gradient change is deemed an important mechanism for valley branching, Mobile fluvial gradients likely decreased 63 km landward of the estuary, at the head of the bay-head delta; however, the fluvial system did not branch until it flowed across the newly exposed Mobile Bay area (Fig. 14). All of the paleovalleys branch at the northern estuarine shorelines in Mississippi Sound and Mobile Bay. This location coincides with where valley-flank topography becomes broader and fluvial systems

are less restricted (Fig. 14), which facilitated valley branching. During the lowstand, when sea level exposed the steeper continental shelf break, the channels incised and left behind the complex alluvial morphology mapped here. The complex morphology mapped on the sequence boundaries is also influenced by the position and morphology of the underlying paleovalleys. Feeder Systems to Alabama Shelf-Edge Deltas Merging paleogeographic maps of the continental shelf by Kindinger et al. (1994) and Bartek et al. (2004) with this study yields better constraints on the source of sediment to shelf-margin deltas. Some of the previously mapped offshore valleys and their associated shelf-margin deltas predate the Stage 2 lowstand on the basis of valley depth, position of valleys mapped in this study relative to those previously mapped on the shelf, a detailed chronostratigraphy, and a regional seismic line collected along the shelf break by Roberts et al. (2004). It is likely that the East and West Mobile deltas of Bartek et al. (2004) and the offshore valleys of Kindinger et al. (1994) and Bartek et al. (2004) are associated with Sequence Boundary B, which may have formed during the Stage 6 lowstand or possibly the Stage 8 lowstand as proposed by Roberts et al. (2004). The La Batre Paleovalley mapped in Mississippi Sound lies directly headward of the Pascagoula Paleovalley mapped on the shelf by Kindinger et al. (1994) and Bartek et al. (2004), with only 7 km separating the two datasets. It is likely that the previously named Pascagoula Paleovalley offshore is truly the La Batre Paleovalley. If these correlations are valid, during the Stage 6 or 8 lowstand the East Mobile Delta, as mapped by Bart and Anderson (2004) and Bartek et al. (2004), was nourished by the East Mobile Valley with an additional

154


JSR

FIG. 13.— Cross section through the axis of the Fowl (B–B9) and La Batre (C–C9) valleys based on seismic sections (short strike-oriented line drawings of seismic data clipped from longer profiles) and core MS-04-1. Between sequence boundaries A and B, the Fowl Valley is filled with a continuous section of alluvial and bayhead-delta sediments whereas the La Batre Valley is filled only with a continuous section of bay-head-delta sediments. Alluvial sediments at the base of the La Batre Valley may have been deposited and subsequently removed by the bayhead delta diastem. Figure 2 shows the location of cross sections.

contribution from the Fish–Magnolia Valley while the West Mobile Delta, as mapped by Bartek et al. (2004), was nourished by the West Mobile Valley and its Mississippi Sound extension with significant contributions from the Dog, Fowl, and La Batre systems. Furthermore, during the Stage 2 lowstand the East and West Mobile valleys may have formed a braided network of channels offshore as proposed by Young (2002) similar to the Pearl Paleovalley of Kindinger et al. (1988). These systems likely nourished the Lagniappe Delta as proposed by Kindinger et al. (1994), Bartek et al. (2004), Fillon et al. (2004), and Roberts et al. (2004). SUMMARY AND CONCLUSIONS

During the lowstand associated with Sequence Boundary B, the La Batre, Fowl, and Mobile–Tensaw fluvial systems incised valleys across Mobile Bay and Mississippi Sound (Fig. 15A). During the subsequent

transgression, the Fowl and Mobile valleys were filled, from bottom to top, with alluvial, bay-head delta, and central-basin sediments. A prominent bay-head delta diastem separates alluvial from bay-head delta sediments and likely removed the entire alluvial section from the La Batre valley (Fig. 15B). During the Stage 2 lowstand, the La Batre, Fowl, and Mobile–Tensaw fluvial systems incised and generally reoccupied the earlier valleys (Fig. 15C). During the Stage 2 to 1 sea-level rise, bay-head deltas backstepped across Mobile Bay and Mississippi Sound in response to inundation of low-gradient topography. Stage 2 valleys are filled with central-basin sediments; no basal alluvial sediments were recognized. However, alluvial sediments could have been deposited and then subsequently eroded by bay-ravinement processes associated with the rapid shoreline transgression event, or are thin (below seismic resolution) and discontinuous (not sampled). Valleys mapped on Sequence Boundary B, interpreted to have formed during Oxygen Isotope Stage 6, exhibit significantly greater erosion depth

