Late Aptian to Late Albian Sea-Level Fluctuations Constrained by Geochemical and Biological Evidence (Nahr Umr Formation, Oman)

ABSTRACT The Aptian to Albian interval of the vast Arabian Platform was studied in northern Oman (Nahr Umr Formation) for its paleoceanographic and sea-level record. The sections indicate seven transgressive-regressive cycles overprinted by small-scale sea-level fluctuations. The peak of regression lead to emergence and exposure of the sea floor and overprinted marine hardground surfaces. The subaerial overprint of marine hardgrounds is difficult to recognize in the field. However, four independent lines of evidence document emergence. These are: (1) Pronounced negative excursions in carbon and partly in oxygen isotope composition beneath hardbottoms. They are attributed to the influx of light, soil-derived carbon and light oxygen from meteoric water. (2) Sparite-filled rudists beneath hardbottoms contain primary brackish-water fluid inclusions of very low salinity interpreted as cement precipitation in the phreatic marine/meteoric mixing zone or "pollution" of meteoric freshwater by residual salt. (3) Symbiont-bearing and thus light-dependent orbitolinids display small, conical morphotypes in the shallow, well illuminated water prior to exposure and hardbottom formation. Large, flat morphotypes are characteristic of the transgressive shales above those surfaces and document deeper, less illuminated waters. (4) The presence of mycorrhiza (Microcodium). The exposure surfaces overlie facies-incomplete shallowing-upward successions or subtidal sediments, but unequivocal evidence for erosion before or during emergence is lacking. Correlation of transgressive-regressive events in Oman with other sea-level curves suggests the record of spasmodic regional tectonism combined with a strong eustatic component.