Lower Silurian Hot Shales Adnan Aqrawi

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261 Journal of Petroleum Geology, Vol. 32(3), July 2009, pp 261-270 © 2009 The Authors. Journal compilation © 2009 Scientific Press Ltd LOWER SILURIAN “HOT SHALES” IN JORDAN: A NEW DEPOSITIONAL MODEL D. K. Loydell a* , A. Butcher a , J. Frýda b , S. Lüning c and M. Fowler a Data are presented from the Batra Formation (also known as the Mudawwara Shale Formation) of a core from well BG-14 in the Batna el Ghoul area, southern Jordan, which enable a new depositional model to be proposed for the middle Rhuddanian (lower Llandovery, Silurian) “hot shale” which may be applicable to other Arabian and North African “hot shales” of similar stratigraphical age. This “hot shale” probably results from rapid early burial of organic carbon associated with a minor regression during which anoxic bottom conditions were maintained for most, but not all, of the time. Evidence for regression comes from (1) increased sediment grain size within the “hot shale” by comparison with underlying shales; (2) palynological changes including a decrease in acritarch species diversity; an increase in the relative abundance of sphaeromorphs, veryhachiids with three processes and acritarchs with short, simple processes; and a decrease in the relative abundance of acanthomorphs; (3) a positive δ 13 C org excursion (other Late Ordovician and Silurian positive δ 13 C org excursions occur during regressions); and (4) very brief intervals of oxygenation (associated with sediment influx) reflected in the preservation of graptolites as three-dimensional pyrite internal moulds, rather than as flattened periderm. The minor regression reflects a eustatic sea-level fall, evidence for which has recently been presented from several regions, including Arctic Canada, Bohemia and Scotland. The BG-14 “hot shale” is shown to be thicker than estimated in previous studies. Previous TOC measurements from the upper part of the “hot shale” were affected by the weathering of overlying strata in the BG-14 core. ICP-MS measurements show that uranium content is high in these weathered levels, extending the stratigraphical extent of the “hot shale” interval into the middle Rhuddanian. Depositional models such as that presented here rely on a robust biostratigraphical framework; in the Ordovician and Silurian of Arabia and North Africa, this can be provided by graptolites and chitinozoans. a School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL. b Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic. c RWE Dea, Überseering 40, 22297 Hamburg, Germany. * Corresponding author: email: [email protected] Key words: “hot shale”, Jordan, Silurian, Llandovery, graptolite, palynology, acritarch, carbon isotope. INTRODUCTION Recent high resolution graptolite biostratigraphical studies (Lüning et al., 2005; Loydell, 2007a) have demonstrated that the so-called “lower hot shale” in the lower Silurian of Jordan is in fact represented by two stratigraphically separate “hot shale” intervals: one in the lower Rhuddanian (lower Llandovery) ascensus-acuminatus graptolite Biozone; and the other at a higher level, in the middle Rhuddanian. The purpose of this paper is to present evidence for a new depositional model for the middle Rhuddanian “hot shale” in core from well BG-14, southern Jordan, which we demonstrate is thicker than previously estimated. The evidence and discussion will focus on palynology and carbon isotopes, with a brief description of graptolite preservation and sedimentology.

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Transcript of Lower Silurian Hot Shales Adnan Aqrawi

Page 1: Lower Silurian Hot Shales Adnan Aqrawi

261Journal of Petroleum Geology, Vol. 32(3), July 2009, pp 261-270

© 2009 The Authors. Journal compilation © 2009 Scientific Press Ltd

LOWER SILURIAN “HOT SHALES”IN JORDAN: A NEW DEPOSITIONAL MODEL

D. K. Loydella*, A. Butchera, J. Frýdab, S. Lüningc and M. Fowlera

Data are presented from the Batra Formation (also known as the Mudawwara Shale Formation)of a core from well BG-14 in the Batna el Ghoul area, southern Jordan, which enable a newdepositional model to be proposed for the middle Rhuddanian (lower Llandovery, Silurian) “hotshale” which may be applicable to other Arabian and North African “hot shales” of similarstratigraphical age. This “hot shale” probably results from rapid early burial of organic carbonassociated with a minor regression during which anoxic bottom conditions were maintained formost, but not all, of the time.

Evidence for regression comes from (1) increased sediment grain size within the “hot shale” bycomparison with underlying shales; (2) palynological changes including a decrease in acritarchspecies diversity; an increase in the relative abundance of sphaeromorphs, veryhachiids with threeprocesses and acritarchs with short, simple processes; and a decrease in the relative abundance ofacanthomorphs; (3) a positive δ13Corg excursion (other Late Ordovician and Silurian positive δ13Corgexcursions occur during regressions); and (4) very brief intervals of oxygenation (associated withsediment influx) reflected in the preservation of graptolites as three-dimensional pyrite internalmoulds, rather than as flattened periderm.

The minor regression reflects a eustatic sea-level fall, evidence for which has recently beenpresented from several regions, including Arctic Canada, Bohemia and Scotland. The BG-14 “hotshale” is shown to be thicker than estimated in previous studies. Previous TOC measurements fromthe upper part of the “hot shale” were affected by the weathering of overlying strata in the BG-14core. ICP-MS measurements show that uranium content is high in these weathered levels, extendingthe stratigraphical extent of the “hot shale” interval into the middle Rhuddanian.

