Patterns of Foraminiferal Abundance and Diversity: Implications for Sequence Stratigraphic Analysis

ARMENTROUT, JOHN M., Mobil Research and Development Corporation, Dallas, TX, and RONALD J. ECHOLS and THOMAS D. LEE, Mobil Exploration and Producing Services Inc., Dallas, TX

Analysis of depositional cycles and sequence stratigraphic frameworks utilizes unconformities and marine flooding surfaces to define cycle boundaries. Biostratigraphic data help in the identification of stratigraphic condensed sections, unconformities, and marine flooding surfaces. Condensed sections have high microfossil abundance and diversity, and the deepest paleobathymetry within a single depositional cycle. Unconformities are associated with missing fossil assemblages, low microfossil abundance, and rapid shifts in paleobathymetry. Transgressive surfaces are associated with high microfossil abundance and deepening paleobathymetry.

Patterns of microfossil abundance and diversity, and biofacies, combined with wireline log motifs, are also useful in identifying depositional systems tracts. The transgressive systems tract is characterized by upward-increasing microfossil abundance and diversity, deepening paleobathymetry, and upward-increasing wire-line log values of gamma ray and spontaneous potential. The regressive highstand systems tract is characterized by upward-decreasing microfossil abundance, shallowing paleobathymetry, and upward-decreasing wireline log values of gamma ray and spontaneous potential. The condensed section, associated with the maximum flooding surface, separates the transgressive and highstand systems tracts. It is characterized by very high microfossil abundance, abundant burrowing, authig nic minerals such as glauconite and siderite, and high wireline log gamma ray values. The lowstand systems tract has deeper water biofacies assemblages with high microfossil abundance associated with the sediment starved top of the basin floor fan, top of the slope fan, and top of the prograding complex. These more local condensed intervals do not correlate over wide areas.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)