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Abstract: Comprehensive, Quantitative Micropaleontological Analysis as a Tool for Paleoenvironmental Interpretation and Sequence Stratigraphy, with an Example from the Yegua Formation, Southeast Texas

Martin B. Lagoe, Thomas B. Layman

Traditional industrial approaches to biostratigraphy and paleoenvironmental analysis largely use only a small portion of the available microfossil assemblage, concentrating on various marker taxa ("tops" of index fossils and paleoenvironmental guide fossils). Sequence-stratigraphic approaches may place more emphasis on the entire assemblage, but efficient analytical strategies still need to be developed to extract maximum information from micropaleontological data. Microfossil assemblages are produced by three types of processes: (1) in-situ accumulation of taxa living at the sample site; (2) post-mortem transport of specimens into and out of the sample site ("down-slope transport"), and (3) taphonomic/diagenetic processes such as dissolution, which can alter taxon proportions. Recogn zing and evaluating the effects of these processes on the microfossil assemblage can lead to a better geological interpretation. We propose an analytical strategy to address these issues, consisting of (1) bulk faunal descriptors (faunal abundance, preservation, diversity, planktic microfossil abundance) combined with lithologic information (e.g., abundance of glauconite) to identify broad paleoenvironmental patterns; (2) biofacies definition based on cluster analysis and factor analysis of the entire microfossil data set to refine these patterns; (3) interpretation and modeling of biofacies trends using detrended reciprocal averaging, and (4) analysis of faunal mixing patterns using polytopic vector analysis. We apply this analytical strategy to foraminiferal data from the middle Eocene Yegua Formation of southeast Texas. Seven biofacies are recognized along a short, three-well, dip transect, representing paleoenvironments ranging from marginal marine delta plain to outer neritic muddy shelf. The detailed "paleoenvironmental fabric" defined by these biofacies helps to recognize genetic sequence boundaries, higher frequency cyclicity, and aspects of depositional systems and paleoenvironments not apparent from analysis of well logs and marker fossils alone.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994