[American Journal of Science, Vol. 309, January, 2009, P. 85–90, DOI 10.2475/01.2009.03]

COMMENT OMAN CHRONOSTRATIGRAPHY (Comment on “Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman” by Samuel A. Bowring, John P. Grotzinger, Daniel J. Condon, Jahandar Ramezani, Mark J. Newall and Philip A. Allen, American Journal of Science, v. 307, p. 1097–1145.) ERWAN LE GUERROUE´*, RUBEN RIEU** and ANDREA COZZI*** Bowring and others (2007) presented a compilation of high-precision U-Pb zircon ages (both detrital and from ash beds) of the exceptionally well preserved Late Neoproterozoic-Early Cambrian Huqf Supergroup of Oman. Given the general lack of precise geochronological data of the Late Neoproterozoic, the data of Bowring and others (2007) represent an important contribution to the understanding and time calibration of this period of Earth history, which records some highly enigmatic features such as the occurrence of low-latitude glaciations, extreme perturbations of the carbon cycle and the evolution of metazoans. The work by Bowring and others (2007) puts the Huqf Supergroup on the map as one of the geochronologically best constrained sections of its age and makes it an invaluable comparison for other Late Neoproterozoic successions around the world. Given the global significance of the Huqf Supergroup of Oman, we would like to clarify two important issues that arise from the Bowring and others (2007) article: (1) an inconsistency in the correlation presented by Bowring and others between glaciations recorded in northern and southern Oman and (2) the highly speculative inference of a long-lived unconformity at the Khufai-Shuram boundary. correlation of the mirbat group with the huqf supergroup stratigraphy

The correlation of the Huqf Supergroup outcrops of northern (Jabal Akhdar), central (Huqf area) and southern Oman (Mirbat) has been a topic of discussion. The Jabal Akhdar and Huqf area outcrops have been always confidently correlated thanks to the availability of abundant sedimentological, chemostratigraphic and geochronological data (Gorin and others, 1982; Loosveld and others, 1996; McCarron, ms, 2000; Cozzi and others, 2004a, 2004b; Le Guerroue´ and others, 2006a, 2006c; Rieu and others, 2006, 2007; Allen, 2007). Until recent times, the correlation of the Jabal Akhdar and Huqf outcrops with the ones in the south of Oman (Mirbat area), devoid of any direct age dating and chemostratigraphic profiles, has been more problematic and prone to speculation. In figure 2 of Bowring and others (2007) the Arkahawl and Marsham Formations of the Mirbat Group are correlated with the Nafun Group in the subsurface of southern Oman, which in northern Oman is directly overlying the Fiq glacial deposits. Later, however, this correlation is contradicted. On page 1109, they support the idea of Rieu and others (2006) that the Ayn Formation glacial deposits that directly underlie the Arkahawl Formation are in fact more likely to correlate with the ca. 712 Ma Ghubrah glacial deposits in northern Oman, rather than with the younger

*Geosciences-Rennes (UMR 6118 -CNRS), 263 Avenue du General Leclerc. CS 74205, Universite´ de Rennes 1, 35042 Rennes Cedex, France; [email protected] **Repsol YPF Exploration & Production, C/ Orense 34, 3™ planta 28020 Madrid, Spain; [email protected] ***Eni India Ltd., Eros Tower, Nehru Place, 110019 New Delhi, India; [email protected]

