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Revista de la Asociación Argentina de Sedimentología

versión impresa ISSN 1853-6360


BOSSI, Gerardo E. Una alternativa en aloestratigrafia. Rev. Asoc. Argent. Sedimentol. [online]. 1998, vol.5, n.2, pp.71-95. ISSN 1853-6360.

The unconformity-bounded stratigraphic units due to the outside nature of the limits are called allounits. The unconformities should be significant as hiatal or erosive intervals in a basin or series of related basins. The defined allounits persist as far as the limiting discontinuities are traceable. According to Chang (1975), the hiatal or erosive intervals within the unit have much less signification than the discontinuities defining the limits. Because, the unconformity-bounded units are defined by surfaces generated by allocthonous non-depositional processes, they are included in the Allostratigraphy (Fig. 1, and see Walker, 1992; Posamentier & James, 1993). Allostratigraphy may be defined as the branch of Stratigraphy where the bodies of sedimentary rocks of a particular basin or related basins are classified by major breaks of the sedimentary record, recognized by the resultant unconformities and not by the sedimentary filling itself (Fig. 1). In 1983, The North American Comission for Stratigraphic Nomenclature (NACSN) defined the Alloformation as the fundamental unconformity-bounded unit, with the subdivisions as Allomembers, and groupings in Allogroups, and also proposed an application methodology (Fig. 4). The concept of Alloformation is similar (ISG, Salvador, 1994) to synthem (Chang, 1975, from Latin and Greek, "syn" meaning "together" and "thema" meaning "the deposit of"). The divisions in Chang (1975) are the subsynthem, interthem and mesothem. In the ISG (Salvador, 1994), the synthems are subdivided in "subsynthems" and grouped in "supersynthems". According to Chang's original paper the scale of a synthem is comparable in time duration with a Period and hence of higher order of magnitud than NACSN's Alloformation. The ISG proposal (Salvador, 1994, pg. 46, figure 4, reproduced as figure 2 in this paper) do not define the scale, but says "the unconformity bounded units may include any number of other kinds of stratigraphic units (lithostratigraphic, biostratigraphic, chronostratigraphic, magnetostrati-graphic, and so on) from a few to many, both in vertical and/or lateral sucession (Fig. 4)". Wheeler's (1959) definition of sequence "as a preserved stratal succession which is unconformably separated from overlying and uderlying rocks" is very close to the NACSN (1983) and should be considered the fundamental definition for units sepated by unconformities. The article 71 of the "Codigo Argentino de Estratigrafía" (CAE, 1992) selected the Sequence or Miosynthem (from Latin and Greek "meio" or "mio" meaning "small", "minor" or "lesser" and "thema" meaning "the deposit of") as the fundamental unit for sucessions bounded by unconformities. The CAE has incorporated a modified version of parasequences as subunits of the Miosynthems and a model for the Sequence (Fig. 3A), very close to the sequence stratigraphy model of Mitchum et al. (1977). This attitude brought in the Sequence Stratigraphy, although modified, into the formal stratigraphic practice. Depositional sequence limits are defined according to contact geometric relations between rock packages and lateral projections or correlative conformities, that differentiate the rock body as a genetic unit. The limits may be discontinuities or continuities. There are some remarks about the CAE made by González Bonorino (1993) which deserve consideration: 1) The sequence stratigraphy method of analysis of sedimentary sucessions lays in the mutual relations between genetically linked rock packages (systems tracts) which in turn are influenced by relative sea-level fluctuations (Fig. 3A). 2) The limiting unconformities (Sequence Boundaries) and/or their correlative (lateral) conformities are involved in time constrained episodes. 3) The parasequences as minor divisions of depositional sequences should not be included in the formal stratigraphy because, among other reasons, its dependence with the sea level steps marking the flooding of the platform. Parasequences and higher order sequences are interlocked concepts where interpretation plays a major role. The present day sequence stratigraphy framework does not support a hierarchical classification of sequences recognized as 1st to 7th order. Most of these orders considered in the literature are the result of different analysis approaches and process-response theories. It is unlike that sequences at all orders correspond to a "Russian doll" stacked set (Carter et al., 1991, pg. 45). The "Secuencia" (Sequence) or "Miosintema" (Miosynthem) of CAE (1992) is a new unit not equal to the synthem and myosinthem of Chang (1975) and/or the miosynthem of International Stratigraphic Guide (ISSC, 1987 and ISG, Salvador, 1994). Implicit in the EXXON Group (Posamentier & James, 1993) definitions of the basic attributes of a depositional sequence, are the cyclic nature of the succession and the chronostratigraphic frame that helps to reinforce the lithologic prediction. The systems tract and parasequences are part of the same body of evidence (or rocks) but taken in different approaches. The correlative conformities are defined in seismic lines as reflector planes assumed as parallel to time lines and connected with the sequence boundaries of the platform. Both elements are formed as a response to sea level relative falls. All these concepts are integrated in the modelling of the basin and in the prediction analysis that conveys into the definition of the depositional sequences. It is obvious that a procedure where lithological contains, unconformities, correlative conformities and sea level response modelling are combined, should not be incorporated into the formal allostratigraphic classification. The main diagnostic criteria for the definition of unconformity-bounded units is the presence of significant discontinuities. The term significant is obviously highly subjective (ISG, Salvador, 1994). The lithology is relevant but not diagnostic. It is important to define clearly the nature, effects and extention of the unconformities that are going to be used as limits. Walker (1992) adhered to the concept of correlative conformities to extend the vality of the allounits beyond the limits of the proved discontinuities. The NACSN (1983) did not touched the question but it seemed to accept a "lateral continuity" in a local well document context (see Figure 7 of NACSN, 1983, reproduced here as Figure 4). There is no gain of clarity incorporating a non-diagnostic concept as correlative conformity in the definition of an allounit. Most of the lateral conformities defined in marine facies are actually paraconformities or set of paraconformities (i.e.: the condensed section, Galloway, 1989b). It is important for the allounits to be related in the most simple way with the lithostratigraphic units defined in the same basin tract or scenario. It should be stressed that it is necessary to count with an objective methodology to define the limits. The present paper not only discusses a extended methodology but also the definitions and therminology for discontinuities and basic concepts in Spanish. It is convenient that the alloformations associated in an Allogroup, have physical contact or be parts of a related basin filling episode The Sequence Stratigraphy should continue to be used as a prediction tool, where lithologic, facial, genetic and temporal assets could be freely managed. Used in this way it is a formidable interpretation tool specially in the Petroleum Industry. The Allostratigraphic methodology discussed in this paper as an extension of the original NASCN (1983) is simple and direct. There are no genetic,temporal or interpretative questions involved in the definition of the units, their subdivisions or groupings. The fundamental unit, the Alloformation, has a scale of similar volume than the Formation in Lithostratigraphy. In very particular cases, some Alloformations may be named as Formations or viceversa. A classification scheme well founded in reproducible observations is always lasting. Interpretations and models may be changed as many times as it is necessary.

Palabras clave : Allostratigraphy; Alloformations; Synthems; Methodology; Allostratigraphic guide.

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