S. Al-Hajri

New evidence for land plants from the lower Middle Ordovician of Saudi Arabia

Macerations of Middle Ordovician (Llanvirnian) shales from Saudi Arabia yield an assemblage of spores of probable land plants (cryptospores), acritarchs, and chitinozoa. The production of sporopollenin-containing, sporelike tetrads is considered a fundamental character of the embryophytes, because no extant algae produce spores of this type. No trilete spores were found at this horizon, reinforcing previous assertions that obligate meiotic tetrads predate the earliest trilete spores. Sporomorph tetrads and dyads, in conjunction with cuticlelike fragments, were probably derived from terrestrial plants at a bryophyte grade. Although there are reports of possibly older cryptospores, the Hanadir assemblage described herein clearly establishes their presence by Llanvirnian time.

Biostratigraphy and palaeogeographic affinities of Early Silurian chitinozoans from central Saudi Arabia

Palynological investigations carried out on subsurface material have demonstrated the occurrence of rich and well diversified Llandovery chitinozoan assemblages in the Qusaiba and Sharawra members of the Qalibah Formation in Saudi Arabia. Both core and cuttings samples from wells located in the central part of Saudi Arabia have been investigated. The biozonation and stratigraphic correlations proposed here are exclusively based on core material. Eight local chitinozoan biozones are distinguished within the investigated Llandovery subsurface strata. Because of significant contamination due to caving in the cuttings samples these are only used to control the extension of the biozones in the intervals devoid of core samples. Several species of the most abundantly recorded chitinozoans in the Early Silurian of Saudi Arabia are typical components of contemporaneous chitinozoan assemblages from the subsurface of Libya, Algeria and to a lesser extent from outcrops in northwestern Spain (e.g. Plectochitina paraguayensis, Conochitina edjelensis, C. alargada, C. elongata, Pterochitina deichai, Belonechitina paravitrea). In addition to these typical North Gondwanan species, a few fairly ubiquitous forms are helpful for inter-palaeocontinental correlations (e.g. Spinachitina fragilis, S. maennili). The relationship existing between North Gondwanan chitinozoan assemblages and the Baltic ones are quantified for Aeronian time by calculating their respective coefficient of similarity. A great similarity exists between North Gondwana and Saudi Arabia chitinozoan populations from the Middle Llandovery. On the other hand, moderate to fairly low similarities are noticed with the contemporaneous chitinozoan populations from Baltica. Such results are consistent with the existence of a wide mid-European Rheic Ocean implying a remote latitudinal position between Northern Gondwana and Baltica and consequently contrasting climatic conditions prevailing on both margins of this ocean. So far, no typical Afro-Brazilian chitinozoan species (e.g. Pogonochitina djalmai) have been recorded in the Saudi assemblages. Although data on the Llandovery chitinozoan assemblages from Asia are still limited, some affinities, at the specific level, between the Saudi and the Chinese material are observed.

Biostratigraphy of the Ordovician chitinozoa of northwestern Saudi Arabia

Abstract Chitinozoans from water and exploratory wells located in northwestern Saudi Arabia have been investigated. A local biozonation is suggested, representing the Lower to Upper Ordovician interval in Arabia, and compared with that of the Northern Gondwana. In general, the chitinozoan faunas belong to the Gondwanan realm but latest Ordovician chitinozoans seem to have been distributed in a more cosmopolitan manner. Ranges of the species encountered have been established, and new forms are left under open nomenclature until a future publication. These microfossils provide a fairly good indication of the age of the investigated Lower Palaeozoic succession and, therefore, allow an accurate definition of the lithostratigraphic units, which has led to the revision of the chronostratigraphy of this area. These biostratigraphic data demonstrate the occurrence of an important stratigraphical gap in some areas at the end of the Ordovician.

Regional distribution and chronostratigraphy of the Qusaiba member of the Qalibah formation in the Nafud Basin, Northwestern Saudia Arabia

The Qusaiba Member of the Qalibah Formation (Silurian age) is a thick sequence consisting of massive dark gray shales. Only the very basal shales are black and highly organic rich. The basal {open_quote}Hot Shale{close_quote} unit is a prolific source correlated to all the Paleozoic discoveries in Central and Eastern Saudi Arabia. In the Nafud Basin, Northwest Saudi Arabia, this same sequence acts as both source and seal to the underlying Ordovician reservoir, making the understanding of this sequence essential to the current exploration program underway in the region. Acquiring high resolution blostratigraphy and detailed source rock analysis substantially contributed to the better understanding of the regional depositional setting of the seal, source, and the reservoirs in the area. Graptolites and chitinozoans zonation were utilized to fine-tune the chronostratigraphy and to further subdivide this huge basin into more manageable subbasins. Regional chronostratigraphic analysis has indicated that the early Qusaiba transgression took place swiftly and covered all of the Arabian plate during early Rhuddanian time. Only high areas not covered by the Rhuddanian transgression and areas that experienced later uplift and erosion lack the Rhuddanian deposits.