Agriopleura Morphotypes of the Lower Aptian Shu’aiba Formation of Saudi Arabia

Abstract

The Shu’aiba Formation of the Arabian Platform consists of Aptian carbonates in which rudists are important biocomponents. It is an important hydrocarbon reservoir. Semi quantitative micropalaeontologyand macropalaeontological observations from cores have enabled three-dimensional interpretation of biofacies and lithofacies that have guided optimal reservoir exploitation.In addition to the presence of Offneria murgensis MASSE, O. cf. nicolinae (MAINELLI), Glossomyophorus costatus MASSE et al., Oedomyophorus shaybahensis SKELTON, and Horiopleura cf. distefanoiBOEHM the Shu’aiba Formation contains Agriopleura cf. blumenbachi(STUDER) and A. cf. marticensis (d’ORBIGNY). Although species of Agriopleura are locally present over short sections below the Glossomyophorus costatus accumulations of the middle part of the formation, they are very well represented in its upper part, where they are typically associated with the moderately deep marine benthonic foraminifera Praechrysalidina infracretacea, Vercorsella arenata, Debarina hahounerensis and Palorbitolina lenticularis.Agriopleura cf. blumenbachi and A. cf. marticensis display contrastingmorphologies. Agriopleura cf. marticensis is cone-shaped, with a fixed valve expansion angle of 30–35º, and with average right valve dimensions of 3.5 cm length and 2.5 cm maximum diameter. A recently observed “clinger” form is, however, 70 cm long. A. cf. blumenbachiis elongate, almost tube-like, with a very low rate of fixed valve diameter increase, of less than 10º with average right valve dimensions of 6.5 cm length and 2.2 cm maximum diameter.The two forms are present in distinct biofacies. Both Agriopleura facies are considered to compose a single depositional cycle, and it is possible that the elongate shape of Agriopleura cf. blumenbachi is better adapted to the slightly higher rate of sedimentation associated with the relatively rapid sea level rise at the base of each new depositionalcycle. The cone-shape of A. cf. marticensis is possibly adapted to lower rates of sedimentation during the relatively slower rate of sea level rise, and ultimate fall of the upper part of each depositional cycle.