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Joelson Lima Soares, Universidade Federal do Amazonas (Brazil)
Afonso C. R. Nogueira, Universidade Federal do Pará (Brazil)
Ozineide da Costa Menezes, Universidade Federal do Amazonas (Brazil)
Alcides da N. Sial, Universidade Federal de Pernambuco (Brazil)
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C and O isotopic analyses were carried out in three samples of peloidal dolograinstone with megaripple bedding (previously described as tepee-like structures) from the upper portion of a cap dolostone exposed in the Mirassol d'Oeste region, southwestern Amazon Craton, Brazil. The complex megaripples are locally composed of migratory and aggradational crests, beside of climbing wave-ripple cross lamination and planar to quasi-planar lamination related to the oscillatory and unidirectional (combined) flow (storms?). The megaripple laminations which are some millimeters to rarely 1 cm thick, formed by peloids and macropeloids with porosity filled by fine dolomite cement or bitumen conferring a streaky pattern to the rock.
Systematic sampling of peloidal dolostone laminae with different thickness, avoiding the bituminous fraction, for isotopic analysis allowed the comparison of the C and O values with the organic matter content. C isotope values which range from -4‰ to -5‰ are related to the thickness of the laminae. In general, thick laminae produced by migratory bedform exhibit enriched δ 13C values between -4,5‰ and -4‰ while thin aggradational laminae show depleted values of -5,5‰ to -5‰ and can reflect expressive organic matter preservation. This observation suggests that the minimum fractionation of C associated to the megaripple bedded dolostone seems to be influenced by flow regime and consequent carbonate accumulation with preservation of organic matter. This work initiates a debate on significance of δ 13C excursions in carbonate rocks with preserved hydrodinamic structures.
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