Complex siliceous concretions in the Jurassic Morrison Formation, Church Rock, New Mexico, USA: Implications of inorganic factors in ichnological interpretations

Abstract

Complex concretions in the Jurassic Morrison Formation in New Mexico, USA, were studied to elucidate their enigmatic origin. The concretions, mostly in the 0.1–2 m height range, are characterized by a microcrystalline quartz matrix and many tiny (4–8 mm diameter) cylindrical protrusions. A common architecture revealed by weathering is a vertical, cylindrical concentric casing around a homogenous core. Concretion morphologies include irregular bodies and larger horizontal cylinders with surficial protrusions. Sometimes cores and/or protrusions are absent. Micromorphological data and open grain packing indicate concretions formed in unconsolidated sediments. Light and scanning electron microscopy shows gradational contacts between the siliceous concretions and host sandstones. Point counts of concretions and host rock show more cement and intergranular volume in the concretions, but fewer grains and primary pores. Most concretions are cemented by microporous, microcrystalline quartz, chlorite, kaolinite, and illite-smectite. X-ray fluorescence and instrumental neutron activation analyses indicate that silica was added to form the concretions. Previous models for the concretions include burrows, termite nests and rhizocretion. However, these interpretations are challenged by geochemical, macromorphological and micromorphological data presented in this study. Infiltration, possibly due to hydrothermal activity, may have added silica, but explanations for several morphological details remain elusive. These data imply that inorganic diagenetic factors can be important in ichnological interpretations.