GeoArabia, Vol. 8, No. 2, 2003 Gulf PetroLink, Bahrain Lithological and structural evolution, Qatar Lithological and structural evolution of the northern sector of Dukhan anticline, Qatar, during the early Tertiary: with special reference to sequence stratigraphic bounding surfaces Harald G. Dill, Sobhi Nasir and Hamad Al-Saad ABSTRACT Early Tertiary sediments are widely exposed in Qatar and only in coastal areas and a few places inland do Holocene deposits mask them. The variable resistance to weathering of the Tertiary sediments is responsible for the general low-relief landscape interspersed with flat-topped hills, the most prominent of which are in western Qatar where they are the surface expression of the Dukhan anticline. The early Tertiary was a time of shallow- marine sedimentation and several transgressive and regressive cycles occurred. Sedimentation began on a dolomitic carbonate shelf, which gave way to a mixed carbonate- siliciclastic shelf that became increasingly calcitic. Coarsening-upward sequences with fine-grained muddy sediments at the base and grainstones and boundstones on top attested to a cyclic change in the energy regime. Small-scale cycles and major unconformities resulted from eustatic sea-level changes. In the Dukhan study area, the most prominent sequence boundary was near the Ypresian-Lutetian boundary. Following this hiatus, a Lutetian transgressive system tract evolved that was terminated by a maximum flooding surface correlated with the named MFS Pg20. Minor unconformities and NE-trending faults of post-Miocene age resulted from the interplay of local diapiric salt movements in the Dukhan anticline and a regionally changing stress field. Extensional faulting was succeeded by a compressional phase that caused the reactivation of some normal faults as steeply dipping reverse faults. Diagenetic processes and the pervasive etching of the landscape formed ferrous, siliceous, and gypsiferous duricrusts. Phosphate was reworked and concentrated above the Ypresian-Lutetian unconformity. Ferricretes were derived from iron-bearing phyllosilicates and disulfides. Ferric iron-oxide hydrates were the result of oxidizing conditions but their parent material furnished evidence of less-oxygenated conditions and a rising sea level during the Lutetian transgression. The studies provided information on the economic potential of aggregates, clay, hydraulic binders, and building stones, and the prediction of subsidence-prone areas. INTRODUCTION The Qatar Peninsula projects northward into the Arabian Gulf as the surface expression of the Qatar Arch (Figure 1). The Peninsula is of low relief and is characterized by flat-topped hills or mesas that rarely exceed 100 m in altitude. The low relief and the few field-based investigations of the structural geology and lithology led previous workers (e.g. Sugden, 1962; Noweir and El-Kassas, 1988; Al-Hinai et al., 1997) into thinking that there had been little structural disturbance in Qatar during the recent geological past (Cavelier, 1970). The Cenozoic structural geology is, however, by no means simple and studies of the more recent structural evolution of the Arabian Gulf Region make reference to hydrocarbon migration and accumulation (Vaslet et al., 1991; Weijermars, 1998, 1999a,b; Stenger et al., 2003). Our study was the lithological and structural evolution of the Dukhan anticline, a major hydrocarbon- bearing structure in western Qatar (Figure 1). The work provided an insight into the interaction between lithology and structural evolution during the early Tertiary. Close attention was paid to the nature of the bounding surfaces of depositional sequences and their distribution in the sedimentary record, together with marker lithologies and geometry. Such bounding surfaces denote chronological events that sometimes tell more about the evolution of the geological successions than the beds themselves (Ollier and Pain, 1997; McNeil et al., 2001). 201 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/8/2/201/4553877/dill.pdf by guest on 02 October 2021 Dill et al. QATAR PENINSULA 50 E 51 52 53 27 Abu Sa'fah IRAN 27 N 3H-2 Qatif Pars South Arabian Gulf Al- Al-Rayyan Shaheen Al-Rayyan Balal (Bahram) Dammam BAHRAIN Alpha 2 North Field Manama Abqaiq Reshadat (Rostam) 26 Awali Al-Khaleej 26 Resalat Najwat (Rakhsh) Najem Salman Bul MaydanMaydan Mahzam Mahzam (Sassan) SIMSIMA Hanine ARCH X ABAUG ABAUG North Dome SYNCLINE SYNCLINE Abu ARCH Idd Al-Shargi Al Bukhoosh E E Study South Dome N N I I Area L L Umm C C X Doha Shaif N N Shaif E E Y Y N N N N S S I I A A L L A A H H Caspian C C I I K K QATAR Sea W SAUDI W T T L L U U Bu N N A A D D 26N A A S ARABIA S Haseer 0 km300 QATAR QATAR Med Sea Umm Al Salsal 25 km Satah Bin Nashef Arzanah Sath Al Razboot