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Illustrative camera lucida images of Rhuddanian graptolites from the Qusaiba Member of Saudi Arabia A-Normalograptus cf. jideliensis; B-Akidograptus ascensus; C-Normalograptus medius; D-Normalograptus lubricus; ENormalograptus normalis; F-Normalograptus? cf. lungmaensis; G-Neodiplograptus lanceolatus; H-Neodiplograptus aff. daedalus; I-Normalograptus? cf. persculptus; J-Normalograptus? parvulus; scale bars are 1 mm
Source publication
Analysis of Silurian graptolite assemblages from 1017 sample horizons in 132 cores (from 65 boreholes) through the Qusaiba Member, Qalibah Formation of Saudi Arabia, provides a refined graptolite biostratigraphy for the Arabian Peninsula comparable in its resolution to that from the British Isles and the Czech Republic. Over 150 graptolite species...
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Citations
... Recently, the oil and gas resources from the Hot Shale were summarized by Williams et al. (2016). ...
... 24 A location map of the Llandovery (Silurian) boreholes in the Arabian Peninsula (afterFig. 2ofWilliams et al., 2016) ...
... 25 A global correlation chart of Llandovery graptolite biozones (afterFig. 14ofWilliams et al., 2016) ...
Regional correlation of the Wufeng and Lungmachi formations is based on four correlative series across the Yangtze Platform. A global view of the latest Ordovician to early Silurian black shales is based on main regions and sections from the low latitudinal Gondwana and the periphery of the high latitudinal Gondwana.
... Organic-rich graptolitic shales were abundant in the lower Silurian sequences across northern Africa, Middle East and Arabia (Klemme and Ulmishek, 1991;Lüning et al., 2000;Farooqui et al., 2012;Melchin et al., 2013;Kaminski and Perdana, 2020). These "hot shales" are capped by organically lean graptolite-bearing shales, and finally by localized sandstone bodies (Wıllıams et al., 2016;Abbas et al., 2017). Organic-rich graptolite shales were also deposited in the Cadomia promontory of Gondwana in France (Robardet et al., 1994;Tetard et al., 2014). ...
This paper discusses the paleogeography and plate tectonics of the Silurian using five global and twelve regional scale maps. These maps illustrate the distribution of major
tectonic elements such as subduction zones, spreading centres, transform faults and main volcanic areas 435 and 425 Ma ago. The regional maps show the global paleogeographic configuration combined with palaeoenvironmental and palaeolithofacies distribution. The paleogeographic analysis shows that the continents of Gondwana, Laurentia, Laurussia, Siberia, South China and North China were separated by the Iapetus, Rheic, Paleoasian (Prototethys), Paleotethys, Panthalassa and Mongol-Okhotsk oceans. Spreading of the Rheic and Paleotethys oceans constituted the main Silurian extensional event. The Scythian-Turan-South Kazakhstan-Junggar-Tarim-North China chain of continents moved north-westwards, and this led to a gradual narrowing of the Paleoasian ocean. The Caledonian-Scandian orogeny that resulted from the collision of Baltica, Avalonia and Laurentia constituted the main convergent event during Silurian times.
... Neodiplograptus parajanus (Štorch, 1983) is also one of the key species commonly used to correlate within and between the Gondwanan and peri-Gondwanan regions (Štorch, 1996). Štorch (1996) indicated the range of Neodiplograptus parajanus (Štorch, 1983) to be from the middle to upper Akidograptus ascensus-Parakidograptus acuminatus Biozone and coincidentally similar findings had been recorded in Jordan (Loydell, 2007) and Saudi Arabia Saparin and Ismail-Ordovician-Silurian Graptolites of NW Malaysia (Williams et al., 2016). As a result, it is possible to further constrain the current Langkawi interval to the middle-upper part of the Akidograptus ascensus-Parakidograptus acuminatus Biozone, at least until more specimens are recovered. ...
Graptolites from the latest Ordovician to the earliest Silurian rocks of northwest Peninsular Malaysia are described and reviewed. The fossils were collected previously by C. R. Jones and presently by the authors inside the black mudstones from the basal section of Tanjung Dendang Formation in Pulau Langgun, Langkawi, which comprises assemblages from the Hirnantian Metabolograptus extraordinarius Biozone to the Rhuddanian Akidograptus ascensus – Parakidograptus acuminatus Biozone. The latest Ordovician strata also include a Hirnantia fauna bed between the Metabolograptus extraordinarius and Metabolograptus persculptus biozones, in which shelly fossils such as Mucronaspis sp. could be recovered. A revised graptolite biozonation is proposed for the latest Ordovician to the earliest Silurian succession of northwest Peninsular Malaysia. This interval is significant for understanding the extent of mass extinction events happening right at the end of the Ordovician period and subsequent faunal change in the region.
