Fluvial Geomorphology of the Lower Pearl River Valley, LA and Mi

Pleistocene and Holocene Fluvial Systems of the Lower Pearl River, Mississippi and Louisiana, USA

Paul V. Heinrich

Louisiana Geological Survey, 3079 Energy, Coast, and Environment Bldg., Louisiana State University,

Baton Rouge, LA 70803

ABSTRACT The terraces of the lower Pearl River valley and adjacent Prairie Allogroup exhibit

the paleochannels of four Pleistocene fluvial systems of the Pearl River. The oldest of these fluvial systems consists of relict sinuous courses and channel segments that form meander belts exhibited by the Hammond alloformation of the Prairie Allogroup west of the Pearl River. Associated with these relict river courses is a large crevasse splay com-plex. On the Mississippi side of the Pearl River, the surface of the Prairie Allogroup exhibits younger, but poorly preserved fluvial ridges. Within the southeastern corner of St. Tammany Parish, an isolated, deeply-incised, relict channel course appears to repre-sent another even younger Pleistocene fluvial system. The youngest of the Pleistocene fluvial systems consists of relict courses and channel segments exhibited by the terrace surfaces of the alloformations comprising the Deweyville Allogroup. Channel widths and radii that greatly exceed those of the modern Pearl River characterize these relict courses and channel segments. Within the lower Pearl River valley, the modern Pearl River exhibits an anastomosing channel system.

INTRODUCTION The Quaternary geology of the northern Gulf coastal plain has been the subject of increasingly detailed stud-

ies. One focus has been the study of individual river valleys, e.g., the Amite, Colorado, Mississippi, Trinity, Sa-bine, and other rivers. Of these major coastal rivers, the Pearl River, Louisiana and Mississippi, remains one of the few for which detailed studies have not been published. Thus, as part of ongoing research, detailed geologic mapping of the lower Pearl River valley region was created for an area encompassing latitude 30°7’30” to 30°45’30” and longitude -89°30’00” to -90°00’00”.

METHODS For this study, a detailed geologic map of the lower Pearl River valley was first compiled from unpublished

geologic maps (Heinrich and McCulloh, 2000a, 2000b, 2001, 2002; McCulloh and Heinrich, 2000, 2001a, 2001b, 2002), and published geologic maps (Heinrich et al., 2004). Then, shaded-relief and color-contoured maps, cre-ated from LIght Detection And Ranging (LIDAR) Digital Elevation Models (DEMs) available from CADGIS Research Laboratory (2006), were used to modify this geologic map. Parts of Mississippi lacking LIDAR and

Heinrich, P. V., 2006, Pleistocene and Holocene fluvial systems of the lower Pearl River, Mississippi and Louisiana, USA: Gulf Coast Association of Geological Societies Transactions, v. 56, p. 267-278.

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other coverage were remapped using shaded-relief and color-contoured maps produced from U.S. Geological Survey 1:24:000 map DEMs in consultation with 1:24,000 scale topographic maps and Pellegrin (1978).

Second, the relict landforms exhibited by Pleistocene units were mapped. Some of these relict landforms were compiled from “feature layers” created for Heinrich and McCulloh (2000a, 2000b, 2001, 2002) and McCul-loh and Heinrich (2000, 2001a, 2001b, 2002). They were also mapped using shaded and colored relief images made from LIDAR and 1:24:000 map DEMs; parish and county soil surveys; 1950 United States Agricultural Stabilization and Conservation Service (ASCS) aerial photography; digital orthophoto quarter-quadrangles (DOQQ), and 1:24,000 topographic maps.

Profiles of terraces associated with Pleistocene geologic units were constructed using LIDAR and 1:24:000 map DEMs. The elevation and Universal Transverse Mercator (UTM) coordinates for a series of points along the surface of each terrace were determined using Global Mapper 6.0.1. For these terraces, north-south profiles and gradients were calculated using this data. The profiles were constructed along a north-south line based on their UTM northings.

RESULTS This geologic mapping revealed three major pre-Holocene stratigraphic units (Fig. 1). The oldest unit, the

Citronelle Formation, underlies the ridge and ravine topography that comprises the uplands within the study area. As previously mapped, i.e., Saucier and Snead (1989), the Prairie Allogroup underlies most of the local coastal plain. Finally, intermediate in elevation and stratigraphic position between the Prairie Allogroup and the modern floodplains are terraces underlain by the Deweyville Allogroup. The Pearl River floodplain consists of Holocene meander belts, backswamps, and deltaic plain of the Pearl River and partially buried terraces of the Deweyville Allogroup.

