ABSTRACT A high resolution geochronology is possible in Plio-Pleistocene sediments of the outer continental shelf-slope of the Gulf of Mexico (GOM) and other marine basins using isotope chronostratigraphy. This study of Outer Continental Shelf (OCS) Well G-1267 No. A-1 from South Timbalier block 198 represents the first publication of specific δ 18 O stable isotope data from a GOM exploration well which compares the surface and bottom water signals from the same site. Forty seven (47) δ 18 O isotope stages are identified in the Plio-Pleistocene section of South Timbalier 198 when integrated with the available biostratigraphy. δ 18 O stages 24-27 and 48-60 are separated by a hiatuses from 0.9 Ma to 1.41 Ma. δ 18 O stages 60-104 are missing due to a hiatus from 1.75 Ma to 3.15 Ma. δ 18 O stages 104-133 are defined in the lower section of the well. This chronostratigraphy makes it possible to derive a detailed record of sediment accumulation rate changes. Comparison of the oxygen isotope records to the resistivity and SP logs reveals a relationship between sand deposition and sea level transgressions and lowstands. Power spectral analysis of the isotope records indicates that the frequencies defined by the Milankovitch orbital parameters are a dominant component of both the benthic and planktonic δ 18 O data sets. In addition, the benthic δ 18 O record provides evidence for entrainment of isotopically negative meltwater into bottom water turbidity or density flows as well as several rapid changes in paleobathymetry.

ABSTRACT Fine-grained (<63 μm) carbonate from Plio-Pleistocene exploration wells, offshore Louisiana and Trinidad, has δ 13 CPDB signatures of about -50 to -10 ‰. In this paper we explore two hypotheses that appear to account for these negative δ 13 C anomalies. The first hypothesis is that the negative δ 13 C signatures observed in Green Canyon block, offshore Louisiana, are due to authigenic subsurface precipitation of fine-grained carbonate as a record of past migration pathways of hydrocarbons through subsurface Plio-Pleistocene sands. The second hypothesis is that the authigenic δ 13 C signatures record precipitation at or near the sediment-water interface and thus record the past locations of hydrocarbon seeps on the northcentral margin of the Gulf of Mexico. Preliminary geochemical and petrographic evidence is presented to examine these hypotheses. Within the limitations of not knowing past pore fluid chemistries, we also attempt to use the δ 18 O signatures of the <63 μ m carbonate to estimate the paleotemperatures, and perhaps paleodepths and timing, of emplacement of the negative δ 13 C signatures. The potential that this isotopically negative carbonate acts as a geochemical recorder or tracer of hydrocarbon movement makes documentation of this phenomenon important to offshore exploration efforts.

Abstract: The stable oxygen isotope record for the Cenozoic is characterized by a series of large third-order steps of +1 per mil superimposed on a long-term second-order trend. This second-order trend accounts for a δ 18 O change of nearly +4 per mil from the early Eocene into the Neogene. The second- and third-order changes in the δ 18 O signal are driven primarily by a combination of glacio-eustatic sea-level and ocean paleotemperature changes. These changes are global responses to evolving circulation and climate patterns. Timing of the δ 18 O events is in good agreement with the seismically defined changes in the coastal-onlap curve (Vail and others, 1977). Agreement in the timing of events supports a common mechanism, perhaps that glaciation is apparent throughout much of the record and certainly intensified beginning in the Neogene. Agreement is not good between the magnitudes of apparent changes in sea level using the EXXON onlap record and oceanic δ 18 O events. Consideration of the δ 18 O, ice volume, and sea-level relationships during the Pleistocene suggests that sinusoidal eustatics, i.e., the rise and fall of sea level being equal, is not a good assumption at fourth- and fifth-order sea-level events. Although interpretation of the δ 18 O record is not without its assumptions and limitations, it offers an independent geochemical check on seismically defined changes in stratal patterns.