... It should be mentioned that 'Hot Shales' are only within the lower part of the Qusaiba Member (Hayton et al. 2017). In central and southern Saudi Arabia, the Qusaiba Member is of Llandoverian age (Le H eriss e et al. 1995;Zalasiewicz et al. 2007;Williams et al. 2016;Breuer et al. 2017 Stump et al. 1995). In Iran, the Sarchahan Formation has been identified in some wells in Coastal Fars (Darang #1, Zirreh #1, and Kuh-e Siah #1; Ghavidel-Syooki 1996). ...
A siliciclastic succession is exposed at the Faraghan mountain, northern Persian Gulf, southeastern Iran. A detailed, high-resolution palynological analysis was performed on the Zardkuh, Seyahou, Dargaz, and Sarchahan formations to verify the exact age and palaeogeographic position of the High Zagros Mountains. Two hundred surface samples from this succession were collected and analysed. Most samples yielded abundant and well-preserved chitinozoans, acritarchs, scolecodonts, and cryptospores. Fifty-three chitinozoan species (21 genera) were identified that permitted to establish the Eremochitina brevis, Desmochitina ornensis, Belonechitina henryi, Siphonochitina formosa, Belonechitina robusta, Tanuchitina fistulosa, Acanthochitina barbata, Armoricochitina nigerica, Ancyrochitina merga, Tanuchitina elongata, Spinachitina oulebsiri, and Spinachitina fragilis biozones in ascending stratigraphic order as was previously established for the North Gondwana Domain. These results indicate that the Zagros Mountain Belt of Iran was part of the North Gondwana palaeo-province during the Ordovician-Silurian. On the other hand, these chitinozoan biozones and other associated fauna (e.g., graptolites, trilobites, brachiopods, conodonts) suggest a late Early Ordovician (Floian)-Early Silurian (Rhuddanian) age range, with unconformities, for this succession. Based on chitinozoan biozones, a distinctive hiatus is present between the Zardkuh and Seyahou formations at Faraghan mountain, which corresponds to the absence of jenkinsi-tanvillensis chitinozoan biozones, encompassing the latest Middle Ordovician (latest Darriwilian: stage slice Dw3) and the early Late Ordovician (Sandbian: the stage slices Sa1 to Sa2 and time slices 5a to 5b). Two chitinozoan species were erected: Belonechitina bifurcaspina sp. nov., Angochitina persianense sp. nov., and Eremochitina cf. brevis was left in open nomenclature.
... Recently, a research impetus driven by development of unconventional exploration resulted in major advances in the understanding of the depositional history, stratigraphic architecture and thermal maturity evolution of the Qusaiba Shales (Hayton et al., 2017). Depositional models have heavily relied on the establishment of accurate chronostratigraphy based on graptolite and palynological biostratigraphy (Boukhamsin et al., 2013;Williams et al., 2016). Extensive palynological investigations of thousands of core and cuttings samples from hundreds of subsurface sections through the Qusaiba Shales in Saudi Arabia have demonstrated the unique role of palynology in the age-dating and paleoenvironmental reconstructions of these organic-rich sediments (Paris and Al-Hajri, 1995;Le Hérissé, 2000;Paris et al., 2015;Le Hérissé et al., 2015). ...
... The samples were selected as to cover a large range of maturities; this was evaluated based on previous studies on Qusaiba thermal maturity (e.g., İnan et al., 2017), and regional geology. All samples were thoroughly investigated for biostratigraphy using both palynomorphs and graptolites and reliable age-assignment were obtained (Table 1); details of biostratigraphic analysis are not discussed herein but are presented in Boukhamsin et al. (2013), Williams et al. (2016) and Hayton et al. (2017), and include previous studies from Le Hérissé (2000) and Miller and Melvin (2005). ...