Prairie Allogroup

The Prairie Allogroup consists of a collection of late Pleistocene depositional sequences of alloformation rank (Autin et al., 1991; Heinrich et al., 2004). The sediments constituting the Prairie Allogroup accumulated within a diverse suite of coastal plain settings, i.e., fluvial (meander-belt, backswamp, and braided-stream), collu-vial, estuarine, deltaic, and shallow marine environments. These sediments largely accumulated over a consider-able part of the late Pleistocene (Sangamon to Wisconsin) (Autin et al., 1991; Otvos, 1997, 2005; McCulloh et al., 2003; Heinrich et al., 2004).

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Figure 1. Geologic map of the Lower Pearl River valley region. (FACING PAGE) Accompanying legend.

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The surface of the Prairie Allogroup forms a coastal terrace along the northwest coast of the Gulf of Mexico from a point about 67 mi (110 km) south of the Rio Grande within Mexico over to at least Mobile Bay, Alabama. This surface is the lowest continuous terrace lying above Holocene coastal and flood plains. This relatively un-dissected terrace exhibits constructional topography that is more poorly preserved than exhibited by terraces of the Deweyville Allogroup and lacking on older Pleistocene surfaces. It comprises multiple stratigraphic units of alloformation rank (Saucier and Snead, 1989; Autin et al., 1991; Dubar et al., 1991; Winker 1991).

Louisiana

On the Louisiana side of the study area, the Prairie Allogroup consists entirely of the Hammond alloforma-

tion, an informal allostratigraphic unit (Heinrich et al., 2004). It is a redefinition of the Hammond terrace of Mat-son (1916) as an allostratigraphic unit from its original usage as a morphostratigraphic unit. It consists of Wis-consin coastal plain deposits lacking a loess cover (McCulloh et al., 2003; Heinrich et al., 2004)

The surface of the Hammond alloformation is a 10 to 25 mi (16 to 40 km) wide coast-wise terrace, which extends eastward from the eastern valley wall of Mississippi Alluvial Valley across the Feliciana parishes to at least the Pearl River. It is the lowest and best preserved of the Pleistocene terraces found between the Mississippi and Pearl rivers. It exhibits moderately to poorly preserved relict constructional landforms. These landforms include relict river courses, meander loops, ridge and swale topography, coastal ridges, and beach ridges. In some areas, they include valley walls and flood plains of entrenched valleys. Faulting has displaced the surface of the Hammond alloformation, creating numerous fault-line scarps (Mossa and Autin, 1989).

Within the study area and west of the Pearl River, the surface of the Prairie Allogroup consists of two terrace segments (Fig. 1). One terrace segment consists of a narrow band extending about 2.5 to 3.5 mi (4 to 5.6 km) south from the southern valley wall of Bogue Chitto River between the communities of Bush and Dave, Louisi-ana. Starting at an elevation over 95 ft (29 m), this segment dips gulfward at 7.0 ft/mi (1.3 m/km) (Fig. 2). It lacks any readily recognizable constructional landforms except for curvilinear hills and ridges that are suggestive of badly degraded relict fluvial landforms.

West of the Pearl River, the other terrace segment consists almost entirely of a single gulfward-sloping sur-face (Figs. 1 and 2). Except adjacent to the Pearl River valley, its northern edge laps against deeply dissected uplands underlain by the Citronelle Formation (Cullinan, 1969; Mossa and Autin, 1989). This terrace slopes gulfward at about 2.6 ft/mi (0.5 m/km) along the edge of the Pearl River valley and about 3.3 ft/mile (0.6 m/km) further west.

This terrace segment exhibits moderately to poorly preserved relict fluvial channels and ridge and swale topography. They occur as sinuous dark tonals in 1950 1:20,000-scale Agricultural Stabilization and Conserva-tion Service aerial photography and sinuous patterns in the distribution of soils as mapped by Trahan et al. (1990). Field inspection showed that they are typically permanently wet, swamp-filled areas that lie as much as 3 ft (1 m) below the surface of the Hammond alloformation. They form well-defined courses, i.e., courses A, B, C, D, and E of Figure 1, radiating outward and gulfward from the area of the community of Talisheek, Louisiana. Within the westernmost study area, narrow anastomosing channels extend westward from a relict course D. They spread westward and drop in elevation into a relict interchannel basin lying west of the Pearl River channels.

One relict river course, course F, distinctly differs from the other mapped paleochannels. It cuts across the southeast corner of the surface of the Hammond alloformation (Fig. 1). Unlike other mapped relict courses, it occupies an entrenched valley. Although partially filled with backswamp, its floor lies 4 to 5 ft (1.2 to 1.5 m) below the surface of the Hammond alloformation. This entrenched valley varies in width from 400 to 600 ft (120 to 180 m).

