Latest Quaternary Stratigraphic Framework of the Mississippi River Delta Region

Kulp, Mark,1 Howell, Paul,2 Adiau, Sandra,2 Penland, Shea,1 Kindinger, Jack,3 and Williams, S. Jeffress4

1Coastal Research Laboratory University of New Orleans, New Orleans, Louisiana 70148 2Department of Geological Sciences, University of Kentucky, Lexington, Kentucky 40503 3United States Geological Survey, Coastal and Marine Geology Program, Woods Hole, Massachusetts 02543 4United States Geological Survey, Center for Coastal and Regional Marine Studies, St. Petersburg, Florida 33701

Abstract Previous researchers separated the uppermost Quaternary stratigraphy of the Mississippi River delta region into two major lithofacies. The stratigraphically lower of these, “substratum,” primarily consists of coarse-grained sediment deposited within lowstand-incised stream valleys. Relatively finer-grained “topstratum” overlies substratum; above interfluves, topstratum directly overlies weathered late Pleistocene sediments. However, the onshore to offshore distribution and architecture of these lithofacies was not well constrained. This study integrates published and unpublished lithostratigraphic data with high-resolution seismic profiles from the continental shelf to aid in mapping the regional distribution of major substratum deposits and thickness of topstratum sediments. A transgressive sand sheet commonly marks the base of the topstratum deposits, providing a stratigraphic marker to aid in regional lithostratigraphic correlations. Radiocarbon- dated deposits and boreholes tied to oxygen isotope chronologies provide chronostratigraphic control. Excel- lent correlation between these multiple datasets has been found to exist, enabling construction of regional isopachous and structural elevation maps and cross sections detailing elements of the Late Quaternary stratigraphy.

Introduction Upper Quaternary stratigraphic relationships in the Mississippi River delta region, north-central Gulf of Mex- ico, have been a topic of investigation for at least five decades (e.g., Fisk, 1944; Kolb and van Lopik, 1958; Frazier, 1967; Suter 1986), contributing significantly toward current knowledge of sedimentation patterns on fluvio-deltaic dominated, continental margins and the associated role of glacio-eustatic fluctuations. The most recent, regional studies covering both the onshore and offshore Late Quaternary (latest Wisconsinan-Holocene) stratigraphy date from the seminal work of Fisk and colleagues (Fisk, 1944; Fisk and McFarlan, 1955; McFarlan, 1961). However, subsequent investigations have been more geographically restricted (e.g., Coleman and Roberts, 1988a; Kindinger, 1988; Stanley et al., 1996; Sydrow and Roberts, 1996) and, despite the availability of appropriate data, a full onshore-to-offshore linkage of latest Quaternary stratigraphic units has not been previously completed. This study builds upon and expands earlier findings by integrating published and unpublished stratigraphic data sets to establish a regional, onshore-offshore lithostratigraphic correlation of uppermost Quaternary sediments of the region. Several thousand kilometers of high-resolution seismic reflection profiles, more than 800 onshore and offshore boreholes, and numerous cross sections spanning 50 years of research constitute the database. The strata of primary interest in this paper have been deposited since a late Wisconsinan lowstand of approximately 120 m (394 ft) below modern sea level 18,000 yr B.P. (Fairbanks, 1989). This dominantly fine-grained interval is herein termed the “topstratum lithosome,” specifically building upon the work of Fisk (1944) who lithostratigraph- ically differentiated the uppermost Quaternary stratigraphy. Fisk’s stratigraphically youngest, fine-grained topstratum represented deposition in fluvial, deltaic, and shelf environments concomitant with and subsequent to full marine flooding of the continental shelf following the latest Wisconsinan lowstand, whereas the subjacent, coarser-grained substratum was suggestive of fluvial deposition within lowstand river valleys. More clearly defining the distribution

Gulf Coast Association of Geological Societies Transactions, Volume 52, 2002 573 Kulp et al. and character of these lithofacies is critical to future stratigraphic studies and to an evaluation of existing models that describe the regional late Quaternary sedimentation patterns.

