Mesozoic Clastics Fm
Type Locality and Naming
Proposed by Forbes et al., 2011 (reference in "Compiler" entry below). Column: not assigned – local. "Internally, within PDO the Mesozoic Clastics Formation was previously loosely defined as the Mesozoic Clastics Group. The unit refers to a sequence of continental clastics encountered along the Eastern flank of the South Oman Salt Basin. The spatial-temporal genetic relationship of the Formation has been fairly unconstrained: it is bounded by the latest Aptian – Albian Nahr Umr Fm above, and the Upper Permian – Early Triassic Khuff Fm below. Because of its ‘often barren nature’, conventional biostratigraphic age dating remained broadly within a Jurassic to Lower Cretaceous range. The Upper Cretaceous is discounted by the existing age constraint given by the overlying Nahr Umr Fm (latest Aptian at base)." Type section is Dimeet-1. Additional reference sections are Al Burj-4, Barah-1, and Marmul-287. All wells are in South Oman.
Synonym:
Lithology and Thickness
The Mesozoic Clastics Fm is an interbedded sequence of sandstones (locally pebbly and argillaceous), siltstones, shales and conglomerates. The basal part (2–40 m) of the Mesozoic Clastics. consists of poorly sorted, very fine to very coarse, angular to sub-rounded, chert-free sandstones (Basal A sands of Osterloff, 2000a). Above this lower unit the clastics are chert-rich sandstones and conglomerates with greenish grey siltstones and red/purple shales. It can be confused with the Gharif Fm, from which it differs by the presence of chert, (except in the basal sandstones). The shales of the Mesozoic Clastics are similar in colour to the Khuff Fm and Gharif Fm shales. Khuff Fm shales can be micaceous but it is generally impossible to differentiate between them.
Relationships and Distribution
Lower contact
Upper contact
It is overlain everywhere by the Nahr Umr Formation. The upper boundary is generally placed at the first appearance of either a red, sandy silty claystone (which sometimes contains chert fragments) or poorly sorted sandstone with a large chert component.
Regional extent
The unit is confined to South Oman in an approximately 300 km long NE-trending depositional low. This is associated with salt withdrawal along the Eastern Flank of the South Oman Salt Basin. [No GeoJSON yet assigned.]
GeoJSON
Fossils
"The larger portion of the Formation is barren and unless sidewall core or core recovery is available, cavings from the highly productive Nahr Umr Fm can mask in-situ, ditch-derived, assemblages. Osterloff (2000a) summarizes and reinterprets results from several contractor and internal PDO studies. He indicates that the evidence is both ‘piece-meal’ and in need of further interrogation. Dated by palynological recovery, mainly terrestrially derived sporomorphs. The evidence is both patchy and, in places, open to interpretation. Future work may extend the age limits of this Formation. . . . The biostratigraphic age range is based primarily on terrestial sporomorphs. Meaningful marine (palynological) taxa are rare and significant, in-situ, microfaunal recovery is virtually absent. Miospore assemblages tend to be dominated by simple smooth trilete spores (Cyathidites/Deltoidospora) or, to a lesser extent, the pollen Classopollis/Corollina gp. and Spheripollenites spp. Other elements, usually in small numbers, include ornamented trilete spores such as representatives of the genera Trilobosporites/ Appendicisporites, Cicatricosisporites and Aequitriradites plus the pollen Callialasporites spp. (including C. dampieri and C. trilobatus). The marker pollen grain Dicheiropollis etruscus, occurs in at least three wells, notably in high numbers at the top of the Mesozoic Clastics section in well Barah-1."
Age
Depositional setting
The Mesozoic Clastics are continental deposits with thick units of coarse-clastic channel fills including conglomerates with high clay matrix content. The lack of sorting of the sandstones, their irregularity of Dipmeter readings and ?lateral discontinuity suggests rapid (proximal) deposition. The varicolored claystones are predominantly red-brown, red-purple with locally mottled green hues. The wide variation in colors and the mottled nature of the claystones supports the interpretation of paleosols in a mid-lower alluvial plain setting. Rare lignitic sediments have been reported from core, but conditions seem not suitably anaerobic enough to develop wider paralic, coal-forming conditions, indicating perhaps a more arid rather than humid environment. The presence of terrestrially derived miospores and absence of marine microplankton and microfaunal evidence confirms the continental setting. However, towards the northern and southern fringes of the main Mesozoic Clastics depocentre, in-situ marine indicators have been noted, although in some instances marine evidence could be the result of caving from the overlying shallow marine Nahr Umr Fm in ditch cuttings.
Additional Information
Assigned provisionally as Hauterivian to Early – mid Late Aptian. "From more recent work, based on variable palynology recovery, the age of the unit may be better constrained, but it is evident that the Formation could have a complex, long ranging depositional history. Osterloff (2000a) favors a Late Barremian to early Late Aptian age. The data presented, is however, not conclusive in this respect and the age may in fact range to Hauterivian or older. In a strict time correlative sense these clastic rocks are potentially age-equivalent to the Habshan Fm, Lekhwair Fm, Kharaib Fm and even Shu’aiba Fm. The bulk probably equates to the Lekhwair Fm and Kharaib Fm, and in particular with the more argillaceous Lekhwair Fm, e.g., in South Oman the Lekhwair Formation has sand recorded at its base. The Triassic – Jurassic clastics of the Mafraq Fm occupy an older stratigraphic position, confined to North Oman, but also unconformably overlying the Khuff Fm , becoming younger to the south. It is possible that Mafraq Fm and Mesozoic Clastics Fm represent preserved end-members of an original time transgressive sequence of clastics that developed on top of the Khuff unconformity, with the older clastics eroded or non-deposited in the south and preserved in the north, and the younger clastics only developed in the south, preserved in salt withdrawal lows at the edge of the South Oman salt Basin. . . . Clearly the Mesozoic Clastics Formation is a complex set of sediments, with respect to both spatial and temporal distribution. It will remain difficult to date and future age limit revisions are to be expected."
Age span:
Beginning stage: Hauterivian
Fraction up in beginning stage: 0.0
Beginning date (Ma):
Ending stage: Aptian
Fraction up in ending stage: 0.8
Ending date (Ma):
Depositional setting:
The Mesozoic Clastics are continental deposits with thick units of coarse-clastic channel fills including conglomerates with high clay matrix content. The lack of sorting of the sandstones, their irregularity of Dipmeter readings and ?lateral discontinuity suggests rapid (proximal) deposition. The varicolored claystones are predominantly red-brown, red-purple with locally mottled green hues. The wide variation in colors and the mottled nature of the claystones supports the interpretation of paleosols in a mid-lower alluvial plain setting. Rare lignitic sediments have been reported from core, but conditions seem not suitably anaerobic enough to develop wider paralic, coal-forming conditions, indicating perhaps a more arid rather than humid environment. The presence of terrestrially derived miospores and absence of marine microplankton and microfaunal evidence confirms the continental setting. However, towards the northern and southern fringes of the main Mesozoic Clastics depocentre, in-situ marine indicators have been noted, although in some instances marine evidence could be the result of caving from the overlying shallow marine Nahr Umr Fm in ditch cuttings.
Depositional-pattern:
Additional Information
Compiler
From Forbes, G.A., Hansen, H.S.M., and Shreurs, J., 2010. Lexicon of Oman: Subsurface Stratigraphy. Gulf Petrolink, 371 pp. (plus enclosures and CD);