... The Table 1 Depth intervals, age, biozones, and calculated/measured thermal maturity for the Qusaiba shales cored in the studied wells. Biozonation from Hayton et al. (2017) and present study; correlation between palynozonation and graptolite zonation is according to Hayton et al. (2017), p. 869) and Williams et al. (2016).VRE converted from graptolite reflectance measurements VRE%=Gro% x 0.8 (İnan et al., 2016). *VRE obtained from Tmax conversion (İnan et al., 2016) based on VRE%=(0.018 ...
The early Silurian Qusaiba Shales are proven source rocks of the many Paleozoic petroleum systems of Saudi Arabia. In this study, a new, simple and cost-effective method for semi-quantitative analysis and graphical representation of palynological assemblage composition (PZC Phytoplankton–Zooplankton–Cryptospore ternary diagram) and a new visual thermal alteration index (IPA–TAI Integrated Palynomorph–Amorphous organic matter Thermal Alteration Index) integrating amorphous organic matter and palynomorph color, were developed based on the palynological/palynofacies analysis of some selected wells penetrating the early Silurian Qusaiba Shales throughout Saudi Arabia. The methods were tested against independent paleoenvironmental and thermal maturity indicators mainly based on geochemical parameters, confirming their validity as effective alternative methods for optical kerogen analysis and interpretation. The thermal maturity values estimated for core samples in the Qusaiba Shales from 13 wells in Saudi Arabia showed a strong correlation with Vitrinite Reflectance Equivalent (VRE%) maturity values determined from graptolite reflectance and pyrolysis Tmax measurements in the entire range of maturities (immature to overmature stages), proving that the present visual thermal alteration index can be successfully employed as a time efficient and reliable method for estimating thermal maturity in the Qusaiba Shales of Saudi Arabia. The present results show that the Qusaiba Shales were deposited in mostly mid- to outer shelf, and occasionally nearshore marine environments under prevailing dysoxic and anoxic water conditions, resulting in accumulation and preservation of organic matter.
... Those species in the Laotian graptolite assemblages that do have a restricted geographical distribution are known only from mid-to highlatitude peri-Gondwanan and/or Gondwanan localities. Glyptograptus dufkai has been recorded previously from the Czech Republic (Štorch, 1992) and Spain (Štorch et al., in press), and Sudburigraptus cortoghianensis from Sardinia (Štorch and Serpagli, 1993), Jordan (Loydell, 2007a), the Montagne Noire, France (Štorch and Feist, 2008) and Saudi Arabia (Williams et al., 2016a(Williams et al., , 2016b. Streptograptus pericoi, from the Namphuc Formation is also a typical (peri-) Gondwanan species, recorded from localities in Spain (Štorch, 1998;Loydell et al., 2015). ...
Graptolites are described from the Llandovery of the Sepon mine area, central Laos, part of the Truong Son Terrane. The palaeobiogeographical affinities of the Rhuddanian graptolites are with peri-Gondwanan Europe and Arabia rather than equatorial regions such as South China and Laurentia (Arctic Canada). A review of previous palaeontological research on the middle Palaeozoic of the Indochina terranes reveals an often contradictory picture with vertebrates suggesting close proximity of South China to the Truong Son Terrane in the Devonian and invertebrates and radiolarians providing evidence for a palaeogeographical barrier between the two. Graptolites from the Mojiang area, Yunnan are typical of low latitude Silurian faunas and suggest that this area (Simao or Loei Terrane) was separate from the Truong Son Terrane.
... In Uzbekistan, Korenʼ & Melchin (2000) recognized a N. lubricus Subzone in the lower part of their ascensus Biozone, again below the lowest occurrence of A. ascensus. In Saudi Arabia, Williams et al. (2016) recorded N. lubricus in association with N. parvulus and N. bifurcatus Loydell, 2007 in a distinct lubricus Biozone recognized by them below the lowest akidograptid occurrences. It is obvious that, in some regions at least, the appearance of N. lubricus preceded the formal base of the ascensus Biozone defined by the FAD of the nominal taxon. ...
... The graptolite assemblage from the uppermost samples of the Estana section has been reported from almost every graptolite-bearing Ordovician-Silurian boundary section of peri-Gondwanan Europe by Štorch (1996). It is typically found in the lower and lower-middle parts of the combined ascensusacuminatus Biozone in other European and Spanish sections (Štorch, 1996) and was recently recorded in Saudi Arabia (Williams et al. 2016). Faunal elements from Estana that are new to peri-Gondwanan Europe are few, but K. bicaudatus, K. lanpherei and Hirsutograptus sp. ...
