Ricardo M. Palma

Evidence of precessional and eccentricity orbital cycles in a Tithonian source rock: The mid-outer carbonate ramp of the Vaca Muerta Formation, northern Neuquen Basin, Argentina

Basin to mid-ramp cyclic facies of the Tithonian Vaca Muerta Formation are exposed in the Loncoche Creek section of the Neuquen Basin, Mendoza province, Argentina. This unit is characterized by a decimeter-scale rhythmic alternation of marls, shales and limestones and extends from the lower Tithonian to the upper Berriasian. Cyclostratigraphic studies based on a detailed facies analysis allowed the identification of cyclic patterns with frequencies within the Milankovitch band. According to biostratigraphic data, the dominant cycle in the studied section has a period of 20 k.y., which correlates with the Earths axis precession element. Spectral analysis based on a series of compacted and decompacted cycle thickness identified a subordinate frequency of about 90 to 120 k.y., which we interpret as the modulation of the precessional cycle caused by the Earths orbital eccentricity. The strength of the precession signal, together with the absence of a well-defined cyclicity attributable to the obliquity orbital cycle (i.e., 40 k.y.), is in agreement with previous data from the Northern Hemisphere.

Crustacean microcoprolites from the Upper Jurassic-Lower Cretaceous of the Neuquén Basin, Argentina: Systematics and biostratigraphic implications

As a result of a microfacial study in the outer and middle ramp deposits of the Vaca Muerta Formation (lower Tithonian-upper Valanginian), four ichnotaxa of crustacean microcoprolites are described: Palaxius azulensis Kietzmann isp. nov., Palaxius caracuraensis Kietzmann isp. nov., Helicerina? isp. A. aff. Helicerina siciliana and Helicerina isp. B. They represent one of the first records of crustacean microcoprolites for the Neuquén Basin and Argentina. Helicerina is reported for the first time from the Upper Jurassic and Lower Cretaceous of South America. It is inferred that Palaxius ichnospecies were produced by callianassids, while Helicerina ichnospecies could be produced by decapods of Mecochiridae, Erymidae, and/or Nephropidae affinity. Two assemblages of crustacean microcoprolites are recognised, a middle Tithonian to lower Berriasian Palaxius-dominated assemblage and an early to late Valanginian Helicerina-dominated assemblage.

Environmental implications of oncoids and associated sediments from the Remoredo Formation (Lower Jurassic) Mendoza, Argentina

Abstract Lower Jurassic deposits of the middle member (19 m) of the Remoredo Formation at Arroyo Montanesito, southwestern Mendoza Province (Neuquen Basin, Argentina), and its oncoids are described. Four lithofacies constitute the lacustrine deposits of the Remoredo Formation: (1) laminated green to gray calcareous shales; (2) laminated to massive dark gray mudstones; (3) massive to upward-coarsening dark gray oncolite wackestones; and (4) massive dark gray packstones with abundant macro-oncolites. According to layer configuration, two types of oncoids were recognized: (a) those having distinctly banded, well defined concentric and continuous lamination (Type I); and (b) those having a vertical arrangement of irregular and discontinuous laminae (Type I 3 –I). The continuous micritic laminations suggest growth during agitated conditions, and the discontinuous laminations reflect short periods of non agitation or tranquility in the water mass. The oncolites are associated with a well preserved faunal assemblage of ostracods ( Darwinula sp. and possible members of the Subfamily Iliocypridinae ) and some fragments of bivalves that are indicative of shallow water conditions. The facies association is a coarsening-upward sequence interpreted as the product of deposition in a shallow nearshore lacustrine environment, and a progressive infilling by pyroclastic subaereal flows.

