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Background and objectives.  Glauconite-rich “greensands” are common in both siliciclastic and calcarentic marine deposits from the Cambrian to the Recent, but their depositional interpretation and sequence stratigraphic implications are frequently unclear.  Condensed stratigraphic sections often display an appreciable amount of biogenic reworking in glauconite-rich sediments.  The principal scientific objective of this research project is to interpret the depositional conditions associated with the formation of greensands based on ichnologic observations and other sedimentologic and mineralogic data.  Trace fossil associations, ichnofacies and ichnofabrics are providing important clues for discerning the paleoenvironmental settings and sequence stratigraphic contexts in which greensands have formed.            

First year findings.  Research activities in the first year of this PRF project focused on two widely separated parts of the geologic column in three different localities – Cambrian greensands in central Texas and southern Wisconsin, and Eocene greensands in eastern Texas – including field work in Texas and Wisconsin and laboratory work at the University of Utah.  The 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 (Cambrian) in central Texas exhibits a low diversity of trace fossils and an upward increase in burrowing to a maximum ichnofabric index (ii5).  Greensands containing up to 95% glauconitic minerals 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 (Cambrian) in southern Wisconsin also exhibits a low diversity of trace fossils and a gradual increase in burrowing to a maximum ichnofabric index (ii5).  Greensands containing up to 90% glauconitic minerals 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 ichnofabrics associated with the glauconitic greensands, and they probably represent comparable paleoenvironments characterized by the Skolithos (and possibly Cruziana) ichnofacies.  The Eocene example exhibits a glauconite-rich sedimentary unit that was linked with a marine transgression in a terrigenous sedimentary setting.  The Main Glauconite Bed (MGB) of the Stone City Member of the Crockett Formation (Eocene) in eastern Texas was selected for evaluation of the ichnologic and sequence stratigraphic implications because of its rich trace fossil and body fossil content.  In the first year of this PRF project, geochemical and mineralogic attributes of the Eocene glauconitic minerals 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) were initiated.  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.  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, which apparently represents a marine flooding surface.  Initial indications suggest a transgressive-regressive systems tract interpretation in which the glaucony grains, trace fossils and body fossils represent a time-averaged association produced during a shoreline migration.  Formal presentations of the first-year findings from this research were presented at the 2009 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).

Second year findings.  Research activities in the second year of this PRF project involved continuing geochemical analyses (microprobe, QEMSCAN and XRD) of samples from the three field sites that were visited in the first year of the project, as well as petrographic examination of thin sections of greensand specimens from several stratigraphic levels at all three sites.  Numerous clay mounts were prepared and analyzed using XRD to identify the clay minerals in the greensands, and “Species Identification Protocol” (SIP) data for employment in the QEMSCAN analyses were developed using the electron microprobe.  Analytical results indicate that the green minerals in the Texas and Wisconsin strata are not all alike, and they include both true glauconite and other related chlorite-like clay minerals.  These mineralogic observations somewhat complicate the paleoenvironmental interpretation of authigenic minerals in the various greensand examples studied in this project, so geochemical and mineralogic analyses will be continuing in the months to come in the geochemical research facilities at the University of Utah.  Trace fossil specimens from all three sites were examined, described and photographed in the ichnology lab at the University of Utah.  The Cambrian trace fossil suites consist mainly of burrows made by worms (Skolithos, Planolites, Palaeophycus and Diplocraterion), whereas the Eocene trace fossil suites consist mainly of burrows made by decapod crustaceans (Thalassinoides, Gyrolithes and Spongeliomorpha).  Despite the differences in the ichnotaxa, the paleoenvironmental settings of the Cambrian and Eocene sites were similar – i.e., nearshore to offshore, shallow-marine environments on a tectonically stable sea bottom that was subjected repeated fluctuations of sea level, perhaps due to eustatic changes.  Additional field work was accomplished at the Eocene site in eastern Texas during the second year of the project.  Also, some nearby field localities in Utah and Wyoming were visited briefly, because published descriptions of Jurassic and Cretaceous greensands suggested that those examples may exhibit some significant similarities in ichnology, sedimentology and mineralogy to those already studied in Texas and Wisconsin.