Conein Cone Petrography

Most cone-in-cone structures are composed of fibrous calcite and films of argillaceous material. The cone structure consists of nested and interfering cone-shaped, plumose aggregates of calcite fibers. The individual fibers are generally less than 6 ^m wide but may be up to 3 cm long; plumose aggregates of fibers may attain 7 cm in length. The long axes of the plumose aggregates are oriented parallel to the long axes of the cones. Individual cones and cone segments are separated by argillaceous films that are up to 0.2 mm thick. Thinner argillaceous films may separate individual plumose aggregates of fibrous calcite and define the conical layering of the cone-in-cone structure. Small amounts of carbonaceous matter that can also be associated with pyrite and/or marcasite are commonly concentrated along the argillaceous films. Locally, the argillaceous material has been concentrated in angular patches corresponding to the filling of the annular rings around the cone cups. These fillings mark the termination of the fibers of a plumose aggregate against those of an adjacent aggregate or cone.

Fig. 3. Silicified cone-in-cone structures from the Hamada region southwest of Erfoud (Morocco). The sample is a loose fragment from a thick individual concretionary layer. Note that striae on cone surfaces are either straight (marked S) or fan-like (F). Scale bar length: 1 cm.

The cones have a tendency to break apart along the striated argillaceous films, resulting in elongated, curved and tapered fragments called "leaves". These characteristics have led some workers to interpret the spaces filled by argillaceous material as fractures (Tarr 1932; Gilman and Metzger 1967; Gilman 1968; Selles-Martinez 1994). Supporting evidence for this interpretation would be the apparent offset of cones and conic scales on the counterpart of the striated surfaces, as well as the telescoping of cones outward with respect to adjacent external cones.

Rare siliceous cone-in-cone structures have been reported in the literature and interpreted as the result of silica replacement of calcite structures (Woodland 1964). However, Becq-Giraudon (1990) proposed a primary origin for the siliceous cone structures of the Lower Ordovician in the Montagne Noir (Massif Central, France).

Table 1. Important characteristics of cone-in-cone structures.

Cone-in-cone

Host rock

shales, sandstones and pedogenic rocks

Associated feaures

septaria, carbonate concretions

Size of cones

mm to dm

Composition of cones

calcite, quartz (replacement)

Cones defined by

calcite fibers, concentrations of argillaceous material, fractures?

Orientation of cone axes

perpendicular to bedding

Orientation of cone apices

toward substrate layer: upward and downward

Apical angles

14° to 100°

Striae on cones

straight, radial from apices

Formation time

unknown, up to million years?

Origin

early diagenesis? pressure solution? crack-seal mechanism?

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