The Pennsylvanian-Permian Fountain Formation is an arkosic conglomeratic sandstone that was deposited in fluvial environments along the eastern flanks of the ancestral Rocky Mountains. The formation owes its pinkish red color to hematite cement that was precipitated early in its diagenetic history. Within the formation are whitened strata that crosscut laminations and facies boundaries, indicating that they are the result of a post depositional process. Whitened features are seen in core, indicating that they are not caused by modern weathering processes. Whitened strata similar to those present in the Fountain Formation are usually the result of the migration of reducing fluids. These fluids reduce and remove hematite cement leaving the fluid migration pathways whitened. Fluids that can cause large-scale reduction and removal of iron oxides include basinal aqueous brines and hydrocarbons.
Whitening within the Fountain Formation appears in a predictable stratigraphically-controlled manner and is most common in coarse channel sandstone facies that are adjacent to laterally continuous paleosol mudstones. The predictable distribution of whitened strata in outcrop suggests that fluid followed preferential pathways. Outcrop analysis indicates that these pathways are closely associated with thin paleosol mudstones and overbank deposits that seem to have focused the paleo-fluids that then flowed laterally along them in the coarser channel sandstones. Laterally continuous paleosol mudstones therefore may have played an important role in determining the spatial location of paleo-fluid migration pathways. Fluids moved through the formation as stringers that took up less than 15% of the total rock volume.
The Fountain Formation has a complex diagenetic history and has undergone multiple stages of cementation. A late stage dolomite cement contains organic matter, hydrocarbon inclusions, and is associated with bitumen. This cement is restricted to whitened strata and likely precipitated from a hydrocarbon-bearing fluid. The hydrocarbon-bearing fluid may have been the fluid that was responsible for whitening sections of the Fountain. Fluid inclusion data indicate that the precipitation of this cement took place after the formation was buried to a depth of at least 1.3km, which would have been during or after Laramide deformation. The presence of bitumen and hydrocarbon inclusions in strata that were not buried to hydrocarbon generating depths indicates that the hydrocarbon-bearing fluid likely migrated through the formation from deeper in the basin.
The amount of whitening in outcrop decreases in the northern study sites and may be related to a decrease in coarse channel sandstone facies. The lesser abundance of those facies at northern study sites may be because those sites were further from the sources for coarse material and were associated with lower energy environments. Although there is less whitened rock at the northern sites, the amount of fluid that passed through them may have been similar to the amount of fluid that passed through the southern sites. Evidence of this is a higher amount of feldspar alteration in whitened strata in the northern site, which may have been caused by more fluid flow per volume of rock because there were fewer coarse channel facies to act as conduits.
|Commitee:||Ridley, John, Sale, Thomas|
|School:||Colorado State University|
|School Location:||United States -- Colorado|
|Source:||MAI 52/02M(E), Masters Abstracts International|
|Keywords:||Denver basin, Diagenesis, Facies, Fountain formation, Paleo-fluid|
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