The surface of the Hammond alloformation exhibits other landforms on its southern edge (Fig. 1). First, Heinrich (2005) noted a relict delta-like feature defined by beach ridges on its eastern side and arcurate topog-raphic highs on its western edge. Recent mapping shows that it is cut by a couple of relict river channels. Closer to the mouth of the Pearl River, two coastal ridges occur on the surface of the Hammond alloformation (Fig. 1). They are ridges about 3 to 10 ft (1 to 3 m) high and 700 to 1,000 ft (210 to 300 m) wide. Crests of one partially-buried ridge form small islands within the Holocene coastal marshes. Finally, fault-line scarps occur along the southern edge of this terrace between and south of Big Branch and Lacombe, Louisiana, and southeast of Slidell, Louisiana (Fig. 1). In the Big Branch - Lacombe area, they displace the terrace by about 10 to 12 ft (3 to 4 m). Southeast of Slidell, Louisiana, the fault-line scarp displaces a coastal ridge by 6 ft (2 m).

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Mississippi On the Mississippi side of the Pearl River, the Prairie Allogroup underlies three terrace segments (Fig. 1).

The northernmost terrace segment is about 2.2 mi (3.5 km) long and 0.5 to 0.8 mi (0.8 to 1.3 km) wide. Its sur-face consists entirely of truncated relict alluvial fans. Later, lateral migration and entrenchment of the Pearl River has destroyed the alluvial plain, to which these alluvial fans were graded. Ongoing mapping indicates that this terrace segment is typical of other surviving terrace segments of the Prairie Allogroup lying north of the study area along the Pearl River.

Further south, the next terrace segment of the Prairie Allogroup is 20 mi (32 km) long and 3.5 to 4.5 mi (5.6 to 7.2 km) (Fig. 1). Picayune, Mississippi lies on its northern end. This terrace segment has an undulating sur-face that dips gulfward at 1.4 ft/mi (0.3 m/km) on the average (Fig. 2). Its surface lies from 3 to 10 ft (1 to 3 m) below the level of the Hammond alloformation lying within the Louisiana side of the Pearl River valley. Al-though lower in elevation and presumably younger than the surface of the Hammond alloformation within Lou-isiana, this segment exhibits more poorly preserved landforms. They consist of badly degraded fluvial ridges and what are likely very poorly preserved relict channels. The fluvial ridges are typically 4 to 10 ft (1.2 to 3 m) high and 700 to 1,600 ft (210 to 490 m) wide. The presence of numerous gravel pits associated with them indicates that these ridges contain graveliferous sediments. The southern edge of this terrace segment includes a 1 to 2 mi (1.6 to 3.2 km) wide “step” in surface mapped by Pellegrin (1978) as his Texas Flats sequence.

Within the Mississippi side of the study area, the southernmost terrace segment consists of a 5 to 7 mi (8 to 11 km) wide strip (Fig. 1). It disappears beneath Holocene marshes along its southern edge. It northern edge consists of the wall of an entrenched valley. This terrace segment exhibits two well-defined coastal ridges within its northern half and the ridge of the Ingleside-Biloxi beach trend along its southern edge.

Deweyville Allogroup Starting with Bernard (1950), the Deweyville Allogroup has been recognized as consisting of fluvial sedi-

ments intermediate in age and stratigraphic position between sediments underlying modern floodplains and the Prairie Allogroup. The Deweyville Allogroup forms terraces lying between modern floodplains and the surface of the Prairie Allogroup found along the valley walls of coastal river systems. Downstream, these terraces dip beneath the floodplains and sediments of these rivers before they reach the Gulf of Mexico. Typically, the ter-races of the Deweyville Allogroup exhibit well-preserved relict channels that are substantially larger than those seen on the adjacent modern flood plains. Various authors including Aten (1983), Blum et al. (1995), and Mor-ton et al. (1996) have recognized that the Deweyville Allogroup consists of stratigraphic units of alloformation rank.

Gum Bayou Alloformation

The Gum Bayou alloformation is the oldest alloformation comprising the Deweyville Allogroup within the

study area (Heinrich et al., 2004). Gum Bayou, for which this alloformation is named, occupies relict meander scars of this alloformation. Within the western part of the study area, the Gum Bayou alloformation underlies a series of terrace segments found along a 10.4 mi (16.7 km) long stretch of the western wall of the Pearl River Valley, from just north of Hickory, Louisiana to just south of St. Joe, Louisiana (Fig. 1). These terraces form a single surface with a gulfward slope of about 2.2 ft/mi (0.4 m/km) that lies about 10 to 14 ft (3 to 4.3 m) below the surface of the Hammond alloformation into which it has cut (Fig. 2). They exhibit the well-preserved over-size relict meander loops which often characterize the surface of the Deweyville Allogroup.

