Incised Pleistocene terraces bound the basin to 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 three to four 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, 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.

Sea-level rise flooded the "Pontchartrain Embayment" between 6,000 to 4,000 BP (Fig. 9) and deposited transgressive nearshore or lagoonal/estuarine sediments across the area over the ravinement surface. A stillstand at ~4,000 BP initiated formation of a barrier shoreline. Sediments eroding from Pleistocene terraces and Pearl River delta shores to the east of the Pontchartrain Embayment, combined with sandy material moving westward along the Alabama-Mississippi shore, built the large recurved spit and barrier (Pine Island Barrier Trend; Fig 10) that bounds the southeast part of the basin. Back-barrier deposits and shell reefs partially filled the open estuary. The next depositional event (at ~3,000 BP) was the eastward progradation of the St. Bernard delta complex and other subdeltas from the Mississippi River valley. Deltaic sediments enclosed the Pontchartrain Basin to the south and eventually covered the Pine Island Barrier, forming Lake Pontchartrain. At that time, the basin began to accumulate prodelta, delta front, and crevasse deposits. Cypress swamps and fresh water marshes formed in the upper basin and intermediate to saline marshes formed in the lower basin. From ~3,000 BP to the present, active growth faults have also influenced basin geometry and geomorphology, particularly along the north shore.