Sedimentation model of piggyback basins: Cenozoic examples of San Juan Precordillera, Argentina

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Sedimentation model of piggyback basins: Cenozoic examples 1

of San Juan Precordillera, Argentina. 2

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J. Surianoa*, C. O. Limarinoa, A. M. Tedescoa and M. S. Alonsoa 4

5 a IGeBA-Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y 6

Naturales, Universidad de Buenos Aires-CONICET 7

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Address: Intendente Güiraldes 2160 - Ciudad Universitaria-Pab.II-CABA-Argentina- 9

C1428EGA - Tel. (++54 +11) 4576-3300 10

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*Corresponding autor e-mail: [email protected] 12

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number of words of text: 7369 14

references: 67 15

tables and figures: 17 16

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Abbreviated title: Sedimentation model of piggyback basins 18

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Abstract 20

Piggyback basins are one of the most important sediment storage for foredeep 21

basins within foreland basin system, so that, to understand the dynamics of its sediment 22

accumulation and controlling allocyclic changes is essential. 23

Three alluvial systems are proposed here to depict sediment movement along the 24

piggyback basin: piedmont, axial and transference system. We propose the 25

differentiation between open continental piggyback basins that include a transference 26

system which is able to deliver sediment to the foredeep; and closed piggyback basins 27

which are isolated. 28

Two idealized models of sedimentation in piggyback basins are proposed. For 29

open piggyback basins we identify four stages: 1. Incision stage, 2. Confined low 30

accommodation system tract, 3. High accommodation system tract, 4. Unconfined low 31

accommodation system tract. Meanwhile two stages are proposed for closed ones: 1. 32

High accommodation system tract and 2. Low accommodation system tract. 33

Sedimentation model of piggyback basins

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To test these models, Quaternary deposits and a Miocene unit are analyzed. The 34

first one controlled by climatic changes, and the second one is related to tectonic 35

activity in the Precordillera. 36

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(MAIN TEXT) 38

Most of the research in foreland basin systems (DeCelles & Giles 1996) has 39

been focused on the evolution of the foredeep region and its relation with the orogenic 40

wedge and the forebulge areas (Jordan 1981; Beaumont et al. 1988; Flemings & Jordan 41

1989; Mitrovica 1989; Holt & Stern 1994; Johnson & Beaumont 1995; Jordan 1995; 42

DeCelles & Giles 1996; Limarino et al. 2001; DeCelles & Horton 2003; among others). 43

These models have been useful to understand the sedimentary patterns in the foredeep 44

as well as to establish the close relationship between tectonism and sedimentary filling. 45

Even though they are a major storage of sediments which can feed the foredeep, 46

much less attention has been paid to piggyback basins, developed between thrust sheets, 47

particularly the continental ones. Therefore, understanding the sedimentary dynamics of 48

piggyback areas is important because: 1. Piggyback basins are significant reservoir of 49

sediments exported periodically to the foreland (DeCelles & Giles 1996; Horton & 50

DeCelles 2001 and Suriano & Limarino 2006), 2. The volume of exported sediments 51

from piggyback basins may even exceed the volume supplied by the orogenic front to 52

the foredeep, 3. The stratigraphic record gives direct information about the uplifting of 53

the different thrusts sheets and the advance of the clastic wedge, 4. Part of the foreland 54

basin record which is eroded as the orogenic front advances, can be preserved in the 55

piggyback basins. 56

In this contribution the piggyback basins located along the Precordillera in the 57

Argentinian Andes (San Juan Province, 30º to 31º South Latitude, Fig. 1) are analyzed. 58

This study intends to characterize the sedimentation patterns and dynamics of 59

piggyback basins as well as to propose a new classification and to establish a sequence 60

stratigraphic model. In order to accomplish these goals, the work is divided in two 61

sections. Firstly, a stratigraphic model based on the changes in the accommodation 62

space is built, taking into account the analysis of piggyback basins, their definition, 63

main sedimentologic features, sedimentary environments, stacking patterns and 64

relationship with allocyclic controls. In the second part, the proposed model is tested 65

analyzing two stratigraphic successions (Quaternary deposits and Miocene Cuesta del 66

Viento Formation) of the Jachal river area, controlled by climatic and tectonics changes. 67


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Piggyback basin concept 69

The piggyback basins were defined by Ori & Friend (1984) as “basins that are 70

formed and filled up while being carried on top of thrust sheets”. This term is similar to 71

“satellite basins” proposed by Ricci Lucchi (1986). Thrust top basins or parched basins 72

(Turner 1990; Doglioni et al. 1999; Butler & Grasso 1993; Butler et al. 1995; Bonini et 73

al., 1999 and Hesse et al. 2010) have a related meaning although they indicate a more 74

specific arrangement associated to basins that are formed and filled up while a portion 75

of foreland basin is dismantling. According to the above mentioned, piggyback and 76

satellite basins are synonymous, and they are related to thrust top and parched basins. 77

Years later, piggyback basins were included by DeCelles & Giles (1996) in their wedge 78

top depozone within the foreland system. Gugliotta (2012) divides the wedge top 79

depozone in inner- and outer- sequences. In this paper, we follow this terminology, 80

using the term inner-wedgetop depozone for the piggyback area, which designates 81

basins separated from the foreland by thrust sheets or anticlines while the outer- 82

wedgetop depozone is where blind thrust structures have no superficial expression. This 83

last zone transitionally passes into the foredeep. 84

85

Sedimentary dynamics of piggyback basins 86

To understand the auto and allocyclic factors that could affect sedimentation in 87

piggyback basins, it is essential inspect the sediment movement pattern all along the 88

basin. Based on the studied examples, the pattern of sediment circulation in continental 89

piggyback basins is here related to three alluvial systems (Fig. 2). The first one 90

corresponds to the piedmont systems, which convey poorly sorted breccias and 91

conglomerates directly from the mountain front to the floor of the basin. This 92

environment includes taluses, different kinds of colluvial fans (Bilkra & Nemec, 1998), 93

alluvial fans and alluvial slopes. In the here studied case, four piedmont associations 94

related to accommodation space were recognized (Fig. 3, Suriano & Limarino, 2009). 95

As the piedmont area has permanent availability of material from the mountain front, 96

the movement of sediment is regulated by axial systems and, if present, by the 97

transference system. 98

Sediment transport along the elongate axis of the piggyback basins is 99

represented by longitudinal, frequently braided channels (the collector river-conoid 100

systems of Suriano & Limarino, 2009). As it was shown by Suriano & Limarino (2009), 101