Geologic mapping revealed that the surface of the Dismal Swamp sequence of Pellegrin (1978) consists of two surfaces. The lower surface is an entrenched valley, which joins the Pearl River valley at almost right angles and cuts across the surface of the Prairie Allogroup (Fig. 1). Because the entrenched valley surface lies close to the level of the Gum Bayou alloformation and exhibits well-developed oversize meander loops, the fill underly-ing this entrenched valley is assigned to it. The floodplain of this valley is about 3 to 5 mi (5 to 8 km) wide and extends 15 mi (24 km) to the east-northeast where it enters Bay St. Louis estuary. There, this former valley of the

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Pearl River has been partially submerged to form Bay St. Louis where it gradually turns to the southeast and out into the Gulf of Mexico. Its remaining course is hidden by thick Holocene deltaic deposits of the St. Bernard delta, which obscure the surface and subsurface stratigraphy of the continental shelf offshore of Bay St. Louis (Roberts et al., 2004).

Mitchell Hammock Alloformation The Mitchell Hammock alloformation is the youngest subdivision of the Deweyville Allogroup within the

study area. Its surface consists of multiple paired and unpaired terrace segments, which occur along both the Louisiana and Mississippi sides of the Pearl River valley (Figs. 1 and 2). On the west side of the Pearl River valley, the topographically highest of these terrace segments lie 10 to 13 ft (3 to 4 m) below the surface of the Gum Bayou alloformation and about 25 ft (7.6 m) below the surface of the Hammond alloformation. The terrace segments occur over a range of about 15 ft (4.6 m) in elevation and exhibit the oversize relict meander loops, which characterize the Deweyville Allogroup. Even terraces associated with Hobolochitto Creek, found west and north of Picayune, Mississippi possess relict oversize meander loops. The gradient of the terrace segments of Mitchell Hammock alloformation varies considerably. The southern terrace segments have a gulfward slope of about 1.6 ft/mi (0.3 m/km). Within the area of Picayune, their slope varies from 2.6 to 3.1 ft/mi (0.5 to 0.6 m/km). Further north their slope decreases to 2.3 ft/mi (0.4 m/km).

Modern Flood and Delta Plains The modern alluvial and deltaic plains of the Pearl River occupy its valley. The Pearl River delta consists of

a low-lying, flat plain, lying between two Pearl River channels, composed of cypress swamp grading gulfward into intermediate and brackish water marsh with tidal channels (Fig. 1). Near the transition from alluvial to del-taic plain, the degree of meandering of Pearl River channels and the number of separate meander belts increase. Further upstream, multiple channels of the Pearl and Bogue Chitto rivers form an anastomosing river system. The slope of the modern floodplain varies greatly within the study area from 0.1 ft/mi (0.02 m/km) for its deltaic plain to 1.5 ft/mi (0.3 m/km) in the northern part of the study area.

DISCUSSION

Detailed mapping of the lower Pearl River region revealed the virtual absence of pre-Sangamon age deposits

within the Pearl River valley. Except for a few possible high-level terrace surfaces along Pearl River tributaries in Mississippi, a complete absence of deposits assignable to the Intermediate allogroup were found. Within the study area, the Prairie Allogroup typically onlaps directly onto deeply dissected Pliocene Citronelle Formation (Cullinan, 1969; Otvos, 1997). North of the study area within the Pearl River valley, ongoing mapping found erosional remnants assignable to the Intermediate allogroup. They consist of large, deeply-eroded alluvial fans, whose base, together with the Pearl River alluvial plain to which they were graded, have been completely re-moved by erosion. The widespread destruction of pre-Sangamon sediments and surfaces north of the study area and their absence within it creates a significant obstacle to the reconstruction of the early and middle Pleistocene prehistory of the Pearl River.

Prairie Allogroup The subdivisions and relict landforms of the Prairie Allogroup mapped within the study area revealed a suc-

cession of Pearl River fluvial systems. The oldest of these fluvial systems consists of relict fluvial courses A, B, C, D, and E exhibited by the surface of the Prairie Allogroup lying west of the Pearl River valley (Fig. 1). The radiating nature of these relict courses indicate this part the Prairie Allogroup consists almost entirely a large alluvial cone built by the lateral shifting of the Pearl River at points between Talsheek and St. Tammany Corner,

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Louisiana. Further north, between Bush and Dave, Louisiana, the Bogue Chitto River concurrently built a much smaller alluvial apron out onto this late Pleistocene alluvial plain of the Pearl River.

