**5. Sedimentary model**

For the development of the paleoenvironmental model that involves the genesis of Member P and part of Members D2 and D3 [6, 7], the different sedimentation moments were divided into seven temporarily consecutive stages.

#### **5.1 Stage I**

The first sedimentation event is made up of fine clastic deposits, represented by the architectural elements FF, SG, CR and CH (FF) (**Figures 4** and **5**). The element FF represented by claystone, tabular bodies and of great extension, corresponds to wide flood plains. The interdigitation between FF and SG

#### **Figure 2.**

*(a) Bodies exhibiting internally ripple-drift laminations, the pen measures 15 cm; (b) level with internally parallel lamination and concretions (indicated by green arrows); c) level with diffuse sedimentary folds, the cane measures 1 m; d) level that presents scaling structures and where, in addition, the direction of flow is indicated; e) Accretional lapilli; f) outcrop showing intercalations of levels with medium (M) and coarse (G) lapilli; g) outcrop where intercalations of tabular to slightly irregular levels are observed, which internally present sheets with variations in the proportion of pumiceous clasts; h) fluid exhaust structure, the pickaxe measures 33 cm; i) outcrop where one level of fall are observed (indicated by green arrow); the person is 1.65 m tall; j) level that is crossed by a pipe that presents elutriation of fines and a vertical trajectory (indicated by green arrow); k) scheme where the structures found in the field are indicated with a table and from which the distance to the emitter focus can be inferred (modified from [19]); l) tabular body exhibiting internally tangential cross-lamination; m) level with parallel lamination structures and chute and pool.*

indicates that these floodplains are intermittently invaded by hyperconcentrated flows, represented by approximately tabular bodies of great thickness (more than 1.5 m), probably amalgamated. The origin of these mass movements (sheet flood) would be associated with humid times, where the pluvial discharge peaks generate floods, which can transport considerable amounts of sand and pellets. These laminar processes [38] can sometimes generate deposits of more than 2 m thick [4]. In an arid climate or dry seasons, the plains would have dried out, generating levels with desiccation cracks observed in the Fm lithofacies.

*Miocene Volcaniclastic Environments Developed in the Distal Sector of the Bermejo Basin… DOI: http://dx.doi.org/10.5772/intechopen.99081*



#### **Table 3.**

*Table of clastic lithofacies and their codes adapting from the proposal by [13, 14].*


*Miocene Volcaniclastic Environments Developed in the Distal Sector of the Bermejo Basin… DOI: http://dx.doi.org/10.5772/intechopen.99081*


**Table 4.**

*Table of volcaniclastic lithofacies and their codes adapting from the proposal by [13, 14].*

In some sectors, interacting with the flood plains, sandy channels develop, of little thickness (between 50 and 70 cm) and tabular to slightly lenticular geometries, interpreted as probable channel systems belonging to crevasses (corresponding to the architectural element CR), originated by erosion of the edges of the main channel during flood events. These channeled systems are sometimes laterally amalgamated, a circumstance that could indicate the topographic compensation of different episodes of flooding-breakdown of the sloping-crevasse formation. The channels are filled by deposits of sand bars (SB), originated from currents with low flow regimes. Interdigitated with FF and SB, sporadically, there are bodies with concave bases filled with fine material (silts and clays), interpreted as deposits of abandoned channels CH (FF). Based on the architecture, spatial relationships and interrelationship that the element FF, CR and CH(FF) present, considered as overbank deposits, it is possible to indirectly infer that the canal system that would have originated them would correspond to an anastomosed fluvial system [13].

#### **5.2 Stage II**

Gradually, this sedimentary system began to be influenced by the volcanism of the region (**Figure 5**). The record of this volcanic activity begins in the study area

#### **Figure 3.**

*(a) Outcrop where the lithofacies Fm and Gmsv are indicated; b) figure b corresponds to an enlargement of figure a, where lahars levels are observed with inverse gradation and also very diffuse lamination; c) outcrop where claystone levels are observed that internally present pumiceous clasts and, in addition, a level of fine sandstone that presents diffuse parallel lamination, the pen measures 14 cm; d) level featuring parallel lamination; e) outcrop where the Sh and FI facies are indicated; f) figure f corresponds to an enlargement of figure e, where the load structures are better observed (indicated with yellow arrows); g) desiccation crack structures; h) level internally featuring planar cross lamination, glasses are 13 cm wide; i) outcrop where the lithofacies Gptv and Shv are indicated. The Gtv lithofacies internally presents tangential cross-stratification, denoted by levels with varying proportions of pumiceous and claystone clasts; j) outcrop where the lithofacies Spsv is indicated, which internally presents planar cross-lamination; k) outcrop where the lithofacies Gmsv, Sm and Fm are indicated; l) scheme of figure k, where the load structures and pinch and swell are indicated.*

with falling pyroclastic deposits (facies association PF). The facies association PF (**Figure 4**) is interpreted as a product of the gravitational fall of material from pyroclastic clouds, formed during high-energy explosive eruptions. The energy condition mentioned above would be indicated by the grain size (ash and lapilli) that the lithofacies mT and pmL present [17]. The contribution of pyroclastic material would have caused changes in the dynamics of the fluvial system, by observing a decrease in the size and frequency of crevasse systems. These changes *Miocene Volcaniclastic Environments Developed in the Distal Sector of the Bermejo Basin… DOI: http://dx.doi.org/10.5772/intechopen.99081*



#### **Table 5.**

*Table of facies associations (pyroclastic facies) and architectural elements (clastic and volcaniclastic facies).*

could be associated with the migration of the river system and/or the loss of its identity, as it has to transport a greater sedimentary load (pyroclastic), under the same tectonic and climatic conditions. In this way, interdigitated volcaniclastic deposits (represented by crevasses) begin to appear in the sedimentary record with the facies association PF and the FF architectural element, the latter element sometimes carrying few pumiceous clasts.

