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

The correlation chart and references in this post have been updated as of November 28, 2020. In the chart, the mean maximum reflectance correlation of Malinconico has been changed and is based now on her (my) data from the prehnite-pumpellyite metamorphic terrane of northern Maine, USA, rather than a compilation with other published mean maximum graptolite reflectance published data. The mean random and mean maximum graptolite reflectance correlations of Luo et al. (2020) and the mean random equation of Bertrand and Malo (2012) have been added. Inclusion of Kübler Indices and associated metapelitic (anchizone, etc) zones have been removed (see text below).

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.

Silurian monograptid graptolites from Lawler Ridge, several miles north of Millinocket, Maine. Mean maximum reflectance 10.6% + 1.0 (n=20).

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 determine the diagenetic level or organic maturity of rocks that, either due to a marine environment of deposition (EOD) or age older than the flourishing 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/subgreenschist metamorphism, the level of metamorphism between sedimentary rocks and greenschist facies metamorphic rocks where some diagenetic indicators may no longer be applicable and where big micas and flashy garnets, staurolites 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 urge to correlate graptolite reflectance values to the well-known or more standard vitrinite-reflectance scale has been problematic for a number of reasons. Firstly, 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, such as solid bitumen/scolecodont/chitinozoan reflectance, conodont alteration indices (CAI), and Rock-Eval pyrolysis Tmax, that are found in or that can be applied to both vitrinite-bearing and graptolite-bearing rocks. However, a drawback particularly of using intermediaries such as CAI and subgreenschist mineral facies is the wide qualitative range of the categories within those indicators. 

Secondly, the percent graptolite reflectance has been reported in different forms: 

1) mean maximum reflectance: on each of numerous specimens in a single polished whole-rock sample, under polarized light (polarizer in light path), the microscopic stage is rotated to the orientation of the maximum reflectance of the highly anisotropic graptolite exoskeleton and then the reflectance is recorded; a mean and standard deviation is calculated from all the maximum reflectances for that sample; 

2) maximum-maximum or true maximum reflectance which is just the single highest maximum reflectance value of all specimens measured on a sample; no standard deviation; 

3) mean random reflectance, polarized light: the instantaneous measured value with no stage rotation is collected on numerous specimens in a sample, mean and standard deviation calculated; the range of reflectances collected can be quite variable depending on degree of anisotropy, and standard deviation can be large;

4) mean random reflectance, in non-polarized light. The reflectance of the non-polarized surface is theoretically an average of the anisotropic range of that specimen’s surface reflectivity. With a large enough number of measurements, the mean random reflectances in both polarized and non-polarized light should be equal, but the standard deviation in polarized light will be larger. 

Besides correlation to thermal maturation indices commonly used in sedimentary rocks and oil-gas exploration (CAI, vitrinite reflectance/coal rank), since graptolite reflectance has been used in zeolite/prehnite-pumpellyite subgreenschist terranes, proper correlation with related terminology such as diagenesis, anchizone/epizone, very-low-grade/low-grade metamorphism is also an issue. Discussions on the limits of the anchizone based on illite crystallinity and relationship to mineral facies and coal rank (bituminous/anthracite) have been ongoing for decades. In 2007, the International Union of Geological Sciences (IUGS) published a correlation classification for very-low to low-grade metamorphic rocks (Árkai and others, 2007), with vitrinite reflectance, diagenetic to low-grade metamorphic zones, subgreenschist mineral facies, and boundaries for metapelitic zones (diagenetic zone, anchizone, epizone) based on the illite crystallinity Kübler Index (KI). Lower and upper KI limits of the anchizone were 0.42-0.25˚ delta 2theta, respectively. Many of the boundaries were gradational or covered a range of values, but it appeared, to me, that there was international consensus. 

So, in 2016, partly for a couple papers still (and even now in 2020) in draft form and partly in response to a petroleum industry colleague asking how well constrained the vitrinite/graptolite correlations are, I made a huge spread sheet with the IUGS 2007 very-low-metamorphic indicators chart, CAI, and vitrinite-graptolite reflectance correlations from numerous collected graptolite reflectance papers (most source references following). From that, I posted in this blog entry, a correlation chart of metamorphic grade, mineral facies, KI with metapelitic zone, coal rank with vitrinite reflectance, and CAI, plus the mean random graptolite reflectance of Bertrand (1990) and Petersen et al. (2013), and mean maximum graptolite reflectance that was a consensus compilation of my own data from northern Maine and several publications that reported mean maximum graptolite reflectance data.

However, since my 2016 chart, there has been an increasing interest in graptolite reflectance and publication of important papers, particularly from China and Denmark. The surge in interest is due to large unconventional petroleum discoveries in Lower Paleozoic black shales. I am also finally getting around to finalizing my graptolite reflectance study in the prehnite-pumpellyite region of northern Maine. Coincidently, however, there has been changes in standardization used for illite crystallinity studies that has changed the KI limits of the anchizone (Warr and Ferreiro Mählmann, 2015) plus continuing discussion on the variability of the boundaries of the anchizone relative to mineral facies and to coal ranks in different regions (Ferreiro Mählmann and Frey, 2012; Warr and Cox, 2016).

