Review of the Geologic History of the Pontchartrain Basin, Northern ...

Journal of Coastal Research

SI

54

12–22

West Palm Beach, Florida

Fall 2009

Review of the Geologic History of the Pontchartrain Basin, Northern Gulf of Mexico James Flocks{, Mark Kulp{, Jackie Smith{, and S. Jeffress Williams1 { U.S. Geological Survey 600 4th Street South, St. Petersburg, FL 33701, U.S.A. [email protected]

{ University of New Orleans Department of Earth and Environmental Sciences 2000 Lakefront, New Orleans, LA 70148, U.S.A.

1 U.S. Geological Survey Woods Hole Science Center 384 Wood Hole Road, Woods Hole, MA 02543, U.S.A.

ABSTRACT FLOCKS, J.; KULP, M.; SMITH, J., and WILLIAMS, S.J., 2009. Review of the geologic history of the Pontchartrain Basin, northern Gulf of Mexico. Journal of Coastal Research, SI(54), 12–22. West Palm Beach (Florida), ISSN 0749-0208. The Pontchartrain Basin extends over 44,000 km2 from northern Mississippi to the Gulf of Mexico and includes one of the largest and most important estuarine systems in the United States. The basin supports a variety of environments, from woodlands in the north to wetlands in the south, and a growing socioeconomic infrastructure that has led to rapid development of the southern half of the basin over the past two centuries. To properly administer this infrastructure, managers need to understand the complex geologic framework of the basin and how it will respond to continued sea-level rise, variable rates and magnitudes of land subsidence, and human alteration of the landscape. This article summarizes the body of work that describes the regional evolution and stratigraphic architecture of the Pontchartrain Basin. The northern two-thirds of the basin is underlain by a stratigraphy of undifferentiated sands and clays deposited throughout the Plio-Pleistocene by glacially influenced rivers. These deposits were weathered and incised by rivers during sea-level low stands, forming a series of terraces that increase with age from south to north. The southern third of the basin is composed of estuaries formed during the Holocene, while shoreline processes created a series of sandy barriers that restricted communication to the Gulf of Mexico. The Mississippi River completed the geologic development of the basin by building a sequence of subdelta lobes along this southern margin over the past 5000 years, further sealing it from the open Gulf of Mexico. Presently, the modern Mississippi River bypasses the estuarine environment and only contributes sediments during flood events when the river overtops the levee system. Sea-level rise, subsidence within the Holocene delta-plain deposits, and movement along numerous fault systems are the active natural processes that continue to affect basin geomorphology. Louisiana, Mississippi, coastal plain, Mississippi River delta plain, Lake Pontchartrain, New Orleans, Pleistocene terraces, Pearl River, Pine Island Barrier, St. Bernard Delta.

ADDITIONAL INDEX WORDS:

INTRODUCTION The Pontchartrain Basin is located in southeast Louisiana on the eastern edge of the modern Mississippi River delta plain (Figure 1) and supports agriculture, a vital shipping and industrial corridor, and one of the most important fisheries in the country. The basin is 350 km north to south and 200 km wide, and drains a diverse landscape of woodlands and wetlands. In the southernmost part of the basin, Lake Pontchartrain, the St. Bernard marshes, and Chandeleur Sound collectively define one of the largest and most important estuarine systems along the northern Gulf of Mexico basin. Nearly one-third of the population of Louisiana lives within the 16 Louisiana parishes of the Pontchartrain Basin. The largest percentage of this population resides along a corridor that extends across the southern edge of Lake Pontchartrain. The modern morphology of the Pontchartrain Basin is the result of geologic processes that began 60 Ma, but primarily have occurred since 100 ka. The northern portion of the drainage basin is underlain predominantly by Cenozoic DOI: 10.2112/SI54-013.1.

strata, with progressively younger units filling the basin toward the south. A series of south-dipping Plio-Pleistocene terraces occupies the northern basin (Figure 2), whereas the Mississippi River delta plain forms the southern boundary. Holocene deltaic progradation, regional subsidence, and barrier-island formation during sea-level transgression are the fundamental processes that have shaped the basin. In human perspective, the Mississippi delta plain and lower Pontchartrain Basin are experiencing rapid land loss due to subsidence, global sea-level rise, and human alteration of the landscape (Barras et al., 2003). Periodic tropical storms affect the coastal zone, causing landscape change, loss of infrastructure, and even life. Presently, shoreline and wetland integrity has been compromised to the point where flood protection and coastal restoration are key components in protecting the coastal zone. Coastal-management strategy recognizes multiple lines of defense that use the natural landscape to promote an environment resistant to degradation (CPRA, 2009). Thus, understanding the evolution of this landscape is key to supporting successful coastal management. During the last 70 years, numerous investigations have developed a comprehensive understanding of the basin characteristics, geologic

Geologic Evolution of the Pontchartrain Basin, Summary of Research

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Figure 2. Diagram modified from McFarlan and LeRoy (1988) shows the relation between the Pleistocene terraces and the down-dipping Quaternary deposits below the Mississippi River Delta plain. The cross section is conceptual but would extend from the upper Pontchartrain Basin south across the Mississippi River delta plain and offshore.