Depositional models such as that presented here rely on a robust biostratigraphical framework;in the Ordovician and Silurian of Arabia and North Africa, this can be provided by graptolites andchitinozoans.

a School of Earth and Environmental Sciences, Universityof Portsmouth, Burnaby Road, Portsmouth PO1 3QL.b Czech Geological Survey, Klárov 3, 118 21 Praha 1,Czech Republic.c RWE Dea, Überseering 40, 22297 Hamburg, Germany.* Corresponding author: email: [email protected]

Key words: “hot shale”, Jordan, Silurian, Llandovery,graptolite, palynology, acritarch, carbon isotope.

INTRODUCTION

Recent high resolution graptolite biostratigraphicalstudies (Lüning et al., 2005; Loydell, 2007a) havedemonstrated that the so-called “lower hot shale” inthe lower Silurian of Jordan is in fact represented bytwo stratigraphically separate “hot shale” intervals:

one in the lower Rhuddanian (lower Llandovery)ascensus-acuminatus graptolite Biozone; and theother at a higher level, in the middle Rhuddanian. Thepurpose of this paper is to present evidence for a newdepositional model for the middle Rhuddanian “hotshale” in core from well BG-14, southern Jordan,which we demonstrate is thicker than previouslyestimated. The evidence and discussion will focus onpalynology and carbon isotopes, with a briefdescription of graptolite preservation andsedimentology.

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MATERIALS AND METHODS

The shallow kaolinite exploration well BG-14 islocated at 29º33.574’N, 35º58.528’E in the Batna ElGhoul area of southern Jordan (Fig. 1). Core from theBatra Formation (also known as the MudawwaraFormation; see Lüning et al., 2005, fig. 2) from thiswell has already been the subject of extensivepalaeontological study, with publications on itsgraptolites (Loydell, 2007a) and chitinozoans(Butcher, 2009). It has also featured in discussions ofdepositional models for “hot shales” in Jordan (Lüninget al.,2005, 2006; Armstrong et al., 2005, 2006, 2009).

Samples of core were collected by one of theauthors (S.L.) for geochemical and graptolitebiostratigraphical studies (Lüning et al., 2005;Loydell, 2007a). The graptolitic core material wassubsampled (by D.K.L.) for palynological, carbonisotope and ICP-MS analyses. The observationsconcerning sedimentology and graptolite preservationbelow are based on these graptolitic samples and onthe sedimentological descriptions of the core byArmstrong et al. (2005, 2009).

Samples were analysed for U (and Th) by ICP-MS following fusion dissolution. 0.25 gm aliquots ofdried powder were mixed intimately with 0.75 gmJohnson Matthey Spectroflux 100B meta/tetraborateflux and fused in graphite at 1000° C for 30 minutes.The fusion beads were then dissolved in 10%, trace-metal grade nitric acid. The solutions were filteredand made to volume with 18MΩ deionised water to afinal acid concentration of c. 5%. These were analysedon an Agilent 7500ce ICP-MS at the University ofPortsmouth, using the octopole reaction cell in He

mode. Calibrations were defined with bespokedilutions of Merck® multi-element standard solution.Accuracy was monitored with accompanyingdigestions of USGS reference material SCo-1 (Codyshale), and precision with replicate analyses. The SCo-1 preparation gave values of 3.0 and 10.0 ppm U andTh respectively, for comparison with recommendedvalues of 3.0 and 9.7 ppm (Potts et al., 1992).Precision (replicate preparations of the same powder)was better than 5% r.s.d.

Palynological samples were processed usingstandard HCl-HF-HCl techniques (Sutherland, 1994),with the organic fraction separated using sodiumpolytungstate (Gelsthorpe, 2002). The acritarch datapresented below are based upon counts of at least 250acritarchs per sample undertaken on a Jenavalpalynological microscope.

δ13Corg and TOC analyses were conducted usingthe methods outlined by Loydell and Frýda (2007).Hand specimens were cut and a few milligrammes ofrock powder were taken from a few grammes of freshsample. Before analyses, rock powders weredecarbonatized with 10% HCl at 40°C for severalhours, then washed and dried. About 20 milligrammesof rock powder were used for TOC and about 10milligrammes for isotope analyses. Samples werecombusted in a Fisons 1108 elemental analyzercoupled on-line to a Finnigan Mat 251 massspectrometer via a ConFlo interface. As referencematerial, NBS 22 (Gulf oil, with δ13C value -29.75‰)and acetanilid (Analytical Microanalysis, UK) weremeasured. Accuracy and precision were controlled byreplicate measurements of laboratory standards andwere better than ±0.1‰(1σ) for total carbon isotope

Fig. 1. Location of kaolinite well BG-14 in southern Jordan (map modified from Loydell, 2007a).

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analyses and better than ±0.02% (1σ) for total organiccarbon content.

DEFINITION, THICKNESS ANDSTRATIGRAPHICAL EXTENT OF THE“HOT SHALE”

Lüning et al. (2005, fig. 11) referred to all of the stratabetween 28.5 m and 46.8 m in core from well BG-14as “hot shale”. In a study of the E1-NC174 core,Murzuq Basin, Libya, however, Lüning et al. (2003,fig. 2) restricted the term “hot shale” to the intervalcontaining the highest TOC and uranium values. Weuse this more restricted definition here.

