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Figure 2.19 Strained Cambrian trilobites from Himalaya. (Courtesy of Nigel Hughes.)

Can the actual color of fossils help us understand the geological history of an area? The investigation of thermal maturation is now a routine petroleum exploration technique. A number of groups of microfossils change color with changing paleotemperature (Table 2.2). The upper end of the thermally-induced color range has proved useful in mapping metamorphic zones in orogenic belts. Conodonts in particular (see p. 429) are useful thermal indicators. They change color from light amber to gray to black and white, and eventually translucent, on a scale of con-odont alteration indices (CAI values) from 1 to 8, through a temperature range from about 60 to 600°C. Carbonaceous organisms, including the graptolites (see p. 412), also show color changes, as does vitrinite derived from plant material. These changes have also been documented in detail for acritarchs (see p. 216), where acritarch alteration indices (AAI values) range from 1 to 5. Spores and pollen have spore color indices (SCI values) ranging from 1 to 10, with colors ranging from colorless to pale yellow through to black. Other groups such as phosphatic microbrachiopods and chitinozoans show similar prospects, but their color changes have yet to be calibrated with precise paleotemperatures. Paleotemper-atures can also help predict the oil and gas window, usually located at depths between 2.5 and 3.5 km, and thus have important application to hydrocarbon exploration.

Box 2.8 Scandinavian Caledonides

Mountain belts are a source of all sorts of exciting and significant fossil assemblages. The Scandinavian Caledonides are no exception. This mountain belt stretches for some 1800 km from north to southwest Norway, never exceeding a width of 300 km. It developed during a so-called Wilson cycle (the opening, closing and subsequent destruction of an ancient ocean, named after J. Tuzo Wilson) culminating in the collision of the Baltic plate with those of Avalonia (England, Wales and parts of eastern North America and north central Europe) and then Laurentia (cratonic North America). During its transit from high to low latitudes in the Early Paleozoic, Baltica rotated anticlockwise and first captured terranes adjacent to the craton itself with Baltic faunas, followed by island terranes from within the Iapetus Ocean, with endemic taxa, and finally island complexes that were marginal to the Laurentian plate with North American faunas (Harper 2001). The mountain belt in its pile of thrust sheets thus stores much of the biogeographic history of the Iapetus Ocean and its marginal terranes (Fig. 2.20). Moreover during the Late Silurian-Devonian, as the mountain belt continued to rise, marginal basins contained remarkable marine marginal biotas with spectacular eurypterid faunas. Adjacent basins, for example in Scotland, contain some of the earliest land arthropods and plants. So the collision of plates and the generation of a huge mountain belt was not entirely a destructive process. It has helped preserve key evidence for an ancient ocean with diverse and endemic faunas that helped contribute to the great Ordovician biodiversification event (see p. 253) while its later non-marine basins hold critical information on the early development of life on land (see p. 442).

Figure 2.20 The Scandinavian Caledonides showing the pre-drift positions of some of the various thrust sheet complexes. During the Early Ordovician (EO) the most seaward, upper parts of the higher thrust sheets (Storen Nappe) contained North American marginal faunas, whereas the lower parts of these thrust sheets (Köli Nappe) contained Celtic (oceanic) type faunas. The lower parts of the nappe pile (e.g. the Valdres Nappe) have Baltic faunas. The Wenlock-Ludlow (MS) marginal molasse deposits (Old Red Sandstone (ORS) facies), for example at Ringerike, have spectacular marine marginal faunas.

Figure 2.20 The Scandinavian Caledonides showing the pre-drift positions of some of the various thrust sheet complexes. During the Early Ordovician (EO) the most seaward, upper parts of the higher thrust sheets (Storen Nappe) contained North American marginal faunas, whereas the lower parts of these thrust sheets (Köli Nappe) contained Celtic (oceanic) type faunas. The lower parts of the nappe pile (e.g. the Valdres Nappe) have Baltic faunas. The Wenlock-Ludlow (MS) marginal molasse deposits (Old Red Sandstone (ORS) facies), for example at Ringerike, have spectacular marine marginal faunas.

Table 2.2 Various measures of thermal maturation. Color changes recorded in conodonts (CAI), together with corresponding values for vitrinite reflectance and the translucency index of palynmorphs, are related to the oil and gas window and metamorphic grades and zones. (Based on Jones, G.L. 1992. Terra Nova 4.)

Table 2.2 Various measures of thermal maturation. Color changes recorded in conodonts (CAI), together with corresponding values for vitrinite reflectance and the translucency index of palynmorphs, are related to the oil and gas window and metamorphic grades and zones. (Based on Jones, G.L. 1992. Terra Nova 4.)

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