Arabian UNITED ARAB Shield N EMIRATES Ghasha 0 km 50 UAE km Arabian Sea Gulf of Aden53 50 51 52 Figure 1: The major structural elements of the Qatar Peninsula (modified from Cavelier, 1970). Based upon these lithological and structural analyses, we correlated the early Tertiary strata of the anticline with charts of worldwide eustatic sea-level changes. The principles for these eustatic and tectonic charts were elaborated by Vail et al. (1977, 1991) and van Wagoner et al. (1988), who defined various criteria for the recognition of the depositional sequences and their sequence boundaries and correlative conformities. On a regional scale, chronostratigraphic and lithostratigraphic sections in Sharland et al. (2001) were checked for the correlative value and used for stratigraphic fine tuning in the study area. The early Tertiary sedimentary series have been marginalized as not worthy of in-depth studies. However, although they do not contain hydrocarbons they may be of interest to exploration geologists in search of industrial minerals (see Qatar General Petroleum Corporation and Amoco Qatar Petroleum Company, 1991). In this respect, our detailed geological mapping and litho-sequence stratigraphic interpretation created a framework for the geologic setting of non-metallic deposits, such as cement stones and drilling mud materials, and the detection of karst-induced sinkholes. This sort of information is essential in the economic development of a country (Harben and Kuzvart, 1996). The survey may also help tackle some lithological and structural problems related to the uplift of the Dukhan anticline, which is host to one of the major oil fields in the Gulf. 202 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/8/2/201/4553877/dill.pdf by guest on 02 October 2021 Lithological and structural evolution, Qatar Geological Setting STRATIGRAPHY The Dukhan anticline on the west coast of Age For- Litho- General Description mation logy Qatar (Figure 1) is host to the most important on-shore oil field in Qatar. It has been the subject of numerous investigations by Conglomerate engineers, stratigraphers, and paleontologists Late Miocene (Qatar General Petroleum Corporation and Pliocene/ Amoco Qatar Petroleum Company, 1991; El Interbedded Beialy and Al-Hitmi, 1994; Hewaidy and Al- limestone and marl Hitmi, 1993a,b,c, 1994, 1999; Al-Saad and Ibrahim, 2002; and other literature cited). Early Miocene Figure 2 outlines the sedimentary units Argillaceous limestone exposed on the Qatar Peninsula and Figure 3 shows the geological setting of the study area in the northern part of the Dukhan anticline. Dolomitic limestone A more detailed picture of previous stratigraphic studies carried out in the Dukhan area is given in Table 1. The table Fossiliferous limestone illustrates the stratigraphic subdivision of the early Tertiary rocks of the Dukhan anticline Middle and provides an overview of the Nummulitic limestone nomenclature applied to the Eocene successions in Qatar. An Eocene age has been Eocene assigned to the sedimentary rocks of the Rus Marly limestone and Dammam formations that are exposed in the uppermost parts of the anticline (Cavelier, 1970; Abu-Zeid and Boukary, 1984). Dolomitic limestone The Middle Eocene Dammam Formation is Rus Dammam Dam Hofuf Early Gypsum interbedded succeeded by the Miocene Dam Formation with marl and they are separated by an Oligocene hiatus in the sedimentary record (Al-Saad and Figure 2: Stratigraphy of the study area. Ibrahim, 2002). Our present level of knowledge of the early Tertiary sediments that roof the Dukhan anticline was provided mainly by a transect through Jebel Dukhan (Cavelier, 1970; Abu-Zeid and Boukary, 1984; Boukary, 1985). Methodology Our study takes an approach that is different from that of previous workers, for example Cavelier (1970), Abu-Zeid and Boukary (1984), and Boukary (1985), by emphasizing both lithology and structural geology. Much of our work was field based. The objective was not to go through the Jebel Dukhan section again and refine the stratigraphy, but to provide easy-to-read and easily accessible maps and cross-sections that may be helpful in the search and exploitation of industrial minerals. Fieldwork was based on 1:10,000 scale mapping, measurements at more than 160 stations, and various traverses across the Dukhan anticline. The geological and contour maps (Figure 3) were created on a digital elevation model that was converted into a grid file by the Centre of GIS, Doha. The sedimentary succession was divided into four stratigraphic units (Figure 4) that are traceable throughout the study area and allow for a comparison of facies, genetic interpretations, and correlation by means of sequence stratigraphy (Vail et al., 1977; Haq et al.,