Associated with relict course D is a large anastomosing channel system (Fig. 1). The degree of development of this channel indicates, at the least, that this area was the location of repeated crevassing by floodwaters while course D was active. The development of the anastomosing channel system suggests it might even represent a failed avulsion by the Pearl River in its attempt to establish a new channel, within the interchannel basin to the west (Makaske, 2001).

The alluvial cone appears to have been abandoned when sea level was lower than present. That relict chan-nels cut through a delta-like feature indicates that this feature is not a real delta. That some of the relict courses disappear under coastal marshes show that the alluvial cone was graded to a sea level below present, at the time it was abandoned. As the surface of this alluvial cone lies below a remnant of the Ingleside-Biloxi beach trend lying west of the Pearl River, this alluvial cone must postdate its formation. However, the beach ridges demon-strate that at some time during the construction of this alluvial cone, local relative sea level was close to modern sea level for a short period of time.

As plotted in Figure 2, the optically-stimulated luminescence (OSL) dates of Otvos (2005) are too internally and stratigraphically inconsistent to be useful in determining the age of this alluvial cone and other Pearl River Pleistocene surfaces. For example, the surface of the Hammond alloformation and correlative terrace segments of the Gum Bayou alloformation yielded internally inconsistent OSL dates that differed by over 50,000 yr. Al-though vertically separated by 10 to 14 ft (3 to 4.3 m), these alloformations also yielded statistically-identical OSL dates. The presence of relict Pearl River courses, specifically relict course A, which traverses the length of the alluvial cone, and the absence of any significant topographic breaks, disproves the hypothesis that it consists of multiple terraces welded seamlessly together as suggested by Otvos (2005). In another example, a single ter-race segment of the Mitchell Hammock alloformation, lying along the west valley wall of the Pearl River, yielded closely spaced OSL samples with dates that differ by about 74,000 yr. In addition, the oldest OSL date from this terrace segment is about 46,000 yr older than an adjacent upstream terrace segment, which lies 9 ft (3 m) in ele-vation above it and must be older than it (Fig. 2). The development of thick biomantles within the surfaces of the Pleistocene units might provide one explanation for some of the inconsistencies and contradictions between OSL dates from the Pearl River valley. As discussed in detail by Johnson (1990) and Johnson et al. (2005), relatively stable geomorphic surfaces, such as the terraces of the Prairie and Deweyville allogroups, are subject to intensive and continuous reworking by bioturbation. In sediments underlying a terrace, bioturbation creates a surficial layer of sediments, called a biomantle, in which sediments are dynamically modified by the upward transfer, of-ten to the surface, of silt, sand, and fine gravel by invertebrates and vertebrates; biomixing of sediments from above and below; and cratering of the surface by tree-throws and formation of large animal burrows. Within the biomantle, sediment that has been exposed to sunlight, and younger eolian sediments are continuously mixed downward into older terrace sediments. Depending on the depth at which a sample was taken and local variations the depth and intensity of bioturbation, the formation of a biomantle can result in apparent standard aliquot OSL dates that differ significantly in age from the time when they originally accumulated (Bateman et al., 2003; Fre-derick et al., 2005). Depending on extremely local variations in the intensity and type of bioturbation, the depth to which bioturbation might modify OSL dates obtained using the standard aliquot method is at least 5 ft (1.5 m) and possibly much deeper than that by many ft (C. D. Frederick, 2006, personal communication).

The next youngest fluvial system, relict course F, lies east of the Pearl River. Because it is deeply en-trenched into the surface of the Hammond alloformation and has different size meander loop radii than other rel-ict courses found on the Prairie surface, it is considered to be a separate and younger fluvial system. The location of the narrowly entrenched valley of this fluvial system indicates that it was likely abandoned during the initial entrenchment and abandonment of the alluvial cone that forms a large part of the study area west of the Pearl River valley.

East of the Pearl River valley, the surface of the Prairie Allogroup on which lies Picayune, Mississippi, ex-hibits the poorly preserved courses of an even younger fluvial system. Although poorly preserved, the fluvial ridges are indicative of river courses that are less sinuous than those mapped west of the Pearl River valley. Be-cause this terrace segment lies below the level of the alluvial cone on the east side of the present valley, it is in-ferred that the Pearl River had abandoned and entrenched a shallow valley into it at the time that this fluvial sys-tem was active.