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Fiq glacial diamictites. In order to clarify this apparent confusion it is important to add to the discussion of the age of the Mirbat Group the wide range of evidence recently put forward by Rieu and others (2006, 2007) which evidently has not been assimilated by Bowring and others. By integrating new lithostratigraphic-chemostratigraphic data and new U-Pb detrital zircon ages from the Mirbat Group with subsurface and outcrop data from elsewhere in Oman, Rieu and others (2007) showed that the Mirbat Group is best correlated with the Abu Mahara Group of the Huqf Supergroup. This interpretation is based on (1) the good match between the Mirbat Group and Abu Mahara Group stratigraphy in the subsurface of southern Oman, and distinct differences to the laterally persistent siliciclastic-carbonate cycles in the Nafun Group throughout Oman; (2) the new documentation of a younger glacial event represented by the Shareef Formation at the top of the Mirbat Group (not mentioned in Bowring and others, 2007); (3) carbon isotopic and lithological characteristics of the ‘cap’ carbonates overlying the Fiq and Ayn glacial deposits; (4) distinct differences between the detrital zircon populations of the Arkahawl and Marsham Formations and of the lower Nafun Group, but the resemblance of the Arkahawl and Marsham detrital zircons to those from pre-Fiq stratigraphy. Thus, taking into account the uncertainties in correlating glacial deposits over large distances, the Ayn glacial deposits in south Oman can be confidently correlated to the Ghubrah diamictites in northern Oman, whereas the younger Shareef glacial may be correlated to the Fiq Fm. It is also important to notice that, based on the evidence mentioned above, it is unlikely that the Shareef glaciation correlates with the major unconformity at the Khufai-Shuram formations boundary, postulated by Bowring and others, 2007. the nature, age and significance of the khufai-shuram boundary

Bowring and others (2007) interpreted the Khufai-Shuram boundary as a major time gap within the Nafun Group stratigraphy, possibly lasting for 10’s of Myr, judging from their figure 11. The detrital zircon age constraint provided by Bowring and others (2007) consists of only two detrital zircons (out of a total of 9 dated zircons) with an approximate age of 620Ma (621Ma and 625 Ma according to their Appendix table). Bowring and others (2007, p. 1124) are cautious about the maximum age determination (206Pb/238U, 2␴) using the youngest coherent age population of 609 ⫾ 7Ma determined by Le Guerroue´ and others (2006c), who, in contrast to Bowring and others (2007), dated several more detrital zircons from samples taken at the top of the Khufai Fm (Huqf area, n⫽35) and at the base of the Shuram Fm (Jabal Akhdar, n⫽21). Surprisingly, Bowring and others (2007) do not mention that Le Guerroue´ and others (2006c) reported 1␴ 206Pb/238U ages for three zircons (zircon n. 3, 14 and 16) from the Top Khufai Fm sample of 600 ⫾ 10 Ma, 601 ⫾ 9 Ma and 599 ⫾ 8 Ma respectively, and these constrain much more tightly than the two grains of Bowring and others (2007) the maximum age for the Khufai-Shuram boundary as ⱕ600Ma. In addition, this maximum age is in good agreement with the approximate ‘age’ calculated for the Khufai-Shuram boundary by decompacting the Nafun stratigraphy in 14 exploratory wells drilled by Petroleum Development Oman and clustering around 600 Ma (Le Guerroue´ and others, 2006c). Furthermore, Bowring and others (2007) predicted the age of the Khufai-Shuram boundary based on the extrapolation of correlative geochronologically-calibrated tie points on the C-isotope curve from the Miqrat well (their fig. 10) which penetrated shallow-water facies of both Khufai and Shuram formations. This methodology does not take into account differential compaction effects between the carbonate Buah Fm and the siliciclastic Shuram Fm which is very significant in the Oman basin (Le Guerroue´ and others, 2006c). Moreover, the use of solely the Miqrat well biases their estimate as this well is only representative of one part of the basin (basin margin) and does not illustrate the wide stratigraphic thickness variability of the Nafun Group

