By: Eng. Eliseo Popolizio.
Centro de Geociencias Aplicadas.
Universidad Nacional del Nordeste.
Av. Las Heras 727. Resistencia, Chaco. Argentina.
The aim of the following paper is to remark the importance that the reconnaissance of the types of fluvial nets in studies of aerial photointerpretation has. On the other hand, we intend to present a model of classification which was previously elaborated by the author and which allows, based on basic types of nets, to establish combinative models. In our opinion this is fundamental since it avoids the multiplicity and diversity of terms that the various authors use to refer to them. It was elaborated for studying the area known as “Submeridional Lowlands” in the province of Chaco and Santa Fe (Argentina) and it has been ever since used in every later paper carried out in the Centre of Applied Geosciences of the Universidad Nacional del Nordeste.
It is known that the integration of the drainage channels of the waters spatially determine a model known as drainage net, which has essential importance in geomorphological studies and in those of photointerpretation.
In fact, the characteristics of a net may reveal considerable data related to the landforms and its behaviour, and on the other hand, since they are easily observed in the aerophotographs, they have a great significance in the reconnaissance of the geomorphological characteristics.
It is necessary to take into account that the components of a net may be integrated or unintegrated and therefore it is convenient to set a legend to differentiate both cases.
Consequently we will designate as InN the spatially and functionally integrated nets, and as UnN those that are unintegrated. (Fig. Nº1).
In the same way the nets may be convergent or divergent. In order to indicate this in the corresponding legend we will use a super index plus or super index minus, respectively.
It is also necessary to consider that in the aerial photographs and in the field works it is possible to recognize ancient nets or paleomodels originated in conditions different from those at present. In those cases we will put the letter P before. Thus, the legend PInN+ would stand for convergent, integrated paleonet.
Finally there are other nets that are buried by sediments or that are underground. However they can be recognized in the aerial photographs and in that case we would put the letters Ph (Phantom) before.
We can not overlook the fact that the scale factor is fundamental in the characterization and reconnaissance of the types of nets. In the following paper the references are related to scales comprised between 1: 75,000 and 1: 20,000 which are quite common. It implies that the concepts included in the following paper may be applied to other scales and to the satellite images with due reservations in each case.
The basic net models used in this classification are indicated in Figs. Nº 1 and 2.
The basic net models are indicated by letters in sub index position, and when the nets represent combinations the corresponding letters are placed one after the other, separated by commas and in the order of importance they have in the combination, as we will see it later.
It is characterized by presenting a steady increase in the tributaries, from the mouth to the sources, usually following a seriate pattern as regards the number of courses of different order, e.g. 2n, i.e. each segment is divided in 2 and so on.
It is generally considered unsequent ,from a genetic standpoint, i.e. it is not controlled by the structure so that there are not preferential lineaments that may affect the drainage channels.
It can be said that there exists an erosive isotropy which may be caused by lithological or structural homogeneity as well as by the existence of a weathering layer, or soil, that may hide, due to its thickness, the structural influence.
It is typical of areas with high pluviosity, mostly because it is there where the weathering and the edaphogenesis originate regoliths and deep grounds, which veil the litho-structural conditions making it difficult to translate them into the lineament of the courses that are part of the net.
However, in many cases, a model on a small scale (e.g. 1: 100,000) may appear as dendritic, while observed on a larger scale (e.g. 1: 25,000) may reflect certain structural conditioning which is not distinguishable in the first ones. In general, the smaller the scale is (that is to say, the larger the area covered is) the more the litho-structural factor will prevail over those of the surface structure of the landscape.
It must not be overlooked that the dominant factor in the genesis of this model is homogeneity or surface isotropy. Therefore it may develop on rocks or homogeneous structures, even though when there are no conditions of excessive pluviosity.
The horizontal or sub-horizontal sedimentary rocks of the type of clay, silt and sand or mixtures of them, offer the best conditions for the development of this model.
The compact sedimentary and the sub-horizontal metamorphic rocks may present a similar model but the presence of diaclases or the effects of the schistocity degenerate it into a complex model: dendritic-angular.
The plutonic rocks and the metamorphic rocks of the hypozone, due to their high degree of homogeneity, when observed on small scales present this model. When this model is thoroughly analysed it always reflects a complex dendritic-angular model as a consequence of the diaclases and faults that frequently affect these rocks.
This model may appear in any of its four varieties, i.e.: Integrated, Unintegrated, Convergent and Divergent.
At present, in arid or semiarid areas it often appears as unintegrated and responds to a decrease of the precipitations, which could have been formerly higher thus originating integrated models.
The divergent model is also quite common in the debris alluvial fans and in the typical deltas, where there may also appear unintegrated models as paleoclimatic relicts.
The essential characteristic that differentiates it from the dendritic net is the very acute angle that the affluents make. It may also present a seriate model.
This model is frequently associated to the surface isotropy and in such event the observations made for the above mentioned net are applicable. However there is always a strong conditioning associated to the topographic slope. If the latter is very marked the net may develop on non-homogeneous rocks.
