R.A. Ellison

Invited comment on Wray & Gale's 'The palaeoenvironment and stratigraphy of the Late Cretaceous chalks'

Despite the context of this reply we would wish to echo the tributes to Jake Hancock expressed elsewhere in this volume. His contribution in papers over many years (e.g. 1961, 1972, 1975, 1991, 2000) is considerable and, in many cases, still relevant. We would also like to thank Wray and Gale for demonstrating the breadth of research into the understanding of the Chalk and its environment of deposition. Our major concern is with the inaccuracy, incorrect assumptions and misrepresentations implied and stated by Wray & Gale (2006) in their discussion of the modern Chalk lithostratigraphical framework applied to British Geological Survey (BGS) maps and demonstrated in many published documents. This reply gives the BGS an opportunity to restate in brief the lithostratigraphical framework for the Chalk Group of England and to deal with specific points raised by Wray and Gale. Our reply is prompted in part by the implications in Wray & Gale that the geological mapping of the Chalk Group in England is carried out in a haphazard and uncontrolled manner without scientific rigour and is of ‘little value’. This implication is refuted absolutely. In fact, it is a test of the robustness of the framework that a large number of field geologists can apply the scheme across southern England and provide such a powerful predictive tool for the practical benefit of the nation. A further verification of the framework’s pertinence, over and above its application to pure scientific endeavours, is its widespread use in applied geological studies. For example, the framework provides the foundation for investigations into the structure of the Chalk, its engineering characteristics and hazards and, perhaps most importantly of all, provides the key and new impetus to studies, including predictive modelling, of the hydrogeology of the UK’s largest and most important aquifer. As stated by Wray & Gale, the development of the regional Chalk lithostratigraphies in the late 1970s to the mid-1980s (Wood & Smith 1978; Mortimore 1983, 1986; Jarvis & Woodroof 1984; Robinson 1986) made it apparent that the traditional tripartite scheme embodied by Jukes-Browne & Hill (1903, 1904) did not delineate fully the lithological variation in the Chalk. This inadequacy led to the development of a more detailed and broadly applicable framework by BGS. The modern lithostratigraphical framework builds upon the expertise of a large number of field geologists. It was presented in a model (Bristow et al. 1997) that was modified at a workshop of the UK’s Chalk experts in 1999. It belongs to all those who contributed and signed up to it, and is the ‘agreed lithostratigraphical framework’ (Rawson et al. 2001). The outline framework with minor additions at member level is given in Figure 1. The full framework report for the Chalk Group of the UK, approved by the Geological Society of London’s Stratigraphy Commission (GSLSC), appears in Hopson (2005) and is available (in pdf format) as a free download from the BGS website (http:// www.bgs.ac.uk) as a joint BGS/GSLSC publication.

The Jurassic–Cretaceous depositional and tectonic evolution of the southernwestern margin of the Neotethys Ocean, Northern Oman and United Arab Emirates

The concept that the autochthonous, parautochthonous and allochthonous Permian–Cretaceous sequences in the United Arab Emirates (UAE) and Oman record the transition from platform, slope to basin sedimentation within the southern part of Neotethys has been fundamental to the interpretation of the geological history of the region. The results of a major geological mapping programme of the UAE, carried out by the British Geological Survey for the Federal Government of the UAE, coupled with the detailed examination of key sections within northern Oman has led to a re-evaluation of the geological evolution of this region. This detailed study has led to a greater appreciation of the sedimentology and depositional setting of the sediments laid down along the northeastern Arabian continental margin during the Jurassic to Cretaceous, allowing a more refined model of Neotethys Ocean basin evolution to be established. The model charts the progressive breakup of the Arabian continental margin and closure of Neotethys during the mid to late Cretaceous and is divided into three main stages: Stage 1—Initial rifting and formation of the Neotethys Ocean, followed by a prolonged period of stable, passive margin sedimentation which extended from the Permian to Late Jurassic times; Stage 2—Uplift and erosion of the shelf margin during the Late Jurassic to Early Cretaceous, coincident with increased carbonate-clastic sedimentation in the outer ramp, distal slope and basinal areas; Stage 3—Increased instability during the Late Cretaceous leading to the breakup of the platform margin and foreland basin sedimentation accompanying the obduction of the Oman-UAE ophiolite. Data obtained for the upper part of the platform and platform margin to slope successions has revealed that the topography of the “shelf”-slope-basinal margin was more subdued than previously thought, with this more gentle ramp margin morphology persisting until early to mid-Cretaceous times when the platform margin started to become unstable during ophiolite obduction. The thrust-repeated allochthonous sedimentary rocks of the Hamrat Duru Group were deposited on the outer platform margin/lower slope rise to basinal plain of this basin margin and includes the dismembered remains of two turbidite fan systems which fed carbonate-rich detritus into deeper parts of the ocean. A re-evaluation of the chert-rich sequences, previously equated with deposition on the abyssal plain of Neotethys, has led to the conclusion that they may record sedimentation at a much shallower level within a starved ocean basin, possibly in a mid-ramp (above storm wave base) to outer ramp setting. A marked change in basin dynamics occurred during the mid-Cretaceous leading to the development of a shallow ramp basin margin in Oman with terrestrial to shallow marine sedimentary rocks interdigitating with red siliceous mudstones. By contrast, the contemporaneous succession in the Dibba Zone of the UAE indicates considerable instability on a steep shelf break. This instability is recorded by the presence of several major olistostrome deposits within the Aruma Group of the UAE which are thought to have been generated in advance of the rapidly obducting Oman-UAE ophiolite.