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Deweyville Allogroup The Gum Bayou and Mitchell Hammock alloformations represent a younger fluvial system characterized by

well-preserved channels with widths and radii of curvature greater than the modern river system. On the east side of the Pearl River, the surface of the Gum Bayou alloformation consists of a relict Pearl River valley cutting across and entrenched into the surface of the Prairie Allogroup. This relict course departs the modern Pearl River valley just south of the Stennis Space Center and flowed east-northeast to Bay St. Louis. There, it turned south-east onto the modern continental shelf. The narrow step, the Texas Flats of Pellegrin (1978), in the surface of the Prairie Allogroup paralleling this entrenched valley appears to be evidence of the initial diversion and entrench-ment of this Pearl River course.

Later, the Pearl River reestablished its course within its modern valley as evidenced by the distributions of the terraces of the Mitchell Hammock alloformation. The numerous unpaired and paired terrace segments on both sides of the Pearl River valley modern valley record a period of relatively unstable Pleistocene floodplain development. Because the terraces of the Mitchell Hammock alloformation have steeper gradients than the ter-races of the Gum Bayou alloformation, the former either merges with or buries the latter upstream within the northern part of the study area, even although the surfaces of the former lie stratigraphically below the level of the latter. The steeper gradient of Mitchell Hammock alloformation is likely the result of fluvial rather than tec-tonic processes as the older surfaces of the Prairie Allogroup have gentler and more consistent gradients than it. The apparent lack of noticeable tectonic steepening is consistent with the study area lying south of the limit of gulfward tilting as mapped by Burnett (1982).

Modern Flood and Deltaic Plains The modern Pearl River deltaic and flood plains reflect the infilling of a valley entrenched during late Pleis-

tocene low sea level stands. During the late Holocene, the modern Pearl River delta has prograded to fill the es-tuary that was created by the flooding of its late Pleistocene valley. The anastomosing nature of its floodplain within the study area appears to reflect the continuing aggradation of a low-gradient fluvial system created by rising base levels (Makaske, 2001).

Geologic mapping found that the OSL dates of Otvos (2005) from within the Pearl River floodplain came from low-lying terraces of the Deweyville Allogroup (Fig. 2). Although partially buried in the floodplain of the Pearl River, the fine detail provided by the LIDAR DEMs clearly delineates the oversize meander loops of these terraces and, in many cases, their truncated edges. None of the OSL dates within the lower Pearl River valley published by Otvos (2005) came from what he has mapped as his “Lower Valley Terrace.” Instead, the OSL samples came from partially-buried terrace segments of the Deweyville Allogroup, which are part of his “Upper Valley Terrace.” As a result, at this time there is a lack of evidence for the presence of late Wisconsin sediments immediately underlying the surface of the Holocene meanderbelts and backswamps of the Pearl River.

CONCLUSIONS

Detailed geologic mapping of the area of the Pearl River valley revealed a complex sequence of late Pleisto-

cene fluvial systems. It appears that that almost all of the early and middle Pleistocene deposits and geomorphic surfaces have been removing by repeated entrenchment and lateral migration of the Pearl River during the late Pleistocene, making reconstruction of events during this time difficult, if not impossible. The oldest recognizable Pearl River fluvial system consists of multiple relict river courses occupying a large alluvial cone largely graded to a lower stand of sea level that postdates the Ingleside-Biloxi beach ridge system. Younger fluvial systems reflect the late Pleistocene entrenchment of the surface of the Prairie Allogroup. One relict course, course F, reflects an initial stage of downcutting into the alluvial cone. Further entrenchment of the Pearl River is evi-denced by poorly preserved fluvial ridges on lower terraces of the Prairie Allogroup. Terraces segments of the Gum Bayou and Mitchell Hammock alloformations record intermediate positions of the Pearl River floodplain. The surface of Gum Bayou alloformation provides evidence of a spectacular diversion of the Pearl River east-ward out of the Pearl River’s present valley to Bay St. Louis. The modern floodplain reflects the accumulation of

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deltaic and fluvial sediments within a valley deeply entrenched into the surfaces of the Prairie and Deweyville allogroups. The precise ages of the Pleistocene fluvial systems remains uncertain.

ACKNOWLEDGMENTS Geologic mapping, which initiated this research and forms a substantial part of the mapping compiled for

this paper, was funded by the U.S. Geological Survey’s STATEMAP program under award numbers 00HQAG0122, 01HQAG0056, and 02HQAG0035. I thank Byron Miller for reviewing a draft of this paper.

