AmericanChemicalSociety.com

Objectives. Glauconite-rich sandstones (greensands) are common throughout the geologic record, but their depositional interpretation is frequently unclear. Passive-margin condensed sections often are characterized by glauconitic sediments and biogenic reworking. Such sequences sometimes are interpreted to reflect sediment-starved, terrigenous, marine paleoenvironments. The principal objective of this research project is to address the depositional interpretation of greensands with application to key bounding surfaces through the study of ichnologic observations and other data. Trace fossil associations, ichnofacies and ichnofabrics offer some promising clues for discerning the paleoenvironments and sequence stratigraphic contexts in which greensands have formed. Research activities in the first year of this PRF project have focused on two widely separated parts of the geologic column in three different localities – in Cambrian greensands in central Texas and southern Wisconsin, and Eocene greensands in eastern Texas – including field work in Texas and Wisconsin and laboratory analytical work at the University of Utah. Cambrian Examples. Two Cambrian greensand sequences were examined and compared on the basis of trace fossils, ichnofacies, ichnofabric, glauconite, sedimentology and primary sedimentary structures. The Lion Mountain Member of the Upper Riley Formation in central Texas exhibits a low diversity of trace fossils and an upward increase in burrowing within beds to a maximum ichnofabric index (ii5). Greensands containing up to 95% glauconite exist in both the laminated and bioturbated zones. The occurrence of cross bedding and scour surfaces indicates a dynamic sedimentary marine environment within wave base. The Reno Member of the Lone Rock Formation in southern Wisconsin also exhibits a low diversity of trace fossils and a gradual increase in burrowing within each bed to a maximum ichnofabric index (ii5). Greensands containing up to 90% glauconite are concentrated within flat pebble conglomerates at the base of each bed. A typical bed includes flat pebble conglomerate at the base grading to flat and hummocky laminations with increasing bioturbation upward and topped by an erosional contact. The Cambrian sites in Texas and Wisconsin exhibit similar ichnofacies and “lamscram” ichnofabrics associated with the glauconitic greensands, and they probably represent comparable paleoenvironments characterized by “r-selected” ichnocoenoses in the Skolithos (and possibly Cruziana) ichnofacies.

Eocene Examples. Glauconite-rich sedimentary units often are linked with marine transgressions, particularly in association with condensed sections in terrigenous depositional environments. The presence of “glaucony” (i.e., glauconite and other related green minerals) can be a powerful interpretative tool when characterized as to maturity, spatial and temporal distribution, and origin. Trace fossils, ichnofacies and ichnofabrics can augment our understanding of glaucony-rich zones. The Main Glauconite Bed (MGB) of the Stone City Member (Crockett Formation) in eastern Texas is Eocene in age. This unit was selected for evaluation of the ichnologic and sequence stratigraphic implications because of its rich body fossil and trace fossil content. Previous studies of the MGB by other workers have addressed the faunal communities based on the body fossil assemblages and the sedimentology based on the sediment attributes and primary sedimentary structures. In the first year of this PRF project, geochemical and mineralogic attributes of the Eocene glaucony were characterized in detail by means of electron microprobe analysis and thin section microscopy. Additional analyses using Quantitative Evaluation of Materials with Scanning Electron Microscopy (QEMSCAN) and x-ray diffraction (XRD) are currently in progress. Preliminary results indicate that the green grains in the MGB are chlorite-like minerals rather than true glauconite in the strict sense; thus, it is appropriate to refer to the green grains with the more inclusive term “glaucony”. The glaucony within the MGB is predominately pelletal in form and is indicative of an in situ reworked origin rather than an allochthonous origin accompanied by appreciable transport. Trace fossils observed within the MGB include the crustacean burrows, Thalassinoides and Gyrolithes, while Spongeliomorpha was identified in association within a concretionary zone some two meters below the MGB. Fish (stingray?) trace fossils, Piscichnus, occur in strata above the MGB. This sequence exhibits a moderate ichnodiversity, and it probably represents the Cruziana ichnofacies. Ichnofabric index range is from ii1 to ii5. The Stone City Member also contains a concretionary zone of ferruginized crustacean burrows. This may represent a firmground or marine flooding surface. Initial indications suggest a transgressive-regressive systems tract interpretation in which the glaucony grains, body fossils and trace fossils represent a time-averaged association produced during a shoreline migration.

Presentations. Formal presentations (as oral papers and technical posters) of the current findings from this research already have been presented in 2009 at the Geological Society of America Rocky Mountain Section meeting (in Orem, UT), American Association of Petroleum Geologists national convention (in Denver, CO), Tenth International Ichnofabric Workshop (in Jiaozuo, Henan, China) and Geological Society of America national meeting (in Portland, OR).