Figure 1. Map of the 44,000 km2 Pontchartrain Basin with watersheds outlined and labeled in blue italics. An east–west trending dashed line marks the southern edge of the Pleistocene terraces, which comprise the upper two-thirds of the basin. This dashed line marks an erosional margin of pre-Holocene strata against which Holocene deltaic and marine sediments onlap. The generally west-trending alignment of the boundary between the upland Pleistocene terraces and Holocene strata approximately coincides with the alignment of the Baton Rouge–Denhams Springs fault trend. Background map is from the NOAA coastal-relief model.

evolution, and processes driving modern geomorphic and ecosystem change (Table 1). The purpose of this paper is to present a synthesis of the geologic history and current understanding of the Pontchartrain Basin.

SETTING The coastal-plain landscape of the northern Gulf of Mexico is punctuated by the Mississippi River delta plain, which is composed of an amalgamation of shallow-water delta complexes created as the river shifted from one dominant course to another. Distribution of these complexes is controlled by river gradient, receiving-basin dimension, and stratigraphic variability within the delta plain (Aslan, Autin, and Blum, 2005; Coleman, 1981; Roberts, 1997). The Pontchartrain Basin is the first of a series of much smaller watersheds situated to the east of the modern Mississippi River trend (Figure 1). This marginal

deltaic basin is unique in that it marks the transition within several physical and biologic environments: (1) The Holocene deposits of the delta plain are adjacent to Pleistocene and older formations of the upland terrace; (2) these terraces drain into coastal wetlands and estuaries, whereas delta plain drainage is dominated by Mississippi River flow offshore; and (3) the landscape changes from swamps and wetlands of the delta plain to rolling woodlands of the terraced uplands. These transitions are reflected in the diverse socioeconomic infrastructure within the basin that shifts from population centers supporting transportation and industrial facilities in the south to agricultural-based communities in the north. The 44,000-km2 basin contains five major rivers (Amite, Tickfaw, Tangipahoa, Tchefuncte, and Pearl) that drain southward across terraces formed by Plio-Pleistocene strata (Figure 1). The major water bodies of the lower basin include Lakes Maurepas, Pontchartrain, and Borgne. The largest system is the Pearl River, which bypasses much of the lower estuaries and empties directly into Lake Borgne. The other rivers drain into Lake Maurepas and the much larger Lake Pontchartrain. Two narrow inlets (Chef Menteur and Rigolets) on the eastern shore connect Lake Pontchartrain to Lake Borgne and the Gulf of Mexico. These three estuarine water bodies (Lakes Maurepas, Pontchartrain, and Borgne) are the predominant environmental assets of the Pontchartrain Basin, covering slightly more than 8000 km2 of the watershed. South of these estuaries, the Biloxi marsh of the Mississippi River delta plain constrains communication between the basin and the Gulf of Mexico.

Geologic History The origin of the Pontchartrain Basin is tied to the formation of the Gulf of Mexico during the early Mesozoic, when regional rifting created a series of basins south of the North American craton. These basins provided accommodation space for massive amounts of clastic deposition from rivers draining the craton’s interior. The landscape of the

Journal of Coastal Research, Special Issue No. 54, 2009

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Table 1. List of selected references pertaining to the geologic history of the Pontchartrain Basin. Theme/Authors

Subject

Pontchartrain Basin Kindinger et al., 1997 Darnell, 1962 Seed et al., 2006 Saucier, 1963 LeBlanc, 1949 Otvos and Howatt, 1992

Geologic framework Ecologic history Geologic history of New Orleans area (Ch. 2) Recent geomorphic history Physiographic features, southern Louisiana Late Quaternary coastal evolution

Pleistocene geology Autin, 1996 Saucier, 1994 Otvos and Howatt, 1992

Pleistocene history of the lower Mississippi Valley Geologic history of the lower Mississippi Valley Late Pleistocene chronostratigraphy

Structure, faulting, and subsidence Combellas-Bigot and Galloway, 2002 Dokka, 2006 Kulp, 2000 Gagliano, 2005 Dixon et al., 2006 McFarlan and LeRoy, 1998 Salvador, 1991 Lopez et al., 1997

Mid-Miocene geologic framework Modern tectonic subsidence Flexural subsidence, Holocene stratigraphy Fault motion Modern subsidence rates Pleistocene terraces Origin of Gulf of Mexico basin Lake Pontchartrain fault trends and motion

Sea level change and sediment transport Combellas-Bigot and Galloway, 2002 Liu and Galloway, 1997 Xinxia and Galloway, 2002 Tornqvist et al., 2006

