The Early Ordovician Middle Shale Member (Am3) of the Amdeh Formation and further evidence of conodont faunas from the Sultanate of Oman

Abstract

The Middle Shale Member of the Amdeh Formation is interpreted to be of Early Ordovician age based on its trace fossils, stratigraphic context and a newly discovered fauna of conodonts. The member abruptly overlies the Lower Quartzite Member, which may be Early Cambrian, and passes gradationally-upward into the Upper Quartzite Member, which is probably Early–Middle Ordovician. The 542.5 m thick Middle Shale Member can be divided into two parts: a shaly lower part, and a sandy upper part that contains an influx of heavy minerals. Bioturbation by marine trace fossils is one of the most obvious characteristics of the member. The shales and sandstones are interpreted to be of Cruziana and Skolithos ichnofacies and represent shallow-marine shelf, shoreface, beach and coastal deposits. Sparse shelly fossils occur in the sandy upper part, principally bivalves, inarticulate brachiopods, ostracods and conodonts. The small assemblage of conodonts includes elements interpreted to be Tremadocian (Tetraprioniodus, Drepanoistodus, Drepanodus, Scolopodus, ?Tropodus, Semiacontiodus and Teridontus), and others which may be Floian or ancestral forms of Floian taxa (Balognathidae gen. et sp. indet. A & B and aff. Erraticodon). No acritarchs have been recovered, probably due to high temperatures experienced during burial to >6 km. It is likely that the Middle Shale Member is the seaward equivalent of the Mabrouk and Barakat formations, and an outcrop gamma-ray log supports such a correlation. The trace fossils, sedimentology, conodont fauna and the general lack of macrofossils are in keeping with the regional Tremadocian–Floian of the Arabian margin of Gondwana.

Tectonostratigraphy of the Late Neoproterozoic in the Eastern Arabian Plate – a revision of the Ediacaran geological evolution in the Central Oman Mountains (Jabal Al Akhdar, Sultanate of Oman)

“…An early Palaeozoic rifting interval produced accommodation space for the deposition of the ∼3 km thick Cambro-Ordovician Amdeh Formation (Lovelock et al, 1981;Oterdoom et al, 1999;Heward et al, 2018).…”

Deformation of the Cambro-Ordovician Amdeh Formation (Members 1 and 2): Characteristics, Origins, and Stratigraphic Significance (Wadi Amdeh, Saih Hatat Dome, Oman Mountains)

“…Since there is evidence for two Early Cambrian folding events in the Jabal Akhdar area [1] and the possibility of a lower Cambrian age of the Amdeh Formation (Figure 2) [5], we want to determine whether the two oldest Amdeh members show the same fold pattern as the Cryogenian to earliest Cambrian rocks of the Jabal Akhdar Dome. If the lower Amdeh members would have experienced the same tectonic evolution as the Cryogenian to earliest Cambrian rocks of the Jabal Akhdar area, we could demonstrate that the Saih Hatat Dome was also affected by the Cadomian and/or Angudan orogenies, and the age of the Amdeh Formation would include a corresponding early Cambrian age.…”

“…Since there is evidence for two Early Cambrian folding events in the Jabal Akhdar area [1] and the possibility of a lower Cambrian age of the Amdeh Formation (Figure 2) [5], we want to determine whether the two oldest Amdeh members show the same fold pattern as the Cryogenian to earliest We will summarize our field data related to the folds and deformation at the Hiyam and Amdeh contact. Our interpretations will focus on comparing the fold data of the Amdeh Formation with those of the Cadomian and Angudan folds of the Jabal Akhdar Dome and the possible maximum stratigraphic age of the Amdeh Formation.…”

“…We studied the folds of the 3.4 km thick Amdeh Formation of the Saih Hatat Dome (Figure 1) which has been considered to be Ordovician in age [4]. Recently, it was suggested that the five members of the formation (Am 1 to Am 5) can be correlated with subsurface formations to the SSW to the interior of Oman ranging from the Cambrian to the Middle Ordovician (Figure 2) ( [5], their Figure 2). However, correlation of the oldest Amdeh Member (Am 1) is less certain than for the other Amdeh members (Figure 2).…”

“…This member may or may not partly correspond to the Fortunian (Lower Terreneuvian) Karim Formation (Figure 2). Lateral correlation of the Amdeh Formation (Saih Hatat Dome) with lithologically similar units in the subsurface of Oman (Ghaba Salt Basin), drawn after [5]. Note that the correlation of Am 1 is less certain than that of Am 2-Am 5.…”