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Abstract

A biostratigraphical review of eight exploration boreholes located within the De Soto Canyon protraction area in the Gulf of Mexico yields a repeatable and predictive evolutionary and paleoecological sequence with implications to paleogeography. The Oxfordian section within these boreholes contains primitive planktic foraminifera such as Globuligerina oxfordiana. Near the end of the Kimmeridgian (or slightly above the nannofossil Calcivascularis cassidyi extinction), nannofossils are of low abundance, and dominated by Cyclagelosphaera spp. Weakly developed benthic foraminifera abundance gives rise to Reinholdella A which is coincident with a nannofossil dominance switch to Polycostella spp. Planktic foraminifera are not observed in this section

In the overlying section, the extinction of nanno-fossil genus Polycostella, the origination and dominance of Nannoconus, and minute benthic foraminifera gradually increase. Here, the suggested datum, Polycostella beckmanii extinction, is observed consistently higher than the Reinholdella A extinction in the early Tithonian. The fossil assemblage change through this section suggests a change in water masses, which has implications to major reorganization in oceanic circulation.

The Lower Cretaceous continues with multiple nannofossil originations that persist into the Valangin-ian. Here, a significant, diverse, and abundant benthic foraminifera and ostracod assemblage occurs in multiple, rapid abundance increases followed by gradual upward decreases, suggesting cyclical change in the shallower, upslope paleoenvironments. The cause of cyclical changes is unclear and may be the result of sea level change, progradation, and/or changes in ocean composition.

The Hauterivian to Aptian section varies greatly in thickness with the maximum thickness in the northern De Soto Canyon area and thinning to the south. Nannoconus continues to dominate the nannofossil assemblage through the Aptian; benthic foraminifera and ostracods disappear rapidly during the Hauterivian and remaining sparse until the Albian when there is an increase of Nezzazata spp. The significance of these fossil sequences and respective assemblages are discussed in a paleoecological and paleogeographical context, which has implications to depositional history and correlation.

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