Mid-Miocene geologic framework Oligocene depositional environments Miocene depositional history Holocene relative sea-level curves

Barrier island systems Otvos, 1978; Otvos and Giardino, 2004 Saucier, 1963 Stapor and Stone, 2004

Holocene barrier-island trends Recent geomorphic history New Orleans barrier trend

Mississippi delta plain Fisk et al., 1954 Fisk, 1944, 1947 Coleman and Gagliano, 1964 Frazier, 1967 Penland, Suter, and Boyd, 1988 Tornqvist et al., 1996

Sedimentary environments Deltaic environments Cyclic sedimentation Holocene depositional history Modern processes Mississippi Delta chronology

Pontchartrain Basin is much younger, however, with the present morphology having developed since the Miocene through sea-level fluctuations and delta progradations. The following summary provides a history of basin evolution beginning with early Gulf Coast development.

Structural Development of the Northern Gulf of Mexico The Gulf of Mexico formed during the late Triassic when a series of grabens opened through tensional deformation corresponding to the breakup of Pangea into North American and South American–African plates. Throughout the Meso-

Figure 3. Graph on left shows eustatic sea-level curve for Neogene cycles (modified from Haq, Hardenbol, and Vail, 1988). Quaternary sea-level curve for the Gulf of Mexico (right) on the basis of radiocarbon-dated sealevel stand indicators from 23 data sources (modified from Balsillie and Donoghue, 2004). Depositional events that took place in the Pontchartrain Basin are superimposed on the sea-level curves.

zoic, deep-water shelf facies (limestones and shales) characterized the study area (Salvador, 1987). Episodic progradations of clastic fronts during the Cenozoic built southward into the basin, supplied by sediment sourced from the Laramide and Appalachian uplifts (Kulp, 2000). This southward progradation of terrigenous clastics produced thick accumulations of sediments that were accommodated by subsidence within the basin through thermal cooling and sedimentary isostatic loading, locally as much as 14 km thick (Galloway et al., 1991; Salvador, 1991). Continued subsidence, fault motion, and salt-diapirism modified these units and influenced the overall Cenozoic architecture. Clastic-sediment flux into the northern Gulf basin continued through the mid-Miocene (15–12 Ma) in response to changing sea level (Figure 3) (Liu and Galloway, 1997; Moore, 1982). These depositional events are preserved in the subsurface of southern Louisiana as thick deltaic deposits interbedded with strata of marine origin (Combellas-Bigot and Galloway, 2002). Incipient contributions from the Ohio and Tennessee basins, combined with established drainage from the Ozark Plateau and the Arkansas and Red rivers, increased sediment supply to the gulf via the ancestral Mississippi River (Saucier, 1994). Fillon and Lawless (1999) note that most of the shelf progradation along the Gulf Coast during this period occurred in the area of eastern Louisiana, where a coastal marshland extended from midbasin to the present shoreline (McFarlan and LeRoy, 1988).

Pleistocene Development Three weathered Plio-Pleistocene units form terraces that outcrop in Louisiana and Mississippi, north of Lake Pontchartrain (Figure 4). These terraces comprise the upper twothirds of the Pontchartrain Basin and are commonly



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Figure 5. Block diagram of the middle Pontchartrain Basin (modified from Kolb and Saucier, 1982; Seed et al., 2006). Early to mid-Holocene barrier-island trends of the Pontchartrain Basin are shown as by arrows. The Baton Rouge–Denham Springs (BR-DS) northwest to southeast trending fault zone is shown along the north shore of Lake Pontchartrain.

Figure 4. Geologic map of the Pontchartrain Basin, showing fault zones and distribution of the clay-rich Pleistocene terraces [boundaries from the Louisiana Geological Survey, and McFarlan and LeRoy (1988)]; Mississippi interpretations from the Mississippi Department of Environmental Quality. Fault locations from Gagliano (2005).

identified at the surface as stiff oxidized clays (Fisk, 1944; Kolb, Smith, and Silva, 1975). This generalization stops at the surface, however, because the terraces form a complex arrangement of fluvial and marine facies, deposited over numerous glacio-eustatic sea-level cycles, where flooding sequences were followed by thousands of years of aerial exposure and fluvial entrenchment (Autin et al., 1991; Kolb, Smith, and Silva, 1975; Otvos and Howatt, 1991). Because of the variability within these deposits, controversy exists over the nomenclature, timing, and stratigraphic correlation between the units (Otvos and Howatt, 1992). Glacial cycles throughout the Plio-Pleistocene were the primary mechanism by which both deposition and variability within the terrace deposits occurred, through direct fluvial aggradation and valley fill, and reworking during eustatic sea-level rise. However, structural downwarping of the sequences during deposition, followed by mid-Wisconsinan regional uplift, contributed to terrace evolution (Fisk and McFarlan, 1955; McFarlan and LeRoy, 1988). The lithology of the terrace sequences is described as stiff clays underlain by undifferentiated sands and gravels as much as 1000 m thick.

