Outcrop sedimentology of the Natih Formation, northern Oman: A field guide to selected outcrops in the Adam Foothills and Al Jabal al Akhdar areas

Abstract

This field guide describes eleven outcrops of the Natih Formation in the Al Jabal al Akhdar-Jabal Shams and Adam Foothills areas, not far from Nizwa, at the foot of the Oman Mountains. The outcrops have been chosen for their accessibility, as well as for the fairly complete picture of the Natih Formation, which they piece together. To visit all eleven outcrops requires several days and the use of 4-wheel-drive vehicles, but the locations offer no serious physical difficulty, nor long hiking, to gain access. The outcrop descriptions follow in stratigraphic order from the lower to the upper Natih members, roving back-and-forth across the outcrop area. Much of the detailed account of the sedimentology and stratigraphy of the Natih that has been used as the basis of this field guide, has been given previously by van Buchem et al. (1996, 2002), Grélaud (2005), Schwab et al. (2005) and Grélaud et al. (2006). The observations and interpretations given here come in part from those studies, but this paper is also largely the product of a subsequent project that was carried out for the Fahud Studies Team of Petroleum Development Oman (PDO), to provide detailed sedimentology and high-resolution sequence stratigraphy of the Natih Formation for further development of the Fahud field (Homewood et al. 2006). In this respect, this field guide is not so much intended to be an original contribution in terms of the science concerning the Natih Formation. The intent is to provide the ways-and-means for all to gain a first-hand personal understanding of the rocks we have enjoyed working on. Following the outcrop descriptions, a general section provides a discussion on facies and facies associations in terms of the constraints of sequence stratigraphy, sea-level change and clay influx on the carbonate factory. With the incorporation of limited subsurface seismic and well data, geobodies and depositional assemblages, the three-dimensional objects that form the stratigraphic packages at outcrop and seismic scales, respectively, are also discussed. Facies are thus interpreted not only in terms of depositional environments, but are also placed within both geometrical (geobody, depositional assemblage) and sequence-stratigraphic frameworks. In the conclusion, it is argued that a deeper understanding of the Natih Formation has been gained by comparison of outcrop data with subsurface data, and by contrast with modern analogs. The detail required to apply what was learned from outcrop to the nearby subsurface, in a practical manner (but also properly to reconstruct the successive Natih scenarios), requires building several facies models. This is in contrast to giving a single composite picture of Natih facies distribution in space and time, under one single facies model.

Sequence Stratigraphy, Palaeogeography and Petroleum Plays of the Cenomanian – Turonian Succession of the Arabian Plate: An Updated Synthesis

“…Siliciclastic units within the Cenomanian became less areally extensive through time (Davies et al, 2002(Davies et al, , 2019 but occur particularly along the western margin of the Arabian Platform. These units include the Wara sandstones and Ahmadi shales in Kuwait, southern Iraq and Saudi Arabia (van Bellen et al, 1959;Davies et al, 2002;Alsharhan et al, 2014;Youssef et al, 2014Youssef et al, , 2019Al-Anzi and Tourqui, 2019) and fine-grained siliciclastics within the Natih Formation of Oman (Droste and Van Steenwinkel, 2004;Homewood et al, 2008). In southern Iraq and Kuwait, the Upper Ahmadi Member represents the last significant siliciclastic pulse within the Cenomanian before the Mishrif carbonate platform became established (Davies et al, 2002(Davies et al, , 2019.…”

“…K130 (middle -late Cenomanian): A major sequence boundary occurs around the lower/middle Cenomanian boundary and is associated with erosion and channel incision (e.g. Grelaud et al, 2006Grelaud et al, , 2010Homewood et al, 2008;Sharp et al, 2010). This was followed by sea-level rise associated with a middle Cenomanian maximum flooding surface and its subsequent highstand.…”

“…Outcrops of the mid-Cretaceous stratigraphic interval occur in the Al Hajar Mountains and the Jebel Akhdar area in northern Oman and in the Zagros Mountains of SW Iran, and provide an opportunity to build a reference model that can be applied in the subsurface and regionally across the Arabian Plate. These outcrops have been studied extensively: in Oman by van Buchem et al (1996van Buchem et al ( , 2002, Grelaud et al, (2006Grelaud et al, ( , 2010; Homewood et al (2008) and Razin et al (2017); and in Iran by Razin et al (2010), Vincent et al (2015), Navidtalab et al (2019), Navidtalab et al (2020) and Kalanat et al (2021). The outcrops are valuable for reference as they enable depositional sequences to be identified in age-constrained sections and correlated into the subsurface, and allow the depositional morphology to be characterized in three dimensions.…”

“…The Natih Formation is divided into seven members, lettered A-G from top to bottom (Tschopp, 1967), which have distinctive wireline log characteristics (Forbes et al, 2010). Notwithstanding that lateral facies variations due to the presence of intrashelf basins may cause uncertainty in the precise position of lithostratigraphic boundaries (Vicedo and Piuz, 2017), these seven members can be traced from the subsurface into the outcrop belt with fairly high confidence (van Buchem et al, 2002;Homewood et al, 2008;Razin et al, 2017) and can be correlated regionally (Fig. 2).…”

“…The Cenomanian-Turonian of the Arabian Plate has been extensively investigated at the outcrop scale (e.g. van Buchem et al, 2002;Homewood et al, 2008;Razin et al, 2010;Kalanat et al, 2021), at the field scale (e.g. Jordan et al, 1985;Videtich et al, 1988;Neo, 1996;Cantrell et al, 2020), and at the scale of individual intrashelf basins (e.g.…”