Numerical chronology of Pleistocene coastal plain and valley development; extensive aggradation during glacial low sea-levels

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Quaternary International 135 (2005) 91–113

Numerical chronology of Pleistocene coastal plain and valley development; extensive aggradation during glacial low sea-levels Ervin G. Otvos Department of Coastal Sciences, University of Southern Mississippi, Ocean Springs, MS 39566-7000, USA

Abstract Based on more than 60 newly acquired luminescence ages, field information, and sediment granulometry data, a comprehensive chronostratigraphic framework is presented for Pleistocene alluvial coastal plain and valley terraces and coastal barrier trends on the northern Gulf of Mexico between Texas and NW Florida. Luminescence ages 216–188 ka from the second youngest coastal terrace coincide with the highstand during the second youngest Pleistocene Marine Isotope Stage (MIS 7a). TL ages from the oldest Prairie alluvium and OSL-ages from the Gulfport barriers; 124–116 ka, are consistent with the Sangamon interglacial substage, MIS 5e. Forest assemblages of the last two Pleistocene highstands are very similar to the present hard pine-dominated arboreal flora, established under warm-temperate, humid conditions. Ages of the youngest, Prairie–Beaumont coastal terrace overlap with the Eowisconsin (MIS 5d–5a) and Wisconsin (MIS 4 and 3) stages, associated with a sea-level range of 80 to 30 m. Coastal plain aggradation, not limited to interglacial highstands occurred during the much longer preglacial and glacial low sea-level stages. The Prairie–Beaumont coastal plain is a collage of seamlessly merged surfaces, aggraded between ca.135 and 30 ka in several alluviation stages. Periodically dry conditions, associated with enhanced slope erosion and consequent increase in sheetwash, colluvial, and fluvial sediment flux may have induced substantial aggradation at significant distances inland from the coeval shorelines during depressed preglacial and glacial sea-levels. This balance between erosion and sediment delivery may explain the diminished control of lower Eowisconsin and Wisconsin sea-levels (base-levels) on coastal entrenchment and aggradation. The combined effects of alluvial aggradation and subsequent uplift, modified by surface erosion produced the present coastal plain topography. An early post-Sangamon phase of deep entrenchment involved the Amite, Sabine, Neches, and the Pearl Valleys in Eowisconsin and early Wisconsin times. Except for the universal impact of the deep Last Glacial Maximum (LGM) entrenchment, the alluviation–incision cycles that produced two to four sets of terraces of aggradational and strath origin did not occur in all valleys synchronously and with identical effects. In certain valleys terrace aggradation coincided with the LGM record lowstand and the following late Wisconsin transgressive deglacial hemicycle. In contrast with intensive valley filling, inferred for the MIS 6 glacial interval, Holocene backfilling is far from complete in most late glacial entrenched valleys. r 2004 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The terrace stratigraphy of the Plio-Pleistocene northern Gulf of Mexico coastal plain (Table 1) has been subjected to increasingly detailed studies (Doering, 1935, 1956; Fisk, 1938, 1944; DuBar et al., 1991). Until now, coastal plain studies were frustrated by the inability to create a comprehensive chronology based on numerical ages. The reported numerical ages are from late Pleistocene alluvial coastal and barrier units, located between SE Texas and northwest Florida.

Aggradation events during interglacial (Sangamon), preglacial (Eowisconsin) and glacial (Wisconsin) stages and substages were correlated with corresponding phases of elevated and low sea-levels (base-levels). They were used (a) to establish a framework of numerical chronostratigraphy for Pleistocene coastal plain and valley terrace units; (b) to relate this chronology to global oxygen isotope stages and substages; (c) to attempt correlation between eustatic sea-level positions and coastal barrier and alluvial aggradation stages.

1040-6182/$ - see front matter r 2004 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2004.10.026

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Table 1 Gulf coastal plain Pliocene and Pleistocene stratigraphic and morphostratigraphic units Prairie Terrace (Beaumont, in Texas) youngest, Gulfward Pleistocene terrace, underlain by Prairie Fm (alluvium) Biloxi Fm (inner neritic, paralic) Gulfport Fm (coastal barrier ridge trend; Ingleside barriers on Texas coast) Montgomery Terrace and Formation (alluvium with subordinate paralic beds). Pleistocene Bentley Terrace and Formation (alluvium). Pleistocene Citronelle Terrace and Formation (Williana in Texas) oldest, most landward terrace. Late Pliocene alluvium and extensive paralic beds

2. Materials and methods The study is based on more than 60 thermoluminescence (TL) and optical luminescence (OSL) ages from alluvial, coastal barrier, and eolian sand samples, collected in recent years in borrow pits, occasionally from natural exposures. TL-dating (Aitken, 1985; Berger, 1988; Huntley et al., 1993b) was performed at University of Wollongong, Australia. Sediment samples were taken by filling and then capping 10-cm PVC tubes at 30 cm or more below ground surface to avoid exposure to sunlight. Ionizing irradiation of mineral grains generates free electrons, some of which become trapped at impurities or structural defects within the grains. Age determination depends on the energy stored in this way by the grains buried in a given sedimentary unit. The 90–125 mm quartz grains were utilized in TL dating. Ages were determined by a regenerative technique, modified by Readhead (1988). The tabulated TL ages are based on the actual moisture content of the collected samples but values based on 10% assumed moisture content were also calculated. Quartz grains of the 90–250 mm size fraction were used for optical dating. This work was performed at the Research Laboratory for Archaeology, University of Oxford. Equivalent dose (De) determination utilized the single aliquot regenerative-dose (SAR) technique of Murray and Roberts (1998), as improved by Murray and Wintle (2000). While the contribution of Rb to the radiation generally is small, the ‘‘light’’ HF etch that was utilized is relevant when the annual radiation flux level is low. Because the sampled sediment units originally were covered by thicker overburden, it was assumed that the cosmic radiation dose received in the past was half of what would have been received at present-day burial depth. The U, Th, and K concentrations of the samples were determined by instrumental neutron activation analysis (INAA). Based on formulae by Prescott and Hutton (1994), the cosmic-ray dose-rate contribution was estimated as a function of geomagnetic latitude, altitude, and overburden. The luminescence signal of sand grains deposited underwater in alluvial settings is more accurately determined by the OSL method than by

TL. More detailed sampling and future improvements in dating methods could greatly refine and expand the use of numerical chronostratigraphy in the region. Anomalous dose rates, diagenetic chemical changes, biological, eolian, and slope processes that cause postdepositional mixing, as well as erosive re-exposure of dune and beach sands to sunlight, not infrequently result in unrealistic luminescence ages. In a limited number of samples, primarily 6-P (Fig. 2) and # 24, B-1, B-2, and B-4 (Fig. 3) bleaching or other postdepositional changes are suspected in the unrealistically old or young ages. Recognized by sample behavior during analytical procedures and/or by chronological inconsistencies, the highly anomalous ages, although documented in the enclosed tabulation, have been excluded from the final evaluation. Compared with the wide standard deviation range of most luminescence ages (Tables 2–6), isotope substages and stages are generally represented by narrower age ranges. The desired perfect match between these sets of values is seldom achievable. Overlap between broad luminescence age ranges of the luminescence samples and the age ranges of two or more consecutive isotope stages and/or substages, each represented by different climate conditions and sea-levels, was common. The relative scarcity of available surface outcrops with appropriate sandy lithology did preclude more detailed mapping of the luminescence-dated sedimentary units.

3. Coastal plain terrace stratigraphy Following Hayes and Kennedy (1903) in Texas, and Doering (1935) who described siliciclastic alluvial deposits that underlie landward rising terrace stairsteps, Fisk (1938, 1944) and Bernard (1950) were the first to establish and map the currently used morphostratigraphic subdivisions of the coastal plain. The gently sloping terrace summit surfaces are delineated by erosion-, occasionally fault-defined and erosionally modified seaward-facing scarps (Fig. 1). Fisk associated terrace scarp development with erosional phases that coincided with glacial lowstands that separated marine

ARTICLE IN PRESS E.G. Otvos / Quaternary International 135 (2005) 91–113 Table 2 Pre-Sangamon coastal terrace ages Sample number

Sample location USGS Quadrangle

Table 4 Prairie–Beaumont alluvial coastal plain ages, Sangamon to late Wisconsin Date, TL/OSL (ka)

(a) Pleistocene Intermediate Coastal Terraces, TL dates (Figs. 2 and 3; Bentley—Be; Montgomery—M; P—false Prairie date) 1-Be Longville, LA 4277 2-Be Glenmora, LA 411479 1-M Buna, TX 216789 2-M Biloxi, MS 210727 2-M Biloxi, MS 221722 3-M Three Rivers, MS 188724 6-P Franklin Lake, TX 363725 (b) Pleistocene Intermed. Terrace and Gulfport Barrier (B-) OSL dates (Fig. 3) 4-M Three Rivers, MS 176.5732.1 B-3 Biloxi, MS 117.2712.4 B-5a Gautier-S, MS 124.0710.8 B-6 Gulf Breeze, FL 116.179.1 Detailed tabulation of location and analytical data available from the authors data archives.

