Geol. Mag.: page 1 of 13 c Cambridge University Press 2016 1 doi:10.1017/S0016756816000832

1 Cambrian–Ordovician conodonts from slump deposits of the 2 Argentine Precordillera: new insights into its passive margin 3 development

∗ 4 GUSTAVO G. VOLDMAN †‡, JUAN L. ALONSO§, LUIS P. FERNÁNDEZ§, ∗ ∗ 5 ALDO L. BANCHIG¶, GUILLERMO L. ALBANESI ‡, GLADYS ORTEGA 6 & RAÚL CARDÓ¶|| ∗ 7 CONICET, Museo de Paleontología, CIGEA, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional 8 de Córdoba, X5016GCB Córdoba, Argentina 9 ‡CICTERRA (CONICET-UNC), Córdoba, Argentina 10 §Departamento de Geología, Universidad de Oviedo, 33005 Oviedo, Spain 11 ¶Facultad de Ciencias Exactas y Naturales, Universidad Nacional de San Juan, 5400 San Juan, Argentina 12 ||Servicio Geológico Minero Argentino (SEGEMAR), 5400 San Juan, Argentina

13 (Received 22 January 2016; accepted 26 July 2016)

14 Abstract – The Los Sombreros Formation represents the western continental margin slope deposits of 15 the Argentine Precordillera, a sub-terrane accreted to Gondwana as part of the Cuyania Terrane in early 16 Palaeozoic times. The age of these gravity-driven deposits is controversial and, therefore, a precise 17 biostratigraphic scheme is essential to reveal the evolution of the continental margin. New conodont 18 samplings along with sedimentological and structural analysis carried out in the Los Sombreros 19 Formation in the La Invernada Range provide clues to its depositional framework. The sedimentary 20 succession is made up of dominantly calciturbidites, carbonate breccias and conglomerates, along with 21 mudstones that represent the pelagic/hemipelagic background sedimentation. It displays hectometric 22 to outcrop-scale slump folds with variable hinge-line orientations and pinch-and-swell structures, 23 evidencing soft-sediment deformation, consistent with a slope to base-of-slope setting. Three limestone 24 samples from this succession include conodonts referable to the pandemic Hirsutodontus simplex 25 Subzone of the Cordylodus intermedius Zone (upper Furongian, Cambrian) and from the Macerodus 26 dianae Zone (upper Tremadocian, Ordovician), implying that a slope connected the shallow-water 27 shelf with a deep-water (oceanic) basin at least since late Cambrian times. The conodont faunas show 28 affinities to coeval assemblages from outer shelf and slope environments around Laurentia yet they 29 are not conclusive to postulate a geographic origin for the Precordillera. The thermal alteration of the 30 conodonts is consistent with sedimentary burial and nappe stacking in this sector of the Precordillera. 31 32 Keywords: conodont, slope facies, slump, Los Sombreros Formation, Argentine Precordillera, Cambrian, Ordovician.

33 1. Introduction The transition between the Central and Western 51 Precordillera is represented by the disorganized 52 34 In the external zone of the Andean orogen, the Ar- deposits of the Los Sombreros Formation (Cuerda, 53 35 gentine Precordillera records a series of tectono- Cingolani & Varela, 1983; Banchig, Keller & Milana, 54 36 stratigraphic events related to the building of 1990) and the Corralito Formation (Furque & Caballé, 55 37 Gondwana during early to middle Palaeozoic times 1988; Furque et al. 1990), recording the slope of a 56 38 (e.g. Cawood, 2005; Rapela et al. 2016). Eastern, Cent- continental margin. However, the complex structure 57 39 ral and Western domains have classically been distin- and the scarcity of fossils challenge against a precise 58 40 guished after their structural and stratigraphic features depositional scheme, whilst Ordovician and Devonian 59 41 (Ortiz & Zambrano, 1981; Baldis et al. 1982). The East- ages have been proposed for the Los Sombreros 60 42 ern and Central Precordillera involve a large passive- Formation (e.g. Benedetto & Vaccari, 1992; Voldman, 61 43 margin carbonate platform, Cambro-Ordovician in age, Albanesi & Ramos, 2009; Peralta, 2013). For instance, 62 44 which is overlain by Middle Ordovician siliciclastic Peralta (2013) considered that all the disorganized 63 45 foreland-basin deposits that reach up to the Devo- deposits slid in Devonian times, after sedimentation 64 46 nian Punta Negra Formation. Conversely, the West- of the Punta Negra Formation. Thus, constraining the 65 47 ern Precordillera exhibits a deep-water succession, spatio-temporal framework of the mélanges is essential 66 48 with ocean-floor sedimentary rocks containing pillow for understanding the geotectonic evolution of the 67 49 lavas and mafic–ultramafic bodies in the westernmost passive continental margin of the early Palaeozoic 68 50 sections. Precordillera. 69 In the present study, new conodont findings along 70 †Author for correspondence: [email protected] with structural, stratigraphic and sedimentological data 71 2 G. G. VOLDMAN AND OTHERS