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across the whole of Oman as illustrated by Le Guerroue´ and others (2006c; table 1). Decompaction effects as well as stratigraphic thickness variability can greatly affect extrapolated age estimates. Le Guerroue´ and others (2006c) showed that the KhufaiShuram boundary age varies between 598 and 613 Ma only based on the variation of these parameters. Therefore, Bowring and others (2007) estimate based on a single well stratigraphy is prone to error. Le Guerroue´ and others (2006c) ⬃600 Ma age remains the best estimate for the Khufai-Shuram boundary at present and is consistent with both the new geochronological data presented by Bowring and others (2007) and those previously published by Le Guerroue´ and others (2006c). Bowring and others (2007) criticism of the post-rift thermal relaxation model applied by Le Guerroue´ and others (2006c) for modeling accommodation space creation during the Nafun Group rests on this sentence “they assume that the Shuram/Khufai boundary cannot be much older than 600 Ma, based on a single detrital grain from the upper Khufai with a 206Pb/238U date of 600 ⫾ 20 Ma (2 ␴)”. As pointed out before in this discussion, Le Guerroue´ and others (2006c) in fact used as an age constraint for the Khufai-Shuram boundary three detrital zircon ages all of them pointing to a ⱕ600 Ma age for the boundary; this was not an age chosen ad hoc by subsidence modeling as envisaged by Bowring and others (2007). The presence of a long-lived Oman-wide major unconformity at the KhufaiShuram boundary is furthermore unsupported by numerous published field studies (Kapp and Llewellyn, 1965; Tschopp, 1967; Glennie and others, 1974; Rabu and others, 1986; Hughes-Clarke, 1988; Rabu, ms, 1988; Burns and Matter, 1993; Rabu and others, 1993; Burns and others, 1994; Cozzi and Al-Siyabi, 2004; Cozzi and others, 2004b; Allen and others, 2005; Le Guerroue´ and others, 2006a, 2006b, 2006c; Allen, 2007). None of these studies reported compelling field evidence, from both deepwater (Jabal Akhdar) and, most importantly, shallow-water sections (Huqf area), of such a major break in sedimentation. Given the time scale of the unconformity suggested by Bowring and others (2007), one would expect to find ample evidence of subaerial exposure at the top of the Khufai Fm shallow-water ramp, and major incision in the outer ramp facies. Instead, as clearly documented by Le Guerroue´ and others (2006a) and Allen and others (2005), the Khufai-Shuram boundary in the Huqf area marks the transition from a peritidal to a storm-influenced shelf depositional setting through a significant flooding surface, outlined locally by the presence of a meter scale transgressive oolite bed. This is exactly the opposite of a several Myr-long unconformity. In the Jabal Akhdar the Khufai-Shuram transition is gradual (fig. 1 and fig. 2) and only locally punctuated by m-wide sandstone channels showing (minor erosive contacts) which also reworked loose carbonate material and produced soft clasts with embayed, irregular clast margins, an additional indication of the absence of subaerial exposure of the Khufai ramp. This transition occurs over a few meters of stratigraphy from where the onset of siliciclastic input marks the declining productivity of the Khufai carbonate ramp (fig. 2; Le Guerroue´ and others, 2006a). The presence of a gradual transition between Khufai and Shuram lithologies is supported by the gamma-ray record through the Khufai-Shuram boundary in both the Jabal Akhdar and Huqf areas. The GR profiles conspicuously lack the sharp jumps in values that would be expected if a major (10’s of Myr) non-depositional surface were present at this stratigraphic position (fig. 1). This pattern is also well illustrated by all the ␦13C profiles provided by Le Guerroue´ and others (2006a) in the Jabal Akhdar area (their fig. 15) as well as in the Huqf area (their fig. 16). In support of their postulated long-lived unconformity at the Khufai-Shuram boundary, Bowring and others (2007) mention unpublished Petroleum Development Oman reports illustrating angularity and incision between the two formations observed in 2D seismic lines located to the north of the Huqf area. Re-examination of these

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Fig. 1. Chemostratigraphic and gamma ray log in detail from Log-12 in the Wadi Bani Awf of the Jabal Akhdar, north Oman (original material in Le Guerroue´ and others, 2006a). Note gradual transition from the Khufai Formation to the Shuram Formation.

seismic data combined with a new well-to-seismic calibration shows that these incised valleys are contained well within the Ghadir Manqil Formation (subsurface name for the glacially-influenced Fiq and Ghubrah formations) and therefore they are much older than the Khufai-Shuram boundary. Recent subsurface studies (Cozzi and Al-Siyabi, 2004; Le Guerroue´ and others, 2006a) also failed to identify any significant angular relationship between the Khufai and the Shuram formations. In light of this work we do not find any support whatsoever for the major unconformity postulated to be present at the Khufai-Shuram boundary by Bowring and others (2007). conclusions