REFERENCES CITED

Aten, L. E., 1983, Indians of the upper Texas coast: Academic Press, New York, 370 p.. Autin, W. J., S. F. Burns, B. J. Miller, R. T. Saucier, and J. I. Snead, 1991, Quaternary geology of the lower Mississippi

valley, in R. B. Morrison, ed., The geology of North America, v. K-2, Quaternary nonglacial geology, Contermi-nous U.S.: Geological Society of America, Boulder, Colorado p. 547-581.

Bateman, M. D., C. D. Frederick, M. K. Jaiswal, and A. K. Singhvi, 2003, Investigations into the potential effects

of pedoturbation on luminescence dating: Quaternary Science Reviews, v. 22, p. 1,169-1,176. Bernard, H. A., 1950, Quaternary geology of southeast Texas: Ph.D. dissertation, Louisiana State University,

Baton Rouge, 165 p. Blum, M. D., R. A. Morton, and J. M. Durbin, 1995, “Deweyville” terraces and deposits of the Texas Gulf Coastal

Plain: Gulf Coast Association of Geological Societies Transactions, v. 45, p. 53-60. Burnett, A. W., 1982, Alluvial stream response to neotectonics in the lower Mississippi valley: Master’s thesis, Univer-

sity of Colorado, Boulder, 181 p. LSU CADGIS Research Laboratory, 2006, Atlas: The Louisiana statewide GIS, <http://atlas.lsu.edu/> Accessed Janu-

ary 2006. Cullinan, T. A., 1969, Contributions to the geology of Washington, and St. Tammany parishes, Louisiana: Ph.D. dis-

sertation, Tulane University, New Orleans, Louisiana, 287 p. Dubar, J. L., T. E. Ewing, E. L. Lundelius, Jr., E. G. Otvos, and C. D. Winker, 1991, Quaternary geology of the Gulf of

Mexico Plain, in R. B. Morrison, ed., Quaternary nonglacial geology, conterminous U.S.: Geological Society of America, The geology of North America, v. K-2, p. 583-610.

Frederick, C. D., M. D. Bateman, A. Carr, D. E. Peter, and M. Wilder, 2005, Anatomy of a biomantle: OSL dating

results of sandy sites in central Florida: Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 154.

Heinrich, P. V., 2005, Contrasting Pleistocene and Holocene fluvial systems of the lower Pearl River, Louisiana and

Mississippi, USA: Geological Society of America Abstracts with Programs, v. 37, no. 2, p. 41 Heinrich, P. V., and R. M. McCulloh, 2000a, Lacombe, LA, 1:24,000-scale geologic quadrangle map: Louisiana Geo-

logical Survey, Baton Rouge, scale 1:24,000. Heinrich, P. V., and R. M. McCulloh, 2000b, Hickory, LA, 1:24,000-scale geologic quadrangle map: Louisiana Geo-

logical Survey, Baton Rouge, scale 1:24,000. Heinrich, P. V., and R. M. McCulloh, 2001, Hasswood, LA, 1:24,000-scale geologic quadrangle map: Louisiana Geo-

logical Survey, Baton Rouge, scale 1:24,000.

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Heinrich, P. V., and R. M. McCulloh, 2002, Industrial, LA, 1:24,000-scale geologic quadrangle map: Louisiana Geo-logical Survey, Baton Rouge, scale 1:24,000.

Heinrich, P. V., R. P. McCulloh, and J. Snead, 2004, Gulfport 30 x 60 minute geologic quadrangle: Louisiana Geologi-

cal Survey, Baton Rouge, scale 1:100,000. Johnson, D. L., 1990, Biomantle evolution and the redistribution of earth material and artifacts: Soil Science, v. 149,

p. 84-102. Johnson, D. L., J. E. J. Domier, and D. N. Johnson, 2005, Reflections on the nature of soil and its biomantle: Annals of

the Association of American Geographers, v. 95, p. 11-31. Makaske, B., 2001, Anastomosing rivers: A review of their classification, origin, and sedimentary products: Earth-

Science Reviews, v. 53, p. 149-196. Matson, G. C., 1916, The Pliocene Citronelle Formation of the Gulf Coastal Plain: U.S. Geological Survey Profes-

sional Paper 98-L, p. 167-192. McCulloh, R. P., and P. V. Heinrich, 2000, St. Tammany, LA, 1:24,000-scale geologic quadrangle map: Louisiana