The TOC (Fig. 2) and Rock-Eval S2 curves for wellBG-14 (Lüning et al., 2005, fig. 11) show a sharpincrease from a depth of 35.0 m to their highest valuesat 33.5 m, suggesting that the base of the “hot shale”is located between these depths, in strata belongingeither to the uppermost ascensus-acuminatusgraptolite Biozone or lower vesiculosus graptolite

Biozone (Fig. 2). Both the TOC (Fig. 2) and S2 values(Lüning et al., 2005, fig. 11) decline sharply between30.5 m and 30.0 m, and reach almost zero at 28.5 m.However, signs of weathering are apparent above the30 m level within the core. The weathering front maybe quite sharp: at 30 m the rock is grey, and bothpalynomorphs and graptolites are well-preserved; at27.5 m, however, although still laminated andgraptolitic, the rock is no longer grey but is a purplishbrown (Fig. 3B); the periderm of the graptolites isless well preserved and the palynomorphs present areoxidized. Although the uranium concentration at adepth of 30.0 m is at its maximum (in Libyan “hotshales” TOC and U have been demonstrated to showa broadly positive correlation; Lüning et al., 2003), itis possible that the TOC value at this depth may besignificantly less than its original value. Zalasiewiczet al. (2007, p. 16) similarly noted that in core fromwell Qusaiba-1 in the Al-Qasim area, Saudi Arabia,the top 97 ft (29.56 m) were barren of palynomorphsbecause of weathering. The very low TOC values

Fig. 2. Curves showing variations in total organic carbon (TOC), uranium (U), and δ13Corg within the BatraFormation in the BG-14 core. The first TOC curve is from Lüning et al. (2005). The two samples exhibitingcoarser grain size are indicated by “silt”. Graptolite biozonation is after Loydell (2007a).

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recorded by Lüning et al. (2005) at 28.5 m and above inwell BG-14 therefore may not reflect original values butvalues that have been reduced by weathering.

Lüning et al. (2003) demonstrated that it is possibleto identify originally organic-rich shales in weatheredexposures using the uranium content, determined usingspectral gamma-ray measurements. The samplesavailable from core from well BG-14 were too smallfor gamma-ray measurements to be made. Instead,direct measurement of uranium content was conductedby ICP-MS analysis. The results (Fig. 2) show uraniumvalues rising to a peak of 25.5 ppm in the 30.0 msample, and declining to 11.3 ppm at 27.5 m, a valueidentical to that within the “hot shale” at 32.5 m whereTOC is 10.5%. This strongly suggests that the “hotshale” in the BG-14 core is significantly thicker thanoriginally suggested by Lüning et al. (2005), andextends well into the middle Rhuddanian vesiculosusgraptolite Biozone.

PALYNOLOGY

Chitinozoans and acritarchs are used extensively inPalaeozoic biostratigraphical and palaeoenvironmentalstudies. Descriptions of the chitinozoans from the BG-14 core and a discussion of their biostratigraphicalsignificance appear in Butcher (2009) and here webriefly discuss only chitinozoan abundance. Samplesfrom above a depth of 30 m have been affected byweathering, thus limiting analysis to the lower part ofthe “hot shale” and the strata below it.

Chitinozoan abundanceChitinozoan abundance, expressed as number ofchitinozoans per gramme of rock processed (Fig. 4),appears to be highly variable within “hot shales”. Forexample, Cole (1994) recorded that chitinozoans wereabundant in samples from the high gamma-ray part ofthe Qusaiba Shale in Saudi Arabia, and Paris et al.(1995, p. 77) recorded up to 3000 chitinozoans pergm from a “hot shale” in central Saudi Arabia.However, Batten (1996, p. 1031) noted thatchitinozoans show generally low abundance anddiversity in organic-rich shales. Within the BG-14 corechitinozoan abundance is very variable: the lowestabundance figures, 104 and 196 chitinozoans pergramme, occur within the “hot shale” and aresubstantially lower than the abundances recordedbelow (670–4762 chitinozoans/gm). This suggestseither a reduction in the number of chitinozoans beingdeposited within the “hot shale” or their dilution dueto the enhanced preservation of other organic matteror to a higher sedimentation rate. Trends in acritarchabundance (see below) parallel those of chitinozoanabundance (Fig. 4), suggesting that the same factor(s)similarly affected the abundance of both groups.

Acritarch abundance and diversityAcritarch abundance fluctuates in core BG-14 (Fig.4), but is high (c. 4,400 to c. 36,000 / gm) throughout.The “hot shale” contains both the highest and lowestabundances of acritarchs. What was not encounteredin core BG-14 was an interval such as that describedby Le Herissé (2000) from the lower Rhuddanian ofSaudi Arabia which, despite yielding abundantgraptolites, chitinozoans and other organic matter,did not contain acritarchs. Similarly, Vecoli et al.(2009) recorded an interval, the lower part of whichis of Rhuddanian age, in which acritarchs are‘virtually absent’. Le Herissé’s (2000) acritarch-freesamples were from the lower part of a “hot shale”;Vecoli et al.’s (2009) acritarch-poor samples werefrom below and within a “hot shale”.

