VITRINITE REFLECTANCE

Vitrinite reflectance measurements were obtained for almost all of the samples. However, for a high proportion of samples, the readings come from a small number of phytoclasts, and in some cases, from only a single plant entity. It is clear that plant and tissue types are the main control over the values obtained rather than thermal history. The degree of compaction of the vitrinite may be a better indication of cover and heating than vitrinite reflectance for this suite of samples. The variation in reflectance of various tissue types precludes correlating vitrinite reflectance with thermal regimes, primarily because measured borehole temperatures and temperatures calculated from the observed sediment thermal conductivities and range in basement heat flow (Fig. F2) are too low to have caused a marked affect on vitrinite reflectance. In contrast, where reflectances exceed ~0.45%, these values probably result from maturation effects. However, texturally mature vitrinite can develop either due to compaction associated with the early stages of physico-chemical coalification or to gelification in the peat stage. The presence of relatively massive structures at moderate to shallow depths (e.g., Plate P1, fig. 3 and fig. 4) suggests that gelification within the peat stage must have been extensive. It is, however, possible that the more gelified tissues were more likely to be preserved after transport and sinking through the water column. In particular, the less massive tissues may have tended to float rather than sink to be included within the sediments.

No clear trend in vitrinite reflectance exists within or among individual well sections. Even textural features do not show any marked trends. For example, Plate P2, figure 1, illustrates texturally immature vitrinite but is from one of the deeper samples in the suite. There is, however, a weak tendency for the more open cell structures such as those illustrated in Plate P3, to occur at shallow to moderate depths. The lack of trends within vitrinite reflectance suggest that formation temperatures even in the deeper samples have not exceeded ~50C. Temperatures above this level are required before vitrinite reflectance shows any marked increase from the levels associated with woody material preserved within peats.

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