CONE-IN-CONE STRUCTURES
Cone-in-cone structures are strange features found as
interbeds in fine-grained siliciclastic rocks and sometimes found in the outer
layers of concretions. They consists of sets of conical structures
stacked up within each other. Mineralogically, they are often composed of
finely-crystalline fibrous calcite. Numerous hypotheses have been
proposed over the years that try to explain cone-in-cone structures. None
has emerged as the most popular or most likely (see Lugli et al., 2005 for a
long list of proposed formation mechanisms). These structures remain a
mystery.
Cone-in-cone structure (3.3 cm across), composed of finely fibrous
calcite. This weathered sample is likely derived from an interbed in the
Ohio Shale outcrop belt (Frasnian-Famennian, Upper Devonian) of central Ohio,
USA
Cone-in-cone structure
Specimen
owned by Nicole Byrd.
For more info. on cone-in-cone structures, see (as
examples):
Melichar & Shkovira (2001) - Case study of the
cone-in-cone structure based on Czech and Crimean samples. GeoLines
13.
Lugli et al. (2005) - Silicified cone-in-cone
structures from Erfoud (Morocco): a comparison with impact-generated shatter
cones. in Impact tectonics. Impact Studies 6:
81-110.