Latest Quaternary Sedimentary Response During the late Wisconsinan sea-level fall, drainage systems of the northern Gulf basin responded by extending across the continental shelf and/or incising into older sediments (Fisk, 1944; Suter, 1986; Kindinger, 1988). Subaerial exposure at maximum lowstand created an expanded coastal plain; point-source fluvial delivery to the outer shelf and upper slope created offlapping, shelf-edge deltaic sequences between 60 m and 150 m (197 ft and 492 ft) thick (Suter, 1986; Kindinger, 1988; Sydrow and Roberts, 1996). Subaerial exposure and pedogenetic processes within stream valleys and on interfluves created mature soil profiles (Fisk, 1944; Kolb and Van Lopik, 1958; Morton and Price, 1987). Color-mottled, silt-rich clay and clay-rich silt of yellow-brown, reddish-brown, and gray hues; calcareous concre- tions; crack-fills; root traces; and Fe/Mn oxide lenses lithologically distinguish the level of subaerial weathering (Fisk, 1944; Stanley et al., 1996; Sydrow and Roberts, 1996). Historically, this weathered horizon has been referred to as the “Prairie surface” because of suggested updip correlation to a coast-parallel outcrop of the late Pleistocene Prai- rie Formation (Fisk and McFarlan, 1955). The erosional surface created above Pleistocene strata is referred to here as the late Wisconsinan unconformity (sensu Stanley et al., 1996). Toward the late Wisconsinan shelfbreak, where shorter or nonexistent exposure produced an ill-defined erosional surface, the unconformity is difficult to discern; downdip of the late Wisconsinan shelf break the surface is a poorly defined correlative conformity below shelf-edge deltaic strata (Suter, 1986; Sydrow and Roberts, 1996). This study addresses the sedimentary package deposited shoreward of the late Wisconsinan shelf-edge.

Substratum Shelf deposition during the late Wisconsinan lowstand and early sea-level rise was primarily restricted to incised valleys within braided fluvial systems (Fisk and McFarlan, 1955; Coleman et al., 1991). This depositional phase produced a lithologically distinct unit, dominated by gravel and sand-rich sediments referred to as substratum (Fisk, 1944). Grain-size analysis of multiple lithofacies above the late Wisconsinan unconformity has revealed that only substratum contains sediments with a coarsest first percentile greater than 350 µm (Kuecher, 1994). Substratum deposits are not well studied but generally exhibit a fining-upward stratigraphy (Saucier, 1994).

Topstratum Lowstand, braided fluvial systems adjusted to subsequent base-level rise by evolving toward meandering regimes (Coleman et al., 1991). Maturation of backswamp and floodplain environments enhanced aggradational deposition, likely accounting for the upward-fining grain size of the substratum interval. Organic-rich and fine-grained sediments of brackish origin above coarse-grained, channel-fill sediments typically indicate initial drowning, inception of estuarine environments, and a major depositional change within progressively flooded stream valleys (Suter et al., 1987; Sydrow and Roberts, 1996). Above interfluves, this marine incursion is marked by a marine ravinement and/or transgressive marine facies created during marine reworking of underlying sediments at the transgressing shoreline. The transgressive marine facies has been referred to as nearshore Gulf facies, bay-sound facies, transgressive sand sheet, and a coalesced strandplain deposit (Kolb and Van Lopik, 1958; Frazier, 1974; May et al., 1984; Stanley et al., 1996). From the modern birdfoot depocenter (Balize subdelta) to below New Orleans, the lithofacies consists of shell-rich, sandy and silty sediment that exists as a locally continuous “sheet,” as well as infill of local topography on the underlying unconformity. Fine-grained, shell-rich sediment below the Balize depocenter passes northward into sandier and more silt-rich sediment, the facies terminates northward as shell and mud-rich sediment at a mid-Holocene, beach/barrier sand (Pine Island Beach Trend) near New Orleans (Saucier, 1994; Stanley et al., 1996). Radiocarbon-dated shells within the facies below the Balize depocenter indicate a maximum age of 15,500 yr B.P. for the lithofacies (Morgan et al., 1963), whereas dates from the Pine Island Beach Trend range from 5,500 to 6,000 yr B.P. (Saucier, 1994). Below chenier-plain deposits of the western Louisiana coastline the facies has been dated at between 4,500 and 6,000 yr B.P. (Coleman, 1966). Progradation of Mississippi River-fed deltaic complexes has resulted in burial of the basal transgressive facies across much of the northern shelf, creating a regressive sequence of deltaic deposits above the transgressive facies and/or marine ravinement. Initial progradation of Holocene deltas from the western margin of the Mississippi alluvial valley (Maringouin/Teche delta plain) began by at least 7,000 yrs B.P. (Frazier, 1974; Boyd et al., 1989). Although

