Interpreting water depth from benthic foram species and assemblages
Paleontologists (especially micropaleontologists specializing in foraminifera) are often asked how we can interpret water depth from benthic foram species and assemblages. In short, “Benthic foraminifera are environmentally controlled ‘facies fossils’ and their tops frequently coincide with abrupt environmental (and sometimes lithological) changes associated with facies changes.” (Lowman, 1949 pp. 1975-1983). This allows paleontologists to link certain benthic foram species to certain facies that are, in turn, linked to a depositional regime with inferred water depths based on those facies.
Furthermore, this principle allowed the construction of paleo bathymetric zonations and paleobathymetric maps, based on benthic foraminifera species that would indicate water depth at the time of deposition by indicating a facies/depositional change. One of the earliest examples is Crouch (1955); however, many others were developed including Phleger (1960), Alberts (1972), Poag (1981), and Murray (2006). Each of these had a local or semi-regional focus; however, Murray (2006) represents a global synthesis (Martin 2013). These “zones” indicating water depth become associated with facies changes (i.e. sand types, shale) and would come to be used as key paleobathymetric indicators. Overtime these zones, Paleo-Data, Inc.’s Zone 4, for example, indicates an Upper Slope (Upper bathyal) environment that indicates water depths from 600’-1500’. This zone can be associated with thick, high-quality sands, and can be targeted as a ‘sweet spot’ for excellent reservoirs.
Benthic foraminifera that indicates changing bathymetries is used to produce paleobathymetric maps (see GoM Deposystems on this page for more info) that give users a visual look of how shelf and slope positions have changed throughout time. These maps are great tools for those trying to decide the best geographical placement for a wellbore when targeting a specific depositional regime from a specific geological time period.