Stratigraphic versus environmental significance of Permian serrated conodonts around the Cisuralian-Guadalupian boundary: new evidence from Oman

摘要

Conodonts from the cephalopod limestones of Rustaq and Ba'ad, Oman, have for many years been dated as Wordina because of the presence of the ammonoid, Waagenoceras. Revised definitions for Guadalupian stages and major differences of conodont morphology among apparently coeval faunas necessitate a review of this age assignment. The lack of serration within the Oman conodont faunas as well as from Waagenoceras-bearing limestone blocks in Sicily differs markedly from conodonts in the Guadalupian stratotype sections of West Texas. These differences have previously been interpreted to be the result of upwelling 'cold botton-water' that may have restricted the distribution of serrated, warm-water Jinogondolella species. However, our smaples from Oman are dominated by species with a carinal configuration that is comparable to gondolellids from the Equatorial Warm Waater Province (EWWP). Abundant species include Mesogondolella siciliensis in the lower ammonoid beds and Mesogondolella idahoensis lamberti in the upper ammonoid beds. If cold bottom-water currents were present at Oman then these taxa must have been pelagic, living in warm surficial water. In contrast, the North Cool Water Province (NCWP) includes coeval condonts that have a different carinal configuration and are dominated by long-ranging Mesogondolella idahoensis idahoensis. These taxa and their morphologic variations represent a geographic cline between the EWWP and NCWP. In addition, Early Roadian serrated gondolellids have been recognized in temperate cool-water settings in the Jilin Province, NE China, the Phosphoria Basin, Idaho, USA, and the Sverdrup Basin, Arctic Canada. These serrated forms extend avove the Lower Roadian in only the Phosphoria Basin, indicating a distribution into temperatures cooler than previously thought, at least during the Early Guadalupian. Other condonts from the deep-water cephalopod limestones of Oman include rare, shallow warm-water sweetognathids, suggesting that these too must have been pelagic; if true, then cold bottom-waters probably played little role in conodont distribution. The identified conodont tata at Oman best correlate with the uppermost Kungurian at the Luodian section of South China and in the Guadalupe Mountains of West Texas, suggesting that the lack of serration is not a result of cold water but rather of a stratigraphic position prior to the evolution and expansion of serrated gondolelids in the Roadian. It is possible that other paleoecologic controls affected distribution, and an alternative age assignment of Early Roadian is considered for these conodonts. The ammonoids, including advanced species of Waagenoceras, suggest that the best correlation is with the third limestone of the Worl Formation (Willis Ranch Member), which correlates with the Upper Roadian according to ratified global stage definitions. There is still an apparent discrepancy between conodont and ammonoid ages, but discounting previous Wordian ages has considerably reduced this. Associated fusulinaceans cannot resolve this argument because of profound provincialism, but they can contributes to correlation of the global stages with Tethyan stages. It is here suggested that the Kungurian correlates with the Upper Bolorian to Upper Murgabian and that the Roadian correlates with the Lower Midian or Upper Murgabian and Lower Midian.