Cone-in-cone structure

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Cone-in-cone structures are secondary sedimentary structures that are formed in the course of diagenesis . They consist of concentric cones of fibrous calcite , more rarely of plaster of paris , siderite or pyrite, nested with one another and one inside the other . The nested cones turn in layers towards the tips and are often crossed to one another. There are many theoretical approaches to explain their origin, whereby a displacement mechanism through recrystallization , the timing of which is still controversial, is most likely.

history

Cone-in-cone structure in lime

Cone-in-cone structures, derived from the English cone for cone (literally cone in cone ), have been known since the study by D. Ure (1793). The scientific term was probably first used by William Lonsdale in 1832 and was in use no later than 1860 under Henry Clifton Sorby .

Attempts have been made to unravel the origins of cone-in-cone structures since the very beginning. Up until the beginning of the 19th century, the structures were even considered to be fossils ( Cophinus dubius ). Strangely enough, one of the very early inorganic explanations agreed with the now accepted model of displacement. In Shaub (1937) the models that were common up to that point were presented.

At the time, the prevailing view was that cone-in-cone structures were created through volume reduction (shrinkage) - a very slow process that took place with water being pressed out in poorly packed, unconsolidated, water-saturated sediment.

For his part, Shaub suggested that partially developed conical surfaces also adjust automatically through the overload pressure of the overlying sediment, as long as lateral pressure relief is ensured. Shaub also pointed out that the explanatory models of his time were inadequate and also questioned the efficiency of the crystallization pressure in the growth of the structures, which called the currently accepted displacement model into question.

Description and occurrence

Cone-in-cone structures in lime

Cone-in-cone structures are actually pretty distinctive with their nested cone arrangement. The composition of the cones is variable and depends on their educational environment. The vast majority of cone-in-cone structures are made up of calcite, with the individual calcite cones being separated from one another by fine layers of clay. Cones made of aragonite , gypsum, anchorite , siderite, sideroplesite , pistomesite or pyrite are rarer . The interior of the cone usually consists of long carbonate fibers (aragonite or calcite) or undulant single crystals, the c-axis of which practically coincides with the cone axis and is approximately perpendicular to the stratification. The limestone finely divided in the marl may have served as the carbonate source. Often the surface of the cone also has a finely stepped transverse wrinkle (English cone step ) and occasionally the inside of the cone protrudes like a nail head (hence the name nail limestone ).

Cone-in-cone structures are usually arranged in layers or lenses, the thickness of which can vary between 1 and 15 centimeters (rarely up to 50 centimeters). The layers usually start from limestone areas such as Schilllage and open from these against the adjacent rock. The cones themselves reach heights of several millimeters to (rarely) 20 centimeters. The opening angle of the cones varies between 15 and 120 °, whereby the individual cones in sandstones are flatter (70 to 120 °) than in marls (15 to 70 °). However, cone-in-cone structures can also be placed around concretions , with the cone tips pointing in the direction of concretion.

Cone-cone-in-structures occur worldwide mainly in limestone and marl on, such as in named after them Nagelkalken and tooting marl . Also in calcitically cemented sandstones or as limestone layers in shale . In Dedolomites (decalcitized Dolomites ) they are rare. Occasionally they even appear in coals . The structures are known from the beginning of the Mesoproterozoic ( Riphäum or Calymmium ). In some positions of the Phanerozoic they occur so frequently that they are used stratigraphically for correlation purposes.

Likelihood of confusion

Cones of rays from the Charlevoix crater in Quebec, Canada

There is a certain risk of confusion with the cones of rays that arise during impact events . In contrast to the radiation cones of the cone tips, however, do not show Cone-cone-in structure according to above, but as well as vertically downward into the lying with deviations of 5 to 15 ° from the vertical. Their persistent welts (striae) run parallel in contrast to the diverging welts in cones of rays.

Emergence

Very large cone-in-cone structure in the limestone marl of the Ligérien , Dordogne, France

Explanations for the development of cone-in-cone structures can be assigned to five topics:

  • Volume increase through the conversion of aragonite to calcite, with the original aragonite cones spreading open and apart, thereby allowing clay to penetrate
  • Pressure release in the sediment due to increasing load, insoluble clay residues remain
  • Breaking of mineral aggregates in areas of high load pressure, whereby cracks can appear due to the decreasing pore pressure
  • Formation in the early diagnostic stage through extensive mineral growth - the growth of the crystals creates pressure directed sideways into the sediment, cone-shaped aggregates of fibrous calcite form, which move the originally clay-rich sediment, deform it and concentrate it into clay layers
  • According to Gilman and Metzger, cone-in-cone structures form due to the growth of fibrous aragonite, which displaces the still plastic clay. This interpretation is very similar to the displacement mechanism (English displacive crystal growth mechanism ) listed in the previous point , which now seems to find general approval as an explanatory model for cone-in-cone structures.

