Cambrian trace fossils from North Africa and their contribution to Gondwana's paleobiogeography and depositional history

摘要

Cambrian successions in northern Africa are widely distributed in the subsurface, but of limited exposure. Over large areas they are dominated by nonmarine deposits free of any body fossil content, which hampers strati graphic and paleogeographic conclusions for subdivision and interregional correlation. Hitherto, the occurrence of trace fossils was mentioned sporadically, but research focused on this special issue was practically absent until recent years. Here we report results of the first ichnological investigation combined with sedimentological observations from the northern Sahara Desert (Hasawnah Formation, Al Qarqaf Arch, Jebel Hasawnah mountain region, west-central Libya), and we summarize the recent state of knowledge on the Cambrian trace-fossil record in northern Africa in general and compare this with the adjacent Middle East succession in Jordan and Israel. Five trace-fossil assemblages of low diversity are present in the Al Qarqaf Arch, belonging to the Skolithos and Cruziana ichnofacies, namely Skolithos assemblage, Diplocraterion assemblage, Rusophycus assemblage, Psammichnites assemblage and Treptichnus assemblage. Fossil content and sedimentological criteria point to a predominantly nonmarine setting for the Hasawnah Formation only sporadically affected by limited marine influence. The depositional area represents an unusually large fluvial to estuarine system with a transition to marginal-marine and finally to fully marine conditions. Due to the new findings and by co-occurrence of trace fossils and body fossils in Jordan and Israel, an ecostratigraphic approach is suggested for the Cambrian of the North Africa Middle East sector of western Gondwana. According to this, a stratigraphic position of the exposed North African Cambrian successions within the (early) Middle Cambrian interval is concluded. Based on distributed trilobite trace fossils we suggest an "aegypticus-burjensis level" as a useful stratigraphic tool for correlation in this North African Middle East paleogeographic region. Due to paleontological and lithofacies data a North-to-South gradient from low or moderate (Jordan and Israel) to high southern latitude positions (Egypt and subsequently Libya) during the Cambrian is concluded. In general, the presented successful methodology exemplifies the combination of critical evaluation of the ichnological and ichnostratigraphic Cruziana concept with sedimentological facies investigation of early Paleozoic transitional land sea areas hitherto assumed to be un-fossiliferous or unworthy of detailed research. The so concluded data are of great value for geological applied aspects, as field work, mapping, correlation of strata, reconstruction of transregional depositional history, and paleogeographic models.