Table 3 Gulfport barrier ages Sample number

Sample location USGS Quadrangle

Gulfport Barrier TL dates (Fig. 3) B-1 English Lookout, MS B-2 Gulfport-N, MS B-4 Biloxi, MS B-5b Gautier-S, MS

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Date, TL/OSL (ka)

160717 103.0710 38.873.7 94.779.9

Detailed tabulation of location and analytical data available from the authors data archives.

highstand phases of terrace aggradation. The younger terrace surfaces, closer to the Gulf are more gently inclined than the older terraces located further landward. Descending Gulfward, the Pliocene–Pleistocene sequence includes the major stratigraphic units and coastal surfaces (Fisk, 1938; Louisiana Geological Survey, 1984; Otvos, 1991a) listed in Table 1.

Sample number

Sample location USGS Quadrangle

Date, TL/OSL (ka)

(a) Pleistocene Prairie Terrace TL dates SW LA to NW FL (Figs. 2 and 3) 3a Moss Bluff, LA 135.077.0 7 Mamou, LA 61.575.4 13 Carencro, LA 54.476.2 16 Port Hudson, LA 65.274.3 17 Hatcherville, LA 30.273.8 18 Pine Grove, LA 50.475.3 19 Watson, LA 48.474.2 20a Denham Springs, LA 44.175.0 21 Denham Springs, LA 30.772.5 22 Denham Springs, MS 32.173.1 23a Hammond, LA 44.975.7 24 St.Tammany, LA 194714 25 Slidell, LA 50.574.1 26 Sun, LA 135.0718.0 27 Hickory, LA 119.076.0 28 Hickory, LA 113.0712.0 29 Nicholson, MS-LA 124.0712.0 30 Gulfport-N, MS 90.1711.3 33 Gulf Shores, AL 499.1 34 Springfield, FL 27.972.5 (b) Pleistocene Prairie– Beaumont Terrace OSL dates LA and TX (Figs. 2 and 3) 1 Winnie, NW, TX 102.378.3 2 Sulphur, LA 91.279.0 3b Lake Charles, LA 88.477.6 4 Lake Charles, LA 106.879.0 5 Indian Village, LA 74.175.6 6 Lake Arthur, LA 76.177.9 8 Basile, LA 36.272.5 9 Evangeline, LA 101.078.1 10 Abbeville-W, LA 89.779.8 11 Branch, LA 101.079.1 12 Opelousas, LA 59.674.7 14 Carencro, LA 86.0714.2 15 Milton, FL 84.6710.0 20b Denham Springs, LA 36.372.5 23b Hammond, LA 33.372.7 24 St.Tammany, LA 194.0714.0 31 Ocean Springs, MS 179.3720.7 32 Gautier, MS 130.0713.6 Detailed tabulation of location and analytical data available from the authors data archives.

3.1. Citronelle terrace and formation The dominantly coarse sandy-gravelly lithology, characterized by oxidized orange-red, orange-pink colors, also includes substantial heterolithic and sandy mud intervals. The Citronelle (Figs. 1–3) occupies a broad coastal region between Texas and SW Georgia. Proposing a Pleistocene age, Fisk (1938) and others rejected Matson’s (1916) original Pliocene assignment. A Pleistocene or Plio–Pleistocene age was adopted in several publications, including Doering (1956), Louisi-

ana Geological Survey (1989), and Saucier (1994). The widespread estuarine-nearshore marine depositional facies of the Citronelle 475 km inland from the present shoreline provide evidence for a major transgression, much more extensive and invasive than those of the Pleistocene Epoch. In addition, the 220+ km shorenormal width and the related record +180 m inland elevation of alluvial Citronelle beds indicate its prolonged existence and a slow, landward increasing regional uplift. These factors are more compatible

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Table 5 Sabine, Amite River, and Calcasieu Valley terrace ages Sample number

Sample location USGS Quadrangle

Date, TL/OSL (ka)

(a) Sabine (S-) and Amite (A-) Valley Terraces TL dates (b-valley floor sample) (Figs. 8 and 10) S-3 Echo, LA-TX 37.173.4 Sb-4 Evans, LA-TX 21.172.2 Sb-5 Merryville-S, LA-TX 9.370.8 Sb-7 Starks, LA-TX 13.071.3 A-1 Watson, LA 17.471.3 A-2a Watson, LA 14.571.3 A-2b Watson, LA 17.470.9 A-3 Watson, LA 18.071.1 Ab4 Watson, LA 14.770.8 Ab-5 Watson, LA 48.474.9 Ab-6 Denham Springs, LA 6.770.6 (b) Sabine Valley (Fig. 8), Calcasieu Valley (symbols as in Table 5a) S-1 Merryville-N, LA S-2 Hartburg, TX Sb-6 Merryville-S, TX-LA C-1 Le Blanc, LA C-2 Le Blanc, LA

(Fig. 2) OSL dates 23.471.2 24.471.3 2.370.2 27.871.8 0.8370.06

Detailed tabulation of location and analytical data available from the authors data archives.

Table 6 Pearl River Valley TL and OSL ages Valley entrenchment and terrace aggradation phases Sample number

Sample location USGS Quadrangle

Date, TL/OSL (ka)

(a) Pearl River Valley, MS-LA TL dates Letter by site number: U— upper/ L—lower terrace (Fig. 13) 3U Angie, LA 40.672.2 4L Angie, LA 34.772.5 5U Bogalusa-E, LA 75.174.6 6U Bogalusa-E, LA 34.871.9 7U Sun, LA 54.275.4 8U Industrial, MS 31.972.5 9U Haaswood, LA-MS 46.773.6 10L Henleyfield, LA-MS 6.870.6 11L Industrial, MS 19.771.4 12L Industrial, MS 18.071.4 13L Nicholson, MS-LA 41.874.4 14L Nicholson, MS-LA 13.671.4 15L Nicholson, MS-LA 88.1711.0 same 85.8710.7 (b) Pearl River Valley, MS-LA, OSL dates (Letters, as in Table 6a; Fig. 13) 1U Morgantown, MS 41.472.6 2U Columbia-S, MS 32.273.2 Detailed tabulation of location and analytical data available from the authors data archives.

with a Pliocene age. Correlation with fossiliferous Pliocene units in south Georgia and northern Florida (Huddlestun, 1984) and the presence of Sciadopytis

(Japanese umbrella pine) pollen in the Citronelle further strengthen the age evidence (Otvos, 1997, 1998, 2004b). 3.2. Intermediate terraces Restricted to isolated areas in southwest Louisiana and adjacent east Texas (Bernard and LeBlanc, 1965), the combined, 5–50 km wide Bentley and Montgomery terrace zone was mapped as ‘‘Intermediate Terraces’’ between south Texas and Alabama (Louisiana Geological Survey, 1984, 1989). The wider Montgomery Terrace occupies a semicontinuous belt between SE Texas and SW Alabama (Figs. 2 and 3). Intensive postMontgomery uplift and erosion along the coast may account for the total absence of these terraces east of Mobile Bay (Otvos, 1991a). Covered by Prairie deposits, the Montgomery alluvium includes at least one paralic lithosome in SW Louisiana (Otvos, 1991b). Terrace deposits contain 0.8–1.0 m thick, partly carbonized woody lenses in flood plain alluvium north of the Big Ridge faultline scarp in Mississippi. The enclosed plant assemblage greatly resembles the extant hard pine-dominated warm-temperate arboreal flora of this humid coastal region. The assemblage is dominated by Pinus elliotti (slash pine), P. taeda (loblolly pine), and P. glabra (spruce pine). Quercus (oak), Carya (hickory), Ilex (holly), Liquidambra (sweetgum), Castanaea (chestnut), and Myrica (bayberry or myrtle) pollen occur in minor amounts (Otvos, 1997). 3.2.1. Intermediate terrace ages and glacio-eustatic sealevels A single non-finite age (4277 ka; 1-Be; Fig. 2; Table 2a), was derived from an outcrop in Louisiana, mapped as Bentley by Holland et al. (1952). Luminescence ages that correspond to MIS 9 interglacial highstand ages in the Bahamas (338–303 ka; Hearty, 1998) and Bermuda (373 ka; Hearty and Vacher, 1994) came from much younger coastal plain bluff sequences at the Colorado Delta of south Texas (Fig. 6; Table 1 in Blum and Price, 1998). A comparable age was obtained also from a southeast Texas borrow pit (present study; Site #6-P, 363725 ka; Fig. 2; Table 2a). These highly unrealistic ages are ca. 200 ka older than the real age of the Beaumont alluvium in which they apparently originated. Five luminescence ages, ranging between 216 and 176 ka originated in sand and silty sand units at three Mississippi and Texas sites (Figs. 1–3; Table 2a and b). They partially overlap with the age range of MIS 7a highstand, 216–194 ka (Pillans, 1994; Pillans et al., 1988; Hearty, 1998; Huntley et al., 1993a; Hearty and Kindler, 1997; Fig. 4). Sea-level was at or slightly above its current position (Chappell and Shackleton, 1986; Shackleton, 1987; Hearty, 1998; Fig. 5).

