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

Field Observations and Some Experimental Results

Black pebbles can be found along subsurface Pleistocene unconformities in south Florida (Perkins 1977) and the Bahamas (Beach and Ginsburg 1980). They also occur on the Holocene subaerial unconformity² presently forming on the surface of Pleistocene limestones throughout south Florida and much of the Caribbean. These unconformities, easily mistaken for diastems, invariably are marked by the presence of soilstone crusts or calcretes (Kornicker 1958, Multer and Hoffmeister 1968, Barthel 1974, Perkins 1977, Robbin and Stipp 1979, Beach and Ginsburg 1980, Pierson 1982, Strasser 1984, and Williams 1985).

The most common black pebbles consist of angular fragments of soilstone crusts (Figs. 5.1 A, B, C). They often occur cemented within unblackened in situ soilstone crusts, as shown in Figure 5.1B, or just above such crusts (Fig. 5.1 A). The more striking examples occur in solution holes where they are imbedded in a cemented multicolored breccia composed of unblackened soilstone pebbles, both blackened and unblackened limestone fragments, corals, and mollusc shells (Figs. 5.2A, B). The vertical walls of these solution pits are often lined with laminated reddish-brown soilstone crust 1 to 3 cm thick, which is usually continuous with adjacent horizontal crusts. Most black pebbles occur in association with thin soilstone crusts often sandwiched in marine limestone sequences. They also occur in young eolian carbonates 10 to 20 m above sealevel. Figure 5.3A shows black pebbles of cemented pelletal and ooid grainstone in a soilstone-lined solution pit approximately 20 m above sealevel. Figure 5.3B and C are photomicrographs showing the contact between the black pebbles and unblackened matrix in which they are imbedded. Brown in situ soilstone crusts sometimes contain discontinuous blackened laminae 1 to 2 m thick (Fig. 5.4). Significance of these black laminae and their relationship with black pebbles will be discussed later.

Figure 5.1A. (Top) Poorly cemented soilstone crust with blackened lithoclasts from Ramrod Key, Florida. Note shading from dark to light in lithoclast above scale. Matrix is angular, light tan to rusty brown in color. 5.1B. (Middle) Angular blackened lithoclasts in brown laminated soilstone crust from Ramrod Key, Florida. 5.1C. (Bottom) Laminated soilstone crust exposed at edge of artificial canal on Ramrod Key where overlying soil has been stripped away. Thickness of crust at center of photo is about 8 cm. Top of outcrop to water level is about 45 cm. [Larger Image]

Figure 5.2A. (Top) A soilstone crust-lined solution pit exposed alongside artificial canal on Ramrod Key, Florida. Note soil and poorly cemented breccia in solution pit. Depth of solution pit is approximately 40 cm. 5.2B. (Bottom) Well-cemented multicolored breccia in solution pit from top of Q3 Unit of Perkins (1977) approximately 20 ft (6 m) below sealevel (from quarry tailings on Big Pine Key, Florida). Note soilstone crust lining at far left of specimen. Blackened fragments include soilstone crust and fossiliferous Key Largo Limestone. [Larger image]

Figure 5.3A. (Top) Blackened lithoclasts in karst solution pit breccia in cross-bedded eolian grainstone approximately 20 m above sealevel. Eolian host rock is late Pleistocene and has never been submerged. Note fractures in black pebble, indicating fracturing took place after emplacement in breccia. Length of key 6 cm. 5.3B. (Middle) Thin section of contact between oolitic black pebble and reddish-brown eolian grain stone to left. Blackening of the oolitic grainstone pebble is due to blackening of individual ooid grains. Cement is clear blocky calcite spar of vadose origin. 5.3C. (Bottom) Another pebble from the same exposure showing contact between blackened soilstone crust pebble at right and reddish-brown carbonate grainstone (highly altered by soil processes) to left. Scale bar 400 µm. [Larger image]

Figure 5.4. Detail of soilstone crust from quarry on Long Key (Florida Keys) showing normal brown laminations between two blackened laminae. Top black layer (arrow) is discontinuous. Basal black layer grades to normal brown color at right where layer enters depression in underlying rock. Unblackened portion of this layer was protected from fire in depression. [Larger image]

Table 5.1 Time, temperature heating experiments in jeweler's kiln. [Larger image]

Figure 5.5A. (Top left) Modern ooids heated for one-half hour in kiln. (a) Control, not heated. (b) Heated to 176°C. (c) heated to 400°C. (d) heated to 625°C. Note in (d) that outer surface of ooids is calcined (white color) but inner parts of individual ooids are dark grey to black. 5.5B. (Top right) Fragment of well-cemented coralline limestone (a) and Porites coral (b) heated together at approximately 400°C for one-half hour. Note only the coral blackened. 5.5C. (Bottom) Soilstone crusts: (a) heated to approximately 400°C for one-half hour. (b) Unheated. [Larger image]

Although there are also rounded and bored black pebbles that have been reworked in a marine environment, the main thrust of this chapter is to explain how irregularly shaped fractured pebbles, which clearly have experienced little transport, can become intermixed with unblackened counterparts.