Lower Ordovician limestones displaying platform fa- 107 cies not represented elsewhere (e.g. Bordonaro, 2003). 108 The Los Sombreros Formation displays ubiquitous 109 extensional structures that result in block-in-matrix 110 fabric in some places as a consequence of submar- 111 ine sliding (Alonso et al. 2008). The main outcrop belt 112 of the mélange extends along the eastern flank of the 113 Tontal Range (Fig. 1), where the formation is more than 114 1000 m thick in the Seca Creek type section (Cuerda, 115 Cingolani & Varela, 1983; Cuerda et al. 1986). The 116 outcrops continue patchily to the north up to the Jáchal 117 River area (Benedetto & Vaccari, 1992), whereas its 118 southern prolongation is represented by the Estancia 119 San Isidro Formation in Mendoza, which exhibits giant 120 Cambrian limestone blocks enclosed in a green shaly 121 matrix of Darriwilian age (Keller, 1999; Heredia & 122 Beresi, 2004;Ortegaet al. 2007). Thus, the slope fa- 123 cies of the Precordillera extends over 300 km with N–S 124 Figure 1. (Colour online) Location map of the Argentine Pre- orientation (Fig. 1). 125 cordillera fold-and-thrust belt showing the studied locality and the western, central and eastern morphostructural domains. Base image derived from Shuttle Radar Topography Mission (SRTM). 3. Previous biostratigraphic studies of the Los 126 Sombreros Formation 127

72 for the Los Sombreros Formation in the La Inver- The Los Sombreros Formation was originally referred 128 73 nada Range are presented. The record of the Hir- to the Lower Ordovician Series based on graptolite re- 129 74 sutodontus simplex Subzone of the Cordylodus inter- cords from its lower third (Cuerda et al. 1985). Later 130 75 medius Zone (upper Furongian, Cambrian) and the trilobite findings revealed the presence of lower and 131 76 Macerodus dianae Zone (upper Tremadocian, Ordovi- middle Cambrian rocks (Bordonaro & Baldis, 1987; 132 77 cian) in gravity-flow deposits with synsedimentary de- Bordonaro & Banchig, 1990), interpreted as outer- 133 78 formation features suggests the existence of a slope platform resedimented blocks in a Lower – Middle 134 79 in late Cambrian – Early Ordovician times. Moreover, Ordovician succession with autochthonous conodonts 135 80 the defined high-resolution conodont biostratigraphy and graptolites (Benedetto & Vaccari, 1992; Bordon- 136 81 improves the intrabasinal correlation with the Precor- aro, 2003). Lehnert (1994) described the first con- 137 82 dilleran carbonate platform as well as with other re- odont assemblage of the Cordylodus proavus Zone 138 83 gions of the world. (upper Furongian) in the Precordillera, from out- 139 crops of the Los Sombreros Formation in the Tontal 140 Range. The conodont association includes Cordylodus 141 84 2. Geological setting primitivus Bagnoli, Barnes & Stevens, Cordylodus 142 85 The Argentine Precordillera of La Rioja, San Juan and proavus Müller and Eoconodontus notchpeakensis 143 86 Mendoza provinces is a c. 80 km wide foreland fold- Miller. These conodont elements derive from a cal- 144 87 and-thrust belt that involves Palaeozoic to Cenozoic cisiltite lens that overlies shales with graptolite speci- 145 88 rocks, which extends between 29º and 33º S in the An- mens that demonstrate its allochthonous character. 146 89 dean foothills above the shallow subduction segment Albanesi, Ortega & Hünicken (1995) proposed that 147 90 of the Nazca plate (Heim, 1952; Allmendinger et al. the conformable stratigraphic contact between the Los 148 91 1990; Gosen, 1992; Ramos, Cristallini & Pérez, 2002) Sombreros Formation and the overlying Yerba Loca 149 92 (Fig. 1). The Precordilleran Cambrian–Ordovician car- Formation at Ancaucha Creek, 10 km northwest of 150 93 bonate platform sequence is unique to South America Jáchal city, is early Darriwilian in age. They determ- 151 94 and makes up part of a larger region of the Andean ined that the top of the Los Sombreros Formation at this 152 95 foothills of western Argentina that is referred to as the locality is lower Darriwilian at the most, by consider- 153 96 Cuyania composite terrane (Ramos, 1995). ing the presence of the index conodont Baltoniodus 154 97 At the continental slope and rise, to the west of the clavatus Stouge & Bagnoli from the basal beds of the 155 98 Precordilleran carbonate platform, the Los Sombreros Yerba Loca Formation and the absence of the key spe- 156 99 mélange is a mudstone-dominated deep-water disor- cies Paroistodus horridus (Barnes & Poplawski), which 157 100 ganized unit, containing a diverse array of rocks derived appears c. 40 m above the Yerba Loca base. 158 101 from the platform, the slope and the basement. These Further fossil findings include upper Cambrian, 159 102 rocks include arkosic sandstones, megabreccias, con- Tremadocian, Floian and Darriwilian conodont faunas 160 103 glomerates with well-rounded basement-derived meta- (Voldman, Albanesi & Ramos, 2009; Voldman et al. 161 104 morphic and igneous pebbles to boulders, thin-bedded 2014) as well as graptolites referable to the uppermost 162 105 fine-grained limestones, carbonate breccias and blocks Tremadocian, Floian and Sandbian stages (e.g. A. L. 163 106 up to several hectometres in size of lower Cambrian to Banchig, unpub. Ph.D. thesis, Univ. Nacional San Juan, 164 Conodonts from slump deposits of the Precordillera 3