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these systems are defined as conoids, and the associated collector river, as axial 102

distributary systems. 103

Finally, the transference systems (Fig. 2) connect piggyback basins among them 104

and to foreland basin. These transference systems can be represented by different kinds 105

of rivers or even by open lacustrine arrangements 106

Regarding this occurrences, we propose that continental piggyback basins (Type 107

2 of Wagerich 2001) can be classified in two main types, open and closed (Fig. 4). The 108

open piggyback basins are those including a transference system which connect 109

between them and is able to export sediment to the foreland. Instead, closed piggyback 110

basins do not have a transference system, which make them endorreic. 111

A piggyback basin can shift from open to close and viceversa due to allocyclic 112

factors; not only tectonics, directly related to the evolution of the fold and thrust belt, 113

but also climatic modifications which affect the erosion capability of the transference 114

system. 115

Open piggyback basins could change from highly efficient to inefficient (or vice 116

versa) according to the efficacy of major sediment exporter system formed by the 117

fluvial systems inside the piggyback basin and transference system. Highly efficient 118

transference systems are those able to export all the sediment delivered by piggyback 119

basins to the foredeep (Fig. 4). On the contrary, a transference system is inefficient if it 120

is not competent to transport the material from the piggyback, to the foredeep (Fig. 4) 121

which leads to the storaging of sediment inside the piggyback basin. 122

Before building a sedimentary model for piggyback basins it is important to 123

examine the factors that control their patterns of sedimentation. They can be local, if 124

they affect each basin individually; or regional, if they have influence over the entire 125

orogenic wedge. Another equally significant aspect to take into account is if those 126

factors affect the accommodation space (A) and/or the sediment supply (S). These 127

factors and their effects are shown in the figure 5. 128

A local factor that increases the relationship A/S is the upstream thrust 129

movement (Fig. 5a). This factor produces an increase in the accommodation space in 130

the footwall as well a higher sediment supply from de hanging wall. Another way of 131

increasing accommodation space is a local base level rise (Fig. 5b and c). This could be 132

generated by a sediment dam within the basin (Fig. 5b) or by activity of the downstream 133

thrust (Fig. 5c). Efficacy of this last factor is limited as major movements produce 134

narrowing and shallowing of the basins, which drops the accommodation space (Fig. 5d, 135


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Talling et al. 1995). Lower local A/S ratio could also be generated by a downstream 136

river capture (Fig. 5f). 137

There are also regional factors affecting the A/S relationship. Increasing of A/S 138

ratio due to subsidence is related to the structural arrangement and dynamics of the 139

orogenic wedge, to tectonic loading and to slab pull (Fig. 5g). Dropping of 140

accommodation space is instead associated to periods of tectonic quietness and isostatic 141

rebound (Fig. 5i). Fluvial equilibrium profile, independently of their driving force, 142

could rise (Fig. 5h) or fall (Fig. 5j), causing an increase or decrease in the 143

accommodation space. Finally, dramatic changes in the amount of sediment production, 144

due to climatic factors or the occurrence of explosive volcanism, should be taken into 145

account. 146

The piggyback sedimentary models here presented base on the principles of 147

sequence stratigraphy in continental environments (e.g. Shanley & McCabe 1994; Quirk 148

1996; Dahle et al. 1997; Dalrymple et al. 1998; Blum & Törnqvist 2000; Miall 2002). 149

According to Dahle et al. (1997) two major stages can be recognized in continental 150

basins disconnected of the sea level: 1. High accommodation system tract and 2. Low 151

accommodation system tract. This proposal detaches system tracts from sea level and 152

relates them to changes in accommodation space (Catuneanu 2006). Following Shanley 153

& McCabe (1994) and Dalrymple et al. (1998), discontinuities generated by fluvial 154

incision are here used as sequence boundaries. Two kinds of erosive bounding surfaces 155

were recognized according to their degree of confinement (confined or unconfined). 156

Finally the existence of sedimentological evidence of the presence of a fluvial 157

transference system can be used to differentiate open from closed piggyback basins, a 158

concept that is also incorporated in the proposed model. 159

160

Model for piggyback basins 161

Using the elements mentioned above we constructed a sedimentary model for 162

continental piggyback basins that is based on the temporal variations of accommodation 163

space and sediment supply. These changes are based in four supporting elements: 1. 164

Basin subsidence, 2. Tectonic activity in the fold and thrust belt, 3. Changes in sediment 165

amount by climatic changes and/or major explosive volcanism. 166

As for basin subsidence, in piggyback basins has been considered short-lived 167

and of much smaller magnitude than the one at associated foredeep (Xie & Heller 168

2009). In fact, the subsidence due to tectonic loading, a major mechanism in foreland 169


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basins (Jordan 1981 and Naylor & Sinclair 2008), reaches lower values in piggyback 170

areas (Zoetemeijer et al. 1993 and Xie & Heller 2009). The key factor in the creation of 171

local accommodation space in most piggyback basins is the tectonic activity in the 172

upstream thrust sheet (tectonically induced accommodation, Fig. 5a), which in turn 173

produces the migration of the depocenter (Bonini 1999 and Zoetemeijer et al. 1993). In 174

this sense, migration of the depocenters in piggyback basins has been associated with 175

passive rotation of the internal thrusts due to the successive external thrusts activation 176

(Bonini et al. 1999 and Roure 2008) or active thrusting. In this last case, migration of 177

the depocentre towards the foredeep has been interpreted like in-phase thrusting 178

sequences while hinterland-directed depocentres migration seems to be related to out-179

of-phase thrusting (Zoetemeijer et al. 1993). In the same way Bonini et al. (1999) 180

showed that the depocenter migration is controlled by the thrust having the highest 181

displacement rate. 182

Sequence deposition is essentially ruled by the above resumed factors, which 183

affect the relationship A/S. On this basis we propose a sequence stratigraphy model for 184

open piggyback basins involving: 1. Incision stage, 2. Confined low accommodation 185

system tract, 3. High accommodation system tract, 4. Unconfined low accommodation 186

system tract or overfill stage (Fig. 6). 187

The first stage or incision stage begins with an important drop in the equilibrium 188

profile (a in Fig. 6) that produces incision in the transference system, generates the 189

entrenchment of the piedmont system and the development of an incision surface all 190

across the basin (Incision stage, a in Fig. 6). As Shanley & McCabe (1994) pointed out, 191

even in high incision events some remnant deposits can be found in the sides of the 192

valley due to short moments of stabilization during this mainly erosive phase. 193

