Middle to early Late Devonian Miospore Biostratigraphy of Saudi Arabia
Published:January 01, 2000
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Stanislas Loboziak, 2000. "Middle to early Late Devonian Miospore Biostratigraphy of Saudi Arabia", Stratigraphic Palynology of the Palaeozoic of Saudi Arabia, Sa’id Al-Hajri, Bernard Owens
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Middle to early Late Devonian miospores of both southern Euramerica and western Gondwana affinities are identified from cuttings of well S-462 in the northern part of Saudi Arabia. Their stratigraphic distribution is compared to well established miospore zonations currently in use for the Devonian, and utilised to suggest an outline palynozonation applicable to western Gondwana.
The present paper originated from a miospore analysis of cuttings samples from well S-462 (Figure 1) in the northern part of Saudi Arabia, located at 30°30′27″N and 40°24′54″Έ, as part of a broader joint Saudi Aramco-Commission Internationale de Microflore du Paléozoïque (CIMP) project on the Palaeozoic palynostratigraphy of the Kingdom of Saudi Arabia.
Well preserved miospore populations were recovered from all of the samples investigated from the interval between 1,465.0-1,490.0 feet (ft) to 2,780.0-2,806.0 ft. They were sometimes abundant, diverse in composition and mostly previously known from Devonian sections of southern Euramerican and western Gondwanan areas. However, an important proportion of these palynofloras is composed of morphologically simple forms (mainly belonging to the genera Retusotriletes and Apiculiretusispora), of poor biostratigraphic resolution. These forms have no particular stratigraphic significance, except for local correlations, for example, those based on remarkable quantitative distribution patterns. Certain taxa relevant to miospore-based comparisons and datings with reference to currently used miospore biozonations are also present, but unfortunately they are much rarer.
The stratigraphic distribution of the main identified taxa is listed in Figure 2. Several index species from the standard Devonian palynozonations have been observed which permit the recognition of distinctive palynological events. However, because of problems related to caving processes, first occurrences of critical taxa are accepted with caution and, therefore, the biozonation proposed herein must be regarded as still provisional. It is partly based on the presence of some zonal species of standard Devonian miospore zonations (Richardson and McGregor, 1986; Streel et al., 1987) but also evaluates data from some species so far known only from western Gondwanan regions. The first occurrences and ranges of these species have been evaluated in a recent publication by comparison with miospore successions from the type marine Devonian of the Ardenne-Rhenish regions (Loboziak and Streel, 1995a).
The oldest assemblage recovered was from the interval below the sample at 2,440.0-2,490.0 ft. It contains Acinosporites acanthomammillatus (from sample at 2,705.0-2,755.0 ft) and A. macrospinosus (from sample at 2,550.0-2,600.0 ft) whose first occurrence defines the base of the A. acanthomammillatus-D. devonicus (AD) Oppel Zone of the Ardenne-Rhenish regions zonation (Streel et al., 1987, p. 216, Figure 5). It also contains the so far exclusively Gondwanan species Craspedispora ghadamisensis, Grandispora permulta [now considered as a senior synonym of Grandispora riegelii (Loboziak et al., 1999)], G. gabesensis and Camarozonotriletes ?concavus. Their appearance is interpreted, by comparison with the Ardenne-Rhenish miospore zonation, in North African and Brazilian equivalents (Loboziak and Streel, 1995a, p. 153, Figure 2; S. Loboziak, unpublished documents) to represent a transitional interval between AD and the underlying A. apiculatus-G. protea (AP) Oppel Zone. Together with these taxa, some age-diagnostic miospore species of Euramerican affinity, such as Chelinospora timanica, Verrucosisporites premnus and V. scurrus, all considered to first appear within the same transitional zone, are also recorded. Because of the lack of any younger diagnostic species, a similar age is therefore proposed for this assemblage. It corresponds to part of the velatus-langii and devonicus-naumovii Assemblage Zones of Richardson and McGregor’s (1986) miospore zonation for the Old Red Sandstone Continent and adjacent regions (Streel et al., 1987, Figure 13). According to Streel and Loboziak (1996, p. 577, Text-Figure 3), the transitional stratigraphic interval between the AP and AD Oppel Zones is within the costatus costatus conodont Zone of late early Eifelian age.
