La Laja Research Articles

Cambrian sponge spicules and Chancelloriid sclerites from the Argentine Precordillera: A Review

Spicules of sponges and sclerites of chancelloriids have been collected from in situ Cambrian sections of the La Laja Formation (San Juan Precordillera). They have also been collected from carbonate olistoliths, which were emplaced from the Cambrian units in the Early Ordovician Los Sombreros Formation (San Juan Precordillera) and Empozada Formation (Mendoza Precordillera). The Cambrian La Laja Formation sequences were deposited in a shallow carbonate platform in the eastern part of the Precordillera. The Cambrian carbonate rocks, which make up the olistoliths in the Los Sombreros and Empozada Formations, record deposition in the slope environments which developed in the western Precordillera. Cambrian Porifera and sponge-like chancelloriid assemblages are known mainly from fragments of skeletal nets, dissociated spicules and sclerites, which enable us to define two assemblages. The first assemblage of spicules was defined from the upper Lower to Middle Cambrian sequences of the La Laja Formation. This assemblage consists of a variety of stauractines and sclerites of Chancelloria WALCOTT. The family Protospongiidae is represented by triradiate prodianes, pentactines and hexactines, all belonging to Kiwetinokia WALCOTT. The second assemblage, which consists of isolated hexactines, pentactines, monaxons and skeletal nets, was defined in the Cambrian derived olistholiths of the Los Sombreros and Empozada Formations. Early hexactinellid Protospongiidae with body preservation (Diagoniella ? and Kiwetinokia) also occur. Demosponges have a very limited record in the Cambrian of the Precordillera. Anthaspidellid sponges had been reported from the La Laja Formation and now from the San Martin olistolith (Empozada Formation). The occurrences of these two assemblages of spicules and sclerites in both the Cambrian platform and slope facies of the Precordillera contribute additional data for a better understanding of the relationships between the eastern and the western facies assemblages, which developed in the Precordillera during the Cambrian. Therefore, they are useful for paleoenvironmental and paleogeographical interpretations of the Cambrian depositional framework.

Systematic, morphometric and palaeobiogeographic study of Blainia gregaria Walcott, 1916 (Trilobita, Ptychopariida), Middle Cambrian of the Precordillera of western Argentina

The ptychoparioid trilobite Blainia gregaria Walcott, 1916 is described for the first time from the late middle Cambrian (Series 3) of the Argentine Precordillera. It occurs commonly in the upper part of La Laja Formation, a carbonate platform succession that crops out in the Precordillera Oriental of San Juan where it reaches a thickness of some 600 m. Sections were sampled on cerro Tres Marías in Sierra de Marquesado and in quebrada de Zonda at the northern termination of Sierra Chica de Zonda. More than 1200 specimens were recovered from 22 stratigraphic levels over a thickness of 155 m, located in the uppermost 15 m of the Soldano Member, the approximately 100 m thick Rivadavia Member, and in the lower 40 m of the Juan Pobre Member. Our biometric study dealt with 205 cranidia and 365 pygidia from cerro Tres Marías. Measured dimensions of a subset of this collection provide bivariate datasets that in some characters show broad scatter plots and relatively low linear correlation coefficients, indicative of rather wide intraspecific morphological variation. Six morphotypes (three cranidial and three pygidial), recognized mainly on the basis of shape, occur together in the same collections. This evidence for an unusually high degree of variability leads to a proposed synonymy of some 30 previously named species in North America belonging to two genera, Blainia Walcott, 1916 and Glyphaspis Poulsen, 1927, the latter considered a junior synonym. The biogeographic distribution of B. gregaria, as redefined here, shows that it was an endemic Laurentian species that inhabited the inner parts of the carbonate platform that rimmed the craton and mantled portions of its interior. Its presence in western Argentina is further evidence for the very close faunal relationship between the Precordilleran terrane, known as Cuyania, and Laurentia during the Cambrian. Copyright © 2012 John Wiley & Sons, Ltd.