The confined low accommodation system tract can be separated in two sub-194

stages. The first one takes place following the incision stage when there is a time of 195

relative instability during which the river profile is close to equilibrium, generating the 196

infilling of incised transference system deposits (b in Fig. 6). In this stage deposition of 197

coarsening-up incised channel complex can be observed (Catuneanu 2006). The amount 198

of sediment involved during this stage relates to the time span of this unstable situation 199

and therefore it is not always recognized. The next stage occurs when the river profile 200

reaches or it is lightly above the equilibrium profile. At that point slow aggradation 201

takes place in the previously eroded piggyback area (c in Fig. 6). This stage is denoted 202


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by the presence of amalgamated high-energy braided channels, generated by the 203

transference system and also by limited aggradation in the piggyback area. 204

If the rise of the equilibrium profile continues, the transference system produces 205

high aggradation in its alluvial plain, promoting high equilibrium profiles in the axial 206

and piedmont systems and the consequent filling of the piggyback basins (high 207

accommodation system tract, d in Fig. 6). As the sediment supply from the mountain 208

front is more or less continuous, if no new accommodation space is created, the basin 209

tends to fill up, reaching the unconfined low accommodation system tract (or overfill 210

stage, e in Fig. 6). 211

For closed piggyback basins, only two stages are proposed. Initially, when the 212

basin is created, A/S ratio is high so there is also high accommodation space. This stage 213

is dominated by fine grained playa lake deposits (f in Fig.6) with an external ring of 214

coarse piedmont facies. Again, if there is no newly created accommodation space, the 215

basin tends to fill up, and the overfill stage is developed. This stage is represented by 216

the progradation of low energy piedmont association (2 in Fig.3), like alluvial slopes 217

and fluid flow-dominated colluvial fans and a marked decrease of the area previously 218

occupied by playa lake deposits. The late stage of this system tract is characterized by a 219

decrease in sediment supply associated with an extensive aeolian reworking and 220

development of a desert pavement. 221

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Stratigraphic context 223

Between 30°S and 31°S, the Andean orogen is made by five morphotectonic 224

units (Fig. 1a): 1. Western and higher Cordillera de los Andes, 2. The wide 225

intermontane Rodeo-Iglesia Basin, 3. The fold and fault thrust belt of Precordillera, 4. 226

The Bermejo Basin and 5. The Pampean Ranges. In this area Cordillera Frontal (part of 227

1) is composed of a Carboniferous metasedimentary rocks (Cerro de Agua Negra 228

Formation), Triassic batholiths (Llambias & Sato 1990) and Triassic to Miocene 229

volcanic rocks (Ramos 1999). To the east, the wide Rodeo-Iglesia Basin is developed, 230

where tertiary volcaniclastic rocks are exposed. 231

The Precordillera belt is divided into three structural provinces (Fig. 1a): 232

Western, Central, and Eastern (Ortiz & Zambrano 1981). The studied area is located 233

within Western Precordillera (Fig. 1b), which has been built since Early Miocene 234

(Jordan et al. 1993; Jordan et al. 2001; Cardozo & Jordan 2001 and Alonso et al. 2011) 235

by eastward migration of the orogenic front. The thin skinned fold and thrust belt of 236


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western Precordillera expose Early Paleozoic rocks sheets that contain the Miocene to 237

Holocene piggyback sedimentation. In particular, we studied the Quaternary piggyback 238

basins along the Jáchal River (Fig. 1b and 7) and the Cuesta del Viento Formation, a 239

Miocene unit deposited within the same stratigraphic context (Fig. 7). Western 240

Precordillera is characterized by its west vergence thick skinned structures (Zapata & 241

Allmendinger 1996). Finally, Bermejo Basin separates the Precordillera from the 242

basement uplifted blocks of Pampean Ranges. 243

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Dynamics of the Quaternary piggyback basins of Jáchal river area 245

In order to apply the models described to the Jáchal river area, the Quaternary 246

filling of the piggyback basins was analyzed in detail. Taking into account the presence 247

of incision surfaces, resulting in decametric to metric scale relief, the sedimentary infill 248

of the four Jáchal river piggyback basins (La Tranca, Caracol, Zanja Honda and 249

Pachimoco) was divided in four depositional sequences (Fig. 8). The Pleistocene-250

Holocene sequences studied here do not show evidence of significant tectonic 251

disturbance. This suggests that tectonic uplift of thrust sheets was not a main 252

mechanism to produce changes in the accommodation space during Pleistocene-253

Holocene times. Moreover, the lack of evidence of volcanic activity (Kay et al. 1988) 254

sets climate changes as a major allocyclic control for sedimentation in these basins. In 255

addition sequences age’s fits with regional climate changes (Iriondo 1999). 256

257

Sequence I: Pediment 259

Depositional Sequences 258

The base of this unit is formed by a low relief erosive pedimentation surface on 260

Miocene and Paleozoic deformed rocks. Over this unconformity a relatively thin layer 261

(up to 10 meter thick) of Quaternary deposits is found (Fig. 8). These deposits are 262

dominated by monomictic breccias of piedemont accumulations with minor 263

participation of lenses of polimictic conglomerates. 264

The monomictic breccias are fine grained and occur mainly as clast-supported 265

tabular, massive or horizontally stratified beds. Lenticular bodies of massive mud-266

supported breccias interbedded with clast-supported breccias are scarce. The latter show 267

planar cross-bedding or clast imbrications, and it is worth mentioning that their scarcity 268

does not allow a statistical measurement of paleocurrents. Clasts are pebbles, mainly 269

slates (more than 95%) and basic volcanic rocks derived from the Yerba Loca 270


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Formation which indicates local provenance as it is the sole unit outcropping in the 271

Precordillera in the area. Deposits are interpreted as formed in alluvial slopes (Smith, 272

2000) with minor participation of colluvial fans (Bilkra & Nemec, 1998), so they can be 273

interpreted as a low energy piedmont association (2 in Fig.3). 274

The infrequent polimictic conglomerates are coarse, well-rounded and clast-275

supported. They show imbrication, horizontal and planar cross-bedding. They appear as 276

lenticular to lentiform beds and are interpreted as gravel bars in a multichannelized 277

fluvial system. Cobbles are dominated by granites and acid volcanics fragments. These 278

lithologies are widely present at the Colangüil Cordillera located to the west (Fig. 1b), 279

indicating an external provenance. An ancient transference system (ancient Jáchal river) 280

draining the Precordillera from west (Cordillera de los Andes) to east (present day 281

foredeep) can be interpreted from clast composition of this conglomerate unit. 282