The next palynological event is the first occurrence of Geminospora lemurata in sample at 2,440.0-2,490.0 ft (We consider the presence of G. lemurata as well as G. piliformis respectively in samples at 2,780.0-2,806.0 ft and at 2,020.0-2,070.0 ft is due to reworking.). In the Ardenne-Rhenish miospore zonation, this appearance defines the base of the G. lemurata (Lem) Interval Zone, the youngest subdivision of the AD Oppel Zone. It is located within the ensensis-obliquimarginatus conodont Zone (Loboziak et al., 1991), close to but above the base of the hemiansatus Zone which defines the base of the Givetian (Walliser et al., 1995). This species is also an eponym of the lemurata-magnificus Assemblage Zone of Richardson and McGregor (1986). All taxa noted in the underlying samples, and particularly the large-sized spiny zonate and pseudosaccate miospores of the genera Grandispora, Samarisporites and Craspedispora are still present in this assemblage. However of greater stratigraphic interest is the occurrence and proliferation of patinate forms with highly irregular verrucate to baculate sculpture, which are attributed in Figure 2 to Chelinospora sp.
The next miospore event corresponds to the first occurrence of Samarisporites triangulatus in the sample at 2,120.0-2,170.0 ft (or at 2,020.0-2,070.0 ft). The first entry of this species characterises the base of the S. triangulatus-A. ancyrea (TA) Oppel Zone of the Ardenne-Rhenish regions, which in the Eifel region is demonstrated to be located (Loboziak et al., 1991) within the ensensis-bipennatus conodont Zone, of late early Givetian age. This Oppel zone is equivalent (Streel et al., 1987, Figure 13) to the upper part of the lemurata-magnificus Assemblage Zone, and possibly, to the lowest part of the optivus-triangulatus Assemblage Zone of Richardson and McGregor (1986).
This assemblage is also characterised by the first occurrence of several new forms, such as patinate specimens with rugulate to reticulate sculpture (Cymbosporites catillus and C. cyathus) and Samarisporites sp. E sensuStreel and Loboziak (1987), a zonate form close to S. triangulatus but possessing a wider zona and coarser, denser sculpture. Most of the large-sized zonate and pseudosaccate taxa found in older samples are still present, but they are generally less abundant in this younger assemblage.
The next and youngest palynological event corresponds to the first occurrence of Geminospora piliformis in the sample at 1,810.0-1,830.0 ft. This species was first described from the Brazilian Paraná Basin (Loboziak et al., 1988), and has been only recorded since in western Gondwana (Loboziak and Streel, 1995a). There it is considered to appear within the range of Verrucosisporites bulliferus, near the limit between the regional equivalents of the V. bulliferus-C. jekhovskyi (BJ) and the overlying V. bulliferus-L. media (BM) Oppel Zones. In terms of the western European conodont zonation, this limit is between the transitans and the Lower hassi Biozones of late early Frasnian age (Streel and Loboziak, 1996, Text-Figure 3). The transitional stratigraphic interval between the BJ and BM Oppel Zones correlates (Streel et al., 1987, Figure 13) with the lower part of the ovalis-bulliferus Assemblage Zone sensuRichardson and McGregor (1986).
This assemblage is also characterised by numerous bi- or multifurcate spores belonging to the genus Ancyrospora (e.g., A. langii) and Hystricosporites (e.g., H. blessii and H. multifurcatus.). On the contrary, the large camerate miospores without bifurcate ornaments are either absent or only scarcely represented.