Most of this material was deposited when the Mississippi River volume of flow was higher across a relatively steeper gradient (Kolb, Smith, and Silva, 1975). The terraces, referred to as the Upland (High), Intermediate, and Prairie Terraces (Figure 4), become progressively older toward the north, away from the modern Mississippi River depocenter. The graviliferous deposits of the Upland Terrace are the oldest, believed to be a product of braided-stream systems that traversed the area during the Pliocene (Saucier, 1994). Also known as the Citronelle Formation, this apron of alluvial deposits extends from Texas to Florida (Cureau, Snowden, and Rucker, 1991). The landscape is entrenched by a dendritic drainage pattern with a high surface gradient of about 2 m km21. Because of the lack of marine fauna, the High Terrace complex is difficult to date, but it is believed that the duration of the deposit spans 1.7 mya and includes two cycles of erosion (Saucier, 1994). Autin (1996) identifies a soil layer within the Upland Complex that indicates a hot, humid forest setting that existed over timescales of 100 kya. A late Pleistocene sea-level high stand (marine-isotope Stage 5e) is believed to have led to the development of a beach trend identified from drill cores collected in the northern portion of Lake Pontchartrain. The Miltons Island barrier trend (Figure 5) is indicated to be a relict transgressive shoreline of the Pontchartrain embayment (Saucier, 1963). Although displaced vertically by faulting and/or subsidence, the trend correlates with surface ridges (Houston and Ingleside) in western Louisiana and Texas (Saucier, 1994). In contrast, Otvos and Howatt (1992) describe the ridges as interfluves carved into the overlying terrace (Prairie). Timing and duration of the Intermediate Terrace are poorly constrained and are generally inferred by its stratigraphic position between the Upland and Prairie complexes to range from 1.3 to 0.8 Ma (Saucier, 1994). The terrace occupies a narrow band across the Florida Parishes of Louisiana (Figure 4), where it was identified by Fisk (1938) as the Montgom-


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Figure 6. Seismic profile along the south shore of Lake Pontchartrain (see Figure 5 for location). Reflection profile reveals horizontal parallel reflectors representing Holocene strata underlain by more chaotic reflection patterns interpreted as a Pleistocene fluvial channel. Modified from Roth (1999).

ery Terrace and correlated by Otvos and Howatt (1992) to the Irene and Perkins Terraces elsewhere across the coastal Gulf of Mexico. They describe the unit containing both alluvial and brackish-marine deposits. By the early Pleistocene, the Mississippi alluvial valley was accommodating the increased fluvial discharge of the waning glacial period, and the area from Arkansas to Alabama is described as a sandy floodplain with meandering stream systems. In the Pontchartrain Basin area, sediments eroded from the Upland complex provided valley-fill near the coastal plain, whereas the Intermediate complex is composed of over 100 m of sand and gravel, overlain by oxidized sandy clays (Kolb, Smith, and Silva, 1975). Regional uplift through the Pleistocene preserved this thin section from erosion by stream migration (Otvos and Howatt, 1992). The youngest terrace is the Prairie complex, deposited between 120 ka and the last glacial maximum (18 ka) (Autin, 1996; Fisk and McFarlan, 1955). The complex rests alongside the modern Mississippi River deposits in the southern portion of the basin (Figure 4) and is classified as a morphostratigraphic unit that consists of fluvial, colluvial, deltaic, estuarine, and marine deposits (Autin et al., 1991; Fisk, 1944; Seed et al., 2006). Earlier studies describe the structure as an earlyWisconsinan uplift as much as 90 m thick (Cureau, Snowden, and Rucker, 1991; Fisk, 1944; Saucier, 1963). However, faults are not detected in seismic profiles that cross the Holocene contact with the terrace, indicating the terrace may be a

depositional feature (Lopez, 1991). Outcropping along the northern shore, the downwarped Prairie Terrace surface dips to the south, where it is encountered at approximately 15 m below present sea level along the south shore of Lake Pontchartrain (Figure 5). Several weathered surfaces within the terrace complex have been identified in borings beneath New Orleans, evidence that the deposits existed through numerous sea-level fluctuations (Saucier, 1994). During the most recent glacial period (marine-isotope Stage 2, 22–18 kYBP), sea level fell to approximately 120 m below present (Figure 3) (Anderson et al., 2004; Balsillie and Donoghue, 2004; Fairbanks, 1989) and the Prairie Terrace formation was exposed. Across the Gulf Coast, drainage systems incised into Pleistocene sediments, forming entrenched valleys. Some of the larger systems, such as the Mississippi (to the west of its present location), Mobile, and Pearl, likely extended to the shelf edge and contributed toward the formation of shelf-margin deltas (Darnell, 1962; Fisk, 1944; Morton and Suter, 1996). In Lake Pontchartrain, incisions created by the extension of coastal stream systems during the low stand have been documented on a seismic profile along the southern edge of the lake (Figure 6). Subsequent sea-level rise flooded the shelf, forming a ravinement surface, and the valley systems were truncated and completely buried beneath Holocene deltaic and marine sediments (Fisk, 1944; Saucier, 1963).



