Jurassic and cretaceous radiolarian biostratigraphy of the Pichakun and Hawasina basins
Résumé
The Pichakun Nappes in Iran and the Hawasina Nappes in Oman are excellently preserved examples of deep-water Mesozoic basins developed on the southern Tethyan margin. Jurassic to Cretaceous successions consisting of radiolarian cherts, shales and carbonate gravity-flow deposits are beautifully exposed. Because carbonate plankton are absent in these sucessions and scarce benthos occur only in resedimented limestones, radiolarians are virtually the only fossils enabling age determination. The proximal-to-distal facies reconstruction is based on 5 sections from the Neyriz area in the Zagros Range and 15 sections from the Sumeini, Hamrat Duru and Al Aridh groups in the Oman Mountains. Several correlative intervals dominated by radiolarian cherts and shales were recognized in the Lower Jurassic to Upper Cretaceous successions. The oldest package of radiolarian chert is Pliensbachian to early Toarcian in age. This interval is recorded in Oman whereas in Iran the sediments of this age are not exposed. An increase of siliceous deposits is then recognized in the upper Aalenian to upper Bajocian and in the Callovian to Berriasian with a maximum in the middle-upper Oxfordian to lower Tithonian. The Callovian to Berriasian record is complete in Iran but includes a stratigraphic gap in some sections from Oman. The most prominent hiatus is documented in the Al Aridh Group, where the upper Bajocian-lower Bathonian and Tithonian cherts are in direct contact. The most distal succession records a continuous sedimentation of cherts and shales from the Callovian to the Barremian. The youngest siliceous interval is assigned to the Aptian to Cenomanian or lower Turonian and, in both regions, includes megabreccias composed of platform-limestone blocks. Overall, the Jurassic to Cretaceous successions of the Pichakun and Hawasina basins correlate well with Mesozoic basins of the western Mediterranean, especially those that were located close to a carbonate platform. In these basins the areal extent of radiolarian chert and/or shale was determined primarily by the amount of carbonate supply from the adjacent platforms. The intervals with prevailing background sedimentation are roughly synchronous in the Southern Alps (Belluno Basin), Dinarides (Slovenian and Budva basins), Hellenides (Pindos Basin), and in the Pichakun and Hawasina basins. The minimum extent of radiolarian chert is recorded in the Bathonian, which is regionally characterized by oolitic megabeds. However, a distinct difference is recorded in pelagic-facies changes just below the Jurassic-Cretaceous boundary. In the Pichakun and Hawasina basins shales are rather abruptly overlain by pure radiolarian chert, whereas in the more western Tethyan basins a transition from chert to pelagic limestone (biancone or maiolica facies) occurs at this level. The mid-Cretaceous silica-rich deposits are again a Tethyan-wide phenomenon and in the correlated basins contain limestone breccias related to regional tectonic events.