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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.

The 10 m-scale shallowing-up Neoproterozoic carbonate cycles at Qarn Alam (Oman) provide a record of microbial textures and the communities responsible for them. This is documented for four major microbialite facies. Despite their age, these microbialites show extremely fine preservation of microbial fossils and mineral associations (primary calcite and dolomite with minor phosphate, glauconite, palygorskite, hematite and goethite) and they are the record of a suite of microbial communities, from pellicular biofilms (planar laminites) to mats and gels (crinkly laminites, and stromatolitic layered and massive thrombolites) and a more complex microbial community in bushy thrombolites possibly involving a sponge-like form. Mineralized extra-cellular polymeric substances (EPS) resembles that of modern microbial mats. The mineral associations, as well as cathodoluminescence attributes, indicate oxic to suboxic conditions during deposition and early diagenesis for planar laminites and crinkly laminites, but more evaporitic to saline conditions during development of thrombolites of the upper part of a cycle. Early cementation under variable redox conditions sealed the organomineralized phases.