Adams Bay Mounds

The Adams Bay Mounds site (archaeological site 16PL8) in Plaquemines Parish Louisiana appears to be positioned on the footwall of the Bastian Bay fault. The construction of the mounds appears to coincide with a historical delta lobe of the Mississippi River roughly 500 years ago.

David Frazier named this delta lobe #13 - the Mississippi River lobe of the Plaquemines - Modern Delta Complex. The modern delta lobe, which is Frazier’s delta lobe #16 and commonly called the Balize lobe, did not exist at this time. The Mississippi River lobe has also been called the Bayou Robinson lobe by Flocks et al, 2006. It is likely that the discharge of the river was then shared between this delta complex and one in Terrebonne and Lafourche Parishes over the span of a few centuries. The sharing of the discharge of the river may have been a swapping back and forth and/or times of equally-shared flow.

Woody Gagliano mapped the Bastian Bay fault by combining interpretations of aerial photography, seismic data and sediment borings. Both the Bastian Bay fault and what Woody called the Empire fault to the north appear to have a structural relationship with the distributary channels of the historical delta lobe. The channels cross the faults at a high angle. They appear to be straighter on the footwall segments of the faults with a tendency to bifurcate on the hanging wall.

It would seem that inhabitation of this area would have more favorable when the majority of the river’s flow was being delivered to the delta lobes to the west. During periods of lower river flow or full abandonment of the delta lobe these areas would have been less prone to river flooding and had greater proximity to productive salt marsh environments. Gagliano shows the position of other mound sites in the area. Those along the natural levee of Bayou Robinson appear to be taking advantage of the elevation of the levee. It may be possible that footwall uplift along the Bastian Bay fault may have offered an elevation advantage that encouraged the construction of the Adams Bay and Buras Mounds.

Gagliano interpreted significant vertical movement on the Bastian Bay and Empire faults as a primary cause of wetlands loss in this area. The surface traces of the eastern ends of these faults were confirmed in the publication INFLUENCE OF GROWTH FAULTS ON COASTAL FLUVIAL SYSTEMS: Examples from the late Miocene to the Recent Mississippi River Delta (Armstrong et al, 2014). The longer term historical movement of these faults was documented by the report FAULT RELATED CHANGES IN LOUISIANA COASTAL GEOMETRY: Governor’s Applied Coastal Research and Development Program Final Report Contract #C162522 (Dawers and Martin, 2006)

Although the apparent elevation advantage of being on the footwall of the Bastian Bay fault has allowed the mounds to remain emergent for the past centuries, they are progressively succumbing to the forces of subsidence and erosion. In theory mounds like this may have been scattered all across the St. Bernard delta lobes that once extended beyond the Chandeleur Islands. If they existed, those mounds would have long since succumbed to subsidence and erosion. The remnant deposits of the St. Bernard delta lobe are now thirty feet below sea level.

This video does a beautiful job of capturing the beauty of the marshes in this area and the pain of watching a part of human culture being consumed by nature.