Delamaran Trilobites from the La Laja Formation, San Juan, Argentina

The La Laja Formation lies exposed almost continuously along the western margin of two hill ranges, i.e., the Sierra Chica de Zonda and Sierra de Villicum. This limestone unit bears a remarkable fauna of Cambrian trilobites. In this work we analyse the Delamaran trilobite fauna from the upper part of the El Estero Member and the lower third of the Soldano Member of the La Laja Formation. Taxa described are Glossopleura walcotti Poulsen, Glossopleura lodensis (Clark), Fieldaspis sp., and Prozacanthoides zondaensis n. sp. The Poliella denticulata, Mexicella mexicana?, and Glossopleura walcotti Zones are documented; these had been previously included in a hiatus. Trilobites-based correlations are established between the La Laja Formation and olistoliths of the Western Precordillera. Additional correlations are also suggested between the Precordillera and different localities in Laurentia. Finally, we discuss the biogeographic implications of the described material. Delamaran taxa reported from the La Laja Formation are characteristic and so far restricted to Laurentia. Analysis of the trilobite fauna from this unit supports the allochtonous origin of the Precordillera.

Sedimentology and paleoenvironments of the La Laja Formation (Cambrian), Quebrada La Laja, Sierra Chica de Zonda, San Juan, Argentina.

The La Laja Formation is a key unit regarding the hypothesis of the Argentine Precordillera as a Laurentia-derived allochtonous terrane in the south central Andes. Together with the Cerro Totora Formation it comprises the oldest unit exposed at the base of the Lower Paleozoic carbonate platform of the Precordillera. According to previous work these units record the rifting-drifting history of this terrane exotic to Gondwana. The La Laja Formation contrasts with the rest of the overlying units of the Cambro-Ordovician carbonate platform by being partly mixed carbonate-siliciclastic. A detailed facies analysis of the five recognized members in the reference section at Quebrada La Laja (Sierra Chica de Zonda, San Juan Province) allow the recognition of 19 microfacies grouped into four main environmentally significant associations: 1) a storm-influenced, deep subtidal environment with variable influx of fine siliciclastic sediments; 2) shallow subtidal, 3) mixed shallow subtidal and 4) shallow subtidal to intertidal occasionally with well developed high-frequency tidal flat cycles. No deeper basinal or slope facies were found. In contrast, the unit largely records shallow-marine facies with some evidence of subaerial exposure. Medium to coarse, calcareous, feldspar-rich sandstones and sandy limestones characterize the El Estero Member. The base of the Soldano Member shows profuse development of cyclicity with capping oolitic shoals. This member and the upper Juan Pobre Member contain deeper subtidal intervals, below the storm weather wave base, with high percentages of fine terrigenous material represented by shaly marls and nodular limestones. Skeletal-rich and oolite-rich storm beds are recorded within them. Relative shallowing to subaerial environments recorded within the Rivadavia and Las Torres members, with the development of ribbon limestones, intraclastic rudstones, microkarstic surfaces and exposure breccias. Herringbone cross-bedded oolite shoals are common at the top of the uppermost Las Torres Member. The transition into the overlying Zonda Formation is represented by an abrupt rearrangement of the depositional systems and development of meter-scale microbial-rich peritidal cycles. Several orders of superposed cyclicity are recorded within the La Laja Formation. Larger-scale cycles ('Grand Cycles') are inferred from alternating members with important amounts of siliciclastics, whereas meter-scale cycles indicate higher frequency superposed mechanisms. Altogether these show a complex depositional history linking environmental and tectono-eustatic signatures.

El Puerto de La Laja (Huelva), como Bien de Interés Cultural en la categoría de Sitio Histórico

The Harbor of La Laja, it is a real example of recovery architecture of an ancient miner village for rural tourism exploitation. It is one of the possible bases of actions in the region of mines of the province of Huelva, to offer an alternative to sun and beach tourism, for knowledge of other cultures and as a development plan of areas that have been affected by the decline of mining in our province. Part of the village of the Harbor of La Laja has been currently restored for its exploitation for rural tourism, which is why the architecture in a way has tried to coordinate former arrangement of houses with fulfilled improvements for this type of use.

