Core samples from the QSIM-801 water well, drilled in central Saudi Arabia, cover a 93-foot interval spanning the transition between the Sajir Member of the Saq Formation, that consists mainly of sandstones of tidal sand flat environments, and the Hanadir Member of the Qasim Formation, characterized by argillaceous graptolitic mudstones, corresponding to a tidal delta front. The samples contain abundant, exceptionally well-preserved and diverse palynomorphs, which include cryptospores, acritarchs and chitinozoans, other problematic organic-walled microfossils as well as other organic particles such as cuticle-like fragments. The studied interval is biostratigraphically well constrained by the presence of chitinozoans of the formosa and pissotensis Zones of late-early to late Darriwilian age (Middle Ordovician) in the uppermost Saq Formation and Hanadir Member. The biostratigraphic age of the Sajir Member considered to span the Dapingian–Darriwilian boundary, is re-discussed based on the results herein. The uppermost part of the Sajir Member yielded the ichnofossil, Phycodes fusiforme. Acritarch assemblages from the Sajir Member of the Saq Formation are poorly diversified and dominated by sphaeromorphs. More diverse assemblages of acritarchs, associated with enigmatic forms, occur in the Hanadir Member of the Qasim Formation. The contact between the two formations and the transition between the palynomorph assemblages are sharp, suggesting a stratigraphic hiatus. A quantitative analysis allows us to discuss the paleoenvironmental changes and possibly climatic changes associated with an hypothesis of ice house conditions during this period. Among the diagnostic acritarch taxa observed are Frankea breviuscula, F. longiuscula, Baltisphaeridium ternatum, Dasydorus cirritus, Dicrodiacrodium ancoriforme, Poikilofusa ciliaris, Pterospermopsis colbathii and Uncinisphaera fusticula. These are associated with other typical forms known to range across the Lower–Middle Ordovician boundary, such as Aremoricanium rigaudae, Aureotesta clathrata, Barakella fortunata, B. rara, Baltisphaeridium klabavense, Glaucotesta latiramosa and Striatotheca spp. Galeate and peteinoid acritarchs are also well represented, as well as tiny forms of ultraplanctonic size. Three new species of acritarchs are proposed: Frankea longiuscula var. darriwilense var. nov, Micrhystridium regulum sp. nov, and Tyrannus proteus sp. nov. Repeated occurrences throughout the section of cryptospores, problematic microfossils such as organic filaments, cuticle-like tissues, striated and pigmented leiospheres frequently in clusters, are interpreted to reflect recurrent terrestrial and freshwater inputs in the depositional environment. Single-specimen, high-resolution analyses using Confocal Laser Scanning Microscopy on the enigmatic form Tyrannus proteus sp. nov. show fluorescence emission spectra and microstructural properties significantly different from those of typical marine acritarchs from the same levels.