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Fig. 1. Composite basinward cross section across the four Pliocene–Quaternary coastal plain terraces, northern Gulf of Mexico. No scale. This combination of all the major coastal landforms is absent from specific shore-normal cross sections with highly variable elevation and width values.

Fig. 2. Pleistocene luminescence sample locations, northwestern Gulf coastal plain. OSL and TL ages (Tables 1 and 2), Symbols: Be—Bentley sample; 1-M: Montgomery: samples; 6-P: sample with unrealistic Prairie age. Fannett barrier sector, Ingleside Trend, SW of Beaumont, TX. C-: Calcasieu valley terrace samples. Prairie terrace samples are only numbered (Tables 1 and 3). Surface subdivisions based on Louisiana Geological Survey (1989).

4. Late Pleistocene terrace ages and sea-levels 4.1. MIS 6-5e transgression and Sangamon highstand units Matching the record MIS 2 Wisconsin lowstand following a major decline in Stage MIS 6, eustatic sea-

level may have stood at 125 to 129 m (Gischler et al., 2000). The inferred depth of entrenchment beneath the Montgomery surface, inferred for the MIS 6 lowstand was comparable with that associated with the MIS 2 lowstand. The constraining topographic configuration of the heavily dissected Citronelle uplands indicates that the Pearl, Amite, and Tangipahoa river valleys have

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Fig. 3. Pleistocene luminescence sample locations, northeastern Gulf coastal plain. OSL and TL-ages (Tables 1–4). Symbols: M—Montgomery alluvium; B—Gulfport barrier trend. The Prairie samples are only numbered.

Fig. 4. Standard deviation ranges of OSL and TL ages: Prairie–Beaumont coastal plain and valley terraces.

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Fig. 5. Late Pleistocene sea-level curve, compiled from Shackleton (1987); Chapman and Shackleton (1999) Huntley et al. (1993a), Toscano and Lundberg (1999), van Andel and Tzedakis (1996), Gischler et al. (2000). Special data points: I—Based on Shackleton (1987); Huntley et al. (1993a); van Andel and Tzedakis (1996); II—Rodriguez et al. (2000) 15 m; III—Toscano and Lundberg (1999); Cabioch and Ayliffe (2001) display a shallower curve between 80–25 ka.

occupied their present positions throughout the Pleistocene Epoch, at least since the entrenchment episode. The fine-grained, highly fossiliferous Biloxi Formation (Otvos, 1975) represents the earliest transgressivehighstand unit in MIS 5e. These dominantly paralicnearshore, occasionally inner neritic deposits underlie 2.0–6.0 m thick Sangamon and Eowisconsin alluvium in coastal Louisiana and Mississippi (e.g., Featherman, 1872, p.106; Fisk, 1948; Jones, 1956; Fig. 8 in Otvos and Howat, 1997; Otvos, unpubl. data), and northwest Florida (Otvos, 1992, 1997). Overlain by Gulfport–Ingleside coastal barriers and silty–sandy Prairie–Beaumont alluvium the Biloxi occurs along and at varying distances inland from the Sangamon highstand shoreline (Fig. 1). It extends seaward beneath the Holocene island chain (Otvos and Giardino, 2004). The linear character of both Sangamon barrier trends suggest that coastal plain valleys, inferred to have been deeply entrenched during the MIS 6 record lowstand, became completely filled during the transgressive hemicycle that preceded the MIS 5e highstand. The coastal surface thus provided a level foundation for the overlying Sangamon and postSangamon units. In contrast, the Gulf coastal streams have filled only a portion of their entrenched late Wisconsin valleys during the current transgressive deglacial hemicycle and highstand. 4.2. Prairie Terrace and the Sangamon complex The 1–35 km wide Prairie Terrace surface, represented by a gently Gulfward-inclined coastal surface

(Figs. 1–3), underlain by silty–sandy, not infrequently gravelly deposits of the Prairie Formation. Rising from below sea-level, the Prairie surface reaches 21+ m elevation inland, occasionally displaying imprinted oversized relict meander loops. This surface widens to 50–100 km in Texas where, designated as the Beaumont it is underlain by a thick alluvial sequence, intercalated with multiple paralic sediment intervals. Cooke (1931, 1945) named the NW Florida–Atlantic coast correlative of the Prairie coastal surface, the Pamlico Terrace. Bluish-gray, sandy-muddy, occasionally clayey, generally o6 m thick fossil-rich paralic, in seaward direction occasionally neritic deposits occur beneath the seaward fringe of the Prairie–Beaumont surface and the Gulfport–Ingleside barriers (Otvos, 1975, 1992, 1997; Gohn, 2001). Designated as the Biloxi Formation (Otvos, 1975, 1992) it extends Gulfward and underlies the barrier islands (Otvos and Giardino, 2004). Two borrow pits on the southwestern Mississippi coast (sec. 28-T7S-R12W and sec. 37-T7S-R16W) exposed plant fossils. The second pit, near Sangamondated Prairie Site #29 (Table 4a), yielded pine macrofossils, including cones and bark. Pollen count in samples taken from the two pits included 40–67% pine pollen with minor amounts of Quercus (oak), Liquidambar (sweetgum), Populus (poplar), Myrica (bayberry), Ilex (holly), and other arboreal genera, as well as several non-arboreal taxa (Otvos, 1991a; G. Chmura and J. Wrenn, written communications, 2004).

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4.3. Sangamon coastal barrier trends and luminescence ages Members of the Ingleside barrier chain (Price, 1933, 1958; Otvos and Howat, 1996) mark seaward prograded interglacial shoreline positions (Figs. 2 and 6; Otvos, 2001a,b). Located along the Prairie coastal plain, the Gulfport ridges flank the mainland shore between the Pearl River, Mississippi, and the eastern Florida Panhandle. Reaching +5 to +9 m elevation, the individual barrier ridge sectors are 0.5–2.0 km wide and 6–13 m thick. Frequent callianassid burrow tubes of ghost shrimp indicate intertidal-subtidal sand facies, saturated by organics-enriched humate matter. The dark-brown semiconsolidated sandstone lenses, commonly intercalated with light gray and white barrier sands reflect widespread postdepositional groundwater effects (Swanson and Palacas, 1965). Composed of moderately sorted subtidal sands that grade upward into very well sorted intertidal sand units, occasionally topped by eolian cross-strata, top the barrier intervals. Several of the Pleistocene barrier sectors in Mississippi, Alabama and NW Florida are capped by an erosionally subdued, degraded strandplain topography (Otvos, 2000). The Fannett barrier is one of only three ridgeplain-capped barrier sectors in the Texas Ingleside Trend (Figs. 2,6 and 7, and Otvos and Howat, 1996). Masking the Sangamon ridgeplain surfaces, most barrier sectors in NW Florida and Alabama are covered by Wisconsin and early Holocene eolian sand sheets and dunes (Otvos, 2004a). Based on previous stratigraphic subdivisions of the Pleistocene Epoch (Fisk, 1938, 1944; Bernard and

LeBlanc, 1965), the Ingleside barriers have been assigned to an assumed mid-Wisconsin interglacial marine highstand (Shideler, 1986; Saucier, 1994). In addition, several sets of topographic ridges in the south Louisiana coastal plain were held to be post-Ingleside coastal barrier chains. Except for the Ingleside Fannett barrier (Fig. 2), none of the ridge-like topographic

Fig. 7. Ingleside barrier sectors, south Texas coast: 1—Seadrift; 2— Aransas, St. Charles (NWR Bluff outcrop; Aransas Refuge); 3— Ingleside, Live Oak; 4—Encinal, Flour Bluff (Otvos and Howat, 1996).

