17 December, 2008

A carbonate tempestite?

UPDATE: I managed to make a polished section of this specimen and in order to avoid confusing my visitors I would like to point out that this is not a tempestite, as previously thought, but most likely nothing more than a coarse sheet of unconsolidated and resedimented lime sand.

Today while measuring joints in a quarry I also took a few samples. One of the samples I took includes a wonderful shell horizon with upward decreasing grain size and an erosive base that I interprete as a storm deposit/tempestite. To let you form your own opinion I included both the edited and unedited image.

without comment

image including description

You will notice the sharp base contact and the concentration of shells and larger grains towards the lower contact and a decrease in grain size towards the top. While the lower part is a relatively finer-grained oolite (average 0.5mm) with only few bioclasts the storm deposit is composed of larger grains (2mm and more), mainly shell fragments, oncoids and larger ooids, few aggregate grains and few crinoid columnals among other, unidentifyable bioclastic debris. At the base I believe to recognise a clast of the underlying finer-grained oolite. Unfortunately I was unable to also extract the uppermost centimeter of this. The varied composition of the coarser grained layer leads me to assume that most particles were transported into the site of deposition from other parts of the carbonate platform.

Tempestites form when up-stirred sediments is deposited from storm aggitated water when the storm subsides or the suspension reaches water below the storm-wave base. That's very generalised of course and you may find more information also on how to distinguish tempestites from turbidites here.

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