Late Ordovician Research Articles

Polychronous formation of the ophiolite association in the Tekturmas zone of Central Kazakhstan inferred from geochronological and biostratigraphic data

Plagiogranites and conodonts from chert intercalations in basalts of the ophiolite association in the Tekturmas zone of Central Kazakhstan were subjected to the U‒Pb geochronological and stratigraphic investigations, respectively. The age of plagiogranite crystallization is estimated to be 489 ± 8 Ma corresponding to the stratigraphic interval spanning from the uppermost Upper Cambrian to the lower Tremadocian. Conodonts from cherts of the Kuzek Formation are distributed along the section interval from the uppermost part of the Darriwilian (Middle Ordovician) to the lower part of the Sandbian (Upper Ordovician), which corresponds to the period of 457‒460 Ma. It is revealed that the formation of the ophiolite section in the Tekturmas zone was a multistage process lasting from the Late Cambrian to the initial Late Ordovician.

Rusophycus by Any Other Name Is Still Rusophycus carleyi

ABSTRACTRusophycus carleyi (James, 1885) is an ichnospecies that encompasses a variety of sizes and morphological variations that can be attributed to substrate-organism interaction(s) and taphonomic variation(s). R. carleyi ranges from the Cambrian to the Late Ordovician, with a cosmopolitan (global) distribution. Four ichnospecies are identified as junior synonyms of R. carleyi: morgati, moyensis, polonicus and radialis.

Depositional environment and organic matter accumulation of Upper Ordovician–Lower Silurian marine shale in the Upper Yangtze Platform, South China

The main controlling factors of organic matter accumulation in the Upper Ordovician Wufeng–Lower Silurian Longmaxi Formations are complex and remain highly controversial. This study investigates the vertical variation of total organic carbon (TOC) content as well as major and trace element concentrations of four Ordovician–Silurian transition sections from the Upper Yangtze Platform of South China to reconstruct the paleoenvironment of these deposits and to improve our understanding of those factors that have influenced organic matter accumulation in these deposits.The residual TOC content of the Wufeng Formation averages 3.2% and ranges from 0.12 to 6.0%. The overlying lower Longmaxi Formation displays higher TOC content (avg. 4.4%), followed upsection by consistent and lower values that average 1.6% in the upper Longmaxi Formation. The concentration and covariation of redox-sensitive trace elements (Mo, U and V) suggest that organic-rich intervals of the Wufeng Formation accumulated under predominantly anoxic conditions. Organic-rich horizons of the lower Longmaxi Formation were deposited under strongly anoxic to euxinic conditions, whereas organic-poor intervals of the upper Longmaxi Formation accumulated under suboxic conditions. Positive correlations between redox proxies and TOC contents suggest that organic matter accumulation was predominantly controlled by preservation. Barium excess (Baxs) values indicate high paleoproductivity throughout the entire depositional sequence, with an increase in the lower Longmaxi Formation. Increased productivity may have been induced by enhanced P recycling, as evidenced by elevated Corg/Ptot ratios. Mo–U covariation and Mo/TOC values reveal that the Wufeng Formation was deposited under extremely restricted conditions, whereas the Longmaxi Formation accumulated under moderately restricted conditions. During the Late Ordovician, the extremely restricted nature of ocean circulation on the Upper Yangtze Platform in tandem with enhanced stratification of the water column promoted anoxic conditions favorable for the preservation of organic matter. During Early Silurian time, organic matter accumulation was principally controlled by changes in sea level, which affected terrigenous flux, redox conditions, and the degree of nutrition recycling.

A geometric model of faulted detachment folding with pure shear and its application in the Tarim Basin, NW China

We present an improved geometric model of faulted detachment folding with pure shear that is characterized by core thickening and a ramp-discordant backlimb. The model includes a two-stage evolution: 1) detachment folding involving pure shear with fixed hinges, and 2) faulted detachment folding, in which the core of anticline thrusts above a break-through fault in forelimb by limb rotation. The growth strata patterns of the model are also discussed with respect to factors such as limb rotation, tectonic uplift rate, and sedimentation rate. A thrust-related fold, called a TBE thrust fold, in the Tarim Basin in NW China, is analyzed as an example of the theoretical model. The result indicates that the TBE thrust fold has undergone a two-stage evolution with shortening of a few hundred meters. Both the theoretical model and the actual example indicate that the shortening in the detachment folding stage takes up a large proportion of the total shortening. The structural restoration of the TBE thrust fold also provides new evidence that the formation of a series of thin-skinned structures in the SE Tarim Basin initiated in the Late Ordovician. The model may be applicable to low-amplitude faulted detachment folds.

