A. Misi et J. Veizer, NEOPROTEROZOIC CARBONATE SEQUENCES OF THE UNA GROUP, IRECE BASIN, BRAZIL - CHEMOSTRATIGRAPHY, AGE AND CORRELATIONS, Precambrian research, 89(1-2), 1998, pp. 87-100
The Una Group, Irec (e) over cap Basin (Bahia state) is correlative wi
th the better known Bambui Group of the S (a) over tilde o Francisco B
asin (Minas Gerais state); these basins are remnants of widespread car
bonate platform that covers > 300000 km(2) of the S (a) over tilde o F
rancisco Craton in Brazil. The Irec (e) over cap area contains a well-
controlled stratigraphic succession hosting economic phosphate deposit
s and sulfide mineralization. Una Group strata comprise a basal silici
clastic sequence, mostly composed of glacial diamictites (Bebedouro Fo
rmation), followed by a carbonate succession that is several hundred m
eters thick, the Salitre Formation. An angular unconformity separates
the two units. The carbonates of the Salitre Formation are characteriz
ed by Sr-87/Sr-86 ratios ranging between 0.70745 and 0.71776, and delt
a(13)C values ranging between -4.4 and +9.4 parts per thousand PDB. Th
e carbonates also contain beds and nodules of barite and gypsum with h
ighly positive delta(34)S values (+25.2 to +32.8 parts per thousand CD
T). The 'best preserved' carbonate samples selected on the basis of th
eir Mn and Sr concentrations as well as their ratios, yield Sr-87/Sr-8
6 values of 0.70745-0.70765. Comparison of these values with the secul
ar strontium isotopic curve for the Neoproterozoic seawater suggests a
600 (to 670) Ma age for the Salitre Formation and for its correlative
units in the S (a) over tilde o Francisco Basin. (C) 1998 Elsevier Sc
ience B.V.