Dropstone

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Dropstone, Itu , Brazil

A dropstone (English from to drop : to fall and stone : stone) is an isolated rock fragment from pebble to block size that has come to be deposited within fine-grained sediments . The deposition method is characterized not by normal transport along the sea or lake bed, but by a vertical fall through the body of water.

background

Evidence of a rock fragment being deposited as dropstone is its isolated occurrence in fine-grained sediments, along with a depression on the underside created by the impact, and evidence of ejected material on the edges of the fragment. Subsequently deposited sediment material covers the stone and its crater without disturbance. These signs are most clearly seen in finely stratified sediment.

Mechanisms of origin

While deposition from the melting or sliding of rock from icebergs or glaciers has been viewed by many geologists as one of the most common causes of the formation of dropstones, dropstones are created by a whole range of sedimentary processes.

Glaciers and icebergs

Glaciers absorb rocks from the subsurface as they flow and are often showered with rock debris. When they get to the coast, glaciers begin to calve and icebergs peel off their foreheads. These can drift for a long time, slowly melting or shifting so that rock debris sinks into the water and falls into the mud on the bottom.

Unlike attachments , in Till ( boulder clay are common), the deposition is done by glacial always drop Stones in the water, either in the sea (marine) or lakes (lakustrisch). Typical of such dropstones is the often very fine-grained sediment in which they occur, and which indicates a deposition environment that is not very energetic. Examples of such glacial dropstones are known from the sea off Greenland.

Volcanoes

During volcanic eruptions , blocks of stone can be thrown several kilometers through the air as volcanic bombs by the force of the eruption. When these hit a lake or the sea, they sink to the bottom and are trapped in the sediment as dropstones.

Dropstones produced in this way are relatively rare, as most of the ejected stones end up on land or near the coast, where it is unlikely that they will be preserved. Only with larger eruptions do rocks fly far enough to land in fine-grained sediment and be recognizable as dropstones.

Turbidite

Dropstones can be created by the effects of strong turbid currents on the sea floor, which lead to the formation of turbidites .

Man-sized blocks have been found in geologically young, finely stratified deposits from the Eocene near Jamaica , which has not been covered by glaciers since its formation. Although turbidity currents are believed to be the cause of the deposition of these blocks, no other deposits were found in the vicinity caused by the turbidity currents.

Driftwood and other biological causes

Stones can travel long distances as components of floating wood and plant masses that have been transported into the sea by rivers, for example. If the vegetable raft dissolves, the stones also sink to the bottom. In the vicinity of such dropstones, plant remains can usually be found that sank to the ground with the stone. Finally, dropstones can also come from the stomach contents of larger marine life.

Individual evidence

  1. ↑ JL Kirschvink: Late Proterozoic low-latitude glaciation: the snowball Earth. In: JW Schopf, C. Klein (Ed.): The Proterozoic Biosphere. Cambridge University Press, Cambridge 1992, pp. 51-52.
  2. ^ Paul F. Hoffman et al.: A Neoproterozoic Snowball Earth. In: Science. Volume 281, No. 5381, August 28, 1998, pp. 1342-1346. Retrieved April 19, 2008.
  3. ^ A b N. Eyles, N. Januszczak: 'Zipper-rift': A tectonic model for Neoproterozoic glaciations during the breakup of Rodinia after 750 Ma. (en., pdf 4 MB). In: Earth Science Reviews. 65 (1-2), 2004, pp. 1-73. Retrieved April 19, 2008.
  4. ↑ JS Eldrett, IC Harding, PA Wilson, E. Butler, AP Roberts: Continental ice in Greenland during the Eocene and Oligocene. In: Nature. 2007 ( short version ). Retrieved April 19, 2008.
  5. ↑ SK Donovan, RK Pickerill: Dropstones: their origin and significance: a comment. In: Palaeogeography, Palaeoclimatology, Palaeoecology. Volume 131, No. 1-2, 1997, pp. 175-178. (Brief overview: doi: 10.1016 / S0031-0182 (96) 00150-2 ). Retrieved April 19, 2008.
  6. ^ MR Bennett, P. Doyle, AE Mather: Dropstones: their origin and significance. In: Palaeogeography, Palaeoclimatology, Palaeoecology. Volume 121, No. 3, 1996, pp. 331-339. (Short version: doi: 10.1016 / j.physletb.2003.10.071 )

Web links

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