Parameters controlling sonic velocities in a mixed carbonate‐siliciclastics Permian shelf‐margin (upper San Andres formation, Last Chance Canyon, New Mexico)

We have measured the acoustic properties and mineralogic composition of 48 rock specimens from mixed carbonate‐siliciclastic outcrops of the Permian upper San Andres formation in Last Chance Canyon, New Mexico. The goals were: (1) identify and model the parameters controlling the sonic velocities; (2) assess the influence of postburial diagenesis on the acoustic velocities. The variation in sonic velocity in the 0 to 25% porosity range is primarily controlled by porosity, and secondly by the ratio of carbonate‐siliciclastic material. Linear multivariate fitting resulted in a velocity‐porosity‐carbonate content transform that accurately predicts sonic velocity at different effective stresses. The slope of the velocity‐porosity transform steepens with increasing carbonate content, which may be explained by the higher velocity of carbonate minerals. Another reason may be the property of carbonate minerals to form more perfect intercrystalline boundaries that improve the transmission properties of acoustic waves and are less sensitive to changes in effective stress. The velocity ratio [Formula: see text] is an excellent tool to discriminate between predominantly calcitic lithologies (ratio between 1.8 and 1.95) and predominantly dolomitic and quartz‐rich lithologies (ratio between 1.65 and 1.8). Gardner's experimental curve overestimates, and the velocity‐porosity transforms by Wyllie and Raymer underestimate, the observed sonic velocities, probably because they do not account for variations in texture, carbonate mineralogy, and pore geometry. Petrographic observations show that postburial diagenesis is minor and does not seem to significanfly affect porosity. Therefore, the outcrop data set can be regarded as a proxy for the subsurface analog. These findings underline the significantly more complex acoustic behavior in mixed carbonate‐siliciclastic sedimentary rocks than in pure siliciclastics where mineralogic composition explains most of the observed relationships between porosity and sonic velocity.

EVOLUÇÃO GEOLÓGICA DOS GRUPOS CANASTRA E IBIÁ NA REGIÃO ENTRE COROMANDEL E GUARDA-MOR, MG

The stratigraphic column established for the Canastra Group, in the Alto Paranaíba region in the centralportion of the Brasília Fold Belt, represents a regressive megasequence of sediments deposited on an openseacontinental platform. The following units occur from base to top: a turbiditic sequence with arenaceousand argillaceous intercalations; an arenaceous sequence containing storm beds; and a clay-rich arenaceoussequence with structures reflecting shallow marine and tidal flat environments. The Ibiá Group overlies theCanastra Group on an erosional disconformity. Its base consists of a metadiamictite of glacial origin (CubatãoFormation) while the uppermost unit (Rio Verde Formation) comprises an extensive and monotonous packageof banded calcareous phyllites. The contact between these two formations is gradational and both units havesimilar mineralogic composition. The Canastra and Ibiá Groups were affected by a single event of progressivedeformation, E1, during the Brasiliano Cycle, manifested as two distinct compressional stages. The earliestand most compressive stage, D1, is represented by thrust tectonics, associated with greenschist faciesmetamorphism. The second stage of deformation, D2, is characterized by a dominant component of pureshear in a ductile-brittle regime. When compared with other similar units in the geochronological record, thecharacteristics of the studied units show that the Ibiá Group metadiamictites are contemporaneous with theJequitaí tillite, with a probable age of deposition between 900 and 1000 Ma and the Canastra Group istherefore older than 1000 Ma.