FLUID FLOW AND REGIONAL DEFORMATION HISTORY OF FRONT RANGE AND DENVER-JULESBURG BASIN, CO REGION: INSIGHTS FROM PETROGRAPHIC AND GEOCHEMICAL ANALYSES OF FRACTURE-FILLING CALCITE

The Colorado Front Range and adjacent Denver-Julesburg Basin host a complex fracture network due to an extensive history of deformation initiated from regional flexure during the Sevier orogeny in the Mid-Cretaceous. Subsequent deformation by orogenesis and basin formation during the Laramide orogeny and later regional uplift adds complexity to this history. Fluid flow episodes are intimately linked with tectonism as the generated fracture network (fractures, joints, faults, open stylolites) acts as conduits for flow. However, there are presently no constraints on the origin, temperature, composition, and timing of associated fluid flow events. We address this through the integration of petrography and geochemical analyses: laser ablation ICP-MS U-Pb geochronology, fluid inclusion microthermometry, stable isotope and strontium isotope analysis of fracture-filling calcite cements hosted in a normal fault system in the Niobrara Fm. Samples were obtained from drill-core (Wattenberg Field, CO) and outcrop exposures near Denver, Colorado Springs, and Canon City, CO.

Initial results obtained from several fracture fill types (e.g., crack-seal-slip, crack-seal and crack fill cements, tectonic stylolite-associated, fault breccias, and joints) are consistent with multiple pulses of fluid flow, possibly coeval with the regional history of orogenesis. Earliest fluid flow phases are likely associated with extensional faulting occurring east of the forebulge during the Sevier orogeny, followed by a period of high heat flow with uplift, compression, and plutonism during the Laramide orogeny in the latest Cretaceous. Latest stage fluid flows are attributed to regional uplift and unroofing during the latest Cenozoic resulting in cement precipitation in more shallow diagenetic environments. We postulate carbon and oxygen isotopic data will be consistent with a meteoric water origin for fluids, whereas strontium isotopes will allow distinction of interaction with radiogenic host rocks. These new results provide a robust history of tectonism and associated fluid flow events that may be applied to future tectonic studies and georesource exploration.