Blackened Limestone Pebbles: Fire at Subaerial Unconformities

By Eugene A. Shinn and Barbara H. Lidz

This article was published in Paleokarst pages 117-131, published by Springer-Verlag 1988.

Abstract

Introduction

Field Observations & Some Experimental Results

Discussion

Submarine Blackening & Accumulation of Salt-and-Pepper Sands

Conclusions

Acknowledgements

References

Abstract

Irregularly shaped blackened limestone pebbles mixed with similar but unblackened material at unconformities have long presented a mystery to geologists examining Tertiary and Holocene limestones in the Caribbean. How can irregularly shaped limestone pebbles showing no signs of lateral transport be mixed together, especially when a distant or underlying source is invariably absent?

The blackened pebbles generally are composed of soilstone crust, lightly lithified grainstone, or multicomponent limestones, and may occur at subaerial unconformities in marine or subaerial (eolian) limestones. The most common examples occur as multicolored breccias in karst potholes, which are abundant throughout the Caribbean.

We propose that selective blackening is caused by "instantaneous" forest fire heating. Simple experiments showed that thorough blackening can occur in one-half hour at temperatures between 400° and 500° C. Heating experiments showed that only those limestones that are commonly black in nature blackened when heated to temperatures similar to those of forest fires. Blackening, restricted mainly to individual sand-size skeletal grains, also occurs under subtidal conditions, and the product of this process should not be confused with fire-blackened limestone pebbles. Correct identification can be useful for distinguishing between submarine diastems and unconformities and subaerial unconformities in ancient limestones.