GEOMAGNETIC INVESTIGATION OF SANDSTONE DIKES OF THE COLORADO FRONT RANGE, FOR DETERMINATION OF AGE AND MODE OF EMPLACEMENT

Clastic dikes hosted by Proterozoic crystalline rocks of the Colorado Front Range present a long-pondered geological enigma (Cross, 1894, GSAB, 5, 525) due to their size, spatial extent, unknown age and uncertainties about the sediment source and mode of emplacement. The sub-vertical to vertical dikes strike NW-SE, attain widths >10m, and outcrop along ~70 km near the trace of the Ute Pass fault. The dikes consist of sub-rounded to rounded sand-sized quartz, detrital magnetite, and angular feldspar (<5%). Cements include minimal clay and varying amounts of hematite. Grading and sorting are generally absent, but where evident, sedimentary layering is subparallel to dike margins. Occasional lags of rounded pebbles, 1-5 cm in length, are found, as well as elongate angular xenoliths of host rock (Pike’s Peak Granite). Detrital zircons yield U-Pb ages determinant of >1.0 Ga. We present paleomagnetic, rock magnetic, and anisotropy of magnetic susceptibility (AMS) results from twelve sandstone dike localities. Characteristic remnant magnetization of the dikes should provide a lower age limit on our estimate of dike emplacement or remagnetization by means of a paleomagnetic pole position. IRM acquisition experiments reveal a mixture of Fe-Ti oxide phases, likely low-Ti magnetite and hematite, as the principal magnetic phases. Laboratory unblocking temperatures for most samples are >600°C, with an indication of a secondary chemical magnetization carried by hematite. Preliminary paleomagnetic results reveal a thermoremanent magnetization carried by magnetite yielding a NW magnetic declination with shallow inclination, similar to results obtained from remagnetized Precambrian and Paleozoic rocks in the region (Geissman & Harlan 2002 EPSL, 203, 905). AMS results for three dikes yield strongly prolate magnetic ellipsoids. The magnetic lineation (K1) trends NW, parallel to the dike margins, with shallow to moderate plunge, and average bulk susceptibilities (Km) are low (6.39E-5 SI). The primary sedimentary textures and AMS characteristics point to instantaneous emplacement due to elevated pore- fluid pressures arising from high lithostatic or tectonic loads. We introduce the hypothesis that overpressure was induced by ice damming or ice loading in a glaciogenic setting. A seismogenic trigger cannot be ruled out.