Taphonomic analysis of fossil concentrations from La Manga Formation (Oxfordian), Neuquén Basin, Mendoza Province, Argentina

Griphaeid-dominated fossil concentrations of La Manga Formation are characterized on the basis of taphonomic, sedimentologic, stratigraphic and palaeoecologic criteria. These concentrations are present in the basal deposits (Unit 1) of three outcrops in the locality of Bardas Blancas. Their attributes allow distinguishing seven different fossil accumulations: A) Channelized fossil concentrations of Gryphaea sp.; B) Shell beds of Gryphaea sp. highly reworked; C) Shell beds of Gryphaea sp. moderately reworked; D) Accumulations of Gryphaea sp. with biomicrite to biopelmicrite matrix; E) Shell detritus; F) Accumulations produced by bottom currents; and G) Accumulations found in clumps. Accumulations A-C were deposited by high energy processes (storm waves and currents) in the upper middle ramp. Accumulations D and E were produced by high energy processes (storm waves and currents) but the final depositions were produced out of suspension in low energy environments, in lower middle ramp settings. Accumulations F were generated by weak bottom currents, meanwhile accumulations G correspond to gryphaeids associations reworked by bioturbators or by hydraulic processes after variable periods of permanence in sediment/water interface. Both are interpreted as lower middle or outer ramp. This interpretation suggests that these deposits that in previous works were interpreted as outer ramp settings, characterize middle and outer ramp settings. The vertical distribution of fossil concentrations from upper middle ramp to lower middle and outer ramp settings evidences a continuously growing accommodation space and allows to interpret the Unit 1 as part of a transgressive systems tract.

Shell bed stacking patterns in the Chachao Formation (early Valanginian) in Malargüe area, Mendoza province, Neuquen Basin-Argentina

The Lower Cretaceous Chachao Formation in the Malargüe anticline area consists of wackestone, packstone, and minor grainstone and mudstone rich in benthic fauna which were deposited in a shallow carbonate ramp. These shell beds are composed of a lowdiversity molluscan fauna mostly dominated by oysters, other benthic pelecypods (Eryphila sp.,Ptychomya sp.,Pecten sp.,Pinna sp.,Trigonia sp.,Cucullaea sp.), serpulids (Sarcinella sp.,Parsimonia sp.), and occasional ammonites (Olcosthepanus curacoensis).Most skeletal accumulations range from 15 to 70 cm in thickness. Shell beds exhibit a sharp, flat and commonly undulating base. In some cases the tops were bioturbated principally byThalassinoides isp. Some shell beds show a densely packed biofabric (bioclastic-supported) but most of them have dispersed or loosely packed biofabric (mud-supported). Bioclasts include articulated or disarticulated shells, fragmented valves and shell debris. The analysis of shell concentrations of the Chachao Formation allow to recognize four taphofacies according to their taphonomic attributes.Different kind of processes, such as low-energy storm currents and waves, were interpreted, whose deposits are arranged into a coarsening-and thickening-upward and then fining- and thinning upward trends. According to their biostratinomic features shell beds of Chachao Formation have been interpreted as parautochthonous to autochthonous skeletal concentrations, mainly of sedimentologic origin.Towards the end of the Valanginian, the drowning of the carbonate ramp took place, related to a relative sea level rise and environmental stress. This drowning event was recorded by the alternating thin- to medium bedded dark gray carbonate and black shales (Agrio Formation) of Late Valanginian-Late Hauterivian age.

Diagenesis of a bioclastic oyster deposit from the Lower Cretaceous (Chachao Formation). Neuquén basin. Mendoza Province, Argentina

