Sunday, May 1, 2016

Graptolite reflectance and correlation with other diagenetic and very-low-grade metamorphic indicators

My first independent organic petrology research project, in the late 1980's-early 1990's, was a graptolite reflectance study of the low- to very-low-grade metamorphic region of the northern US Appalachians in northern Maine. The goals were to 1) test the applicability of the technique, used in other anchizone regions, to these prehnite-pumpellyite grade rocks, and 2) outline in more detail the regional patterns or trends in metamorphism.

Graptolites are extinct colonial marine invertebrates of the Phylum Hemichordata with a geologic age range from Cambrian to Carboniferous. They derive their name from the pencil-mark appearance of preserved periderm on shales (Graptolithus= rock writing). Since the mid-1970's, the reflectance of graptolites, in a similar fashion to vitrinite reflectance, has been used to access the diagenetic level or organic maturity of rocks that, either due to a marine environment of deposition (EOD) or age older than the advent of land plants (pre-Silurian), lack vitrinite derived from woody plant matter. Graptolite reflectance has been applied to both petroleum source rock evaluation and analysis of patterns of anchizone metamorphism, the anchizone being the level of metamorphism between sedimentary rocks and greenschist facies where some diagenetic indicators may no longer be applicable and where big micas and flashy garnets, staurolite etc have not yet appeared. (In one talk in ~1992, I did compare the anchizone to the Neutral Zone of Star Trek, which separates Romulan space from Federation space; the anchizone is the area where neither those looking for liquid hydrocarbons nor those studying traditional metamorphic petrology care to go.)

However, the correlation of graptolite reflectance values to vitrinite reflectance has been problematic since vitrinite and graptolites are hardly ever found in the same rock, due again to age of rock or EOD. Correlation has been made through intermediaries: thermal maturity indicators found in or that can be applied to both vitrinite-bearing and graptolite-bearing rocks. Also, microscopically, some authors reported "maximum reflectance" in which, under polarized light (polarizer in light path), the microscopic stage is rotated to the orientation of the maximum reflectance of the highly anisotropic graptolite periderm and then reflectance is recorded. The mean of many maximum reflectance measurements on a rock sample can, therefore, be higher, with higher standard deviation, than "random reflectance", measured in non-polarized light. Correlation of graptolite reflectance with other indicators, such as conodont alteration indices (CAI), and with mineral-based metamorphic zone boundaries was also not clear, partly due to differing interpretations of the latter. But, fortunately, in 2007, the International Union of Geological Sciences (IUGS) published a classification for very-low to low grade metamorphic rocks (Árkai and others, 2007), which includes vitrinite reflectance, and standardizes boundaries for mineralogically-based diagenetic to low-grade metamorphic zones.

So a few years ago, partly for a couple papers still (even now) in draft form and partly in response to a petroleum industry colleague asking how well constrained the vitrinite/graptolite correlations are, I made a big spread sheet with the IUGS 2007 very-low-metamorphic indicators chart and correlation interpretations from numerous collected graptolite reflectance papers (references following). From that, I made the table BELOW showing THREE correlative relationships of graptolite reflectance: two from studies reporting random reflectance, but that use different intermediaries to relate graptolite to vitrinite reflectance and are from different geological provinces, and a third correlation of graptolite mean maximum reflectance.

TABLE (click on it to enlarge): Correlation of metamorphic facies, Kübler indices, vitrinite reflectance, and coal grade from the IUGS Subcommission on the Systematics of Metamorphic Rocks (Árkai et al., 2007); metapelitic zones to vitrinite reflectance and Kübler index (Abad, 2007); CAI (conodont alteration index) to vitrinite reflectance (Repetski et al., 2008); mean random graptolite reflectance, non-polarized light (to vitrinite reflectance through chitinozoan reflectance: Bertrand, 1990); mean random graptolite reflectance, non-polarized light (to vitrinite reflectance through RockEval pyrolysis Tmax: Petersen et al., 2013);  mean maximum graptolite reflectance to CAI or metamorphic facies (compilation of results from Oliver, 1988; Goodarzi, 1990; Goodarzi et al., 1992; Wang et al., 1993; Malinconico, 1992, unpublished data).