Sequence II: Quaternary incised valley 283

The lower boundary of Sequence II is a high relief surface carved on Quaternary 284

deposits of Sequence I, Miocene or Paleozoic rocks (Fig. 8 and 9a). Deposits are few 285

and scattered, with relatively small thicknesses (up to 25 meters). This unit is dominated 286

by polimitic conglomerates with scarce intercalations of gravelly sandstones. 287

The polimictic conglomerates are coarse to very coarse and clast-supported, with 288

erosive base and occur as massive or planar cross-bedded beds. The clasts (sized up to 289

boulders) are rounded to well rounded, compositions are granite and acid-volcanics 290

(reflecting external provenance), with little participation of greenish gray and yellow 291

sandstones, quartz and greenish slates. The gravelly sandstones have planar or trough 292

cross bedding. 293

The described facies are interpreted as ancient Jáchal River deposits, as they are 294

similar to the present Jáchal River, sharing the provenance, lithology and sedimentary 295

structures. 296

Occasionally, breccias associated with taluses, coluvial fans and collector rivers 297

(piedmont and axial systems) are interbedded within the fluvial conglomerate. 298

We correlate this sequence with an important erosive event responsible for the 299

generation of the lowest Quaternary valley floor, which can be traced up to Llano del 300

Médano, an upstream site (Fig.1b), where the incision is evidenced by several hundred 301

meters of relief between the oldest Quaternary deposits and the valley floor (Suriano & 302

Limarino, 2006). The eroded material was probably exported from the piggyback area 303


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and formed two megafans (Damanti, 1993 and Horton & De Celles, 2001) that can be 304

currently seen at the mouth of Huaco and Jáchal Rivers in the Bermejo Valley (Fig. 10). 305

Sequence III: Sedimentary dams 306

The lower boundary of this sequence largely fits the incision marking the base of 307

sequence II (Fig. 8). Above this surface, there is one of the thickest and well preserved 308

quaternary units in the area characterized by the presence of conspicuous intermontane 309

shallow lacustrine sediments (Colombo et al., 2000; Colombo et al. 2005; Suriano & 310

Limarino, 2005; Suriano & Limarino, 2008; Colombo et al., 2009 and Suriano & 311

Limarino, 2009). Deposits of these natural dams can be found in Rodeo, La Tranca, 312

Caracol and Zanja Honda basins, all of them with similar characteristics. Three main 313

facies associations could be recognized in the lacustrine system: 1. Siltstones and fine 314

sandstones, 2. Monomictic roughly stratified breccias and 3. Polimictic conglomerates. 315

The fine grained association is dominant and it is formed mainly by muddy and 316

sandy lithofacies (1 in Fig. 11a y c) with scarce breccias. Tabular beds of whitish gray 317

mudstones with horizontal lamination or massive are abundant. Mudstones commonly 318

show root marks and mudcraks and there are also some levels with shells of freshwater 319

gastropods (Colombo et al. 2005) and others with plant remains. Brown shales with 320

high contents of organic matter are present though scarce. Gypsum levels are present 321

towards the top. Light brown sandstones are massive or with ripple lamination, and 322

some planar cross bedded structures are also found. 323

Fine grained lithofacies are interpreted as produced by both decantation and 324

underflows which according to May et al. (1999) can take place in shallow lacustrine 325

environments. The ephemeral nature of the lake system is inferred by the root marks, 326

mudcracks and evaporite deposits at the top of the association (May et al., 1999 and 327

Colombo et al., 2009). Massive sandstones represent rapid deposition from sheet floods. 328

Horizontally laminated sandy lithofacies record the migration of subaqueous micro and 329

mesoforms and represent the coastal zone of the lake when dominant. 330

The palynological assemblage of these lacustrine deposits has been studied by 331

Colombo et al. (2009), who proposed periods of heavy rain, alternating with episodes of 332

extreme aridity in a warm climate for the base of the section turning arid towards the 333

top. Colombo et al. (2005) dated gastropods shells and organic matter by 14C technique 334

obtaining ages from 8930 + 50 (near the base of the lacustrine deposits) to 6497 + 45 335

yBP (in the upper part). These ages indicate that the lacustrine system persisted for at 336

least 2700 years. 337


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This fine grained association alternates with breccia levels. Roughly stratified 338

matrix and clast-supported monomictic breccias, appear as tabular massive beds. Some 339

of them also show imbricated clasts, normal gradation or planar cross bedded 340

stratification. Massive and laminated mudstones occur to the top of some bodies. 341

Breccias are clearly dominant downstream (2 in Fig. 11c), and are interpreted as 342

conoids (axial distributaries systems). As they always occur downstream from the 343

lacustrine deposits, these breccias are interpreted as resulting from the dam closure (Fig. 344

11d, Colombo et al. 2000; Suriano & Limarino 2005 and Colombo et al. 2009). 345

While downstream the muddy lacustrine association pinches out into conoid 346

facies, upstream intercalations of polimictic conglomerates become frequent (at left in 347

Fig. 11c), forming the last facies association. The beds of polimictic conglomerate form 348

biconvex beds from 2 to 7 m in thickness. They show horizontal, planar cross bedded or 349

massive structures. Between the conglomeratic beds, sandy facies are common, forming 350

beds up to 1 m with a coarsening upward arrangement (3 in Fig. 11b). 351

The interpretation of these deposits and its lateral variations are shown in figure 352

12. Fine grained sediments are found at the center zone of the lake (Fig. 11a). In the 353

upstream area, the transference system (ancient-Jáchal river) flowing into the lake 354

generated a microdeltaic arrangement (Fig 11b). Figure 11 c shows the natural dam 355

deposits represented by the light gray fine lacustrine sediments that laterally pinches out 356

to the downstream conoid breccias (dark gray) responsible for the dam closure. The 357

above mentioned arrangement is related to a high equilibrium profile in the transference 358

system due to the damming, that also caused a local level rise in the whole piggyback 359

basin, so that aggradation was favored. This led to the deposition of thick units in 360

conoids and collector rivers (axial systems) as well as in the piedmont areas. 361

