**1. Introduction**

The study area is located southwest of Campo de Talampaya, La Rioja Province, Argentina. In this sector, there is a topographic high called Alto de San Nicolás, in which the Grupo San Nicolás [1] emerges. The San Nicolás Group is made up of the Rio Mañero and Desencuentro Formations, both of continental origin (**Figure 1b** and **c**), which together have a thickness greater than 2000 meters [4]. The pyroclastic sediments in these units were dated by [5], in 15,0 1,2 Ma and 7,54 1,56 Ma placing this sequence in the middle-late Miocene.

The Desencuentro Formation was divided into four informal Members named D1, D2, D3 and D4 [1, 2, 4] based on their facies and paleoenvironmental evolution, emphasizing the description of clastic sediments. However [6, 7], the study and interpretation of the important pyroclastic deposits present within this Formation, calling it Member P. These authors point out that member P is interdigitated within members D2 and D3. Paleontological and paleoenvironmental papers indicate that

**Figure 1.**

*(a) Topographic map and its environments with map of South America and Argentina; (b) Local geology of study area (modified of [2, 3]); (c) Detailed geological map of study area.*

the prevailing climate for that period was warm, seasonal with torrential rains [1, 2, 8].

The objective of this contribution is to determine the prevailing paleoenvironmental conditions during the late Miocene and their evolution, through detailed sedimentological studies, in the middle section of the Desencuentro Formation in Campo de Talampaya, La Rioja Province, Argentina.

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

### **2. Geology and tectonic evolution**

Since the beginning of the Andean Orogeny (Maastrichtian-Danian), the extensional basins of southwestern South America have changed their tectonic configuration. Most of them went from extensional to compressive regimes, and those near the elevated front became foreland basins (Austral Basin, Neuquina, Cuyana, and Bermejo Basin, etc.).

The Alto de San Nicolás Group deposits in what was the distal sector of the Bermejo basin correspond to this stage of sedimentation of a predominantly continental character [1, 4]. The basal section of the Alto de San Nicolás Group is conditioned by the elevation of the Cordillera Frontal and by the ascent and migration of the continuous and folded belt in the Precordillera. On the other hand, the upper section is associated with the elevation of the Sierras Pampeanas, more precisely with the Sierra de Velasco [1]. Finally, with the ascent between the end of the Miocene and the Pliocene of the Sierra de Valle Fértil, Sierra Morada and Sierra de Los Tarjados produces the disconnection of the most distal parts of the basin (Campo de Talampaya Basin) with the main depocenter of Bermejo [9].

Currently, in the central-western sector of Argentina (Pampean segment), the low inclination (5°-10°) with which the Nazca Plate subducts below the South American Plate conditions the tectonics of the sector [10, 11]. This Pampean segment is characterized by the development of foreland basins, which are fragmented by elevated basement blocks and originate intermountain depressions. Among the foreland intermontane basin that develops on the Pampean segment is the Campo de Talampaya which is related to the uplift of the Sierra de Los Tarjados [1].

In the southwestern sector of Campo de Talampaya (**Figure 1a,b** and **c**), there is a topographic high that extends in a NE–SW direction called Alto de San Nicolás (**Figure 1a,b** and **c**), which consists of a succession of continental Neogene (Alto de San Nicolás Group) age strata, 2320 m thick, which also exhibit a dip to the east that varies between 5° and 21° [1, 4]. In this sector, the Alto de San Nicolás Group (**Figure 1a,b** and **c**) comprises the Río Mañero and Desencuentro Formations. The base of this stratigraphic unit rests paraconformity or by angular unconformity upon Cerro Rajado Formation (Cretaceous?) depending on which is the sector of the Campo de Talampaya [4]. Discordant on the neogenic units are an alluvial fan, ephemeral fluvial systems, and eolian deposits of the Quaternary [12].