sion and gradationally overlie a breccia/conglomerate 206 division. Breccias/conglomerates contain mainly clasts 207 of mudstones, peloidal grainstones and recrystallized 208 limestones, with minor amounts of chert and quartz 209 clasts. They constitute beds that are massive or graded, 210 and that may be gradationally overlain by a calciturbid- 211 ite; some exceptional examples of stratified (laminated) 212 beds also occur (Fig. 3c). The whole succession lacks in 213 situ shallow-water faunas, with the exception of scarce 214 lingulids, and exhibits slight bioturbation restricted to 215 the peloidal/calcisphere limestones. Remarkably, it dis- 216 Figure 2. (Colour online) Geological map of the study area with plays hectometric slump folds, extensional faults and 217 fossiliferous sampling points. lateral transitions between intact and faulted/brecciated 218 beds (Fig. 3a, d). 219 165 1995; Banchig & Moya, 2002;Ortegaet al. 2014). As a whole, most of these facies are indicative of sed- 220 166 Additionally, Astini, Thomas & Yochelson (2004) imentation from gravity flows, from cohesive debris 221 167 identified the enigmatic fossil Salterella maccullochi flows to low-density turbidity currents (see Lowe, 222 168 (Murchison) in the Ancaucha Olistolith of the Los Som- 1982; McIlreath & James, 1984; Mutti et al. 1999, 223 169 breros Formation, composed of inner-shelf deposits amongst others). Fine-grained, peloidal/calcisphere 224 170 that they interpreted as lower Cambrian synrift strata. mudstones probably represent the pelagic/hemipelagic 225 background sedimentation. This interpretation is com- 226 patible with the synsedimentary deformation features 227 171 4. Study area and methods (see also Section 5.a.1. below), all indicating a slope 228 to base-of-slope setting. The relatively deep-water en- 229 172 The La Invernada Range constitutes a critical region to vironment is also suggested by the absence of in situ 230 173 investigate the architecture and kinematic development shallow-water fauna and the scarcity of bioturbation. 231 174 of the disorganized deposits of the Los Sombreros The slumped succession at the Aguada de La Cueva 232 175 Formation. This range lies to the north of the Tontal Creek section is unconformably overlain by grey shales 233 176 Range, extending for c. 60 km with a N–S trend with poorly preserved graptolites, including Archicli- 234 177 (Fig. 1). It displays E-verging thrusts and related folds, macograptus sp., Dicellograptus sp., Dicranograptus 235 178 involving a lower Palaeozoic succession that also sp., Nemagraptus sp. and Reteograptus speciosus Har- 236 179 includes the Siluro-Devonian mélange deposits of the ris, which suggest a late Darriwilian to Sandbian age 237 180 Corralito Formation, which crop out exclusively in for the upper part of the Los Sombreros Formation 238 181 this range. In order to determine the age of the Los in this area (Sample AMGRAP, Figs 2, 4). These 239 182 Sombreros Formation at the Aguada de La Cueva graptolitic shales are in turn unconformably over- 240 183 Creek section (Fig. 2), nine limestone samples (14 kg lain by sandstone–mudstone alternations with scarce 241 184 in total) were collected and processed following the calcarenites and interbedded conglomerates and cal- 242 185 standard techniques to recover conodont elements careous breccias of the Sierra de La Invernada Forma- 243 186 (Stone, 1987). Three samples were productive, yield- tion (Furque et al. 1990), which are Middle–Late Or- 244 187 ing 80 specimens that are housed under the repository dovician in age (Ortega et al. 2008). 245 188 codes CORD-MP 50734 to 50814 in the Museo de The Los Sombreros Formation is carried on a W- 246 189 Paleontología, Facultad de Ciencias Exactas, Físicas dipping thrust surface onto the Siluro-Devonian Cor- 247 190 y Naturales, Universidad Nacional de Córdoba, Ar- ralito mélange (Fig. 2), which comprises greenish grey 248 191 gentina. Field work also included structural mapping shales and coquinas with extensional features, such as 249 192 and stratigraphic and sedimentological studies, during boudins, as well as limestone blocks of the San Juan 250 193 which kinematic data were acquired and rocks were Formation. The thrust surface displays Cʹ-type shear 251 194 described in terms of their bedding and lithology and bands and striations, recording an eastward movement 252 195 sampled for thin-section preparation. of the hanging wall. It is probably Andean in age be- 253 cause it is located at the toe of the Invernada Range and 254 196 5. Results therefore should have been responsible for the uplift of 255 this modern geomorphic feature. However, this thrust 256 197 5.a. Stratigraphy and structure could be the result of rejuvenation of an older Chanic 257 198 In the Aguada de La Cueva Creek area, the Los or Gondwanic thrust during Andean times, which com- 258 199 Sombreros Formation consists of a c. 500–600 m monly occurs in the Argentine Precordillera (Ramos, 259 200 thick succession of dominantly limestones, carbon- Vujovich & Dallmeyer, 1996; Alonso et al. 2005). 260 201 ate breccias and conglomerates and subordinate shales 202 (Fig. 2). The limestones are thin bedded, varying 5.a.1. Synsedimentary deformation 261 203 from peloidal/calcisphere mudstones and wackestones 204 to graded and laminated calclithites (calciturbidites) Regarding the internal structure of the Los Sombreros 262 205 (Fig. 3a, b), which may pass upwards into a shale divi- Formation, the most conspicuous features in the study 263 4 G. G. VOLDMAN AND OTHERS