574 Latest Quaternary Stratigraphic Framework of the Mississippi River Delta Region earlier deltaic strata have been suggested to exist on the central mid shelf, burial and marine reworking of these early units complicates recognition. River avulsions resulted in successive development of the St. Bernard, LaFourche, Plaquemines, and Balize delta complexes (Frazier, 1967). Except for the shelf-margin Balize delta, these deltas have been termed shelf-phase deltas because of their thin (generally less than 50 m; 164 ft), widespread distribution related to progradation into shallow, shelf waters. Beyond the progradational limits of the Holocene deltas, limited sediment input is currently creating a condensed section above the transgressive marine facies on the continental shelf (Loutit et al., 1988).

Approach

The purpose of this study is to delineate the distribution and thickness of the topstratum sedimentary package, which represents a significant change in depositional style related to marine flooding following the late Wisconsinan lowstand. Locally this change in deposition is indicated or marked by stratigraphic discontinuities such as the late Wisconsinan unconformity, the topstratum-substratum lithologic contact, marine ravinement, and/or a transgressive marine facies. Collectively or individually, these features can be used to isolate and identify much of the sedimentary package deposited subsequent to late Wisconsinan lowstand.

Data Sources

Onshore Boreholes

Onshore-borehole data used in this study were taken from atlases published by the U.S. Army Corps of Engi- neers (USACE) (Dunbar et al., 1994; Dunbar et al., 1995). Each atlas contains 15-minute quadrangle maps (1:62,500) covering part of the delta plain; borehole locations are mapped on each quadrangle and accompanied by select stratigraphic cross sections with unit descriptions and/or interpreted depositional environments. A total of 680 boreholes were used in this study (Fig. 1).

Offshore-Platform Boreholes

In the late 1980’s, researchers at Louisiana State University compiled a stratigraphic database of offshore-platform boreholes covering much of the continental shelf and slope (Fig. 1) (Coleman and Roberts, 1988a,b). Analysis of the boreholes revealed a vertical repetition of sedimentary units that was used for shelf-wide correlation of strata deposited within the last 250,000 years. In a general sense, the lithologic boundaries reflect “condensed” (odd-numbered units) and “expanded” (even-numbered units) sedimentary intervals deposited during times of rising to highstand sea level and falling to lowstand sea level, respectively. A regional chronostratigraphic framework for the units was suggested on the basis of oxygen-isotope data for several boreholes correlated to an isotope chronology. Most important to this study are the stratigraphically youngest units 1 and 2 of the dataset, representative of deposition during late Wisconsinan sea-level fall and subsequent rise. Isotopically, unit 1 was deposited in the last 12,500 years and appears to record initial transgressive deposition and Holocene regressive deltaic deposition. Radiocarbon-dated shells from the basal transgressive facies of unit 1 indicate a maximum age of between 15,000 and 11,500 yr B.P.

-31° Pleistocene Pleistocene Uplands Fig. 3 Uplands -30° Figure 1. Base map of study area showing distribution of onshore USACE boreholes (red), offshore-platform boreholes (blue), and seismic data -29° used in this study. Lines of cross section refer to Fig. 2 figures discussed in the text. N 0 100 Fig. 5 Fig. 4 -28° km -94° -92° -90° -88°

575 Kulp et al. Seismic Surveys

Five high-resolution seismic surveys acquired in the 1980’s were used in this study to aid in characterization of the Upper Quaternary stratigraphic framework (Fig. 1). A variety of equipment was used to produce the seismic data: a 400-joule minisparker, ORE Geopulse Boomer System, and a 3.5 khz sub-bottom profiler. Penetration depth for the low-frequency data was typically greater than 50 m (164 ft). The commonly used seismic velocity of 1500 m s-1 (3921 ft s-1) for water and the Upper Quaternary strata was used to convert travel times to depths and thickness (e.g., Suter, 1986; Kindinger, 1988; Sydrow and Roberts, 1996). Reflector relationships, findings of other workers, and lithologic control from borehole data were used to establish regional correlations. Numerous other data sources such as maps, cross sections, and boreholes used in this study were taken from Fisk (1944), Fisk and McClelland (1959), Frazier (1967, 1974), Suter (1986), Kindinger (1988), Otvos (1992), Syd- row and Roberts (1996), and Stanley et al. (1996).

