Al Khlata Fm

The Al Khlata represents the oldest rocks deposited after the Mid-Carboniferous (Al-Husseini, 2008) or base Haushi, ‘Hercynean’ unconformity and comprises the lower part of the Haushi Gr. The unit corresponds with the Arabian Plate Glaciation 3 in Sharland et al. (2001). The Al Khlata Fm is defined to include all glacially-related deposits of the Permian – Carboniferous glaciations in Oman. It is named by Levell et al. (1982) after the outcrops in Wadi Al Khlata, north and south branches, in the southern Al Huqf outcrop area. However,, "only ca. 50 m of section is seen at outcrop (P5 and P1 members only) compared with the thicker, stratigraphically more complete subsurface reference section, Rahab-2 well in South Oman." Column: Oman Subsurface, Oman Outcrop

Synonym: Prior to 1982, the Al Khlata Formation was known, informally, as the Lower Haushi Fm or the Marmul Fm in South and Central Oman and the Ghaba Fm in North Oman. These names appear in some early publications (e.g. as illustrated in Gorin et al., 1982).

"The Al Khlata Formation consists of a complex mix of clastic lithologies, involving considerable lateral and vertical changes in facies, lithology and thickness. In South Oman, these changes are most pronounced and occur over short distances. The lithologies range from coarse- to fine- grained diamictites, conglomerates, gravels, pebbly sands, sands, silts, silty shales and shales. There are also some very distinctive glacial facies associated with the diamictites such as rythmically laminated (varve-like) shales with dropstones and climbing ripple cross-laminated sands. There is also a lack of bioturbation in the shales and macroscopic plant debris except very near the top of the Formation. The pebbles, cobbles and boulders (up to metre-size in some diamictites) may be faceted and striated and are of a very varied composition. The diamictites are grey, argillaceous/silty/slightly sandy with a variety of rock fragments. Huqf-like lithologies are often seen, but a suite of acidic, plutonic igneous rocks, mainly granites and granophyres is the dominant rock fragment type. Volcanic clasts are also common, including feldspar porphyries, trachytes and various chertified devitrified lithologies. Similarly, the grains in the sands are varied, with very common lithic and arkosic sands. Pure quartz sands also occur, but micas, other than in lithic grains, are rare. From south to north in Oman, the coarser-grained elements tend to disappear and the sediments become generally more mature (more quartzose), better- sorted and less laterally-variable. …

The Al Khlata was initially subdivided into four sedimentary sequences by Levell et al. (1988). Osterloff et al. (2004b) illustrates the lithostratigraphical scheme that subsequently developed in PDO based on palynological calibration. The four informal units, in ascending order are: P9, P5 and P1, where the P1 is split further by the recognition of an upper Rahab Shale unit. This naming convention originates from Al Khlata Production Unit names, e.g., AK P9. The numbers highlight the link to, and the importance of, palynology as they relate directly to the database codes of the zonal marker miospore species (from Palynozones 2159, 2165 and 2141). Although strictly a subunit of the P1 ‘Member’ the P1 Rahab is worthy of separate consideration by virtue of its unique and distinctive character and it is described separately (see Rahab Shale Fm entry)."

Three Palynozones with six Palyno-subzones and two transitional assemblages. Penney et al. (2008) place the 2165-2159 boundary at the Asselian – Gzhelian (Permian – Carboniferous) boundary and do not range the Al Khlata below a tentative late Moscovian. Previously Penney and Stephenson (2006) and Osterloff et al. (2004b) have argued that the Al Khlata could range as old as Bashkirian (see Haushi Group age discussion). The basal barren zone adds further uncertainty to the maximum interpreted age of the Al Khlata. Such large uncertainties apply to these ages because there is no direct palynological correlation possible with the type Russian sections. Also no other fossil groups have been identified in the Al Khlata to corroborate these zonal age assignments. It is significant that no marine fossils are found in the Al Khlata, which is consistent with a terrestrial, fluvio-lacustrine setting. Reworked Devonian (Misfar Gr) sporomorphs occur regularly, especially in Palynozone 2159.

The Al Khlata palynological succession is detailed in Penney and Stephenson (2006), and Penney et al. (2008) and discussed in Osterloff et al. (2004b). The latter also summarises the warming climatic story inherent in the Al Khlata with reference to the miospore content, e.g. potentially cold, dry tundra conditions at P9 ending in the melt out, warmer, more diverse vegetation, conditions associated with the P1 Rahab lacustrine phase. Correlation with sequences in Saudi Arabia is provided by comparison with the OSP palynological Zones OSPZ2 to OSPZ1 of Stephenson et al. (2003) (see also Penney et al., 2008).