With the exception of the highest sample (at 30.0m), acritarch diversity declines upwards within the“hot shale” interval (Fig. 4). Acritarch diversity hasbeen found to decline in Early Palaeozoic shelfenvironments in a shorewards direction. For example,Dorning (1981) recorded 25–60 species offshore butonly 5–15 species in nearshore environments.Similarly, Li et al. (2004) recorded a decline from40 acritarch species on an ‘offshore shelf’ to 10species in nearshore environments. Thus a fall in sea-level, generating a basinward advance of theshoreline, would be expected to be associated with areduction in acritarch diversity. Molyneux (2009)emphasized that acritarch diversity also declines asfacies pass from shelfal to basinal. The Rhuddanianstrata of Jordan were deposited on the broad northernGondwanan shelf, however (Lüning et al., 2003;Armstrong et al., 2005), suggesting that a decline indiversity resulting from a transition from shelf tobasin was unlikely. The decline in species diversityrecorded in the BG-14 “hot shale” is thereforeinferred to result from regression.

Relative abundance of sphaeromorphsAn increase in the relative abundance ofsphaeromorphs in acritarch assemblages is widelyrecognised as indicating shallowing, either along aprofile from offshore to inshore shelf (e.g. Dorning,1981; Li et al., 2004; Stricanne et al., 2004) or withina regessive sequence (e.g. Jacobsen, 1979;Richardson and Rasul, 1990). In core BG-14,sphaeromorph relative abundance increasessignificantly upwards into and within the “hot shale”(Fig. 4), suggesting shallowing.

Variation in the numberof processes in veryhachiidsRichardson and Rasul (1990) and Le Hérissé (2002)observed that veryhachiids (simple process-bearingpolygonal acritarchs) with three processes were more

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Fig. 3. BG-14 core samples.A: BGS FOR 5359, depth 43.4 m, from below the “hot shale”; note abundant, flattened graptolite rhabdosomesshowing current alignment.B : BGS FOR 5363, depth 27.5 m, weathered sample with very low TOC, but 11.3 ppm uranium, indicating thatthis level is still within the “hot shale”.C: BGS FOR 5378a, depth 32.5 m, Normalograptus normalis proximal end, internal mould preserved in pyrite inthree dimensions, from within the “hot shale”; photographed under ethanol.D: BGS FOR 5410, depth 37.5 m, from below the “hot shale”.E: BGS FOR 5383, depth 33.5 m, from within the “hot shale”; note the coarser grain size, darker colour(perhaps reflecting the higher organic content) and flattened graptolites.F: BGS FOR 5368, depth 28.5 m; photographed under ethanol; note the sharp boundary (arrowed) between thepaler oxic/turbiditic mudstone and the darker graptolitic shale and the extensive pyrite; graptolites in the 28.5m sample are preserved three-dimensionally as pyrite internal moulds.A, B, D, E are × 0.95; C is × 7.6, F is × 2.375. Specimens are housed in the British Geological Survey, Keyworth.

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abundant in nearshore environments, while those withfour processes were more common further offshore.Mullins et al. (2004), however, considered that bothforms “may have had broad palaeoenvironmentaltolerances and that their ratio, therefore, may not be areliable indicator of distance from shoreline”. The BG-14 data (Fig. 4) shows high relative abundance ofVeryhachium with three processes within the “hot shale”.

Relative abundance of acanthomorphsStricanne et al. (2004) noted that the abundance ofacanthomorph acritarchs increases towards the distalshelf. Within the BG-14 “hot shale” the relativeabundance of acanthomorphs declines upwards (Fig.4), again suggesting shallowing in the depositionalenvironment.

Acritarch process length and complexityFor reasons unknown, acritarchs with long, complexprocesses characterize offshore environments (e.g. Al-Ameri, 1983; Li et al., 2004; Stricanne et al., 2004,2006), while acritarchs with short, simple processesare more common in nearer shore environments. Thetrend shown in core BG-14 (Fig. 4, right hand column),with a marked increase in the relative abundance of

acritarchs with short simple processes, suggestsshallowing during deposition of the “hot shale”.

SummaryThe palynological evidence presented above isconsistent in indicating that the “hot shale” in coreBG-14 was deposited during a regression.

CARBON ISOTOPES

In core BG-14, δ13Corg values are close to -31‰ (range-31.12 to -30.92‰) in the lowest six samples (46.8m to 42.5 m) analysed (Fig. 2). Above this, valuesrise, but fluctuate between -30.46‰ and -9.92‰at depths between 39.8 m and 30.9 m with a sharprise to -29.01‰ and -28.82‰ at 30.0 m and 28.5 m,respectively (Fig. 2). This positive excursion thereforedeveloped in two phases: a positive shift of 0.5–1‰late in the ascensus-acuminatus Zone, followed by asharp positive shift of more than 1.5‰ in thevesiculosus Zone. A similar pattern can be seen inthe δ13Corg data presented by Armstrong et al. (2005,fig. 10; 2009, fig. 6).

In graptolitic shales, a consistent relationship hasbeen recognised between positive δ13Corg excursions

Fig. 4. Palynomorph abundance data versus depth in the Batra Formation in the BG-14 core. Curves show:absolute abundance per gramme of chitinozoans and acritarchs; acritarch species diversity (expressed asnumber of species); relative abundance of sphaeromorphs (spherical unornamented acritarchs, calculatedagainst acanthomorphs, polygonomorphs and netromorphs); relative abundance of Veryhachium speciesbearing three processes (as compared with those bearing four); relative abundance of acanthomorph (non-polygonomorph, process-bearing acritarchs, calculated against sphaeromorphs, polygonomorphs andnetromorphs); relative abundance of acritarch species bearing short, simple processes (calculated againstthose with long simple, long complex, and short complex processes). Graptolite biozonation is after Loydell(2007a).

