Neptunian Dike

Neptunian dikes (Neptunian fissures, Neptunian corridors) are former fissures of the sea ​​floor filled with sediment , which occur mainly in carbonate rocks .

Word origin

The word neptunian comes from English and means 'submarine'. It relates to the submarine formation of the crevices and their fillings. Today it is mainly used for carbonate rocks. Initially, the combination neptunian dike was used in 1896 by Alexis P. Pavlov for sedimentary channels of the Lower Cretaceous , which appear as sandstone entrances in clay sediments.

Form and spread

Neptunian dikes are usually perpendicular or at an angle to the layering , but they can also appear parallel to the layer.

The width of Neptunian dikes is in the centimeter to meter range. They can extend several meters, in some cases a hundred meters or more, and the length can be several kilometers. The walls of the columns are parallel to each other and have a straight to undulating surface. In some cases there is only one column, in other cases there are whole sets of columns. The crevices often occur in solidified surfaces ( hardgrounds ) under geological discordances .

The crevices are filled with new sediment soon or for some time after their formation . This can be done under water (mud or similar filler material) as well as in the open air (e.g. dune sand ). Submarine crevices are often tied to carbonate platforms, but they can also arise on the continental slope or in basins .

Formation of the columns

The crevices that are later filled with sediment can be divided into four classes: primary crevices, synsedimentary crevices, epigenetic and tectonic crevices. The mechanisms are naturally almost the same as those for the formation of crevices in sedimentary dikes .

Primary crevices form mainly around a coral reef . They correspond to the cavities and crevices between large blocks of coral debris or large reef formers. They are mostly filled with the debris that results from the erosion of the reef.

Synsedimentary crevices are formed during sedimentation, primarily as a result of sediment slipping on inclined subsoil (slumping), often as a result of the vibrations caused by earthquakes . Furthermore, crevices can also arise from different settlements of sediment under a cover layer or over basement structures. Another mechanism for the formation of crevices is the drying up of shallow sea areas or lakes and the formation of dry cracks .

After deposition and solidification, epigenetic fissures can arise from karstification or dissolution processes on the earth's surface.

Tectonic fissures are mainly formed by an expansion of the subsoil, mostly as a result of block tilting and disturbance activity during the deposition of sediments. A special form is the injection of sediment material as a result of increased pore water pressure from below through the stratification or laterally into the space between strata surfaces.

filling

Neptunian corridor (foil print), Rübeland cave restaurant

Neptunian dikes are filled with sediments such as clay or sand and fossil remains, and there are usually different generations of cement .

Sometimes the filling is colored differently than the older flanking rock in which they occur, making them easily recognizable. In other cases it can hardly be distinguished from the surrounding rock and requires examination of thin sections under the microscope , or a precise analysis of the fossil contents . When the water flows through the crevice at high speeds, ripple marks and similar sedimentary structures can develop.

The fillings of submarine and onshore crevices can be distinguished on the basis of characteristic features. For example, crevices created and filled on land do not contain marine fossils, in some cases contain remains of soil formation , and the type of cement indicates that it was formed on the free surface of the earth.

Most of the time, a sediment dike does not change after it has been formed and filled. In rare cases, however, the filling of the gaps can develop further by tearing open again, so that several generations of filling material can occur. More frequent, however, is the change caused by loosening and karst processes when the rock is on the surface.

Fossil guide

On the walls of the crevices, which consist of solidified sediment or solid rock, sometimes stuck aquatic organisms such as brachiopods or algae lived , which were later spilled by mud. In most cases, however, the fossilized animals were washed up dead in the crevices.

In some fissures, the long-term deposition of fossils such as ammonites , mussels and snails have formed real fossil deposits. The fossils are then often present without the intermediate sediments present outside the crevice; the sequence that can be verified on the basis of the fossils is then present in a greatly shortened sequence (condensed profile). In the filling of the crevices, microfossils are usually to be found that were alive at the time this filling was deposited and often represent the only evidence of a younger age of the filling.

Occurrence

Neptunian dykes are common. Examples of Neptunian Dykes can be found in the Dachstein limestone in the Eastern Alps or in the Elbingeröder complex in the Harz Mountains . Further examples are known from Devonian limestones in Australia or from reef complexes in Permian sediments on the American Gulf coast and off Mexico. Perhaps a modern example of Neptunian Dykes are some of the so-called Blue Holes of the Bahamas.

literature

• Erik Flügel: Microfacies of carbonate rocks. Springer Verlag, 2004, ISBN 9783540220169 . Pp. 217-223: Sediment Filled Fissures: Neptunian Dikes and Fissure Fills.

References and comments

1. ↑ predominantly used in English in German-language texts.
2. ↑ Collective of authors: Lexicon of Geosciences, Volume III, Spectrum Akademischer Verlag, Heidelberg, Berlin 2000, ISBN 3-8274-0422-3 , p. 465.
3. ↑ Wing, p. 217.
4. ^ AP Pavlov: Dikes of Oligocene Sandstone in Russia. Geological Magazine, Vol. 3, February 1896, pp. 49-53.
5. ^ Ned Frost, Dan Carpenter and Charles Kerans: Platform-Margin Trajectory as a Control on Neptunian Dike Distributions, Devonian Reef Complexes, Canning Basin, Western Australia. ( Memento of the original from December 3, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Fractured Reservoir Controls, 2006 AAPG International Conference and Exhibition, (November 5-8, 2006) Technical Program, Lecture Summaries.