It can be assumed that crystal growth starts in the partially solidified sediment. As the cone-in-cone structure grows, it then requires more and more space in the sediment and thus exerts pressure on its surroundings. The differential pressure, in turn, is responsible for the ultimate conical shape of the structure, which spreads out against the surrounding pressure gradient.

This displacement mechanism due to the crystal growth suggests that the majority of the precipitation should have taken place very early and not too deeply. However, depleted 18 O values ​​in the cone structures also allow the conclusion that they grew much later and below several hundred meters of sediment cover.

Conclusion

Even after more than 200 years, there is still no satisfactory, comprehensive explanation for cone-in-cone structures for their formation. This is not surprising since the structures can be designed very differently and variably. In addition, the physico-chemical processes in the freshly deposited sediment, including diagenetic compaction, are very varied and complex in nature. The structures are likely to result from the interplay of the following factors:

  • Chemism of the sediment and the surrounding sea water
  • Temperature conditions
  • Hydroplastic behavior taking into account pore water and load pressure (pressure solution)
  • Tectonic influence (crack formation, offsets, stylolithization)

Kolokol'tsev (2002) provides an interesting new approach based on Ilya Prigogine's theory of self-organization . Accordingly, cone-in-cone structures are dissipative structures that spontaneously set up under a temperature gradient at the expense of heat and mass transfer. Strictly speaking, this is a metasomatic hypothesis based on convective fluid exchange.

Examples

Examples of cone-in-cone structures can be found at the following locations:

See also

Individual evidence

  1. ^ Ure, D .: The history of Rutherglen and East Kilbride . David Niven, Glasgow, UK 1793, p. 334 .
  2. ↑ a b Kolokol'tsev, VG: The Cone-in-Cone Structure and Its Origin . In: Lithology and Mineral Resources . tape 37 , no. 6 , 2002, pp. 323-335 .
  3. ^ Lonsdale, W .: Series 2. On the Oolitic District of Bath, vol. 3 . In: Transactions of the Geological Society . London 1832, p. 241-276 .
  4. ^ Sorby, HC: On the origin of "cone-in-cone" . In: Transactions of Sections, Geology . British Association for the Advancement of Science, Report of the 29th Meeting, 1860, pp. 124 .
  5. ↑ Shaub, BM: Origin of Cone-In-Cone and its Bearing on the Origin of Concretions and Septaria . In: American Journal of Science . 1937, p. 331-344 .
  6. ↑ Mozley, P .: diagenetic structures . Ed .: Middleton, GV, Encyclopedia of Sediments and Sedimentary Rocks. Kluwer Academic Publishers, Dordrecht 2003, p. 219-225 .
  7. ^ Carstens, H .: Early diagenetic cone-in-cone structures in pyrite concretions . In: Journal of Sedimentary Petrology . tape 55 , 1984, pp. 105-108 .
  8. ^ Franks, PC: Nature, origin and significance of cone-in-cone structures in the Kiowa formation (Early Creataceous), north-central Kansas . In: Journal of Sedimentary Petrology . tape 39 , 1969, p. 1438-1454 .
  9. ↑ Kowal-Linka, M .: Origin of cone-in-cone calcite veins during calcitization of dolomites and their subsequent diagenesis: A case study from the Gogolin Formation (Middle Triassic), SW Poland . In: Sedimentary Geology . tape 224 , 2010, p. 54-64 .
  10. ^ French, BM: Traces of catastrophe . Lunar and Planetary Institute, 1998, p. 36-40 .
  11. ^ A b Fairbridge, RW and Rampino, M .: Diagenetic Structures . In: Middleton, GV (Ed.): Encyclopedia of Sediments and Sedimentary Rocks . Kluwer Academic Publishers, 2003, ISBN 1-4020-0872-4 , pp. 219-225 .
  12. ^ Gillman, RA and Metzger, WJ: Cone-in-cone concretions from western New York . In: Journal of Sedimentary Petrology . Volume 37, 1967, p. 87-95 .
  13. ↑ Lugli, S., Reimold, WU and Koeberl, C .: Silicified Cone-in-Cone Structures from Erfoud (Morocco): A Comparison with Impact-Generated Shatter Cones . In: Koeberl, C. and Henkel Impact Tctonics (eds.): Impact Studies . Vol. 6. Springer, Heidelberg 2005, ISBN 3-540-24181-7 , pp. 82-109 .
  14. ↑ Woodland, Bertram G .: The nature and origin of cone-in-cone structure . In: Fieldiana: Geology . Volume 13, No. 4 . Chicago Natural History Museum, 1964.