Fig. 6. Sangamon interglacial Ingleside and Gulfport barrier chains on northern Gulf of Mexico, with coastal stream Gulfport barrier locations (Tables 2b and 3). PB—Pensacola Bay Bluff outcrop, FL.

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features assigned to an eastern Ingleside trend (Price, 1958) and to younger Pleistocene barrier chains further Gulfward (Saucier, 1994) are remnants of coastal barrier ridges. Instead, they are topographic interfluves, sculpted by surface erosion from alluvial deposits (Otvos and Howat, 1997). Scarps and bluffs cut in alluvial beds along tectonic lineaments also contributed to the misidentification of these landforms (Otvos, 1999; Otvos and Howat, 1997). Sediment and stratigraphic evidence disproves the existence of late Pleistocene barrier trends in south Louisiana (Fig. 2) and of relict wave-cut scarps at other coastal locations as well. Saucier and others associated assumed sea-level indicators with landforms displayed in Montgomery and Prairie–Beaumont surfaces in four coastal states (in Otvos, 1995, 1999). The new luminescence ages provide the most direct chronological link between coastal lithosomes and highstand stages (Figs. 3,6 and 7). Three OSL ages from Mississippi and NW Florida Gulfport barrier ridges (B-3, B-5a, B-6; Table 2b) range between 124 and 116 ka. They partially overlap with the published ages of the MIS 5e highstand. A few highly anomalous TL barrier ages (Sites #B-1; B-4; B 5/b; Table 3) include a 39 ka value (Site B-4; Fig. 3) that may have originated in the Wisconsin eolian sand veneer over a Gulfport barrier sector in west Biloxi. Eolian Wisconsin inland sandsheet and dune deposits are known from the Louisiana and Alabama coastal plain but not yet from Mississippi (Otvos, 2004a). The highest exposures of shallow subtidal Ophiomorpha shrimp trace fossils in the Belle Fontaine barrier bluffs (Site B-3; Fig. 3) mark the minimum elevation of the Sangamon highstand at +1.5–2.0 m at this site (Otvos, 1991a, 1997). Another Sangamon luminescence age from the Gulf Breeze bluff on Pensacola Bay (116 ka; B-6; Table 2b, Fig. 6) was associated with a cross-stratified Sangamon dune interval, buried under thick Wisconsin eolian sand carpet (Sample B-6, Table 2b; Otvos, 2004a). Amino acid racemization analyses on Chione bivalves, collected from drillcores at the base of the Ingleside Live Oak barrier (Otvos and Howat, 1996) produced 0.8 alloisoleucine/isoleucine ratios that provided aminostratigraphic correlation with Sangamon marine terraces in Baja California, Mexico (J. Wehmiller, written communication, 1993). ESR (electron spin resonance) ages 24:0  9:4 and 56:6  8:1 ka (H.P. Schwartz, written communication, 1995) were obtained from Mammut americanum (American mastodon) bones. They were emplaced in lacustrine deposits that filled pond basins, originated by earlier deflation in two Ingleside barrier sectors (Otvos and Howat, 1996). Coupled with global sea-level evidence and Gulfport barrier ages, the ESR values lend additional support to the pre-Wisconsin age of the barrier trend.

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Blum and Carter (2000) reinterpreted the 1–3 km wide Gulfward margins of the south-central Texas Pleistocene Ingleside barrier sectors as representing Holocene strandplains. They postulated a mid Holocene transgression and record highstand that led to the seaward progradation of the much younger strandplains. However, drillcore information from the Ingleside sectors (Otvos and Howat, 1996; Otvos, 2004c) and luminescence ages 52.3–9.8 ka (Carter, 2000) provides no basis for such a transgressive sediment interval. No mid Holocene ages were reported from the Gulfward flank of the Ingleside barrier sectors. The late Pleistocene OSL ages apparently originated in shallow subsurface Ingleside barrier lithosomes, repeatedly subjected to partial bleaching by solar re-exposure during eolian and colluvial reworking. 4.4. Prairie– Beaumont alluvium (Figs. 1– 5; Tables 4a and b) As the Ingleside barrier chain, the Prairie–Beaumont coastal terrace surfaces and directly underlying alluvium have also been linked in the past to the hypothetical mid-Wisconsin interglacial highstand (Fisk, 1944; Bernard and LeBlanc, 1965; Saucier, 1991; Delcourt and Delcourt, 1996). Saucier (1991, 1994) and Autin et al. (1991) designated a ‘‘Prairie Complex’’, consisting of two marine highstand units. Lacking numerical ages, Saucier assigned his lower Prairie to the Sangamon Interglacial (MIS 5e) and the upper Prairie either to a relatively warm Eowisconsin substage (MIS 5a or 5c), or the middle Wisconsin; alternately to the brief late Wisconsin Farmdalian ‘‘interglacial’’ (interstade). The existence of this two-tier sequence, interspersed with diagnostic paleosol horizons was never demonstrated in specific field exposures and locations. 4.4.1. Sangamon (MIS 5e) alluvial deposits Following the Fiskian concept that marine highstands, associated with high ultimate base-levels are indispensable in alluvial aggradation of coastal plains, a Sangamon interglacial age has been earlier proposed for most of the Prairie interval (Otvos, 1971, 1975, 1991a). Luminescence ages 130–117 ka from Prairie coastal plain deposits located along the lower Pearl Valley partially overlap with the MIS 5e age range (Figs. 3 and 6; Table 3). In addition to forming the present land surface along the Pearl Valley of Mississippi and Louisiana, yet undated interglacial alluvial strata probably also underlie the post-Sangamon alluvium. According to high-precision uranium-series dating by various authors, the Substage 5e interglacial interval ranged between 135–132 ka and 118–114 ka (Chen et al., 1991; van Andel and Tzedakis, 1996; Winograd et al., 1997; Gischler et al., 2000; Hearty and Kaufman, 2000; Nichol, 2002; Cutler et al., 2003). Muhs et al. (2002)

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constrained the duration of the stand, higher than the present between 128 and 116 ka. Neumann and Hearty (1996) and Hearty and Neumann (2001) suggest a +2.5–3.0 m level early in MIS 5e that rose briefly to +6.0 to 8.5 m near the end of the substage. 4.4.2. Eowisconsin (MIS 5d– a) Prairie– Beaumont Deposits and sea-level positions Luminescence ages indicate extensive Prairie coastal plain aggradation during preglacial and glacial stages, with occasionally drier conditions. Increased surface erosion provided abundant sediment flux at times of drier conditions and lowered sea-levels. During times of depressed mid- and late Wisconsin base-levels prior to the Last Glacial Maximum (LGM), the large fluvial sediment load, supplemented by colluvial, and slopewash contribution prevented or mitigated both valley and coastal plain entrenchment. Luminescence dating of the Pleistocene alluvium identified three intervals, broadly correlatable with eustatic sea-level stages and substages of MIS 5–3. Substages MIS 5d–5a represent alternating cooler and warmer climate phases between 113 and 76 (74) ka (Richmond and Fullerton, 1986; Cutler et al., 2003). Frustrating correlation between relatively high sea-levels and the luminescence ages of the alluvial deposits, the broad one standard deviation error ranges tend to overlap with at least two isotope stages or substages (Figs. 4 and 5). Several meters of oxidized gray to yellowish-brown sandy silt, sandy mud, and mud, deposited in overbank flood plain facies directly underlie dated Eowisconsin alluvial surfaces. Very fine-to-fine grained cross-stratified sands of Sangamon and/or Eowisconsin age may directly underlie these units. The Sangamon–Eowisconsin boundary was unrecognizable in field exposures. However, the depths of Sangamon Biloxi deposits beneath the Prairie–Beaumont land surface do define the maximum thickness of the Eowisconsin alluvium in given exposures and drillholes. Such an example is the 6 m thick alluvial sandy mud interval that may also include Sangamon-age alluvium above the fossiliferous Biloxi Formation at Abbeville, SW Louisiana (sec. 41-12S-3E). This interval is capped by Eowisconsin-dated deposits that underlie the Mississippi River meander belt surface. Heavy mineral studies identified three major fluvial sources and corresponding Eowisconsin and Wisconsin flood plain sectors that underlie the Prairie coastal plain of SW Louisiana (Mange and Otvos, in press). Luminescence ages, 101–76 ka from nearby sites suggest that the oversized meander loops incised in the Prairie coastal surface ca. 12 km east of the Pearl Estuary (Fig. 3) also formed during post-Sangamon alluviation. An Eowisconsin Beaumont age was determined in SE Texas (Site #1, Fig. 2; Table 4a). Durbin et al. (1997) and Blum and Price (1998) reported three Beaumont ages with a

