Associação vertical de fácies e análise de elementos arquitecturais: concepções concorrentes e complementares na caracterização de ambientes aluviais

P. D. Ferreira-Júnior, P. T. A. Castro

Abstract


There are two ways to study sedimentary rocks in outcrops: the vertical facies succession analysis and the architectural element analysis. The main difference between them is related to how detailed the results are. This paper deals with methodological aspects of both methods, trying to point out the advantages and disadvantages of using each one in alluvial rocks. In order to make this research it was selected as fieldwork object the Neocretaceous sedimentary rocks from Uberaba Formation, Bauru Group, of Paraná Basin, Southeast Brazil. From using both methods we can define these rocks as formed by a braided fluvial system with a high mobility channel complex.
When the vertical facies succession analysis is used, several steps must be used from outcrop analysis to depositional system inrepretation. The collect of outcrop data and the necessity of an useful technical language are the base over which the facies concept was established. The vertical facies succession and facies association based on their genetic and environmental relations are used to define depositional systems. However, the establishment of vertical succession of facies by itself can not be used to characterise a depositional system.
In the same way, comparison among facies groups from outcrops and those presented as summary tables of each depositional systems that appear in several publications must be avoid because they simply ignore some characteristics of each example. Vertical successions of facies are usually showed as bidimentional graphic logs, being wide used since the 1970’s.
In 1980, the establishment of the architectural element analysis method improved the sedimentary rock description and analysis of outcrops. Instead of using only sedimentary rock aspects (facies) this method pays attention on the discontinuity surfaces such as set boundaries that usually occurs in outcrops. The main goal of architectural element analysis method is to identify discontinuity surfaces that can be used to limit and identify genetically related sedimentary bodies in alluvial rocks. These sedimentary bodies, hierarchically ordered are known as architectural elements and macroforms.
It is possible to obtain a tridimentional perspective of sedimentary rocks and consequently their geometry by using architectural elements and macroforms defined in outcrops associated with paleocurrent measurements of bedforms.
The Architectural elements analysis offer more advantages to the user when compared with vertical facies succession analysis. This is mainly because architectural element analysis turns possible to determine sandy body geometry, local variation of the alluvial depositional style and the direction of sedimentary growing and/or developing. These informations are useful in mineral deposit prospection such as in placer deposits and in petroleum prospection also.
When compared, both the vertical facies succession and architectural element analysis methods they seem to be competing and complementing at the same time.


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Keywords


Facies, architectural elements, macroform, alluvial, depositional models.

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