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

Submarine Blackening and Accumulation of Salt-and-Pepper Sands

Intertidal and subtidal blackening of fossils and other carbonate materials has been reported from numerous areas of Holocene deposition (for example, Van Straaten 1954, Ginsburg 1957, Houbolt 1957, Bush 1958, Sugden 1966, Macintyre 1967, Maiklem 1967, Shinn et al. 1969, Pilkey et al. 1969, Shinn 1973, Wagner and van der Togt 1973). On the basis of equivocal data, these investigators have cited both organic matter and/or iron or manganese sulfide as the blackening agent. Pilkey et al. (1969) proved the necessity of reducing conditions in an experiment in which modern and fossil shells from the beach were buried in a North Carolina salt marsh. In just three weeks, most modern shells turned black. Fossil shells did not blacken. Shells placed in distilled water through which H₂S was bubbled also turned black. Color was thought to be caused by reduced organic matter.

Subtidally blackened grains and fossils often occur together as a kind of salt-and-pepper accumulation that may or may not include crossbedding. Salt-and-pepper sands commonly accumulate just above a contact, whether it be a diastem produced by submarine hardground formation or a drowned subaerial unconformity. Laterally migrating tidal channels erode blackened grains from their outer banks and redeposit them with unblackened counterparts, usually as a lag deposit above an underlying unconformity. Some channels that do not reach underlying unconformities produce their own diastems as they migrate.

Migrating mudbanks also cause mixing of blackened and unblackened materials. Fischer (1961) and PiIkey et al. (1969) showed how blackened grains concentrate as a result of landward migration of barrier islands along the eastern United States.

The blackened grains in Table 5.2 (Florida Bay near Crane Key and the Marquesas core samples) are considered to be of subtidal origin. Recognizing that blackening can occur both above and below the water, it seems important, then, that the two different kinds be distinguishable. Correct identification of subaerial fire-blackened pebbles on an unconformity could be useful for recognizing subaerial unconformities. Recognition of marine black grains may accurately distinguish submarine diastems. Difficulties are likely to arise, however, when a subaerial unconformity is inundated by a marine transgression, thus mixing two kinds of blackened material together. The following criteria are considered useful for separating the two.

1. Most grains are rounded and predominantly in the sand-size range.
2. Grains are often whole and skeletal, such as foraminifera, or they may be broken fossil fragments, such as molluscan pieces, all imbedded in a light-colored matrix.
3. Blackened grains occur with unblackened counterparts, as do subaerial pebbles, but they are in the sand-size range and produce a mixture resembling salt and pepper.
4. They frequently occur above an unconformity or diastem. Subaerial black pebbles also occur above unconformities but not diastems.
5. Accumulations are graded (fining upward) and the ratio of black to white grains decreases upward.
6. Black grains are current-sorted and sometimes cross-laminated.

1. Black material is usually in the pebble-size range.
2. Blackened lithoclasts are generally angular and some show evidence of having fractured in situ.
3. Black pebbles often occur in multicolored breccias with no particular grading.
4. Black pebbles may occur at the top of pedogenic sequences, especially soilstone crusts. Crusts may contain patchily distributed, millimeter-thick black layers.
5. Some individual blackened pebbles may show gradation of blackening from white through grey to black.

Many of these criteria overlap; therefore, the entire sedimentary sequence should be considered when making an environmental interpretation. Since present chemical analyses are of little use in correctly identifying the environment or even the cause of blackening, it is proposed that more detailed study is needed. A simple chemical technique, once developed, could be invaluable for distinguishing subaerial and subtidal environments.

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