FLUID FLOW THROUGH JURASSIC SANDSTONES IN THE PARADOX BASIN, COLORADO: SYNDEPOSITIONAL, DIAGENETIC OR LATER?

Timing of diagenetic fluid flow are important because they are linked with hydrocarbon migration in many cases. Using magnetic fabric and paleomagnetic methods, we investigate the Middle-Late Jurassic sandstones from the Paradox Basin, Colorado to determine the direction of fluid flow and, if possible, its timing. This study focuses on the Entrada Sandstone, the Wanakah Formation, and the Tidwell Member of the Morrison Formation. We show that the anisotropy of magnetic susceptibility (AMS) of these sandstones is carried primarily by iron (oxyhydr)oxides. These fabrics, measured on horizontally bedded sandstones, are systematically oblique with respect to the local bedding (average tilt of ~ 50° SE). These fabrics are also remarkably consistent between specimens from two study sites, probably reflecting a large-scale fluid migration event. The magnetic carriers occupy pore spaces between siliciclastic grains, and the AMS originates from pore shape anisotropy. A tectonic origin for the fabric can be ruled out because the sandstones are macroscopically undeformed and bear no microscopic evidence for plastic deformation. The magnetic assemblage in each specimen is constrained using low-field AMS, isothermal remanent magnetization (IRM), and thermal demagnetization of the natural remanent magnetization (NRM). Our data show that the AMS fabric, contrarily to our initial expectation, does not reflect depositional paleocurrent directions but instead results from post-depositional downdip flow of iron-rich diagenetic fluids. SEM-BSE images provide documentation on the shape of iron (oxyhydr)oxide grains in pore spaces. Thermal demagnetization behavior is highly variable between specimens in terms of stability, number of components and directions. This type of record suggests a complex magnetization history primarily through chemical remanent magnetization (CRM) processes which record multiple fluid flow events. The preliminary results are used to determine whether the bulk of this CRM was acquired in Jurassic (shortly after deposition), Late Cretaceous (during regional uplift), or even more recently during the Quaternary downcutting of the Gunnison River. Overall, this study further demonstrates the potential of rock magnetic and paleomagnetic methods to resolve complex fluid migration in porous rocks.