Late Ordovician to early Silurian acritarchs from the Qusaiba-1 shallow core hole, central Saudi Arabia

摘要

Well-preserved acritarchs are documented from Upper Ordovician and lower Silurian sections in the Qusaiba-1 shallow core hole of central Saudi Arabia. Sixty-nine genera comprising 68 named species and 62 forms under open nomenclature were recorded from forty core samples.ududAt the base of the Upper Ordovician and lower Silurian succession in Qusaiba-1 is the Quwarah Member of the Qasim Formation. This is overlain by glacio-marine deposits of the Sarah Formation, which are overlain in turn by the Qusaiba Member of the Qalibah Formation. Four distinct acritarch assemblages are informally numbered 1 to 4 from the base of the core upwards. Assemblage 1 is from the Quwarah Member, and is independently dated by Chitinozoa as being late Katian to early Hirnantian in age (Late Ordovician). The assemblage contains a number of new species, plus species reported from low-latitude Late Ordovician Laurentia and Baltica as well as the Gondwanan margin. Assemblage 2 is from a glacitectonite at the base of the Sarah Formation and is early Hirnantian in age. Assemblage 3, from the Baq'a Shale Member of the Sarah Formation, is also Hirnantian in age and is characterized by a stratigraphically admixed Ordovician palynoflora. Assemblage 4 is restricted to three samples from the Qusaiba Member in the lowermost part of the Qalibah Formation and is dated as Rhuddanian (earliest Silurian). The highest of the three samples that comprise Assemblage 4 is from the same level as a gamma ray peak at 254.8 ft.ududReworking of Middle Ordovician forms is evident in Assemblage 3 and is attributed to processes of glacial erosion and resedimentation during glacial melting. Reworked specimens are probably from the Hanadir Member and possibly also the Kahfah Member of the Qasim Formation. The extent of later Ordovician reworking in Assemblage 3, for example reworking from the Quwarah Member, is unclear. However, given that glacial erosion extended to levels below the Quwarah Member, Late Ordovician palynomorphs present in Assemblage 3 might also be reworked. The extent of any reworking in assemblages 1 and 2 is uncertain. There is no evidence for reworking in Assemblage 4.ududTwo new acritarch genera, five new species and one new combination are proposed: Dorsennidium polorum (Miller and Eames, 1982) comb. nov., Falavia magniretifera gen. et sp. nov., Inflatarium trilobatum gen. et sp. nov., Nexosarium mansouri sp. nov., Orthosphaeridium orthogonium sp. nov. and Tunisphaeridium bicaudatum sp. nov. Samples from the same set were used for chitinozoan, scolecodont and miospore studies (this volume). Eurypterid and graptolite remains are also presentud