Detrital zircon analysis from the Neoproterozoic–Cambrian sedimentary cover (Cuyania terrane), Sierra de Pie de Palo, Argentina: Evidence of a rift and passive margin system?

Fil: Naipauer, Maximiliano. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Trilobite-based biostratigraphic model (biofacies and biozonation) for the Middle Cambrian carbonate platform of the Argentine Precordillera

A preliminary biostratigraphic model that combines trilobite biofacies and biozonation is proposed for the early Marjumian (Middle Cambrian) of the Precordillera of western Argentina. This interval involves a variety of paleogeographic–paleoenvironmental settings across the carbonate platform, ranging from its interior to exterior areas beyond the platform margin and including both open-marine and near-shore environments. This model is based on three stratigraphic sections of the basal part of the Alojamiento Formation, which crops out in the Cordon del Alojamiento (Provincia de Mendoza) and Cordon de Santa Clara (Provincia de San Juan); and three sections of the Soldano Member of La Laja Formation in the Sierra Chica de Zonda (Provincia de San Juan). Three biofacies were recognized on the basis of trilobite associations that show a marked environmental distribution: 1) The Ptychagnostidae Biofacies is represented by an assemblage exclusively dominated by agnostoids, most of them belonging to Ptychagnostus; this assemblage occurs in mudstones and calcareous shales deposited below storm wave-base seaward of the platform margin; 2) the Peronopsidae Biofacies also developed in the external areas but it occurs in limestones deposited between fair-weather and storm wave-base; this assemblage is likewise dominated by agnostoids, but instead belonging to Peronopsis, with the rest of the fauna comprising polymeroids (Dolichometopidae, Oryctocephalidae, Zacanthoididae and Alokistocaridae); 3) the Alokistocaridae Biofacies occurs in grainstones, mudstones and calcareous shales deposited between fair-weather and storm wave-base under changeable conditions located near the platform margin and in the interior. It is dominated by alokistocarids whose relative abundance increases landward from 52% to 58% and to 95%. The remaining components belong to Dorypygidae, Zacanthoididae and Oryctocephalidae; Eodiscidae and Peronopsidae comprise minor proportions in the most seaward location. Due to the contrasting, environmentally mediated associations, these sections belong to three essentially synchronous biozones erected previously for the Middle Cambrian of western Laurentia, the Ptychagnostus gibbus, Oryctocephalus and Ehmaniella Biozones.

Paleoenvironmental and Carbon-Oxygen Isotope Record of Middle Cambrian Carbonates (La Laja Formation) in the Argentine Precordillera

Abstract The La Laja Formation (Early to Middle Cambrian) is one of the oldest units exposed at the base of the lower Paleozoic carbonate platform of the Argentina Precordillera. This is a key unit regarding the hypothesis of the Precordillera as a Laurentia-derived allochthonous terrane currently located in the south-central Andes. According to the faunal affinity and stratigraphic development of the thick Cambrian carbonate bank, the Argentine Precordillera would have been attached to Laurentia. The La Laja Formation contrasts with the rest of the overlying units of the Cambro-Ordovician carbonate platform by being partly mixed carbonates–siliciclastics. This dominantly shallow subtidal unit is internally arranged into several Grand Cycles indicating a complex environmental mosaic, probably with local depocenters related to variable subsidence. This unit records the stabilization of the rifted margin of the Precordillera terrane, prior to the broadening of the carbonate sedimentation during the passive-margin drifting stage. A high-resolution δ13C and δ18O isotope study, in concert with a detailed paleoenvironmental analysis, was carried out to better understand both environmental and chronostratigraphic evolution of the La Laja Formation. Three δ13C positive excursions were recorded; the first one at the Glossopleura biozone within the Soldano Member, the second beginning at the base of the Rivadavia Member, and the third during deposition of the Las Torres Member. Comparisons with other Middle Cambrian curves, in the Precordillera and elsewhere (Rocky Mountains and Great Basin, U.S.A., the western Hunan Province in south China, and the Amadeus, Georgina, and Daly basins in Australia) suggest a global control on these excursions. Mechanisms to produce these positive excursions could be related to high bio-productivity and increased burial of Corg (organic carbon) produced by high nutrient influx to the ocean associated with a relative sea-level fall. Local environmental controls could have in part altered the original isotopic signal.

