Fácies e isótopos de carbono e oxigênio da Formação Nobres, Neoproterozóico da Faixa Paraguai Norte, Mato Grosso.

Abstract

The lastest stages of orogenic evolution of the Northern Paraguay Belt, south of the Amazon Craton, are related to a closure of a passive margin in the Neoproterozoic-Cambrian transition. In this basin, the predominant carbonate sedimentation of the Araras Group was succeeded by siliciclastic deposits of the Alto Paraguay Group. Events that led to the inhibition of carbonate sedimentation were partially recorded in the Nobres Formation, uppermost unit of the Araras Group. In the southern part of the Northern Paraguay Belt, in the Cáceres region, state of Mato Grosso, Central Brazil, the Nobres Formation overlies in unconformity the dolostones of the Serra do Quilombo Formation, and is overlain unconformably by coastal siliciclastic deposits of the Raizama Formation of the Alto Paraguai Group. The Nobres Formation consists of shallowing / brining upward small-scale arid peritidal cycles subdivided into two members: (i) lower member, composed of massive dolostone, intraclastic dolopackstones with enterolithic and megaripple bedding, wavy lamination with mud drapes, dolostone with silicified evaporite molds and domic to stratiform stromatolites, interpreted as tidal flat/sabkha deposits; and (ii) upper member, consisting of massive dolostone, dolomitic sandstone with megaripple bedding, wavy lamination with mud drapes, domic to stratiform and wrinkled stromatolites, dolostones with silicified evaporates molds, sandstones with low angle and climbing ripple cross lamination and mudstones with even parallel and wavy lamination, interpreted as mixed tidal flat deposits. The chemostratigraphic framework of Nobre Formation includes values of 13C and 18O between -2.19 and 0, 27‰VPDB and -7.42 and -4.25‰VPDB, respectively. The δ13C values are primary and representative of the original composition of Neoproterozoic seawater. The analysis of the δ13C stratigraphic curve in combination with the facies interpretation and following the Ediacaran stratified ocean model, allowed the identification of four isotopic patterns of the studied succession: type I, related to the storm wave to storm influenced shallow platform of Serra do Quilombo, with δ13C values near zero ranging from -0.27 to 0.4 ‰VPDB interpreted as a mixing of stratified waters of the ocean; type II, which refers to the transition between Serra do Quilombo and Nobres formations, with δ13C values ranging from -0.12 to 0.41 ‰VPDB, whose positive trend is interpreted as the fixation of 12C by biological activity; type III, defined as the typical isotopic signal of the Nobres Formation, trending and ranging from uniform values δ13C from -1.80 to -0.13‰VPDB related to changes in sea level; and type IV, that consist of crinkled pattern with δ13C values from -2.19 to -0.73‰VPDB indicative of isotopic reequilibrium of peritidal waters. The stacking of peritidal cycles, up to 200 m thick, suggests continuous and recurrent generation of accommodation with probably linked to tectonic subsidence. The siliciclastic inflow in the end of the deposition of the Nobres Formation, hindered the carbonate sedimentation, and it is attributed to uplift of source areas linked to the initial phase of the closure of Clymente Ocean, during the Pampean-Araguaia orogeny at the Neoproterozoic-Cambrian limit.