... Normalograptus rhizinus, which also occurs in the upper persculptus Biozone in China and southern Scotland, made its lowest occurrence two samples below the lowest occurrence of A. ascensus in the Estana section and ranges well into the lower part of the ascensus-acuminatus Biozone. Normalograptus lubricus, described by Chen & Lin (1978) from the lowermost ascensus-bicaudatus Biozone of China and recorded from the same level in southern Scotland (Fan, Melchin & Williams, 2005) and Arctic Canada (Melchin, McCracken & Oliff, 1991), gives its name to a subzone in Uzbekistan (Koren' & Melchin, 2000) and a biozone in Saudi Arabia (Williams et al. 2016). It is confined to the uppermost part of the newly established parvulus Biozone, still below the lowest occurrence of akidograptids. ...
An Ordovician–Silurian boundary section marked by an uninterrupted, relatively high rate of black shale sedimentation and abundant, diverse graptolites is described from the south-central Pyrenees. The structurally simple Estana section comprises the uppermost part of the quartzite-dominated Bar Formation and overlying black shales of late Hirnantian and early Rhuddanian age, which have been dated by graptolites to the upper Metabolograptus persculptus and lower–middle Akidograptus ascensus–Parakidograptus acuminatus biozones. Due to the absence of M. persculptus , a Metabolograptus parvulus Biozone correlative with the upper part of the persculptus Biozone is recognized below the lowest occurrence of akidograptids, which indicate the base of the Silurian System. The graptolite fauna comprise 27 species including Normalograptus minor , N. lubricus , N. rhizinus , Hirsutograptus , Korenograptus bifurcus , K. bicaudatus, K. lanpherei and Nd. shanchongensis , most of which were formerly considered to be endemic to the low-latitude palaeobiogeographical province of China, Siberia and northern North America. Two new species, N. baridaensis and N. ednae , are described. The succession of graptolite assemblages in the Estana section, and occurrence of several cosmopolitan taxa in its parvulus and lower ascensus – acuminatus biozones that are unknown elsewhere in peri-Gondwanan Europe, suggest that strata immediately surrounding the Ordovician–Silurian boundary may be absent, highly condensed or oxic and barren of graptolites in other sections of northwestern peri-Gondwana. Common graptolite synrhabdosomes and abnormal rhabdosomes may indicate some environmental stress in the parvulus Biozone, although the rather uniform black shale lithology, total organic carbon content and δ¹³ C org values suggest uninterrupted sedimentation under stable, anoxic conditions.
... The Jordanian specimens are from the upper ascensus-acuminatus graptolite Zone [ 7 ]. The stratigraphically lowest Silurian graptolite assemblages (lubricus graptolite Zone and lower ascensus-acuminatus graptolite Zone) from Saudi Arabia contain M. parvulus and N. targuii [ 13 ]. The stratigraphical range of N. targuii in the E1-NC174 core, Murzuq Basin, Libya [ 3 ] is within the lower tilokensis graptolite Zone, which is correlated with the peri-Gondwanan lower ascensus graptolite Zone [ 14 ]. ...
The graptolite-bearing interval in well A1-NC101, Libya indicates a Hirnantian age (persculptus Biozone) for the lowermost Tanezzuft Fm based on the presence and stratigraphical distribution of N.? pseudovenustus. In the C1-NC101 core, the presence of N. inazaouae (recognised for the first time outside Algeria) and M. parvulus indicates either the uppermost Ordovician or the lowermost Silurian, but the revised total stratigraphical range of N. targuii (ascensus-acuminatus Biozone) suggests an earliest Rhuddanian age. Several specimens of N. targuii are longer than those previously recorded. Pure quartz arenites of the uppermost Mamuniyat Fm (well A1-NC101) were deposited during the end-Hirnantian deglaciation probably in a nearshore marine environment. Their provenance was associated with sedimentary recycling of mature sands that were formed across North Gondwana in Cambrian and pre-glacial Ordovician times.
... The nomenclature proposed by Hayton et al. (2015 and2017) is based on a four letter identifier composed of Q (Qusaiba Member); stage (RRhuddanian, A -Aeronian or T -Telychian); and HS or GS (hot shale or gray shale respectively). For example QAHS -equates to Qusaiba Aeronian hot shale and QTGS -equates to Qusaiba Telychian gray shale The four letter graptolite biozone identifier, while known (Boukhamsin and Hooker 2013 and various operational reports) due to the excellent graptolite record as described by Williams et al. (2016), has not been utilized in this study due to the limited number of samples, however, subdivisions have been used where necessary to reflect distinct changes in the geological character of the lower Qusaiba Member. ...