JSR


155

FIG. 14.—Digital elevation model showing a decrease in gradient of the Alabama (east) and Tombigbee (west) rivers as they converge to form the Mobile–Tensaw river system at the bay-head delta. Gradients down the axis of paleovalleys, mapped on Sequence Boundary B and the Stage 2 Sequence Boundary, are low. Valley-flank topography becomes broader and fluvial systems are less restricted towards the south. Valleys also branch towards the south. The northern two cross sections are based on borings across the Interstate-60 (top; based on 75 borings) and Interstate-10 (based on 14 borings; Fig. 7) bridges collected by the State of Alabama Highway Department (May 1976). Notice vertical scale change at 0 m.

156

JSR


FIG. 15.— Cartoon east–west cross section extending through northern Mississippi Sound and south-central Mobile Bay illustrating the evolution of the area during the late Quaternary. A) During the lowstand associated with the formation of Sequence Boundary B, the La Batre (LBV), Fowl (FLV), and Mobile (Mississippi Sound Mobile Valley (MSMV), West Mobile Valley (WMV), and East Mobile Valley (EMV)) fluvial systems incised and branched seaward. B) These valleys were filled with alluvial, bay-head delta, and central basin sediments during the subsequent transgression. C) During the Stage 2 lowstand, the fluvial systems generally reoccupied previous valleys and during the subsequent Stage 2 to 1 transgression filled with centralbasin sediments.

and width than Stage 2 valleys. This is probably because Stage 2 sea level was higher, and the lowstand was of shorter duration than Stage 6. The morphology of Stage 6 valleys may indicate that fluvial systems carried higher sediment loads to the Mississippi–Alabama– Florida shelf break during the Stage 6 lowstand as opposed to the Stage 2 lowstand and may explain deposition of multiple associated lowstand deltas. Greater erosion depth, larger width, and increased sediment discharge across the shelf during the Stage 6 lowstand as opposed to the Stage 2 lowstand was recognized along the west Louisiana and east Texas coast (Wellner et al. 2004) and the central Texas coast (Abdulah et al. 2004). Antecedent topography is important in controlling valley morphology and fill architecture. Fluvial systems branched seaward during the falling limb of the sea-level curve in response to low-gradient coastal-plain topography being exposed. These systems likely coalesced farther seaward as gradients increased before feeding shelf-edge deltas (Sager et al. 1999; Kindinger 1988; Roberts et al. 2004; and Bartek et al. 2004). During the Stage 2 to 1 sea-level rise, inundation of low-gradient topography induced rapid shoreline transgression, which caused bayhead deltas to backstep across Mississippi Sound and Mobile Bay. The piedmont Mobile–Tensaw and coastal-plain Fowl and La Batre valley systems persisted through at least two sea-level cycles. Reoccupa-

tion of older valleys during Stage 2 could have been caused by the formation of topographic lows during the highstand above valleys mapped on Sequence Boundary B in response to greater compaction of these younger valley-fill sediments than sediments in adjacent interfluve areas. Underlying valley-fill sediments may also have been less indurated and more erodible than the adjacent older interfluve strata, promoting reincision during Stage 2. The presence of compound valley-fill sequences in all valley systems examined suggests that differentiation between coastal-plain and piedmont incised valleys cannot be based on valley-fill architecture alone. ACKNOWLEDGMENTS

