Stratigraphic palynology of Devonian boreholes from northern Saudi Arabia
Résumé
This extended abstract describes well-preserved palynomorph assemblages recovered from Devonian strata in five shallow boreholes in the northern part of Saudi Arabia (Fig. 1). These fully cored holes overlap stratigraphically to form a 1,640 ft composite section spanning the upper Tawil, Jauf and lower Jubah formations (Fig. 2). Only the Jauf and lower Jubah, represented by 188 samples taken over a 1,300 ft stratigraphic interval, are addressed here as the sandy Tawil lithologies are unsuitable for palynological analysis. The 900 feet of alternating of siliciclastic and carbonate units, that together comprise the five members of the Jauf Formation, were sampled comprehensively. However, the Jubah Formation, like the Tawil Formation, is mostly sandstone and is productive palynologically at only two levels separated by a 300 ft barren interval. The age of the sampled succession is assessed using index species from the established Euramerican Devonian palynozonations of Richardson & McGregor (1986) and Streel et alii (1987), and is shown to span the late Early and Middle Devonian. Miospores dominate the palynological assemblages of most samples. Trilete spores are abundant, whereas monolete spores and cryptospores are less common. At certain levels within the Hammamiyat and Murayr members of the upper Jauf Formation, leiospheres and acritarchs predominate. A specific leiosphaerid typifies the D3B palynozonal event that occurs in both northern and eastern Saudi Arabia (Al-Hajri et alii, 1999). Moreover, within these assemblages there are other algal palynomorphs and perhaps a dinoflagellate-like cyst of uncertain affinity (? Arpylorus sp.). However, other specimen of this form must be found before a reliable determination can be made. Undescribed chitinozoans also occur at one level. Figure 2 depicts the palynostratigraphy of the Jauf-Jubah succession. The oldest productive samples are from the Sha'iba Member of the Jauf Formation in the BAQA-2 borehole. These assemblages include the trilete spores Verrucosisporites polygonalis, Dictyotriletes emsiensis, D. subgranifer and Brochotriletes foveolatus (Fig. 3) and therefore can be referred to the west European Su Interval Zone of Streel et alii (1987). On that basis they are assigned a late Pragian age. Monolete spores (Fig. 3) occur in the upper part of the Sha'iba Member in BAQA-2 and range to the top of the Jauf Formation. The overlying Qasr and Subbat members present in the BAQA-1 borehole also contain trilete spores such as Emphanisporites schultzii, Synorisporites papillensis and Knoxisporites riondae (Fig. 3). As the upper limit of the ranges of these species is confined to the Emsian an early to middle Emsian age is proposed for the upper Sha'iba-Qasr-Subbat interval. Hence, the upper Sha'iba monolete spores are among the oldest known since the only other Early Devonian records found are from the early Emsian of Antarctica (Kemp, 1972) and late Emsian of Russia (Arkhangelskaya, 1985). The pseudosaccate miospore Rhabdosporites minutus occurs first in the upper Subbat Member of JNDL-4. Its occurrence marks the base of the Interval Zone Min of Oppel Zone FD (Streel et alii, 1987). The same levels contain V. polygonalis and Clivosispora verrucata (Fig. 3) species that become extinct before the Emsian/Eifelian boundary. Camarozonotriletes sextantii (Fig. 3) makes its first inception in the overlying Hammamiyat Member of JNDL-4. Miospore abundance and species diversity are low in the Hammamiyat Member of the JNDL-3 section, so no palynozonal boundaries can be defined. The Murayr Member in JNDL-3 is more productive and seems to be of late Emsian age, based on the extinction of C. sextantii, V. polygonalis and Retusotriletes maculatus (Fig. 3) in the upper part of this unit. The miospore assemblages show a marked increase in diversity, as well as a change in composition, in the lower part of the Jubah Formation as found in the JNDL-1 borehole. This characteristic allowed early workers (Hemer & Nygreen, 1967) to recognize a Devonian (Givetian-Frasnian) section (now termed Jubah Formation) younger than the marine, lower Middle and Lower Devonian Jauf Formation known up to that time. Simple trilete spores like Retusotriletes and Apiculiretusispora are much less abundant in the lower part of the Jubah Formation than in the underlying succession, and more complexly structured and ornamented species such as Acinosporites apiculatus and Grandispora protea appear for the first time. They characterize the AP Oppel Zone of Streel et alii (1987) and in the sequences of western Europe appear just prior to the Emsian/Eifelian boundary. As C. verrucata is still present in the lower part of the Jubah Formation that portion of the formation may be of latest Emsian age. The distinctive camerate spores Geminospora lemurata, Grandispora protea, G. douglastownense and the zonate miospore Samarisporites eximius (Fig. 3) occur in the upper part of JNDL-1. The first occurrence of G. lemurata defines the base of the Lem Interval Zone of Streel et alii (1987) within their AD Oppel Zone and for this reason the upper part of the JNDL-1 succession is dated Givetian. Similar assemblages have been described from the Jubah Formation in well TRBH-1, approximately 150 km north of the BAQA boreholes (Loboziak & Streel, 1995), and from borehole S-462 located some 100 km north of the JNDL boreholes (Loboziak, 2000). Palynological events characterized by an abundance of a monospecific leiosphaerid (Fig. 4.1-3) occur in the Hammamiyat and Murayr members of the Jauf Formation in JNDL-3 and JNDL-4. At some levels, this taxon constitutes almost the whole of the palynological assemblage. These leiosphaerid-rich, low-diversity assemblages can be related to Palynosubzone D3B defined by Al-Hajri et alii (1999). Here, this subzone is not as a unique event but is represented by a series of pulses that span some 400 ft of section. In the current study, assemblages of this kind may also include a variety of large aquatic palynomorphs (Fig. 4) that can comprise 10-30% of an assemblage. Most are probably green algae and represent unnamed prasinophyte phycomata (Le Hérissé, pers. comm.). As recognized here, the D3B episode is restricted to the Min Interval Zone of western Europe and thus is middle to late Emsian in age. This palynological study determines with great precision the age of the Jauf Formation in northern Saudi Arabia (late Pragian to late Emsian). The Pragian/Emsian boundary is very probably in the upper part of the Sha'iba Member while the Emsian/Eifelian boundary could be in the lower part of the Jubah Formation. However, taxa characteristic of the Eifelian are absent probably because of the 300 ft sample gap in the lower Jubah section of JNDL-1. The youngest samples from the Jubah Formation are Givetian in age. The D3B interval in the JNDL boreholes comprises a series of marine pulses throughout the Hammamiyat and Murayr members of the upper Jauf Formation. Palynostratigraphically they equate with the FD Min western European zones of middle Emsian age. The 400 ft stratigraphic span of the D3B episode is much thicker than it is in eastern Saudi Arabia where it is represented normally by but a few tens of feet. Moreover, in the JNDL boreholes the assemblage is accompanied by many other marine palynomorphs that are rarely evident in the east. Monolete spores appear in the upper Sha'iba Member and range to the top of the Jauf Formation. Their appearance in the earliest Emsian as reported here is among the oldest known global occurrences of this tracheophytic morphotype. The stratigraphic palynology presented here is mainly based on species that are common to the established Euramerican zonations. A palynostratigraphic scheme grounded on species endemic to western Gondwana is under development (Breuer et alii, in preparation) and should lead to more refined Early and Middle Devonian correlations in the Arabian Peninsula.
Domaines
Stratigraphie
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