SHATTER CONES IN THE FRONT RANGE: EVIDENCE FOR MULTIPLE IMPACTS

Shatter cones and other shock metamorphic features have been discovered at a number of sites in the central and southern front range mountains of central Colorado (Duncan, 2012). Subsequent field work has yielded new shatter cone exposures as well additional discoveries at previously worked sites. All shock metamorphism is located within circular topographic structures and likely mark the locations of central uplifts or rings. Eight such features have been named and investigated in the field and range from 5 to 25 km and cover a 2500 square km area.

All shatter cones meet the characteristics as defined by French (1900). Shocked rocks in the study area have undergone moderate degrees of weathering (chemical and biological/lichens) as well as hydrothermal alteration in some cases that has obliterated or reduced the quality of striations. Perfect cones are uncommon.

Most host rocks are feldspar-rich granitoids and highly susceptible to chemical weathering. The highly fractured character of the impact target rocks enhances weathering further. However the gross cone shape is preserved to some degree in most cases. Older exposures have are typically modified by lichens to the extent that striations have been completely removed in some cases leaving behind only a “bald” cone-shaped mass of rock. Close examination of a site typically leads to areas of better preservation where striated cone surfaces are preserved.

Weathered shatter cone surfaces at several sites are covered with 2-4 mm, round, white splotches. Examination with a hand lens reveals that the white splotches are actually small, parasitic shatter cones developed in coarse quartz crystals. Quartz’s stability favors preservation of the small cones even in highly altered granitoids.

Where cone axes intersect exposed joints at high angles (near or at 90 degrees) a cross sectional view of the shatter cones is produced on fracture surface. This perspective results in circular to elliptical shatter cone patterns on the joint surface with smaller parasitic cones converging within a larger “parent” cone. The circular parent-cone patterns are commonly preserved and identified even in highly weathered rocks.

Recognition of weathering patterns in shatter cones is an important tool in finding additional older impact structures worldwide.