Funding for this project was provided by the National Science Foundation EAR-0107650, the Gulf Coast Association of Geological Societies, and the Hooks Research Fund. Jerry Bowling, Tom Creech, Craig Meyer, Joe Lambert, and Robin Mattheus, from the University of Alabama, and Alex Simms and Chip Anderson from Rice University, provided valuable assistance in the field. Thanks goes to Jack Kindinger for providing access to the USGS seismic data. This manuscript was influenced by useful discussions with Louis Bartek, Robert Dalrymple, James MacEachern, Harry Roberts, Brian Willis, Rob Wellner, and Lesli Wood. Constructive reviews by Gary Hampson, Alessandro Amorosi, and Martin Gibling are greatly appreciated and improved the paper.

JSR

SEAWARD BRANCHING OF COASTAL-PLAIN AND PIEDMONT INCISED-VALLEY SYSTEMS REFERENCES

ABDULAH, K.C., ANDERSON, J.B., SNOW, J.N., AND HOLDFORD-JACK, L., 2004, The Late Quaternary Brazos and Colorado deltas, offshore Texas—Their evolution and the factors that controlled their deposition, in Anderson, J.B., and Fillon, R.H., eds., Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico: SEPM, Special Publication 79, p. 237–271. ALLEN, G.P., AND POSAMENTIER, H.W., 1991, Facies and the stratal patterns in incised valleys: examples from the recent Gironde Estuary (France) and the Cretaceous Viking Formation (abstract): American Association of Petroleum Geologists, Annual Meeting, Program with Abstracts, p. 534. ALLEN, G.P., AND POSAMENTIER, H.W., 1993, Sequence stratigraphy and facies model of an incised valley fill: Gironde Estuary, France: Journal of Sedimentary Petrology, v. 63, p. 378–391. AMOROSI, A., CENTINEO, M.C., COLALONGO, M.L., AND FIORINI, F., 2005, Millennialscale depositional cycles from the Holocene of the Po Plain, Italy: Marine Geology, v. 222–223, p. 7–18. ANDERSON, J.B., RODRIGUEZ, A.B., ABDULAH, K.C., FILLON, R.H., BANDFIELD, L.A., MCKEOWN, H.A., AND WELLNER, J.S., 2004, Late Quaternary stratigraphic evolution of the northern Gulf of Mexico margin: a synthesis, in Anderson, J.B., and Fillon, R.H., eds., Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico: SEPM, Special Publication 79, p. 1–23. BARD, E., HAMELIN, B., FAIRBANKS, R.G., AND ZINDLER, A., 1990, Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U–Th ages from Barbados corals: Nature, v. 345, p. 405–410. BARD, E., HAMELIN, B., ARNOLD, M., MONTAGGONI, L., CABIOCH, G., FAURE, G., AND ROUGERIE, 1996, Deglacial sea-level record from Tahiti corals and the timing of global meltwater discharge: Nature, v. 282, p. 241–244. BART, P.J., AND ANDERSON, J.B., 2004, Late Quaternary stratigraphic evolution of the Alabama and West Florida outer continental shelf, in Anderson, J.B., and Fillon, R.H., eds., Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico: SEPM, Special Publication 79, p. 43–53. BARTEK, L.R., CABOTE, B.S., YOUNG, T., AND SCHROEDER, W., 2004, Sequence stratigraphy of a continental margin subjected to low-energy and low-sediment supply environmental boundary conditions: late Pleistocene–Holocene deposition offshore Alabama, U.S.A., in Anderson, J.B., and Fillon, R.H., eds., Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico: SEPM, Special Publication 79, p. 85–109. BLUM, M.D., SIVERS, A.E., ZAYAC, T., AND GOBLE, R., 2003, Middle Holocene sea-level and evolution of the Gulf of Mexico coast: Gulf Coast Association of Geological Societies, Transactions, v. 53, p. 266–277. BOONE, P.A., 1973, Depositional systems of Alabama, Mississippi, and western Florida coastal zone: Gulf Coast Association of Geological Societies, Transactions, v. 23, p. 266–277. BOYD, R., AND HONIG, C., 1992, Estuarine sedimentation of the eastern shore of Nova Scotia: Journal of Sedimentary Petrology, v. 62, p. 569–583. BUCK, K.F., OLSON, H.C., AND AUSTIN, J.A., 1999, Paleoenvironmental evidence for the latest Pleistocene sea-level fluctuations on the New Jersey continental shelf: combining high-resolution sequence stratigraphy and foraminiferal analysis: Marine Geology, v. 154, p. 287–304. CABOTE, B.S., 1998, Seismic stratigraphy and stochastic models of Late Quaternary lowstand and transgressive deposition, Mobile incised valley system, offshore Alabama [Unpublished M.S. Thesis]: University of Alabama, Tuscaloosa, 311 p. CHAPPELL, J., OMURA, A., ESAT, T., MCCULLOCH, M., PANDILL, J., OTA, Y., AND PILLANS, B., 1996, Reconciliation of late Quaternary sea-levels derived from coral terraces at Huon Peninsula with deep sea oxygen isotope records: Earth and Planetary Science Letters, v. 141, p. 227–236. DALRYMPLE, R.W., ZAITLIN, B.A., AND BOYD, R., 1992, Estuarine facies models: conceptual basis and stratigraphic implications: Journal of Sedimentary Petrology, v. 62, p. 1130–1146. DALRYMPLE, R.W., BOYD, R., AND ZAITLIN, B.A., 1994, History of research, types and internal organization of incised-valley systems: introduction to volume, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 1–10. DURAN, D.M., 2006, Incised-valley fill architecture controlled by the interplay between valley morphology and sea-level rise in Weeks Bay, Alabama [Unpublished M.S. Thesis]: University of Alabama, Tuscaloosa, 89 p. DAVIES, D.J., AND HUMMEL, R.L., 1994, Lithofacies evolution from transgressive to highstand systems tracts, Holocene of the Alabama Coastal Zone: Gulf Coast Association of Geological Societies, Transactions, v. 44, p. 145–153. FILLON, R.H., KOHL, B., AND ROBERTS, H.H., 2004, Late Quaternary deposition and paleobathymetry at the shelf–slope transition, ancestral Mobile River delta complex, northeastern Gulf of Mexico, in Anderson, J.B., and Fillon, R.H., eds., Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico: SEPM, Special Publication 79, p. 111–141. GASTALDO, R.A., 1989, Preliminary observations on phytotaphonomic assemblages in a subtropical/temperate Holocene bayhead delta: Mobile Delta, Gulf Coastal Plain, Alabama: Review of Palaeobotany and Palynology, v. 58, p. 61–83. GROOTES, P.M., 1983, Radioisotopes in the Holocene, in Wright, H.E., ed., Late Quaternary environments of the United States, Volume 2, The Holocene: Minneapolis, University of Minnesota Press, p. 86–105.

