High-Resolution Calcareous Nannofossil Biostratigraphy of High-Energy Sediments from the Ursa and Brazos-Trinity Minibasins (Gulf of Mexico) during the Latest Quaternary: Preliminary Results from IODP Expedition 308

A. Shumnyk¹, P.B. Flemings², J. Behrmann³, C. John⁴, and Expedition 308 Shipboard Scientific Party
¹ Florida State University, Tallahassee, FL
² Penn State Univesity, University Park, PA
³ Albert-Ludwigs-Universitat Freiburg, Freiburg, Germany
⁴ University of California, Santa Cruz, CA

Uppermost Pleistocene–Holocene sediments recovered from the Ursa and Brazos-Trinity Minibasins consist primarily of various types of mass transport deposits (MTDs) and show distinctive cyclical patterns in the distributions of some twenty calcareous nannofossils. The well-preserved nannofossils are rare in the high-energy deposits (generally fine sands) but are abundant in pelagic and hemipelagic sediments. Reworked assemblages are usually more abundant than those in-situ except in the hemipelagic sections. The cyclic patterns, which are more evident in the Ursa Minibasin, indicate periodic influxes of sediments associated with mass-transport events. As a result it is possible to age-date the sediments and develop a high-resolution nannofossil stratigraphy.

The oldest sediment drilled in the Brazos-Trinity Minibasin IV is dated at ~150 Ka based on the absence of Helicosphaera inversa; this section spans the Holocene-upper Pleistocene interval from marine-isotope stages (MIS) 1 to 6. The oldest deposits recovered from the Ursa Minibasin span MIS 1-4 within the Emiliana huxleyi Acme Zone (QAZ1), and are younger than ~60 Ka.

Quantitative analysis of the assemblage distributions provides the basis for a high-resolution nannofossil stratigraphy. As a result, the Emiliana huxleyi Acme Zone (QAZ1) and the Transitional Nannofossil Zone (QAZ2) with its two A and B subzones have been identified in Brazos-Trinity Minibasin IV. On the other hand, the Emiliana huxleyi Acme Zone (QAZ1) with its two A and B subzones are distinguished in the Ursa section. Five nannofossil intervals from A-1 throughout A-5 could be resolved within Subzone A at the Ursa sites. A number of stratigraphic units can now be correlated within the minibasins using this high-resolution nannofossil stratigraphy despite the prevalence of MTDs.