A very comprehensive correlation chart (their Figure 26) and discussion of numerous visual organic maturity Indicators by Hartkopf-Fröder and others was published in 2015. They included coal rank, reflectance of vitrinite, graptolites and other zooclasts, coloration of conodonts, spores/pollen and other microfossils, and hydrocarbon generation zones, but did not include mineral metamorphic facies. Three graptolite reflectance scales are in their chart: Petersen and others (2013) and two by Bertrand and colleagues. Their bibliography includes graptolite reflectance citations (such as several by Bertrand) that are not listed below.

These tables do not solve the graptolite/vitrinite reflectance correlation problem. They do, however, show state of the current publicly available knowledge.

and references for other diagenetic to very-low-metamorphic indicators
used in making the correlation table (in bold)

Abad, I., 2007, Physical meaning and applications of the illite Kübler index: measuring reaction progress in low-grade metamorphism, in Nieto, F., and Jiménez-Millán, J., eds., Diagenesis and Low-Temperature Metamorphism. Theory, Methods and Regional Aspects: Seminarios de la Sociedad Española de Mineralogia, v. 3, p. 53-64.
Árkai, P., Sassi, F., Desmons, J., 2007, Very low- to low-grade metamorphic rocks (Chapter 2.5), in Fettes, D., and Desmons, J., eds., Metamorphic Rocks: A Classification and Glossary of Terms (Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks): Cambridge, UK, Cambridge University Press, p. 36-42.
Bertrand, R., 1990, Correlations among the reflectances of vitrinite, chitinozoans, graptolites, and scolecodonts: Organic Geochemistry, v. 15, no. 6, p. 565-574.
Bertrand, R., and Heroux, Y., 1987, Chitinozoan, graptolite and scolecodont reflectance as an alternative to vitrinite and pyrobitumen reflectance in Ordovician and Silurian strata, Anticosti Island, Quebec, Canada, American Association of Petroleum Geologists Bulletin, v. 71, p. 951-957.
Bustin, R., M., Link, D., and Goodarzi, F., 1989, Optical properties and chemistry of graptolite periderm following laboratory simulated maturation: Organic Geochemistry, v. 14, p. 355-364.
Cardott, B. J., and Kidwai, M. A., 1991, Graptolite reflectance as a potential thermal-maturation indicator, in K. S. Johnson, ed., Late Cambrian-Ordovician geology of the southern Midcontinent, 1989 symposium: Oklahoma Geological Survey Circular 92, p. 203-209.
Clausen, C.-D. and Teichmüller, M., 1982, Die Bedeutung der Graptolithenfragmente im Paläozoikum von Soest-Erwitte für Stratigraphie und Inkohlung: Fortschritte in der Geologie von Rheinland und Westfalen, v. 30, p. 145-167.
Cole, G. A., 1994, Graptolite-chitinozoan reflectance and its relationship to other geochemical maturity indicators in the Silurian Qusaiba shale, Saudi Arabia: Energy & Fuels., v. 8, p. 1443-1459.
Goodarzi, F., 1984, Organic petrology of graptolite fragments from Turkey: Marine and Petroleum Geology, v. 1, p. 202-210.
Goodarzi, F., 1984, Organic petrology of graptolite fragments from Turkey: Marine and Petroleum Geology, v. 1, p. 202-210
Goodarzi, F., 1985, Dispersion of optical properties of graptolite epiderms in increase maturity in early Paleozoic organic sediment: Fuel, v. 64, p. 1735-1740.
Goodarzi, F., 1990, Graptolite reflectance and thermal maturity of Lower Paleozoic rocks, in V. F. Nuccio and C. E. Barker, eds., Applications of thermal maturity studies to energy exploration: SEPM, Rocky Mountain Section, p. 19-22.
Goodarzi, F., Gentzis, T., Harrison, C., and Thorsteinsson, R., 1992, The significance of graptolite reflectance in regional thermal maturity studies, Queen Elizabeth islands, Arctic Canada: Organic Geochemistry, v. 18, no. 3., p. 347-357.