157

HEAP, A.D., AND NICHOL, S.L., 1997, The influence of limited accommodation space on the stratigraphy of an incised-valley succession: Weiti River estuary, New Zealand: Marine Geology, v. 144, p. 229–252. HUGHEN, K.A., BAILLIE, M.G.L., BARD, E., BAYLISS, A., BECK, J.W., BERTRAND, C.J.H., BLACKWELL, P.G., BUCK, C.E., BURR, G.S., CUTLER, K.B., DAMON, P.E., EDWARDS, R.L., FAIRBANKS, R.G., FRIEDRICH, M., GUILDERSON, T.P., KROMER, B., MCCORMAC, F.G., MANNING, S.W., BRONK RAMSEY, C., REIMER, P.J., REIMER, R.W., REMMELE, S., SOUTHON, J.R., STUIVER, M., TALAMO, S., TAYLOR, F.W., VAN DER PLICHT, J., AND WEYHENMEYER, C.E., 2004, Marine04 Marine radiocarbon age calibration 26–0 ka BP: Radiocarbon, v. 46, p. 1059–1086. HUMMEL, R.L., 1996, Holocene Geologic History of the West Alabama Inner Continental Shelf, Alabama: Tuscaloosa, Geological Survey of Alabama, Circular 189, 131 p. HUMMEL, R.L., AND PARKER, S.J., 1995a, Holocene Geologic History of Mississippi Sound, Alabama: Tuscaloosa, Geological Survey of Alabama, Circular 185, 91 p. HUMMEL, R.L., AND PARKER, S.J., 1995b, Holocene Geologic History of Mobile Bay, Alabama: Tuscaloosa, Geological Survey of Alabama, Circular 186, 97 p. JENSEN, R.E., 1983, Mississippi Sound Wave–Hindcast Study: Vicksburg, US Army Corps of Engineers, Waterways Experiment Station, Technical Report HL-83-8, 56 p. KINDINGER, J.L., 1988, Seismic stratigraphy of the Mississippi–Alabama shelf and upper continental slope: Marine Geology, v. 83, p. 79–94. KINDINGER, J.L., 1989a, Depositional history of the Lagniappe Delta, northern Gulf of Mexico: Geo-Marine Letters, v. 9, p. 59–66. KINDINGER, J.L., 1989b, Upper Pleistocene to Recent shelf and upper slope deposits of offshore Mississippi–Alabama: SEPM, Gulf Coast Section, Seventh Annual Research Conference, p. 163–174. KINDINGER, J.L., BALSON, P.S., AND FLOCKS, J.G., 1994, Stratigraphy of the Mississippi– Alabama shelf and the Mobile River incised-valley system, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 83–95. KNOWLES, S.J., 1989, Analysis of ship channeling at Ship Island, Mississippi, in Stauble, D.K., and Magoon, O.T., eds., Barrier Islands: Process and Management: Charleston, American Society of Engineers, p. 238–252. KRAMER, K.A., 1990, Late Pleistocene to Holocene geologic evolution of the Grande Batture headland area, Jackson County, Mississippi, Starkville [unpublished M.S. Thesis]: Mississippi State University, Mississippi State, 165 p. MARS, J.C., SHULTZ, A.W., AND SCHROEDER, W.W., 1992, Stratigraphy and Holocene evolution of Mobile Bay in southwestern Alabama: Gulf Coast Association of Geological Societies, Transactions, v. 42, p. 529–542. MAY, E.B., 1976, Holocene Sediments of Mobile Bay, Alabama: Alabama Marine Resources, Bulletin 11, 25 p. MCBRIDE, R.A., BYRNES, M.R., PENLAND, S., POPE, D.L., AND KINDINGER, J.L., 1991, Geomorphic history, geologic