Tsunamis in a Stormy Sea: Middle Cambrian Inner-Shelf Limestones of Western Argentina

Abstract Coarse-grained strata deposited on continental shelves and epeiric seas in some tens of meters' water depth are conventionally ascribed simply to storms of varying intensity which interrupted the settling out of suspended mud. Surprisingly, the possibility of tsunamis is virtually never evaluated even though they might, indeed should, have been common in many regions. Portions of the Middle Cambrian La Laja Formation of the Argentine Precordillera formed in such a submerged epeiric platform or shelf. They comprise a suite of three limestone facies: variably argillaceous burrowed mudstone, bioclastic grainstone, and intraclastic conglomerate composed of pieces of mudstone including cemented haloes around burrows. Sporadically developed eocrinoid meadows and trilobite populations are recorded respectively by disarticulated ossicles and usually broken sclerites. The latter indicate that, generally, exoskeletons became fragmented by repeated bottom turbulence. Hence, the mudstones and grainstones show that, before stabilization by calcite cementation which began beneath the sediment–water interface and away from burrow margins, physical reworking of the surface sediment by storms was so frequent that it did not leave a specific signature of individual events. By contrast, scattered lenticular beds of intraclastic conglomerate were due to anomalous, brief episodes of deep scour and strong oscillatory flow from powerful wave action. These conglomerates then escaped subsequent reworking. Because they are abruptly interbedded and encased within uniform mudstone, and there are no stratigraphic patterns, allochthonous material, or unconformities to indicate sea-level fluctuation, the events that created the conglomerates were more likely tsunamis, not storms. They could have been unleashed by faulting elsewhere on the shelf or rifting beyond the shelf margin to the (present-day) west to southwest. Those originating in the rift basin may have been diffracted by marginal shoals and attenuated by the shallowness of the platform which caused the waves to break some distance offshore. In any case, the study area was not affected by backwash. Recognition of tsunami-laid beds urges a reexamination of the standard tempestite paradigm with its use of coarse-grained strata as a simple measure of storm frequency and intensity.

Zircon and whole‐rock Nd‐Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera

AbstractGraptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛNd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; TDM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (<30%) component that was likely derived from a high‐μ (U/Pb) terrane.

Napaeus lajaensis sp. nov. (Gastropoda: Pulmonata: Enidae) from a Quaternary Aeolian Deposit of Northeast Tenerife, Canary Islands

Napaeus lajaensis sp. nov. is the oldest Napaeus species found in the Canary Islands, with more than 130 ka. It is described from a Pleistocene aeolian deposit intercalated between two basaltic lava flows located at Mancha de La Laja (Tenerife Island). The new species is characterized mainly by the presence of two very prominent, spiral, semicylindrical ribs on the body whorl shell. The stratigraphic setting and taphonomic features of the land snail association to which N. lajaensis belongs, were also shown.