"The Al Khlata deposits are highly heterogeneous with a wide range of thicknesses. Pinching out over highs, the Al Khlata thickness can exceed 800 m in localised depocentres (Osterloff et al., 2004b). Sediments are dominated by glaciodeltaic and glaciolacustrine lithofacies associations, with subordinate glaciofluvial deposits. These were all deposited in proglacial environments, probably during deglaciation phases, based on the absence of features that can be readily attributed to the direct action of ice (Aitken et al., 2004; Aitken and Clark, 2007). During periods of glacial (re-)advance, much of the previously deposited sediments were scoured, either directly by the action of ice or, more probably, by proglacial meltwaters. As such the Al Khlata represents numerous glacial and warm interglacial periods. Deposition primarily occurred during the shift from glacial to interglacial periods and was dominated by ice melt-out deposition. Subsequent glacial advances severely eroded previous deposits. Only a fraction of the sediments initially laid down are preserved in the subsurface, with extensive ‘missing time’. Striated glacial pavements occur in the Al Huqf outcrops (Braakman et al., 1982), with crag and tail features indicating ice movement towards the northeast. These may reflect the earliest, most extensive and coldest glacial periods, with little or no deposits preserved. Detrital zircons, striae, paleo-current and diamictite provenance data from the Al Huqf outcrops and selected subsurface sections (Martin et al., 2008) indicate that the oldest deposits (P9) are sourced from far away, most likely carried by a relatively large-scale ice cap/sheet. A characteristic feature of this succession in the subsurface of southeast Oman is the high frequency of erosional valleys observed within it. Valleys have also been observed in younger deposits and one should be careful assigning age without a detailed palynological check. Detrital zircons from younger sediments (P5) suggest more local source areas, possibly from glaciers on local highs. In the outcrops of the Al Huqf this sequence is found in erosional valleys, but in the subsurface this P5 unit is often completely missing (Aitken et al., 2004; Aitken and Clark, 2007). It does seem that P5 represents the most fragmented and variably distributed of the three Al Khlata units, particularly on the Eastern Flank (Penney et al., 2006 and Osterloff et al., 2004b). Renewed glaciation in the early Permian (P1) resulted in the widespread expansion of ice sheets across Gondwana. Detrital zircons suggest sediments derived from Yemen and Saudi Arabia in southern Oman and local sources in northern Oman. This could also explain other observations made in outcrops of the Al Huqf area reported by Heward (1986), Al Belushi (1996) and Al Belushi et al. (1996) suggesting that ice sheets may have moved from highlands in the northeast towards the southwest in addition to the overall picture of a glaciation from the southwest. A blanketing diamictite unit (of glaciolacustrine origin) can be recognised on a regional scale in southeast Oman at the base of the youngest Al Khlata (P1) level and it is overlain by at least two large-scale lacustrine cycles (the Rahab Shale) comprising mainly siltstones, which are locally rhythmically laminated. Dropstones observed in these siltstones decrease upwards in frequency. These lacustrine cycles were deposited during the final stages of deglaciation."

"The earliest published account of rocks of the Al Khlata Formation in the Al Huqf outcrops of Central Oman (but not identified as glacial deposits) is by Thesiger (1948, reported by Game, 1950) as part of his extensive travels through Arabia. He states, “the infrequent ridges were chiefly composed of limestone, but here, for the first time we found outcrops of red granite and of gabbro, and many fragments of porphyry, jasper, and rhyolite”. Possible glacial sediments had been noted in the 1950s by geologists of the Iraq Petroleum Company (e.g. Hudson, 1958). They interpreted the outcropping rocks as diamictites with a glaciogenic origin as subaquous rain-out deposits. In the subsurface, the Al Khlata Formation was discovered in 1956, by the Dhofar Cities Services Petroleum Company (DCSPC), in well Marmul-1. The heavy oil encountered discouraged further exploration. It was not until the 1980s when renewed interest associated with a successful exploration boom of the Eastern Flank of the South Oman Salt Basin resulted in further studies. The remote Al Huqf outcrops were ‘rediscovered’ and the rocks unmistakably identified as glacial deposits soon thereafter by Braakman et al. (1982) and subsequently others. Levell et al. (1988), Al-Belushi et al. (1996) and Martin et al. (2008) more fully discuss the nature, composition and origin of the Formation."