Goodarzi, F., and Norford, B. S., 1985, Graptolites as indicators of the temperature histories of rocks: International Journal of Coal Geology, v. 11, p. 127-141.
Hartkopf-Fröder, C., Königshof, P., Littke, R., Schwarzbauer, J., 2015, Optical thermal maturity parameters and organic geochemical alteration at low grade diagenesis to anchimetamorphism: A Review: International Journal of Coal Geology, v. 150-151, p. 74-119.
Hower, J. C., and Malinconico, M. L., 2000, Organic metamorphism in Middle Ordovician carbonates, Lebanon Valley nappe, Pennsylvania:  International Journal of Coal Geology, v. 42, p. 221-230.
Kurylowicz, L. E., Ozimic, S., McKirdy, D. M., Kantsler, A. J. and Cook, A. C., 1976, Reservoir and source rock potential of the Larapinta Group, Amadeus Basin, Central Australia: Australian Petroleum Exploration Association Journal, v. 16, p. 44-65.
Malinconico, M. L., 1992, Graptolite reflectance in the prehnite- pumpellyite zone, northern Maine, U.S.A.: Organic Geochemistry, v. 18, p. 263-271.
Malinconico, M. L., 1993, Reflectance cross-plot analysis of graptolites from the anchi-metamorphic region of northern Maine, USA: Organic Geochemistry, v. 20, p. 197-207.
Oliver, G. J. H., 1988, Arenig to Wenlock regional metamorphism in the paratectonic Caledonides of the British Isles- a review, in Harris, A. L. I., and Fettes, D. J., eds., The Caledonian-Appalachian Orogen: Geological Society (London) Special Publication 38, p. 347-363.
Petersen, H. I., Schovsbo, N. H., Nielsen, A. T., 2013, Reflectance measurements of zooclasts and solid bitumen in Lower Paleozoic shales, southern Scandinavia: Correlation to vitrinite reflectance: International Journal of Coal Geology, v. 114 , p. 1-18.
Rantitsch, G., 1995, Coalification and graphitization of graptolites in the anchizone and lower epizone: International Journal of Coal Geology, v. 27, p. 1-22.
Repetski, J. E., Ryder, R. T., Weary, D. J., Harris, A. G, and Trippi, M. H., 2008, Thermal maturity patterns (CAI and %Ro) in Upper Ordovician and Lower-Middle Devonian rocks of  the Appalachian basin: A major revision of USGS Map I-917-E using new subsurface collections: U.S. Geological Survey Scientific Investigations Map 3006, one CD-ROM.
Riediger, C., Goodarzi, F., and MacQueen, R. W., 1989, Graptolites as indicators of regional maturity in lower Paleozoic sediments, Selwyn Basin, Yukon and Northwest Territories, Canada: Canadian Journal of Earth Sciences, v. 26, p. 2003-2015.
Ruble, T. E., Knowles, W. R., Selleck, B. W., Wylie, A. S., 2013, Assessment of thermal maturation in outcrop samples of the Utica Shale, northern Appalachian basin, New York: American Association of Petroleum Geologists 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, AAPG Search and Discovery Article #90163 (; accessed August 2013). 
Taylor, G. H., Teichmüller, M., Davis, A., Diessel, C. F. K., Littke, R., Robert, P., 1998, Organic petrology: Gebrüder Borntraeger, Berlin, 704 pages.
Teichmüller, M., 1978, Nachweis von Graptolithen-Periderm in geschieferten Gesteinen mit Hilfe kohlenpetrologischer Methoden: Neues Jahrbuch für Geologie und Paläontologie, Mh. 7, 430-447.
Wang, X. F., Hoffknecht, A., Xiao, J. X., Chen S. Q., Li Z. H., Brocke, R. B., and Erdtmann, B-D., 1993, Graptolite, chitinozoan, and scolecodont reflectances and their use as indicators of thermal maturity: Acta Geologica Sinica, v. 6, no. 1, p. 93-105.
Yang, C., and Hesse, R., 1993, Diagenesis and anchimetamorphism in an overthrust belt, external domain of the Taconian Orogen, southern Canadian Appalachians—II. Paleogeothermal gradients derived from maturation of different types of organic matter: Organic Geochemistry, v. 20, p. 381-403.

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