CLASTIC INJECTION FEATURES IN ALLUVIAL DEPOSITS OF THE PENNSYLVANIAN FOUNTAIN FORMATION (MANITOU SPRINGS, COLORADO)

Outcrops of the Fountain Fm are scenically exposed at the Garden of the Gods, Colorado. Two distinctive features, recently interpreted as the result of thermal contraction, appear in the vicinity of Balanced Rock, one of the park’s premiere attractions: a) conglomeratic, tapering-upward, subvertical clastic dikes, up to 0.8 m thick and 2.5 m tall, with exposed lateral extents up to 12 m; b) smaller-scale sandstone dikes, projecting up to 35 cm upward and downward from a relatively clean sandstone interval sandwiched between more matrix-rich beds. Fourteen larger dikes, all with NNW/SSE orientations, were recognized in the study area. Five of these occur along the fault planes of synsedimentary normal faults, with offsets of less than 1 m. The smaller dikes consistently occur ~1 m above the larger ones and typically form a polygonal pattern where they intersect bedding planes. The two sets of dikes are not directly continuous; though the smaller ones originate in the same interval breached by the larger dikes. They co-occupy an outcrop area of ~0.5 km² within a 6 m interval of channelled sandstone and conglomerate beds alternating with beds of similar lithology that are richer in fines. The entire interval is part of a thick succession of alluvial fan deposits that display various types of soft-sediment deformation features.

The general abundance of soft-sediment deformation features in the succession, the co-occurrence of the two dike types in a restricted geographical area, and the parallel orientation of the larger dikes and normal faults, in addition to the specific morphologies of these features, indicate that all the clastic dikes were generated in a syn-sedimentary lateral spreading event involving liquefaction and fluidization of recently deposited sediment in the shallow subsurface. It appears that the larger dikes were generated from liquefaction of an underlying coarse interval. Coarse sediment was entrained in water escaping upward from the zone of liquefaction. Less permeable layers were breached by hydraulic pressure, aided by faults and fractures formed as surficial sediments spread toward the collapsing zone of liquefaction. More permeable layers accommodated lateral fluid movement, resulting in secondary fluidization and clastic injection as hydraulic pressure rose in these layers.