Preservation of large titanosaur sauropods in overbank fluvial facies: A case study in the Cretaceous of Argentina

Patagonia exhibits a particularly abundant record of Cretaceous dinosaurs with worldwide relevance. Although paleontological studies are relatively numerous, few include taphonomic information about these faunas. This contribution provides the first detailed sedimentological and taphonomical analyses of a dinosaur bone quarry from northern Neuquén Basin. At Arroyo Seco (Mendoza Province, Argentina), a large parautochthonous/autochthonous accumulation of articulated and disarticulated bones that represent several sauropod individuals has been discovered. The fossil remains, assigned to Mendozasaurus neguyelap González Riga, correspond to a large (18–27-m long) sauropod titanosaur collected in the strata of the Río Neuquén Subgroup (late Turoronian–late Coniacian). A taphonomic viewpoint recognizes a two-fold division into biostratinomic and fossil-diagenetic processes. Biostratinomic processes include (1) subaerial biodegradation of sauropod carcasses on well-drained floodplains, (2) partial or total skeletal disarticulation, (3) reorientation of bones by sporadic overbank flows, and (4) subaerial weathering. Fossil-diagenetic processes include (1) plastic deformation of bones, (2) initial permineralization with hematite, (3) fracturing and brittle deformation due to lithostatic pressure; (4) secondary permineralization with calcite in vascular canals and fractures, and (5) postfossilization bone weathering. This type of bone concentration, also present in Rincón de los Sauces (northern Patagonia), suggests that overbank facies tended to accumulate large titanosaur bones. This taphonomic mode, referred to as “overbank bone assemblages”, outlines the potential of crevasse splay facies as important sources of paleontological data in Cretaceous meandering fluvial systems.     

Tidally dominated, rimmed-shelf facies of the Picún Leufú Formation (Jurassic/Cretaceous boundary) in southwest Gondwana, Neuquén Basin, Argentina

A palaeoenvironmental model for the Picún Leufú Formation (Jurassic/Cretaceous boundary), which crops out in the Neuquén Basin, Argentina, on the southwestern margin of Gondwana, is presented in this paper for the first time. Detailed stratigraphic sections exposed along National Road 40 where it crosses the Picún Leufú Creek (type locality) and in the Cerrito Caracoles area, were examined and sampled. Based on a combination of the sedimentological data obtained (facies/microfacies analysis) and the relationship between benthic macrofaunas and their taphonomic attributes, it is concluded that the formation reflects a tidally dominated, rimmed-shelf setting characterized by prograding bars dissected by channels and thick lagoonal facies with shoal developments. In the Cerrito Caracoles area, where only the basal part of the formation is exposed, it is interpreted to have been deposited in a shallow subtidal marine environment in which shelf margin facies with patch reefs have been recognized.     

New Early Cretaceous palaeomagnetic pole from Córdoba Province (Argentina): revision of previous studies and implications for the South American database

A continental sequence of red beds and interbedded basaltic layers crops out in the Sierra Chica of Córdoba Province, Argentina (31.5°S, 64.4°W). This succession was deposited in a half-graben basin during the Early Cretaceous. We have carried out a palaeomagnetic survey on outcrops of this basin (147 sites in seven localities). From an analysis of IRM acquisition curves and detailed demagnetization behaviour, three different magnetic components are identified in the volcanic rocks: components A, B and X are carried by single- or pseudo-single-domain (titano) magnetite, haematite and multidomain magnetite, respectively. Component A is interpreted as a primary component of magnetization because it passes conglomerate, contact, tilt and reversal tests. The carrier of the primary magnetization, fine-grained (titano)magnetite, is present in basalts with a high degree of deuteric oxidation. This kind of oxidation is interpreted to have occurred during cooling. Components B and X are discarded because they are interpreted as recent magnetizations. In the sedimentary rocks, haematite and magnetite are identified as the carriers of remanence. Both minerals carry the same component, which passes a reversal test. The calculated palaeomagnetic pole, based on 55 sites, is Lat. 86.0°S, Long. 75.9°E ( A ₉₅=3.3, K =35). This palaeomagnetic pole supersedes four with anomalous positions reported in previous papers.     

