Incised Pleistocene terraces bound the basin to the north, the Mississippi River delta plain to the south-southwest, and the Chandeleur Islands to the south-southeast. Over the last 150 years, urban growth of New Orleans and the north shore communities and associated exploitation of natural resources have severely altered the environmental quality of the basin. In 1994, the USGS began a multidisciplinary evaluation of the geology, geomorphology, coastal processes, and environmental quality of the Pontchartrain Basin for use by Federal, State and local officials in coastal management and restoration planning.

Existing geological information has been integrated with newly acquired bathymetry, high-resolution seismic and sonar profiles, vibracores, boxcores, and geochemical data to develop a geologic history and record of sediment distribution of the basin (fig. 1, fig. 2).

The Pontchartrain Basin has a complex depositional history, a result of sedimentary processes controlled by sea-level change. During the late Wisconsin lowstand, the region was entrenched by rivers. A buried incised channel of the ancestral Mississippi River, identified from seismic profiles (see fig. 3, A-A’ ), underlies the southern margin of Lake Pontchartrain. The incision is 3 to 4 km across, was cut to a depth of 40 m, and can be traced from west to east into Lake Pontchartrain (fig. 4). Sea-level rise during deglaciation truncated the filled paleochannel and the surrounding region of the Pontchartrain Basin. The ravinement surface is a sharp contact with siderite nodules and has been identified from vibracores and boring and interpreted from seismic profiles as the Pleistocene-Holocene contact (fig. 5, fig. 6, fig. 7). The late Pleistocene unit is typically described as a stiff, olive-gray to light grayish-yellow clay that is highly bioturbated. The burrows are filled with oxidized organics or sand and silt. The structure contour map (fig. 4) of the Pleistocene surface shows the contact is shallow (near the sediment surface, 2 m below sea level) in northeast Lake Pontchartrain and deeper (20 m below sea level) to the southwest. Cross section A-A’ (fig. 4) illustrates how the Pleistocene contact crops out along the northeast shore and dips to the southwest.

Due in part to structure of the Pleistocene surface and in part to deltaic deposition from the Mississippi River, Holocene sedimentation has occurred in at least five episodes. Holocene sediments in Lake Pontchartrain were differentially deposited to the north (~0.3 m thick; fig. 5 and fig. 8) and south (~5.0 m thick). Of the five episodes, two were not extensive and could not be mapped, but three units above the Pleistocene-Holocene contact have been delineated (fig. 7). As the basin flooded, the first sediment unit began to accumulate in topographic lows in the west-central part of the lake near the Mississippi River (fig. 4). The oldest Holocene unit (B₂, thickness of 2.0+ m, fig. 5) is described as soft gray clay sporadically interbedded with fine sand, horizontal laminations, isolated lenticular bedding, sand-filled burrows, and rafted organics. Unit B₂ is completely burrowed. Unit B₁ overlies unit B₂ and is composed of a soft, silty, fine sand interbedded with clay and (or) clay clastics, shell fragments, and small rafted organics (coffee grounds). Unit B₁ averages 1.0 m thick. The uppermost unit in the section, A₁ (>1.0 m thick), is a burrowed, soft dark gray to grayish-brown clay with massive bedding containing small shells and shell fragments. Unit A₁ is possibly a disturbed zone as a result of heavy commercial shell dredge activity from the 1930's to 1990. Each of these units has sharp basal contact. Units A₁ and B₁ have the largest areal distribution.

Figure 1