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Figure 7. Seismic profile and interpretation from south Lake Pontchartrain (see Figure 5 for location) reveals a succession of horizontal parallel reflectors. Correlation of these reflector geometries to sediment cores indicates a stratigraphy of open-bay deposits overlain by sandy barrier-island strata (Pine Island) and horizontally bedded lacustrine deposits. This overall upward change in strata reflects an increase in the presence of restricted environments as the estuary becomes increasingly segregated from the Gulf of Mexico (modified from Roth, 1999).

Holocene Development Deglaciation from about 18 ka to 4 ka led to a rapid rise in sea level across the low relief of the Louisiana shelf (Saucier, 1994). This period was exceptionally dynamic along the Gulf of Mexico coast, with flooding and infilling of incised drainage systems and rapid shoreline retreat of headlands. Flooding of the southern Pontchartrain Basin formed an embayment that extended west to a longitude parallel with Baton Rouge (Fisk, 1947; Saucier, 1963), closely following a fault system along the northern margin of the lakes (Figures 4 and 5). South of this fault system, Lopez (1991) suggested that incipient Lakes Maurepas, Pontchartrain, and Borgne developed in response to subsidence along the fault trend, where the Prairie Formation dropped to below present sea level. Radiocarbon dates from the earliest Holocene deposits indicate that sea level was near present elevation by approximately 7 ka (Figure 3) (Stapor and Stone, 2004). As sea level rose and the Mississippi River Delta began to prograde, shallow-marine and fluvial sediments were deposited on the oxidized Pleistocene ravinement surface (Figure 6).

Early Holocene The early Pontchartrain embayment was a shallow open bay that supported tidal-flat species (Darnell, 1962). Within these

strata, oyster shells are present beneath brackish fauna, indicating that the water body was more saline during its initial stages of formation (Seed et al., 2006). Willard et al. (2000) report that a polyhaline environment existed prior to 4.2 ka. Faulting and subsidence at regional and local scales due to sediment loading (Saucier, 1963) collectively contributed toward a reduction in elevation of the Pontchartrain embayment. While the shoreline neared its present position, abundant sand resources and littoral transport led to the formation of barrier-island systems along the northern Gulf Coast, from Mobile Bay to south of Lake Pontchartrain. The trend extends along the southern shore of Lake Pontchartrain (sometimes referred to as the New Orleans barrier trend), where it is buried by the Mississippi River Delta deposits of the St. Bernard lobe (Corbeille, 1962; DeWindt, 1976; Saucier, 1963; Treadwell, 1955). The trend is a continuation of the barrier islands still present off Mississippi (Petite Bois, Horn, Ship, and Cat islands) but extended inland to the modern-day mainland. As a result of preservation by burial (Figure 7), this western trend predates deposits found on the Mississippi barrier islands, which respond to current climatic conditions (Otvos and Giardino, 2004). These western deposits have been exposed along the southern shoreline of Lake Pontchartrain and are thickest below the area of New Orleans (10–11 m). The


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presence of Ophiomorpha burrows and a massive texture in core samples from this area indicate subtidal deposition in shoreface and spit environments (Otvos, 1978), although marine fossils and terrestrial-mineral assemblages collected from the barrier sands point to dual provenances. Several sources for the New Orleans barrier trend have been put forth: Saucier (1994) suggests a Pearl River source, whereas Otvos and Giardino (2004) extend their source eastward to Dauphin Island. Stapor and Stone (2004) point to shell assemblages in the sand deposits below New Orleans as evidence of a sand source closer than the Pearl or Mobile rivers. They suggest that a brief sea-level fall at approximately 4.1 ka produced a nearshore-shelf source, and onshore transport during the subsequent sea-level resurgence contributed to the construction of the New Orleans trend. Up to this time, delta deposits from the Mississippi River had not prograded into this area, and oyster reefs existed in open-bay conditions as the barrier systems progressed westward unimpeded. By 3.9 ka, salinities in the area had begun to decrease, heralding the arrival of the St. Bernard Delta (Otvos and Giardino, 2004; Saucier, 1994).