Growth of red gurnard (Teleostei: Triglidae) from Pegasus bay,Canterbury, New Zealand

Growth of the red gurnard, Chelidonichthys kumu (Lesson and Garnot), from Pegasus Bay, Canterbury, was measured during 1966–67. Otoliths were used as an indicator of fish growth; mean length‐at‐age data were obtained from back‐calculated fish lengths at the time of formation of successive annual rings in the otoliths. Growth in length was found to be adequately expressed by the von Bertalanffy growth equation :

l_t = 52.0 [1 ‐ e‐^{0.406 (t‐o.291)}]

(where l_t is the fork length in cm at age t). The length: weight relationship was:

w = 78.56 × 10^‐4 l ^3.072

(where w is the weight in grams). From this relationship, growth in weight was described by the equation:

w_t = 1469 [1 ‐ e‐0.406 (t‐0.291)]³.     

Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina

The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine‐grained biotite + plagioclase, that lacks a well‐defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1‐related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine‐grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser‐grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H⁺ and/or H₂O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system.     

Post glacial lacustrine event sedimentation in an ancient mountain setting: Carboniferous Lake Malanzán (Western Argentina)

Lacustrine deposits of the Malanzán Formation record sedimentation in a small and narrow mountain paleovalley. Lake Malanzán was one of several water bodies formed in the Paganzo Basin during the Late Carboniferous deglaciation. Five sedimentary facies have been recognized. Facies A (Dropstones-bearing laminated mudstones) records deposition from suspension fall-out and probably underflow currents coupled with ice-rafting processes in a basin lake setting. Facies B (Ripple cross-laminated sandstones and siltstones) was deposited from low density turbidity currents in a lobe fringe environment. Facies C (Massive or graded sandstones) is thought to represent sedimentation from high and low density turbidity currents in sand lobes. Facies D (Folded sandstones and siltstones) was formed from slumping in proximal lobe environments. Facies E (Wave-rippled sandstones) records wave reworking of sands supplied by turbidity currents above wave base level.The Lake Malanzán succession is formed by stacked turbidite sand lobe deposits. These lobes were probably formed in proximal lacustrine settings, most likely relatively high gradient slopes. Paleocurrents indicate a dominant direction from cratonic areas to the WSW. Although the overall sequence shows a regressive trend from basin fine-grained deposits to deltaic and braided fluvial facies, individual lobe packages lack of definite vertical trends in bed thickness and grain size. This fact suggests aggradation from multiple-point sources, rather than progradation from single-point sources. Sedimentologic and paleoecologic evidence indicate high depositional rate and sediment supply. Deposition within the lake was largely dominated by event sedimentation. Low diversity trace fossil assemblages of opportunistic invertebrates indicate recolonization of event beds under stressed conditions.Three stages of lake evolutionary history have been distinguished. The vertical replacement of braided fluvial deposits by basinal facies indicates high subsidence and a lacustrine transgressive episode. This flooding event was probably linked to a notable base level rise during postglacial times. The second evolutionary stage was typified by the formation of sand turbidite lobes from downslope mass-movements. Lake history culminates with the progradation of deltaic and braided fluvial systems     

The Gondwanan Carboniferous-Permian Boundary Revisited: New Data from Australia and Argentina

