Surfaces with a long history: the Aptian top Shu’aiba Formation unconformity, Sultanate of Oman

摘要

Discontinuity surfaces in shallow-marine carbonate successions may represent significant time gaps in the geological record of ancient epeiric-neritic seas. Understanding the hidden geological information contained in major discontinuities is thus of key significance in palaeo-environmental analysis, sequence stratigraphy, reconstructions of sea-level change and basin evolution. In the present paper, the Aptian top Lower Shu’aiba Formation discontinuity in the Sultanate ofOman is taken as a prominent example of a regionally extensive (>100 000 km~2) surface with a long (up to 10 Myr) and complex geological history. The top Shu’aiba discontinuity formed on the topographically elevated domain of the Oman platform and represents in essence the Late Aptian time interval. Coeval carbonates in the intrashelf Bab Basin and oceanic rim indicate forced regression and sequence-wise, gradual down-stepping. Available regional, sedimentological, sequence-stratigraphic, petrographic, palaeontological and geochemical evidence from outcrops and cored wells in Oman is summarized, in part complemented by new data, and reviewed in a process-oriented context. In the field, the discontinuity is expressed as a low relief, stained surface with evidence for a marine hardground stage being dominant. Indistinct features that indicate a transient meteoric precursor stage (isotope shifts, meteoric cements, circumgranular cracks, etc.) are present but their interpretation requires careful and detailed work. This feature is remarkable, as a series of relative sea-level falls with amplitudes of up to several tens of metres from the Early to Late Aptian boundary to the end of the Aptian are reported from the Middle East and elsewhere. Despite the palaeogeographic position of the study area in the tropical climate zone, evidence of deep-cutting karst features, characteristic for many long-term exposure surfaces worldwide is scarce. Acknowledging the fact that the modern world offers no genuine analogues for the Lower Aptian carbonate system in Oman, morphological similarities between actualistic, wave-eroded coastal terraces and the top Shu’aiba discontinuity are discussed critically. This discussion may imply that, during an exposure time of several million years, the top Shu’aiba discontinuity experienced repeated stages of shallow flooding and emergence, with each transgression removing portions of the underlying rock record. The data shown here exemplify the complexity of hiatal surfaces in epeiric-neritic carbonates and may serve as a case example for other major discontinuities.