Sequence IV Jáchal River 362

This stage is developed over an important incision surface which corresponds to 363

the present Jáchal river valley (Fig. 8) and includes two sub-sequences. The first sub-364

sequence (IVa) involves the highest terraced levels dated by Colombo et al. (2005) in 365

La Tranca area between 978 ± 20 yBP and 525 ± 32 yBP. This unit is composed by two 366

types of deposits, the first one is characterized by clast-supported conglomerates and 367

sandstones with lenticular to lentiform geometry, that represents channel facies. The 368

second type of deposits corresponds to tabular mudstones floodplain beds. 369

The conglomerates of the channel facies have clasts of granite and acid volcanic 370

rocks, indicating a foreign provenance similar to those of the Stage II, but clasts have a 371


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smaller mean diameter (minor to 30 cm). There are two types of channel arrangements 372

according to their hierarchy; the first one is dominated by polimictic clast-supported 373

conglomerates. Some of the beds begin with massive or imbricate conglomerates, but 374

overlaying clast-supported conglomerates are a conspicuous lithofacies, with planar 375

cross bedding and horizontal stratification. On top of beds, gravelly sandstones and 376

sandstones with planar cross bedded and horizontal lamination are found. This 377

arrangement corresponds to migrating transversal and longitudinal gravelly bars (Hein 378

& Walker, 1977). Considering the geometry of the beds, they are interpreted as 379

migrating in a conglomeratic channel of low-to medium-sinuosity. 380

The second type of channel facies (Fig. 9b) comprises gravelly-sandstone or 381

sandstone lentiform to lenticular beds. Massive or horizontally laminated fine grained 382

conglomerates or gravelly-sandstones are found at the base. Above them, sandstones 383

with planar and trough cross bedded stratification occur. This association is interpreted 384

as deposited by sinuous channels in which longitudinal gravel bars and 2D and 3D sand 385

dunes migrated. 386

Floodplain beds are the dominant facies of the sub-sequence IVa. They are 387

formed by mudstones and subordinated sandstones. Mudstones show horizontal 388

lamination or ripple marks either massive structures. They are greenish or light brown 389

and have intense root bioturbation. Sporadic beds of planar cross bedded, horizontally 390

laminated or massive sandstones are intercalated within mudstones. The fine grained 391

lithofacies were formed by decantation or low energy fluid flows in the waning of 392

floods. These floodplains (Fig. 9b) show different degrees of maturity (according to 393

Reinfelds & Nanson 1993), which range from incipient to well evolved, this last one 394

with important development of paleosols. During the flood events, the sandstones reach 395

these areas with migration of megaripples, plane bed or sudden deposition by 396

competence loss. 397

In summary, this association is characterized by different hierarchy channels 398

with low to intermediate sinuosity with and extensive development of confined 399

floodplains (Nadon 1994) or cohesive low-energy floodplains (Nanson & Croke 1992). 400

Therefore we interpret the subsequence IVa as an anastomosed fluvial system (Smith & 401

Smith 1980; Nadon 1994), which represents the Early Holocene dynamics of the Jáchal 402

river. 403

The sub-sequence IVa is best developed in Pachimoco area (Fig. 2), where a 404

wider piggyback basin, allowed for the deposition of large amounts of fine grained 405


13

deposits (Pachimoco Formation after Furque 1979) of ancient-Jáchal river. The sub-406

sequence IVb includes the recent and present deposits of the Jáchal river, younger than 407

525 + 32 yBP. Its lower limit is marked by a low relief surface (up to 5 m high) carved 408

into the sub-sequence IVa deposits (Fig. 8). Above this surface, youngest terraces and 409

active channels with scarce floodplains of the Jáchal river are found. Beside the incision 410

surface between them, the difference among sub-sequences IV a and b is the minor 411

participation of fine grained floodplain deposits in the sub-sequence IVb that could be 412

described as a bed load dominated system intermediate between braided and wandering 413

(Miall 1996). 414

415

Accomodation stages in the Quaternary Precordillera’s piggyback basins

In this section, we present the evolution of the Quaternary sequences in context 417

of temporal changes in relative accommodation space and sediment supply. The 418

scenario before the deposition of sequence I is a regional-scale erosive surface 419

(pediment) carved into Paleozoic to Miocene rocks (Figs. 9a, 12 y 13). This low relief 420

surface indicates a long erosive period. Its origin cannot be stated unequivocally but 421

may be associated with a period of relative tectonic quiescence of the fold and fault 422

thrust belt. Tectonic quiescence could have been related to an isostatic rebound phase 423

due erosion, during which a slow but continuous fall in the equilibrium profile (Fig. 13) 424

led to an important erosive event throughout the entire orogenic wedge, generating a 425

pedimentation surface all over the piggyback area. That situation was followed by a 426

small rise in the equilibrium profile which produced a relatively thin layer of breccias 427

and conglomerates of sequence I deposited in an unconfined low accommodation 428

system tract (overfill stage Fig. 12). 429

416

Then, long term degradation in the piggyback area which generated the present 430

valley of the Jáchal river took place. This phase had probably begun in late Pleistocene 431

and involved an intensive erosion period, with a low degree of discontinuous stages of 432

deposition in isolated patches which were gathered within the sequence II (incision 433

stage, Figs. 12, 13 and 14a). 434

The origin of sequence II is related to deglaciation times that took place after the 435

late glacial maximum (Isotopic stage 2, Iriondo & Kröling 1996 and Iriondo 1999), 436

when the Cordillera de los Andes was glaciated (above 3500 meters). During the 437

deglaciation (between 15,500 and 16,500 yBp, Iriondo & Kroling 1996) melted ice 438

generated high discharge in rivers that drained the Andes, including the transference 439


14

systems across Precordillera (i.e. Huaco and Jáchal rivers) all of them with catchment 440

areas including large portions of glaciated regions. The climate amelioration caused a 441

higher flow of the transference fluvial systems that increased its efficiency and 442

produced the fall of the equilibrium profile (Fig.13). As a result of this the huge 443

volumes of sediments eroded, promoting the deposition of large megafans (DeCelles & 444

Cavazza 1999, Horton & DeCelles 2001) in the foredeep. According to Damanti (1993), 445

the megafans of Huaco and Jáchal reach areas of 700 and 1400 km2 with catchments 446

areas of 7100 and 27700 km2 respectively. Due to the huge amount of sediment 447

delivered to the foredeep, the Bermejo river had to divert its course next to the Pampean 448

range. 449

Sequence III deposits, represent the end of the deglatiation phase little before 450