On the causes of mass extinctions

The temporal link between large igneous province (LIP) eruptions and at least half of the major extinctions of the Phanerozoic implies that large scale volcanism is the main driver of mass extinction. Here we review almost twenty biotic crises between the early Cambrian and end Cretaceous and explore potential causal mechanisms. Most extinctions are associated with global warming and proximal killers such as marine anoxia (including the Early/Middle Cambrian, the Late Ordovician, the intra-Silurian, intra-Devonian, end-Permian, and Early Jurassic crises). Many, but not all of these are accompanied by large negative carbon isotope excursions, supporting a volcanogenic origin. Most post-Silurian biocrises affected both terrestrial and marine biospheres, suggesting that atmospheric processes were crucial in driving global extinctions. Volcanogenic-atmospheric kill mechanisms include ocean acidification, toxic metal poisoning, acid rain, and ozone damage and consequent increased UV-B radiation, volcanic darkness, cooling and photosynthetic shutdown, each of which has been implicated in numerous events. Intriguingly, some of the most voluminous LIPs such as the oceanic plateaus of the Cretaceous were emplaced with minimal faunal losses and so volume of magma is not the only factor governing LIP lethality. The missing link might be continental configuration because the best examples of the LIP/extinction relationship occurred during the time of Pangaea. Many of the proximal kill mechanisms in LIP/extinction scenarios are also potential effects of bolide impact, including cooling, warming, acidification and ozone destruction. However, the absence of convincing temporal links between impacts and extinctions other than the Chicxulub-Cretaceous example, suggests that impacts are not the main driver of extinctions. With numerous competing extinction scenarios, and the realisation that some of the purported environmental stresses may once again be driving mass extinction, we explore how experimental biology might inform our understanding of ancient extinctions as well as future crises.

Disentangling the record of diagenesis, local redox conditions, and global seawater chemistry during the latest Ordovician glaciation

The Late Ordovician stratigraphic record integrates glacio-eustatic processes, water-column redox conditions and carbon cycle dynamics. This complex stratigraphic record, however, is dominated by deposits from epeiric seas that are susceptible to local physical and chemical processes decoupled from the open ocean. This study contributes a unique deep water basinal perspective to the Late Ordovician (Hirnantian) glacial record and the perturbations in seawater chemistry that may have contributed to the Hirnantian mass extinction event. We analyze recently drilled cores and outcrop samples from the upper Vinini Formation in central Nevada and report combined trace- and major element geochemistry, Fe speciation (FePy/FeHR and FeHR/FeT), and stable isotope chemostratigraphy (δ13COrg and δ34SPy). Measurements of paired samples from outcrop and core reveal that reactive Fe is preserved mainly as pyrite in core samples, while outcrop samples have been significantly altered as pyrite has been oxidized and remobilized by modern weathering processes. Fe speciation in the more pristine core samples indicates persistent deep water anoxia, at least locally through the Late Ordovician, in contrast to the prevailing interpretation of increased Hirnantian water column oxygenation in shallower environments. Deep water redox conditions were likely decoupled from shallower environments by a basinal shift in organic matter export driven by decreasing rates of organic matter degradation and decreasing shelf areas. The variable magnitude in the record of the Hirnantian carbon isotope excursion may be explained by this increased storage of isotopically light carbon in the deep ocean which, in combination with increased glacio-eustatic restriction, would strengthen lateral- and vertical gradients in seawater chemistry. We adopt multivariate statistical methods to deconstruct the spatial and temporal re-organization of seawater chemistry during the Hirnantian glaciation and attempt to isolate the latent magnitude and global perturbation in the carbon cycle. We speculate, using a two component mixing model and residual estimates from principal component analysis, that the secular open ocean Hirnantian C isotope excursion possibly amounts to only ∼ +1.5‰. Such an increase could be mechanistically driven by the combination of sea-level fall, persistent deep water anoxia, and cooler glacial temperatures that increased the organic carbon burial efficiency in the deeper basins.