Source characterization of the San Juan (Argentina) crustal earthquakes of 15 January 1944 (M w 7.0) and 11 June 1952 (M w 6.8)

The backarc region of the Andes in the vicinity of San Juan, Argentina, is one of the most seismically active fold and thrust belt regions in the world. Four large damaging crustal earthquakes (1894, 1944, 1952 and 1977) occurred during the last 111 yr between 30°S and 32°S. We have determined the source parameters for two of these important earthquakes, the 1944 and 1952 events, using historic seismic records. The earthquake on 15 January 1944 had an epicentral location between the eastern thin-skinned Precordillera fold and thrust belt and the thick-skinned Sierras Pampeanas basement-cored uplifts. The 11 June 1952 earthquake occurred in the eastern Precordillera about 35 km southwest of the 1944 epicenter location. The P-wave first motions, long-period teleseismic P waveform modeling, and SV/SH amplitude ratio indicate a thrust focal mechanism for the 1944 event (strike N45°E, dip 35° to the southeast, and rake 110°) with M 0 = 3.01 × 10 19 N m and M w = 7.0. The 1952 earthquake focal mechanism solution indicates a more oblique mechanism (strike N40°E, dip 75° to the southeast, and rake 30°) with M 0 = 2.20 × 10 19 N m and M w = 6.8. Both the 1944 and 1952 earthquakes have focal depths < 12 km and simple source time functions with one pulse of moment release with durations of 10 s and 8 s, respectively. Both the shallow focal depth and the east-dipping fault plane in the focal mechanism solution for the 1944 earthquake are consistent with the parameters observed along the La Laja fault in the frontal part of the eastern Precordillera that generated a 6–8-km-long coseismic surface rupture. The 1952 earthquake focal mechanism solution and its shallow source depth suggest it is related to faults in the eastern Precordillera, but a particular fault association is difficult. The 1944 earthquake was clearly the most destructive event because its proximity to the most populated area in San Juan, large size and shallow focal depth.

The Early Paleozoic history of the Cuyania (greater Precordillera) terrane of western Argentina: evidence from geochronology of detrital zircons from Middle Cambrian sandstones

U-Pb geochronology of large detrital zircons populations is a powerful tool for interpreting sandstone provenance. Here, it is applied to three Middle Cambrian sandstones from the Precordillera of Argentina with the purpose of using the provenance interpretations to test paleogeographic and paleotectonic models proposed for the Cuyania or Precordillera terrane. Two samples from the La Laja Formation have distinctive detrital zircon age distributions. All zircon grains fall within unimodal populations of 1688-1200 Ma in one sample and 1559- 1316 Ma in the other. Of these grains, 23% and 65%, respectively, are within the age range of the North American magmatic gap (1610-1490 Ma), indicating a non-Laurentian provenance. A very different sample was taken from a sandstone interval in a large olistolith within the Estancia San Isidro Formation. Its zircon population is dominated by a single, prominent 615-511 Ma age cluster, which is indicative of a provenance in a Brasiliano orogenic belt. The absence of zircons with Grenvillian ages (1200 to 950 Ma) is difficult to reconcile with paleogeographic and geotectonic models in which Cuyania rifted from Laurentia in Cambrian or Ordovician time. The data are most consistent with models in which Cuyania rifted from the southern margin of West Gondwana. Given a Cambrian association with Gondwana and a post-Ordovician arrival at its present position in Gondwana, the Cuyania terrane must have migrated along the southern and western margins of Gondwana during the Ordovician Period.

C and Sr isotopic evolution of carbonate sequences in NW Argentina: Implications for a probable Precambrian-Cambrian transition