Bowring and others (2007) paper is a benchmark in terms of highly reliable absolute and detrital zircon ages for the Huqf Supergroup of Oman, making it a

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Fig. 2. Field picture presenting the Khufai Shuram boundary from Log-9 in the Wadi Sahtan of the Jabal Akhdar (original material in Le Guerroue´ and others, 2006a). The sedimentary nature of the Khufai Shuram contact is made of a gradual increase in siliciclastics inputs and starvation of the Khufai carbonate ramp. Hammer for scale (50 cm).

worldwide reference for the Late Cryogenian-Ediacaran-Early Cambrian. Bowring and others (2007) stratigraphic synthesis, however, fails to account for sedimentological, stratigraphic and further geochronological data in two respects: the correlation of glacial units from north to south Oman, and the interpretation of a major unconformity at the Khufai-Shuram boundary. The former is problematical since it is critically important to the debate of extreme climate change in ’deep time’. The latter is problematical since the Khufai-Shuram boundary records the largest negative ␦13C excursion of Earth history and therefore represents a potential key chemostratigraphic marker for the Ediacaran Period. Until a direct age date from the Shuram Fm is obtained, the interpretation of Le Guerroue´ and others (2006a, 2006b, 2006c) of a period of essentially continuous sedimentation through the Nafun Group with a Khufai-Shuram boundary at approximately 600 Ma should be given no less credit that the unconformity model of Bowring and others (2007). References Allen, P. A., 2007, The Huqf Supergroup of Oman: Basin development and context for Neoproterozoic glaciation: Earth-Science Reviews, v. 84, Nos. 3– 4, p. 139 –185, doi:10.1016/j.earscirev.2007.06.005. Allen, P. A., Grotzinger, J. P., Le Guerroue´, E., Cozzi, A., Al Siyabi, H., and Newall, M., 2005, Neoproterozoic geology of the Jabal Akhdar and core from subsurface: International Association of Sedimentologists, Field trip guide, IAS regional meeting, Oman 2005, 56 p. Bowring, S. A., Grotzinger, J. P., Condon, D. J., Ramezani, J., Newall, M., and Allen, P. A., 2007, Geochronologic constraints of the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman: American Journal of Science, v. 307, p. 1097–1145, doi:10.2475/10.2007.01. Burns, S. J., and Matter, A., 1993, Carbon isotopic record of the latest Proterozoic from Oman: Eclogae Geologicae Helvetiae, v. 86, No. 2, p. 595– 607. Burns, S. J., Haudenschild, U., and Matter, A., 1994, The strontium isotopic composition of carbonates from the late Precambrian (⬃ 560-540 Ma) Huqf Group of Oman: Chemical Geology, v. 111, Nos. 1– 4, p. 269 –282, doi:10.1016/0009-2541(94)90094-9. Cozzi, A., and Al-Siyabi, H. A., 2004, Sedimentology and play potential of the late Neoproterozoic Buah Carbonates of Oman: GeoArabia, v. 9, No. 4, p. 11–36. Cozzi, A., Allen, P. A., and Grotzinger, J. P., 2004a, Understanding carbonate ramp dynamics using ␦13C profiles: examples from the Neoproterozoic Buah Formation of Oman: Terra Nova, v. 16, No. 2, p. 62– 67, doi:10.1111/j.1365-3121.2004.00528.x. Cozzi, A., Grotzinger, J. P., and Allen, P. A., 2004b, Evolution of a terminal Neoproterozoic carbonate ramp system (Buah Formation, Sultanate of Oman): Effects of basement paleotopography: Geological Society of America Bulletin, v. 116, No. 11/12, p. 1367–1384.

