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Basin Geology - Geology Contributor: Kindinger
Pontchartrain Basin geology has been shaped by its depositional history, which has been controlled by eustatic (global) fluctuations in sea level. Basin depositional history happened in a two-phase process over two time periods, Pleistocene and Holocene. This review relies heavily on the work done by Roth (1999).
Pleistocene sediments of southeast Louisiana outcrop north of the Lake Pontchartrain Basin in the form of three east-west trending shore-parallel terraces. They comprise the High, Intermediate, and Prairie Terraces (Fig. 1). Each terrace formed as the result of glacial and interglacial processes (rise and fall of sea level), leaving the sedimentary sequences preserved as relicts farther landward than the younger developing systems. Each terrace can be distinguished based upon sediments, geomorphic features, and topographic position (Autin et al., 1991). Older terraces are exposed topographically higher than younger terraces. Approaching the coast from the north there is a decrease in elevation, and relative age, of the terraces. Near the coast the Holocene Mississippi deltaic plain overlies the Pleistocene deposits.
The High Terraces Complex are the oldest and topographically highest in the Florida Parishes in Figure 1. They consist of silts, sands, and gravel deposits and tend to have an erosional topography that have dendritic drainage patterns with relatively steep gradients (Mossa and Autin, 1986).
The Intermediate Complex is named based on its topographic position and relative age between the High Terraces and Prairie Complexes, where it outcrops in a narrow belt 16 to 24 km (10 to 15 mi) wide (Saucier, 1974). The Intermediate Complex has gently rolling hills that consist of a fining-upward sequence (coarser sediments overlain by finer sediments) overlain by laminated clays, a geosol and loess (Autin et al., 1991). The complex is believed to be about 107 m (350 ft) thick and composed of oxidized, tan and red silty and sandy clays which grade into sand and gravel at depth (Kolb et al., 1975). The terrace dips southward at about 1 to 2 m (3 to 5 ft) per mile and is continuous beneath the Prairie Complex (Saucier, 1974). Drainage trends southward with dendritic to parallel patterns and lower channel gradients than in the High Terraces (Mossa and Autin, 1986).
The Prairie Terrace is the youngest terrace in Southeast Louisiana, representing the first well defined shore-parallel terrace above the Holocene deltaic plain and the first Pleistocene formation beneath the Holocene deltaic plain (Autin et al., 1991). The Prairie Terrace has been divided into the Eunice and Oberlein Terraces (Doering, 1956) and have been referred to it as the Beaumont Terrace (Parsons, 1967).
Prairie Terrace has low relief with elevations of 0 to 38 m (0 to 125 ft) across the Florida Parishes (Autin et al., 1991). Highest surface elevations of Prairie sediments are the result of regional landward uplift associated with the Wiggin's Arch (Otvos, 1991a). Overall, the Prairie Terrace dips to the south-southwest with a slope that varies from 0.6 to 3.0 m (2 to 10 ft) per mile and averages 1.5 m (5 ft) per mile north of the structural hinge line (Saucier, 1963). North of Lake Maurepas and Lake Pontchartrain the Prairie slopes southward from an elevation of 18.3 m (60 ft) above mean sea level at the Intermediate Terrace to sea level, a slope of approximately 1.2 m (4 ft) per mile (Kolb et al., 1975). Slope on the Prairie surface is often reversed in a belt 0.8 to 1.6 km (0.5 to 1.0 mi) wide south of the Baton Rouge-Denham Springs faults (Durham and Peeples, 1956) where the Prairie surface disappears beneath the Holocene deltaic plain.
Parallel to dendritic drainage patterns cut across the Prairie surface with low channel gradients (Mossa and Autin, 1986). Sizable Prairie streams had large meanders as evidenced by preserved meander scars on the Prairie surface, particularly near the Amite River (Fisk, 1951; Otvos, 1971; Autin et al., 1991). These relict channel scars are not seen on the older terraces. Large streams currently flow in the channels developed in Pleistocene times, leaving interfleuve areas flat undissected and poorly drained (Saucier, 1963; Autin et al., 1991).