Insight from early coral–stromatoporoid intergrowth, Late Ordovician of China

One of the earliest endosymbiotic associations with stromatoporoids occurs in the Late Ordovician Xiazhen Formation of southeastern China. Bajgolia, an auloporid tabulate coral characterized by dichotomous branching due to longitudinal fission, is represented by free-living as well as endobiontic forms in various lithofacies representing a wide range of environments. Only two of 11 stromatoporoid genera (Clathrodictyon and Ecclimadictyon) hosted Bajgolia, mainly in reef and related facies. Bajgolia–stromatoporoid associations occur occasionally in the lower part of the formation, but eventually become persistent in the upper part. Such associations were initiated by larval settlement of the coral on the growth surface of the stromatoporoid. Growth of Bajgolia usually kept pace with its host, but the coral's ability to change growth direction and grow faster prevented its envelopment and termination by the stromatoporoid, allowing the establishment and recurrence of an ongoing endosymbiotic relationship between the two organisms. Endobiontic Bajgolia was able to survive with its corallites protruding from the host; in some cases, the growth form of the stromatoporoid changed in response to the coral. The relationships between Bajgolia and stromatoporoids were probably commensal, but there is also evidence for mutualism and/or parasitism. Bajgolia–stromatoporoid associations represent an important stage in the development of complex ecological relationships and community structure, prior to the common and widespread syringoporid (“caunopore tubes”)–stromatoporoid associations in the Siluro-Devonian.

A new Lagerstätte from the Late Ordovician Big Hill Formation, Upper Peninsula, Michigan

A new exceptionally preserved marginal marine biota is reported from the Late Ordovician Big Hill Formation of Stonington Peninsula in Michigan's Upper Peninsula. The new Lagerstätte hosts a moderately diverse fauna of medusae, linguloid brachiopods, non-mineralized arthropods and orthocone nautiloids, alongside dasycladalean green algae. The biota is similar to those of Lagerstätten from the Late Ordovician of Canada, revealing an extensive distribution of a distinctive marginal marine palaeocommunity in Laurentia at this time. The Big Hill biota extends the geographical range of exceptionally preserved Late Ordovician faunas in Laurentia and indicates that further examples remain to be discovered.

Earliest symbiotic rugosans in cystoporate bryozoan Ceramopora intercellata Bassler, 1911 from Late Ordovician of Estonia (Baltica)

The earliest known endobiotic rugose corals are recorded in the Katian of Estonia. Multiple rugosans were partially embedded in colonies of the cystoporate bryozoan Ceramopora intercellata Bassler, 1911, leaving only their apertures free on the bryozoan growth surface. Bodophyllum sp. and Lambelasma sp. are rugosans that formed a symbiotic association with C. intercellata which may have been mutualistic. Rugosans presumably benefitted from growth within the stable substrate provided by the bryozoan, while bryozoans presumably benefitted by protection against some types of predators. Symbiosis between rugosans and the bryozoan Ceramopora intercellata was most likely facultative.

An unusual sponge – microbe – synsedimentary cement framework in a Late Ordovician reef, Southampton Island (Nunavut, Canada)

A large, resistant buildup at the top of the Upper Ordovician (Hirnantian?) Red Head Rapids Formation on Southampton Island (Nunavut, Arctic Canada) is dominated by massive boundstone and cementstone facies. These massive facies have more in common with the sponge–microbial reefs that dominated worldwide in the Early Ordovician, including the following primary components: early calcified sponge material, microbial elements, and synsedimentary cement. A close spatial relationship between sponge and microbial framework elements suggests that a poorly preserved decaying sponge framework provided substrates for the attachment and development of microbes and that the microbes played essential roles as reef consolidators. Centimetre-scale colonial metazoans are present and locally intergrown with the sponge and microbial components. Other mound-dwelling invertebrates or calcareous algae are rare. Although altered now to calcite, cement fabrics suggest that aragonite was ubiquitous as seafloor precipitate. Prior to its subaerial exposure in the latest Ordovician, the Red Head Rapids Formation buildup developed on the margin of a shallow-marine evaporative epicratonic basin where a diverse community of reef-building metazoans was unable to flourish.

High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician.

