Landslides in the Coal Hill area, Kane County, Utah (Thesis and maps)

The Coal Hill area comprises about 4 square miles along State Highway #15, approximately halfway between Mt.Carmel Junction and Zion National Park. Principal drainage of the area is by Meadow Creek and Little Meadow Creek. The Coal Hill area is composed of Jurassic and Cretaceous sedimentary rocks having a regional northeast dip of 1° to 2°. Rocks of the Jurassic System are represented by the Carmel and Winsor Formations which comprise the San Rafael Group in this region. The Cretaceous System is represented by The Dakota-Cedar Mountain Formation, and the Tropic Formation. Locally these sedimentary rocks are overlain by Recent alluvium along Meadow Creek and by colluvial deposits of the various landslides. The Coal Hill area is characterized by an instability of slopes, primarily in shale beds of the lower Tropic Formation. Landslides of various types and magnitudes are dominate in the topography on the slopes adjacent to Meadow Creek. An active landslide where the present State Highway #15 crosses Coal Hill, designated as the ""Coal Hill Slide"", has imposed considerable highway maintenance problems since 1928. The highway route is currently being relocated to pass above the active part of the slide and across an inactive part. The slide at Coal Hill is of the composite slump-block type, com posed of several stages of backward rotational movement along sliding surfaces which are concave upward. The foot and toe of the slide have recently degenerated into a series of earth- flows. Logs of drill holes and seismic refraction profiles from the State Road Commission were used to determine depth of the sliding contact at several locations. The subsurface information confirmed the surface evidence relating to the mechanics of movement. Movements in the Coal Hill slide result from a failure zone in bentonitic shales of the basal Tropic Formation. Ground water has lowered the shearing resistance of the shales to a limit where failure occurs. The resulting sliding movement has effected both overlying and underlying beds. Water is supplied to the shale beds primarily by a fault zone and associated fracture system along the east flank of the slide area. Active dissection and removal of material by Meadow Creek promotes movement in the lower portions of the slide. A larger landslide, approximately one mile downstream from the Coal Hill slide, also involves beds in the lower Tropic Formation. At the head of the slide, marked by a conspicuous ""slump scar"", blocks of lower Tropic shale, coal, and sandstone have slumped and collapsed along local fractures. This movement has been ""triggered"" by extensive burning in coal beds, resulting in a collapse of overlying sediments. The flanks of the slide consist of steep debris flows in which blocks of sandstone, shale, and coal have been transported in a matrix of wet shaly material. The lower portions of the slide area contain debris flows and talus-creeps. A study of aerial photos revealed that the principal movements in this slide area have occurred since 1939. Several isolated slides are present along slopes to the south of Meadow Creek. These slides are of the debris avalanche type and are caused by failure in the colluvial cover along the steep slopes. The colluvium is composed of disintegrated shale and coal from lower Tropic beds. On slopes lying to the north of Meadow Creek, the down- slope creeping action of colluvium has imposed highway maintenance problems. The colluvium, comprising about a square mile, is creeping on an eroded surface of Jurassic and Cretaceous beds primarily from the force of gravity. The source of the colluvium appears to be from older landslides which once occurred in Tropic beds on higher slopes to the north. Dissection of the lower limits of the colluvial material by Meadow Creek has caused several local slides. Age relationships of slide debris to the alluvium of Meadow Creek indicate that most of the instability of slopes in the Coal Hill area has developed within the past 1300 years. The unstable slope conditions arise from the favorable stratigraphy of the lower Tropic Formation supplemented by local structural control, effects of ground water, climate, and the Recent geomorphological history of Meadow Creek.