Journal of the Geological Society, London, Vol. 143, 1986, pp. 219-220. Printed in Northern Ireland Organic microfossil geothermal alteration and interpretation of regional tectonic provinces K. J. DORNING Pallab Research, International palynological consultants, 58 Robertson Road, Shefield S6 SDX, UK Abstract: Optical analyses for colour, transparency and reflectance of all organic fossils may be used €or palaeotemperaturedata. Of theorganic-walled fossils, acritarchs, chitinozoans, dinoflagellate cysts,graptolites, pollen and spores are of particularvalue in geothermometry. At increasing temperatures, fossil organic material shows progressive changes in colouration and increasing reflec- tance. Palaeoternperatures of sedimentary rocks may be derived from the optical characteristics of the main organic fossil groups recorded. Regional palaeotemperature values are associated with variations in overburden thickness and heat flow. Regional tectonic provinces typically record a restricted range of palaeotemperature values that may be related to sediment thickness and phases of igneous and tectonic activity. There is potential for the preservation of the organic 1. Bacteria, blue-green algae, green algae. Coccoid and material from most organisms in the fossil record. Many of filamentous algae are common in shallow water environ- the organic materials are sufficiently stable for the structure ments.Remains of bacteriaprobably form a significant to be preserved in fine detail over thousands of millions of element of the fine organic debrisformed from organic years,though theparts of organismscontaining complex decay in anoxic sediments. The colourless to pale yellow organic materials including polymers such as sporopollenin materials are essentially of cellulosic composition and and chitin are considerably more resistant to decay than soft rapidly change in colour through increasingly dark browns tissues. The organic fossils thatare regularly recorded in to dark grey at relatively low palaeotemperatures. great abundance overlong stratigraphical intervals are of 2. Acritarchsand dinoflagellate cysts. These groups prime importance in geothermometryand include the form the main elements of the marine microflora. The acritarchs, dinoflagellate cysts, pollen and spores. The acritarchs contain many unicellular organisms of uncertain organic fossils regularly recorded in moderate abundance affinity. The vast majority are algal cysts, with probable include the chitinozoans,graptolites and scolecodonts. affinities in the Pyrrhophyta or Chlorophyta. Dinoflagellate Bacteria, blue-greenalgae, green algae, fungi, protists, cysts of the Peridiniales are frequently recorded in marine chitinousforaminifera, melanosclerites, crustaceans, cheli- sediments fromthe Triassic toRecent, with freshwater cerates and insects may be used for supplementary data. forms known from the Portlandian toRecent. Unaltered Fromthe Precambrian Recent,to acritarchs are acritarchs and dinoflagellate cysts are colourless to pale regularly recorded in marinesediments, with bacteria, yellow, and show colour changes through increasingly dark blue-green algae, green algae, fungi and protists preserved browns to grey or black,depending on wall thickness. sporadically. Theother common organic fossils have Forms with a thin, single, smooth wall, such as restricted stratigraphical ranges: dinoflagellate cysts, Triassic Micrhystridium and Veryhachium, are moresuitable for toRecent; chitinozoans, Tremadocto Late Devonian; palaeotemperature analysis. Forms with more than one wall graptolites, Cambrianto Carboniferous;scolecodonts, often show differences in colouration between the inner and Tremadoc to Recent. outer walls. All fossil organic material undergoes thermal alteration Legal1 al.et (1981) proposed an acritarch colour atincreased temperatures. The temperature range for the alteration index based on colour changes in sphaeromorph changes varies dueto the variety in composition of the acritarchs at low temperatures from 60-90 "C. Progressive original organicmaterials. While all organicmaterials colour changes inacanthomorphitic acritarchs have been generally show a colour change through dark brown to black recorded at much higher palaeotemperatures. In the British and both increased translucency and reflectance, the Isles this includes regional areas with equivalent conodont different organicmaterials exhibit a different colour, colour alteration index values (Epstein et al. 1976) up to 5 transparency and reflectance at any given palaeotempera- (Dorning unpublished data). ture. The optical changes are a result of slow carbonization, 3. Spores, pollen, fungi and plant fragments. Terrestrial particularly the loss of molecular hydrogen and oxygen from