Silicified Cone-in-Cone Structures from Erfoud (Morocco): A Comparison with Impact-Generated Shatter Cones

Abstract.

Several geological features, including sedimentary cone-in-cone structures and percussion marks, may resemble impact-generated shatter cones. Especially inexperienced workers may mistake such features for impact deformation. In 1997, our group investigated an alleged occurrence of shatter cones in the Hamada area of southeastern Morocco and found that these are actually cone-in-cone structures, probably from the Lower Visean Merdani Formation. Here, a detailed discussion of cone-in-cone structures, as well as a short review of shatter cone characteristics, is presented in an effort to clarify some distinguishing criteria. Important differences include: (1) Shatter cone striations are of distinctly roundish shape, whereas cone-in-cone striae are step-like; (2) shatter cones never show scaled surfaces; (3) broken cone-in-cone structures invariably produce one surface with striated cone features, but its opposite side would display scaled cone cups; (4) shatter cones do not telescope out of the bedding-plane, as cone-in-cone structures may do; (5) at the thin section scale, the internal structure of cone-in-cone features is well preserved, even after complete silicification of the primary carbonate. Thus, careful observations should allow unambiguous decision whether certain rocks contain shatter cones or cone-in-cone structures.