The Desencuentro Formation (upper Miocene) consists of a clastic sequence that overlaps the Río Mañero Formation through a transitional passage, which consists of sandstones, mudstones, few conglomerate lenses, and tuff levels deposited in a saline mud-flat environment which are finally covered by sand-flat/mudflat [1]. This unit was divided into four informal Members (D1, D2, D3, and D4) emphasizing the description of the widely developed clastic sediments [1, 2, 4], but not in the case of the rocks of volcanic origin, since these, they are only mentioned as participants in the sedimentary record. Subsequently, a new member (called P) is included for the Desencuentro Formation, depending on its pyroclastic/ volcaniclastic character, leaving the stratigraphic column composed of D1, D2, P and D3, D4 [6]. The recognition of Member P indicates characteristic volcanic participation during the deposition of the Desencuentro Formation in the distal sector of the Bermejo Basin [7].

### **3. Methodology**

Five field works were performed in the middle section of Alto de San Nicolás, where traditional field techniques were used: description of detailed stratigraphic profiles (lithology, grain size, contacts, geometry, color, and sedimentary structures/biogenic were the data taken) and measurement of strike and dip of strata with Brunton compass. Subsequently, the different lithofacies were classified and interpreted, which were grouped into associations of facies and architectural elements to interpret the depositional environments. In the case of clastic and volcaniclastic lithofacies, the associations will be interpreted as architectural elements, following what was stated by [13, 14]. Pyroclastic lithofacies, their genetic processes will be interpreted in a general way, following the methodology of [15] and other authors [16, 17].

The identified pyroclastic lithofacies were coded adapting the proposal of [15]. The capital letter is used for the grain size classes and the lower case for the sedimentary structures: (T) tuffs, (L) lapillita, (m) massive, (b) lamination, (db) diffuse lamination, (//b) parallel lamination, (xb) cross-lamination, (ob) sinusoidal rippledrift lamination, (dob) diffuse sinusoidal ripple-drift lamination, (r) deformational structures, (ch) chute and pool, (esc) climbing structures, (acc) accretional lapilli. For the clastic and volcaniclastic lithofacies, they were coded by adapting the scheme of [13, 14]. Also, for these lithofacies the capital letter is used for the grain size classes and the lower case for the sedimentary structures and identification of volcaniclastic rocks: (F) claystone, (S) sandstone, (G) conglomerate; (m) massive, (pl) planar cross-lamination, (ps) planar cross-stratification (t) tangential cross-stratification, (h) parallel lamination, (v) volcaniclastic (s) matrix supported.

In this work, sedimentary rocks without the content of volcanic fragments will be called clastic rocks. Following the scheme [18], rocks or pyroclastic deposits will be called those that demonstrate a mode of fragmentation, transport and deposition [fall, surge or flow] directly related to volcanic activity. The term volcaniclastic will be used, following the criteria of [18], to refer to those deposits that have a connection with volcanism, but that result from the transport and deposition of exogenous cycle agents (mudflows, river currents, etc.), regardless of the fragmentation mechanism (during the eruption or if they are new particles formed by the weathering of older rocks). For this reason, for their classification, they will be considered as clastic rocks to which the qualifying noun volcaniclastic will be added.

For the granulometric classification of the sedimentary particles, the Udden-Wentworth classification and Phi scale were used and for the pyroclastic particles the modified proposal of [17] was used.

#### **4. Sedimentological and paleoenvironmental analysis**

A total of **23** facies were identified in the deposits of the middle section of the Desencuentro Formation in Alto de San Nicolás, which were grouped into three facies associations (pyroclastic facies) and six architectural elements (clastic and volcaniclastic facies). The nomenclature used in the identification of lithofacies is indicated in **Tables 1**–**4**, together with the sedimentary structures and other characteristics of each one; and the terminology applied for the associations of facies (DPS, WPS and PF) and architectural elements (FF, CH(FF), CR, CH and SG) is indicated in **Table 5**. The different interpretations that were inferred from the identified lithofacies, associations of facies and architectural elements are briefly described below:

The presence of sismites (fluid leaks, flaming structures, ball pillow, etc.) in the sediments originated by the humid pyroclastic surges, allows us to infer, following what was stated by [19] that the sedimentation area would be in a distal sector with respect to the eruptive center (**Figure 2k**).

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



#### **Table 1.**

*Table of pyroclastic lithofacies and their codes adapting from the proposal by [15].*


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


**Table 2.**

*Table of pyroclastic lithofacies and their codes adapting from the proposal by [15].*