It should be pointed out that the anchizone is defined specifically by illite crystallinity: Kisch wrote in 1990 (p. 42), “The main argument against such re-definitions […in terms of the metamorphic grade in associated materials…] is that the notion of the anchizone is intractably bound to phyllosilicate mineralogy in clastic sedimentary rocks, and particularly defined in terms of illite ‘crystallinity’…Such re-definition of the anchizone in terms other than illite ‘crystallinity’ should therefore be rejected.” In the Alps, there is a range of vitrinite reflectance values associated with the lower and upper boundaries of the anchizone depending on geothermal gradient or heating rate indicating that there are material differences in the chemical and mechanical kinetics of the transformation of clay crystallinity and the physiochemical structure of vitrinite. In addition, the influence of pressure varies between clay and organic matter. Warr and Cox (2016) also reported that in the famous zeolite to prehnite-pumpellyite facies metamorphic region of New Zealand, the epizone, previously interpreted to be equivalent to chlorite-grade greenschist metamorphism, should now include pumpellyite-actinolite facies, formerly in the anchizone.

Therefore, my new 2020 table below, does not include any reference to Kübler Index and related metapelitic zones, despite my fondness for the term “anchizone”. I so far have only found one researcher that reports KI and associated graptolite reflectance (Rantitsch 1995, 1997). Most graptolite reflectance practitioners are working in gas exploration, rather than very-low-grade metamorphic terrane studies, and the co-existing indicators in my area of northern Maine are CAI and zeolite/prehnite-pumpellyite mineral facies. 

The new table BELOW has FIVE correlative relationships of graptolite reflectance: three from studies reporting random reflectance, but that use different intermediaries to relate graptolite to vitrinite reflectance and are from different geological provinces, and two correlations of graptolite mean maximum reflectance.

TABLE (click on it to enlarge): Correlation of metamorphic grade, mineral facies, coal rank, vitrinite reflectance from the IUGS Subcommission on the Systematics of Metamorphic Rocks (Árkai et al., 2007); general boundaries of zeolite facies to vitrinite reflectance (Kisch, 1981); CAI (conodont alteration index) to vitrinite reflectance (Repetski et al., 2008); mean random graptolite reflectance, non-polarized light, from equation based on natural and heat-treated graptolites plus previously published sources (Luo et al., 2020); 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 combining results in Bertand, 1990, 1993: Bertrand and Malo, 2012); mean random graptolite reflectance, non-polarized light (to vitrinite reflectance through RockEval pyrolysis Tmax: Petersen et al., 2013); mean maximum graptolite reflectance equation based on natural and heat-treated graptolites plus previously published sources (Luo et al., 2020); mean maximum graptolite reflectance to CAI (Bradley et al., 2000)  and metamorphic facies (Richter and Roy, 1976) of northern Maine, USA (Malinconico, 1992, 1993, unpublished data).

Another useful correlation chart is that of Hartkopf-Fröder (2015; their Figure 26). They comprehensively include not just coal rank, reflectance of vitrinite and graptolites and CAI, but other zooclasts, coloration of spores/pollen and other microfossils, and hydrocarbon generation zones. They did not include mineral metamorphic facies, which were important for my work. Three graptolite reflectance scales are in their chart: Petersen and others (2013) and two by Bertrand and colleagues (1990, 2012); I included those in my chart. Their bibliography includes graptolite reflectance citations (such as several by Bertrand) that are not listed below. Another bibliographic list (Microsoft Word document) of “Zooclast Reflectance” citations is on the website of The Society for Organic Petrology (TSOP) (https://www.tsop.org/references.html). Luo et al. (2020) have a useful graph (Figure 13) for visualization that plots several graptolite-vitrinite reflectance correlations against each other. 

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

Selected GRAPTOLITE REFLECTANCE 

and diagenetic to very-low-metamorphic indicator references 

including those examined for construction of the correlation table.

(Citations specifically mentioned or used in the table are in bold.) 

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., 1993, Standardization of solid bitumen reflectance to vitrinite in some Paleozoic sequences of Canada, in F. Goodarzi and R.W. Macqueen, eds., Geochemistry and petrology of bitumen with respect to hydrocarbon generation and mineralization: Energy Sources, v. 15, p. 269-287.

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.

Bertrand, R., and Malo M., 2012, Dispersed organic matter reflectance and thermal maturation in four hydrocarbon exploration wells in the Hudson Bay Basin: regional implications: Geological Survey of Canada, Open File 7066, 52 p. http://publications.gc.ca/collections/collection_2012/rncan-nrcan/M183-2-7066-eng.pdf 

Bradley, D. C., Tucker, R. D., Lux, D.; Harris, A. G., and McGregor, D. C., 2000, Migration of the Acadian orogen and foreland basin across the northern Appalachians of Maine and adjacent areas: U.S. Geological Survey Professional Paper 1624, 49 p. 

