Neogene Benthic Foraminiferal Biofacies, Paleobathymetry, and Paleoenvironments of a Gulf of Mexico Transect

Abstract

Abstract We document Neogene benthic foraminiferal biofacies changes on a depth transect of six Gulf of Mexico industry wells (sidewall and cutting samples) that is oblique to the coast and extends from Main Pass to Green Canyon (offshore Alabama to Louisiana, USA). Calcareous nannofossil and planktonic foraminiferal biostratigraphic control provides the framework to make interwell comparisons of the benthic foraminiferal biofacies; these comparisons provide the basis for paleobathymetric and paleoenvironmental interpretations and to identify useful benthic foraminiferal biostratigraphic markers in this region. Benthic foraminiferal faunas indicate that Neogene paleodepths were slightly shallower at the eastern wells and deepened towards the western wells. Calcareous benthic foraminiferal biofacies dominated by Uvigerina spp. indicate that paleoenvironments characterized by low-oxygen conditions and/or high productivity occurred periodically during deposition along the transect. This is supported by repeated occurrences of a distinctive assemblage of agglutinated foraminifera (known as the “Agua Salada Fauna”) that is typical of dysaerobic environments. Evidence of intensified low-oxygen/high-productivity environments are recorded in lowermost Middle Miocene sediments at some locations and are present at all wells in Upper Miocene deposits. In Upper Miocene-Pliocene sediments, oxygen levels appear to have been related to paleodepth, indicating that the development of lower oxygen conditions was the result of an expansion or migration of the oxygen minimum zone. Our results are consistent with a global cause for the expansion of the oxygen minumum zone during the Late Miocene and widespread increase in export production. Our study shows that despite problems in well cuttings (cavings, inconsistent sampling) and complications in regional salt and fault tectonics, well transects can provide coherent benthic foraminiferal biofacies patterns that reveal paleobathymetric and paleoenvironmental changes in the Gulf of Mexico.