Microfossils as thermal metamorphic indicators

Microfossils with a mineral skeleton are commonly composed of high or low magnesium calcite or calcium phosphate whereas palynomorphs are composed of organic materials such as sporopollenin, chitin and pseudochitin. These materials, though highly resistant, are susceptible to weathering, reworking by erosion, oxidation and to thermal metamorphism. Pristine palynomorphs have transparent to very pale green-yellow walls and often have to be stained to see the material under the microscope. Less well preserved fossil material can range in colour from yellow to black. It is this colour change which can be used as an indicator of the thermal metamorphic history of a rock, as outlined below. The primary factors affecting the colour of fossil palynomorphs are oxidation during weathering, heat related to depth of burial or contact metamorphism, and length of exposure to heat. Oxidation can initially remove the fine detail and ornamentation, ultimately removing the paly-nomorphs entirely. Oxidation is the prime cause of the absence of palynomorphs from reddened shales and sandstones.

When subjected to heating, owing to increased depth of burial or to contact metamorphism, organic matter undergoes a series of irreversible chemical and physical changes best seen in the transformation of peat to coal. Similarly, dispersed organic matter in sediments undergoes similar changes with the loss of H and O and the concomitant increase in C during diagenesis (up to 50°C), or catagenesis (50-150°C) and metagenesis (150-200°C) and finally metamorphism above 250°C. Experimental heating of organic matter in inert or reducing atmospheres requires higher temperatures than in air to affect the same colour change. Pressure alone does not cause carbonization. These physical and chemical processes also affect organic matter trapped within the mineralized skeletal components of all fossils. This is particularly true for the conodonts which contain sufficient organic material to colour the biogenic apatite yellow.

Experimentally derived temperature ranges have been assigned to the rather subjective scale of colour changes for various groups of palynomorphs and conodonts (Fig. 5.1). Palynomorphs associated with these chemical changes show a change in colour of the wall from transparent through yellows, to browns and finally to black. Small differences do occur between groups. For example, pristine acritarchs and dinoflagellates are almost transparent and take more heat to darken than do the already darker spores and pollen. The conodont colour alteration index (CAI: Epstein et al. 1977) scale for conodonts has been extended beyond black (300°C) through grey (CAI 6-7, 360-720°C) to colourless at temperatures greater than 600°C (Rejebian et al. 1987).

Figure 5.1 compares the various palynomorph thermal maturity indices against other commonly measured thermal parameters. The economically important oil window is indicated by mid-brown colours in most organic indices. Darker colours than this indicate petroleum source rocks will have generated gas. Although mainly used in hydrocarbon exploration microfossil coloration has been successfully applied in unravelling the geological histories of sedimentary basins (e.g. Robert 1988) and ancient orogens (e.g. Bergstrom 1981), in base metal and mineral exploration, and for the tracking of ancient hotspots and geothermal energy studies (e.g. Nowlan & Barnes 1987). A comprehensive review on sedimentary organic matter can be found in Tyson (1995).

50 Ro

ZONES OF PETROLEUM GENERATION AND DESTRUCTION

ORGANIC

AMORPHOUS (OIL) LIPTINITIC

50 Ro

ZONES OF PETROLEUM GENERATION AND DESTRUCTION

ORGANIC

AMORPHOUS (OIL) LIPTINITIC

Thermal Alteration Index Spore

[50 C Temp, ranges CAI 60

Steranes AAI TAI 0 0 0

10

1

1-5

2

4-5

---

3

a

«00

3 Z

c. o

o

3

Fig. 5.1 Comparision of the main indicators of thermal maturity with the zones of petroleum generation and destruction. AAI, acritarch alteration index; CAI, conodont colour alteration index; SCI, spore coloration index; TAI, thermal alteration index.

REFERENCES

Bergström, S.M. 1981. Conodonts as paleotemperature tools in Ordovician rocks of the Caledonides and adjacent areas in Scandinavia and the British Isles. Geol. Fören. Stockholm Förhandl 102, 337-392.

Epstein, A.G., Epstein, J.B. & Harris, L.D. 1977. Conodont Color Alteration - an index to organic metamorphism. US Geological Survey Professional Paper 995, 27pp.

Nowlan, G.S. & Barnes, C.R. 1987. Thermal maturation of Paleozoic strata in eastern Canada from conodont colour alteration (CAI) data with implications for burial history, tectonic evolution, hotspot tracks and mineral and hydrocarbon exploration. Bulletin. Geological Survey of Canada 367, 47pp.

Rejebian, V.A., Harris, A.G. & Hueber, J.S. 1987. Condont color and textural alteration - an index to regional meta-morphism and hydrothermal alteration. Bulletin. Geological Society of America 99, 471-479.

Robert, P. 1988. Organic Metamorphism and Geothermal History. Elf Aquitaine/D. Reidel, Dordrecht.

Tyson, R.V. 1995. Sedimentary Organic Matter: organic facies andpalynofacies. Chapman & Hall, London.

PART 2

The rise of the biosphere

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    How microfossil works as thermal metamorphic indicators?
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