8930 ± 50 yBp to beyond 6.497 ± 45 yBp (Colombo et al., 2005), when a dramatic fall 451

in the flow rates of the transference rivers occurred. This situation favored high rates of 452

aggradation (up to 150 m thickness) and the occurrence of high accomodation system 453

tract (Fig. 12 and 13) in the piggyback basins. Note that these ages, according to our 454

interpretation, are related to climatic amelioration proposed by Iriondo & Kroling 455

(1996). During this period, the loss of efficiency of the transference system together 456

with an increase in sediment supply from piggyback basins could have produced the 457

advance of these axial systems (conoid-collector rivers) over the transference system 458

valley. Besides, it would have led to the formation of the numerous sedimentary dams 459

(La Tranca, Caracol and Zanja Honda), recorded at the margins of the present Jáchal 460

river valley (Figs. 11c y 14b). The increase of sediment supply can be related to the 461

humid period associated with the deglaciation proposed by García et al. (1999) for the 462

southern Precordillera. It is also consistent with the pollen record of the base of the 463

lacustrine sediments in the area studied by Colombo et al. (2009). Although, thick 464

monomictic breccias, from piedmont and axial systems, were deposited in the 465

piggyback area; it is important to point out that, despite the local high sediment supply 466

and the low efficiency of the transference system, the overfill stage was never reached. 467

This was probably due to the large accommodation space created during the previous 468

stage (deglaciation, Fig. 12 and 13). 469

A new incision surface, that contains terraces and the present day alluvial belt of 470

the Jachal river, was carved into the lacustrine deposits of the sequence III, which form 471

the lower boundary of sequence VIa. 472


15

The origin of the change in the accommodation space between sequences III and 473

IVa is not clear but may have resulted from the adjustment of the transference system to 474

its regional base level fall (the Bermejo River). This was associated with the end of 475

lacustrine system sedimentation (the local-base level of the piggyback basins in 476

sequence III), that could be related to the broken of dams by connection with the 477

foredeep (regional-base level) and/or the readjustment to the new base level (probably 478

falling by a continued subsidence in the foredeep). For this reason the Jáchal River 479

produced an incision in its valley in order to catch up with its new base level 480

(degradation stage in the base of the sequence IV, Figs. 8, 12 and 13). This period of 481

degradation, with very limited or no aggradation, also reached the piggyback area. From 482

978 ± 20 yBp to 525 ± 32 yBp (Colombo et al. 2005) the Jáchal River had a short 483

aggradation period recorded by sequence IVa, corresponding to a high accomodation 484

system tract (Fig. 12). A minor incision surface (Figs. 8 and 12) separates sequence IVa 485

from the present-day fluvial deposits of the Jachal river, only interrupted by short 486

episodes of aggradation limited to the transference system, that form sequence IVb. 487

488

Ancient record of piggyback basin sedimentation in La Tranca 489

The proposed model for piggyback sedimentation can also be applied to ancient 490

settings. As an example, we analyzed the Cuesta del Viento Formation, a Miocene unit, 491

described and proposed by Suriano et al. (2011), located within the same geographic 492

context that the previous Quaternary basins. These are the oldest synorogenic strata in 493

the area and record the transition from outer-wedge top to inner- wedgetop (piggyback 494

basin) sedimentation. Its upper section was dated as Lower Miocene (19.5 + 1.1 Ma and 495

19.1 + 1.3 Ma) by Jordan et al. (1993). 496

497

This unit was studied in detail by Suriano et al. (2011) who have defined six 499

facies association that are here summarized. 500

Sedimentary environments 498

Facies association 1 (chaotic breccias) 501

This facies association comprises monomitic breccias, with metamorphic clasts 502

from the Yerba Loca Formation. They are dominated by coarse grained chaotic matrix-503

supported massive breccias (Fig. 9c) in beds up to 60 cm thick. Massive and imbricated 504

clast-supported breccias in lenticular amalgamated beds also appear. It is interpreted as 505


16

high energy piedmont arrangement composed by hyperconcentrated flow-dominated 506

colluvial fans, with some participation of taluses (1 in Fig. 3). 507

Facies association 2 (breccias) 508

This facies association is composed of breccias with the same provenance of 509

those of facies association 1, sandstones and mudstones (Fig. 15). It is dominated by 510

clast-supported massive and imbricated breccias. Matrix-supported breccias appear as 511

tabular bodies with massive or strong parallel fabric deposited by hyperconcentrated 512

flows (Bilkra & Nemec 1998). Sandstones and gravelly-sandstones locally appear with 513

massive, planar cross bedded or horizontal lamination. These lithofacies are more 514

common to the top of the section (Fig. 15). Massive and laminated mudstones also 515

appear as milimetric mud drapes or as continuous centimeter-thick levels. 516

This facies association is interpreted as braided river deposits, in particular a 517

transitional system between gravel-bed braided with sediment-gravity-flow and shallow 518

gravel-bed braided types of Miall (1996). The full interpretation of this facies 519

association depends upon its geomorphic context (Suriano et al. 2011). At both, the 520

bottom and the top of the section (Fig. 15) it appears as braided channels developed in 521

piedmonts, colluvial fans and alluvial slopes, associated to proximal facies. On the 522

contrary, in the middle part of the unit an alternation of transference and dam facies 523

occurs, allowing their interpretation as collector river deposits (axial systems). These 524

interpretations are supported by scarce clast imbrications observed (Fig. 15), not enough 525

to make a statistical analysis. 526

Facies association 3 (polimictic conglomerates) 527

This unit is comprises lenses of coarse to fine grained clast-supported 528

conglomerates and sandstones. The conglomerates bearing granites, mudstones, slates, 529

acid and ultrabasic volcanic clasts, record a mixture of local and foreign provenance 530

(Suriano et al. 2011). The conglomeratic lenses occur as massive beds, or with 531

imbricated or horizontally stratified structures, thus indicating the deposition of 532

transversal gravel bars (Hein & Walker 1977). Subordinately massive or horizontally 533

laminated sandstones appear as bar tops. Tabular beds of sandstones and mudstones, 534

massive or laminated deposited in a floodplain environment are also present. 535

This facies association is interpreted as a low-sinuosity river with channels of 536

multiple hierarchies, similar to the deep gravel-bed braided river of Miall (1996) or the 537