framework, and hard mineral resources of the Petit Bois Pass area, Mississippi–Alabama: SEPM, Gulf Coast Section, 12th Annual Research Conference, Program and Abstracts, p. 116–127. MCPHERSON, R.M., 1970, The hydrography of Mobile Bay and Mississippi Sound, Alabama: Alabama Marine Science Institute, Marine Science Journal, v. 1, p. 1–83. METCALF, M.J., AND RODRIGUEZ, A.B., 2003, Sedimentary facies and Holocene evolution of the Mobile bay-head delta, Alabama: Proceedings, International Conference on Coastal Sediments 2003, 8 p. NEUMANN, C.J., CRY, E.C., AND JARVENIN, B., 1981, Tropical cyclones of the north Atlantic Ocean 1871–1980: Ashville, United States Department of Commerce, National Climate Center, 174 p. NICHOL, S.L., BOYD, R., AND PENLAND, S., 1996, Sequence stratigraphy of a coastal-plain incised valley estuary: Lake Calcasieu, Louisiana: Journal of Sedimentary Research, v. 66, p. 847–857. NICHOLS, M.M., 1991, Zonation and sedimentology of estuarine facies in an incisedvalley, wave dominated, microtidal setting, in Smith, D.G., Reinson, G.E., and Zaitlin, B.A., eds., Clastic Tidal Sedimentology: Canadian Society of Petroleum Geologists, Memoir 16, p. 41–58. OTVOS, E.G., 2004, Holocene Gulf levels: recognition issues and an updated sea-level curve: Journal of Coastal Research, v. 20, p. 680–699. OTVOS, E.G., 2005, Holocene coastal barriers, Gulf of Mexico: chronology, evolution, and sea-level relationships: Journal of Coastal Research, Rhodes Fairbridge Festschrift Volume, Special Issue 42, p. 141–163. REIMER, P.J., BAILLIE, M.G.L., BARD, E., BAYLISS, A., BECK, J.W., BERTRAND, C.J.H., BLACKWELL, P.G., BUCK, C.E., BURR, G.S., CUTLER, K.B., DAMON, P.E., EDWARDS, R.L., FAIRBANKS, R.G., FRIEDRICH, M., GUILDERSON, T.P., HOGG, A.G., HUGHEN, K.A., KROMER, B., MCCORMAC, F.G., MANNING, S.W., RAMSEY, C.B., REIMER, R.W., REMMELE, S., SOUTHON, J.R., STUIVER, M., TALAMO, S., TAYLOR, F.W., VAN DER PLICHT, J., AND WEYHENMEYER, C.E., 2004, IntCal04 Terrestrial radiocarbon age calibration 26–0 ka BP.: Radiocarbon, v. 46, p. 1029–1058. RITCHIE, B.D., GAWTHORPE, R.L., AND HARDY, S., 2004, Three-dimensional numerical modeling of deltaic depositional sequences 2: influence of local controls: Journal of Sedimentary Research, v. 74, p. 221–238. ROBERTS, H.H., FILLON, R., KOHL, B., ROBALIN, J., AND SYDOW, J., 2004, Depositional architecture of the Lagniappe Delta: sediment characteristics, timing of depositional events, and temporal relationship with adjacent shelf-edge deltas, in Anderson, J.B., and Fillon, R.H., eds., Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico: SEPM, Special Publication 79, p. 143–188. RODRIGUEZ, A.B., AND MEYER, C., 2006, Sea-level variation during the Holocene deduced from the morphologic and stratigraphic evolution of Morgan Peninsula, Alabama, U.S.A.: Journal of Sedimentary Research, v. 76, p. 257–269.

