ABSTRACT: Paleoenvironmental Significance of Foraminiferal Biofacies in Genetic Stratigraphic Sequences of the Yegua Formation (Middle Eocene), Southeast Texas

LAYMAN, THOMAS B., and MARTIN B. LAGOE, University of Texas at Austin, Austin, TX

A major question concerning sequence analysis involves the level of precision attainable with "standard" subsurface datasets (SP-resistivity logs, mudlogs, ditch samples). This study addresses that problem using a downdip transect through the Yegua Formation in Montgomery and Harris Counties, southeast Texas. Detailed biofacies analysis of foraminifera from 30-ft composite ditch samples provides a more refined paleoenvironmental framework within which to interpret correlations, lithologic information, and well log motifs. This yields better interpretations of depositional systems, thus improving sequence analyses as well. Quantitative analysis (cluster analysis, ordination analysis) helps identify five major biofacies in the Yegua Formation: (1) Ammobaculites hockleyensis (marginal ma ine), (2) Florilus hantkeni (inner neritic), (3) Textularia spp. (inner to middle neritic), (4) Eponides mexicanus (middle neritic), and (5) Uvigerina spp. (outer neritic). Planktic/benthic foraminiferal ratios and diversity analysis supplement the biofacies patterns and lead to the recognition of two genetic stratigraphic sequences. Two episodes of deltaic progradation are characterized by brackish-marine biofacies, low-diversity faunas, and a scarcity or absence of planktic foraminifera. Flooding events are easily recognized from shoreward shifts in middle to outer neritic biofacies and increases in diversity and numbers of planktic foraminifera. Planktic foraminifera within these flooding events place the Yegua in zone P14 (upper-middle Eocene). In addition, the lower Yegua sequence i cludes delta plain deposits that were more marine-influenced than the upper sequence. This study demonstrates that detailed micropaleontological analysis can reveal stratigraphic and paleoenvironmental complexities that are not readily apparent from a lithologic/well log analysis alone.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)