Map showing political boundaries in the horn of Africa with digital elevation data for the country of Ethiopia with the study area outlined.

The Tambien Group of northern Ethiopia is a succession of early Neoproterozoic mixed carbonates and siliciclastic sediments. These sedimentary rocks were deposited from ca. 820 to 715 million years ago in the time period leading up to the first low-latitude glacial event of the Neoproterozoic (the Sturtian Snowball Earth). The Tambien Group was deposited atop the volcanic sequence of the Tsaliet Group that records the formation of the juvenile crust of the coalescing Arabian-Nubian continental shield (the continental crust that underlies much of northeast African and the Arabian Peninsula).

This project (supported by NSF grant EAR-1325230) seeks to develop integrated chemostratigraphic, paleomagnetic and U-Pb geochronological data from the Tambien Group of northern Ethiopia to provide a new backbone for stratigraphic correlation in the 100 million years preceding Cryogenian glaciation.  Our overarching goal is to understand how and when potentially unique geochemical and paleogeographic boundary conditions were developed that allowed for global glaciation at the onset of the Cryogenian Period.  Fundamental to this goal is the ability to use precise U-Pb ages to determine how quickly specific global changes occurred. Without a quantitative understanding of rates, we cannot hope to move towards understanding the processes responsible for specific changes, or even the background states that characterized this critical time interval in Earth history.

A tuff with pumice fragments on the left from the Tambien Group contains prismatic zircon crystals that can be dated to high precision through U/Pb geochronology.

While carbon (δ¹³C) and strontium (⁸⁷Sr/⁸⁶Sr) isotope, magnetostratigraphic, and paleontological records exist for the early Neoproterozoic Era, and are the primary tools used for correlation of stratigraphic sections around the world, radiometric calibration is limited.  Most carbonate platforms well-suited for the development of δ¹³C and ⁸⁷Sr/⁸⁶Sr data are developed on passive margins far from explosive volcanic activity that might deliver zircon-rich ash to the basin. The Tambien Group of northern Ethiopia is ideal for solving this problem because it is a thick carbonate-containing succession deposited in a back-arc basin proximal to active volcanism. Zircons from these ashes can be dated very precisely through uranium-lead geochronology and this work is ongoing at Dan Condon’s lab at the British Geological Survey and in Blair Schoene’s lab at Princeton University. The ability to identify and date multiple ash beds throughout the stratigraphy provides an amazing opportunity that is rare in Proterozoic stratigraphy to develop many ages from a sedimentary basin from which we can also obtain data about changing ocean chemistry.

Field work to date throughout the Tambien Group has led to a entirely new framework for correlation of the physical stratigraphy across the basin (described in a paper currently in review at GEOLOGY). Currently developed chemostratigraphy and geochronology data support a correlation between a carbon cycle perturbation recorded in the Tambien Group and the Bitter Springs Stage we have been studying in AustraliaMarissa Tremblay (graduate student here at UC Berkeley) has been working to develop new ⁸⁷Sr/⁸⁶Sr isotopic data in order to add additional time-constrained data to the global composite curve. By quantifying the rate of isotopic change proposed mechanisms for global change in the lead-up to the Sturtian glaciation can be critically evaluated.

Sediments of the Tambien Group were deposited in the time period leading up to the first Neoproterozoic ice age. The record of Proterozoic Earth history from the basin can be constrained in age due to the presence of abundant units of volcanic ash interspersed within the sediments.

This material is based upon work supported by the National Science Foundation Division of Earth Sciences under Award No. 1325230. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.