FREILE, DEBORAH* AND MELANIE DEVORE. Geology, Berry College, Mt. Berry, GA 30149, Department of Biological and Environmental Sciences, Georgia College and State University, Milledgeville, GA 31061-0490. - Utility of petrographic thin section analysis of fossil plant material and associated sediments in taphonomic studies: An example from the Oligocene of Texas.
Petrographic thin sections of the late Oligocene Catahoula Formation
in Huntsville, TX show well preserved plant fragments including some
with clear indications of permineralization. The plant fragments show
primarily a reddish-brown low grade thermal maturation. Authigenic
feldspar crystals, some within the organic fragments, are clearly
visible. Other cements include chert and quartz. The highly variable
lithology of the samples indicates the potential of a multiple
provenance rock. Some grains are clearly volcanic in origin
(including clear examples of volcanic glass clasts), others are
clearly detrital (principally subrounded quartz and chert grains),
while a few other quartz grains show the characteristic undulose
extinction patterns of metamorphic quartz. The rock samples are
either finely parallel laminated, very fine to fine grained, angular
to subangular, and moderately sorted OR wavy laminated, fine to medium
grained, subangular to subrounded, and moderately to well sorted.
Accessory minerals include feldspars-exhibiting good twinning, altered
micas, zeolites and nascent stage glauconitic globules. Some
specimens show cementation by opal and clays; probably related to the
diagenesis of volcanic ashes into silica and smectite. The Catahoula
Fm. has been described as a primarily terrigenous, fluvial, deltaic,
marginal unit. The presence of glauconite is an indication of shelf
(60-550m depth) marine environments and/or a transgressive episode.
The late Oligocene was a time of transgression after a period of
lowest (-200 to -350 m relative to present) sea level. The rapid rise
of sea level at the end of the Oligocene, increased erosion and
sedimentation to the shelf, and catastrophic flooding events may all
have contributed to the unusual circumstances in which glauconite is
preserved as authigenic crystals within quartz grains and cements
within the plant fragments. This contradicts previous descriptions
suggesting that the fine laminations present in some members of the
formation represent a terrestrial, oxbow lake, fluvial environment.
Key words: Oligocene, Paleobotany, taphonomy