Martin B. Lagoe, Lynette Y. Holdford, John B. Anderson, Ken Abdulah
The late Quaternary depositional history of the East Texas Shelf is being investigated with a combination of high-resolution seismic and continuously cored platform borings. Foraminiferal biofacies are an important paleoenvironmental tool in placing this depositional history within a sequence stratigraphic framework. We describe here an approach to modeling foraminiferal distributions within upper Quaternary sediments in order to maximize paleoenvironmental information from these assemblages. The approach is applied to data from three borehole sections in the Brazos offshore area (boreholes B146, B106 and BA24) and the results correlated to boreholes in the Galveston offshore area (boreholes B343 and B349). High-resolution sampling (ca. 1-3 m sample spacing) of the 100-12 m deep boreholes allows us to paleoenvironmentally characterize the entire high-resolution seismic section at compatible scales between different data sets.
Foraminiferal distributions are investigated using bulk faunal descriptors and quantitative multivariate analysis. The Quaternary relationships are calibrated using modern foraminiferal data from the East Texas Shelf. Bulk faunal descriptors include faunal abundance, species diversity, planktic foraminiferal abundance and abundance of reworked (especially Cretaceous) foraminifera. These data provide information about general paleoenvironmental trends (e.g. sediment accumulation rate, broad paleobathymetric patterns, depositional processes) which are further refined by the multivariate quantitative analysis. Benthic foraminiferal distributions are analyzed by cluster analysis, detrended reciprocal averaging and polytopic vector analysis. Cluster analysis yields information about genera faunal similarity patterns. Detrended recriprocal averaging allows us to model the paleobathymetric significance of benthic taxa in glacial vs. interglacial intervals, thus increasing the precision of paleobathymetric estimates. Polytopic vector analysis "unmixes" faunal distributions influenced by post-mortem transport, thus yielding information about depositional processes.
The paleoenvironmental history of the Brazos offshore area derived from the foraminiferal data matches well with seismic geometries recognized in the area. Paleobathymetry fluctuates from marginal marine to outer neritic and the resulting paleoenvironmental curves illustrate a complex history of eustatic effects combined with delta lobe switching.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995