The Lower Cretaceous Chachao Formation in the Malargüe anticline area consists of wackestone, packstone, and minor grainstone and mudstone rich in benthonic fauna that were deposited in a carbonate ramp. The carbonate diagenesis in the Valanginian Chachao Formation contains many processes with conspicuous effects, including micritization, dissolution, neomorphism, and cementation. The early diagenetic process is characterized by micritization, dissolution and mineralogic stabilization of components, and earlier cement phase represented by micrite cement and isopachous calcite cement, which have petrographic characteristics consistent with precipitation in a marine-phreatic diagenetic environment. Later diagenetic phenomena include granular calcite and syntaxial cement. Both of cement types are interpreted as typical of a meteoric-phreatic environment. Concentric-zoning pattern of alternating dull, and blotchy- to bright luminescent zones is interpreted as being caused by a decrease in redox potential (Eh), under conditions of a progressive marine burial meteoric-phreatic diagenetic environment. Geochemical data (Sr++, Na+, Mg++, Fe++, Mn++) and SEM features of the micrite suggest that original calcareous mud could have been calcite dominated (CDP). The δ18O composition of the granular calcite cement ranging from −2.84‰ to −4.27‰ PDB and the δ13C values of the cement between −2.46‰ and −3.50‰ PDB are compatible with precipitation from a fluid that evolved meteoric-phreatic composition. The high depleted δ18O values of theGryphaea oyster shells can be related to the dilution of the marine water with a fresh water influx, whereas shells with the heaviest δ13C isotopic compositions are probably related to the original marine signal, which suggest a closed diagenetic system for carbon.

Facies, microfacies, and diagenesis of late Callovian-early Oxfordian carbonates (La Manga Formation) in the west central Argentinean high andes

At 32° 40′ south latitude the Andean orogen shows a typical thin-skinned fold and thrust belt that exposes the Mesozoic backarc basin fill. The late Callovian-early Oxfordian limestones of the La Manga Formation were studied based on lithofacies, microfacies, and diagenetic processes in order to interpret the paleoenvironment depositional setting. Five lithofacies can be recognized within the La Manga Formation: (A) stromatolite mudstone, (B) massive and laminated carbonate mudstone, (C) massive wackestone, (D) coralline floatstone, and (E) oolitic-skeletal packstone-grainstone. The vertical succession reveals six closely related microfacies grouped as follows: (1) stromatolite mudstone, (2) peloidal wackestone, (3) bioclastic peloidal wackestone, (4) oolitic bioclastic packstone, (5) coral skeletal floatstone, and (6) oolitic-intraclastic grainstone.Diagenetic events, listed in sequence, include: 1) dissolution of high-Mg calcite and aragonite shells; 2) compaction; 3) pore filling; and 4) pressure-solution. Three types of cement, isopachous, granular calcite and syntaxial overgrowth, were identified. Based on CL petrography the cement sequence changes from non-luminescent to brightly luminescent, suggesting variations within the pore fluid system. Paleotemperature during deposition is estimated at 26°C according to stable isotope analyses on oyster shells.Lithofacies and microfacies data suggest that an intertidal-lower supratidal environment (lithofacies A and microfacies 1) was followed by a deepening succession (lithofacies B, C, and microfacies 2,3), followed by a shallowing upward succession (lithofacies D, E, and microfacies 2,3), followed by a shallowing upward succession (lithofacies D, E, and microfacies 4, 5, 6). This facies assemblage is interpreted as part of a transgressive system tract and a high-stand succession. The sequence represents an upward-coarsening deposit on a shallow carbonate platform.

Analysis of carbonate microfacies in the Chachao Formation (cretaceous), Barda Blanca-malargüe, Mendoza province-Argentina: A cluster analytic approach