It has been hypothesized that predecessors of today's bryophytes significantly increased global chemical weathering in the Late Ordovician, thus reducing atmospheric CO2 concentration and contributing to climate cooling and an interval of glaciations. Studies that try to quantify the enhancement of weathering by non-vascular vegetation, however, are usually limited to small areas and low numbers of species, which hampers extrapolating to the global scale and to past climatic conditions. Here we present a spatially explicit modelling approach to simulate global weathering by non-vascular vegetation in the Late Ordovician. We estimate a potential global weathering flux of 2.8 (km3 rock) yr−1, defined here as volume of primary minerals affected by chemical transformation. This is around three times larger than today's global chemical weathering flux. Moreover, we find that simulated weathering is highly sensitive to atmospheric CO2 concentration. This implies a strong negative feedback between weathering by non-vascular vegetation and Ordovician climate.

Xianella: a new mat‐forming calcified cyanobacterium from the Middle–Late Ordovician of North China

AbstractXianella hongii gen. et sp. nov. is described from the Middle–Late Ordovician of Shaanxi, China and interpreted as a calcified cyanobacterial sheath. Xianella filaments formed cable‐like strands that constructed thick fenestral layers. The specimens occur in metre‐sized limestone blocks, possibly derived from local collapse of a reefal platform margin. In combination with micrite, some of which is intraclastic and peloidal, Xianella created thick and extensive stacks of layered calcified fenestral fabric that appear to be synsedimentarily calcified open‐frame mat deposits. The fenestrae range from small, laminose and very irregular, to large equidimensional areas ~2 cm across. Fenestrae with rounded outlines resemble primary gas bubbles observed in present‐day microbial mats. These delicate fabrics are comparable in structure and quality of preservation with those of some Proterozoic silicified stromatolitic mats.

A Late Ordovician age for the Whirlpool and Power Glen formations, New York

A restudy of the palynology of the Whirlpool Formation and Power Glen Formation in New York (USA) yielded a diverse fossil assemblage with cryptospores, glomalean fungi, acritarchs, chitinozoans, scolecodonts, and small carbonaceous fossils. These new data, and particularly the presence of the chitinozoan index fossil Hercochitina crickmayi, combined with emerging stable carbon isotope data, suggest a Late Ordovician (Katian or Hirnantian) age for these formations, which is older than their previously suggested Silurian (Rhuddanian) age.

High-resolution images above the Pampean flat slab of Argentina (31–32°S) from local receiver functions: Implications on regional tectonics

In the flat slab region of the South Central Andes (∼31–32°S), geological observations suggest that the regional crustal structure is inherited from the accretion of different terranes during the Ordovician. These structures were later reactivated, first in extension during the Triassic and later in compression during the Andean uplift since the Miocene. Seismological observations confirmed that those fault structures extend to depth with décollement levels that accommodate current crustal shortening in the region. In order to get better insight on the regional tectonics we computed higher frequency receiver functions (RF) from local slab seismicity of intermediate ∼100 km depth. Using a common conversion point (CCP) stacking method we obtained cross sections showing high vertical resolution crustal structure at the transition between the Precordillera and the Frontal Cordillera. In addition we performed a joint inversion of our high frequency RFs with surface wave data from ambient noise tomography allowing us to constrain absolute seismic wave velocities. Our higher resolution images reveal more structural details down to a depth of 80 km and laterally over the flat slab in good agreement with previous studies. Our results help to better identify very shallow discontinuities in seismic velocities. Recent petrological analyses combined with our high-resolution RF structure correlates with a crustal mafic composition and partial eclogitization in the lower crust. We observe a shift in the crustal structure between the Precordillera (east) and the Frontal Cordillera (west). Regional seismicity and previously determined focal mechanisms superimposed over our images indicate this shifting is a thrust structure extending down to a depth of 40 km. Our results suggest the presence of a master fault between the Cuyania (Western Precordillera) and Chilenia (Frontal Cordillera) terranes that probably accommodates the crustal deformation in the Pampean flat slab region since the Late Ordovician.

Petrogenesis and tectonic implications of the high-K Alamas calc-alkaline granitoids at the northwestern margin of the Tibetan Plateau: Geochemical and Sr–Nd–Hf–O isotope constraints

