Blackened Limestone Pebbles: Fire at Subaerial Unconformities

Abstract

Introduction

Field Observations & Some Experimental Results

Discussion

Submarine Blackening & Accumulation of Salt-and-Pepper Sands

Conclusions

Acknowledgements

References

Introduction

Dark grey to black, irregularly shaped limestone pebbles of millimeter to centimeter size occur at subaerial disconformities and unconformities in limestones of Tertiary and Holocene age throughout the Caribbean. The origin of these pebbles has long been a puzzle to local geologists. In south Florida, many black pebbles are fragments of soilstone crusts often called calcrete¹, but they also include blackened coral, lightly lithified grainstones, or multicomponent limestones. Most blackened pebbles occur in a matrix of similar but unblackened irregularly shaped pebbles. These multicolored breccias are generally concentrated in karst potholes. The intimate mixture of blackened and unblackened irregularly shaped pebbles is puzzling, because an underlying or adjacent grey or black limestone source is always absent, and irregular shapes argue against extensive lateral transport. Although it has been argued that the black pebbles are erosional remnants of a missing overlying bed, we believe they are produced essentially in situ by forest fire-induced heating. Such an interpretation may have useful implications for limestone containing black pebbles in ancient limestone, especially in the Mesozoic, where they are extremely common yet often go undescribed. We point out, however, that there is also a well-documented subtidal blackening process which affects molluscs and produces the familiar "salt-and-pepper" sands phenomenon. Subtidal blackened fossils and sand-size carbonate grains often accumulate at disconformities and unconformities (either submarine or subaerial unconformities subsequently drowned by rising sealevel) and can easily be confused with those of subaerial fire origin. Correct distinction may therefore provide useful clues for identification of subaerially exposed unconformities and disconformities in the geologic record.

In this chapter, we will describe some observations and simple experiments relating to the origin of black pebbles. Later, we will discuss the origin of subtidal blackened grains and fossils, show how they are often mixed with subaerially formed black pebbles, and discuss the dangers of misidentifying their origin. Furthermore, we note that geochemical analysis is usually equivocal for distinguishing between subtidal and subaerially produced black pebbles, grains, and fossils.

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