Stratigraphic Correlations

Eastern Shelf and Delta Plain

Figure 2 is a seismic line that images two major depositional packages east of the Balize delta along the outer continental shelf. The lowermost of these units consists of moderately high-amplitude, sigmoidal reflections with toplap terminations against an overlying truncation surface. These strata are interpreted as a basinward-dipping progradational clinoform set that forms part of a late Pleistocene progradational package, termed the Lagniappe delta (Kindinger, 1988). A continuous high-amplitude reflector with as much as 20 ms of relief truncates underlying Lagniappe clinoforms. This surface can be traced reasonably well, updip to near the landward limit of seismic coverage. On the basis of stratigraphic position, configuration, and absolute dating of strata elsewhere, this laterally extensive surface is interpreted as the late Wisconsinan unconformity. Sedimentary fill within erosional troughs grades vertically from coarse- to medium-grained, sand-rich fluvial sediments to organic-rich, laminated sandy silts with mixed-salinity fauna interpreted as estuarine fill (Sydrow and Roberts, 1996). Separation of the fluvial and estuarine facies in seismic data is difficult, although subtle truncation of wavy-parallel, high-amplitude reflectors in the fluvial intervals by parallel reflectors of the upper estuarine interval has been noted. Beyond the late Wisconsinan stream incisions, the truncation of Lagniappe clinoforms is indicated by a moderately high-amplitude reflection cou- plet that can be traced regionally. Toward the north and east, low-angle, deltaic deposits of the Holocene Balize and St. Bernard subdeltas downlap this surface. Correlation of the lithologic boundary between units 1 and 2 in the offshore boreholes to seismic profiles indicates the presence of the transgressive facies at this level in the seismic profiles (Fig. 2). Farther east on the outer shelf and northward on the mid-shelf, similarly excellent correlations exists between the stratigraphic boundary identified in platform data and the transgressive surface identified in seismic lines. These observations confirm that, along the eastern continental shelf, the lithologic boundary between units 1 and 2 from the platform data can be identified in seismic lines and used collectively to establish regional correlations.

SW NE 0 0 MP 299 MP 298

) (1.5 km NW) ( 4 km SE) Figure 2. Seismic profile and interpretation

s m

( 50 38 from the Acadiana 87-1 data set east of the

sea floor e

m modern deltaic depocenter; platform boreholes

e MP 298 and MP 299 plotted for comparison. )

v 100 75

( t

Profile confirms the excellent stratigraphic

p n

e agreement between offshore borehole data and

u d

o 150 113 s 1 km ravinement surface seismic data for the base of the topstratum

y 75 sea floor 56 a

w Holocene deltaic deposits lithosome. - 100 75

o Late Wisconsinan incision w t Late Pleistocene ve= 23x Prograded Clinoforms 150 113

576 Latest Quaternary Stratigraphic Framework of the Mississippi River Delta Region Across the eastern shelf and delta plain, USACE data indicate that either transgressive marine facies or prodeltaic deposits directly overlay the late Wisconsinan unconformity. There is no indication of late Wisconsinan incision or substratum sedimentary fill. Although some incision is likely present, its apparent absence in the available data may reflect the concentration of large drainage systems farther to the east and west. Figure 3 shows the correlation between several USACE boreholes and platform-borehole units 1 and 2. Correlation between the two data sets of the basinward dipping unconformity and overlying transgressive facies is excellent and emphasizes the utility of the two

data sets to identify the base of the topstratum lithosome.

U 5

5 2

3 .

1 -

4 4

P P

P SE

3 P

NW P

A I

B M

B B

M B M 0 V seafloor 10 Figure 3. Cross section extending offshore from

20 TOPSTRATUM/ the Balize delta. Constructed from USACE bore- UNIT 1

) 30 hole data and offshore borehole data to display

(

h 40 similarity in elevation of late Wisconsinan

e d 50 Late Wisconsinan unconformity of USACE data and base of unit 1 60 unconformity indicated in offshore-platform borehole data. 70 undifferentiated 80 deltaic deposits UPPER PLEISTOCENE/UNIT 2 transgressive facies 0 10km 90 ve= 313x