Figure 3. (Colour online) Field photographs of selected deposits from the Los Sombreros Formation at the Agua de la Cueva Creek. (a) Detail of an interval made of pebble-to-cobble breccias (left of photograph) and thin-bedded limestones (laminated mudstones and graded and laminated calciturbidites). Notice the local disruption of bedding, which laterally passes into undisturbed bedding. Stratigraphic top towards the left. (b) Interval formed of calciturbidites and variably laminated, slightly bioturbated mudstones with peloids and calcispheres. The lowermost bed is a graded calciturbidite evolving from a basal fine-pebble-bearing granulestone to a sand-grade division, whose upper part is faintly laminated and displays smooth undulations. Younging direction is to the top of the photograph. Hammer handle for scale c. 19 cm long. (c) Poorly sorted boulder-to-pebble carbonate breccia/conglomerate, displaying two divisions, a lower graded division and an upper laminated division. Walking pole is c. 120 cm long. Stratigraphic top towards the right. (d) Close-up of an interval made of thin-bedded lime mudstones, commonly laminated, with a variable degree of folding and bed disruption and brecciation. Bedding in the area is parallel to the bed above the scale, which is almost intact. The yellowish matrix surrounding the clasts is mainly dolomitic. Younging direction is to the top of the photograph.

264 area are hectometric folds that can be interpreted as a normal fault), all the above mentioned structures sup- 282 265 slump structures (Fig. 2). These folds do not involve port the interpretation that gravitational collapse and 283 266 the overlying Sierra de La Invernada Formation. As sliding was the cause of the deformation. In Figure 5d, 284 267 well, most of the smaller, outcrop-scale structures are the cut-off lines of the tension fractures (blue) are par- 285 268 slump folds with variable hinge-line orientations and allel to the boudin necks lineation (red), recording the 286 269 pinch-and-swell structures, which record soft-sediment same extension direction. 287 270 deformation (Fig. 5). Tension fractures perpendicular 271 to bedding and normal faults also occur. In the ex- 5.b. Conodont fauna and biostratigraphy 288 272 ample of Figure 5a, b, tension fractures are restricted 273 to the yellowish beds, indicating that these beds under- Three carbonate samples taken at the Aguada de 289 274 went brittle behaviour, while other beds, with pinch- La Cueva Creek section yielded conodonts, which 290 275 and-swell boudinage, were stretched by more ductile constrain the age of the slump deposits of the Los 291 276 deformation, probably because the yellowish beds were Sombreros Formation with a high-resolution biostrati- 292 277 more lithified. So, pinch-and-swell boudins and tension graphy and improve the conodont biozonation scheme 293 278 and shear fractures are more or less coeval and all of of the Precordillera. The taxonomy of the identified spe- 294 279 them imply bed-parallel extension. Although the devel- cies is well known, following descriptions of previous 295 280 opment of the slump fold in Figure 5a, b is prior to ex- authors; therefore, only a brief discussion is presented 296 281 tensional deformation (both fold limbs are truncated by herein. 297 Conodonts from slump deposits of the Precordillera 5

A lime mudstone affected by synsedimentary exten- 298 sional faults (sample AM6) yielded Cordylodus caboti 299 Bagnoli, Barnes & Stevens, C. cf. tortus Barnes, C. 300 intermedius Furnish, C. proavus Müller, C. cf. andresi 301 Viira & Sergeyeva, Drepanoistodus sp., Teridontus na- 302 kamurai (Nogami), Variabiloconus datsonensis (Druce 303 & Jones), Westergaardodina sp. and the index species 304 Hirsutodontus simplex (Druce & Jones) (Fig. 6). The 305 latter species is chronostratigraphically restricted to the 306 Cordylodus intermedius Zone, Stage 10 of the Furong- 307 ian. H. simplex is characterized by a simple cone with 308 circular cross-section and a series of spines scattered 309 mainly on the anterior and lateral sides of the base and 310 cusp (Fig. 6n). 311 The different species of Cordylodus were dis- 312 tinguished by considering the general shape of the 313 elements, the pattern of denticulation (discrete, 314 confluent) and the basal cavity configuration (number 315 Figure 4. Late Darriwilian to Sanbian graptolites from the up- of apices, depth, position and shape of its anterior 316 per levels of the Los Sombreros Formation (Sample AMGRAP, border). As described by Bagnoli, Barnes & Stevens 317 see location in fig. 2). (a) Archiclimacograptus sp., CORD-PZ (1987), the basal cavity of C. caboti (Fig. 6g–i) is not 318 25705; (b) Dicellograptus sp., CORD-PZ 25706; (c) Reteograp- as deep as in C. proavus (Fig. 6a–e), but also extends 319 tus speciosus Harris, CORD-PZ 25707; (d) Nemagraptus sp., above the posterior process. Its basal cavity displays 320 CORD-PZ 25708; (e) Dicranograptus? sp., CORD-PZ 25709. Scale bar: 1 mm. a slightly concave to straight anterior margin that 321

Figure 5. (Colour online) (a, b) Photograph and outcrop sketch showing a slump fold and pinch-and-swell structures in the carbonate succession of the Los Sombreros Formation. Tensional and shear fractures are depicted. See text for explanation, and location in Figure 2. (c) Stereoplot of fold axes in the study area. (d) Boudinaged limestone bed (on the left) and closely spaced microfractures (on the beds located to the right). Hammer for scale: 33 cm. 6 G. G. VOLDMAN AND OTHERS