Mid- to Late Holocene By 4 ka the embayment was becoming progressively more isolated from the Gulf of Mexico because of the closure of its southern outlet by barrier-island systems migrating westward and Mississippi River Delta lobes prograding eastward across the basin. As open-bay conditions became more limited, salinity in the early Lake Pontchartrain shifted toward mesohaline conditions (Willard et al., 2000). As the St. Bernard Delta prograded eastward along the south side of the Pontchartrain Basin, longshore transport of Pine Island sediments began to build up against the delta deposits (Stapor and Stone, 2004), forming the base of the modern Metairie–Gentilly ridge and the Pontchartrain estuary. These deposits are buried by a stratigraphic succession of Holocene lacustrine, marsh, and fluvial sediments (Figure 7). By 2.9 ka, the modern form of Lakes Pontchartrain and Maurepas was in place (Otvos, 1978), completing the modern morphology of the Pontchartrain Basin. Throughout the Quaternary, a combination of riverine processes, subsidence, and marine transgression collectively played a fundamental role in the formation of the Pontchartrain Basin (Coleman and Gagliano, 1964; Frazier, 1967; Lopez, 1991; Otvos, 1978).

Structural Features in the Southern Pontchartrain Basin Fault Systems Several major faults in southern Louisiana have been active since at least Miocene time. Regionally, extensional fractures trend approximately northward across the basin, whereas growth faults trend eastward, aligned with subsidence due to sediment loading and salt displacement (Dokka, 2006; Fisk, 1944; Gagliano, 2005). In the southern half of the Pontchartrain Basin, faults are associated with the inactive south Louisiana tertiary basin growth-fault trend (Lopez et al., 1997). The Baton Rouge–Denham Springs (BR-DS) fault system (Figure 5) is an active fault zone extending northwest to

southeast along the north shore of Lake Pontchartrain (Fisk, 1944; Lopez, 1991; Saucier, 1963). The BR-DS system was first activated in the Miocene (Seed et al., 2006), and seismic profiles crossing Lake Pontchartrain indicate that fault movement has remained active until at least the late Pleistocene (Kindinger et al., 1997). Recent tectonic activity is evident as stratigraphic offset in seismic profiles along the northern boundary of the lake (Lopez, 1991). These larger fault systems have significant impact on the elevation of the Pleistocene surfaces and are likely a controlling factor in estuarine development in the lower basin (Lopez, Penland, and Williams, 1977). There is significant controversy regarding the contribution of growth faults and deep tertiary faults on subsidence within the basin. Throughout the delta plain, the locations of faults, once thought to be ubiquitous in the Holocene substrate, are very difficult to identify in the near-surface record. Several scenarios explain this scarcity: (1) structural flexibility within the Holocene unconsolidated material accommodated centimeter-scale movement over the past 10 kya; (2) active structural movement has ceased since the Tertiary; and (3) fault traces are near-vertical in the Holocene deposits, making them difficult to detect using downward-looking shallow geophysical techniques (e.g., single-channel seismic profiling), and deeper industry data (e.g., three-dimensional seismic cubes) are not designed to examine the top 100 m of the sediment column. Proxies for compaction, such as deriving relative sea-level rise from basal peats (,1.5 mm y21), and velocity measurements through geodetic surveys (,4.6 mm y21), sometimes produce widely different rates of subsidence (Dokka, 2006; Tornqvist et al., 2004). This is in part due to the different time scales involved. Extrapolating short-term (10 years) compaction measurements over long term (10 ka), or vice versa, is not realistic (Dokka et al., 2008; Edrington, 2008). Fault movement and compaction rates are variable and react to changes in stratigraphy and overburden (loading). There are different processes driving subsidence as well. Shallow subsidence is predominantly compaction controlled, whereas deeper subsidence includes numerous processes acting collectively, including faulting, isostatic adjustments, and thermal contraction (Kulp, 2000). But the ‘‘deeper’’ processes, among other occurrences (e.g., fluid withdrawal, differential loading, etc.), also contribute to Holocene subsidence (Dokka, 2006; Meckel, Ten Brink, and Williams, 2007; Morton, Bernier, and Barras, 2006). Within the southern Pontchartrain Basin, some fault systems (e.g., BR-DS) consistently contribute to morphologic change (Gagliano et al., 2003). Recent studies suggest that older tertiary growth-related faults have been inactive since the Oligocene but became reactivated as a result of increased sediment loading during the Pleistocene (Edrington, 2008) and may contribute to the failure of rigid man-made structures, such as the levee system around New Orleans following Hurricane Katrina (Gagliano et al., 2003).