The Alamas granitoid pluton in the eastern part of the Western Kunlun Orogen, the northwestern margin of the Tibetan Plateau, is composed of quartz diorite. Zircon separates from the pluton has SIMS U–Pb age of ∼446Ma. Rocks from the pluton have a narrow range of SiO2 (56.84–62.57wt%), MgO (1.76–2.94wt%), and total alkalis (Na2O+K2O=5.14–9.59wt%), and are metaluminous and high-K calc-alkaline to shoshonitic in composition. They are enriched in light rare earth elements (LREEs), with (La/Yb)N=14–25, and show weakly negative Eu anomalies. These rocks are relatively enriched in Sr (472–676ppm) and Ba (435–2388ppm), and depleted in Nb, Ta, Th, and Ti. Their εNd(t) values range from −6.4 to −8.4, and (87Sr/86Sr)i=0.7184–0.7200. Zircons from the pluton show εHf(t) values of −1.4 to −8.8, and δ18O=6.4–9.0‰. Geochemical data indicate that the granitoids were likely derived from the reworking of an ancient, deep crustal source, influenced by a minor mantle-derived component. Magmatic differentiation was dominated by the fractional crystallization of hornblende, biotite, and accessory minerals such as apatite, allanite, and Fe–Ti oxides. In summary, the Late Ordovician Alamas pluton is an I-type granitoid that was emplaced in a post-collisional environment, suggesting that this tectonic stage had already initiated prior to ∼445Ma.

Glacial onset predated Late Ordovician climate cooling

AbstractThe Ordovician glaciation represents the acme of one of only three major icehouse periods in Earth's Phanerozoic history and is notorious for setting the scene for one of the “big five” mass extinction events. Nevertheless, the mechanisms that drove ice sheet growth remain poorly understood and the final extent of the ice sheet crudely constrained. Here using an Earth system model with an innovative coupling method between ocean, atmosphere, and land ice accounting for climate and ice sheet feedback processes, we report simulations portraying for the first time the detailed evolution of the Ordovician ice sheet. We show that the emergence of the ice sheet happened in two discrete phases. In a counterintuitive sequence of events, the continental ice sheet appeared suddenly in a warm climate. Only during the second act, and set against a background of decreasing atmospheric CO2, followed steeply dropping temperatures and extending sea ice. The comparison with abundant sedimentological, geochemical, and micropaleontological data suggests that glacial onset may have occurred as early as the Middle Ordovician Darriwilian, in agreement with recent studies reporting third‐order glacioeustatic cycles during the same period. The second step in ice sheet growth, typified by a sudden drop in tropical sea surface temperatures by ∼8°C and the further extension of a single, continental‐scale ice sheet over Gondwana, marked the onset of the Hirnantian glacial maximum. By suggesting the presence of an ice sheet over Gondwana throughout most of the Middle and Late Ordovician, our models embrace the emerging paradigm of an “early Paleozoic Ice Age.”

First occurrence of Lesueurilla prima (Gastropoda) from the Middle Ordovician (Darriwilian) of Iran

The near planispiral euomphaloid gastropod Lesueurilla prima (Barrande in Perner) is a typical part of the cold water Mediterranean fauna in the Lower and Middle Ordovician. It is a signature species in strata of this age in the Prague Basin in the Czech Republic, Montagne Noire in France, and Central Morocco. The species is reported for the first time from the Darriwilian of Iran where it occurs in the upper part of the Shirgesht Formation of the Derenjal Mountains north of Tabas in east Central Iran. It occurs with a small species of the bellerophontoid gastropod Sinuites sp., as well as the calymenid trilobite Neseuretinus birmanicus (Reed) and the pliomerid trilobite Ovalocephalus kanlingensis (Zhang). The biogeographic distribution of Lesueurilla suggests that the genus had its origin in cold water, high-latitude peri-Gondwana areas during the Lower Ordovician, spreading to other peri-Gondwana areas and low-latitude areas of North China, Tarim, and finally Baltica, Laurentia and Australia in the late Middle and Upper Ordovician.

Cryptospore and trilete spore assemblages from the Late Ordovician (Katian–Hirnantian) Ghelli Formation, Alborz Mountain Range, Northeastern Iran: Palaeophytogeographic and palaeoclimatic implications

