ABSTRACT

The mostly buried late Holocene barrier ridges of Orleans-Jefferson Parishes belong to two distinct periods of formation. The earlier ridges formed between about 5000-4600 yrs. B.P.; the younger ridges, that extend from southwestern Hancock County, Mississippi, probably formed between 3600-3000 yrs. B.P. Most of the sand was part of sand shoals and bars, that occasionally accreted above sea level. Heavy mineral and foraminiferal data indicate the closeness of subaerial delta lobes during accumulation of the ridges.

Although a brackish water embayment did exist at least since about 4000-3800 yrs. B.P. north of the Orleans Parish subaerial delta belt, Lake Pontchartrain did not originate until about 2900-2800 yrs. B.P. due to further growth of the St. Bernard delta complex. Since its origination, the generally oligohaline lake received natural and artificial influxes of saltier water that significantly affected its biota.

While heavy mineral spectra sensitively reflected Eastern Gulf and Mississippi River influences in the subject area, no significant difference has been found in the range semiquantitative clay mineral spectra between deposits of the Mississippi River, Mississippi Sound and clay beds within and above the Holocene barrier sands. Montmorillonitic clays predominated in all samples.