Western Shelf Data used in this study to map the topstratum lithosome, overlying the marine ravinement and late Wisconsinan unconformity on the western shelf, primarily derive from Suter (1986), but are supplemented by platform boreholes. Seismic mapping by Suter (1986) revealed the subsurface presence of five unconformity-bounded Late Quaternary depositional sequences. The shallowest, regionally significant incision was interpreted as the late Wisconsinan unconformity. Latest Pleistocene sediments below the late Wisconsinan unconformity are similar to those of the eastern shelf, as generally stiff and cohesive sediments with rusty-brown oxidized lenses and other coloration suggestive of sub-aerial exposure (Fisk and McClelland, 1959). Marine transgression across exposed strata reworked lowstand deltaic sediments in a manner analogous to the process described for the eastern shelf. The resulting fine-grained transgressive deposits are sedimentologically similar to the marine-transgressive facies of the eastern shelf, with a thickness of as much as 15 m (49 ft) locally; elsewhere late Pleistocene sediments crop out at the seafloor, with only fine-grained, hemi-pelagic sediment above the unconformity (Suter et al., 1987). From approximately a mid-shelf position toward the modern shoreline, transgression truncated lowstand fluvial and deltaic deposits, creating a regionally extensive ravinement (Suter et al., 1987) at the stratigraphic level discussed for the eastern shelf. The most substantial thickness of sediment overlying the ravinement surface exists in the vicinity of the early Holocene Missis- sippi River deltas (Suter, 1986). Post-transgressive sediments here thicken abruptly to between 10 and 20 m (33 and 66 ft), in part a reflection of Holocene transgression that reworked these early deltas to form subaqueous sand-rich shoals.

Teche Ravinement From the upper mid shelf toward the western Louisiana coastal plain, a notable divergence in elevation locally exists between the unit 1-2 offshore borehole boundary and seismic placement of the transgressive surface. Generally excellent agreement between these data sets elsewhere suggests that the discrepancy is not an artifact of time-depth conversions of the seismic data. Total difference between the seismic pick for the unconformable surface and the platform-borehole data reaches 15 m (49 ft) at the northernmost location of the offshore-platform data below the chenier plain. Comparison of maps presented by Suter (1986) with the offshore-borehole data reveals a similar discrepancy between the seismically and lithologically identified unconformable surface. Furthermore, a borehole on the midshelf from Fisk and McClelland (1959), which identifies the topstratum contact at 49 m (160 ft), closely corresponds to placement of the late Wisconsinan unconformity at 46 m (151 ft) from a nearby seismic line. We suggest that the stratigraphic boundary between unit 1 and 2 of the offshore-borehole data is not equivalent to the base of topstratum in this area and thus does not fully capture the sedimentary package deposited since full marine flooding of the shelf. Existing models for the Holocene Mississippi River delta complexes emphasize that the Maringouin-Teche systems represent the first shelf-phase deltas to prograde beyond the confines of the alluvial valley. Subsequent to

577 Kulp et al. Maringouin-Teche progradation, renewed middle Holocene transgression (~4,000 yr B.P.) generated a marine ravinement surface across the uppermost Teche sediments (Teche ravinement, Penland et al., 1991). Vibracores proximal to the offshore boreholes reveal Holocene deltaic deposits several meters below the unit 1-2 contact identified in the platform boreholes. Locally, the unit 1-2 boundary appears correlative, lithologically and in elevation, to sand-rich sedimentary units nestled within a larger package of well-defined deltaic sediments. A stronger correlation exists for the unit 2-3 boundary of the offshore platform boreholes and the ravinement surface identified in nearby seismic profiles, suggesting that across much of the inner western shelf, the unit 1-2 contact does not fully incorporate the strata deposited since marine waters inundated the late Wisconsinan unconformity. A suggested explanation is that the unit 1-2 contact in this area instead identifies the Teche ravinement along the upper western shelf. Divergence between the seismically identified unconformity and overlying transgressive facies, and the unit 1-2 contact progressively increases toward the modern shoreline, the direction in which the Teche ravinement stratigraphically rises. Furthermore, an elevation of approximately 9 m (29 ft) below sea level near the coastline for the Teche ravinement is virtually the same elevation identified for the unit 1-2 contact (9.1 m; 30 ft) at the most landward platform borehole with an equivalent latitude. Nearby USACE borehole data additionally confirm that the unit 1-2 contact actually lies within the topstratum interval in this area. In conclusion, high-resolution seismic profiles, USACE borehole data, and platform borehole data can be used to establish correlations of the topstratum lithosome across the eastern and western shelf and delta plain. Locally, deposition of the topstratum lithosome began above the late Wisconsinan unconformity or the ravinement-modified equivalent, the basal part of the lithosome marks the initial deposits of transgressive marine lithofacies. Elsewhere a marine ravinement surface truncates the uppermost fill of incised valleys and, in turn, is overlain by transgressive facies that similarly mark the base of the topstratum lithosome.