It frequently appears in the rear side of the hogback relief and also where there exist inclined planes that determine a regional slope.
Alike the previous model, it may also present the four possibilities indicated in Fig. Nº1 and it may also respond to paleomodels, in such event the observations made for the dendritic net are also valid.
It is always the result of a strong structural conditioning due to geoclases or conjugated faults that meet determining almost a right angle. Therefore the tributaries courses meet the main courses with that angular value.
The massifs formed by plutonic or metamorphic rocks, with a high degree of metamorphism, tend to present this model to such an extent that it makes one of the basic patterns for the reconnaissance of this type of rock.
Great caution is required when dealing with small or very small scales in which the lithologic isotropy predominates over the structural discontinuities and then the model may seem dendritic.
Despite of the above, as it has been already commented by many other authors, this model may appear in sedimentary basin as a result of the reflection of the low tectonics. This may happen even when the covering is hundreds of meters thick, which has been observed in the plains in many countries and in Argentina.
It is convenient to say that these conjugated lineaments that almost meet in a right angle form, in South America, two systems: the Amazonian (E-W)- Sao Franciscan (N-S) and the Caribbean (NW-SE)- Brazilian (SW-NE).
There apply to this net all observations made above. It is the result of the overlapping of the two mentioned systems or even of other lineaments of faults or diaclases that control the joining of the fluvial courses. As a result of that the meeting angles of the affluents with the main courses vary from one place to another. For the same reason, it is valid what was mentioned for the previous net: this model on small scales may be mistaken for a dendritic one.
It is characterized by having very long main and subsequent courses while the affluents are short, meet in a right angle and have a obsequent and consequent nature.
It generally responds to a strong structural conditioning caused by inclined and even vertical strata corresponding to monoclinal structures or to the sides or limbos of folded structures.
The affluent courses of greater length follow the direction of dip and therefore, the allow to determine it which is also a key to photointerpretation. The alternation of strata of different resistance to erosion causes a marked surface anisotropy that controls the dominant direction of the net.. That is the reason why the main course follows the direction of the course of the strata. For the same reason a combined parallel-trellis model is frequent.
It is characterized by having rectilinear courses that suddenly change their direction in approximately 90º for a short stretch and then return again to the former direction. It is typical of folded reliefs specially of the type of Appalachians and reflects phenomena of overimposing of a net over a structure, gradually exhumed. However, similar models may occur in parallel nets with capture processes originated by a tributary.
It may also occur over parallel eolian ridges or moraines with the same model, by overimposing or by capture processes. It may also indicate the presence of a fault transversal to the drainage or a geoclase with those characteristics. It is quite common in Appalachian reliefs.
In this case the main courses are arranged as the spokes of a wheel and may logically be convergent or divergent. They respond both to structural and morphological conditions, and therefore they must be analysed with caution before making a judgment about their genesis.
In general, it is going to be originated by any dome-like or circular depression, e.g. the volcanoes, the half dome-like or conical residual reliefs, the half circular structural and eolian depressions or those of differential settlement.
It may be observed on quite different scales, for instance, on a great plate-shaped sedimentary basin or in small seudokarstic depressions. In the latter, it presents a feature that in Argentina was named by some authors as “star-shaped” or “spider” model. However the chance of it being associated to dome-like structural conditionings (salt or clay diapires, laccoliths or small batholith, etc) has given great importance to this model.
It may be the first element to recognize an area potentially rich in minerals, such as the salt domes associated to the petroleum.
In the plains, the karstic or pseudokarstic and eolian depressions and those of half circular lakes or lagoons frequently originate this model.
It is often associated to morphological or structural models similar to those mentioned for the above net, but it requires that the morphology or the structure determine a differential zoning arranged as concentric rings before erosion.
From a structural standpoint these conditions may be the result of circular or concentric faults, or strata arranged in domed or basin-like structures, bevelled by erosion which originates a succession of strata with different resistance to it.
The structural deformations created by laccoliths, lopoliths and intrusions of little batholiths, or by the annular metamorphic zoning may originate this type of net. In some cases the annular shore ridges that are caused by the progressive drying of a lake, and some eroded stratum-volcanoes may determine this model.
Finally we must say that the association between the last types of nets mentioned is frequent thus originating a radio-annular model.
It may respond to a structural conditioning such as the one that occurs in the nose or prow of anticlines or synclines, which have been partially dismantled. It may also appear associated to a morphology of eolian ridges of the parabolic type or rather bordering the frontal moraines of the glaciers.
It is characterized by a series of depressions along the fluvial axis, as if they formed a rosary. They create quite favourable conditions for regulating the drainage since depressions function as regulating reservoirs. It may be conditioned by many factors and it can be mono or polygenetic.
It may be originated by the presence of ancient or exhumed eolian depressions, interconnected by a current fluvial system, the interconnection of the karstic or pseudokarstic depressions and the differential settlements by the compacting of sediments along a fluvial axis; and they frequently represent models in the plains.
The interconnection of lakes of moraine- damming by a fluvial course and at an antropic level, the building of stepped dams may originate this type of net.