The lithology of the Prairie Terrace is difficult to generalize because sediments vary regionally and locally and have multiple depositional environments. As a result, the complexity of Pleistocene terraces is simplified by mapping the terraces as morphostratigraphic entities rather than multiple formations (Otvos, 1991a; Saucier and Snead, 1989). Although Fisk (1944, 1951) was the first to include fluvial, deltaic, and marine sediments in the Prairie Terrace, Autin et al. (1991) developed the idea of a complex, whereby Pleistocene sedimentary sequences and their geomorphology represent more than one depositional environment.
The Prairie Complex as a whole is composed of three major lithostratigraphic formations - the Biloxi, Gulfport, and Prairie Formations. For in-depth discussions of these formations the reader is referred to the following publications: Autin et al., 1991; Durham et al., 1967; Fisk, 1944, 1951; Kolb et al., 1975; Otvos, 1971, 1978, 1991a, 1991b; Saucier, 1963; and Trowbridge, 1954.
Although the Pleistocene age of the Prairie Terrace has been verified by the existence of Pleistocene invertebrate marine faunas and microfauna (Richards, 1939; Bridges, 1939; Anderson and Murray, 1953), narrowing down the age to a particular stage of the Pleistocene has produced considerable debate. The realization of a more complex Quaternary sea-level curve has perpetuated the debate.
Fisk (1944) advocated a Sangamonian interglacial age for the deposition of the Montgomery (Intermediate) Terrace and a Mid-Wisconsin age for the deposition of the Prairie Terrace. This concept was carried over in ensuing studies (Fisk and McFarlan, 1955; Saucier 1963) until Roger Saucier recognized two episodes of glacial outwash in the Mississippi Alluvial Valley that were stratigraphically younger than the Prairie Terrace (Saucier, 1968). Since then, the deposition of the Prairie Terrace has been commonly assigned to the Sangamonian interglacial which is bracketed between 135,000 and 70,000 years before present (Autin et al., 1991; Frye and Leonard, 1965; Gagliano and Thom, 1967; Hoyt et al., 1968; Kolb et al., 1975; Otvos, 1971, 1972; Otvos and Howat, 1991; Saucier, 1974, 1994; Saucier and Fleetwood, 1970; Schnable, 1966). Lines of evidence supporting a Sangamonian age include radiocarbon dating of younger sediments and uranium-thorium dating and Pleistocene fossil assemblages (Russ, 1975).
For other in-depth discussions of Prairie Terrace age the reader is referred to the following publications: Alford et al., 1983, 1985; Autin, 1985, 1986, 1991; Fisk and McFarlan, 1955; Flint, 1963; Gagliano et al., 1982; McFarlan, 1961; Otvos, 1991a, 1991b, 1993; Saucier, 1963, 1977; and Shideler, 1986.
Regardless of depositional timing - of the erosion, entrenchment, oxidation, desiccation and consolidation of the Prairie surface can be attributed to the Late Wisconsin glacial maximum and lowstand of sea level. During the Late Wisconsin glacier growth the shoreline advanced hundreds miles (Fisk and McFarlan, 1955). Several researchers estimate Late Wisconsin glaciation reached a maximum sometime between approximately 35,000 and 15,000 years ago with maximum sea level withdrawal ranging from 90 to 134 meters below present (Blackwelder et al., 1979; Broeker, 1961; Curray, 1960, 1961; Dillon and Oldale, 1978; Fisk and McFarlan. 1955; Flint, 1963; Frazier, 1974; McFarlan, 1961; Penland et al., 1991). Kolb et al., 1975, suggested that maximum glaciation's coldest temperatures and the lowest sea level occurred between 15,000 and 10,000 years before present . The generally accepted dates are that sea level began to rise around 18,000 years ago, leading to the deposition of Holocene sediments in Southeast Louisiana and the Lake Pontchartrain Basin (Morgan, 1977; Penland et al., 1991).
Holocene sediments and sedimentation processes are important because they were essential in the formation of Lake Pontchartrain and adjacent lakes within the Lake Pontchartrain Basin. As previously mentioned, eustatic fluctuations and regional tectonic forces such as subsidence, faulting, and downwarping are responsible for the pre-Holocene development of the basin as a whole. Relative sea-level change caused by delta progradation and subsidence controls Holocene lake development. Refer to the Environmental Overview/Geologic History for discussion of the Holocene and formation of Lake Pontchartrain including adjacent lakes.
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