Upper Precambrian-Lower Cambrian sequences in the Tucuman, Salta and Jujuy provinces, NW Argentina, comprise sandstone, slate, conglomerate and black limestone (Las Tienditas/Volcan Fms.) with abundant Vendian/Tommotian trace fossils in the clastic facies rocks. The Precordillera basin, San Juan province, represents a continuous carbonate sequence belonging to the Lower to Middle Cambrian La Laja Fm. The Pie-de-Palo Range, Pampean Range, characterized by carbonates intercalated with greenschist/amphibolite facies metaclastic rocks, forms a part of the Precordillera basement. The δ13C values in carbonates of the Las Tienditas Fm. show a gradual decrease from the base (+3.4 ‰PDB) to the top with a minimum of −1.6‰ observed at ∼15m from the top, the latter having a higher clay content. Carbonates in a 700m thick section within La Laja Fm. is marked by a slightly positive δ13C values at the base (marly) with a negative anomaly (−2.0‰) at ∼20 m above, followed by a small positive anomaly (+0.5‰) ∼100 m from the base. All the values above this point are around −0.5‰ with a negative anomaly (−2.0‰) recorded at ∼240m above the base. Seawater87Sr/86Sr values define a non-monotonic increase (0.70870–0.71082) through the carbonates in Las Tienditas Fm. while the La Laja Fm carbonates vary from 0.70926 to 0.71030, with higher values at the base. C and Sr isotopes, thus suggest that the Las Tienditas carbonates record the Precambrian-Cambrian transition (∼15m from the top of studied section). The same is also evident at ∼30 m from the base of the La Laja Fm. The narrow range of δ13C variation (−1.4 to +1.3‰) and87Sr/86Sr in the 0.709–0.710 range for the Caucete Group carbonates of the Pie-de-Palo Range, although unequivocally, appear to be in consonance with a Vendian to Tommotian age, reinforced by the presence of the trace fossils Didymaulichnus and Gordia.

Quaternary alluvial terraces in an active tectonic region: the San Juan River Valley, Andean Ranges, San Juan Province, Argentina

The San Juan River, located in San Juan Province (Argentina), crosses the Precordillera and other geologic units including the Ullum tectonic valley and the La Laja Zone between latitudes 31°S and 32°S. The San Juan River is antecedent as is suggested by its two perpendicular segments linked by an almost parallel segment to the main structural trend. Along the Precordillera, the San Juan River valley has many different alluvial fans at the river junctions with its tributaries. The Quaternary alluvial fans display surfaces cut in a series of steps which we consider to be alluvial terraces generated by aggradation and repeated incision episodes. The studied sector includes one area with recent major seismic activity (La Laja Zone), another without major seismic activity (Precordillera area), and a subsident area (Ullum area) where a large lake was formed 6500yr BP. The old San Juan River was captured by the Quebrada de Ullum valley by means of a 25-m incision, which resulted in river-gradient headward erosion. The San Juan River gradient shows some irregularities that, although unrelated to the main structures, are associated with river dynamics, which emphasizes lithologic differences. The main river valley width, the geometry and gradient of each tributary, together with the basement rock lithologies and the size of each local source area are the major factors which control the alluvial terrace generation processes. In the La Laja Zone, where the uppermost terrace is capped by travertine, dating of travertine deposits suggests that the maximum incision rate is 0.9–1mm/yr related to episodic activity on the La Laja Fault.

High bedrock incision rates in the Atenguillo River valley, Jalisco, Western Mexico

Lava flows from three basaltic shield volcanoes preserve Pliocene–Pleistocene river levels in the Atenguillo River basin, western Mexico. K–Ar dates of these basalt flows, together with present and palaeoriver levels, allow calculation of bedrock incision rates at three points along the length of the Atenguillo River: at Volcan La Laja, dated at 0·65 Ma, incision rates are 25 cm ka−1; at Volcan La Cienega (2·2 Ma) incision rates are 23 cm ka−1; and at Volcan El Vigia (2·7 Ma) incision rates are 23 cm ka−1. These high incision rates, as well as two distinct knickpoints along the profile of the Atenguillo River, are related to a base level c hange at the northern end of the basin. The dynamics of this river basin are controlled by the ongoing process of continental fragmentation associated with the opening of the Gulf of California. © 1997 by John Wiley & Sons, Ltd.