Donjek type of Williams & Rust (1969). Due to its lithology and environmental 538


17

interpretation this association is envisaged as a transference system, which represents 539

the ancient-Jáchal River. 540

Facies association 4 (Mudstones with polimictic conglomerates) 541

Two fine-grained facies associations were recognized in this Formation. The 542

facies association 4 Fig. 9d) is formed by coarsening-up cycles up to 20 m thick. The 543

cycles are dominated by massive or slightly laminated light brown mudstones, 544

intercalated with fine grained sandstones. Deposition took place due to alternating 545

processes of sheetfloods (Davis 1938) and decantation. These facies are overlaid by 546

sandstones with ripple cross lamination and planar cross bedded, which are covered in 547

turn by lenses of polimictic conglomerates. Conglomerates composition is similar to 548

that described in the facies association 2, pointing out a mixing of local and external 549

provenance. Conglomerate beds have erosive base and appear as massive or planar 550

cross stratification, deposited by channelized fluid flows. 551

In summary, facies association 4 is interpreted as the occurrence of clastic 552

ephemeral lake (natural damming) associated with microdeltaic bars facies related to 553

ancient-Jáchal river. The fine grained tabular facies are related to lacustrine deposits and 554

the sandstones and conglomerates in coarsening arrangements are interpreted as 555

microdeltaic bars facies in the ancient-Jáchal channels mouths. 556

Facies association 5 (Mudstones with monomictic breccias) 557

This facies association (Fig. 9e and f) is formed by thick tabular beds of light 558

greenish mudstones, dominantly laminated. Coarsening-up arrangements of laminated 559

sands and massive and trough cross bedded breccias, with local provenance appear in a 560

minor proportion. This association is interpreted as a closed ephemeral clastic lacustrine 561

system deposits, dominated by decantation processes and the prograding lobes of 562

sandstone sheetfloods or thin monomictic gravel bars with local provenance 563

(microdeltaic facies with local provenance). 564

Facies association 6 (stratified breccias) 565

It is made of tabular beds of 10 to 20 cm of thickness composed of fine to 566

medium clast-supported breccias, imbricated or massive. Locally, some lenses of planar 567

cross-bedded stratification are found. Some paleocurrents measurements indicate East to 568

West direction of flow. This unit is interpreted as shallow channels with broad 569

interchannel areas dominated by gravel sheetfloods, corresponding to alluvial slopes 570

(Smith 2000) developed in a low-energy piedmont environment (2 in Fig. 5). 571

572


18

The Cuesta del Viento Formation records the earliest stages of the Bermejo 574

Foreland Basin (Jordan et al. 1993; Jordan et al. 2001; Alonso et al. 2011 and Suriano 575

et al. 2011), related to the uplift of the Sierra de La Tranca. The base of the unit (Fig. 15 576

and 16, facies association 1 and 2) corresponds to wedge-top deposition, in the inner 577

wedge-top depocenter, represented by the development of high energy and local 578

provenance piedmont (Figs. 16 and 17 a). Overlying this basal unit, lower-energy 579

piedmont facies were deposited, represented by braided channels of colluvial fans. 580

Finally, the last facies associations deposited in this basin correspond to the ancient-581

Jáchal river, interbeded with colluvial fans (Fig. 16, facies associations 3 and 2). 582

Basin evolution 573

As the Andean deformation advanced to the east, the uplift of the Caracol Range 583

started. This phase represents the onset of the La Tranca area as a piggyback basin 584

configuration, and the deposition of lacustrine and microdeltaic facies recording this 585

change (Fig. 16, facies association 4). The dam was, in this case, related to an increase 586

of the local accommodation space due to the Caracol range uplift (Fig. 17 b). 587

Afterwards, the base level was readjusted to the foreland, probably because the 588

damming was broken, an incision surface was carved (incision stage, Fig. 16) and axial 589

systems prograded in response to a low equilibrium profile on the transference system 590

(confined low accomodation system tract, Fig. 17c). Finally, the major uplift of the 591

Caracol range took place. La Tranca area was isolated forming a closed piggyback basin 592

(Figs. 16 and 17d) recording a high accommodation system tract, registered by muddy 593

lacustrine deposits and a high energy piedmont association (Fig. 16). In the latter phase, 594

low energy piedmont systems (facies association 3) prograded onto playa lake facies 595

producing the filling of the basin (unconfined low accommodation system tract, Fig. 596

16). 597

598

Conclusions 599

The movement of sediments within continental piggyback basins can be 600

sketched as driven by three different types of alluvial-fluvial systems: 1. Piedmont 601

accumulations (talus, different types of colluvial fans and, in wide enough basins, 602

alluvial fans) forming alluvial aprons during mountain denudation, 2 Axial fluvial 603

systems which collect sediments derived from the piedmont area and transport them to 604

transversal rivers, 3. Transference systems formed by large rivers draining the fold and 605

thrust belt and transferring sediments from the piggyback basins to the foredeep region. 606


19

Despite the fact that this model is a simplification of the alluvial-fluvial systems 607

in Precordillera, it is useful to explain the transference of sediments not only within the 608

piggyback basins but also from the piggyback area to the foreland. In addition, the 609

existence of transference fluvial systems (in our case the Jáchal river) enables the 610

recognition of two different types of piggyback: open piggyback basins and closed 611

piggback basins (when the lack of transference fluvial systems prevents the transport of 612

sediments from the piggyback to the foreland, turning the piggyback into an endorheic 613

isolated basin). The evidence obtained from Cenozoic deposits as analyzed in this paper 614

clearly shows that open piggyback basins can undergo temporary closures and, in the 615

same way, closed piggyback areas can be open when a new transference fluvial system 616

developes in the thrust and fold belt. 617

The stratigraphic record of open piggyback basins has been divided in this paper 618

in four major theoretical stages: 1. Incision stage, 2. Confined low accommodation 619

system tract, 3.High accommodation system tract, 4. Unconfined low accommodation 620

system tract or overfill stage. The incision stage is reached when long-lived periods of 621

erosion within the piggyback allow/produce a massive transference of sediments to the 622

foreland. Degradation stages are recorded by incision surfaces. These surfaces can be 623

related not only to changes in subsidence or tectonism in the foreland or in the fold and 624

thrust belt but also to climatic fluctuations significant enough to produce an increase in 625

the sediment transport capacity of the third-order fluvial systems. The high 626

accommodation system tract is characterized by strong aggradation in the piggyback 627

basins mainly related to either low rates of subsidence in the foredeep or inefficiency of 628

the transference fluvial systems. Low-accommodation stages reflect limited capacity of 629

the piggyback to store sediments. The confined stage occurs in subsequent periods of 630

erosion and transference of sediments towards the foreland, recorded as the first fill over 631

the incision surface. Finally, the unconfined stage is associated with basin overfill. 632

