Tohora Tahuhu (Otago) EarthCache
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The cache translation is: Whale Skeletons
Four main geological divisions may be made in the Otago Land District. North-west Otago consists of steeply eroded alpine ranges of Fiordland-type rocks. Central Otago is underlain by peneplaned schists and is a range-and-basin terrain with flat-topped or rolling block-faulted mountain ranges, separated by broad alluviated basins. South-east Otago is low rolling country with elongated north-west – south east fold ridges of Upper Paleozoic and Mesozoic sedimentaries. Limestone is one of these sedimentaries which gives this spot its unique features.
By far the most extensive area in Otago is underlain by chlorite schists, forming the basement to the east and outcropping over much of Central Otago. They are high stress metamorphic rocks, originally greywackes and argillites of the New Zealand Geosyncline. Beds of quartzose conglomerate, sand, and white clays were laid down by slow-flowing rivers and in shallow inland lakes. As inland seas receded limestone deposits, many containing shellfish, dolphins, and whales from the period, were left behind, as is the case at this site.
Fossilisation occurs in a number of different ways:
Permineralization - Permineralization occurs after burial, as the empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater and the minerals precipitate from the groundwater, thus occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce very detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death or soon after the initial decaying process.
Casts and molds - In some cases the original remains of the organism have been completely dissolved or otherwise destroyed. When all that is left is an organism-shaped hole in the rock, it is called an external mold. If this hole is later filled with other minerals, it is a cast. An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a cranium.
Replacement and recrystallization - Replacement occurs when the shell, bone or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material.
Compression fossils - Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state. Often what remains is a carbonaceous film. This chemical change is an expression of diagenesis.
Bioimmuration - Bioimmuration is a type of preservation in which a skeletal organism overgrows or otherwise subsumes another organism, preserving the latter, or an impression of it, within the skeleton. Usually it is a sessile skeletal organism, such as a bryozoan or an oyster, which grows along a substrate, covering other sessile encrusters.
Source: Te Ara & Fossils of New Zealand (2000).
Logging Requirements:
Email the answers to me. You may log the find before I respond. Any logs without answers may be deleted without consultation.
1. During what period was South Otago submerged under the sea?
2. What part of the fossilised whale is on show?
3. When did the deposits of lime containing the fossils form?
4. What other mineral was mined at nearby Clarendon?
5.OPTIONAL: Take a picture of yourself or GPSr looking toward Lake Waihola
Additional Hints
(Decrypt)
Rawbl!