Sponges and chancelloriids from the Cambrian of western Argentina

Anthaspidellid sponges are reported from the La Laja Formation in the Chica de Zonda Range in the Precordillera Oriental, Argentina. These are generically unidentifiable fragments of trabs and more or less well-defined dendroclone spicules. They occur as skeletal elements in small transported fragments and are the only Cambrian occurrence of anthaspidellids known thus far from South America.Walcott (1920) proposed to include an array of dissociated spicules, including triradiate prodiaenes, hexactines, and probable monactine spicules, in the genus Kiwetinokia. Assemblages tentatively identified as Kiwetinokia utahensis? Walcott, 1920, are reported from the Estancia San Martin Formation, of latest Early Cambrian and early Middle Cambrian age from San Isidro Gulch near Mendoza. Similar spicule assemblages occur in the La Laja Formation from the Chica de Zonda Range, in the Precordillera Oriental near San Juan, and in the Los Sombreros Formation in the Tontal Range of the Precordillera Occidental.Sclerites of the sponge-like Chancelloria eros Walcott, 1920, are described from the upper Lower to lower Middle Cambrian La Laja Formation. Chancelloria sclerites are also reported here from rocks of the Middle Cambrian Glossopleura Zone in the San Isidro Formation in Empozada Gulch in the San Isidro area of the Precordillera Austral, west of Mendoza, in Mendoza Province. Sponges and chancelloriids from the Cambrian of Argentina are known basically from dissociated skeletal elements.

Trace fossils from a carboniferous turbiditic lake: Implications for the recognition of additional nonmarine ichnofacies

A high‐diversity ichnofauna from a Late Carboniferous turbiditic glacial lake at Cantera La Laja, northwest Argentina, includes Aulichnites cf. A. parkerensis, cf. Bergaueria isp., Circulichnis nton‐tanus, Cochlichnus anguineus, Gordia marina, Haplotichnus india‐naensis, Helminihoidichnites tenuis, Helminthopsis tenuis, Mermia carickensis, Orchesteropus atavus, Palaeophycus tubularis, Planolites beverleyensis, Rusophycus isp., Treptichnus pollardi, Undichnia britannica, Vndichnia insolencia, string pits, and rhomboidal traces. Fifteen forms are present in fine to very fine sandstones and siltstones deposited by underflow currents, muddy turbidity currents, and suspension sedimentation in the deepest parts of the lake. Underflow and turbidity currents provided food and oxygen to deep‐lake settings, allowing the development of a rich biota. Alternation of fine‐grained laminae favored preservation of delicate and very small biogenic structures. The ichnofauna analyzed is remarkably different from the Sco‐yen...

Hawaiites and related lavas in the Atenguillo graben, western Mexican Volcanic Belt

Hawaiite and mugearite, alkali-olivine basalt, and basaltic andesite have erupted from three shield volcanoes in the Atenguillo graben, Jalisco, Mexico. The youngest shield volcano, La Laja (0.66 Ma), is the focus of this study. Basal pyroclastic surge deposits of basaltic ash, lapilli, and blocks are overlain by seven to eight, 5- to 10-m-thick hawaiite flows. Pillow lavas along a 4-km stretch on the southeast flank of the volcano and lake-bed sediments define a shoreline and lake bottom southward to the town of Atenguillo. Eruption of La Laja apparently dammed the northward-flowing Atenguillo River and formed a lake of as much as 100-m depth. Subsequent fluvial erosion has destroyed the lava dam, exposing the shield succession. Discharge rates for the Atenguillo River and rainfall estimates for the Atenguillo basin both suggest that this lake formed within 20 to 40 yr. Eruption of nearly 10 km 3 of material within 10 to 100 yr indicates an output rate of 0.1 to 1 km 3 /yr, which is larger than any rate calculated for volcanoes in the Mexican Volcanic Belt (MVB), but similar in magnitude to that of Mauna Loa and Kilauea volcanoes (Hawaii). The older volcanoes, El Vigia (2.7 Ma) and Tin Cleto (3.6 Ma), are composed of basaltic-andesite and alkali-olivine basalt, respectively. The composition of these lavas is unusual for a continental-arc setting. Total FeO, TiO 2 , and alkali concentrations are higher than those of basic calc-alkaline lavas from the western MVB. Alkaline earth/light-rare-earth and light-rare-earth/high-field-strength element ratios in these lavas are low relative to those of calc-alkaline lavas but similar to those of oceanic-island basalt values. Such compositional traits coupled with a high magma-output rate are characteristic of oceanic-island volcanoes. Both hydrous, fluid-enriched, sub-arc mantle, and oceanic-island-type mantle components are present beneath western Mexico; contemporaneous subduction and extension of this region of the western MVB allow tapping of both sources and thus the eruption of compositionally diverse lavas.

