Geology of sakaka area northern Saudi Arabia with special emphases on palynology

Abstract

The study area is located at a part of the Shuwayhtyiah Quadrangle and the other part is located at Al-Jawf quadrangle. In this area, the rocks consist of continental to marine clastics, fine to medium-grained sandstone, with a thin shaly layers interbeded. In addition, this area includes another type of a stromatoletic, creamy limestone and a yellowish semi-consolidated limestone rocks with shale, silt, gypsum and some sandy in the form of thin interbeded layers. Ash Shuwayhtyiah Quadrangle is located on the southeastern side of the Sirhan –Turayf basin. A part of the studied area is included in this quadrangle and the rest is located in the Jawf quadrangle, so this area covers the outcrop of the four formations included in this study from bottom to top as follows:- 1- Tawil Formation. 2- Jauf Formation. 3- Jubah Formation. 4- Wasia Formation. The oldest rocks in this sequence is the Devonian Tawil Formation, which is composed mainly of sandstone, The Jauf Formation overlies the Tawil Formation, the contact between them is not exposed in the study area, but it is exposed unconformablly between Qasr limestone, the

second member, of Al-Jauf Formation and The Tawil Formation in a small part exposure at the top of Al-Adarea Mountain, 30 km north west of Dawmat Al-Jandal. The Jauf Formation is divided into five members, Sha’iba, Qasr, Subbat, Hammamiyat and Murayr Member so The Murayr Member replaces the “transitional zone” at the top of the formation.. The Middle Devonian Jubah Formation overlies the Jauf which consists of sandstone and some shale.. The formation is assigned to the because of the presence of Prototaxites sp., a tree-like algae which is restricted to the Devonian period. This fossil has been collected from the lower sandstone previously mapped as Wasia, but the upper part of this sequence is a part of the Cretaceous Wasia because of the fossil (Dinoflagellates), this fossil had been found in a rock samples collected from the studied wells. The thickness of the Jubah Formation is 275 m in Sakaka well and 198 m in Hazm Al- Jalamid well. The Cretaceous Wasia Formation disconformably overlies the Jubah Formation in the central part of Sakaka City and the contact between these two formations is exposed at the bottom of the Za’bal Castle mountain. It can be also seen at the northern part of Sakaka City near Al- Nyhayan farms at the western side of beginning of the Sakaka –Ar’ar highway, to west of the Al-Jubah Garden. Fifty rock samples from the studied wells (Sakaka well and Hazm Al- Jalamid well) were chemically analyzed for the Palynology study, and as a result 193 species were defined 101 pollen grains species, and 92 spores species, these fossils divided to two main microfloral assemblages as follow: 1- Microfloral assemblages no.1 were found in Hazm Al-Jalamid well at depth 550-748 meters and Sakaka well at depth 20 - 295 meters. The palynomorphs in this assemblage consist of about 61.6% pteridophyta, 33.1% gymnospermophyta and 5.3% unidentified palynomorphs. These species are datable as Middle Devonian, the thickness of the first zone in Hazm Al-Jalamid well is 198 m, and 275 in the second zone which refer to Sakaka well, which is the thickness of Jubah Formation. 2- MIcrofloral assemblages no. 2 were found in samples from only one well comprise: Sakaka well at depth 0-20 meters, but it was not exposed in Hazm Al-Jlamid well. The palynomorphs in this assemblage consist of about 49.3% pteridophyta, 26.1% gymnospermophyta, 20.7% angiospermophyta and 3.9% unidentified palynomorphs. This assemblage is distinguished from the first assemblage by the decrease in the percentage frequency of pteridophyta and gymnospermophyta, and the importance index is the first appearance of the angiospermophyta. The palynomorphs of this assemblage is most likely confined to the Late Cretaceous which is 20 m in thickness in Sakaka well as a result.