Echinoids from the upper Qishn Formation (lower Aptian) in the Haushi-Huqf area, Oman Interior Basin, Arabic Peninsula

Abstract The Haushi-Huqf region (central Oman) is a structural high on which the Qishn and Nahr Umr formations represent the upper Barremian–lower Aptian and Albian intervals, respectively, which record some of the transgressive events that covered large portions of the Arabian Peninsula during the mid-Cretaceous. With a complex post-depositional evolution through processes of burial, uplift, tilting and intense denudation, the current landscape allows a glimpse of a virtually exhumed ocean floor, as it would have occurred about 120 myr ago. A NW–SE hiking of about 1200 m in Wadi Bauw, in the southeasterly portion of Jabal Dumur, has allowed the uninterrupted monitoring of the transition of depositional palaeoenvironments from a restricted lagoon to an open-marine platform, below storm wave base. The echinoids Tetragramma deshayesi (Cotteau, 1864), Coenholectypus neocomensis (Gras, 1848) and Pliotoxaster cf. comanchei (Clark, in Clark and Twitchell, 1915) come from the upper part of the Qishn Formation and are dated as early Aptian, based on the presence of the foraminifer Voloshinoides murgensis Luperto Sinni and Masse, 1993 and the rudists Glossomyophorus costatus Masse, Skelton and Sliskovic, 1984, Offneria nicolinae (Mainelli, 1983) and O. murgensis Masse, 1992. All specimens were collected from marly facies in which orbitolinid foraminifera predominate, in what could have been the external portion of an open platform, immediately after the occurrence of carbonate shoals consisting of oncolitic/peloidal grainstones/packstones and rudists that occur in association with bioconstructions of the Lithocodium / Bacinella consortium. Morphological characteristics of the echinoids suggest that, although Tetragramma was epifaunal and Coenholectypus endofaunal, both would have inhabited shallow and protected environments, composed of coarse sandy sediments in an internal lagoon. In contrast, the genus Pliotoxaster would have been better adapted to deeper environments or to settings with lower oxygen levels, i.e., being more closely compatible with the environment it was found in. Thus, it is likely that representatives of Coenholectypus and Tetragramma had been transported from the lagoonal regions and/or near the carbonate shoals to deeper regions with fore-bank characteristics.