115–92 ka range. The Gulfward width of the Beaumont–Prairie apron that surrounds the Sangamon Fannett barrier (Fig. 2) indicates the extent of Eowisconsin alluvial aggradation and progradation. Sea-level briefly reached 45 m, respectively 7 m during substages MIS 5c and 5a (Vacher and Hearty, 1989; Toscano and York, 1992; Hearty and Kindler, 1997; Skene et al., 1998; Simmons et al., 1999). Cabioch and Ayliffe (2001) inferred a peak level of 19 or 6 m during Substage 5c. By 80 ka, the glacio-eustatic highstand may have closely approximated the present one (Muhs et al., 2002). According to high-precision New Guinea and Barbados coral sequence ages, the sea-level during the MIS 5c–5b transition declined from a MIS 5c peak of 17 to 57 m. It rose again to ca. 10 m during the MIS 5b–5a transition (Cutler et al., 2003) and achieved a highstand that lasted for less than 7 ka. The severely depressed sea-levels during Eowisconsin substages MIS 5d and 5b stood between 60 and 50 m (Chappell et al., 1996; Hearty and Kindler, 1997; Chapman and Shackleton, 1999; Toscano and Lundberg, 1999; Winograd, 2001). The MIS 5d lowstand that may have also induced major valley entrenchment, was preceded by a 76–78 m sea-level decline over a period of 15 ka (Fig. 5; Toscano and York, 1992; van Andel and Tzedakis, 1996, p. 484; Chapman and Shackleton, 1999; Hearty and Neumann, 2001). 4.4.3. Sea-levels during isotope stages MIS 3, 4, and ages of Wisconsin alluvial units Sea-level positions during MIS 4 and 3 were significantly lower than in the Eowisconsin substages. Views differ on the magnitude of Wisconsin sea-level changes, including a probable decline early in MIS 4 (Fig. 5). MIS 4 levels generally ranged between 35 and 50 m. Cutler et al. (2003) provided evidence for an even greater sea-level decline from ca. 21 to 81 m during the MIS 5a–MIS 4 transition between 71 and 65 ka. Skene et al. (1998) and Berne´ et al. (2002) report a ca. 60 m low at ca. 55 ka. Cabioch and Ayliffe (2001) identified a sea-level range of 30 to 60 m between 50 and 45 ka in the western Pacific. In contrast, Duncan et al. (2000) infer a 75 m stand at ca. 35 ka in the Atlantic continental shelf. Utilizing radiocarbon ages close to their applicability limit, Rodriguez et al. (2000) interpret a 15 m Gulf highstand between 45 and 37 14C ka BP (Fig. 5). Based on New Guinea and Barbados coral ages, Cutler et al. (2003) identified three MIS 3 sea-level stands between 85 and 74 m during the 61–37 ka interval. Lacking tighter chronological constraints, comparisons of Wisconsin coastal plain ages with coeval sea-level positions provide no close correlations between sea-level stands and coastal plain and stream valley aggradation and terrace incision cycles. Wisconsin ages in Louisiana indicated substantial aggradation during depressed sea-level stages. An early

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alluviation phase of the SW Louisiana Mississippi River meander belt was dated between 60 and 54 ka (Sites 12 and 13), a later phase, dated 36 ka (Site 8, Fig. 2; Table 4b). A 10–12 m thick interval of pebbly granular-coarse to medium sandy tabular and festoon cross-stratified, meander channel deposits are well exposed in a large gravel pit at Site 14 (Fig. 2; Table 4b). Luminescencedated 2–3 m thick sandy overbank muds and muddy sands cap this 6 m thick stacked sandy megaripple sequence. The meander belt surface lies 9–12 m above the present Mississippi flood plain. Mt. Pleasant Bluff exposes a 20 m thick late Pleistocene sequence in the eastern valley wall. Following Saucier and others, Delcourt and Delcourt (1996) tentatively assigned the oldest three channel-fill cycles in the Bluff to ‘‘multiple Sangamonian interglacial highstandsy between 130 and 80 ka.’’ However, a 65 ka age originated near to the toe of the escarpment, 4.5 m above flood plain level (Site 16; Fig. 3, Table 4a) and a 27 14C ka BP age (Autin et al., 1988) 12.3 m above the level of the lower sample. The age of the lower bluff sample is compatible with Wisconsin Prairie coastal plain ages in the Lafayette, La, area (Fig. 2). The 50–30 ka coastal plain age range along Amite and Tangipahoa Valleys (Sites 17–23b and 25, Fig. 3; Tables 4a and b) confirm prolonged aggradation east of the Mississippi only a few millennia prior to the late glacial maximum. Despite their reversed chronologies, two Prairie coastal plain ages obtained in vertical succession (44 and 34 ka; Sites 20a and b, Fig. 3; Tables 4a and b) document a 45 m thick mid-to-late Wisconsin coastal plain interval near Amite River. Similar to Site S-10 (Fig. 13), four-to-five meters of mid-Wisconsin alluvium, dated 46–50 ka, overlies the paralic Biloxi (Site 25, Fig. 3; Table 4a). Ages from the Amite Valley, Mt. Pleasant Bluff (Fig. 3 and Autin et al., 1988), and from Calcasieu Valley (C-1; Fig. 2) suggest continued coastal plain aggradation for a short time after 30 ka, probably only at scattered locations. Indicative of late Wisconsin coastal plain aggradation far to the east, an isolated 28 ka age is reported from a wider portion of the generally very narrow NW Florida Pleistocene coastal plain as well (Site 34; Table 4a). Alluvial aggradation was not confined to narrow, entrenched river valleys even during the Wisconsin glacial stages. Laterally shifting, coalescing, seamlessly merging flood plain surfaces of different ages and fluvial sediment sources combined to create the evenly sloping present coastal plain surface. The aerial extent and elevations of the terrace surfaces and the thickness of underlying alluvial sediment sequences were influenced to greatly differing degrees at different sites by (1) the horizontal distance between actively aggraded inland coastal plain areas and the corresponding palaeoshoreline. Position of the coeval sea-level; (2) the volume of fluvial sediment discharge available for coastal plain

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or valley terrace aggradation; (3) the rate of postdepositional uplift. Increased sediment flux due to enhanced slope erosion was linked to dry climate periods reflected by eolian Pleistocene and early Holocene deposits in the Prairie coastal plain (Otvos and Price, 2001). In combination with both locally and regionally enhanced colluvial and sheetwash sediment transport, abundant fluvial sediment flux from remote semiarid Red River and periglacial–glacial Mississippi River source areas led to flood plain aggradation. This process balanced the effect of lowered base-level that would have otherwise induced widespread entrenchment. This resulted in the infilling of entrenched valleys and aggradation of the broad Prairie coastal plain. Luminescence dating proved that elevated base-level is not exclusive precondition for coastal plain aggradation. Otvos (1971) and Otvos and Price (2001) suggested that alluviation in Louisiana may have continued into and/or resumed in the post-Sangamon lower sea-level period. Alluvial aggradation during depressed sea-levels (Fig. 5) represents ca. 90% of the total time span of Prairie coastal plain aggradation in the course of MIS 5-3.