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at 33.5 m (Fig. 3E) and 32.5 m, where the sedimentgrain size was coarser than in any of the othersamples examined. This also suggests an enhancedsedimentation rate, consistent with the regressioninterpreted from the palynological and δ13Corg resultsabove. At 28.5 m (Fig. 3F), there is evidence eitherfor an input of thick, distal muddy turbidites or foroxygenated bottom waters: there is a clear boundarybetween dark graptolitic shale and paler mudstone.

This sedimentological and fossil preservationalevidence for temporary, more oxygenatedconditions during deposition of the “hot shale”seems counter-intuitive, as oxygenated conditionsare not usually invoked to generate organic-richshales. The implication is either that thesedimentation rate was sufficiently high for a muchgreater than normal amount of organic material tobe buried rapidly below the taphonomically activezone and thus to escape oxidative decay; or, assuggested above, that the intervals with oxygenatedbottom waters were brief interruptions ofpredominantly anoxic bottom-water conditionswhich, under conditions of increased sedimentationrate associated with regression, caused a higher thanusual amount of organic carbon to be buried rapidly.The oxic bottom intervals are conspicuous in thecore because they are associated with an influx ofsediment of different colour/grain size and unusualfossil preservation (for this core).

DISCUSSION

Rhuddanian sea-level changesThe data presented above indicate that the middleRhuddanian “hot shale” in core BG-14 wasdeposited during a regression, a very differentscenario from that envisaged in other depositionalmodels for the Jordanian “hot shales” (Lüning etal., 2003; Armstrong et al., 2005, 2009), whichemphasize deposition during transgression.Armstrong et al. (2009), for example, refer to “aprogressive increase in the sedimentation of 12C-enriched organic matter during progressivedeglaciation”.

The eustatic sea-level curves of both Johnson(1996) and Loydell (1998) show a sustained andsteady rise in eustatic sea-level through theRhuddanian. Evidence presented since thepublication of these sea-level curves has, however,demonstrated a rather more complex history of sea-level change through the Rhuddanian. In particular,Melchin and Holmden (2006, p. 176) referred to asea-level fall recognised in Arctic Canada, AnticostiIsland and Estonia dated to the mid-Rhuddanian;they suggested that this may be a eustatic event.Further support for the eustatic nature of this mid-

and intervals of lowered eustatic sea-level. This has beendemonstrated for the Hirnantian (e.g. Underwood et al.,1997), late Aeronian sedgwickii Zone (e.g. Melchin andHolmden, 2006), early Sheinwoodian (e.g. Loydell andFrýda, 2007) and mid Homerian (Lenz et al., 2006) (seeLoydell (2007b, 2008) for a detailed discussion). In thelight of the palynological evidence above, the positiveδ13Corg excursion recognised in core BG-14 is inferredto reflect a fall in sea-level. Evidence for the eustaticnature of the sea-level fall is discussed below.

GRAPTOLITE PRESERVATION ANDSEDIMENTOLOGY

Within the lower part of core BG-14, below the “hotshale”, graptolites are preserved exclusively as flattenedperiderm. Schieber (2003, p. 5) noted that “under anoxicconditions, with an excess of H2S in the sediment, allthe iron that was released from terrigenous grains wouldbe precipitated in the form of disseminated and tiny ironsulphide grains”. Thus it would not be expected forpyrite to accumulate preferentially withinmicroenvironments such as graptolite rhabdosomes.With oxygenated waters present above the seabed,however, localized growth of pyrite would be expectedto occur within the reducing microenvironmentsprovided by graptolite rhabdosomes, where circulationwould be reduced and perhaps soft tissues were stilldecaying. It is important to recognise, as emphasizedby Schieber (2003), that it is oxic (not anoxic) bottomwaters that “allow for localized accumulation of pyrite”.

It is only within the “hot shale”, at 32.5 m (Fig. 3C)and 28.5 m, that graptolite rhabdosomes are three-dimensionally preserved as a result of pyrite infill. Thissuggests that, perhaps rather surprisingly, conditions atthe seabed were temporarily (and probably very briefly)more oxygenated at times during deposition of the “hotshale”, perhaps associated with an influx of sediment-bearing waters. At 33.5 m (Fig. 3E), 30.9 m, 30.0 m and27. 5 m, also within the “hot shale”, graptolites areflattened, identical in preservation to those below the“hot shale” (Fig. 3A, D), suggesting anoxic bottomconditions. Armstrong et al. (2009, fig. 5) recoveredisorenieratene (XXIII) at a depth of 33.9 m (erroneouslystated to be from the Upper Ordovician) and at otherlevels within core BG-14, indicating photic zone anoxiaduring deposition, again suggesting that the periods ofgreater oxygenation of bottom waters were briefinterruptions in a predominantly anoxic environment.

