ABSTRACT

Avery Island, one of the Five Islands salt domes of south-central Louisiana, is a piercement-type dome that has been uplifted from about 17 km depth. The dome is nearly circular in plan with a maximum elevation approximately 50 m above the surrounding coastal marsh. Dissection has produced a terrain of gullies and steep slopes.

Along N-S and E-W transects, borings of 3.9 to 8.4 m depth reveal loess and silty colluvium less than 3.4 m thick overlying a buried soil developed in late Pleistocene sediments. The late Pleistocene sediments are correlative with those found beneath the Prairie Terraces only a few km to the north. The loess represents a single genetic unit, is thickest on side slopes, and is thin or absent on ridge crests. Around the perimeter of the island, silty colluvium occurs at the base of slopes. This suggests that loess has been stripped and reworked, predominantly from higher elevations. At a pit excavated for fill material, a 0.6 m thick loessial mixing zone overlies 12 + m of sand that shows spotty induration by silica cement. Shear fractures with high-angle average dips occur in the sandy late Pleistocene sediments at this site, and in both loess and late Pleistocene sediments in various core samples. The fractures in the pit have modal strikes of N77E and N53W, and are believed to have developed from extensional stress associated with vertical uplift of the underlying salt.

This study suggests a complex Quaternary geological history for Avery Island. Deposition of late Pleistocene sediments in a low-relief alluvial plain and initiation of soil development occurred prior to the latest emergence of the dome. The stratigraphy of loess and colluvial silt indicates that the island was emergent during loess deposition. The degree of dissection, distribution of colluvium, and shearing of Quaternary sediments reflect continued uplift after loess deposition.