Stromatolites in the mesoproterozoic Itaiacoca Group (se Brazil): paleoenvironmental inferences and comparisons with other similar forms

STROMATOLITES IN THE MESOPROTEROZOIC ITAIACOCA GROUP (SE BRAZIL): PALEOENVIRONMENTAL INFERENCES AND COMPARISONS WITH OTHER SIMILAR FORMS

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ILLIAM SALLUM FILHO and THOMAS R. FAIRCHILD

Instituto de Geociências, USP, S ao Paulo, SP.

Presented by ANTONIO C. ROCHA-CAMPOS

Stromatolites have long been used as paleoenvironmental markers and for correlation within and among diverse Proterozoic basins. Coniform stromatolites of the Mesoproterozoic volcanosedimentary Itaiacoca Group in the Ribeira Belt of southeast Brazil form two types of bioherms south of Itapeva (São Paulo), one tabular and widespread, and the other lenticular and isolated. In both cases the predominance of Conophyton together with the rarity of other forms and with the absence of such shallow-water sedimentological indicators as micro-unconformities, stromatolitic intraclasts, and crinate stratiform stromatolites suggests a relatively deep depositional setting, below fairweather wave base. Morphological differences between the stromatolites apparently reflect differing degrees of luminosity in these two settings. In the more widespread bioherm, the composition (dolomite minimally contaminated by siliciclastics), larger diameters, and frequent lateral coalescence of the columns may all have been favored by relatively well lit, clear, possibly shallower waters. In the isolated bioherm, the generally narrower width and rare lateral coalescence of the stromatolites, plus the abundance of impurities in these calcitic limestones and the very lenticularity of the bioherm, suggest predominantly vertical, light-seeking growth, presumably within poorly lit, more turbid or deeper setting.

These and closely similar stromatolites, also from the Itaiacoca Group about 130 km to the southwest, are attributed to Conophyton garganicum. They differ significantly in their axial zones and lamination from other forms of Conophyton in the Paranoá Group known from the São Francisco Craton 500 km or more to the north. Apparently, Conophyton in these two regions formed in different depositional settings and/or at different times. (December 10, 1999)

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Publication Dates

• Publication in this collection
  05 Jan 2001
• Date of issue
  Dec 2000