ANISOTROPY OF MAGNETIC SUSCEPTIBILITY AS A PROXY FOR EMPLACEMENT PATTERNS OF BASEMENT-HOSTED SANDSTONE DIKES, COLORADO FRONT RANGE
We present new anisotropy of magnetic susceptibility (AMS) data used to determine the alignment of detrital magnetite resulting from shear interaction of fluidized sediment with host-rock boundaries. Selected dikes are 1 to 2 meters wide, outcropping laterally up to 30 meters, with little or no overprint by brittle fracturing or shearing associated with the UPF. Where possible, samples were obtained on paired margins and in transect to test for imbrication of the magnetic foliation plane and directionality of flow. A full description of magnetic fabrics present in the dikes will help to define the emplacement behavior of the dikes, including information about the direction and flow regime of injection. Rock magnetic properties provide information about post-emplacement diagenesis and fluid flow.
AMS fabrics are predominately oblate with the magnetic foliation plane, defined by the K1-K2 axes, parallel to dike or sill margins. Several sites exhibit prolate fabrics with maximum susceptibility axes oriented parallel or sub-parallel to dike margins. One premier sampling site reveals a strongly expressed subhorizontal prolate imbrication fabric.
The results of our study corroborate textural petrography findings in that the dikes formed as injectites with prevalent laminar flow. Evidence points to rapid injection of dike sediment due to elevated pore-fluid pressures arising from high lithostatic or tectonic loads. We hypothesize that high sediment accumulation rates and fluid overpressure occurred in association with sediment-gravity deposition in a narrow fault-controlled trough. Our ongoing investigation extends to the tabular sandstone bodies that represent the remnants of source sands for the dikes that may have undergone liquefaction in response to a seismic or glaciogenic trigger (Siddoway et al. 2013, GSA Field Guide v. 33).