Oldest Arabian glacial rocks?

Glacigene rocks are widespread in the Arabian peninsula, a part of the suite of Permo-Carboniferous glacigene rocks that occur across the former continent of Gondwana. The oldest of these rocks in Oman and Saudi Arabia have long been considered late Carboniferous in age, but new evidence might suggest that they are older, perhaps separated from the main episode of the Gondwana Permo-Carboniferous glaciation by as much as 30 million years.

Arabian glacigene rocks of the Carboniferous-Permian

Glacigene rocks of the Carboniferous-Permian are well known in the Arabian peninsula extending across Oman and Yemen and into southern Saudi Arabia. Perhaps the best known glacigene formation is the Al Khlata Formation which has been the subject of palynological study for almost half a century, mainly because it bears hydrocarbons in the subsurface of interior Oman. The Al Khlata Formation comprises clastic lithologies that range from conglomerates through diamictite, gravels, pebbly sandstones, siltstones to silty shales (Levell et al. 1988).

The formation varies hugely in thickness because it fills a glaciated topography but also because salt dissolution caused syn-depositional sagging to allow great thicknesses to develop (Osterloff et al. 2004). The deposits of Petroleum Development Oman’s (PDO) ‘Production Unit 9’ (P9) are considered the oldest, occupying the deepest parts of the glacially cut valleys and salt withdrawal and dissolution grabens. Similarly very thick deposits known as the Unayzah C member (part of the Unayzah Formation) occur in the southern parts of Saudi Arabia.

These P9 deposits are assigned to PDO palynological biozone 2159 which is divided into two, a lower 2159A and an upper 2159B. Penney et al. (2008) described these biozones concluding that they are Late Pennsylvanian in age. Stephenson et al. (2003) described the similar OSPZ1 zone (Oman and Saudi Arabia Palynological Zone 1), associated with the lower parts of the Al Khlata Formation and the Unayzah C member as being probably Stephanian in age (Fig. 1). Similar dates for the oldest Al Khlata Formation include Moscovian (Martin et al. 2008). The formation, being non-marine and clastic dominated, contains no other fossils but palynomorphs, and apparently no radiometrically datable layers.

Fig. 1. Chronostratigraphy of the Permo-Carboniferous, based on Heckel and Clayton (2006)

A study of the Mukhaizna Field in south-central Oman distinguished five biozones in the Al Khlata Formation working downhole mainly with cuttings samples but also with small amounts of core and sidewall core (Stephenson et al. 2008). The deepest of the biozones appears to provide evidence to that glacial deposition could have occurred in the Mississippian and that this period of deposition was quite separate from later glacial activity.

The top of the highest of the five biozones, Biozone A, is marked by the first downhole increase of Microbaculispora tentula and Cycadopites cymbatus and is associated with high gamma-ray readings indicating the Rahab Member of the Al Khlata Formation. The top of Biozone B is distinguished by the first downhole appearance (FDA) of, amongst others, common cavate-zonate spores. It is associated with shaley diamictites and more distal lacustrine mudstones with considerable lateral variability. The top of Biozone C is marked by the FDA of Anapiculatisporites concinnus but also contains persistent occurrences of Ahrensisporites cristatus, Spelaeotriletes triangulus and Deusilites tentus. It is associated with thick sequences of shaley diamictites that occur throughout the Mukhaizna field with relatively little lateral variability, suggesting very long-lived, large, proglacial or subglacial lakes. A lower subdivision of Biozone C, termed Sub-Biozone C1 is distinguished by very high percentages of cavate-zonate spores.

But the most problematic (and deepest) of the biozones, Biozone D, is very distinct from the others. It is characterised by very low diversity assemblages dominated by monosaccate pollen and Punctatisporites. It also represents rocks of much greater lateral variability in thickness and character (as shown by log character) than rocks assigned to the younger biozones. Stephenson et al. (2008) also showed that a marked palynological discontinuity appears to occur at the Biozone C/D boundary in the sense of an abrupt change in quantitative compositions of assemblages. Subsequent unpublished work has suggested that depocentres in Biozone D times, gained from correlations across the field, were quite independent of those of Biozones A to C.

In view of its problematic character and low palynological diversity, Biozone D core and sidewall samples were processed in larger-than-usual amounts, and extra palynological slides were prepared to ascertain the range of rare taxa, concentrating particularly on those confined to Biozone D. Stephenson et al (2008) noted one taxon – a species of Indotriradites– that occurred in very low numbers, but consistently, in core and sidewall core samples from Biozone D. This taxon has also been recovered from cored sequences of equivalent age to Biozone D in other well sections in Oman (Stephenson, unpublished data). Stephenson et al (2008) collected a population of specimens large enough to name the species Indotriradites fibrosus. However Playford (2016) recently correctly recognised that Indotriradites fibrosus, as described by Stephenson et al. (2008), is unmistakably a junior synonym of I. daemonii Loboziak, Melo, Playford and Streel, 1999.

According to Loboziak et al. (1999) and Melo & Playford (2012), I. daemonii has been recorded from Visean strata of North Africa, the Middle East and Brazil. The taxon is believed to be confined to the mid Visean ‘Mag’ Range Zone in Brazil (Playford, pers. comm., 2018).

This range, however, seems inconsistent with the presence in Biozone D of monosaccate pollen in quite high numbers (Stephenson et al., 2008). According to many authors monosaccate pollen first occurs in the early Namurian in NW Europe (e.g. Owens et al., 2004); in the Rhadames Basin, Libya (Clayton, 1995); in eastern Canada (Utting, 1987); and in Asia (Ouyang, 1996). This apparently synchronous first appearance has prompted the use of monosaccate pollen by many palynologists as a marker for the basal Namurian worldwide (e.g. Jones and Truswell, 1992; Stephenson et al., 2017), i.e. in the early Serpukhovian of the Mississippian (Fig. 1).

The presence of I. daemonii and monosaccate pollen in the same samples in Biozone D could be explained in several ways. Firstly I. daemonii could range higher in Oman than in Brazil. Secondly the Oman specimens of I. daemonii could have been reworked into the Al Khlata Formation from below. The second option seems unlikely because no other reworked spores appear in Biozone D, and the preservation of I. daemonii – a relatively delicate spore - is excellent.

Thus although Biozone D is unlikely to be Visean in age, it could certainly be Mississippian, as opposed to Pennsylvanian which is the age the oldest glacial deposits of Oman have been assigned in the past.

Clearly more work needs to be done to establish the character of assemblages in the deepest glacigene rocks in Oman, but an age that might indicate a significant time separation between Biozone D and younger biozones would be consistent with the Biozone D sequence’s rather distinct character – which suggests a separate (older?) depositional system.

Plate 1 Indotriradites daemonii Loboziak, Melo, Playford and Streel, 1999 from the Al Khlata Formation in the Mukhaizna Field. Location given by England Finder; BGS MPK collection number; diameters given individually in microns (a) E40, MPK 13569, 57 μm; (b) L37, MPK 13570, 58 μm; (c) U24/4, MPK 13571, 60 μm; (d) G23/3, MPK 13572, 58 μm; (e) T15/1, MPK 13573, 72 μm; (f) W23/4, MPK 13574, proximal face missing, 65 μm.

Acknowledgment

Prof. G. Playford for sending his 2015 paper and for subsequent discussions.

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