The identification and correlation of the Carboniferous-Permian (Gzhelian-Asselian) boundary within the sedimentary sequences of Gondwana has always been a topic of debate. Type latest Carboniferous and earliest Permian marine sequences are characterised by warm tropical faunas and come from the Uralian Region of Russia and Kazakhstan. Faunas include conodonts and fusulinid foraminiferids which are prime tools for correlation. Such faunal groups are absent from most Gondwanan sequences where reliance for correlations must be placed primarily on brachiopods, bivalve molluscs and palynology. The Western Australian marine sequences, with their contained ammonoids, provide a pivotal link for the dating and correlating of Early Permian Gondwanan sequences with those of the type regions and their palynostratigraphical record is essential for trans-Australian correlations and correlations elsewhere throughout Gondwanaland.New data from the fully cored DM Tangorin DDHl bore hole, drilled in the Cranky Corner Basin, New South Wales, Australia, reveals a sequence of descending faunal zones. The stratigraphically highest zone with Eurydesrna cordaturn, encompasses the Late Sakmarian (Sterlitamakian). The middle zone with Torniopsis elongata, Sulciplica c r a m and Trigonotreta tangorini straddles the Sterlitamakian-Tastubian boundary, with the palynomorphs Pseudoreticulatispora pseudoreticulata high in the zone and Granulatisporites confluens low in the zone. An impoverished fauna with Trigonotreta nov., low in the Granulatisporites confluens Zone, is probably of latest Asselian or Tastubian age.Significant new data from Argentina has revealed marine faunas from below the occurrence of Granulatisporites confluens. These are considered to be of Asselian age. Outcrops of the Tupe Formation, with a marine fauna, at La Herradura Creek in the western Paganzo Basin, San Juan Province, are best regarded as being of mid to late Asselian age. The Tupe Fauna has been recognised as the Tivertonia jachalensis-Streptorhynchus inaequiornatus Zone. Previously, this fauna was considered to be of Late Carboniferous or Stephanian age. Three faunal associations are known from the Rio del Peii6n Formation, Rio Blanco Basin, La Rioja Province. The middle assemblage with Tivertonia, Costaturnulus, Kochiproductus and Trigonotreta, appears to correlate well with the Tupe Formation fauna. The lower assemblage, with Streptorhynchus, Etherilosia, Costaturnulus, Trigonotreta and a punctate spiriferid, as well as indeterminate productids, probably of Early Asselian age. The youngest assemblage includes a species ofRhynchopora that is close to Rhynchopora australasica from the latest Asselian-early Tastubian of Western Australia. The marine biostratigraphical data from Argentina has enabled a much greater understanding of the earliest Permian marine faunas to be achieved - a story that is apparently absent from the other cold and cool temperate regions of Gondwana.     

Diagenetic model of the Bajo Barreal formation (Cretaceous) in the southwestern flank of the Golfo de San Jorge Basin (Patagonia, Argentina)

This paper analyses the diagenetic evolution of sandstones belonging to the Bajo Barreal Formation (Cretaceous) in the Golfo de San Jorge Basin (Patagonia, Argentina). The Bajo Barreal Formation includes the main reservoirs, which are located along the western area of the basin and is composed of sandstones, conglomerates, mudstones, tuffaceous mudstones and some layers of tuffs. The principal reservoirs comprise medium-to coarse-grained sandstones, which are dominated by feldspathic litharenites and contain minor amounts of litharenites and lithic arkoses. The authigenic minerals include kaolinite, smectite, chlorite, quartz overgrowths, microquartz and calcite, with minor proportions of megaquartz, siderite, analcime, laumontite, feldspar overgrowths and illite/smectite and chlorite/smectite mixed layers. Secondary porosity is much more important than primary porosity and is produced by the dissolution of feldspar, lithic clasts and clay cements. The diagenetic history of the Bajo Barreal sandstones can be divided into seven diagenetic stages, each of which is characterized by a specific assemblage of authigenic minerals and diagenetic processes. Eogenetic conditions occur in stages 1, 2, 3 and 4. Stage 1 corresponds to shallow burial characterized by the physical reduction of primary porosity by compaction; during stage 2, rim clay cements of chlorite, smectite and clinoptilolite, as well as thin quartz overgrowths, were formed. The precipitation of pore-filling cements of kaolinite, chlorite and smectite occurred during stage 3, while stage 4 records the intense dissolution of feldspar, lithic fragments and kaolinite cements. Mesogenesis occurs in diagenetic stages 5 and 6. The former corresponds to a new phase of authigenic kaolinite, while the latter records the significant dissolution of feldspar, lithic clasts and previous cements, which produced the highest values of secondary porosity. Finally, stage 7 corresponds to the highest degree of diagenesis in the Bajo Barreal Formation (mesogenesis), which resulted in the precipitation of cements of zeolites and calcite, as well as quartz and plagioclase overgrowths.