158


RODRIGUEZ, A.B., ANDERSON, J.B., AND SIMMS, A.R., 2005, Incised valley parasequence formation and implications for estuarine evolution: Galveston Estuary, Texas: Journal of Sedimentary Research, v. 75, p. 608–620. RYAN, J.J., 1969, A sedimentological study of Mobile Bay, Alabama: Tallahassee, Florida State University, Sedimentological Research Laboratory, Contribution 30, 110 p. SAGER, W.W., SCHROEDER, W.W., DAVIS, K.S., AND REZAK, R., 1999, A tale of two deltas: seismic mapping of surface sediments on the Mississippi–Alabama outer shelf and implications for recent sea-level fluctuations: Marine Geology, v. 160, p. 119–136. SHACKLETON, N.J., 1987, Oxygen isotopes, ice volume, and sea-level: Quaternary Science Reviews, v. 6, p. 183–190. SHANLEY, K.W., AND MCCABE, P.J., 1994, Perspective on the sequence stratigraphy of continental strata: American Association of Petroleum Geologists, Bulletin, v. 78, p. 544–568. SMITH, W.E., 1988, Geomorphology of the Mobile Delta: Tuscaloosa, Geological Survey of Alabama, Circular 132, 131 p. STUIVER, M., AND REIMER, P.J., 1993, Extended 14C database and revised CALIB radiocarbon calibration program: Radiocarbon, v. 35, p. 215–230. TO¨RNQVIST, T.E., GONZALEZ, J., NEWSCOM, L., VAN DER BORG, K., DE JONG, A.F.M., AND KURNIK, C.W., 2004, Deciphering Holocene sea-level history on the U.S. Gulf Coast: a high resolution record of the Mississippi Delta: Geological Society of America, Bulletin, v. 116, p. 1026–1039. THOMAS, M.A., AND ANDERSON, J.B., 1989, Glacial eustatic controls on seismic sequences and parasequences on the Trinity/Sabine incised valley, Texas continental shelf: Gulf Coast Association of Geological Societies, Transactions, v. 39, p. 563–569. THOMAS, M.A., AND ANDERSON, J.B., 1994, Sea-level controls on the facies architecture of the Trinity/Sabine incised valley-system: Texas continental shelf, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 63–82. UPSHAW, C.F., CREATH, C.W., AND BROOKS, F.L., 1966, Sediments and microfauna off the coasts of Mississippi and adjacent states: Oxford, Mississippi Geological Survey, Bulletin 106, 127 p. VAN WAGONER, J.C., MITCHUM, R.M., CAMPION, K.M., AND RAHMANIAN, V.D., 1990, Siliclastic Sequence Stratigraphy in Well Logs, Cores, and Outcrops: Concepts for High-Resolution Correlation of Time and Facies: American Association of Petroleum Geologists, Methods in Exploration Series, No. 7, 55 p.

