Origin of Conein Cone Structures

The origin of cone-in-cone structures has been strongly debated, but many aspects of their formation have remained unclear. A common definition (Bates and Jackson 1987) states: "The structure appears to be due to pressure aided by crystallization and weathering (solution) along intersecting conical shear zones". The important hypotheses for the formation of calcareous cone-in-cone structures can be divided into two groups: those favoring early displacive formation of concretions in soft sediment and those that emphasize late fracturing of concretions with or without excess pore fluid. Most workers reported the association of cone-in-cone structures with other concretions such as septaria and also with organic-rich sediments. Some authors relate their growth to degradation of organic matter (Brown, 1954) or possible microbial activity (MacKenzie 1972; Aso et al. 1992; McBride et al. 2003), in particular for pedogenic rocks (Aassoumi et al. 1992). In summary, the following proposals have been made (see also Mozley 2003):

Fig. 4. Back of the sample illustrated in Fig. 3 showing annular rings (corrugations) and cone cups left by the removal of cones (marked C and by arrows). Erfoud (Morocco) sample. Scale bar length: 1 cm.

Recrystallization of fibrous aragonite to calcite forming open cone fractures intruded by argillaceous matter (Tarr 1922; Gilman and Metzger 1967).

Differential pressure solution on calcite fibers along conical shear planes induced by the weight of overlying strata (Tarr 1932; Pettijohn 1957).

Settling and volume shrinkage during the slow dewatering of highly saturated and loosely packed subaerially-exposed sediments (Shaub 1937).

Precipitation of fibrous calcite along fractures or in concretionary masses. The fibers would then be deformed by "tractional forces" during deformation of the host strata or during concretionary growth of the calcite

Fig. 5. Another portion on the back of the sample illustrated in Fig. 3 showing relatively wide annular rings. Erfoud (Morocco) sample. Scale bar length: 1 cm.

component (Bonte et al. 1947).

Crystallization of calcite fibers under gravity-induced differential stress, during gradual compaction of the sediment. The angles of cone apices would be determined by the plasticity of the enclosing sediment: large apical angles would indicate relatively low plasticity (Woodland 1964).

Early displacive growth of cone-shaped plumose aggregates of fibrous calcite (Franks 1969).

Diagenetic water-escape structures promoting the "reordering of phyllitic elements within a sediment undergoing diagenetic compaction and which exhibits differences in porosity and competency in its lithological composition; cone in cone can be compared to a schistosity acquired in the realm of hydroplastic deformation" (Becq-Giraudon 1990).

Brittle fracturing of crystalline calcite aggregates, grown by a crack-seal mechanism in overpressured environments, and induced by a decrease in pore pressure of the plastic host sediment (Selles-Martinez 1994).

Shallow burial (<1500 m) precipitation from modified marine pore fluids (Hendry 2002).

Early diagenetic growth (depth <40 m), probably microbially mediated, in sandstone units beneath flooding surfaces and sequence boundaries (McBride et al. 2003).

Fig. 6. Top view of the sample shown in Fig. 3-5. Note telescoping cones (white arrows) and conical surfaces forming cone cups (black arrows). Scale bar length: 1 cm.

Fig. 7. Silicified cone-in-cone structure consisting of nested and interfering cones. Note section of intersecting and overlapping circular arcs of former fibrous calcite forming cone "branches" originally separated by argillaceous material (arrow). The interference of these conical surfaces produces the striated appearance of the cone surfaces. Erfoud (Morocco) sample. Scale bar length: 1 cm.

Fig. 7. Silicified cone-in-cone structure consisting of nested and interfering cones. Note section of intersecting and overlapping circular arcs of former fibrous calcite forming cone "branches" originally separated by argillaceous material (arrow). The interference of these conical surfaces produces the striated appearance of the cone surfaces. Erfoud (Morocco) sample. Scale bar length: 1 cm.

Fig. 8. Silicified cone-in-cone structures from the Hamada region southwest of Erfoud (Morocco). Note wide cone bases. The sample is a loose fragment from a thin individual concretionary layer. Scale bar length: 1 cm.

Fig. 9. Top view of the sample shown in Fig. 8.

Note telescoping cones

(white arrows) and a hollow cone cup (black arrow). Scale bar length: 1 cm.

Fig. 9. Top view of the sample shown in Fig. 8.

Note telescoping cones

(white arrows) and a hollow cone cup (black arrow). Scale bar length: 1 cm.

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