MINERALOGIC CHARACTERIZATION OF GREEN RIVER FORMATION DEBRIS IN WEST SALT CREEK LANDSLIDE, WESTERN COLORADO

On May 25, 2014, a historically unprecedented rock avalanche occurred in the West Salt Creek (WSC) drainage on the north flank of Grand Mesa near Collbran, CO. The slide initiated from a preexisting landslide consisting of the Parachute Creek Mbr. (PCM) of the lacustrine Green River Fm. (Eocene). The PCM debris blocks, which are up to 10 m across, rapidly degraded into piles of rubble within weeks of deposition. However, time-equivalent PCM strata elsewhere in the Piceance Basin are resistant to erosion and are cliff-forming. Thus, the objective of this study is to see how matrix minerals and clay minerals in the slide debris compare to PCM strata elsewhere in the Piceance Basin.

Ten PCM samples from the slide and nearby roadcuts (USFS 121) were analyzed by X-ray diffraction. They consist of soft, laminated, clay-rich marlstone, lean oil shale, and silty claystone. For comparison, a control group of 7 PCM samples were analyzed from Parachute Creek, Anvil Points, Douglas Pass, and Rio Blanco, located 52, 40, 95, and 63 km from the slide area, respectively. The control samples consist of hard marlstone, lean oil shale, and siltstone. Major non-clay minerals in the slide samples are, in decreasing abundance, calcite, quartz, and dolomite, whereas those in the 7 control samples are dolomite, calcite, quartz, albite, K-feldspar, and analcime. Clay minerals in the WSC samples are smectite > illite > kaolinite. In the control group, illite and kaolinite exist in very small amounts and smectite is absent. The most significant mineralogical difference between the WSC and control samples is smectite abundance.

To simulate the disintegration of the PCM slide blocks, 2 slide samples (~2 kg each) were subjected to repeated freeze-thaw cycles (72 and 67 respectively) in the laboratory. Fragmentation by induced frost-wedging began after two or three cycles, with impressive fragmentation and total disintegration after 30 cycles. One marlstone sample (~2.5 kg) from the control group was subjected to 52 cycles, but showed no change. The results suggest that the lithologic and mineralogic characteristics of the PCM in the WSC slide area influenced mechanical stability and were probably contributing factors to the event.