oa The Palaeoenvironmental and Microstratigraphic Significance of Microcoprolites in Saudi Arabian Upper Jurassic Carbonates

Reservoir carbonates of the Arab Formation B and A units and Hith Formation contain beds in which<br>microcoprolites are well preserved. Species of the tubule-bearing, rod-like Favreina are attributed to F.<br>salevensis and F. fontana and represent derivation from a decapod crustacean source. Contorted<br>ribbon-like microcoproliths of Prethecoprolithus centripetalus represent derivation from a mollusc<br>source. Their association with cyanobacterial microgranules, often in stromatolitic layers, and<br>monospecific unornamented cyprid ostracods, Terebella lapilloides and absence of foraminifera<br>suggests a bacterial grazing mode of life within a stressed, marine environment that may have<br>experienced elevated salinity and temperature. The ascending succession from: (a) microfaunallybarren<br>anhydrite lithofacies; (b) microfossil-barren, granular cyanobacterial microbialite -<br>Decastronema / Aeolisaccus biofacies; (c) ostracod biofacies; (d) Favreina-Prethocoprolithus - ostracod<br>- cerithid gastropod biofacies; (e) Trocholina - Redmondoides - Palaeopfenderina - Mangashtia -<br>Clypeina-Salpingoporella - Thaumatoporella biofacies; to (f) concentric ooid biofacies. These facies are<br>considered to represent a parasequence within a shallow marine palaeoenvironment. The alternation of<br>such stressed carbonate units and evaporitic sediments is considered to represent episodic flooding of<br>a playa-like evaporitic basin, in which the foraminiferal biofacies probably represents the maximum<br>flooding event and best circulation of marine water. Microcoproliths provide intra-reservoir stratigraphic<br>events to complement micropalaeontologically sparse carbonates and would be expected to provide<br>valuable micro-biocomponents to assist coiled-tube biosteering of late Jurassic carbonate reservoirs.