plants, including Bryophytes,Pteridophytes and higher the complex organic compounds. plants,produce resistant spores as one means of The original colour, transparency and reflectance of the propagation.Spores and plantfragments are regularly organic fossils varies with differences in composition and recorded in non-marineand nearshore marine sediments thickness. With increasing thickness, fossil organic materials from the Caradoc to Recent. Many of the late Ordovician assumea greater intensity of colourationand increasing and Silurian plantfragments are sheets of cellular tissue. translucency. The organic fossils may begrouped for Thickened tissue suitable for vitrinite reflectance studies is convenienceaccording to similar if not identical optical recorded from thelate Silurian toRecent. The original properties. colour of land plant material is often pale yellow to brown 219 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/1/219/4893456/gsjgs.143.1.0219.pdf by guest on 26 September 2021 220 K. J. DORNING and shows progressive changesthrough dark browns to organic fossils may be undertaken with standard palynologi- black. Gutjahr (1966) examinedpollen andspore trans- cal preparations if no oxidation is employed during lucency in relation to hydrocarbonthermal maturation processing. Oxidation lightens the colour of organic notingincreased translucency of the exines at the higher materials, with mostchange in organic material oflow temperatures. Staplin (1969) expressedpalynomorph col- thermal alteration. Marine organic fossils appear to provide ours as a thermal alteration index (TAI) and this has been more consistent results than those derived from pollen and widely adopted as a scale in visual estimation of pollen and spores. This may be due to greater variation in geochemistry spore colouration. of non-marine sediments and exposure to variable oxidation 4. Chitinozoans and scolecodonts. In low palaeotem- prior to final deposition. Samples from the present day land perature rocks the chitinozoa and scolecodonts are light to surface or geological exposure surface are likely to provide dark brown incolour. The chitinozoan wall andisolated somewhat anomalous results due to partialdestruction of scolecodont elementsare of very variable thickness and the organicmaterials. This is particularly evident in measurements must be made on the same selected part of unaltered to moderate temperature samples, while high to the fossil for meaningful comparative results. At increasing very high thermal alterationmaterial is little affected by palaeotemperaturesthe colourchanges to black or, less weathering. Climate is also critical, for while samples from commonly, grey. polar areas are least affected, those from tropical areas are 5. Graptoliteand eurypterid fragments. The optical most proneto alteration. In extreme cases, all organic changes in fragments of graptolites and eurypterids are material except inertinite may be removed by oxidation. poorly documented,but generally show changesfrom Palaeotemperature values invarious regional tectonic variousshades of brown to black with increasing provinces are associated with variations in the overburden palaeotemperature.The colour of graptolitefragments in thickness, rock conductivity and heat flow. Overburden may palynological assemblages is often only slightly different simply be produced by later sediments in a subsiding basin; from that of the chitinozoans and scolecodonts. it may also be produced by tectonic thrusting of rock over Variation in threshold temperatures and rate of optical the sediments, rapidly introducingagreat thickness of changes in different fossil groups with increasing tempera- overburden. Small areas of higher thermal alteration values ture results in many palynological assemblages showing a are oftenrelated to local high heat flow associated with variety in colouration of the organic fossils. Bacteria and igneous intrusions. algae generally show the greatest alteration, followed by the The use of palaeotemperaturedata from the thermal pollen, spores, chitinozoa and scolecodonts, with the alteration of organicmaterials and vitrinite reflectance is acritarchs and dinoflagellate cysts showing the least. well established in estimatingsource rock maturity in Calibration of the resultsfrom optical analyses to exact hydrocarbon exploration. Palaeotemperature data from the palaeotemperatures is complicated by potentialvariables examination of the optical properties of the organic fossils including sediment lithology, rock pH, humic staining and provides datafor estimation of maximum overburden oxidation. thickness, including that removed by erosion during uplift. Conodontsand other phosphatic fossils including Initialresults from the British Isles suggest