Micropaleontology as Applicable to Relative Coastal Onlap Sequences and Eustatic Cycles, Gulf of Mexico: ABSTRACT

Abstract

High-resolution biostratigraphy integrated with seismically defined relative coastal onlap sequences and eustatic cycles in the Gulf of Mexico permits precise correlation of genetic Pleistocene and Pliocene sequences on seismic records. Planktonic and benthonic foraminifers and calcareous nannofossils analyzed in ten wells in a dip-section in High Island were identified, and their occurrences and abundances were recorded on a checklist of more than 170 species in each well. The highest occurrence of each species may be its evolutionary highest occurrence (mapping horizon time line) or its highest environmental occurrence (''top''). Because benthonic foraminifers are environmentally controlled, they demonstrably follow the lithologic units downdip as the units rise in time, whereas planktonic microfossils follow time lines. Evaluations of the chronostratigraphic relationships between the first down-hole occurrences of these individual species are subsequently achieved by first dividing the stratigraphic column into a series of depositional packages resulting from cyclic sedimentation. These sediments are delimited by eustatic changes in sea level that are regional throughout the Gulf of Mexico. Eustatic changes are recognized on paleobathymetric dept curves interpreted from condensed checklists. The eustatic changes are plotted on cross sections, where their correlation lines produce accurate ''time slices'' of the strratigraphic section. Correlation lines connectingmore » primary and secondary foraminiferal and calcareous nannofossil ''tops'' are evaluated against the pattern of this stratigraphic fabric. These microfossil correlations are integrated with correlations developed by parallel studies involving seismic data. The results of this technique produce a chronostratigraphic interpretation of maximum reliability in a dip or strike sequence.« less