Subsequently, a new volcanic episode represented by the lithofacies bL (association facies DPS) records the activity of dry pyroclastic surges on these plains. The association DPS is interleaved with the FF element. The variations in the content of pumiceous clasts that each of the sheets that make up these pyroclastic deposits (bL) present, allows us to infer that they would have originated from successive surges with oscillations in the populations of clasts, a product of currents fluctuating sustained over time [15].

The plains are areas that are characterized by developing gentle slopes, so a greater contribution of sediments, from pyroclastic falls and successively surges, has probably caused an even greater loss of slope of these plains, being the topographic features very scarce.

#### **5.3 Stage III**

In these plains sedimentary events occur (**Figure 5**) whose characteristics are high fluid discharge and high sedimentary load (SG architectural element), which can be interpreted as lahars and hyperconcentrated flows. The origin of both types of gravitational flows is associated with floods as a result of exceptional rains, which allow the generation of hyperconcentrated currents, which by remobilizing pyroclastic materials generate lahars. The deposits product of these processes of mass movements is characterized by being tabular, of great lateral extension (greater than 25 m) and being internally massive, although sometimes they can present tractive structures (for example, diffuse parallel lamination), which could be explained due to dilution of these flows due to loss of sedimentary head, which results in a different fluid/head ratio of the event.

Sometimes the interdigitation of the Gmsv and Sm lithofacies with the FF element generates bodies that present deformational basal contacts and structures

**Figure 4.** *Composite stratigraphic column and facies of the middle section of the Desencuentro formation. From left to right, the sections from the base to top are described. The lithofacies codes correspondsTables 1–4; and the facies associations and architectural elements to Table 5.*

 *to*

*Miocene Volcaniclastic Environments Developed in the Distal Sector of the Bermejo Basin… DOI: http://dx.doi.org/10.5772/intechopen.99081*

#### **Figure 5.**

*Scheme unscaled outline showing the seven sedimentary stages and their evolution over time.*

*Miocene Volcaniclastic Environments Developed in the Distal Sector of the Bermejo Basin… DOI: http://dx.doi.org/10.5772/intechopen.99081*

such as clastic dikes, pinch and swell, structures in flame, among others; which indicate that these sediments would have been embedded in fluids and that, due to differences in densities and pressures, the aforementioned structures are produced.

#### **5.4 Stage IV**

A new volcanic event, also represented by explosive eruptions, gives rise to dry pyroclastic surges (facies association DPS), which are deposited again on these plains (**Figure 5**). The pyroclastic deposits resulting from this eruptive moment, internally develop sharp tractive structures and high flow regime (e.g. parallel lamination, scaling structures, etc.), which correlate with dry pyroclastic surges, which probably respond to a rapid stacking and amalgamation of successive pyroclastic events. This is indicated by the transitional contacts between the different bodies.

#### **5.5 Stage V**

On top of this pyroclastic sedimentation, a new volcaniclastic sedimentation cycle begins that shows an interuptive period (**Figure 5**). This period of mixed sedimentation is represented by the architectural element FF that interdigitates with SB and SG vertically, which would indicate that the river system would be recovering its old position within the area. Fluvial deposits consist of conglomerate and coarse sandy lithofacies. The development of these deposits could correspond to new crevasses that, by vertical accretion, continue to generate the floodplain.

Later, massive, mantiform and sandy bodies are deposited, whose origin is related to hyperconcentrated flows caused by torrential rains.

#### **5.6 Stage VI**

In this sedimentary stage, the area is influenced by pyroclastic(s) event(s) which are registered as deposits of wet surges (facies association WPS) and dry (facies association DPS) that once again cover the extensive plains (**Figure 5**). The associations of WPS and DPS facies interdigitate with each other, and together present power of approximately 25 to 30 m, with a great absence of clastic sedimentation. In the deposits produced by wet pyroclastic surges, deformational structures develop (e.g. fluid leaks, sedimentary folds, convolute lamination, flame structures, etc.), observed in the dbTacc and rL lithofacies. The origin for these deformational structures is associated with seismic waves; gravity and inertia effects by pyroclastic flows and/or differential gas pressures [30]. Seismic waves can be attributed to contemporary volcanic activity, which causes unconsolidated and plastic sediments to deform and/or undergo liquefaction. However, a probable deformation generated by a rapid stacking of successive surges should not be ruled out.

The facies association DPS corresponds to dry pyroclastic surges, where no deformational structures have been observed, which implies a different behavior when passing seismic waves. This behavior would be related to the lack of interporal fluid in the sediments. In the sedimentary record, the dry and wet surge deposits are interdigitated, however, in the upper terms of the sequence, those of the DPS association prevail.

#### **5.7 Stage VII**

On the surge deposits (DPS and WPS indistinctly), tabular and low-power deposits (4 to 8 cm approximately) develop, interpreted as deposits of gravitational fall from pyroclastic clouds, which present levels with contrasting granulometry (CP association) (**Figures 4** and **5**). This type of deposition is related to nonsustained eruptions that have several pulses of short duration or to partial collapses of the eruptive column [23]. The last pulse of volcanic activity is recorded, in the study area, as a new wet surge event, followed by dry surges.
