Utilizing Abundance Changes of New and Non-Standard Calcareous Nannofossil Taxa to Increase Biostratigraphic Resolution in Expanded Continental Margin Deposits: Examples from the Middle and Lower Miocene of the Gulf of Mexico

Due to their comparatively low resolution and the paucity of species restricted to open-ocean or nearshore environments, the standard calcareous nannofossil zonation schemes generally do not adequately subdivide expanded continental margin deposits. As attention has focused on the reservoir-prone Middle to Lower Miocene transition in the deep-water Gulf of Mexico, several new or often overlooked calcareous nannofossil taxa have proved useful in subdividing Martini (1971) zones NN6 – NN3. These taxa include two forms of Cyclicargolithus labeled “C. floridanus neogammation” and “C. minutus”, a variety of Discoaster sanmiguelensis labeled “Catinaster sp. A”, two forms of Reticulofenestra (R. aff. R. daviesii and R. gartneri), and a form of Umbilicosphaera labeled Umbilicosphaera sp. A. Although these forms are often “lumped” with other species, they are relatively common when care is taken to differentiate them. Utilizing last occurrences and downhole abundance increases (bioevents) of these forms as well as of those more typically identified, 42 bioevents of 19 different taxa were identified in the study interval. These bioevents were used to differentiate four horizons in the lower portion of zone NN6, seven horizons within zone NN5, seven horizons in zone NN4, and three horizons in zone NN3. These horizons are tied to abundance increases correlated to high order Maximum Flooding Surfaces.

Although rare forms are generally not reliable biostratigraphic markers, their highest occurrences can be used to indicate the presence of specific zones. These “flags” include both well-known and new forms. The most useful forms in the study interval include a large variety of Discoaster musicus near the NN6 / NN5 boundary, Helicosphaera obliqua and Triquetrorhabdulus milowii in zone NN5, Helicosphaera euphratis in zone NN4, Reticulofenestra lockeri in the lower portion of zone NN4, and Sphenolithus tintinnabulum and a form of Helicosphaera labeled “H. neorecta” in zone NN3.