Figure 6. (Colour online) Conodont elements from the Hirsutodontus simplex Subzone of the Cordylodus intermedius Zone (upper Furongian, Cambrian), sample AM6: (a–f) Cordylodus proavus Müller, (a) S element, lateral view, CORD-MP 50734; (b) S element, lateral view, CORD-MP 50735; (c) S element, lateral view, CORD-MP 50736; (d) P element, lateral view, CORD-MP 50737; (e) S element, lateral view, CORD-MP 50738; (f) S element, lateral view, CORD-MP 50739. (g–i) Cordylodus caboti Bagnoli, Barnes & Stevens, (g) S element, lateral view, CORD-MP 50740; (h) S element, lateral view, CORD-MP 50741; (i) S element, lateral view, CORD-MP 50742. (j) Cordylodus cf. tortus Barnes, S element, lateral view, CORD-MP 50743. (k–m, o, p) Cordylodus intermedius Furnish, (k) S element, lateral view, CORD-MP 50744; (l) S element, lateral view, CORD-MP 50745; (m) S element, lateral view, CORD-MP 50746; (o) M element, lateral view, CORD-MP 50747; (p) S element, lateral view, CORD-MP 50748. (n) Hirsuto- dontus simplex (Druce & Jones), CORD-MP 50749, (n1) anterolateral view, (n2) posterior view. (q) Westergaardodina sp., lateral view, Conodonts from slump deposits of the Precordillera 7

322 curves near the tip, with its apex centrally located, a new genus for hybrid Teridontus-like species after fur- 374 323 which differentiates it from C. intermedius (Nicoll, ther investigations. The limited number of specimens 375 324 1991; Pyle & Barnes, 2002; Zeballo & Albanesi, recovered does not allow for adequately defining the 376 325 2009). Miller et al. (2003) regarded C. caboti as a latitude of morphologic variability of these primitive 377 326 junior synonym of Cordylodus ‘drucei’ Miller; since forms. Therefore, we cautiously assign a simple sub- 378 327 we cannot follow his criteria in our collection, we symmetrical coniform element with long cusp to T. na- 379 328 maintain the species C. caboti as valid. kamurai (Fig. 6u) and those with short, erect and keeled 380 329 The Cordylodus intermedius Zone is divided into cusps with weak grooves to V.datsonensis, which is re- 381 330 alowerHirsutodontus simplex Subzone and an upper stricted to the Furongian (Fig. 6w). Younger species 382 331 Clavohamulus hintzei Subzone (Miller et al. 2003). The of Variabiloconus are easier to distinguish as they ex- 383 332 lower subzone begins with the First Appearance Datum hibit an increase of ornamentation and development 384 333 (FAD) of H. simplex whereas its top is defined by the of the cusp (Löfgren, Repetski & Ethington 1998; Ze- 385 334 FAD of C. hintzei. The presence of advanced species ballo & Albanesi, 2013a). The Teridontus nakamurai 386 335 of Cordylodus in sample AM6, such as C. caboti and specimen recovered from the Los Sombreros Forma- 387 336 C. intermedius, which are more frequent in the upper tion lacks the typical microstriation of the genus, as 388 337 subzone, in the absence of C. hintzei, suggests an upper previously observed by Lehnert (1994). The genus Or- 389 338 H. simplex Subzone for this stratigraphic level (e.g. minskia also resembles Teridontus and lacks microstri- 390 339 Ross et al. 1997). ation, yet it has a hyaline cusp (Landing, Westrop & 391 340 Cordylodus proavus extends from the upper Cam- Keppie, 2007). 392 341 brian up to the Iapetognathus Zone, indicative of the Two samples obtained from thin-bedded fine-grained 393 342 base of the Ordovician, whereas C. intermedius and calciturbidites interbedded with marlstones and brec- 394 343 C. caboti reach up to the Rossodus manitouensis Zone cias located a few metres stratigraphically above AM6 395 344 (Pyle & Barnes, 2002). Two elements recovered com- were also productive. Sample AM8A yielded one 396 345 pare well to the figured forms of Cordylodus cf. andresi specimen of Macerodus dianae Fåhraeus & Nowlan 397 346 sensu Zeballo & Albanesi (2009)(Fig. 6x), which ex- (Fig. 6s), a distinctive form of the standard North 398 347 hibit a narrower cavity compared to the nominal species American Midcontinent Realm zonation (Ross et al. 399 348 (re-illustrated by Miller et al. 2015). Additionally, C. 1997), whose biostratigraphic range is restricted to 400 349 andresi occurs in older rocks as it is restricted to the a narrow interval of the upper Tremadocian. The 401 350 Hirsutodontus hirsutus and Fryxellodontus inornatus M. dianae Zone correlates with the lower part of 402 351 zones of the C. proavus Zone. Cordylodus cf. tortus is the upper subzone of the Paltodus deltifer Subzone 403 352 strongly asymmetric, with a denticle flexed compared of the Baltoscandian scheme (Webby et al. 2004). 404 353 to the cusp plane and a basal cavity relatively shallow Sample AM8B contains eight elements including Dre- 405 354 with its apex slightly displaced to the posterior region panodus arcuatus Pander, Paltodus aff. inaequalis 406 355 (Zeballo & Albanesi, 2009). (Pander) (Fig. 6y), Rossodus cf. manitouensis Repet- 407 356 The taxonomy of simple cones is complex, partic- ski & Ethington (Fig. 6r), Scolopodus cf. subrex Ji 408 357 ularly owing to their high variability and subtle dia- & Barnes (Fig. 6t) and a cluster of the paracon- 409 358 gnostic features. Variabiliconus shares with Teridontus odont Phakelodus tenuis (Müller). The S. subrex Zone 410 359 a similar apparatus plan, an abrupt junction between the partly correlates with the Macerodus dianae Zone 411 360 hyaline base and albid cusp, and the surface microstri- (Pyle & Barnes, 2002), whereas Rossodus cf. man- 412 361 ation. In particular, Variabiliconus (Oneotodus) dat- itouensis points to a slightly older Ordovician age; 413 362 sonensis closely resembles Teridontus nakamurai but is yet the available material is not sufficient to verify its 414 363 distinguished by the circular basal outline, the upturn- taxonomy. 415 364 ing of the oral termination of the cusp and the presence 365 of shallow furrows associated with carinas (Druce & 5.c. Conodont palaeoecology and palaeobiogeographic 416 366 Jones, 1971; Zeballo & Albanesi, 2009). Miller (1980) considerations 417 367 reassigned Oneotodus datsonensis Druce & Jones to 368 T. nakamurai and to Semiacontiodus nogamii Miller. The recognition of the Furongian Hirsutodontus 418 369 Nicoll (1994) rejected the latter suggestion as well as simplex Subzone of the Cordylodus intermedius Zone 419 370 the emended diagnosis given by Ji & Barnes (1994a), and the Tremadocian Macerodus dianae Zone in 420 371 as they are not applicable to the type species of the the Los Sombreros Formation allows the biostrati- 421 372 genus, which lacks lateral grooves, costae and keels. graphic correlation between the slope facies and the 422 373 Tolmacheva & Abaimova (2009) suggested introducing carbonate-platform domain to be improved, as both 423