Chance observations around outdoor fireplaces at campgrounds throughout the Florida Keys and the Bahamas suggested the fire hypothesis. Native limestones used for campfire containment are generally blackened and show a distinct color gradient and selectivity. They are darkest toward the fire and grade over a distance of several centimeters to their normal color. Soilstone crust fragments show the most darkening, and some limestones resist blackening.

To test whether fires could explain the multitude of shades and colors in nature, we first performed a simple experiment. Various pebbles of Pleistocene limestone, modern corals, shells, and soilstone crusts were placed in an open oil drum and covered with a 30-cm-thick layer of seaweed which was set afire. When the fire cooled a few hours later, we examined the pebbles and found that, as in multicolored breccia-fills, only certain pebbles blackened.

Figure 5.6A. (Top) Well-cemented multicolored breccia from solution pit in Pleistocene Key Largo Formation in Florida Keys. Note blackened angular lithoclasts. Also note in situ fracturing of black pebble. Laminated soilstone crust to left was part of solution pit lining. Soilstone crust is rusty brown in color. Figure 5.6B. (Bottom) Same rock as in A after heating for one-half hour at 400°C. Note darkening of fragment shown by arrow. Soilstone lining at left has turned dark grey. (Note that in black-and-white photographs, rusty brown appears the same as dark grey, thus color change from A to B cannot be fully appreciated.) Notice that some darkened lithoclasts, such as one in center of specimen, became lighter upon heating. [Larger image]

Soilstone crust blackened most thoroughly followed by coral, shells, and those poorly cemented grainstones which still contain considerable aragonite. Well-cemented Pleistocene limestones with essentially no aragonite did not blacken. To test the idea further, a series of tests was run in a jeweler's kiln. The results are summarized in Table 5. 1, and selected examples of experimentally blackened material are shown in Figures 5.5 and 5.6. Thin sections of experimentally blackened soilstone crust appear identical to natural blackened soilstone crust and pebbles (Fig. 5.7). It is impossible to distinguish any difference between normal reddish-brown and blackened soilstone crusts in black-and-white photographs. Laminae of darker (reddish-brown) cryptocrystalline material blacken more than the light brown cryptocrystalline matrix.

Figure 5.7. (Top) Thin section of experimentally blackened soilstone crust (one-half hour at 400°C). Crude laminations of originally reddish-brown cryptocrystalline calcite (color imparted mainly by organic matter) turned black, while less darkly stained portions between dark laminae only turned grey. In hand specimen, as shown in Figure 5.5C, the rock looks completely black. A thin section photo of unheated crust (control) is not shown because in black-and-white photography no differences could be seen (plain light). Scale bar 400 µm. [Larger image]

5.8. (Bottom) Thin section of unblackened soilstone crust from Ramrod Key, Florida showing charred twigs (charcoal). Charcoal scattered throughout 5000-year-old crust indicated natural forest fires occurred before the invasion of modern man. Circular objects are bubbles created during thin section preparation. Scale bar = 400 µm. [Larger image]

The data in Table 5.1 show that blackening begins first in modern ooids heated 2 h between 232° and 288°C. Coral, both modern and Pleistocene, began blackening after 3 h of exposure at 343°C. After 4 h at 399°C, all specimens began blackening but curiously not the soilstone crusts. After 6 h at 510°C, samples attained maximum blackening. Temperatures above 510°C caused lightening of the outer surface, and after 8 h at 566°C, all samples had calcined. Visual aspects of the blackening process are shown in Figures 5.5A, B, and C. In the oil-drum and experiments subsequent to those listed in Table 5.1, we found that maximum darkening could be obtained in just one-half hour. Although temperature was not measured in the oil-drum experiment, additional experiments in the kiln showed that maximum darkening could be obtained at around 400°C in just one-half hour. Results of these later experiments are shown in Figures 5.5A, B, and C.

Table 5.2 Organic carbon content and pyrolysis assay of related pairs of light- and dark-colored carbonate crusts, lithoclasts, and salt-and-pepper carbonate sands. [Larger image]

Naturally blackened and unblackened Pleistocene limestone and experimentally blackened Holocene crust were subjected to LECO and Rock-EVAL pyrolysis, as were subtidal salt-and-pepper sands from Florida Bay near Crane Key (1.5 m of water) and from the base of an 8-m-thick Holocene sediment section (30 cm below sealevel) (Table 5.2).

Another experiment considered significant to our hypothesis was performed on a cemented breccia sample. A slab of the same breccia shown in Figure 5.2B was photographed (Fig. 5.6A), wrapped in aluminum foil (to retard oxidation), and heated in an oven for one-half hour at approximately 400°C. The experiment caused blackening of some previously unblackened breccia pebbles and the in situ soilstone crust which lined the breccia-filled pit (Fig. 5.6B).

²We recognize that this unconformity is part Pleistocene and part Holocene. Subaerial erosion occurred during the last Pleistocene sealevel drop; however, the distinguishing features—soilstone crust and blackened pebbles—developed mainly during the past 5,000 to 10,000 years (Robbin and Stipp 1979).

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