In conclusion the investigated interval is demonstrated to range from the late early Eifelian (AP/AD transition) to the late early Frasnian (BJ/BM transition). Zonal or characteristic species such as for example Chelinopora concinna or Verrucosisporites bulliferus, whose first occurrences respectively characterise the bases of the S. triangulatus-C. concinna (TCo) and V. bulliferus-C. jekhovskyii (BJ) Oppel Zones, which succeed the TA biozone in the Ardenne-Rhenish regions zonation, are however absent or extremely rare. For this reason, a detailed miospore subdivision cannot be established, and only a comprehensive TA-BJ/BM zonal range is herein proposed for the uppermost part of this interval. The TCo and BJ Oppel Zones may be preserved within this interval, but this cannot be demonstrated by the present palynological study. The base of the Givetian, defined by the base of the hemiansatus conodont Zone, is somewhere below the first occurrence of G. lemurata, which is recorded in sample at 2,440.0-2,490.0 ft. The base of the Frasnian, which in western Europe is defined within the falsiovalis conodont Zone by the first occurrence of Ancyrodella rotundiloba, does not correspond to any known miospore-defined biohorizon. This still has to be searched for within the TCo Oppel Zone (Streel et al., in press). In the studied section of well S-462, as noted above, the TCo Zone has not been positively identified. If present at all, it must occur within the TA-BJ/BM zonal interval, i.e., somewhere between the samples at 2,120.0-2,170.0 ft (or at 2,020.0-2,070.0 ft) (the first occurrence of S. triangulatus) and 1,810.0-1,830.0 ft (the first occurrence of G. piliformis).
MIOSPORE ZONAL SCHEME
The purpose now is to develop a zonal scheme for the late Early to early Late Devonian section, using at the same time the criteria defined in southern Euramerica biozonations (Richardson and McGregor, 1986; Streel et al., 1987) and endemic western Gondwanan species. This scheme derives from recently concluded and on-going palynological investigations by the same group of workers in various areas of North Africa (Loboziak and Streel, 1989; Loboziak et al., 1992a) and Brazil (Loboziak et al., 1988, 1992b; 1993; Rodrigues et al., 1995; S. Loboziak, unpublished documents).
By late Early Devonian time, miospores had developed most of their Devonian morphological characteristics (Richardson and McGregor, 1986, Figure 2). A renewal of the microflora occurred in the late Emsian, mainly due to a dramatic increase in size of the specimens as documented by the appearance of large, prominently-spined zonate and pseudosaccate miospores belonging to the Samarisporites/Grandispora complex and the disappearance of several small-sized forms, particularly reticulate elements well known in the Early Devonian.
The large spiny zonate-pseudosaccate miospores first occur and proliferate in the transition between the annulatus-sextantii and the overlying douglastownense-eurypterota Zones of Richardson and McGregor (1986, p. 13, Figure 2). According to Streel et al. (1987, Figure 13) this same interval correlates with the transition between the E. foveolatus-V. dubia (FD) and the succeeding A. apiculatus-G. protea (AP) Oppel Zones of the Ardenne-Rhenish regions. Such miospores are abundant through the Eifelian, attaining peak values in the Givetian and becoming gradually rarer in the Frasnian. Some specimens, mainly belonging to Grandispora incognita, still persist in the phase Zone IV, the highest miospore subdivision of the Frasnian. The implied range extends approximately from the serotinus or costatus patulus to rhenana or linguiformis conodont Zones (Streel and Loboziak, 1996).
Therefore, those large zonate-pseudosaccate miospores are stratigraphically useful for identifying the above mentioned stratigraphic interval. However, they still occur, though quite sparsely, as part of reworked Middle Devonian material in latest Famennian (“Strunian”) marine or glacio-marine sections of Gondwanan regions bearing Retispora lepidophyta.
Several successive miospore-defined events are recorded during the in situ range of the large zonate-pseudosaccate miospores, each of them characterised by the first occurrence of one or more well known taxa. In ascending order, the first event corresponds to the appearance of several endemic zonate and pseudosaccate miospores in western Gondwana areas, such as Grandispora permulta, G. gabesensis and Craspedispora ghadamisensis. This occurs within a stratigraphic interval equivalent, by comparison with the Ardenne-Rhenish zonation, to the AP/AD transition of late early Eifelian age. Of all these species, G. permulta is actually the most representative pseudosaccate in Middle Devonian microflora of western Gondwana.
The second event, within the earliest Givetian, corresponds to the first occurrence of G. lemurata, a species well dispersed all over the world and usually well represented in western Gondwana regions, along with the proliferation of verrucate to baculate patinate taxa.