CORD-MP 50750. (u) Teridontus nakamurai (Nogami), S element, lateral view, CORD-MP 50751. (v) Drepanoistodus sp., S element, lateral view, CORD-MP 50752. (w) Variabiloconus datsonensis (Druce & Jones), CORD-MP 50753, (w1) lateral view, (w2) basal cavity view. (x) Cordylodus cf. andresi Viira & Sergeyeva, S element, lateral view, CORD-MP 50754. (r–t, y) Conodont elements from the Macerodus dianae Zone (upper Tremadocian, Ordovician): (r) Rossodus cf. manitouensis Repetski & Ethington, S element, lateral view, sample AM8B, CORD-MP 50755; (s) Macerodus dianae Fåhraeus & Nowlan, lateral view, sample AM8A, CORD-MP 50756; (t) Scolopodus cf. subrex Ji & Barnes, lateral view, sample AM8B, CORD-MP 50757; (y) Paltodus aff. inaequalis (Pander), lateral view, sample AM8B, CORD-MP 50758. Scale bar: 0.1 mm. 8 G. G. VOLDMAN AND OTHERS

424 zones occur within the La Silla Formation (Lehnert, Santa Rosita formations in the Cordillera Oriental (Ze- 484 425 Miller & Repetski, 1997). In particular, Albanesi, ballo & Albanesi, 2009). 485 426 Cañas & Mango (in press) described thoroughly the Although conodont provinces can be already distin- 486 427 conodont association of the M. dianae Zone for the guished in the late Cambrian (Jeong & Lee, 2000), 487 428 shallow-water carbonates of the La Silla Formation, at most of the palaeogeographic studies are concentrated 488 429 the Cerro Viejo de San Roque section. Hirsutodontus in the Ordovician, when major realms were already 489 430 simplex, whose biostratigraphic range is restricted established by late Tremadocian times (Miller, 1984; 490 431 to the Cordylodus intermedius Zone, was originally Charpentier, 1984). Accordingly, the Precordillera is 491 432 defined in Australia (Druce & Jones, 1971) and identified as a conodont faunal Province of the Temper- 492 433 subsequently recovered from China (Chen & Gong, ate Domain of the Shallow-Sea Realm (or the Open-Sea 493 434 1986; Chen et al. 1988), Laurentia (Miller, 1980; Ross Realm depending on the sedimentary setting) (Albanesi 494 435 et al. 1997; Terfelt, Bagnoli & Stouge, 2012; Miller & Bergström, 2010; Serra & Albanesi, 2013), as it 495 436 et al. 2014), Siberia (Abaimova, 1971, 1975) and NW lacks the typical shallow-water, tropical forms charac- 496 437 Argentina (Zeballo & Albanesi, 2009). Miller (1984) teristic of the Laurentian, Australasian or North China 497 438 suggested that Clavohamulus and Hirsutodontus had a provinces (Bagnoli & Stouge, 1991; Zhen & Percival, 498 439 nektobenthic habit of life and that they preferred warm, 2003). Instead, it is distinguished by cosmopolitan or 499 440 shallow seas. The record of Hirsutodontus in the Los widespread faunas, showing a moderate endemism and 500 441 Sombreros Formation would then indicate reworking diversity when compared with faunas from the Tropical 501 442 of shallow-water deposits into deep-water facies by Domain. The Baltoscandian Province of the Cold Do- 502 443 gravity flows, as observed in the GSSP for the base of main presents lower diversities and higher abundances 503 444 the Ordovician at Green Point, Newfoundland (Cooper, instead (Zhen & Percival, 2003). 504 445 Nowlan & Williams, 2001; Miller et al. 2014). Ac- Macerodus dianae, an index taxon of the late 505 446 cordingly, Clavohamulus hintzei Miller, indicative of Tremadocian, appeared in sample AM8A. It was first 506 447 the upper subzone of Cordylodus intermedius Zone, described from outcrops of the Cow Head Group in 507 448 is present in the shallow-marine facies of the La Silla western Newfoundland, a series of Laurentian slope 508 449 Formation, at the eastern domain of the carbonate deposits fed from the outer shelf and the upper contin- 509 450 platform (Lehnert, Miller & Repetski, 1997). ental