McLaren-Hart Environmental Management, United States

The sedimentary record of Ocean Drilling Program's Core 625B (northeast Gulf of Mexico) contains significant unconformities that represent a record of sea-level change during the Pleistocene. The core may thus serve as a standard for timing of sea-level changes of the western Gulf, the stratigraphic record of which has been distorted by salt and shale tectonics. Utilizing primarily relative abundances of the warm-water Globorotalia menardii complex and cool (temperate)-water G. inflata, we have subdivided the pre-zone W Pleistocene of Core 625B into 17 subzones, resulting in an average duration of approximately 100,000 years per unit. The subzones are recognizable not only in other cores [Eureka Core E67-135 (northeast Gulf of Mexico), DSDP Core 502B (Colombia Basin, Caribbean Sea) and V16-205 (tropical Atlantic)], but also in cuttings from the industrial well Garden Banks Block 412 Unocal No.1 (western Gulf of Mexico), Based on graphic correlation, subzonal boundaries are largely co-eval between sites, and can provide high-resolution biostratigraphic subdivision of the Pleistocene of industrial wells on an operational (day-to-day) basis. Moreover, the subzonation reveals changes in sediment accumulation rate that reflect sequence boundaries at the Zone Y/X ( approximately 0.09 Ma), W/V1 ( approximately 0.2 Ma), V2/V3 ( approximately 0.4 Ma), U/T ( approximately 0.525-0.620 Ma), T3/T4 ( approximately 0.7-0.9 Ma), T4/S1 ( approximately 1.0 Ma), S2/S3-S3/R1 ( approximately 1.2 Ma), R2/R3-R3/Q1 ( approximately 1.4-1.5 Ma), and Q2/P-P/Pliocene ( approximately 1.8-1.9 Ma) boundaries, and a regionally condensed section in Subzone R1 ( approximately 1.3 Ma). Our age assignments for ecozone-based sequence boundaries are in substantial agreement with those of previous workers. Also, the subzonation delineates anomalous paleotops that are reworked, erosionally truncated at sequence boundaries, or delta-depressed as a result of localized sediment influx.