This initial study has identified constraints on the fractional volumes of the four principal clay components within the lower Qusaiba Member, Qalibah Formation. The availability of such constraints will have a significant impact on the petrophysical analysis of the lower Qusaiba Member and associated organic-rich source rocks.
The regionally extensive organic-rich shales within the lower section of the Early Silurian Qusaiba Member, Qalibah Formation are believed to have been the principal source for much of the Paleozoic petroleum system. More recently they have been the focus as a potential unconventional organic-rich shale play, especially in northern Saudi Arabia.
Detailed petrophysical assessment of the Qusaiba Member source rocks has been difficult, in part, due to a lack of understanding related to the mineralogy of these fine-grained shales. Various studies have recently been completed or are underway to try and resolve some of the underlying uncertainties. As part of one of these studies, the clay mineralogy of 42 core samples from 8 wells from across the Kingdom was analyzed and compared to the recently developed chronostratigraphy for the lower Qusaiba Member in northern Saudi Arabia.
By placing the results of the quantitative clay mineralogy into the chronostratigraphic framework, it has been possible to develop first pass relationships to constrain the fractional volumes of illite/smectite, illite mica, chlorite, and kaolinite within the Qusaiba shale.
... For example QAHS -equates to Qusaiba Aeronian hot shale and QTGS -equates to Qusaiba Telychian gray shale. The four letter graptolite biozone identifier, while known due to the excellent graptolite record as described by Williams et al. (2016), has not been utilized in this study due to the limited number of samples, however, subdivisions have been used where necessary to reflect distinct changes in the geological character of the lower Qusaiba Member. ...
Regionally extensive organic-rich shales characterize the lower section of the Early Silurian Qusaiba Member, Qalibah Formation. These shales are believed to have acted as the primary Paleozoic petroleum systems source rock in Saudi Arabia. Based on the interpreted paleoenvironmental/chronostratigraphic variability of these organic-rich hot shales (Hayton et al., 2017), significant variation in the organic matter (OM) composition is expected to be present within these source rocks. With notable exceptions including Cole (1994) and Abu-Ali (2005), most of the OM typing work on the Qusaiba Member source rocks has been largely inferred from bulk geochemical analysis. OM typing inferred from bulk geochemistry (e.g. Rock-Eval) is undoubtedly useful, but as it is a weighted average of the mixture of the individual OM components, organic petrography is required to understand the actual OM composition.
Organic petrography conducted on 47 Qusaiba Member core samples from eight wells across the Kingdom identified 5 major groups of OM: (1) aquatic palynomorphs, like acritarchs (strongly fluorescent), (2) terrestrial palynomorphs, like cryptospores (moderately fluorescent), (3) structureless brownish organic matter (BOM), (4) graptolites and chitinozoa (nonfluorescent), and (5) inertinite and black degraded OM (nonfluorescent). Most samples are dominated by partially intensely degraded, nonfluorescent BOM. In one well much of the BOM shows intense fluorescence, indicating significant quantities of hydrogen-rich OM in this well. In all other wells the abundance of hydrogen-rich OM (group 1 and 2) is usually less than 5% each. As expected, given the observed variation in graptolite occurrence on bedding planes in core, group 4 OM abundance is highly variable.
This is the first study aimed at quantifying the variability in the nature of the OM within the Early Silurian source rocks of Saudi Arabia. The findings show that there is significant variability in the nature of the OM, both laterally and chronostratigraphically. Therefore, great care needs to be taken to ensure that geochemical and especially kinetics work on Qusaiba source rocks takes into account this variability, as this will have significant impact on volumetric resource calculations when integrated with basin modeling.
The identification of BOM as the dominant type of OM in the Qusaiba Member means that this organic matter (of hitherto unknown biological affinity) has played a major role in controlling the hydrocarbon generative potential of these source rocks. This highlights the need for further work to understand its primary composition, biological precursor, and hydrocarbon generative potential. Future work will continue to explore for ways to link the organic matter composition to the chronostratigraphy, so that the organic matter composition and correspondingly the hydrocarbon generative potential of the organic-rich shales of the Qusaiba Member can be determined in a predictive manner to support exploration activities.