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.

Cao, C., Sang, Q., Fang, Y., 2000, The study of graptolite reflectance as the indicator of source-rock maturation in Ordovician and Silurian of Tarim basin, Ordos, Jiangsu areas: v. 39, issue 1, Acta palaeontologica sinica, p. 151-156. (In Chinese; English abstract and figure captions)

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.

Ferreiro Mählmann, R., Frey, M., 2012, Standardisation, calibration and correlation of the Kübler-index and the vitrinite/bituminite reflectance: an inter-laboratory and field related study: Swiss Journal of Geosciences, v. 105: 153-170.

Gentzis, T., T. de Freitas, F. Goodarzi, M. Melchin, and A. Lenz, 1996, Thermal maturity of lower Paleozoic sedimentary successions in Arctic Canada: AAPG Bulletin, v. 80, p. 1065-1084.

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.

Goodarzi, F., and Norford, B. S., 1989, Variation of graptolite reflectance with depth of burial: 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.

Kemp, A. E. S., Oliver, G. H. I. and Baldwin, J. R., 1985, Low-grade metamorphism and accretion tectonic: Southern Uplands terrain, Scotland: Mineralogical Magazine, v. 49, p. 335-344.

Kisch, H.J., 1981, Coal rank and illite crystallinity associated with the zeolite facies of Southland and the pumpellyite-bearing facies of Otago, southern New Zealand: New Zealand Journal of geology and Geophysics, v. 24, p. 349-360.

Kisch, H.J., 1990, Calibration of the anchizone: a cricial comparison of illite 'crystallinity' scale used for definition: Journal of Metamorphic Geology, v. , p. 31-46.

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.

Luo, Q., Goodarzi, F., Zhong, N., Wang, Y., Qiu, N., Skovsted, C. B., Suchy, V., Schovsbo, N. H., Morga, R., Xu, Y., Hao, J., Liu, A., Wu, J., Cao, W., Min, X., Wu, J., 2020, Graptolites as fossil geo-thermometers and source material of hydrocarbons: An overview of four decades of progress: Earth-Science Reviews, v. 200, Article 103000. doi:10.1016/j.earscirev.2019.103000 

Luo, Q., Hao, J., Skovsted, C.B., Luo, P., Khan, I., Wu, J., Zhong, N., 2017. The organic petrology of graptolites and maturity assessment of the Wufeng–Longmaxi Formations from Chongqing, China: insights from reflectance cross-plot analysis: International Journal of Coal Geology, v. 183, p. 161–173. 

Luo, Q., Hao, J., Skovsted, C.B., Xu, Y., Liu, Y., Wu, J., Zhang, S., Wang, W., 2018. Optical characteristics of graptolite-bearing sediments and its implication for thermal maturity assessment: International Journal of Coal Geology, v. 195, p. 386–401. 

Luo, Q., Zhong, N., Dai, N., Zhang, W., 2016. Graptolite-derived organic matter in the Wufeng–Longmaxi Formations (Upper Ordovician–lower Silurian) of southeastern Chongqing, China: implications for gas shale evaluation: International Journal of Coal Geology, v. 153, p. 87–98. 

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.

Rantitsch, G., 1997, Thermal history of the Carnic Alps (Southern Alps, Austria) and its palaeogeographic implications: Tectonophysics, v. 272, p. 213-232.

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.

Richter, D. A., and Roy, D. C., 1976, Prehnite-pumpellyite facies metamorphism in central Aroostook County, Maine, in Lyons, P. C., and Brownlow, A. H., eds, Studied in New England geology: Geological Society of America Memoir 146, p. 239-261.

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 (www.searchandiscovery.com; 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.

Warr, L. N., and Cox, S. C., 2015, Correlating illite (Kübler) and chlorite (Árkai) “crystallinity” indices with metamorphic mineral zones of the South Island, New Zealand: Applied Clay Science, v. 134, p. 164-174. 

Warr, L. N., Ferreiro Mählmann, R., 2015, Recommendations for Kübler Index standardization: Clay Minerals, v. 50, p. 282-285.

Watson, S. W., 1976, The sedimentary geochemistry of the Moffat Shales: a carbonaceous sequence in the Southern Uplands of Scotland [Ph.D. dissertation]: St. Andrews, Scotland, UK, St. Andrews University, 818 pages. (https://research-repository.st-andrews.ac.uk/handle/10023/15471)

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.

Zheng, X., Sanei, H., Schovsbo, N.H., Luo, Q, Wu, J., Zhong, N., Galloway, J.M., Goodarzi, F., 2021, Role of zooclasts in the kerogen type and hydrocarbon potential of the lower Paleozoic Alum Shale: International Journal of Coal Geology, v. 248, doi.org/10.1016/j.coal.2021.103865 (This article does not mention reflectance but is an important discussion of non-granular and granular graptolite texture, generative potential, and relationship to Rock-Eval pyrolysis results.)