Birds of a Tropical Deciduous Forest in Extreme Northwestern Peru

From 14 June to 5 July 1979 we studied the avifauna at two localities in Depto. Tumbes, Peru, near the Ecuadorean border. We obtained specimens of six species and 1 5 subspecies new for Peru, and collected specimens and natural history information on such poorly known taxa as Ortalis erythroptera, Synallaxis tithys, Automolus erythrocephalus, and Thamnophilus zarumae. RESUMEN. Entre el 14 de junio y el 5 de julio de 1979 estudiamos la avifauna de dos localidades cercanas a la frontera con Ecuador en el Departamento de Tumbes, Peru. Obtuvimos especimenes de seis especies y 1 5 subespecies nuevas para el Peru y coleccionamos especimenes e information sobre la historia natural de taxa poco conocida como: Ortalis erythroptera, Synallaxis tithys, Automolus erythrocephalus y Thamnophilus zarumae. In South America west of the Andes, the ranges of many humid tropical forest bird species extend from Colombia south to western Ecuador. The climate of this narrow, coastal corridor becomes much drier in southwestern Ecuador, and many species found in the more humid forests to the north are absent. The dry area extends from northwestern Peru in the Deptos. of Piura and Tumbes into west-central Ecuador, with a narrow arm extending into coastal northwestern Ecuador, and is bounded on the east by the Andean Cordillera. Noting the high endemism of birds, as well as other organisms, in the region, Chapman (1926) designated them as the Equatorial Arid Fauna. This region has been studied very little by ornithologists in recent years. Jelski and Stolzmann visited Tumbes in 1876, going on to collect at Lechugal, and in nearby Ecuador at Palmar (= Palmales; Taczanowski 1877). Collecting in the region was not extensive, however, until just after the turn of the century when several expeditions from the American Museum of Natural History worked in the area of Ecuador just north and east of what is now the Depto. Tumbes, Peru. Collectors from the American Museum spent about 200 field-days in the area between 1913 and 1922 (Chapman 1926). M. A. Carriker, Jr., later collected at La Laja, Peru, now near the Ecuadorean border (Bond 1945). From 14 June to 5 July 1979 a field party (the authors, J. W. Eley, and Manuel Sanchez S.) from the Louisiana State University Museum of Zoology (LSUMZ) studied the birds of northeastern Depto. Tumbes near the Ecuadorean border. The aim of our collecting was to clarify the ranges of the forms that reach their southern limit in this region, which is believed to be at or near the southern terminus of Chapman's Colombian-Pacific Fauna, to gather information on the poorly known species of the area, and to provide the Peruvian government with an initial faunal inventory of the Bosque Nacional de Tumbes. Our camp was ca. 2 km east of the Peruvian military post at El Caucho, on Quebrada Faical, about 24 km southeast of Pampa de Hospital (3°49'S, 80°l 7'W; Fig. 1), at about 400 m elevation. During this period, we also visited Campo Verde (3°51'S, 80°H'W), another military outpost ca. 11 km eastsoutheast of El Caucho and at 750 m elevation (30 June; 1, 3, and 4 July). At these localities we collected six bird species not previously recorded in Peru, and recorded 1 5 subspecies new for the country. Because of the extensive earlier collections in nearby Ecuador, most species that we recorded new for Peru had previously been taken within 100 km of our El Caucho camp. Thus, most records are not major range extensions, and in most cases extend the range westward, not