It implies a complete disorganization of the drainage, though it does not affect the general direction of the flow of the stream which in most cases is easily detected.
Its most frequent forms occur in karstic and pseudokarstic areas, where the depressions and channels may be interconnected in different directions.
It may also occur in plains of glacial erosion or in partially dismantled eolian paleomodels.
It may be conceptually applied to fluvial plains and delta areas where a tangle of channels become intercrossed and may be interconnected to any type of depressions though it cannot be considered a net in a narrow sense.
The term was created by Sc.D. Pierina Pasotti for a quite frequent model in the province of Santa Fe (Argentina), but which also appears in other plain areas and in zones of high relief.
It is a consequence of a structural or morphologic obstacle, that forces a system of parallel courses to change the direction and converge into a central one which succeeds in avoiding the obstacle due to the increase in the erosive capacity. This indicates that the courses ran along an antecedent surface and once the obstacle appeared, due to a tectonic uplift or to differential erosion, said net originated.
It is frequently observed in the cuesta reliefs, in platform areas or areas of uplift of tilted tectonic blocks which indicates the presence of a fault.
As it has been already mentioned, tectonic movements of the basament may take place in the plains although the sedimentary covering may be thick, thus originating this type of net as Sc.D. Pasotti indicated for the province of Santa Fe.
This type of pure net cannot be convergent or divergent. The existence of a net of these characteristics indicates quite an interesting adaptation between the topography and the drainage besides strong surface isotropy.
In fact, the topography must be conditioned to the loss of energy of the drainage due to velocity, so that it may be impossible that the waters overflow the drainage channels. Otherwise it would cause an immediate or gradual integration between them towards a pinnate model. This model does not often appear in its basic or pure form. It may be, for instance, conditioned by structural lineaments originated by inclined strata bevelled by an erosion surface, but in this case, only the main or subsequent courses are frequently parallel, meanwhile the model tends to be of the trellis type. In some cases and on small scales they may be the result of a quite homogeneous regional gradient or the existence of parallel faults.
Despite of all the above, it is important to consider it as a basic model since it occurs in many combinations.
It is characterized by a series of isolated depressions where there may be an input of water or where it can flow in, thus indicating, in this case, the presence or the existence of a subterranean net.
It is typical of the karst and pseudokarst, where the surface and subterranean drainages are closely related. It may also appear in some paleolized and glacial plains over deposits of ground or frontal moraines. By extrapolating, the term cribble net is used to name a group of isolated depressions where there is water, temporarily or permanently. In some plains, such as in Argentine ones, when there is excess of precipitations, these depressions may connect to each other modifying the model.
As it can be seen in Fig. Nº 1 this net has no possibility of being integrated, convergent and or divergent, but in some cases, if the subterranean drainage is taken into account, this possibility may arose in the case of paleonets or phantom nets. However, it is evidently a matter of compound nets.
When working with basic or pure nets there is a possibility to combine them to obtain compound nets. If we take into account that the former are fourteen and they may be combined in groups of two, three or even fourteen, the number of combinations is immense, so that any net may be represented with its corresponding legend written.
In Fig. Nº 3 we indicated a matrix composed by the fourteen nets considered in groups of two, where the diagonal would represent the pure nets and it would be apparently symmetrical. We must remark that it is not so; for instance, in Fig. Nº 4 the characteristics of a dendritic-orthogonal net and its orthogonal-dendritic symmetry are indicated; thus it may be observed not only its topological difference but also the structural characteristics that they would be representing.
In the first case, the net indicates that the main courses are working on, at a greater depth, an underlying isotropic rock of the type of the noncompact sedimentary ones and the less deep secondary courses are controlled by a diaclased compact rock.
In the second case the main courses undergo a tight structural control, due to faults or geoclases, of a massive rock, meanwhile the secondary courses develop over a deep weathered layer that determines surface isotropy. The system also allows, by means of the legend used, to indicate the existence of overlapping nets by means of the plus sign (+). For instance, an integrated dendritic net which has an underlying dendritic-cribble integrated phantom net, both convergent but with different direction, would read:
InNd+ + Ph InNd,c+
The system also allows to indicate, by means of arrows, the evolving tendency of the nets. For instance, an unintegrated cribbled net would become a convergent integrated dendritic rosary net and finally a typical dendritic one, which would read as follows:
UnNc- InNd,ro+ InNd+
Finally and by the way of an example it is indicated in Fig. Nº 5 a schematic model in which by analysing the nets it may be concluded that there is a volcanic structure over a plain of erosion. Under that plain there is an anticlinal whose prow is clearly observed and it is affected by an almost transversal fault that has displaced the structure.
We believe that the classification proposed may be greatly useful, avoiding the variety of names for every type of net by using the combinatory models.
Using the methods on small and large scales, such as in satellite images and aerial photographs, allows to compare the inferences obtained by using different scales, thus achieving a greater approximation to reality, specially when working in relatively large areas.
Finally, it has to be remarked that the system proposed allows to indicate, by means of a legend, the natural or antropic nature which has significant importance in Prospective Geomorphology, essential to application works.
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