Armstrong et al. (2009) described the strata in coreBG-14 as “monotonous, organic carbon-rich black shale,which in thin section comprise laminated black siltstoneto dark grey homogeneous claystone couplets”. Thisdescription is certainly true of the majority of the samplesexamined for their graptolites by Loydell (2007a).Exceptions occur low within the “hot shale”, however,

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Rhuddanian sea-level fall comes from studies ofsuccessions in Bohemia, where a lowstand isrecognised close to the acuminatus/vesiculosusgraptolite Biozone boundary (Štorch, 2006). It isinteresting also to consider the changes insedimentation and graptolite distribution recorded inthe Dob’s Linn section, Scotland. Here, much of thethe ascensus-acuminatus graptolite Biozone isrepresented by easily weathered, graptolite-rich‘shivery shales’ (Lapworth, 1877), which are overlainby hard, thick-bedded black flagstones of theuppermost ascensus-acuminatus and vesiculosusgraptolite biozones (Toghill, 1968) with graptolitesabundant at a few levels, but with intervening strata‘wholly destitute of organic remains’ (Lapworth,1877). This suggests an increase in distal turbiditicinput during the vesiculosus Zone, as would beexpected during a regressive interval.

Carbon isotope and TOC dataMelchin and Holmden (2006) summarized carbonisotope data for the Llandovery. Unfortunately, mostRhuddanian data are from sections with eitherstructural complications or poor graptolitebiostratigraphical control. On Cornwallis Island(Arctic Canada), however, Melchin and Holmden(2006, figs 2, 3) recorded a minor positive δ13Corgexcursion commencing close to the acuminatus/vesiculosus graptolite Biozone boundary. TOC valuesrise (to their highest Llandovery values in the CapeManning section) within the lower part of thevesiculosus Biozone (the biozone is divided into theatavus and acinaces biozones in Arctic Canada). Morerecently, Yan et al. (2009, fig. 2) presented a δ13Corgcurve for the Wangjiawan section, China, showing amarked rise in δ13Corg between the upper acuminatusand vesiculosus graptolite biozones.

A new depositional modelfor “hot shales”The model proposed below is intended to explain thegenesis only of the middle Rhuddanian “hot shale”.There is no published evidence for regressionassociated with the stratigraphically lower “hotshales”, such as those in the upper Hirnantianpersculptus Biozone and in the lower Rhuddanianascensus-acuminatus Biozone of Jordan discussed byLüning et al. (2005). Their origin can be explainedby processes associated with post-glacialtransgression.

The model that we outline here may be applicableto stratigraphically equivalent “hot shales” in NorthAfrica, for example those described from the middleRhuddanian of Libya by Fello et al. (2006). It mustbe stressed that depositional models such as thatpresented here can only be developed when high

resolution biostratigraphical data is available.Graptolites and chitinozoans are the most usefulfossils in this regard in the Ordovician and Silurianof North Africa and the Middle East.

The widespread nature of black graptolitic shalefacies in the Ordovician and Silurian suggests thatEarly Palaeozoic seas were less well oxygenated thanthose later in the Phanerozoic. Only major climaticperturbations (e.g. the Hirnantian glaciation) werecapable of introducing oxygenated waters and thusbioturbated facies into deeper shelf and basinalenvironments for prolonged periods over widegeographical regions. A minor fall in sea-level, suchas that proposed for the mid-Rhuddanian, permittedmaintenance of largely anoxic conditions at the seafloor. A higher sedimentation rate, associated with theregression, resulted in more rapid burial of organicmatter and thus the high TOC values recorded in the“hot shales”. A contributory factor may have been thatregression would have resulted in enhanced terrestrialinput of biolimiting nutrients into the shelf seasbordering Gondwana, thus stimulating increasedorganic productivity and thus a greater amount oforganic material available for rapid burial. This modelis consistent with all of the data available for themiddle Rhuddanian “hot shale” of Jordan.

CONCLUSIONS

There are several different “hot shales” in the UpperOrdovician – lower Silurian succession of Jordan.Deposition of the Hirnantian and lower Rhuddanian“hot shales” appears to be associated withdeglaciation-induced transgression. The focus of thispaper, however, has been the middle Rhuddanian “hotshale” which, by contrast, is inferred to have beendeposited during an interval of lowered eustatic sea-level resulting in regression, the “hot shale” resultingfrom more rapid burial of organic material andperhaps increased primary productivity at this time.We present several lines of evidence for regression:

1. Palynological evidence: acritarch morphologiesindicative of nearer-shore environments(sphaeromorphs; taxa with short, simple processes;veryhachiids with three processes) increase in relativeabundance in the “hot shale”, while those more typicalof offshore environments (acanthomorphs) decline inrelative abundance. Total acritarch species diversityalso declines within the “hot shale”, another indicatorof a shift from a more offshore to a nearer-shoreenvironment of deposition.

2. Organic carbon isotope analyses: a positiveδ13Corg excursion commences within the “hot shale”.In graptolitic shales, a consistent relationship has beenrecognised between positive δ13Corg excursions andintervals of lowered eustatic sea-level.

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3. Sedimentological evidence: the coarsestsedimentary grain size occurs within the “hot shale”,suggesting greater proximity to the shore.

4. Graptolite preservation: three-dimensionalpyrite internal moulds are known only from withinthe “hot shale” (all other graptolites are flattened),suggesting brief periods of oxygenation perhapsassociated with sediment influx during regression.

The interpretation of this evidence anddevelopment of this new model for “hot shale”deposition during regression relies on a solidbiostratigraphical framework provided by graptolitesand chitinozoans.

ACKNOWLEDGEMENTS

The authors thank journal referees Adnan Aqrawi(StatoilHydro) and Petr Štorch (Institute of Geology,Prague) for their constructive criticisms on a previousversion of the manuscript.

