Thermal metamorphism of the organic matter in the Mancos Shale near Crested Butte, Colorado
Ferreira, Justo Camejo
Baker, Donald R.
Master of Arts
The area of this study is located in west central Colorado, near the town of Crested Butte. In this area, Cretaceous and Tertiary sedimentary rocks have been intruded by Tertiary igneous rocks. The results of previous work on the organic geochemistry of the Mancos Shale in this area were somewhat unexpected, since a continuous transition from metamorphosed to non-metamorphosed conditions from the hot "backbone” of the Ruby Range outward was not confirmed. Instead, two zones were defined: one very extensive metamorphosed zone and one non-metamorphosed zone. This led to the interesting possibility of an underlying igneous mass as the agent of the observed metamorphic effects. This being the case, all the expected changes in the organic matter in the Mancos Shale would occur in-a narrow, "critical" transition zone. The main purpose of the present investigation was the study of a series of samples from the "critical" zone in order to achieve a better understanding of the thermal history of the area and its metamorphic effects. A continuous transition between the metamorphosed and the non-metamorphosed zone was found well recorded by: total extractable bitumens; ratio hydrocarbons/organic carbon; ratio resins/organic carbon; coal; ratio n-paraffins/iso-cycloparaffins; kerogen; normal paraffin distribution and pristane/phytane ratio. The total extractable bitumens decrease from the nonmetamorphosed to the metamorphosed zone. The same was observed with the ratios hydrocarbons/organic carbon, resins / organic carbon, n-paraffins/iso-cycloparaffins and pristane/phytane. The cbal increase in rank from the non-metamorphosed zone (bituminous) to the metamorphosed zone (semi-anthracite). The thermal alteration index of the kerogen increases from the non-metamorphosed to the metamorphosed zone, the color changing from light brown to black. The n-paraffin distribution in the C13 to C33 interval Shows a predominance of C17, C18 and C19 paraffins In the metamorphosed zone, while in the non-metamorphosed and in the transition zone the n-paraffins are more evenly distributed. The above results strongly suggest an underlying batholith, to which the entire igneous complex must be related. Other interesting conclusions frpm the present study ares 1) The resins seem to be as sensitive as the hydrocarbons to low grade thermal metamorphism; 2) The relative proportion of pristane and phytane decreases with respect to the normal paraffins, and also with respect to the iso-cycloparaffins; 3) The pristane/phytane ratio decreases with increasing metamorphism.