Late Wisconsinan Incised Alluvial Valley Area The USACE borehole dataset indicates that coarse-grained substratum deposits are present below the topstratum unit in the central study area. A cross section of the delta plain by Autin et al. (1991) suggests that nearshore gulf facies, above the late Wisconsinan unconformity east of the alluvial valley, extend into the alluvial valley at the substratum-topstratum contact. This suggests that the transgressive facies locally marks the substratum-topstratum contact, and that the substratum-topstratum contact of the alluvial valley can be used in conjunction with basal transgressive facies to establish regional lithostratigraphic correlations. A cross section slightly east of the late Wisconsinan axis of incision shows the lithologic correlation between the offshore and USACE borehole data on the eastern alluvial-valley margin (Fig. 4). A thin interval of fluvial sands below the nearshore gulf facies (USACE data) is included as substratum on the cross section, the fluvial sands are more lithologically and genetically similarity to substratum. On the cross section, nearshore gulf facies of the USACE data are at the same elevation of a transgressive shell hash at the base of unit 1 in the platform-borehole data. The base of the nearshore gulf facies in the USACE borehole data marks the contact between the substratum and topstratum. Although specific lithologic details for the nearshore gulf facies are not presented in the USACE atlases, outside the incised valley, the facies commonly contains a large fraction of shell material. The correlations presented in Figure 4 indicate that unit 1 is lithologically equivalent to the topstratum identified in USACE boreholes within the Mississippi River valley, providing a means for regional topstratum correlation.

N 3

3 S

3 1

- -

B B

L L

L L 0 69819 L LB-1 GI48 ST63 ST178 Transgressive sand Figure 4. Onshore-to-offshore cross section Transgressive shell-hash Late Wisconsinan incised east of the late Wisconsinan valley axis. Near- 20 TOPSTRATUM seafloor valley shore gulf facies above substratum in the 40 UNIT 1 onshore data are lithologically similar to off-

DOWNDIP 60 UNIT 2 shore transgressive facies described as shell-

EQUIVALENT )

m OF ( rich clay. Unit 2 identified in offshore borehole SUBSTRATUM UNIT 3 h 80 UPPER t UNIT 4 p SUBSTRATUM

e at this location is equivalent in elevation to d 100 UNIT 5 upper substratum deposits identified in 120 Pleistocene unconformity onshore boreholes. Subjacent units 3, 4, and 5 sand shell-rich clay UNIT 4 ? are equivalent to lower onshore substratum. 140 clay nearshore gulf facies valley-fill sequence silt 0 20 km 160 (intertonguing sand, silt, clay) ve= 454x

578 Latest Quaternary Stratigraphic Framework of the Mississippi River Delta Region Where the substratum-topstratum contact is identified in the USACE data it is indicated as slightly irregular with topography of generally less than 10-m (32-ft) relief. Fisk (1944) suggested that stratigraphic relief at this level resulted from differential scour by distributaries active during topstratum time. Other possibilities are that topstratum deposits might have buried sandbars and banks of upper substratum fluvial systems, or that sandy topstratum deposits were identified as substratum. Approximately 10 m (33 ft) of relief for the base of unit 1 in the offshore-borehole data also exists just seaward of the coastline in the same vicinity. Figure 4 highlights some additional considerations with regard to the Upper Quaternary stratigraphy. Unit 2 of the offshore-platform data was originally interpreted to represent falling and lowstand conditions (Coleman and Rob- erts, 1988a, b). As shown on the cross section, unit 2 on the eastern valley consists primarily of clay; a similar, fine- grained lithology for unit 2 is present in neighboring boreholes. However, where the cross section obliquely crosses the narrow incised late Wisconsinan valley axis, considerably more sand and silt-rich strata are present within unit 2. The interpretation presented here is that unit 2 locally contains sedimentary fill of late lowstand and early sea-level rise within the late Wisconsinan incised valley. Absence of similar strata in nearby boreholes indicates a restricted valley-fill succession, suggestive of a narrow, deeply incised, late Wisconsinan “trench” as originally envisioned by Fisk (1944) from a substantially smaller dataset. Offshore from substratum identified in the USACE boreholes, unit 4 is substantially thicker than elsewhere on the shelf and consists primarily of coarse-grained, generally graveliferous deposits (Coleman et al., 1993). Coleman and Roberts (1988a,b) suggested that unit 4 deposition took place between 59,000 and 71,000 yr B.P. and unit 5 between 71,000 and 128,000 yr B.P. Similar structural elevation of units 4 and 5 to substratum deposits suggests that a part of the onshore substratum strata may have been deposited during earlier, relative sea-level lowstands.

