Palynostratigraphy of Sub-Tuwaiq Mountain Limestone
Outcrops in Al-Arid Escarpments, South Central Saudi
Arabia
By
AL-ASWAD, A.A. and M.N. EL-SABROUTY (2007)
Seventh Meeting of the Saudi Society for Earth Sciences (Geosciences
and Sustainable Development) King ٍSaud University, Riyadh, Kingdom
of Saudia Arabia.
Abstract
Stratigraphic outcrops underlying the Tuwaiq mountain
Limestone between latitudes 18o 30’N and 19o 30’N along the
Al-Arid escarpment, South Central Saudi Arabia are composed
exclusively of siliciclastic rocks. Friable cross-bedded sandstone
showing fining upward sequences is the dominant facies with
subordinate siltstone and shale and rare conglomerate. The
maximum thickness of these is about 124m. The stratigraphy and
palynology of these exposures were studied in six measured
sections along a 70 km stretch. These outcrops have been
designated by Aramco geologists in 1963 as the Khuff Formation
(upper Permian) conformably overlain by the Sudair Shale
(L0wer Triassic) and disconformably overlain by the Tuwaiq
Mountain Limestone (Callovian-Oxfordian?) where the Sudair is
absent. However, the BRGM geologists in 1986 assigned all
rocks in Al-Arid escarpment underlying the Tuwaiq Mountain
Limestone, including the stratotype of the Sudair Shale, to
Dhruma formation on the basis of some Jurassic fossils and
concluded that there are no Early Triassic rocks exposed in the
Al-Arid escarpment. The braided stream depositional model is
the most appropriate fluvial model for these deposits in the area.
Two distinct facies were recognized: a proximal braided stream
dominating in the south and a distal braided stream restricted to
the north; each having its own characteristics. This palynological
study of 49 samples from six measured sections in the Al-Arid
escarpment indicated that these rocks are Late Permian-Middle
Triassic in age. The six sections yielded 27 genera of dispersed
spores and pollen grains. Among the Upper Permian taxa, some
types have a long time range in the Early and Middle Triassic
Epochs: others are characteristics of either Early or Middle
Triassic. Because these rocks have different lithologies to the
Khuff and Dhruma in the north, different sedimentary setting,
different depositional history and different geologic age; it is
proposed here that these rocks be classified as a new
lithostratigraphic unit named here Mughiran Formation (Upper
Permian-Middle Triassic). The stratotype of this unit is
designated at Jabal Mughiran.