The goal of this study is to provide age and paleoenvironmental determinations of the Catahoula Formation in eastern Louisiana. The age of the formation. has been debated since it was established because marine micro- and macrofossils are absent. The Catahoula Fm. and its lithologic equivalents have been traditionally “dated” by their stratigraphic position. Typically, one of two interpretations is made. The Catahoula Fm. interfingers with the Vicksburg Group and is therefore Oligocene in age. Or, the Catahoula Fm. unconformably overlies the Vicksburg Group and is Miocene in age.

However, terrestrial fossils (e.g., palm wood, leaves, freshwater mollusks, and lignite) do occur in the formation. Palynologic study of lignitic shale from outcrops along Big Creek on Sicily Island, Louisiana yielded rich palynomorph assemblages. Pteridophyte spores, pollen, and fungal debris dominate, with rare specimens of freshwater algae, dinoflagellate cysts (dinocysts), and reworked pollen and dinocysts.

The in situ pollen includes ambrosid pollen and that of Helianthus sp., both of which belong to the family Asteraceae. These age-significant species indicate equivalence with the Paly 24 marker (8.68 Ma, early late Miocene, or Tortonian in age) of Shell Offshore Inc. Paly 24 equates to the calcareous nannofossil zones of NN 10 (Martini, 1971) and CN 8B (Okada and Burky, 1980) and to the planktonic foraminifera zone N 16 (Blow, 1969, 1979). Our results permit, for the first time, the direct correlation of the Catahoula Fm. outcrops in eastern Louisiana with the worldwide planktonic foraminifera and nannofossil zonations, and the Gulf Coast biostratigraphic zonation.

A tropical setting is indicated by abundant spores of the freshwater floating ferns Azolla sp. and Magnistriatites howardi , those of Cicatricosisporites spp., and Lygodiumsporites spp., and by the presence of mangrove (e.g., Rhizophora sp.) and palm pollen (e.g., Arcepites sp.). The freshwater floating ferns indicate a quiet, nonmarine depositional setting, such as a bayou or fluvial backwater. The presence of rare Cenozoic lagoonal and estuarine dinocysts (e.g., Homotryblium plectilum , Spiniferites spp.) and mangrove pollen suggest brackish conditions. Thus, deposition of the lignitic shale may have occurred in the upper reaches of an estuary.