Discussion Extensive coverage with high-resolution seismic and lithologic borehole data of the north-central Gulf basin enable regional, onshore-offshore lithologic correlations of uppermost Quaternary strata. In this study, the late Wis- consinan unconformity, marine ravinement, transgressive marine lithofacies, and substratum-topstratum contact have been collectively used to define the base of a dominantly fine-grained “topstratum lithosome.” Deposition of the topstratum lithosome began with marine incursion following late Wisconsinan lowstand; ongoing deposition and erosional processes continue to modify this stratigraphic unit. The relationships between these stratigraphic boundaries and their identification in data of this study are summarized in Figure 5. Figure 5. Diagrammatic cross section showing A A' USACE relationships between the stratigraphic bound- Platform USACE USACE Platform borehole

borehole Seismic borehole borehole borehole Seismic aries picked from the USACE, offshore-platform-

E N

E borehole data, and high-resolution seismic pro-

TOPSTRATUM C O

L files. The erosional “trough” filled with substra- O

SUBSTRATUM H tum represents Sangamonian to late Wisconsinan E

Late Wisconsinan N

E erosion and infilling (units 5 through early 3).

unconformity Late Wisconsinan C O

T Latest Wisconsinan lowstand resulted in incision

incised valley S I

Late Pleistocene E

L across these sediments, creating an incised valley erosional "trough" P that was subsequently filled.

Structural Elevation and Isopach of the Topstratum Lithosome

A structure contour map on the base of the topstratum lithosome is shown in Figure 6. On the western and eastern continental shelf, the structural configuration closely parallels the modern bathymetric profile of each area. A prominent embayment on this surface trends northwestwardly toward the modern alluvial valley of the Mississippi River above the well-defined ancestral alluvial valley containing substratum. Erosional and/or depositional irregular- ity of the topstratum-substratum contact above the late Wisconsinan alluvial valley has been discussed; and, although local variation exists, the 50-m (164-ft) contour has been extended inland from offshore to include relevant data points below the subaerial delta plain. Coleman and Roberts (1988b) similarly projected structure contours of 40 m (131 ft) and less for the base of unit 1 inland; however, their data were limited to offshore boreholes. In contrast, onshore boreholes in this study indicate an inland elevation of approximately 50 m (164 ft).

579 Kulp et al.

-32° N cene Pleisto ds Uplan -31° Figure 6. Structure contour map for the base of the topstratum lithosome. The landward-project- cene Pleisto ing 50-m contour line in the ancient alluvial-val- nds -10 Upla -20 -30° -30 ley area of the delta plain suggests an inland -40 -10 flooding event that initiated accumulation of the -50 -60 -50 topstratum sedimentary package above previ- -20 -70 ously deposited coarser-grained substratum -29° -30 deposits. -40 -50 -60 0 100 -70 -80 -28° -90 ci = 10 m km -94° -92° -90°-90° -89°-88 -88°°

Figure 7 is an isopachous map of the topstratum lithosome, which reveals a topstratum lithosome thickness resembling the distribution of Holocene depocenters identified by Frazier (1967). Relative sea-level rise across Holocene deltaic deposits that originally extended onto the shelf has resulted in marine transgression, reworking and redistribution of these sediments. Thus, original thickness of the transgressed deltas is not preserved; had the pre- modern deltas not been submerged a thicker topstratum lithosome would likely extend farther onto the shelf, and the subaerial delta plain is where most of thick topstratum is now preserved. A relatively thick topstratum interval of slightly more than 120 m (394 ft) exists in the vicinity of the current Balize depocenter. A lower portion of the con- toured sedimentary package here includes fine-grained sediments of prodeltaic origin derived from St. Bernard deltaic deposition that was concentrated farther north. However, the majority of this substantially thick section represents deposition by the Plaquemine and Balize deltas that have been located in the region for approximately 1,000 years (Frazier, 1967).

