Overburden-driven Late Holocene Compaction in the Mississippi Delta

Compaction of unconsolidated Holocene sediments is a major mechanism driving subsidence in large deltas. Previous studies have suggested that compaction-driven subsidence rates in the Mississippi Delta can be as high as 1 cm/yr. A correlation between geodetically derived subsidence rates (mostly from GPS measurements) and the thickness of Holocene sediments in the Mississippi Delta has been used to argue for a causal relationship between the two. So far, no geological data have been used to test this hypothesis.

Here we present rates of natural compaction of Holocene sediments beneath the transition from mouth-bar sand to overbank mud at 1 to 8 m below the surface along the downstream portion of Bayou Lafourche, an abandoned Mississippi River course. We find that these rates, averaged over 600 to 1500 years, are generally <2 mm/yr, smaller than compaction rates (1 to 5 mm/yr) reported at localities farther upstream along Bayou Lafourche. The rates of natural compaction decrease seaward and are positively correlated with the thickness of the overbank mud, but not with the thickness of the underlying Holocene strata. This suggests that the weight of the overburden exerts a first-order control on the measured compaction.

We infer that present-day rates of natural compaction along Bayou Lafourche may be even lower than the mean late Holocene rates because overbank sedimentation ceased when Bayou Lafourche was largely abandoned ~600 years ago. The much higher compaction rates along Bayou Lafourche as inferred from geodetic measurements may be a manifestation of infrastructure overburden, if not sourced from the compaction of the shallowest Holocene strata that is not captured in our data.