The Chachao Formation (Valanginian), which occupies a middle position in the Mendoza Group, is a carbonate unit consisting of packstones and grainstones, with subordinate wackestones and rare mudstones. Its average thickness is 35 meters. Pelecypods, echinoderms, gastropods, crinoids, ostracods, worm tubes, sponge spicules, and foraminifera dominate the fauna in the studied sequence. Rare remains of bryozoans, ammonoids, brachiopod shells, coral fragments, and algae are present too. Phosphatic debris occurs in minor quantities. Pyrite, glauconite and phosphate minerals commonly occlude foraminiferal chambers, ostracods shells, as well as borings in skeletal particles. Taphonomic features enable this association to be considered as parautochthonous in response to compositeevent concentrations. The Barda Blanca sequence was divided into six microfacies in order of relative increasing energy level. Each one is characterized by the dominance of depositional textures, bioclasts and sorting. In order to determine the degree of interrelationships among bioclasts, data were subject to cluster analysis. Two isolated nuclei (calcispheres & sponge spicules/fish scales & coprolites), and a big group integrated by three subgroups were recognized. Each one was distinguished by the existence of pelecypods, echinoderms, gastropods, annelids, ostracods, and foraminifers. Changes in microfacies types and in fauna content reveal considerable variation in the frequency-distribution curves of bioclasts. These variations point out discontinuities, which may indicate that water depth increased with time. These discontinuities suggest the presence of different parasequence boundaries and they could be used as time lines in the reconstruction of the depositional events. The fossil assemblage and deposit suggest an open marine condition below the influence of fairweather waves, and reworking by action of smaller storms (gradient currents) eventually.

High frequency subtidal-peritidal cycles of the Callovian Calabozo Formation (Neuquén Basin, Western Argentina): Preliminary approach

The Calabozo Formation (Cuyo Mesosequence, Neuquen Basin) is made up by a thin, up to 30-m thick Middle Jurassic carbonate-dominated sequence. The minor, thin siliciclastic facies in this formation (i.e. lowermost granule-pebble shoreface conglomerate and fine-grained sandstones interbedded among the lower carbonates) record early transgressive shoreline and shallow lagoonal-subtidal sedimentation, respectively. The major carbonate deposits have been split into six facies and seven microfacies on the base of macroscopic field observation and petrological analysis. The carbonate facies have been grouped into three major assemblages: 1) Tidal flat facies assemblage (including the planar laminated algal boundstone facies; 2) Inner platform facies assemblage (including peloidal and bioclastic wackestone, coral floatstone and bioclastic and intraclastic packstone facies; and 3) Platform margin assemblage (including oolitic-peloidal packstone-grainstone facies). Afossil algal record made up by dasycladacean species (Salpingoporella annulata, Cylindroporella sp. and Heteroporella sp.) and blue-green algae (Cayeuxia (Rivularia) piae, and Cayeuxia (Rivularia) kurdistanensis) has been reported for the first time in this unit. The analysis of the intracycle facies and microfacies arrangements, as well as the nature of the cycle bounding surfaces, enabled one to identify two kinds of cycles. 1) Widespread subtidal cycles capped by shallow to intermediate subtidal facies, with no evidence of subaerial exposure at cycle tops; and 2) a minor subtidal cycle with similar shallowing-upward trend but capped by peritidal facies. The stacking pattern and thickness trend of these cycles was studied to determine possible allocyclic and autocyclic controls on their evolution. The Fischer plots reveal that the deposition of the Calabozo Formation subtidal-peritidal cycles took place under upward decreasing accommodation space conditions which may be related to eustatic lowering.

Lithofacies and depositional environment of Middle Cambrian San Isidro Formation, Mendoza, Argentina

Outcrops of the San Isidro Formation (32 m) in west Mendoza province, consist of monotonous, nearly homogeneous-sequences of dark-gray to dark brown, thin-bedded limestones with shales, siltstones and interbedded medium to fine grained sandstones. This area is known for its fossiliferous localities of Middle Cambrian trilobites. Fossils include trilobites, particularly planktonic agnostids, sponge spicules, foraminifera and calcispheres. Both fossils and sedimentologic characteristics indicate relatively deep-water marine sedimentation.The overall picture of depositional history represents a transgressive-regressive-transgressive sequence, as the result of moderate sea-level change. During the sea-level low-stands siliciclastic sediments were transported on the shelf and into the adjacent basin. It would seem possible that the presence of mudstone-wackestone over the low-stand sandstones represents a rapid sea-level rise.Some aspects about diagenetic effects are discussed with special reference to cementation, neomorphism, dolomitization, pressure-solution, and silicification.