REFERENCES

AL-AMERI, T. K., 1983. Acid resistant microfossils used in thedetermination of Palaeozoic palaeoenvironments in Libya.Palaeogeography, Palaeoclimatology, Palaeoecology, 44, 103–116.

ARMSTRONG, H. A., TURNER, B. R., MAKHLOUF, I. M.,WEEDON, G. P., WILLIAMS, M., AL SMADI, A. and ABUSALAH, A., 2005. Origin, sequence stratigraphy anddepositional environment of an upper Ordovician(Hirnantian) deglacial black shale, Jordan. Palaeogeography,Palaeoclimatology, Palaeoecology, 220, 273–289.

ARMSTRONG, H. A., TURNER, B. R., MAKHLOUF, I. M.,WEEDON, G. P., WILLIAMS, M., AL SMADI, A. and ABUSALAH, A., 2006. Reply to “Origin, sequence stratigraphyand depositional environment of an upper Ordovician(Hirnantian) deglacial black shale, Jordan”. Palaeogeography,Palaeoclimatology, Palaeoecology, 230, 356–360.

ARMSTRONG, H. A., ABBOTT, G. D., TURNER, B. R.,MAKHLOUF, I. M., MUHAMMAD, A. B., PEDENTCHOUK,N. and PETERS, H., 2009. Black shale deposition in an UpperOrdovician-Silurian permanently stratified, peri-glacialbasin, southern Jordan. Palaeogeography, Palaeoclimatology,Palaeoecology, 273, 368–377.

BATTEN, D. J., 1996. Palynofacies and palaeoenvironmentalinterpretation. In: Jansonius, J. and McGregor, D. C. (Eds)Palynology: principles and applications. Association ofStratigraphic Palynologists Foundation, Dallas, 1011–1064.

BUTCHER, A., 2009. Early Llandovery chitinozoans from Jordan.Palaeontology, 52, 593–629.

COLE, G. A., 1994. Graptolite–chitinozoan reflectance and itsrelationship to other geochemical maturity indicators inthe Silurian Qusaiba Shale, Saudi Arabia. Energy & Fuels, 8,1443–1459.

DORNING, K., 1981. Silurian acritarch distribution in theLudlovian shelf sea of South Wales and the WelshBorderland. In: Neale, J. W. and Brasier, M. D. (Eds)Microfossils from Recent and fossil shelf Seas. Ellis Howood,Chichester, 31–36.

FELLO, N., LÜNING, S., ŠTORCH, P. and REDFERN, J., 2006.Identification of early Llandovery (Silurian) anoxic palaeo-depressions at the western margin of the Murzuq Basin

(southwest Libya), based on gamma-ray spectrometry insurface exposures. GeoArabia, 11, 101–118.

GELSTHORPE, D. N., 2002. Testing of palynological processingtechniques: an example using Silurian palynomorphs fromGotland. Journal of Micropalaeontology, 21, 81–86.

JACOBSEN, S. R., 1979. Acritarchs as palaeoenvironmentalindicators in Middle and Upper Ordovician rocks fromKentucky, Ohio and New York. Journal of Paleontology, 53,1197–1212.

JOHNSON, M. E., 1996. Stable cratonic sequences and astandard for Silurian eustasy. Geological Society of AmericaSpecial Paper, 306, 203–211.

LAPWORTH, C., 1877. The Moffat Series. Quarterly Journal ofthe Geological Society of London, 34, 240–346, pls 11–13.

LE HÉRISSÉ, A., 2000. Characteristics of the acritarch recoveryin the early Silurian of Saudi Arabia. In: Al-Hajri, S. andOwens, B. (Eds) Stratigraphic palynology of the Palaeozoicof Saudi Arabia. Gulf PetroLink, Bahrain, 57–81.

LENZ, A. C., NOBLE, P. J., MASIAK, M., POULSON, S. R. andKOZLOWSKA, A., 2006. The lundgreni Extinction Event:integration of paleontological and geochemical data fromArctic Canada. GFF, 128, 153–158.

LI, J., SERVAIS, T., YAN, K. and ZHU, H., 2004. A nearshore–offshore trend in acritarch distribution from the Early–Middle Ordovician of the Yangtze Platform, South China.Review of Palaeobotany and Palynology, 130, 141–161.

LOYDELL, D. K., 1998. Early Silurian sea-level changes. GeologicalMagazine, 135, 447–471.

LOYDELL, D. K., 2007a. Graptolites from the Upper Ordovicianand lower Silurian of Jordan. Special Papers in Palaeontology,78, 1–66.

LOYDELL, D. K., 2007b. Early Silurian positive δ13C excursionsand their relationship to glaciations, sea-level changes andextinction events. Geological Journal, 42, 531–546.

LOYDELL, D. K., 2008. Reply to ‘Early Silurian positive δ13Cexcursions and their relationship to glaciations, sea-levelchanges and extinction events: Discussion’ by Bradley D.Cramer and Axel Munnecke. Geological Journal, 43, 511–515.

LOYDELL, D. K. and FRÝDA, J., 2007. Carbon isotopestratigraphy of the upper Telychian and lower Sheinwoodian(Llandovery–Wenlock, Silurian) of the Banwy River section,Wales. Geological Magazine 144, 1015–1019.

LÜNING, S., CRAIG, J., LOYDELL, D. K., ŠTORCH, P. andFITCHES, B., 2000. Lower Silurian ‘hot shales’ in NorthAfrica and Arabia: regional distribution and depositionalmodel. Earth-Science Reviews, 49, 121–200.