St. Bernard Complex and the Chandeleur Islands The St. Bernard delta complex (Figure 8) of the Mississippi River became active at 3.9 ka (Otvos and Giardino, 2004), as distributaries from the Teche delta complex were abandoned and depocenters shifted eastward (Frazier, 1967; Tornqvist et



Figure 8. Delta lobes of the St. Bernard delta complex formed by the Mississippi River between 4.0 kYBP and 500 YBP. Relative timing of the delta lobes is shown by numbers in brackets and corresponds to the sequence in Frazier (1967). The most recent lobe of the delta complex is Bayou Sauvage, which developed along the southern shore of Lake Pontchartrain and buried part of the Pine Island barrier trend with delta muds (axis of island trend shown by dashed line). The positions of the modern Mississippi barrier and Chandeleur Islands are also shown. Relic barrier islands remain buried beneath wetlands or as ridges: (A) Pearl River Island (4–3.7 ka); (B) Magnolia Ridge (4 ka); (C) Point Clear Island (4.6–4.4 ka); and (D) New Orleans trend (5.0–4.8 ka). Data from Frazier (1967), Otvos (1978), and Otvos and Giardino (2004).

al., 1996). The generally east-trending progradation of the delta complex ended at approximately 2 ka when the Bayou Sauvage subdelta impinged on the southern portion of the Pontchartrain embayment (Figure 8), forming Lake Pontchartrain (Frazier, 1967). This event resulted in burial of the Pine Island barrier-trend sands in the area of New Orleans with silty muds as much as 4 m thick (Figure 7; Coleman and Gagliano, 1964; Stapor and Stone, 2004). Bayou Sauvage persisted as the main distributary for the Mississippi River until about 1 ka, when avulsion into the modern delta began (Frazier, 1964). Otvos and Giardino (2004) observe that increased human activity across the St. Bernard Delta between 1.8 and 0.3 ka followed the abandonment phase of the delta, when expanding brackish-water bodies provided favorable habitat for native food supply. The delta complex extended offshore onto the shallow shelf of the eastern Pontchartrain Basin, eventually providing the sediments that form the Chandeleur Island chain (Penland, Suter, and Boyd, 1988). These islands formed in response to erosion of the delta headland and have not migrated far from the headland origin (,1.5 km; Otvos and Giardino, 2004). The islands occupy a narrow arcuate ridge that is about 4 km wide and rises about 4 m from the Chandeleur Sound (Twichell et al., 2009). Longshore processes, acting primarily on the distributary sands within the delta-front deposits of the St. Bernard complex, gradually concentrated sand deposits along the ridge. Although the island chain consisted of more substantial subareal dunes in the past, a finite source and intense erosion by tropical storms have reduced the main lithosome thickness to 3–5 m of approximately 75% sand (Flocks et al., 2009; Twichell et al., 2009). Following abandonment of the St. Bernard lobe, subsidence became the main process affecting the landscape because the

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Figure 9. Since the Pleistocene, the Pontchartrain estuary has experienced dynamic changes: (A) Sea-level low stand at the end of the Pleistocene exposed the Prairie Terrace complex, which was incised by rivers extending to the shelf edge. (B) During the Holocene transgression, longshore-transport processes led to the development of the Pine Island barrier trend. This mid Holocene barrier-island trend extends from the Pearl River Delta area toward the west where it is located in the modern subsurface of New Orleans. (C) The Pine Island is thought to have ceased development as the St. Bernard Delta complex prograded across the Pontchartrain Basin from the west, effectively closing off the estuary from the Gulf of Mexico and forming the southern shoreline of Lake Pontchartrain. (D) Subsidence and shoreline erosion continue to shape the estuary and expand open-water areas.

modern Balize Delta is far offshore and does not contribute sediments to the Pontchartrain Basin. Although the Pontchartrain estuary was largely restricted from the Gulf of Mexico at this time, subsidence of the St. Bernard deposits offshore are increasing gulf circulation into Lake Borgne. Continued subsidence of the region is linked largely to tectonic activity and sediment loading, as well as to compaction, organic degradation, and fluid extraction (Dokka, 2006; Morton, Bernier, and Barras, 2006).

DISCUSSION Although aggradation of the North American shoreline into the gulf basin occurred throughout the Cenozoic, the shift of deposition from western Louisiana to the east began in the mid-Miocene (Xinxia and Galloway, 2002) with terrace building in the northern Pontchartrain Basin area. Thus, the geomorphologic features present in the Pontchartrain Basin are relatively young, and the most significant evolutionary events to occur in the basin are also the most recent. Pleistocene terraces in the north and Holocene delta deposits in the south are the products of fluvial processes and sea-level rise that ranged in duration from 1 Ma to 1 ka. The Pleistocene terraces comprise the majority of the basin watershed and were built over numerous glacial cycles. Terrace uplift, although not fully understood, was a major component of their formation and preservation. During uplift (and sea-level low stands), base-level extension produced