Geological Survey, Baton Rouge, scale 1:24,000. McCulloh, R. P., and P. V. Heinrich, 2001a, Mandeville, LA, 1:24,000-scale geologic quadrangle map: Louisiana Geo-

logical Survey, Baton Rouge, scale 1:24,000. McCulloh, R. P., and P. V. Heinrich, 2001b, Slidell, LA, 1:24,000-scale geologic quadrangle map: Louisiana Geologi-

cal Survey, Baton Rouge, scale 1:24,000. McCulloh, R. P., P. V. Heinrich and J. Snead, 2003, Ponchatoula 30 x 60 minute geologic quadrangle: Louisiana Geo-

logical Survey, Baton Rouge, scale 1:100,000. McCulloh, R. M., and P. V. Heinrich, 2002, Bush, LA, 1:24,000-scale geologic quadrangle map: Louisiana Geological

Survey, Baton Rouge, scale 1:24,000. Morton, R. A., M. D. Blum, and W. A. White, 1996, Valley fills of coastal plain rivers, southeastern Texas: Gulf Coast

Association of Geological Societies Transactions, v. 46, p. 321-331. Mossa, J., and W. J. Autin, 1989, Quaternary geomorphology and stratigraphy of the Florida parishes, southeastern

Louisiana: Louisiana Geological Survey Guidebook Series 5, 98 p. Otvos, E. G., 1997, Northeastern Gulf Coastal Plain revisited. Neogene and Quaternary units and events — Old and

new concepts: Field Trip Guidebook, Gulf Coast Association of Geological Societies, New Orleans, Louisiana, 143 p.

Otvos, E. G., 2005, Numerical chronology of Pleistocene coastal plain and valley development: Extensive aggradation

during glacial low sea-level stands: Quaternary International, v. 135, p. 91-113. Pellegrin, F. J., 1978, Late Pleistocene-Holocene depositional systems, Hancock County, Mississippi: Master’s thesis,

University of Mississippi, 88 p. Roberts, H. H., R. H. Fillon, B. Kohl, J. Robalin, and J. Sydow, 2004, Depositional architecture of the Lagniappe delta:

Sediment characteristics, timing of depositional events, and temporal relationship with adjacent shelf-edge deltas, in J. B. Anderson, and R. H. Fillon, eds., Late Quaternary stratigraphic evolution of the northern Gulf of Mexico Margin: Society of Economic Paleontologists and Mineralogists Special Publication 79, p. 111-141.

Saucier, R. T., and J. L. Snead, 1989, Quaternary geology of the lower Mississippi valley: Louisiana Geological Sur-

vey, Baton Rouge, scale 1:1,100,000.

277

Heinrich

Trahan, L., J. J. Bradley, L. Morris, and R. Nolde, 1990, Soil survey of St. Tammany Parish: Soil Conservation Ser-vice, U.S. Department of Agriculture, Washington, D.C., 141 p., and 72 plates.

Winker, C. D., 1991, Quaternary geology, northwestern Gulf Coast, in R. B. Morrison, ed., Quaternary nonglacial geol-

ogy, conterminous U.S.: Geological Society of America, The geology of North America, v. K-2, plate 8, scale 1:2,000,000.

278

Erratum to ‘‘Pleistocene and Holocene Fluvial Systems of the Lower Pearl River, Mississippi and Louisiana, USA‘‘

In the legend of figure 1 on page 268 of Heinrich (2006), the Ingleside-Gulfport trend is incorrectly labeled as the “Ingleside-Biloxi trend.” The corrected legend is below.

Reference Cited: Heinrich, P. V., 2006, Pleistocene and Holocene Fluvial Systems of the Lower Pearl River, Mississippi and Louisiana, USA Gulf Coast Association of Geological Societies Transactions (2006) vol. 56, pp. 267‐278.

Lake Pontchartrain

Communities and Towns 1 Bb =Big Branch LC = Lacombe Bu = Bush Ni =Nicholson Co = Colt Pi =Picayune Da = Dave Sj = St. Joe Fl = Florenville Sl = Slidell Hi =Hickory St = St. Tammany In = Industrial Stc = St. Tamnany Comer

Stn = Stennis Space Center Ta = Talisheek

- J

LEGEND

Pliocene Deweyville Allogroup landforms

Citronelle Formation

Pleistocene

4 Gum Bayou alloformation ^\ relict channels

n Mitchell Hammock f^-i^. filled relict channels alloformation - - fluvial ridge

Intermediate Allogroup

- 1 unidentified unit ie Allogroup

Hammond alloformation undifferentiated

I Bogue Chitto apron "Texas flats" Ingleside-Gulfport trend

Pleistocene or Holocene

unnamed coastal

(?I unit (?)

Holocene - meander belts , backswamp 1 1 alluvium

delta plains coastal marshes

coastal ridge fault-line scarp beach ridge

Cr crevasse complex A, B, C, fluvial systems D, E,

other symbols I towns

water

Documents

Fluvial Geomorphology of the Lower Pearl River Valley, LA and Mi