slope (Fåhraeus & Nowlan, 1978; Pohler, Barnes 510 451 The species Teridontus nakamurai has been found & James, 1987). Ji & Barnes (1994a) emended its dia- 511 452 in several lithofacies suggesting a pelagic habit of life gnosis with material from the Boat Harbour Forma- 512 453 (Ji & Barnes, 1994b), eventually restricted to the shelf tion (St George Group) on the Port au Port Penin- 513 454 environments owing to a nektobenthic habit (Miller, sula in western Newfoundland. Macerodus dianae is 514 455 1984). After studying a large conodont collection from recognized in widely separated geographic locations 515 456 the Cordillera Oriental, Zeballo & Albanesi (2013b) of the Great Basin (Ethington & Clark, 1981; Repet- 516 457 recognized an antithetical relationship between the cos- ski, 1982; Ross et al. 1997; Landing et al. 2012), the 517 458 mopolitan genera Variabiloconus and Teridontus, veri- Arctic Archipelago of Canada (G. S. Nowlan, unpub. 518 459 fying that the latter predominates in the deeper parts of Ph.D. thesis, Univ. Waterloo, 1976) and northern Nor- 519 460 the platform. In particular, V. datsonensis is present in way (Lehnert, Stouge & Brandl, 2013). In the Kechika 520 461 NE Australia (Druce & Jones, 1971), Antarctica (Bug- Formation of British Columbia, the Macerodus dianae 521 462 gisch & Repetski, 1987) and NW Argentina (Zeballo Zone is absent and is substituted by the shallow-water 522 463 & Albanesi, 2009). Scolopodus subrex Zone (Pyle & Barnes, 2002). 523 464 The genus Cordylodus was a major component of Rossodus is a typical genus from the Great Basin, 524 465 most slope and platform communities during Furong- characteristic of the North American Midcontinent 525 466 ian and Early Ordovician times. C. proavus has a wide- Province. The latter is approximately equivalent to 526 467 spread geographic distribution and is found in a wide the Laurentian Province of the Tropical Domain, in 527 468 range of lithofacies, which suggests a pelagic habit the Shallow-Sea Realm, distinguished by shelf areas 528 469 of life. In contrast, C. andresi, C. caboti and C. in- ˂ 200 m in depth with high endemism and diversity 529 470 termedius preferred deeper-water environments (lower (Ethington & Clark, 1981; Ross et al. 1997; Zhen 530 471 proximal to distal slope facies), as younger species & Percival, 2003). Rossodus manitouensis Repetski 531 472 of Cordylodus adapted to a nektobenthic mode of life & Ethington has also been documented in China (= 532 473 (Miller, 1984; Zhang & Barnes, 2004). ‘Acodus’ oneotensus sensu An, Du & Gao, 1985; 533 474 The C. intermedius Zone has a wide global distri- Wang, Bergström & Lane, 1996), Korea (Seo, Lee 534 475 bution and has been documented in China (Chen & & Ethington, 1994), Thailand (Agematsu et al. 2008) 535 476 Gong, 1986), Laurentia (Bagnoli, Barnes & Stevens, and Tasmania (R. C. Cantrill, unpub. Ph.D. thesis, 536 477 1987; Barnes, 1988; Miller, 1988; Ross et al. 1997; Univ. Tasmania, 2003). It is also known from the 537 478 Miller et al. 2003) and central Asia (Dubinina, 2000). peri-Gondwanan volcanic arc of the Famatina Sys- 538 479 A correlative conodont assemblage has been retrieved tem, where it occurs along with Drepanodus arcuatus, 539 480 from Australia (Druce & Jones, 1971) and Iran (Müller, Cornuodus longibasis (Lindström), Paltodus deltifer 540 481 1973). In Argentina, it was previously identified in pristinus (Viira), P.cf. subaequalis Pander and Parois- 541 482 the Volcancito Formation of the Famatina System (Al- todus numarcuatus (Lindström), which characterize a 542 483 banesi et al. 2005) and the Cardonal (Rao, 1999) and biofacies dominated by pelagic species from deep/cold 543 Conodonts from slump deposits of the Precordillera 9

