ASSEMBLAGE-BASED BIOZONATIONS: A KEY TOOL IN THE DETECTIONOF REWORKED CALCAREOUS NANNOFOSSILS

摘要

Determining the age of oil-well cuttings samples with microfossils is problematic in wells with a significant amount of reworked fossils. In addition, caving is a common problem in cuttings and a species' base cannot be used with any confidence. If an age-group plot indicates that some of the fossil markers are older than the overall assemblage would suggest, then a biostratigraphic zonation that utilizes assemblage events (e.g., downhole increases, dominance shifts, and morphologic shifts) is needed to correctly determine the age of the sediment. This is especially true if the age of the reworked fossils is only slightly older than the age of the indigenous assemblage. A well from the slope of the Gulf of Mexico sampled 1500 m of Pleistocene section with numerous reworked Pliocene specimens. Utilizing a zonation scheme with 18 horizons based on nine last occurrences and 23 assemblage changes, nine events were identified. Twelve species were interpreted to be reworked. In an example from the upper Pliocene of the Gulf of Mexico, a well was noted to contain a large number of reworked nannofossils from the lower Pliocene and upper Miocene. Many of these reworked species have their extinction points within nannofossil zone NN16 (basal upper Pliocene) or NN15 (uppermost lower Pliocene). A high-resolution, assemblage-based zonation scheme of the Pliocene was used, which utilizes 26 species to define 23 separate horizons. Five upper Pliocene events were identified using the species assemblage changes and highest occurrences. Eighteen species were interpreted as being reworked. A third example is a well from the Gulf of Mexico that contained rare but consistent occurrences of upper Eocene calcareous nannofossils in what was believed to be an Oligocene section. The zonation scheme used to subdivide the section utilizes 28 species to define 27 events within the Oligocene and uppermost upper Eocene. The species were divided into four age groups, which, along with the species' assemblage changes and highest occurrences, were used to identify 11 events within the Oligocene to uppermost Eocene. Fourteen species were determined to be reworked in the section.