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Glennie, K. W., Hughes Clarke, M. W., Moody-Stuart, M., Polaar, W. F. H., and Reinhardt, B. M., 1974, Geology of the Oman Mountains: Verhandelingen van het Koninklijk Nederlands geologisch mijnbouwkundig Genootschaap, v. 31, 423 p. Gorin, G. E., Racz, L. G., and Walter, M. R., 1982, Late Precambrian-Cambrian sediments of Huqf Group, Sultanate of Oman: American Association of Petroleum Geologists Bulletin, v. 66, No. 12, p. 2609 –2627. Hughes-Clarke, M. W., 1988, Stratigraphy and rock unit nomenclature in the oil-producing area of interior Oman: Journal of Petroleum Geology, v. 11, No. 1, p. 5– 60, doi:10.1111/j.1747-5457.1988.tb00800.x. Kapp, H. E., and Llewellyn, P. G., 1965, The geology of the Central Oman Mountains: Petroleum Development Oman Exploration Report S00005-9. Le Guerroue´, E., Allen, P. A., and Cozzi, A., 2006a, Chemostratigraphic and sedimentological framework of the largest negative carbon isotopic excursion in Earth history: The Neoproterozoic Shuram Formation (Nafun Group, Oman): Precambrian Research, v. 146, No. 1-2, p. 68 –92, doi:10.1016/j.precamres. 2006.01.007. –––––– 2006b, Parasequence development in the Ediacaran Shuram Formation (Nafun Group, Oman): High resolution stratigraphic test for primary origin of negative carbon isotopic ratios: Basin Research, v. 18, No. 2, p. 205–210, doi:10.1111/j.1365-2117.2006.00292.x. Le Guerroue´, E., Allen, P. A., Cozzi, A., Etienne, J. L., and Fanning, M., 2006c, 50 Myr recovery from the largest negative ␦13C excursion in the Ediacaran ocean: Terra Nova, v. 18, No. 2, p. 147–153, doi:10.1111/j.1365-3121.2006.00674.x. Loosveld, R. J. H., Bell, A., and Terken, J. J. M., 1996. The tectonic evolution of interior Oman: GeoArabia, v. 1, No. 1, p. 28 –50. McCarron, G. M. E., ms, 2000, The sedimentology and chemostratigraphy of the Nafun Group, Huqf Supergroup, Oman: Oxford, England, Oxford University, Ph. D. thesis, 175 p. Rabu, D., 1988, ms, Ge´ologie de l’autochtone des montagnes d’Oman, la feneˆtre du Jabal Akhdar: Paris, France, Universite´ of Pierre et Marie Curie, Paris 6, Ph. D. thesis and documents BRGM 130. Rabu, D., Be´chennec, F., Beurrier, M., and Hutin, M., 1986. Geological map of Nakhl: Directorate General of minerals, Oman Ministry of Petroleum and Minerals, Sheet NF 40-3E, scale 1:100,000. Rabu, D., Nehlig, P., Roger, J., Bechennec, F., Beurrier, M., Le Metour, J., Bourdillon de Grissac, C., Tegyey, M., Chauvel, J-J., Cavelier, C., al Hazri, H., Juteau, T., Janjou, D., Lemiere, B., Villey, M., and Wyns, R., 1993, Stratigraphy and structure of the Oman Mountains: Orle´ans, BRGM Editions, Document du Bureau de Recherches Ge´ologiques et Minie`res, No. 221, 262 p. Rieu, R., Allen, P. A., Etienne, J. L., Cozzi, A., and Wiechert, U., 2006. A Neoproterozoic glacially influenced basin margin succession and ‘atypical’ cap carbonate associated with bedrock palaeovalleys, Mirbat area, southern Oman: Basin Research, v. 18, No. 4, p. 471– 496, doi:10.1111/j.1365-2117.2006.00304.x. Rieu, R., Allen, P. A., Cozzi, A., Kosler, J., and Bussy, F., 2007. A composite stratigraphy for the Neoproterozoic Huqf Supergroup of Oman: integrating new litho-, chemo- and chronostratigraphic data of the Mirbat area, southern Oman: London, Journal of the Geological Society, v. 164, No. 5, p. 997–1009, doi:10.1144/0016-76492006-114. Tschopp, R. H., 1967, The general geology of Oman: Mexico, Proceedings of the 7th World Petroleum Congress 2, p. 231–242.