544 waters (Albanesi et al. 2005). In the eastern domain of related heat source is not possible to discern as CAI 602 545 the Precordillera, Lehnert, Miller & Repetski (1997) values are relatively low, within the range of the ob- 603 546 described Rossodus aff. manitouensis from shallow- served values in the platform, which can be accounted 604 547 water facies of the La Silla Formation, along with Alox- for solely by sedimentary burial (Voldman, Albanesi & 605 548 oconus cf. propinquus (Furnish), Scolopodus cf. floweri Ramos, 2010). 606 549 Repetski, Paroistodus numarcuatus and Colaptoconus Alternatively, the mafic rocks from the Western Pre- 607 550 quadraplicatus (Branson & Mehl). The authors correl- cordillera produced very restricted thermal anomalies 608 551 ated this conodont assemblage with the Low Diversity in the country rocks given the small volume of single- 609 552 Interval and the lower Macerodus dianae conodont pulsed basalt intrusions, which could only slightly 610 553 biozone in North America (Ross et al. 1997), con- contribute to a regional increment of the heat flux 611 554 sistent with the suggested age for the samples AM8A (Voldman, Albanesi & do Campo, 2008; González- 612 555 and AM8B from the slope facies of the Los Sombreros Menéndez et al. 2013). This is consistent with the 613 556 Formation. metamorphic conditions inferred from the paragen- 614 557 The record of Macerodus dianae in the slope fa- etic associations of the mafic rocks, which suggests 615 558 cies verifies a strong link between the conodont faunas c. 250–350 ºC and 2–3 kbar, with palaeogeothermal 616 − 559 from the Precordillera with those from Laurentia dur- gradients of  30–35 ºCkm 1 (Robinson, Bevins & 617 560 ing Early Ordovician times, demonstrating a connec- Rubinstein, 2005), which are typical of mature pass- 618 561 tion along the borders of the Iapetus Ocean. Accord- ive margins and foreland basins (e.g. Allen & Allen, 619 562 ingly, Lehnert, Miller & Repetski (1997) interpreted 2005). Consequently, the CAI 3 in the studied con- 620 563 that the record of Clavohamulus hintzei in the La odont samples reflects a thermal history related to the 621 564 Silla Formation as well as the faunal similarities at sedimentary burial and nappe stacking of the Western 622 565 the species level with the shallow-water North Amer- Precordillera. 623 566 ican Midcontinent Province was a consequence of the 567 derivation of the Cuyania Terrane from the Ouachita 568 Embayment in Laurentia. Nevertheless, the conodont 6. Conclusions 624 569 faunas do not provide clear evidence to postulate a The new conodont data from the Los Sombreros Form- 625 570 geographic origin for the Precordillera as they show ation in La Invernada Range along with the sedimento- 626 571 dominantly Laurentian affinities again in the Middle logical and structural analysis carried out in the study 627 572 Ordovician, after a gradual immigration of conodonts area show that slope sedimentation and gravity slid- 628 573 from colder regions (Albanesi, 1998; Albanesi & ing has taken place in the Precordillera since at least 629 574 Bergström, 2010). late Cambrian times, contrasting with the current hy- 630 pothesis of a Devonian age for the slope deposits of the 631 575 5.d. Conodont preservation and palaeothermometry Los Sombreros mélange. Moreover, the recognition of 632 the Hirsutodontus simplex Subzone of the Cordylodus 633 576 The Cambrian specimens recovered from sample AM6 intermedius Zone (upper Furongian, Cambrian) and the 634 577 are well preserved and exhibit a conodont colour alter- Macerodus dianae Zone (upper Tremadocian, Ordovi- 635 578 ation index (CAI) 3 that provides some translucency to cian) improves the correlation with the La Silla Forma- 636 579 the conodont elements. The conodonts present smooth tion of the Precordilleran carbonate platform as well as 637 580 surfaces and scarce mineral overgrowths. Microfrac- with regions of Gondwana and other palaeocontinents. 638 581 tures are frequent and are responsible for the lack of The record of the index species Macerodus dianae in 639 582 apices on cusps and denticles. Conodonts recovered the Los Sombreros Formation, as well as Clavohamu- 640 583 from samples AM8A and AM8B also exhibit a CAI lus hintzei in the La Silla Formation, emphasizes the 641 584 3 but display a sugary texture with abundant quartz strong faunal affinity of the shelf environments of the 642 585 overgrowths instead. In this case, the different type of Precordillera with the Laurentian Province of the Trop- 643 586 textural alteration suggests variations in the intensity ical Domain for the late Cambrian – Early Ordovician 644 587 of the diagenetic processes. periods. However, given the wide global distribution of 645 588 Interestingly, previous findings of reworked pre- the studied specimens, the present data are not indic- 646 589 Floian conodonts recovered from the Los Sombreros ative of a geographic origin for the Precordillera. The 647 590 Formation display high CAI values in contrast to the thermal alteration of the studied specimens is consist- 648 591 elements recovered from the host rock. This fact was ent with the sedimentary burial and nappe stacking of 649 592 interpreted as a result of burial-related metamorphism the Western Precordillera. 650 593 and exhumation of the carbonate platform near the su- 594 ture zone of Cuyania with Gondwana, which supplied 595 detritus to the deep-water basin of the Western Precor- Acknowledgements. The authors thank Gian Andrea Pini 651 596 dillera (Voldman, Albanesi & Ramos, 2009). (Università degli Studi di Trieste) for helping with field work. 652 We appreciate Nigel H. Woodcock and an anonymous re- 653 597 The record of Furongian–Lower Ordovician con-  viewer for their constructive suggestions. This study was 654 598 odonts with CAI 3 ( 110–200 ºC; Epstein, Epstein & funded by CONICET, Argentina (RD3646/14, RD2827/15 655 599 Harris, 1977) in the Los Sombreros Formation reflects and PIP 11220130100447CO) and the Ministerio de Eco- 656 600 a simpler burial history instead, if a uniform palaeo- nomía y Competitividad, Spain (project CGL2012-34475). 657 601 geothermal gradient in the basin is considered. A rift- This paper is also a contribution to the IGCP Project 591. 658 10 G. G. VOLDMAN AND OTHERS

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