LÜNING, S., KOLONIC, S., LOYDELL, D. K. and CRAIG, J.,2003. Reconstruction of the original organic richness inweathered Silurian shale outcrops (Murzuq and Kufrabasins, southern Libya). GeoArabia, 8, 299–308.

LÜNING, S., SHAHIN, Y. M., LOYDELL, D., AL-RABI, H. T., MASRI,A., TARAWNEH, B. and KOLONIC, S., 2005. Anatomy of aworld-class source rock: distribution and depositionalmodel of Silurian organic-rich shales in Jordan andimplications for hydrocarbon potential. AAPG Bulletin, 89,1397–1427.

LÜNING, S., LOYDELL, D., ŠTORCH, P., SHAHIN, Y. M. andCRAIG, J., 2006. Origin, sequence stratigraphy anddepositional environment of an upper Ordovician(Hirnantian) deglacial black shale, Jordan – discussion.Palaeogeography, Palaeoclimatology, Palaeoecology, 230, 352–355.

MELCHIN, M. J. and HOLMDEN, C., 2006. Carbon isotopechemostratigraphy of the Llandovery in Arctic Canada:implications for global correlation and sea-level change.GFF, 128, 173–180.

MOLYNEUX, S. G., 2009. Acritarch (marine microplankton)diversity in an Early Ordovician deep-water setting (theSkiddaw Group, northern England): implications for the

Page 10: Lower Silurian Hot Shales Adnan Aqrawi

270 Lower Silurian “hot shales” in Jordan

relationship between sea-level change and phytoplanktondiversity. Palaeogeography, Palaeoclimatology, Palaeoecology,275, 59–76.

MULLINS, G. L., ALDRIDGE, R. J. and SIVETER, D. J., 2004.Microplankton associations, biofacies andpalaeoenvironment of the type lower Ludlow Series,Silurian. Review of Palaeobotany and Palynology, 130, 163–194.

PARIS, F., VERNIERS, J., AL-HAJRI, S. and AL-TAYYAR, H., 1995.Biostratigraphy and palaeogeographic affinities of EarlySilurian chitinozoans from central Saudi Arabia. Review ofPalaeobotany and Palynology, 89, 75–90.

POTTS, P. J., TINDLE, A. G. and WEBB, P. C., 1992. Geochemicalreference material compositions. Whittles Publishing, CRCPress.

RICHARDSON, J. B. and RASUL, S. M., 1990. Palynofacies in alate Silurian regressive sequence in the Welsh Borderlandand Wales. Journal of the Geological Society, London, 147, 675–686.

SCHIEBER, J., 2003. Simple gifts and buried treasures –implications of finding bioturbation and erosion surfacesin black shales. The Sedimentary Record, 1(2), 4–8.

ŠTORCH, P., 2006. Facies development, depositional settingsand sequence stratigraphy across the Ordovician–Silurianboundary: a new perspective from the Barrandian area ofthe Czech Republic. Geological Journal, 41, 163–192.

STRICANNE, L., MUNNECKE, A. and PROSS, J., 2006. Assessingmechanisms of environmental change: palynological signalsacross the late Ludlow (Silurian) positive isotope excursion(δ13C, δ18O) on Gotland, Sweden. Palaeogeography,Palaeoclimatology, Palaeoecology, 230, 1–31.

STRICANNE, L., MUNNECKE, A., PROSS, J. and SERVAIS, T.,

2004. Acritarch distribution along an inshore–offshoretransect in the Gorstian (lower Ludlow) of Gotland,Sweden. Review of Palaeobotany and Palynology, 130, 195–216.

SUTHERLAND, S. J. E., 1994. Ludlow chitinozoans from thetype area and adjacent regions. Monograph of thePalaeontographical Society, 148 (594), 1–104, pls 1–18.

TOGHILL, P., 1968. The graptolite assemblages and zones ofthe Birkhill Shales (lower Silurian) at Dobb’s Linn.Palaeontology, 11, 654–668.

UNDERWOOD, C. J., CROWLEY, S. F., MARSHALL, J. D. andBRENCHLEY, P. J., 1997. High-resolution carbon isotopestratigraphy of the basal Silurian stratotype (Dob’s Linn,Scotland) and its global correlation. Journal of the GeologicalSociety, London, 154, 709–718.

VECOLI, M., RIBOULLEAU, A. and VERSTEEGH, G., 2009.Palynology, organic geochemistry and carbon isotopeanalysis of a latest Ordovician through Silurian clasticsuccession from borehole Tt1, Ghadamis Basin, southernTunisia, North Africa: palaeoenvironmental interpretation.Palaeogeography, Palaeoclimatology, Palaeoecology, 273, 378–394.

YAN, D., CHEN, D., WANG, Q., WANG, J. and WANG, Z.,2009. Carbon and sulphur isotopic anomalies across theOrdovician–Silurian boundary on the Yangtze Platform,South China. Palaeogeography, Palaeoc limatology,Palaeoecology, 274, 32–39.

ZALASIEWICZ, J., WILLIAMS, M., MILLER, M., PAGE, A. andBLACKETT, E., 2007. Early Silurian (Llandovery) graptolitesfrom central Saudi Arabia: first documented record ofTelychian faunas from the Arabian Peninsula. GeoArabia,12, 15–36.