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Geology Summary - August 3, 2005
Tiziana Trabucchi Reporting
During our EVAs on the Island, we saw Limestones, Dolomites with fossils belonging to Allen Bay Fm. The rocks from this formation look like chemical-organogenic marine Limestone. CaCo3 in this type of Limestone is usually Aragonite, it comes from a large number of organisms that build different types of structures. In this way a coral barrier reef can be generated and this seems to be true for many of the Limestones-Dolomites present on the Island. The genus that the palaeontologist (TT) has identified the first day, is Halysites and the range is M.Ordovician-Lower Silurian. This kind of coral colony could live in a reef habitat. The majority of Limestones and Dolomites present on the island are crushed, deteriorated, dissolved and recrystallized. Sometimes it is possible to see compact banks of Limestones Dolomites arranged in strata, like in the Devo Rock case.
Regarding the dolomitization process, it can be caused by different processes the most important being the action of sea water on the Limestones of the reef. There is a Magnesium enrichment due to the reaction between the magnesium salt dissolved in sea water and calcium carbonate of the reef organisms. Very often, even in small bits of these limestones, it is possible to find fossil fractions and in many cases coral tabulae could be observed. In other occasions and often in the circular voids present in the rocks a small coral or part of it could be found and that was enough to imagine the original presence of a whole coral colony. The majority of the corals observed were part of a colony but in some cases it was possible to distinguish single corals. Many other organisms could also be observed and in particular: many algae laminar structures, sponges, ammonites, one crinoid columnar stem, bivalves, nautiloids, actinostroma(stromaporoidea), some others have to be further investigated for a positive identification: crustacea but anyway arthropods, coral algae(rodophite).
Some Limestones were found in association with chert most probably as a part of the formation "Eleanor River Fm". In the area located in the SW of the hab (an area previously called impact breccia and more recently impact melt breccia), two outcrops (20 m from one another) of evaporites were found, probably gypsum/anhydrite. Both the outcrops (probably connected) are in a subvertical position . They seem to be evaporites emerged to the surface due to their lower weight in respect to the contiguous areas. The possible faults interaction in the underground could have also influenced the diapiric emersion of the evaporites. Still in the same area some gray rocks were found that reacted to HCl (and so they showed a carbonatic nature) and looked vaguely like argillaceous schists but they were not. In many cases these samples presented some "fan lineations" that resembled shatter cones. Thin sections will be prepared to verify that possibility. The amount of of shatter cones found was a bit surprising, considering the age of the impact. It was also discovered (with the help of a magnifier) that these rocks are full of micropores.
From the "pedological" point of view, a lot of observed soils presented a poligonal pattern and various shades of colors, due to the presence of Iron hydroxides, like for example Goethite or Limonite.
A part from some rare cases of rocks strata the majority of the rocks are laying down in a chaotic way, their size varies a lot and it doesn't depend on their location. Generally these fragments have an angular shape. All the rocks present a lot of fractures (joints). The nature and appearance of the fractures gives a strong indication of the fact that a mechanical action "crioclastismo e termoclastismo" (freeze-thaw process) caused most of these fractures.
A granular disruption, observed in some cases, reinforced the idea of this mechanical process.
Iron and/or nickel minerals were extensively searched but never found. Some rock fragments with a vivid red encrustment were found and thought to be Realgar (As S).
An x-ray portable spectrophotometer (XRF) was also used and it helped to detect the presence of Calcium (in the majority), Manganese (in some) and Strontium (very few) in the samples analysed.
A lot of collected samples will be further analysed in specialised laboratories.
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