Well-preserved miospore assemblages are recorded from the Late Ordovician (Katian–Hirnantian) Ghelli Formation in the Pelmis Gorge, located in the Alborz Mountain Range, Northeastern Iran. The palynomorphs were extracted from siliciclastic deposits that are accurately dated using marine palynomorphs (acritarchs and chitinozoans). The spore assemblages consist of 14 genera and 28 species (26 cryptospores and 2 trilete spore species). Six new cryptospore species are described: Rimosotetras punctata n.sp., Rimosotetras granulata n.sp., Dyadospora asymmetrica n.sp., Dyadospora verrucata n.sp., Segestrespora iranense n.sp., and Imperfectotriletes persianense n.sp. The study furthers knowledge of the development of the vegetative cover during the Late Ordovician. Various and abundant cryptospores in the Late Ordovician (Katian–Hirnantian) Ghelli Formation are probably related to the augmentation of land-derived sediments either during the global sea-level fall linked to the Late Ordovician glaciation or adaptation of the primitive land plants in a wide range of climatic conditions. These miospore taxa were produced by the earliest primitive land plants, which probably grew close to the shoreline and were washed in from adjacent areas, producing a high volume of miospores. The associated marine palynomorphs consist of acritarchs (13 genera and 18 species), chitinozoans (9 genera and 10 species), prasinophycean algae, scolecodonts, and graptolite remains, which are not discussed in detail herein. The established chitinozoan biozones of this part of the Palaeozoic sequence are the Armoricochitina nigerica Biozone, the Ancyrochitina merga Biozone, the Tanuchitina elongata Biozone, and the Spinachitina oulebsiri Biozone, suggesting a Late Ordovician age (Katian–Hirnantian). These chitinozoan biozones are widely evidenced only in the peri-Gondwanan Domain, indicating that the study area was part of this palaeo-continent in the Late Ordovician.

Biotic invasion, niche stability, and the assembly of regional biotas in deep time: comparison between faunal provinces

AbstractBiotic invasions in the fossil record provide natural experiments for testing hypotheses of niche stability, speciation, and the assembly and diversity of regional biotas. We compare ecological parameters (preferred environment, occupancy, median abundance, rank abundance) of genera shared between faunal provinces during the Richmondian Invasion in the Late Ordovician on the Laurentian continent. Genera that spread from one faunal province to the other during the invasion (invading shared genera) have high Spearman rank correlations (>0.5) in three of four ecological parameters, suggesting a high level of niche stability among invaders. Genera that existed in both regions prior to and following the invasion (noninvading shared genera) have low correlations (<0.3) and suggest niche shift between lineages that diverged at least 8 Myr earlier. Niche shift did not accumulate gradually over this time interval but appears to have occurred in a pulse associated with the onset of the Taconic orogeny and the switch from warm-water to cool-water carbonates in southern Laurentia.

The ages and tectonic setting of the Faja Eruptiva de la Puna Oriental, Ordovician, NW Argentina

The Ordovician Faja Eruptiva de la Puna Oriental is a magmatic, predominantly intrusive belt in the Puna of northwestern Argentina with a N-S extension of ca. 400km. Scarce isotope geochemical ages and biostratigraphic data on some of the folded Faja Eruptiva country rocks assign the magmatism either to the Lower and lower Middle Ordovician, or to the latest Ordovician. Interpretations of origin and tectonic framework of the Faja Eruptiva are controversial and vary between arc, back-arc and collisional-orogenic settings.We present high-resolution La-ICP-MS U–Pb age and Hf isotope data on zircons from 10 plutonic samples covering the magmatic belt along a length of 200km in the northern Argentinian Puna. The xenocrystic and magmatic zircon age data have a wide spread between 2700Ma and 440Ma. Concordia and weighted mean age data document protracted magmatism in two phases between 480 and 460Ma, and between 453 and 444Ma, and constrain the time of the last intrusions at 444±3Ma and at 445±2Ma thus defining this last and main phase of intrusion at 444Ma.εHf(t) values define a main vertical trend centered at 500Ma with εHf(t) values between +3 and −16 indicating significant mixing of juvenile early Paleozoic melts with Paleoproterozoic crustal components. A second trend is formed by zircons with ages between 1.1Ga and c. 500Ma and predominantly positive εHf values between +8 and −3 and originates in juvenile mantle compositions between 1.6 and 1.1Ga. The spread of the zircon and Hf data document that the Faja Eruptiva intrusives have experienced large-scale contamination by the hosting crustal basement. It follows that the basement of the Puna is formed either by the upper Proterozoic-lower Cambrian Puncoviscana Formation as an erosional product of the Proterozoic orogenic belts of SW Amazonia or that the Puna including its Puncoviscana basement is underlain by a crust shaped by these orogenies.The main intrusive event at 444Ma has been linked to the Oclóyic orogeny in the Late Ordovician. The plutons intruded very likely in a sinistral strike-slip regime after the main folding phase of the Oclóyic orogeny had deformed the Ordovician sedimentary country rocks.