5. Valley terraces; aggradation, entrenchment, granulometric relationships Valley terraces play a key role in defining postentrenchment coastal plain chronology. To varying degrees, coastal stream terrace evolution is related to sea-level positions, sediment yield, climatic, and hydrological conditions in a given watershed. Relating them to runoff increase, Barton (1930) described oversized relict meander loops and bight embayments from Sabine River terrace surfaces, respectively, in valley walls. In designating these terraces, Bernard (1950) borrowed the name Deweyville from an adjacent river town. The term then was applied to the Neches, Brazos, Trinity, Colorado, the Pearl (Fig. 6), and other valleys; even to valleys of the continental interior (Saucier and Fleetwood, 1970). Leigh and Feeney (1995) reported similar terraces with large paleomeanders, dated 31–28 ka from the Atlantic coastal plain. High effective precipitation and increased fluvial discharge under cooler climate conditions, characterized by reduced evaporation rates contributed to the aggradation of the ‘‘Deweyville’’ terraces. Precipitation rates and runoff volumes may have declined and terrace aggradation probably slowed during drier climate phases of loess and eolian sand deposition (Otvos, 2004a). Based on Colorado River valley fill lithologies in Texas, Blum and Valastro (1994) ascribed the overwhelmingly coarse-grained sandy-gravelly Pleistocene glacial stage alluvium to lateral accretion by fluvial

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Table 7 Comparison of approximate stream entrenchment and aggradation phases, Gulf coastal valleys. Ages in ka, based on luminescence ages in Tables 2–6 and cited references Valley

Entrenchment Aggradation

Nueces, TX (Durbin et al., 1997) ca. 91–60 Calcasieu, LA 85–40(?) Sabine, TX–LA 100/80–50 Amite, LA pre-50 Pearl, LA–MS 110–60

60–47; 43–40; 35–31 (3 terraces) 30–23(?) 40–20 (2 terraces) 50–30 (coastal plain age) 60–30 (upper terrace)

discharge that remained within ‘‘bankfull’’ stages. In contrast, vertical aggradation of silty-muddy overbank flood plain deposits in flood plains was associated with Holocene floods of ‘‘flashy’’, high-peaked discharge that frequently overstepped the river banks. The temporal associations of the two depositional styles could not be confirmed in the present study. Limited to radiocarbon ages and based on theoretical assumptions, ages that ranged between 100 ka and the mid-Holocene have been suggested by various authors (e.g., Aronow, 1967; Alford and Holmes, 1985; Thomas and Anderson, 1994, p. 40). Luminescence ages of the two most recent Pleistocene coastal surfaces now help in establishing chronological links between Gulf valley and coastal plain development, and glacio-eustatic sea-level positions. Numerical chronologies reflect different aggradation and entrenchment histories in different valleys (Table 7). Episodes of deep entrenchment mark the late Eowisconsin and Wisconsin intervals. Following cessation of coastal plain aggradation shortly after 30 ka (Figs. 2–4; Tables 4a,b), non-synchronous cycles of terrace aggradation-incision involved the Sabine, Pearl, and Calcasieu valleys. Certain upstream valley sectors have experienced terrace aggradation even during the LGM record lowstand. In addition to molding aggradational terraces, erosion by flood currents beveled widespread strath terrace surfaces into underlying alluvial beds. Fluvial and colluvial sediment flux, base-level (sealevel) changes, tectonic influences, fluvial and surface erosion processes control valley terrace evolution to greatly variable degrees. The record Wisconsin lowstand, 120 to 127 m at 22 ka (18.5 14C ka BP; Duncan et al., 2000) influenced all valleys in the entire coastal plain. Because of the brevity of this lowstand phase and the lag time between sea-level decline and final entrenchment depth, the value ranged only between 22 and 45 m in the present lower valley and adjacent nearshore zone (Schnable and Goodell, 1968; Kindinger et al., 1994; Morton et al., 1996; Otvos, 1997). This range is only ca. 18–28% of the LGM record lowstand value. Valley terrace ages, associated with the deglacial transgressive hemicycle ranged between ca. 20.0 and

Entrenchment

Aggradation

47–43; 40–35 23(?)–18 20–18 30–20 35–20 (partly strath lower terrace)

13 to modern (lowest terr.) 18 to modern 18–2 19–6 19–6

0.8 ka (Tables 5a,b and 6a,b). Creek terrace aggradation in the Tunica Hills of Louisiana represents the most striking example. It illustrates the potential for valley aggradation even during the record low Pleistocene base-levels of the LGM lowstand. 5.1. Nueces Valley OSL ages originate from the three elevated Nueces Valley terraces, for which Durbin et al. (1997) applied the conventional ‘‘downstepping’’ model. Provided the terrace ages on which the model is based are sufficiently accurate, this model portrays base-level fluctuations that are unique in magnitude and speed among the dated Gulf coastal valleys. Followed by the 12-m aggradation of the middle terrace lithosome, a 20-m entrenchment of the Eowisconsin coastal plain carved out the upper terrace between 5375 and 4174 ka. Renewed aggradation, followed by valley entrenchment between ca. 40 and 36 ka was shown as having established the 12 m high middle terrace extremely rapidly (Durbin et al., 1997, Fig. 5). Aggradation of the ca. 10-m thick lower terrace lithosome was the next step. The lowest terrace, veneered by Holocene flood plain deposits in the valley floor dated 13.2 ka. 5.2. Sabine River Valley This sizable border stream valley widens from 6–7 to 8–12 km, downstream (Fig. 8). The high walls of the northern valley escarpment, were cut from Citronelle and older Pleistocene deposits, the low bluffs downstream, from the sandy-muddy Prairie–Beaumont alluvium. Utilizing the nearest coastal plain ages of 102–90 ka in Texas and Louisiana (Fig. 2) an Eowisconsin age of the adjacent coastal plain appears plausible. Two terraces, each rising only 2–3 m above their base, occur in isolated locations above flood plain level. Oversized meander bights cut from the valley walls and oversized relict meander loops characterize the valley landforms. The upper 1–3 m sediment intervals in the terraces consist of several m thick very fine sandy-silty

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muds, fine sandy silt, and silty-muddy medium sands of overbank origin. Very well sorted fine sand and coarse silty, very fine to granular muddy medium sand underlies the top sandy mud interval in both terraces. Local summit accordance between terrace areas of significantly different ages suggests strath development. Erosional degradation and dissection followed initial aggradation of the high valley terrace. Anderson et al. (1991) and Thomas and Anderson (1994) identified several pre-Wisconsin sets of entrenched and buried river channels in seismic profiles beneath the inner shelf floor. They suggested an Eowisconsin (MIS 5c) age for the two present elevated terraces. An old radiocarbon age (OxL-1060; Morton et al., 1996, after Pearson et al., 1966) and new luminescence figures, however, provided a 37 ka value for the highest valley terrace and comformable 23.4–21.1 ka ages for the next (‘‘middle’’) terrace (Figs. 8 and 9; Table 5b). Terrace ages indicate continued aggradation near Merryville at the same time when the Sabine and Calcasieu valleys became deeply entrenched beneath the present shore zone and inner shelf during LGM record lowstand. Significant channel entrenchment depths were recorded in the present offshore area, 120 km downstream from this locality (Anderson et al., 1991; Thomas and Anderson, 1994). The flood plain-veneered valley floor represents an erosional strath surface that bevels the semiconsolidated muddy-fine sandy valley fill of latest Wisconsin-to-early Holocene age in the valley floor. Dated 13 and 9 ka at two sites, the valley fill extend slightly above modern flood plain level (Fig. 8). Coeval shoreline elevations were associated with Gulf levels that rose from ca. 50 m to ca. 20 m during this interval. This deglacial alluviation phase is correlative with the interval that emplaced the Colorado Valley fill between 19 and 11 14C ka BP (Blum and Valastro, 1994). 5.3. Amite River Valley

Fig. 8. Luminescence ages, Sabine Valley terraces, Louisiana, Texas (Fig. 12, Table 4).

Flanked by 3–5-m bluffs, cut in the Citronelle, Intermediate, and Prairie coastal terrace deposits, the river’s middle reach occupies a 3 km wide incised valley. Downstream, the valley narrows to 0.9–1.1 km at Denham Springs (Fig. 10). A previous valley incision phase, correlative with the inferred entrenchment of the Sabine and Pearl Valleys during the Stage MIS 6 lowstand, may have incised the Intermediate (Montgomery) coastal terrace lithosome in MIS 7, deposited north of Magnolia (Fig. 3). The only available subsurface data that reveals the depth of entrenchment beneath the coastal plain during MIS 6 and/or MIS 2 came from dam foundations north of Site 17 (Fig. 10). Valley incision into the underlying Neogene clays and muds extended ca. 32 m below the Montgomery terrace surface; 18 m below the present flood plain level (United

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Fig. 9. W-E cross section across Sabine Valley (Fig. 8). Note lower (strath) terrace and surface erosion-related local accordance of elevations between intermediate and high valley terrace summit surfaces.

Fig. 10. Luminescence ages, Amite Valley terraces (Fig. 11, Tables 5a and b) and adjacent Prairie coastal plain (Figs. 3 and 11; Table 5a and b), Louisiana (Citronelle and Valley symbols are inverted).

