Oligocene-Pliocene taxonomy and stratigraphy of the genus Sphenolithus in the circum North Atlantic Basin: Gulf of Mexico and ODP Leg 154.

Abstract

Sphenolithus is secondary only to the genusDiscoaster in Cenozoic nannofossil biostratigraphy, notably of primary importance for the Oligocene to Early Miocene. The research presented herein provides a framework for Oligocene to Middle Pliocene Sphenolithus, where gradations between individual taxa and their stratigraphic ranges are presented. Integration of schemes from BP, Amoco and Arco Vastar, to form a single composite Neogene calcareous nannofossil framework for the Gulf of Mexico basin, began around the turn of the century with focus on refining taxonomic concepts to solve stratigraphic discrepancies between the heritage schemes. Further study and bioevent calibration was accomplished through analyses of deep-water wells in the Gulf of Mexico along with research grounding the integrated framework to the “astronomical clock” of ODP Leg 154 back into the middle Early Oligocene at an average sample resolution of 21 kyr. Forty-two species are considered herein, in addition to the morphologically-similar Ilselithina fusa. Thirteen new species are introduced. Sphenolithus peartiae and Sphenolithus distentus are emended. Clarification of taxonomic concepts by the use of morphologic groups is supplemented with identification tables, drawings, and light photomicrographs. Iterative processes, in terms of size, are stressed.