JSR

WILLIS, B.J., AND GABEL, S.L., 2003, Formation of deep incisions into tide-dominated river deltas: implications for the stratigraphy of the Sego Sandstone, Book Cliffs, Utah, U.S.A.: Journal of Sedimentary Research, v. 73, p. 246–263. WELLNER, J.S., SARZALEJO, S., LAGOE, M., AND ANDERSON, J.B., 2004, Late Quaternary stratigraphic evolution of the west Louisiana–east Texas continental shelf, in Anderson, J.B., and Fillon, R.H., eds., Late Quaternary Stratigraphic Evolution of the Northern Gulf of Mexico: SEPM, Special Publication 79, p. 143–188. WILSON, A.M., 1984, Response modeling of Holocene sediments within the shallow substrate of central Mississippi Sound [unpublished M.S. Thesis]: The University of Mississippi, Oxford, 88 p. YOUNG, T., 2002, Seismic stratigraphy of the offshore Alabama continental shelf [unpublished M.S. Thesis]: The University of Alabama, Tuscaloosa, 353 p. WESTCOTT, W.A., 1993, Geomorphic thresholds and complex response of fluvial systems—some implications for sequence stratigraphy: American Association of Petroleum Geologists, Bulletin, v. 77, p. 1208–1218. WOOD, L.J., ETHRIDGE, F.G., AND SCHUMM, S.A., 1993, An experimental study of the influence of subaqueous shelf angles on the coastal plain and shelf deposits, in Weimer, P., and Posamentier, H.W., eds., Siliciclastic Sequence Stratigraphy—Recent Developments and Applications: American Association of Petroleum Geologists, Memoir 58, p. 381–391. WOOD, L.J., 2004, Predicting tidal sand reservoir architecture using data from modern and ancient depositional systems, in Grammer, G.M., Harris, P.M., and Eberli, G.P., eds., Integration of Outcrop and Modern Analogs in Reservoir Modeling: American Association of Petroleum Geologists, Memoir 80, p. 45–66. WOOD, L.J., AND YOSHIDA, S., 2004, Sequence Stratigraphy and Reservoir Architecture of Tide-Influenced Shoreline Systems in the Late Cretaceous (Campanian) Sego Sandstone Member of the Mancos Shale: Recent Advances in Shoreline–Shelf Stratigraphy: SEPM, Research Conference, Field Guide, Day 2, p. 1–63. ZAITLIN, B.A., DALRYMPLE, R.W., AND BOYD, R., 1994, The stratigraphic organization of incised valley systems associated with relative sea level change, in Dalrymple, R.W., Boyd, R., and Zaitlin, B.A., eds., Incised-Valley Systems: Origin and Sedimentary Sequences: SEPM, Special Publication 51, p. 45–60. Received 27 February 2006; accepted 5 September 2006.