The next horizon, of late early Givetian age, corresponds to the first occurrence of S. triangulatus, another species widely distributed in the Middle Devonian.
The latest event, of early Frasnian age, corresponds to the appearance of pseudosaccate taxa bearing tabulate ornamentation.
These successive miospore events and their comparison with the interregional assemblage zones of the Old Red Sandstone Continent (Richardson and McGregor, 1986) and the interval zones of the Ardenne-Rhenish regions zonation (Streel et al., 1987) are shown in Figure 3. The correlation, demonstrated by Streel and Loboziak (1996, Text-Figure 3) in Ardenne-Rhenish regions, of miospore biohorizons with the standard conodont zones provide accurate stratigraphic dating for those events.
As part of the Saudi Aramco-CIMP project, this paper is a second contribution concerning the miospore analysis in the middle part of the Devonian sequence of Saudi Arabia. The study confirms the presence, within that interval, of well known cosmopolitan zonal and characteristic species, in addition to several species apparently endemic to western Gondwana. It also confirms that the miospore zonal schemes defined in western Europe, which have been successfully extended to several parts of western Gondwana, appear equally applicable to Saudi Arabia (Loboziak and Streel, 1995b).
List of species
Acinosporites acanthomammillatus Richardson, 1965
Acinosporites apiculatus (Streel) Streel, 1967
Acinosporites lindlarensis Riegel, 1968
Acinosporites macrospinosus Richardson, 1965
Ancyrospora langii (Taugourdeau-Lantz) Allen, 1965
Archaeozonotriletes variabilis Naumova emended Allen, 1965
Camarozonotriletes ?concavusLoboziak and Streel, 1989
Camarozonotriletes parvus Owens, 1971
Chelinospora concinna Allen, 1965
Chelinospora ligurata Allen, 1965
Chelinospora timanica (Naumova) Loboziak and Streel, 1989
Craspedispora ghadamisensisLoboziak and Streel, 1989
Cymbosporites catillus Allen, 1965
Cymbosporites cyathus Allen, 1965
Diatomozonotriletes franklinii McGregor and Camfield, 1982
Geminospora lemurata Balme emend Playford, 1983
Geminospora piliformisLoboziak, Streel and Burjack, 1988
Geminospora punctata Owens, 1971
Grandispora douglastownense McGregor, 1973
Grandispora gabesensisLoboziak and Streel, 1989
Grandispora incognita (Kedo) McGregor and Camfield, 1976
Grandispora permulta (Daemon) Loboziak, Streel and Melo, 1999
Grandispora riegeliiLoboziak and Streel, 1989
Grandispora tomentosa Taugourdeau-Lantz, 1957
Hystricosporites blessiiLoboziak and Streel, 1989
Hystricosporites multifurcatus (Winslow) Mortimer and Chaloner, 1967
Perotrilites pannosus Allen, 1965
Retispora lepidophyta (Kedo) Playford, 1976
Rhabdosporites langii (Eisenack) Richardson, 1960
Rhabdosporites minutus Tiwari and Schaarschmidt, 1975
Samarisporites angulatus (Tiwari and Schaarschmidt) Loboziak and Streel, 1988
Samarisporites eximius (Allen) Loboziak and Streel, 1989
Samarisporites praetervisus (Naumova) Allen, 1965
Samarisporites triangulatus Allen, 1965
Samarisporites sp. E Streel and Loboziak, 1987
Verrucosisporites bulliferusRichardson and McGregor, 1986
Verrucosisporites premnus Richardson, 1965
Verrucosisporites scurrus (Naumova) McGregor and Camfield, 1982
The author acknowledges with gratitude the Saudi Arabian Ministry of Petroleum and Mineral Resources and the Saudi Arabian Oil Company for permission to publish this study.
ABOUT THE AUTHOR
Stanislas Loboziak is a researcher at the French National Centre of Scientific Research. His work mainly deals with the palynology of Palaeozoic dispersed miospores and its applications, especially in the field of biostratigraphy. He has contributed to the development of reference scales for the Devonian and Carboniferous of western Europe and to their implementation in comparative studies, particularly with North Africa, Middle East, Central Europe and North and South America.