Reflectance measurements have been carried out on zooclasts (graptolites, chitinozoans, and vase-shaped microfossils) and other organic particles (vitrinite-like particles, porous/granular vitrinite-like particles, and solid bitumen) in Middle Cambrian to Upper Silurian shales from central and southern Sweden and the Danish island of Bornholm (Baltic Sea). The most abundant organic components in all the shales are fragments of graptolites and vitrinite-like particles. The reflectance distribution of these two types of components is largely identical, and it is suggested that the vitrinite-like particles are fragments of graptolites without any recognizable morphology. Reflectance measurements of graptolites and vitrinite-like particles yield well-defined reflectance populations. In samples with average R graptolite and average R vitrinite-like of > 0.75% R o, the reflectance distributions are bimodal because of increasing bireflectance, and the average reflectance value of the well-defined lower reflecting population is arbitrarily used as maturity indicator. Our results suggest that with increasing thermal maturity the reflectance of graptolites increases faster than the predicted vitrinite reflectance. The relationship between graptolite reflectance and equivalent vitrinite reflectance can be expressed by the equation: VR eqv = 0.73 R (graptolite + vitrinite-like)low + 0.16. The ‘gas generation window’, which normally is considered to begin at a vitrinite reflectance of 1.3% R o in post-Lower Paleozoic rocks containing vitrinite, starts, accordingly, at 1.56% R o graptolite reflectance. Porous/granular vitrinite-like particles occur in minor amounts and they may represent graptolite fragments with a non-smooth surface. They generally yield slightly higher reflectance than non-granular vitrinite-like particles and graptolite fragments. The Middle Cambrian to Furongian (upper Cambrian) shales may contain sparse fragments of vase-shaped microfossils (VSM) that seem to follow the maturation trend of chitinozoans. In the present sample set, the reflectance of chitinozoans and VSM is comparable to that of graptolites at the same level of maturity. Reflectance measurements of solid bitumen are a poor maturity indicator, probably because bitumen can have various origins and morphologies and it may not be indigenous to the host rock.
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