-32° N cene Pleisto plands -31° U

cene Pleisto ds Uplan Figure 7. Isopachous map of the topstratum -30° 10 30 40 lithosome. Maximum thickness is approximately 120 m in the vicinity of the modern depocenter. 30 60 50 70 -29° 20

ci = 10 m 0 100 max = 120 m 10 -28° km -94° -92° -90-90°° -89° -88°-88°

Late Quaternary Framework Following an early Sangamon highstand, sea level began falling toward the late Wisconsinan maximum lowstand elevation. Relative highstands and subsequent sea-level drops punctuate the overall record of falling sea level. Multiple erosional and depositional events on the north-central Gulf shelf have been attributed to these sea-level fluctuations (Suter et al., 1987; Coleman and Roberts, 1988a). Onshore to offshore lithostratigraphic correlations of this study indicate that locally, onshore substratum deposits are equivalent to Sangamonian and early- to late-Wisconsinan sediments identified in offshore boreholes (Coleman and Roberts, 1988a; unit 5 through early unit 2). Substratum, 580 Latest Quaternary Stratigraphic Framework of the Mississippi River Delta Region often ascribed to valley infilling associated with the late Wisconsinan sea-level lowstand and rise, is interpreted here as partially resulting from multiple valley-cutting and infilling phases driven by latest Sangamonian and early Wis- consinan sea-level fluctuations. Previous researchers have also suggested that substratum sedimentary fill of the ancient Mississippi alluvial valley is the product of multiple incisions and infilling related to late Pleistocene glacial cycles (Durham, 1962). The late Wisconsinan sea-level fall and subsequent rise is recorded on the shelf as a narrow incised valley and valley-fill succession (Coleman and Roberts, 1988b; late unit 2); within the coarse-grained onshore substratum the latest phase of valley cutting and infilling is very poorly constrained with available data. Initial incision of the Trinity/Sabine incised valley farther west near the Texas-Louisiana border has also been attributed to the Sangamon initiation of falling sea level; subsequent valley infilling occurred during early-to mid- Wisconsinan sea-level changes (Thomas and Anderson, 1994). Sediments deposited within the Trinity/Sabine system during these valley infilling episodes are preserved as fluvial terraces within the overall valley fill. Each terrace was subsequently cut during the next drop in sea level so that a composite record of latest Pleistocene erosion and deposition is preserved within a complex cut and fill stratigraphic architecture similar to that interpreted in this study for the late Pleistocene Mississippi River system. By at least 14,000 yr B.P., deglaciation was in progress and a large volume of glacial meltwater was entering the Gulf basin through the Mississippi River (Ruddiman, 1987). Because of their lower structural elevations incised valleys would have been flooded prior to bordering sections of minimally dissected shelf. During the early segment of the sea-level rise, deltaic deposition was of limited extent on the north-central shelf and was concentrated within the late Wisconsinan incised valley. The structural elevation map presented for the base of the topstratum lithosome suggests that when sea level reached approximately -50 to -40 m (-164 to -131 ft), marine waters encroached inland of the present shoreline well into the late Wisconsinan Mississippi River alluvial valley. This flooding event is lithologically recorded as a transgressive marine facies that can be correlated between onshore and offshore borehole data. The transgressive facies at the base of unit 1 on the mid to outer shelf has been dated to between 15,000 and 11,500 yr B.P. (Coleman and Roberts, 1988b). Radiocarbon dates for areas slightly inland of the 50-m (164 ft) contour constrain the base of topstratum at approximately 10,000 yr B.P. (Fisk and McFarlan, 1955). Late Quaternary sea-level curves for the region generally agree that sea level reached -50 m (-164 ft) between approximately 14,000 and 10,000 yr B.P. (e.g., Nelson and Bray, 1970). Paleogeographic maps additionally indicate inland flooding of the alluvial valley at approximately 12,000 to 10,000 yr B.P (Saucier, 1994). Continued valley filling and sea-level rise eventually allowed the Mississippi River to migrate beyond the confines of the narrow late Wisconsinan incised valley and begin deltaic deposition beyond the valley confines (Boyd et al., 1989).

Acknowledgments Research was supported by Geological Society of America Student Research Grants 5871-96 and 6060-97 to Kulp.

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