States Army, 1997). Pre-Sangamon-to-Eowisconsin infilling of the entrenched valley has established the base of the Prairie coastal plain sequence. On this level

foundation did the Wisconsin alluvium accumulate between 50 and 30 ka (Fig. 3; Tables 5a and b). Entrenched between 3 and 5 m high valley walls, the Amite ceased to aggrade the adjacent coastal plain (Fig. 14; Tables 5a and b). Two terrace surfaces, separated by an intermittently recognizable 1–2 m high scarp were excavated from 1 to 4 m thick yellowish brown and underlying gray sandysilt sequence (Fig. 11). Several m of clean sand underlies the silty valley fill between the escarpment and the entrenched modern flood plain. Autin (1992, 1993) considered this sediment interval the combination of two Holocene alloformations that preceded a third allounit, the presently active valley floor meander belt. Autin’s assumptions on the early Holocene development history of the Amite River system and prolonged stability of a late Holocene meander belt were based on problematical radiocarbon ages. The silty, sandy mud deposits of this interval display no readily recognizable unconformity surfaces, lithological and other features that would distinguish them as two discrete aggradational terraces. The surface used by Autin to separate two aggradational terraces, appears to be an only sporadically recognizable erosional feature; a strath surface, excavated by river floods. Ages obtained from the incised valley match ages of adjacent Wisconsin coastal plain deposits. Luminescence dating established the late Pleistocene age of most lithosomes encountered within the entrenched valley. Two identical ages, 48.4 ka, obtained from two sand samples 1.5–2.0 m beneath the two terrace surfaces provide evidence (Sites 5 and 19; Fig. 11; Tables 5a and b). No well-defined erosional unconformity separated the sand beds from the overlying sandy silt in sandpit exposures. Rafted woody debris, collected from the base of the thick top sandy silt terrace intervals of the terraces was radiocarbon-dated Holocene (Autin, 1992). Considering the subsurface depths of sampling, the

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Fig. 11. W–E cross section across Amite Valley (Fig. 10). Mid-Wisconsin terrace deposits (coarse stipples) overlain by Late Wisconsin alluvium (fine stipples).

radiocarbon ages are unusually young. With one exception, 9.1 14C ka BP, they postdate 3.0 14C ka BP. Two of the samples dated modern. The conflict with the TL values and the unusually deep depths of recovery, 3.5–4.0 m, even of one of the ‘‘modern’’ samples suggest diagenetic alteration or other causes for the anomalous Holocene radiocarbon ages. The latest Pleistocene sandy and sandy silty valley fill that extends slightly above the modern valley floor (e.g., Sites 2 and 3) yielded TL ages between 18.0 and 14.5 ka. They reflect post-LGM Wisconsin valley floor aggradation during the deglacial hemicycle associated with rising base-level. Renewed stream incision and channel erosion resulted in exposure of the valley fill in the land surface. While a luminescence-dated sample taken beneath the highest present flood plain level provided a 6.7 ka age (Site 6; Fig. 10), the Holocene fill appears to be very thin and restricted to the valley bottom. 5.4. Calcasieu Valley The upper valley terrace also displays oversized relict meanders. A 28 ka upper terrace age (C-1; Fig. 2, Table 5b) was obtained from beneath the terrace summit at an elevation 6 m below that of the coastal plain. This age therefore records terrace aggradation following an earlier phase of coastal plain entrenchment. Muddy as well as sandy units alternate in several meters thick, well-exposed deposits. The lower valley terrace, slightly above flood plain level, dated pre-modern. (C-2; Table 5b). 5.5. Tunica Hills Creek Valleys The heavily dissected Tunica Hills, underlain by Citronelle and earlier Neogene deposits and characterized by steep slopes north of Baton Rouge, LA lie directly adjacent to the modern Mississippi meanderplain (NW corner, Fig. 3). Major creeks expose several m thick plant-fossil-bearing terrace deposits

20–66 m above the adjacent Mississippi flood plain. The thick Peoria loess blanket and the spruce flora that existed here for several millennia in the late Wisconsin (Otvos, 1971, 1980; Jackson and Givens, 1994) suggest cool, periodically also dry conditions. Creek terrace aggradation continued, probably intermittently, between 25 and 17 14C ka BP, before and even during the LGM record lowstand (Delcourt and Delcourt, 1977; Otvos, 1984; Table 2 in Jackson and Givens, 1994). As recently as 12 ka the late-glacial Mississippi braidplain that preceded the Holocene meanderplain and kept aggrading since the LGM, still stood 30–36 m below modern flood plain level downstream at nearby Baton Rouge (Saucier, 1994, p. 245). In the face of these greatly depressed base-levels, only significantly diminished creek runoff and increased sediment input from eroding loess and colluvium could explain the extent of valley aggradation this high above and near the Mississippi flood plain. The steep valleys slopes and a sparser vegetation cover induced intensive surface erosion that lead to terrace aggradation. In contrast, intensive valley downcutting characterized the subsequent wetter Holocene phase. 5.6. Tangipahoa Valley Exposures in this valley indicate coastal plain aggradation until at least 33 ka (Figs. 3 and 12). Tycer Hill, a late Wisconsin dune ridge, TL-dated 14 ka (Otvos and Price, 2001) caps the Wisconsin coastal plain in the west of this section. A strath terrace, cut by flood current erosion probably during LGM beveled the Wisconsin Prairie coastal plain in a narrow zone along the stream. 5.7. Lower Pearl Valley downstream from Columbia, MS This deep, 6–12 km wide valley sector drains large parts of central and southwestern Mississippi and

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Fig. 12. Tangipahoa River strath terrace. and erosional scarp cut from mid to late Wisconsin alluvial coastal plain deposits, capped by late Wisconsin Tycer dune hill ; Otvos and Price, 2001). Prairie coastal plain, southeast Louisiana.(Samples 23a and b; Fig. 3, Table 3).

adjacent southeast Louisiana. This fact and the topographic configuration of the surrounding dissected Citronelle upland area that defined the course of the Pearl River suggest that the location of the stream valley remained unchanged throughout the Pleistocene. Declining in height downstream, the valley escarpments in the north rise 27 m above flood plain level. Two terrace units may be confidently traced within the valley. The erosionally degraded, spatially fragmented discontinuous upper valley terrace surface occurs at varying elevations in the central and northern reaches of the valley. It lies 12–18 m below the valley rim and 6–10 m above the valley floor (Figs. 13 and 14; Tables 6a and b) Following the inferred MIS 6 entrenchment phase, the valley was filled by MIS 7e. The Sangamon Prairie coastal plain, dated 135–113 ka in Sites 25–29 along the stream (Fig. 3) was contiguous with the river valley. Falling sea-level and/or reduced sediment flux triggered the subsequent valley incision phase, interspersed with alluviation episodes during the early post-Sangamon. Recurring Eowisconsin aggradation may account for sample ages 88 and 75 ka at Sites 15, respectively 5 (Fig. 13). Eowisconsin aggradation took also place in the adjacent coastal plain in Mississippi (Figs. 8 and 14). Alluvial sediment parcels in the Nueces Valley escarpment, dated 72 ka may reflect a similar history of valley incision and aggradation episodes (Durbin et al., 1997). Renewed valley entrenchment in the Pearl Valley and other locations between ca. 75 and 60 ka may have coincided with significantly lower Wisconsin sealevels (Fig. 5) as well as with changes in sediment input and/or in regional uplift. Aggradation of the upper Pearl Valley terrace between ca. 60 and 30 ka coincided with alluviation

phase of the coastal plain in southeastern Louisiana. The concurrent aggradation of the upper valley terrace and the contiguous Wisconsin Prairie coastal plain is reflected by the smooth continuity of the land surface between Sites 25 (Fig. 3) and S-10 (Fig. 13). Several m of clean sand and sandy alluvial overbank mud underlies the Wisconsin coastal surface at Site S-10. The base of the sequence may include a thin Sangamon alluvial interval. Fossiliferous paralic deposits of the Biloxi Formation underlie the alluvial sequence. The lateral retreat of the valley escarpment by meander bight erosion resulted in narrow strath terraces, cut from Sangamon and Eowisconsin alluvium (e.g., Sites 5 and S-8, Fig. 13). Oversized meander loops and bights associated with upper and lower terrace lithosomes and associated other erosional landforms suggest periods of substantial stream runoff that alternated with relatively drier climate phases of eolian deposition (Otvos and Price, 2001). Local variations in alluvial aggradation, erosion, and nondeposition may account for the significant age and elevation differences of the upper valley terrace lithosomes in the lower Pearl Valley. Late Wisconsin valley entrenchment and terrace erosion after ca. 30 ka may have penetrated below the present valley floor. Also developing strath terraces, erosion has steepened and dissected summit surfaces and slopes of the upper terrace. Exposed in borrow pits and channel banks in the present valley floor, several m of semiconsolidated, locally oxidized muddy overbank deposits underlie the lower terrace. The most continuous terrace surface in the valley, this mildly undulating plain is located slightly above the active stream channels and represents a strath surface cut into mid-Wisconsin and older valley-fill alluvium (Sites 10–15; Fig. 13) by flood currents. The

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Fig. 13. Sample locations with luminescence ages, Pearl Valley terraces and adjacent Prairie coastal plain, Mississippi–Louisiana. (Fig. 3; Table 4).

underlying Wisconsin alluvial fill includes gravelly fine sand, sandy gravel, deposited in river channels, and silty medium overbank sands and muds, often of weakly oxidized, light reddish-brown color. Pre-LGM luminescence ages in the valley floor provide evidence that, the

late Wisconsin entrenchment of the Pearl Valley tended to be relatively shallow. As in the Colorado Valley of Texas (Blum and Valastro, 1994), the Sabine and Amite formed significant alluvial terraces between 21–18 ka, during the record lowstand.

