| Description |
Concretions are mineral masses that form in response to changes in chemical conditions, porosity/permeability conditions, and pore-fluid composition during diagenesis. Large (up to ~1 m diameter spherical/spheroidal iron oxide concretions of the alluvial-fluvial Cretaceous Baseline Formation in southern Nevada provide opportunities to explore concretion formation mechanisms and conditions through investigation of formation- to pore-scale sedimentologic and diagenetic attributes. Description of a measured section through the entirety of the ~1100 m formation thickness forms the context for thin section petrography and point-counting. Stratigraphic architecture and allied analyses indicate that deposition of the Baseline Formation was related to activity on nearby thrust faults. The fluvial fan advanced during uplift of the Willow Tank thrust allochthon and retreated as activity stepped westward to the Muddy Mountain thrust. The diagenetic context was determined through a multi-scale approach utilizing remote sensing mineral mapping, field characterization, and laboratory analyses. Diagenetic patterns occur as coloration and cementation facies depending on the authigenic mineral abundance. Cementation facies comprise ironstone horizon, iron oxide concretionary forms, and carbonate/silica concretionary forms. Relationships of diagenetic facies with lithofacies indicate that coloration patterns and iron oxide cementation facies formed related to depositional processes and basin-scale fluid flow principally in the early diagenetic realm with later alteration during regional iv extension. Field- and laboratory-based characterization of the large spherical concretions reveals that the concretions occur near the transition from a fluvial fan to alluvial plainfluvial environment. The spherical geometries occur within relatively isotropic sediments of channelized facies. The large concretions likely formed within the shallow subsurface during early diagenesis in saturated conditions of pore fluid composition sufficient for the mobilization and transport of iron. Additional diagenetic products indicate relationships with biological materials or activity. These diagenetic features represent a spectrum of biological involvement in cementation processes from physicochemical-driven to biologically mediated. Recognition of biosignatures may be helpful in the search for life elsewhere in the solar system. Finally, this dissertation outlines efforts to leverage new technologies for digital field notebooks. In particular, this study expands the StraboSpot Data Management System to accommodate the specific data types and stratigraphic framework critical to sedimentological and stratigraphic studies. |
