STUDY OF "TERMINAL" DISTRIBUTARY CHANNELS IN FLUVIAL-DOMINATED DELTA FRONT DEPOSITS, USING GROUND-PENETRATING RADAR AND DIGITAL MAPPING TECHNIQUES, CRETACEOUS PANTHER TONGUE SANDSTONE, UTAH, USA

We document the 3-D facies architecture of "terminal" distributary channel deposits within the fluvial-dominated Cretaceous Panther Tongue in central Utah. A series of cliff exposures are oriented at high angles to the southerly progradation direction of the delta deposits in Spring Canyon and Sowbelly Gulch area. The outcrop shows the cross sectional facies architecture of proximal delta front deposits that contain stacked channel facies interbedded with seaward-dipping, marine influenced fine-grained sandstones beds. These deposits are interpreted as "terminal" distributary channels associated with mouth bar deposits. 2-D, 100 MHz and 50 MHz ground-penetrating radar (GPR) lines are used to extend mapping of the facies architecture of channel and bar deposits in 3-D behind the cliff. Channel features with 2-3 meters topography and up to 25 meters wide can be observed in both outcrop and radargrams. Digital mapping allows high precision correlation between outcrops at the northern and southern sides of Spring Canyon and adjacent valleys. A model of the 3-D facies architecture of the proximal environment of the ancient deposits of a river dominated delta shows an area of stacked channels in the northern part of the study area and thinner channel deposits to south. In the southern area, bed surfaces are planar rather than channelized, and are interpreted as distal mouth bar deposits. Modern examples of river-dominated deltas (e.g. Mississippi, Volga, Atchafalaya, and Lena) show distinct channel patterns produced by the equilibrium between fluvial and marine energies. From modern deltas we distinguish many "terminal" distributary channels formed in basins with shallow water that contain numerous bifurcations and decreasing downstream discharge. Ancient "terminal" distributary channels, however, have not been previously described because without lateral outcrop exposures, it is difficult to differentiate these small-scale features within the sandy delta front.