Allan A. Ekdale , University of Utah
The Main Glauconite Bed (MGB) is a trace fossiliferous greensand of the Middle Eocene Stone City Member, Crockett Formation, which crops out along the Brazos River in Burleson County, Texas. This investigation of trace fossils, ichnofabrics, ichnofacies and mineralogy of the greensands at Stone City Bluff contributes significantly to understanding its paleoenvironments. The MGB represents a composite ichnofabric made up of pre-omission, omission and post-omission suites of trace fossils developed in a greensand with mineralogy that changed through time. The central bed contains the pre-omission suite characterized by indistinguishable individual burrows, intense bioturbation, and abundant pellets. This represents an infaunal community dominated by polychaete worms, deposit-feeding pelecypods and carnivorous gastropods. The top of the MGB is characterized by an omission suite of crustacean trace fossils (Thalassinoides, Gyrolithes and Spongeliomorpha). The top of bed shows evidence of post-omission burrowing within the sediment fill of the crustacean burrows and also contains pellets of two varieties (ovoid fecal pellets reworked from the central bed and autochthonous larger pellets stacked within the burrows). This represents a third trace fossil association in the MGB. Thus, the MGB shows sequential occupation of a changing substrate: the softground (pre-omission suite), firmground (omission suite), and firmground (post-omission suite). Subsequently the top of bed was cemented with siderite, and crustacean burrows were preserved as concretions. There is no evidence of bioerosion of a hardground.
The ichnofacies of the MGB at Stone City Bluff changed from the Cruziana to Glossifungites Ichnofacies. The softground central bed with abundant deposit-feeding burrowers and suspension feeders of a typical Cruziana Ichnofacies transformed to a compacted firmground omission surface at the top of bed with subsequent occupation by decapod crustaceans typical of the Glossifungites Ichnofacies. This transition is interpreted as a shallowing marine environment within the MGB, essentially a parasequence transgressive-regressive cycle topped by the omission surface that represents a parasequence boundary. This parasequence occurs within the larger Stone City Member transgressive systems tract, which is topped by a maximum flooding surface.
Mineralogical identifications to confirm the presence of glaucony (glauconite and related marine clays) are important, since glaucony typically forms in an offshore setting in modern marine environments. Glaucony today forms under sediment-starved, fully marine, geochemical conditions on the sea floor at a depth range of 50m to 1000m. If during accumulation of the MGB glaucony did not form, or if original glaucony was diagenetically altered to other clays, either possibility has paleoenvironmental significance.
Mineralogical analyses of the green clays from the MGB employed several methods. XRD (x-ray diffraction) confirmed the presence of smectite along with kaolinite and illite. Electron microprobe analysis of pellets was conducted to assess geochemical composition. Presence of potassium oxide, 2% or higher, was the criterion used to confirm glaucony composition of the greensand. Glaucony was not detected in the central bed of the MGB, but a few glaucony pellets were identified in post-omission burrow fill at the top of bed. The green hue of the greensands at Stone City Bluff is attributed to the high iron oxide content of the pellets and matrix. Iron-rich smectite is a pre-glaucony end member in the glauconitization process. Smectite is the clay mineral before absorption of appreciable amounts of potassium ions, and it also can be a diagenetic product of glaucony minerals. QEMSCAN (Quantitative Evaluation of Materials with Scanning Electron Microscopy) analyses suggest diagenetic change within the pellets, since their detailed mineralogical composition shows various components as if in diagenetic transition. Much of the clay in the MGB is in the form of pellets attributed to ingestion by sediment-feeding worms and/or pelecypods. As sediment passes through the intestinal tract of an animal, there is some influence on its mineralogy, but the degree of organic influence on clay geochemistry is unclear.
The Cambrian greensand sequences in Texas and Wisconsin were examined and compared on the basis of trace fossils, ichnofabric, ichnofacies, glaucony, sedimentology and primary sedimentary structures. The Lion Mountain Member, Upper Riley Formation, in central Texas exhibits a low diversity of trace fossils and an upward increase in burrowing intensity within beds. Greensands containing a high percentage of 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, Lone Rock Formation, in southern Wisconsin also exhibits a low diversity of trace fossils and a gradual upward increase in burrowing within each bed. 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 greensands, and they probably represent comparable paleoenvironments characterized by the Skolithos (and possibly Cruziana) ichnofacies.
This project significantly expanded the experience of the P.I. (A.A. Ekdale) and formed part of the Ph.D. dissertation of a graduate student (Sherie Harding). Findings were presented at the Geological Society of America Rocky Mountain Section meeting in Utah, American Association of Petroleum Geologists national convention in Colorado, Tenth International Ichnofabric Workshop in China, Eleventh International Ichnofabric Workshop in Spain, and Geological Society of America national convention in Oregon.