Interlinked barrier chain and delta lobe development, northern Gulf of Mexico

Abstract

A wealth of new data provides a well-constrained chronology of mid- to late Holocene coastal development in the Louisiana–Mississippi borderland that may also be utilized in a globally applicable sedimentation model. Barrier sand and deltaic mud sequences illustrate a process by which potential ground water and hydrocarbon reservoir rocks accumulated unusually rapidly and were preserved. Set against decelerating Holocene sea-level rise and locally variable subsidence rates, the study provides an example of the interplay between an emerging, prograding, and partially stranded barrier island chain, a sizable estuary, and several extensive delta lobes. Utilizing microfossil fauna-based depositional facies information and archaeological data, absolute dates helped to reconstruct the history of the Alabama–Louisiana barrier chain and deltas between ca. 5.7 and ca. 1.5 14C ka BP. Protected by the substantial dune cover that prevented island submersion, the regional eustastic transgression paradoxically was synchronous with significant progradational barrier and deltaic regression. The earliest barrier islands emerged ca. 4.6–4.4 14C ka BP (ca. 5.7–5.0 cal years) when sea level stood at ca. −1.0 to −1.5 m. These islands isolated Mississippi Sound from the greater Gulf of Mexico. The absence of a lagoonal-inshore sediment interval beneath the islands and the 3–15 m thick basal nearshore marine muddy–sandy unit that blankets the Pleistocene surface refutes the transgressive history of barrier initiation. The islands aggraded on a 3.0–16.5 m thick fine sandy shallow-marine regressive platform lithosome that, in turn, overlies a transgressive muddy–sandy nearshore marine lower sediment interval. Avulsion of the Mississippi River had abruptly reduced nearshore salinities by ca. 3.9–3.7 ka 14C BP. Renewed Mississippi delta growth induced rapid aggradation and progradation on the opposite Pearl River delta-mainland shore as well. Gulf influences have rapidly diminished in areas so affected. A new absolute chronology of the Mississippi–St. Bernard delta lobes constrains these events. Delta growth and mainland progradation first isolated, then severely constricted the Lake Borgne embayment. Accompanied by ongoing subsidence, the western barriers were stranded, then buried. Shoaling, related to St. Bernard delta progradation, interfered with westward littoral drift to maintain Cat Island. Archaeology provided important supplementary data for dating environmental changes. Refuting earlier suggestions that Native colonization rapidly followed delta complex formation, the earliest known Indian sites postdated the start of the associated St. Bernard delta lobe by 1.9–3.1 14C ka.