Tuesday, January 14, 2014

Quote: Werner Breuckner On Cyclic Carbonates

JSR Paper Clips highlights a paper from 1953 on cyclic carbonates with a memorable quote from Werner Breuckner:

“When a detailed study of limestones is made, their interpretation becomes more difficult.”

The 1950's was the time when such sentiments were apt as far as carbonate sediments were concerned. Giant oil deposits in carbonate rocks were being discovered in Texas, Gulf of Mexico and the Middle East. Research on carbonate rocks took on a new focus and urgency.

One can get a view of the importance of this new field in sedimentary geology using Google Ngram Viewer which tracks frequency of word usage in published books. I have used the terms "carbonate platforms" and "sedimentary petrology" to illustrate how the field of carbonate sedimentology gained significance from the late 1950's onwards. 

Werner Breuckner's paper abstract also says something interesting about the state of the science:

Cyclic variation in the CaCO 2 content of Cretaceous calcareous sediments in the Helvetic zone of the Swiss Alps is attributed to temperature changes in the waters of the depositional basin caused by climatic variations, rather than to alternating uplift and subsidence of the basin floor. The significance of cyclic calcareous sedimentation for stratigraphic correlation, as well as paleogeographic and paleoclimatic investigations, is noted. 

He suggests that cyclicity is due to periodic changes in temperature and associated chemical conditions in sea water rather than the alternative of uplift and subsidence of the basin floor. Periodic tectonic movements can lead to depositional cycles but the time scales involved are generally in the hundreds of thousands of years at least. Many carbonate sequences show cyclicity of a much higher frequency measured in tens of thousands of years. 

Today, different causal mechanisms are invoked to explain deposition of these higher frequency cycles. Among them, especially relevant to shallow water intertidal and shelf areas is the autocyclic model of carbonate deposition (Ginsburg 1971) wherein feedbacks between sediment production and accumulation, current and sediment dispersal results in a kind of a playback loop wherein environments of deposition at certain time intervals keep shifting their positions resulting in one burying the other. The result is a repeated  stacking of the same facies sequence or a depositional cycle. The other mechanism is sea level fall and rise due to glacial and interglacial conditions. It was not until the early mid 1970's that geologists began started taking seriously the link between ice ages and carbonate cyclicity, prodded to some extent by J. D. Hays, John Imbrie and N. J. Shackleton's classic paper Variations in the Earth's Orbit: Pacemaker of the Ice Ages. This now has been used to explain the origin of many cyclic carbonate sequences.

Walter Breuckner in the 1950's though was working on a frontier area when a detailed look at carbonates opened up more questions than answers.

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