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dendritic fluvial patterns across the Prairie Terrace upland. Subsequent faulting along the southern terraces dropped terrace elevations below the Mississippi River delta plain. This subsidence is a recent event, as evidenced by the weathered clays of the terrace deposits. The clays north of Lake Pontchartrain have been oxidized and stained buff colors (Otvos and Howatt, 1992), whereas their counterparts to the south are characteristically grayish green (Saucier, 1963), indicating a shorter duration of subaerial exposure before burial by delta deposits. The progradation of the St. Bernard Delta across the southern basin less than 4 ka completely changed the terrace landscape and created the estuarine environment that supports the ecosystem and urban centers of the southern basin. At least two subdelta progradations from west to east closed the Pontchartrain embayment, created the ‘‘Lagniappe’’ extension of wetlands south of the Pearl River, and halted the westward movement of barrier-island systems along the modern shoreline (Otvos and Giardino, 2004). These wetlands today protect the major urban centers from tropical storms that track northward through the Gulf of Mexico. Barrier-island systems that developed shortly before and during this time literally support portions of Orleans Parish (New Orleans) and have protected humans from flood events since archaeological times. Presently, land-elevation change through compaction and fault-related subsidence, along with a relative sea-level rise, are the primary natural forces shaping the basin. As relative sea-level rise continues, and in the absence of major sediment flux to the basin through fluvial processes, the Pontchartrain estuaries will continue toward becoming openwater ecosystems. Modern tectonic subsidence around Lake Pontchartrain is estimated by some studies to range between 5.6 and 29 mm y21 (Dixon et al., 2006; Dokka, 2006). The coupling of subsidence with sea-level rise will force a gradual shift within the coastal basins to polyhaline-bay environments, similar to early Holocene conditions (Willard et al., 2000). Punctuated by intense storms such as Hurricane Camille in 1969 and Hurricane Katrina in 2005, wetland loss will continue within the basin, and the protective land bridge at Pass Manchac and the Rigolets will begin to fragment. FitzGerald et al. (2007) document wetland loss around Barataria Bay in southern Louisiana and note that linked processes, including subsidence, marsh-front erosion, and catastrophic inundation during storms, increase open water at the expense of wetlands. The evolution of Lake Borgne to the southeast can be viewed as a proxy for this transition, and in the future, open-water conditions may exist well into the basin.

CONCLUSION The Pontchartrain Basin encompasses roughly 44,000 km2 of rolling hills, woodlands, estuaries, fresh- and saltwater wetlands that support agricultural, coastal, urban, and industrial communities. This environmental diversity is related to the complex geologic evolution of the basin. Most of the recent evolution occurred during and since the Pleistocene. The prominent stratigraphy is a series of terraces that comprise the northern two-thirds of the basin. These terraces were formed over numerous glacial cycles from the Pliocene through the

Pleistocene and were subjected to fluvial and marine reworking, uplift, and down-warping along fault trends. The deposits dip to the south, beneath the Holocene delta plain. About 18 ka, when the position of sea level was lower and the shoreline was a considerable distance south of the present shoreline, meandering streams incised the northern terraces and carried glacially derived sediments to the shelf edge (Figure 9A). As sea level approached present levels following the last glacial low stand, longshore transport of abundant sand deposited in the coastal zones developed a barrier-island system from Mobile Bay to the Pontchartrain embayment. Since 4 ka, this barrier trend has been migrating westward, unimpeded by delta progradations that were occurring across central Louisiana (Figure 9B). Shoreline deposits associated with this coastal process are found beneath Orleans Parish and extend into Mississippi where they trend offshore along the axis of the modern barrier islands. Continued coastal migration of the barrier trend was cut off by the Mississippi River delta plain, which avulsed from the west. The Pontchartrain embayment was closed off from the south to form the Pontchartrain estuary, as St. Bernard Delta deposits formed the wetlands of the southern parishes and inundated the Pine Island barrier trend (Figure 9C). Future development of the basin is likewise dependant on continued geologic change. This change is most dramatically expressed in wetland loss and shoreline erosion as compaction and fault-related subsidence, coupled with relative sealevel rise, affects the southern portion of the basin. Increasing open-water areas, such as Lake Borgne (Figure 9D), expose major industrial and metropolitan centers to storm impact. The processes that drive this change must be understood to properly manage the coastal zone and protect the ecosystem and human population. Numerous landmark scientific studies across the basin and delta plain have contributed to unraveling the complex evolution of the region. Studies in Cenozoic depositional history, Pleistocene glacial cycles, Holocene delta progradations, and structural development piece together the geologic framework of the basin. The synthesis of these efforts addresses the sequence of events that produced the unique and important estuarine environment that is present today.

ACKNOWLEDGMENTS The summary contained in this report draws on the results of landmark delta-plain studies produced by numerous researchers associated with the Louisiana State University, the University of New Orleans, the U.S. Geological Survey, the U.S. Army Corps of Engineers, and other universities and agencies. The authors would like to thank Duncan FitzGerald, Shea Penland, and Jack Kindinger for their efforts and support in Lake Pontchartrain Basin studies, Jordan Sanford and Chandra Dreher for their contributions, and comments from Charles Holmes, Dave Twichell, and an anonymous reviewer.

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