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Fig. 14. Conceptual W–E cross section across southern Pearl Valley and adjacent Sangamon to Wisconsin Prairie coastal plain units. Rounded luminescence ages compiled from type locations (Fig. 8, Table 4).

landward direction. The oldest Prairie terrace that flanks the valley rim with maximum inclination, while the most recently cut lower valley terrace, displays the least seaward tilt (Fig. 15).

6. Base-level control vs. alluvial aggradation in entrenched valleys and broad coastal plains

Fig. 15. Seaward-inclined terrace slope axial profiles, Pearl Valley, Mississippi–Louisiana (Fig. 13).

Coarse-grained and oxidized semiconsolidated, silty deposits, exposed between active river channels in the broad valley floor represent the erosionally beveled lower terrace strath surface a few m above the active channel banks. Similar to the Sabine flood plain terrace, with the exception of the channel fill, the Holocene sediment sequence is thin to non-existent. The exposed Pleistocene strath surface is underlain by older Pleistocene alluvium, water-covered only during flood seasons. In contrast, several m of transgressive paralic deposits bury the valley floor downstream in the Pearl Estuary (Fig. 13; Otvos, unpublished drill data; Otvos and Giardino, 2004). Axial profiles of the three seaward inclined Pearl Valley surfaces indicate the progressive differential effects of the post-Citronelle uplift that increases in

The aggradation of post-Sangamon terrace alluvium raises the issue of base-level influence on coastal plain aggradation during the depressed post-Sangamon sealevels landward of and well above the coeval shorelines. Base-level studies focused on valley response to rising, stable, and falling sea-levels. Mackin (1948), Posamentier and Vail (1988), Posamentier (2001), and others regarded base-level as the principal determinant of coastal plain stratal architecture. Taking a different view, Gordon and Bridge (1987) emphasized that subsidence, uplift, sediment supply, and other factors play equal or more important roles in coastal alluviation. In stream table experiments, aggradation in incised valleys involved development of shelf-margin deltas at times of stillstand or base-level rise (Koss et al., 1994). In yet another valley model, similar to the present conclusions Leeder and Stewart (1996) demonstrated how increased sediment supply may overcome the influence of low base-levels. While the reasons for significant alluvial aggradation in several entrenched coastal valleys immediately before, during, and following the record LGM lowstand are easier to explain by local conditions, aggradation during depressed MIS 4 and 5 sea-levels across the wide coastal

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plain poses probably a greater challenge. Combination of bathymetric contours from United States Coast and Geodetic Survey nautical charts and estimated sea-levels from the literature (Fig. 5) provide the minimum Eowisconsin and Wisconsin distances between sites of dated coastal terrace alluvium and the corresponding inferred lowstand palaeoshorelines. Proportionally increasing during times of the lower Wisconsin sea-level stages, distances between the present shoreline and projected Eowisconsin palaeo-shorelines ranged from 50 to 160 km in west Louisiana and southeast Texas and from 20 to 90 km in NW Florida. The dated coastal plain sites evidently were sufficiently distant from the coeval shorelines to foster alluvial aggradation and prevent entrenchment.

7. Conclusions Luminescence ages helped to establish a preliminary chronostratigraphic framework for late Pleistocene Gulf coastal and valley terrace aggradation and entrenchment stages. Coastal plain aggradation during the last two interglacial highstand stages represented relatively short time intervals in the development history of the coastal plain. Extensive and prolonged growth coincided predominantly with stages of lower-than-present preglacial and glacial sea-levels. The earliest dated coastal plain deposits include the Montgomery Terrace alluvium, in part coeval with Substage MIS 7a marine highstand between 216 and 188 ka. Sangamon Interglacial barrier development and a phase of coastal plain alluviation between 124 and 113 ka coincided with the MIS 5e Sangamon highstand. Most of Prairie coastal plain alluviation was synchronous not with the brief Sangamon highstand but with depressed preglacial and preglacial Eowisconsin and glacial Wisconsin sea-levels of a much longer time span. Compensating for low base levels of glacial stages, increased sediment flux, aided by intensified surface erosion during drier climate intervals, generally characterized by relatively sparser vegetation cover, have contributed to aggradation in various coastal areas during Eowisconsin (116–74 ka) and Wisconsin (74–30 ka) intermediate-lows and lowstands. As the result, the Prairie land surface is a postdepositionally uplifted patchwork of Sangamon-to-late Wisconsin flood plain sectors. The quantitative data remain elusive when trying to judge respective volumetric contribution from various fluvial sources at different times. Steady post-Citronelle uplift, flood plain aggradation, modified by fluvial and surface erosion processes were the main processes in shaping the present configuration of the Prairie coastal plain. The recognition that times of significant coastal plain aggradation have been closely associated with prolonged periods of sea-level decline

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and lowstand should be incorporated in the lowstand systems tract concept. Post-Sangamon incision of the Pearl, Sabine, Calcasieu, and Nueces valleys deeply entrenched Sangamon and Eowisconsin Prairie coastal plain surfaces. Two to four episodes of Wisconsin valley entrenchment and terrace aggradation between ca. 60 and 18 ka are identifiable by luminescence dating. The last entrenchment episodes of the Mississippi, Amite, Calcasieu, Tangipahoa, and other valleys into the Wisconsin coastal plain deposits postdated 30 ka. Except for the LGM, terrace aggradation and incision episodes were not always coeval between different coastal valleys (Table 7). Preservation of the full-glacial (22–18 ka) alluvium indicates rather moderate entrenchment depths in given valley sectors. The diminished influence of base-level was in part related to the great landward distances between the ancient shorelines and present coastal plain areas where significant postSangamon aggradation did take place. Strath terrace development by erosional beveling characterized certain Amite, Sabine, Tangipahoa, and Pearl valley terrace surfaces. In sharp contrast with valleys, apparently incised during the MIS 6 lowstand but completely filled by the start of the Sangamon Interglacial highstand phase, the entrenched late Wisconsin coastal valleys are still only partially filled after twenty thousand years of sea-level rise. Despite the inferred contrasts in hydrological regimes and depositional styles between Pleistocene glacial stage and Holocene interglacial flood plain sedimentation (Blum and Valastro, 1994; Durbin et al., 1997; Morton et al., 1996), lateral aggradation of coarse clastic flood plain sediments was not confined to Wisconsin glacial stages, assumed to have been essentially characterized by bank-contained flow. The authors reserved vertical aggradation of muddy overbank floodplain deposits by ‘‘flashy’’, high-peaked discharge for Holocene floods. However, such lithological differences are influenced by the granulometric range of the source sediments and by hydrodynamic factors that control alluvial deposition in different facies. By this token, several m thick sand and gravel intervals occur in channel facies of interglacial Sangamon deposits (e.g., Site S-6; Fig. 13). On the other hand, hindering the search for sand-enriched exposures, best suited for luminescence dating, several m thick muddy and sandy-muddy overbank sediments generally dominate not only Sangamon but also most Eowisconsin and Wisconsin coastal plain and valley sequences in the shallow subsurface.

Acknowledgments Luminescence dating was performed by E.J. Rhodes (OSL ages) and D.M. Price (TL ages). Drs. Gail

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Chmura and John Wrenn analyzed the pollen assemblages. The highly constructive and detailed editorial reviews by Drs. C.V. Murray-Wallace and D.J. Huntley were most appreciated. Sincerely thanks are due to NASA’s Commercial Applications Directorate and its Deputy Director, Dr. Marco Giardino for support and financial assistance throughout this and associated projects. The author is greatly indebted to Mrs. Dawne Hard of the Gulf Coast Research Laboratory, Ocean Springs, MS, and Messrs. Kevin Eckhoff and Shannon Ellis, Lockheed-Martin Stennis SC Operations, for the most skillful preparation of computer-generated illustrations.

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