Limestone Turbidites, Zebra Quartz, and Zebra Dolomite

The 2001 OUGS Symposium in Dublin saw a so far unseen number of post symposium field trips. Four field trips to choose from on Monday, and a field trip on Tuesday to a limestone quarry as well as a field trip on Wednesday to the mining district of Avoca. Hereby a few impressions from the field trip to Loughshinny on Monday.

Chevron folds in the Lougshinny Formation (Viséan) a short walk south from Loughshinny Harbour. This compressional folding is of Variscan (or Hercynian) age (c. 290 million years ago).

If the rocks hadn't been folded they might have looked a bit like the rocks to the right. The picture is from quite another location, namely County Clare, Western Ireland, taken from a boat on the Shannon Estuary. It shows turbidites (interbedded with mudstones/siltstones) from the Ross Sandstone Formation Turbidite system of Namurian age, and thus a little bit younger than the Loughshinny formation. The sandstone beds were formed in a deep basin by turbidites coming from a delta area.

The beds above, however, are limestones interbedded with black shales, and the limestone beds are referred to as limestone turbidites.

Limestone turbidites

These limestone turbidites were formed by transport of shelf-derived carbonate sediment downslope towards the basin floor. As can be seen on the picture to the left, they are graded with well-developed internal lamination.

What you see is in fact a fining upward (partial) Bouma sequence, consisting of a lowermost layer of coarse, chaotic clastic sediments (A: massive) overlain by successively finer grained and better stratified sediments with parallel lamination above the coarser grains (B: parallel laminated) and cross-stratification higher up (C: rippled).

Further north - to the north of the Coppermine Fault we saw

Zebra quartz

Banded quartz-filled vugs in bedded platform limestones (also Lougshinny Formation) extensively dolomitized.

And to stay with zebras and dolomite we also saw

Zebra dolomite

- not very spectacular, but still recognizable.

 

Ole Nielsen

Thank you Ireland Branch for some wonderful field trips organized under so difficult circumstances (read Foot and Mouth Disease problems).

In 2002 the OUGS Symposium will be held at Norwich - Fossils: Curios or time machines - 5-7 July.

In 2000 the OUGS Symposium was held at Telford. See Global Worry at Local Quarry.

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