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Revista de la Asociación Argentina de Sedimentología

versión impresa ISSN 1853-6360versión On-line ISSN 0328-1159


MILANA, Juan Pablo. Secuencias aluviales asociadas a variaciones hidrológicas: consideraciones teóricas y ejemplos. Rev. Asoc. Argent. Sedimentol. [online]. 1994, vol.1, n.2, pp.103-124. ISSN 1853-6360.

The mechanisms which can generate depositional sequences in proximal alluvial settings are here analyzed, aided with some case studies. The most likely working mechanic in the making of these proximal alluvial sequences, with no connection to the sea level, is that while tectonism shapes the primary accommodation space, such space is further modulated by changes in the hydrology of the basin, which eventually can be related with climate dynamics. Consequently, it seems highly possible that climate can generate third and fourth order alluvial sequences, if compared with the time-scale of marine depositional sequences. When high mean discharges are recorded, the transport efficiency increases and the stream slopes tend to flatten, implying upstream erosion and gravel progradation (downward migration of the knick point). During low-discharge periods, accumulation on steeper slopes is allowed by a decrease in transport efficiency. The latter causes accumulation in places where erosion prevailed during the earlier half-cycle. So that, gravel retrogradation occurs, which can also be viewed as an upstream migration of the knick point. The two case studies presented were selected to show the geometry of such a model in different sedimentary settings. The first case study, the Zonda terrace sequence, shows how this hydrologic sequences can be reflected in an slightly degradational setting which can be well constrained in their origin because we know the sedimentary agent who shaped them (the San Juan river). As these terraces were deposited and incised during the Pleistocene and the San Juan river finds its discharges largely on melting processes, the most suitable alternative to explain these terraces is in connection with glacio-induced hydrologic changes. Baselevel changes acting in the terrace generation are ruled out because terraces are convergent downstream, in opposition to the geometry of incisions by baselevel changes, as observed in the field, in flume experiments and with mathematical models by other researchers (Cf. Begin et al., 1981). So that, the lack of convergence of the terraces and the lack of evidence of great fluctuations of the local baselevel (the Guanache swamps) invalidates that possibility. Terraces in this area represent the physical extension of the last depositional surface prior to upstream incision and downstream progradation. Consequently, they can be taken as approximated time-equivalents to the initiation of gravel progradation in the basin. The hydrologic history of terrace generation applied to this special case (glacio-related discharge changes) can be synthesized in two cycles, each with four sub stages as follows: Inrerglacial 1: the river begins to accumulate and to acquire a high sloping profile, because the decrease of transport capacity respect to the earlier stage. Glacial Growth I: Due to the lowest transport efficiency, accumulation can occur on the highest slope, meaning that at this time the knick point reaches its uppermost location, causing the main accumulation of the terrace. Glacial I: Larger water reserves (ice) at the drainage basin causes discharges and transport efficiency to increase; the river can transport now the sediments left in the last step. Upstream erosion and gravel progradation begins, and the knick point migrates downstream. Deglaciation I: Maximum erosion along the upper alluvial profile occurs, because the highest transport capacity. Erosion extends downstream beyond of the convergence point (which divides overall degradational from agradational areas) ending where the right equilibrium slope for accumulation is reached. The latter indicates the maximum downstream migration of the knick point, and in this case, this point migrated several kilometers beyond the convergence point. Interglacial 2: Upstream accumulation begins again. Glacial Growth 2: Maximum growth of the alluvium but reaching a lower level than during the Glacial Growth l because the long-term erosional tendency. However, alluvium growth surpasses Deglacial I level up to the convergence point (new location of the knick point), implying a complete filling of erosion downstream and only a fraction upstream of the convergence point. Glacial 2: Begin of upstream erosion and gravel progradation. Deglaciation 2: Maximum erosion up to a lower level as in Desglac. 1, because the progressive uplift. The second study-case is based on the alluvial seismic sequences of the Iglesias Basin and intends to show how the hydrologic cycles can be imaged in seismic depositional sequences (slightly aggradational settings). Although this case-study has a nearly identical tectonic situation to the other, it has interesting differences as (a) there is not a clear convergence point here, and (b) differences on time scale. The lack of convergent truncations in the Iglesias Basin is a direct response of the general basin dynamics, which produces the basin to migrate in a whole to the west, in response to the "pushing" due to the self-similar growth of the rear of the Precordillera tectonic-wedge (Eastern Iglesias Basin is actively faulting). That does not occur in Zonda, where both mountain blocks are active leading the Tulum Valley to be stationary located (although the long-term basin tendency is not seen as in the Iglesias Basin). Differences in temporal scale suggests that the same cyclic process acts at different scales, generating a hierarchy of sequences, as indicated by the hierarchy of incisions at Iglesias. The different time resolution of seismic sequences resembles what happens in marine sequences where high frequency eustatic shifts cannot be surveyed by the seismic method. It is suggested that these hydrologic sequences can he more common than normally assumed. Changes in discharge can also influence alluvial systems in more ways than the simple geometric schemes brought. For example, in pure agradational areas where no major incision occur, intervals of higher channel density can be also well explained by an increased transport capacity, because it leads to a faster channel belt filling and consequent more avulsion occurs. In such way, highly variable sequences can be associated to a constant subsidence rate as demonstrated by other studies in the region (Milana, 1990). For proximal successions as those exemplified, a simple model as that of figure 4 can be applied, noting that no migration of the basin occurs. The theoretically origin of each unconformity (truncation) and their graphic simulation by means of diffusion algorithms fit very well with the characteristics of both field examples. Such a simple model serves as a guide to relate geomorphologic evidences as terraces and basinal grain-size sequences, which are highly related. Also, as in both study-cases, the hydrologic cycles can be related to glacial fluctuations, it may be possible to correlate these cycles with those marine of glacio-eustatic origin. Such link must exist as the hydrology of alluvial systems composes the interface through which water is transferred from continents to oceans.

Palabras clave : Alluvial; Depositional sequences; Hydrology; Terraces.

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