The deposits of close piggyback basins have been divided in two end members 633

stages: 1. High accommodation system tracks and 2. Low accommodation system tracts. 634

In order to examine the utility of the proposed model, two case studies in the 635

Precordillera of San Juan province, Argentina were analyzed: the Pleistocene-Holocene 636

deposits of the Jáchal river piggyback basins and the Oligocene-Miocene Cuesta del 637

Viento Formation. 638

The first example associated with relative relatively quiescent tectonic shows 639

that, changes in subsidence rates along the foreland under these conditions can exert a 640


20

strong control in the equilibrium profiles of the transference systems thus promoting 641

periods of aggradation and degradation of the piggyback. A good example of this 642

situation appears in sequence I which illustrates how the isostatic rebound in the thrust 643

and fold belt controlled the enhancement of aggradation phases. On the other hand, 644

changes in climatic conditions should not be overlooked, In fact, the widespread 645

developement of intermontane lakes (sequence III) as well as the large progradation of 646

megafans (sequence II) into the foreland are linked to postglacial conditions and result 647

good examples on how climate plays an important role in the accumulation patterns in 648

both piggyback and foreland basins. 649

The second case study records the transition between outer- wedgetop depozone 650

to piggyback basin as well as different types of piggyback basin (open to close). 651

We conclude that the here proposed model was useful to analyze the sediments 652

of the Jáchal river area deposited during limited or lacking tectonic activity with 653

climatic controls, as well as the Cuesta del Viento Formation deposited in an active fold 654

and thrust belt. In both cases, the model provided a framework in which piggyback 655

basin evolution could be described and related to the factors that controlled the 656

dynamics of sedimentation. 657

658

Acknowledgements 659

This study was supported by grant BID 1728 OC/AR PICT 20752 of the 660

Agencia de Promoción Científica y Tecnológica (Argentina), the UBACyT 661

20020110300025 and the Departamento de Ciencias Geológicas of the Universidad de 662

Buenos Aires. We would like to thank José Mescua the critical comments that helped 663

to improve our manuscript. The authors acknowledge the critical reading and the 664

valuable proposals suggested by the reviewers (Maisa Tunik and an anonymous one). 665

666

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Figure captions 869

Fig.1 Location maps (a) Morphotectonic units related to the Andean orogen at 30° 870 south latitude, numbers indicate the three major sectors of Precordillera (1, Western; 871 2, Central, and 3 Eastern), the white frame shows the location of figure b. (b) 872 Geological map, where two main domains can be recognized: mainly granitic and 873 volcanic (Cordillera) at west and a metasedimentary one at east (Precordillera), the 874 frame shows the location of fig. 7. 875 Fig.2 Schematic distribution of the sedimentary circulation in piggyback basins. 876 Fig.3 Arid piedmont associations for Precordillera (modified from Suriano & 877 Limarino, 2009). Highest energy corresponds to Association 1 , composed by 878 taluses and colluvial fans with steeper slopes, if there is enough lateral space this 879 association could evolve into association 2. Association 3 is related to gentle slope 880 mountain fronts as much as association 4 which developes in wider valleys and 881 evolved drainages. 882 Fig.4 Piggyback basin classification proposed in this work. 883 Fig.5 Allocyclic controls on piggyback sedimentation. 884 Fig.6 Proposed model for piggyback basins evolution. a) Schematic sedimentary 885 section, b) Transference system profile and its equilibrium profile through time. 886 Numbers related the time in both figures. 887 Fig.7 Detailed geologic map of La Tranca valley. See Fig.1b for general location. 888 Fig.8 Schematic distribution of the Quaternary sequences and ages in the Jáchal 889 valley. 890 Fig.9 a) Quaternary valley carved on Miocene aeolian units, b) Deposits of the 891 sequence IVa in Pachimoco area, the white arrow shows the abundant bioturbation 892 in the floodplain deposits, the red line shows the scale, c) Massive matrix supported 893 chaotic breccias (Bmm) of facies association 1, Cuesta del Viento Formation, d) 894 Mudstones with polimictic conglomerates in coarsening up arrangements of facies 895 association 4, Cuesta del Viento Formation. The red bar is 1.5 m long, e) Mudstones 896 with monomictic breccias in coarsening up arrangements belonging to facies 897 association 5, Cuesta del Viento Formation, f) Close view of the coarsening up 898 cycles of facies association 5, the red bar is 1,5 m long. 899 Fig. 10 Satellite image of Huaco and Jáchal megafans. 900 Fig. 11 Scheme and pictures from the Quaternary lacustrine arrangement. a) Dam-901 lacustrine fine grained association, b) Coarsening up cycles from a micodeltaic 902 system, c) Closure of a dam represented by conoid facies, lateraly related to fine 903 dam-lacustrine facies, d) Sceme of the dam facies, numbers in the scheme 904 correspond to those showed in the photos. 905 Fig. 12 Schematic section of the Quaternary sequences, with systems tracts 906 interpretation and geological events that originates them (right column). 907 Fig. 13 Changes in the accommodation space trough the time, their relationship with 908 different system tracts (at right) and with the Quaternary sequences. The black 909 arrows shows an increase in accommodation space and the white ones a decrease. 910


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Fig. 14 Block diagrams of the Jáchal river related basins. (C2= sediment transport 911 efficiency of the longitudinal fluvial system, C3=sediment transport efficiency of 912 the transference system). a) Sequence II deposition arrangement scheme (Mainly 913 erosive phase, few isolated deposits related), b) Sequence III deposition scheme. 914 Fig. 15 Sedimentary section of Cuesta del Viento Formation and summary of its 915 facies associations, characteristics and interpretation. 916 Fig. 16 Schematic section of the Cuesta del Viento formation, with system tracts 917 interpretation and geological events that originate them (right column). 918 Fig 17 Block diagrams showing the evolution of Cuesta del Viento Formation and 919 their relationship with the facies association (thick arrows represent active 920 thrusting). 921

Sedimentation model of piggyback basins: Cenozoic examples of San Juan Precordillera, Argentina

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