HR: 1340h
AN: PP33B-0943    [Abstracts]
TI: Of Island Arcs and Icebergs? Strontium and Carbon Isotope Stratigraphy of Ethiopian Snowball Earth Sequences
AU: * Miller, N R
EM: miller@utdallas.edu
AF: University of Texas at Dallas, PO Box 830688, Richardson, TX 75083-0688 United States
AU: Stern, R J
EM: rjstern@utdallas.edu
AF: University of Texas at Dallas, PO Box 830688, Richardson, TX 75083-0688 United States
AU: Avigad, D
EM: avigad@vms.huji.ac.il
AF: The Hebrew University of Jerusalem, Jerusalem, Jerusalem, 91904 Israel
AU: Beyth, M
EM: mbeyth@mni.gov.il
AF: Geological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem, 95501 Israel
AU: Kster, D
EM: kuesterreeck@yahoo.de
AF: Mekelle University, PO Box 231, Mekelle, 231 Ethiopia
AU: Gebresilassie, S
EM: sol1703@yahoo.com
AF: Mekelle University, PO Box 231, Mekelle, 231 Ethiopia
AU: Mehari, K
EM: kiros_mehari@yahoo.co.uk
AF: Ezana Mining Development Co., PO Box 788, Mekelle, 788 Ethiopia
AB: Neoproterozoic low-grade metasedimentary exposures uniquely preserved in synclinoria of the Tigrai region of northern Ethiopia offer excellent opportunities to assess the sedimentary response to extreme climatic changes encompassed by the Snowball Earth Hypothesis within the framework of the tectonic evolution of the Arabian-Nubian shield. Basement exposures consist of two major sequences; an older predominantly island arc metavolcanic sequence named the Tsaliet Group, and a younger metasedimentary succession named the Tambien Group. An intervening phyllite unit displays a transitional lithology variously interpreted by previous workers as metasedimentary and metavolcanic. We report Sr and C isotope results for Tambien Group carbonates from three prospective Snowball Earth successions flanking the Mekelle outlier: Negash, Mai Kenetal, and Samre. These localities display excellent exposures of the transition from Tsaliet metavolcaniclastics into distinctive carbonate exposures of the Tambien Group. Distinctive volcanic polymict conglomerates occur close to the basal Tambien contact in the upper Tsaliet Group in two localities. Presence of evaporite pseudomorphs and persistence of microbially laminated lithologies including stromatolites suggests that Tambien carbonate deposition occurred in a shallow marine setting that was initially hypersaline. A distinct black limestone unit, with abundant organic matter and indications of variable energy conditions in a slope setting, occurs higher in the section. Accompanying this transition in gross carbonate depositional style is a pronounced enrichment of carbon isotopes from values between -4 and -1 per mil in the lower interval to values between +4 and +7 per mil in the overlying black limestone. A similar excursion to more radiogenic $^{87}$Sr/$^{87}$Sr compositions is observed from values between 0.7060 to 0.7064 in the lower interval to values "plateauing" between 0.7067 and 0.7069 in the black limestone. Thus far, only the Negash syncline preserves a diamictite of probable glacial origin (i.e., dropstones, faceted clasts, cobbles of diverse composition) where a pronounced negative carbon excursion from values of +7 to -2 per mil accompanies the transition from upper black carbonate to diamictite deposition. The diamictite is the youngest preserved unit within the investigated structures so it is unknown if a superjacent cap carbonate was ever deposited. Zircon evaporation dates from granites intruding the base of the sequence but post-dating early deformation of the entire sequence suggest that the Tambien Group is older than 613Ma, and therefore diamictite genesis is incompatible with younger Varanger, or `Ediacaran' glacial intervals. Ages of ~800 Ma for Tsaliet equivalents in Eritrea provide an upper age constraint. The range of measured Sr isotopic compositions best overlaps Sturtian portions of available reference curves (ca. 720-750 Ma or a little older) and strongly supports the prospect that the Negash diamictite is the product of a Sturtian glacial episode.
DE: 9305 Africa
DE: 9619 Precambrian
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1635 Oceans (4203)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting