Lajas Formation Research Articles

Palaeoenvironments and palaeoclimate of the uppermost Cuyo Group and lowermost Lotena Group at Quebrada Álvarez, Picún Leufú Sub-basin, Patagonia, Argentina: a preliminary study based on palynology

The palynological (palynostratigraphical/palynofacies) analysis of samples from the Middle Jurassic outcrop at the Quebrada Álvarez section located in the Neuquén Basin, Patagonia, Argentina, allowed us to begin to evaluate the palaeoecological significance of the recovered palynofloras and their importance in the evolution of these ecosystems, during the transition between the uppermost Cuyo Group and the lowermost Lotena Group. The Lajas Formation, characterised by the palynofacies type A, could be interpreted as a tide-modified delta front environment. The high abundance and diversity of the palynomorphs identified in the studied samples, contribute to improve the palynological knowledge of the Challacó Formation (palynofacies type B − F). Considering the palaeoecological requirements of the recognised plant families, relatively humid and warm climate conditions could be inferred during the deposition of this unit. Also, the ‘seasonally dry (winterwet)’ biome was interpreted for the first time in the Neuquén Basin. Based on selected key taxa a late Bathonian–early Callovian age is proposed for the Challacó Formation at the Quebrada Álvarez section. The palynological material recorded in this unit suggests the development of a lacustrine environment with cycles of relative contraction and expansion of the water body due to fluctuations between relatively dry and wet conditions. The dominance of phytoclasts, mainly opaque particles, and freshwater algae (Botryococcus) in the Bosque Petrificado Formation (palynofacies type G − I) could suggest the development of a freshwater to brackish lacustrine environment. Based on the dominance of the same group of palynomorphs (chlorophyte algae) in the Challacó and Bosque Petrificado formations, similar palaeoenvironmental conditions would be inferred for these units. From a lithofacies analysis point of view, the Challacó and Bosque Petrificado formations show isopic facies associations in the study locality.

Prograding early to middle Jurassic margin, Neuquén Basin: Topset process stratigraphy and morphodynamic sediment partitioning

AbstractNew Cuyo Group outcrop data complement recently published seismic and well data that detail the progradational character of the Lower‐Middle Jurassic Neuquén Margin (Argentina). The deepwater margin, with shelf topset (Lajas Formation), slope break and deepwater slope to basin‐floor deposits (Los Molles Formation), prograded northward and westward as large‐scale clinoforms (250–500 m high) for 120 km, during 13 My, supplied by sediment mainly from the North Patagonian Massif. The studied Lajas Formation topsets are themselves progradational, with a 500 m‐thick vertical succession of subaqueous delta, shoreface with associated tidal flat, mouth bar, fluvio‐tidal channel and alluvial plain deposits. This pattern is confirmed by facies analysis in five areas of outcrops and well data but is interrupted by frequent marine transgressions that add a short‐term cyclicity to the succession. A new paleogeographic map suggests previous disagreements on process stratigraphy are due to shoreline/paralic strike variability and to increased basinal process impact as the topset/shelf widened. A marked feature of each Lajas topset sequence is a differential partitioning of sand and mud, though not with a simple proximal to distal grain‐size reduction. The sand‐prone outer shelf passes landward to a channelized muddy shoreline environment, and then to a mixed sandy and muddy channelized coastal and alluvial plain. This type of partitioning, important for the characterization and prediction of reservoir quality, can also be recognized on seismic and well log data.

Palynological analysis of sandy hyperpycnal deposits of the Middle Jurassic, Lajas Formation, Neuquén Basin, Argentina

The basal succession of the Lajas Formation at Trasandino section, Arroyo Covunco area, constitutes an excellent example of proximal prodelta facies cut by sediment-waves formed during a complete acceleration-deceleration hyperpycnal discharge cycle. The main outcome of this contribution was to present a new studied locality in which the Lajas Formation outcrops. Nine mudstones and fine-grained heterolites levels deposited during the waning stage, were sampled to palynological analysis. The recovered assemblages are dominated by sporomorphs, which are characterized by a great diversity of the trilete spores. Among them, is interesting to highlight the first mention of Manumia variverrucata and the extension of the last occurrence of Striatella seebergensis until to Callovian in Argentina. Based on selected key taxa a Late Bathonian–Callovian age is proposed for the Lajas Formation at the Trasandino section in this area. The Trasandino section spore assemblages show the greatest similarity with the Arroyo Covunco section spore associations when they are compared with the Lajas Formation of other studied localities. The endemic development of certain types of bryophyte (sensu lato) spores, e.g. Taurocusporites quattrocchiensis, was favored by locally humid conditions inferred at the Arroyo Covunco area. Large abundance of phytoclasts, sporomorphs and fresh-water algae characterize the recovered organic matter suggesting a high input of continental organic particles to the basin, as the result of fluvial-derived density discharges. Three palynofacies type (PT) had been identified which allow to evaluate the hydrodynamic characteristics of the flow, taking into account the differential buoyancy of the opaque particles. The large amount of equidimensional opaque particles recognized at the PT-B, characterizes the beginning of the deceleration phase of the flow and the abundance of blade-shape opaque particles, identified in the PT-A, point out a deposition from the final buoyant plume of hyperpycnal flow. The PT-C shows transitional features between these two conditions.

U–Pb detrital zircon ages in the Lajas Formation at Portada Covunco: Maximum depositional age and provenance implications for the Neuquén Basin, Argentina

The Lajas Formation of the Neuquén Basin (Argentina) was deposited mainly during the Middle Jurassic but with a highly diachronic pattern. The exact time elapsed in the deposition of this unit remains controversial and depends on the location within the basin. Particularly in the Portada Covunco locality, north of the Huincul High, the debate on the age of the Lajas Formation persists. The age of the unit has been assigned by sequence stratigraphy, palynological, and magnetostratigraphic studies. Moreover, previous studies suggest that the Lajas Formation in this locality is composed of two depositional sequences (Sq1 and Sq2) with a change in the composition of the sandstone, indicating different areas of sedimentary supply. The new U–Pb data (LA-ICP-MS) in the detrital zircons allows us to define a maximum depositional age of approximately 163 Ma for Sq1 and 162 Ma for Sq2, suggesting a Callovian age for the Lajas Formation in the studied area. Furthermore, we propose that the provenance pattern in Sq1 was dominated by U–Pb ages of detrital zircons from the Permian–Triassic (peak at 264, 244 and 215 Ma), indicating derivation predominantly from the North Patagonian Massif magmatic units; while Sq2 shows a provenance pattern of detrital zircons associated with the uplift and exhumation of units located to the southwest margin of the basin during the end of the Middle Jurassic. Interestingly, it is not possible to rule out that the Huincul High may be related to the change in the sedimentary supply area; the probable uplift of the Huincul High has resulted in an effective physical barrier that limited sediment input from the extreme southeast during the deposition of Sq2.

Base metal mobility linked to brine and hydrocarbon migration at the Huincul High in the Neuquén Basin, Argentina: Implications for the formation of sediment-hosted base metal deposits

This contribution discusses the discovery of PbZn mineralization in hydrocarbon exploration drill holes hosted in siliciclastic Jurassic rocks below the Cretaceous sediment hosted Cu mineralization located north of the Huincul High. In the Early Cretaceous, hydrocarbon and basinal water expulsion took place from the oil source rocks (Los Molles Formation) into the reservoir rocks Lajas Formation (Middle Jurassic), after the formation of diagenetic quartz overgrowths. Marcasite, pyrite and illite + illite-smectite mixed layer minerals then formed together with hydrocarbons. Marcasite (δ34S −3.5 and −2.2‰) and pyrite (δ34S −11.6 to 18.1‰) precipitated via bacterial sulfate reduction from fluids showing temperatures <80 °C and fluctuating pH conditions. During the Paleogene, Andean tectonism triggered migration of oxidized basinal brine containing dissolved leached Zn and Pb from the Los Molles Formation source rocks and S from the evaporites of the basin. This brine entered in the hydrocarbon reservoir of the Lajas Formation, resulting in the precipitation of sphalerite (δ34S +10.4 to +10.8‰) and galena (δ34S +5.2‰) by TSR at temperatures of 119–123 °C together with siderite. The Andean orogeny may also have triggered the migration of hydrocarbons + brines from the shallowest source rocks (Vaca Muerta Formation) and reservoir rocks (Mulichinco, Lajas and Lotena formations) into the red beds of the upper Cretaceous Neuquén Group, which became bleached as a result. Marcasite (δ34S −36.4 to 18.2‰) and pyrite (δ34S −60.2 to −24‰) precipitated by BSR, while calcite (δ13C −12.3 to −5.6‰) formed due to redox reactions accompanying formation of quartz overgrowths, clay mineral coatings (δD −92.4‰ to −82.8‰ and δ18O 16.2‰ to 18.40‰), and barite (δ34S −5.9‰). Quartz overgrowths and barite precipitated from brines (1.5–10 wt% NaCl equiv.) at temperatures of 91–120 °C, with barite precipitating close to the feeder zones. Calcite cements are more widespread and generated from several pulses of brine and hydrocarbons up flow, evidenced by the wide range of homogenization temperatures (100–185 °C) and diversity of UV fluorescence colors of hydrocarbon-rich fluid inclusions. In subsequent Miocene tectonic events, basinal brines were able to leach Cu from the underlying thick red beds of the basin, entering the Neuquén Group oil reservoir traps or carrier beds where they became reduced. Chalcopyrite and bornite precipitated at temperatures >100 °C close to the feeder zones at the expense of barite as a local sulfur source. Chalcocite group minerals (δ34S −21.3‰ to −7.3‰) precipitated outwards in pore spaces of the sandstones at lower temperatures (<100 °C) through BSR of sulfate in the mineralizing brines. Final exhumation of the Neuquén Group (Upper Miocene-Pliocene) promoted the infiltration of the meteoric water, the oxidation of the sulfides and formation of supergene FeCu, V and U minerals. Isotope geochemistry and fluid inclusion results combined with alteration and ore mineralogy document the processes controlling the distribution of base metals in the Huincul High region that can be applied to evaluate the metal potential in other areas of the Neuquén Basin and further extrapolated to similar geological systems elsewhere.

Reply to Ponce et al. (2020)

The Lajas Formation of the Neuquén Basin (Argentina) was deposited mainly during the Middle Jurassic but with a highly diachronic pattern. The exact time elapsed in the deposition of this unit remains controversial and depends on the location within the basin. Particularly in the Portada Covunco locality, north of the Huincul High, the debate on the age of the Lajas Formation persists. The age of the unit has been assigned by sequence stratigraphy, palynological, and magnetostratigraphic studies. Moreover, previous studies suggest that the Lajas Formation in this locality is composed of two depositional sequences (Sq1 and Sq2) with a change in the composition of the sandstone, indicating different areas of sedimentary supply. The new U–Pb data (LA-ICP-MS) in the detrital zircons allows us to define a maximum depositional age of approximately 163 Ma for Sq1 and 162 Ma for Sq2, suggesting a Callovian age for the Lajas Formation in the studied area. Furthermore, we propose that the provenance pattern in Sq1 was dominated by U–Pb ages of detrital zircons from the Permian–Triassic (peak at 264, 244 and 215 Ma), indicating derivation predominantly from the North Patagonian Massif magmatic units; while Sq2 shows a provenance pattern of detrital zircons associated with the uplift and exhumation of units located to the southwest margin of the basin during the end of the Middle Jurassic. Interestingly, it is not possible to rule out that the Huincul High may be related to the change in the sedimentary supply area; the probable uplift of the Huincul High has resulted in an effective physical barrier that limited sediment input from the extreme southeast during the deposition of Sq2.

Sedimentología e icnología de un delta fluvio-dominado, Formación Lajas (Jurásico Medio), cuenca Neuquina, Argentina

Sedimentologic and ichnologic analysis of the Middle Jurassic Lajas Formation in Bajada de Los Molles area allow recognizing upper offshore-lower shoreface deposits in transition to prodelta, delta front and interdistributary bay succesions. This system is classified as a river-dominated delta due to the presence of distributary channel deposits with huge amounts of particulate organic matter and low diversity and abundance of trace fossils. Hyperpycnites are common in the basal and upper part of the studied section, and form channel-levee and distributary-channel systems, integrated by massive sandstones and load deformation structures, or transitional and recurrent passages of tractive sedimentary structures with abundant particulate organic matter on the foresets. The greatest diversity and abundance of trace fossils are recognized in the upper offshore-lower shoreface deposits where Skolithos and Cruziana ichnofacies occur. Deltaic deposits show trace fossil associations with lower diversity and abundance than the fully marine ones, whereas the hyperpycnite deposits are either unbioturbated or show the lowest diversity and abundance of trace fossils, reflecting the most stressed conditions within the system.

Comment to Pazos et al. (2019) “The oldest record of a tyreophoran track in Gondwana: Geological implications of subaerial exposure in the lower part of the Lajas Formation at the Covunco section (Neuquén Basin), Patagonia, Argentina”

Fil: Ponce, Juan J. Universidad Nacional de Rio Negro. Instituto de Investigacion en Paleobiologia y Geologia; Argentina.

Magnetostratigraphy of a Middle Jurassic delta system (Lajas Formation), Portada Covunco section, southern Neuquén Basin, Argentina

A systematic paleomagnetic study was carried out in the Lajas Formation (Middle Jurassic), cropping out at the Covunco creek in the central part of the Neuquén basin. The Lajas Formation at the Portada Covunco section consists of a succession of conglomerates, sandstones and mudstones that reaches c. 290 m-thick. On the basis of lithological and textural features, sedimentary structures, strata geometries, and fossil content, 14 facies were defined that were grouped in two facies associations within a fluvial-dominated delta system. The age of the Lajas Formation is rather controversial, and has been assigned from sequence stratigraphy and palynological data. Thus from south to north, interpreted ages span the Upper Aalenian–Middle Bathonian through the Callovian. Therefore, magnetostratigraphy was used to better precise the age of the Formation at Portada Covunco. Paleomagnetic data obtained from 29 sampling sites present reliable behaviors. Rock magnetic studies (thermomagnetic curves, K vs H curves, IRM acquisition curves) suggest that the remanence is carried by Ti-poor magnetite. Anisotropy of magnetic susceptibility data show clustering of the Kmax axes produced by the action of paleocurrents. The characteristic remanent magnetization passed a tilt test and a reversal test that prove its primary origin. The resultant magnetostratigraphy is characterized by a reverse-dominant with minor normal polarity intervals. When compared with the Bajocian-Bathonian reference geomagnetic polarity time scale (GTS2016), the best fit is achieved in the Lower through Middle Bathonian (Chrons M41 to M39). A paleomagnetic pole was calculated for Portada Covunco located at 167.1°E, 59.4°S, N = 28, A95 = 3.6°, K = 57.2. When compared with the other available paleopole of the same age −170 Ma-derived from dykes exposed at Mamil Choique, Patagonia, Portada Covunco falls in the same paleomeridian although at a lower latitude. Thus, it is interpreted that the studied section was not subjected to rotations around vertical axis (R = 4° ± 10°), but was compacted, which produced inclination shallowing of the magnetic vector (F = 18° ± 11°). The calculated inclination shallowing and amount of compaction in the section are well within the expected values in medium to fine-grained sandstones. Using the paleomagnetic pole from Mamil Choique as reference, the locality of Portada Covunco would have been located at c. 33°S during the Bathonian, in agreement with paleoclimatic interpretations derived from palynological data.

Stratigraphic evolution of the nearshore to fluvial plain of the Upper Cuyo Group, Neuquén, Argentina

AbstractThe upper portion of the Cuyo Group in the Zapala region, south‐eastern Neuquén Basin (Western Argentina), encompasses marine and transitional deposits (Lajas Formation) overlain by alluvial rocks (Challacó Formation). The Challacó Formation is covered by the Mendoza Group above a second‐order sequence boundary. The present study presents the stratigraphic framework and palaeophysiographic evolution of this Bajocian to Eo‐Calovian interval. The studied succession comprises the following genetic facies associations: (i) offshore and lower shoreface–offshore transition; (ii) lower shoreface; (iii) upper shoreface; iv) intertidal–subtidal; (v) supratidal–intertidal; (vi) braided fluvial to delta plain; (vii) meandering river; and (viii) braided river. The stratigraphic framework embraces four third‐order depositional sequences (C1 to C4) whose boundaries are characterized by the abrupt superposition of proximal over distal facies associations. Sequences C1 to C3 comprise mostly littoral deposits and display well‐defined, small‐scale transgressive–regressive cycles associated with fourth‐order depositional sequences. Such high‐frequency cycles are usually bounded by ravinement surfaces associated with transgressive lags. At last, the depositional sequence C4 delineates an important tectonic reorganization probably associated with an uplift of the Huincul Ridge. This is suggested by an inversion of the transport trend, north‐westward during the deposition of C1 to C3 depositional sequences (Lajas Formation) to a south‐west trend during the deposition of the braided fluvial strata related to the C4 depositional sequence (Challacó Formation).

Discussion of: Ortega-Flores et al. (2018) Provenance analysis of Oligocene sandstone from the Cerro Pelón area, southern Gulf of Mexico.https://doi.org/10.1080/00206814.2018.1476922

ABSTRACTWe discuss the 2018 publication that reports petrographic, heavy mineral data, mineral chemistry, and zircon geochronology for Oligocene sandstones in the Cerro Pelón area in southern Mexico Sureste basin. As the title of their paper says, the goal of their study is to establish the source (s) of the voluminous Cenozoic section in this region, reaching several kilometres in thickness and important as a petroleum system. These authors conclude that Oligocene sandstones of La Laja Formation were mostly sourced from eclogite- to greenschist-facies metasedimentary, metaigneous, and ultramafic rocks of the Guatemala suture complex. Minor contributions from the Chiapas Massif Complex, exposed directly to the south ~60 km of the Cerro Pelón area, were also suggested by the authors. They thus conclude that the Palaeogene stratigraphic record in southeastern Mexico was mostly controlled by the development of the Caribbean–North America plate boundary rather than by orogenic processes at the Pacific margin of North America. Presently, we do not agree with the conclusions of Ortega Flores and colleagues who studied the Cerro Pelón section, thus some discussion is required. Serpentinite bearing Nanchital Conglomerate is well exposed in the Cerro Pelón area, and high- to low-grade metamorphic rocks experienced an uplift in the vicinity of the Cerro Pelón area at the time of deposition of the La Laja Formation. We believe the data are better explained by multiple local sources in southern and eastern Oaxaca as well as sources to the south and southwest, which include the Cenozoic coastal batholith, the Grenvillean/Guichicovi basement complexes, the Chiapas Massif, the Mazatlán schist and other units in the Cuicateco Belt, as well as the Mesozoic cover of these areas (Todos Santos Formation, Cretaceous carbonate rocks, and Paleogene strata such as the Soyaló and Bosque Formations).

The oldest record of a tyreophoran track in Gondwana: Geological implications of subaerial exposure in the lower part of the Lajas Formation at the Covunco section (Neuquén Basin), Patagonia, Argentina

The outcrops of the Lajas Formation at the Covunco section expose a succession that contrasts with the classical localities for the unit, due to subaerial exposure confirmed by a vertebrate track documented in the basal section of the unit. The stacking pattern, facies distribution, subaerial exposure in the previously interpreted prodelta facies and the documented unconformity in the middle part of the unit are incompatible with the continuous deltaic evolution suggested previously for the studied locality. Regardless of the absence of a precise age for the succession, the regional literature of subsurface data, the paleontological remains and the intra-Callovian unconformity at the top of the unit, suggest a Bathonian (late?) up to Early Callovian age for the unit in the area. It is younger than the classical deltaic deposits extensively studied in outcrops or subsurface to the north-east and south of the Huincul High and confirms the diachronism between the south and north of the high and it sheds light on the late evolution of the unit. The younger age in Covunco section is compatible with the suggested age differences. The sedimentary record in the logged section was divided in three parts: a lower section containing marginal-marine deposits with tidal features like sigmoidal structures and muddy and coaly drapes, marine trace fossils (Asteriacites quinquefolius) and bioclastic deposits that are transitionally overlying the Los Molles Formation and end with fluvial deposits containing the vertebrate track. It is documentedat the top of an interval with3D dunes. The track is tridactyl and assigned to cf. Deltapodus. It constitutes the oldest record in Gondwana of a track attributed to tyreophorans. Isaberrysaura (Neoornitischia/Tyreophora) is the candidate producer, taking into account the stratigraphic and geographical proximity of the underlying Los Molles Formation that contains the skeletal remains. The track has some morphological features that suggest a possible swimming-wading behaviour as was previously proposed for stegosaurs. Overlying the fluvial interval, the middle section is composed of transgressive deposits containing abundant bioclastic conglomerates and channels with trunks. The upper section was deposited after a facies dislocation overlying a previously interpreted unconformity that suggests an abrupt change in the sedimentary record, containing pebbly sandstones intercalated with sandstone beds with Skolithos resembling “pipe rocks”. They are replaced upward by fine-grained sandstones with HCS, suggesting open marine conditions over the storm wave base level and a deepening upward trend. Then, the succession becomes sandier and highly bioturbated with abundant Rhizocorallium specimens and some lenticular channels with lateral accretion and abundant symmetrical wave ripple levels developed during a regressive phase.

Shrimps and leaves: Phytodetrital pulses and bioturbation in deposits of a river-dominated delta (Middle Jurassic Lajas Formation, Neuquén Basin, Argentina)

The Middle Jurassic Lajas Formation of the Neuquén Basin (Argentina) records mainly deltaic sedimentation. However, the precise role of the depositional mechanisms involved is still controversial. Well core YPF Nq. Laj-8 records the overall vertical transition from delta-front mouth bars to delta plain. Discrete mouth-bar successions typically comprise a lower unbioturbated sandstone unit that passes upwards into a more intensely bioturbated sandstone unit. Phytodetrital material is abundant through the mouth-bar succession forming distinct layers in the lower units and becoming part of the lining of the burrow walls of Ophiomorpha irregulaire and Gyrolithes isp. in the upper units, representing two successive developmental stages of deltaic sandy mouth bars. Layers rich in phytodetritus exhibiting the same characteristics have been traditionally interpreted in outcrop as mudstone drapes generated by tidal action during slack-water periods. However, more recent research suggests that these layers can be best regarded as produced by river-fed flows, which is consistent with a river-dominated delta model.

Facies and architecture of river-dominated to tide-influenced mouth bars in the lower Lajas Formation (Jurassic), Argentina

The Middle Jurassic Lajas Formation in the Neuquen Basin of Argentina has been considered an ideal analog for tide-dominated deposits because of the widespread presence of tidal sedimentary structures and its heterolithic nature. Outcrops of the Lajas in the Los Molles area provide excellent exposures allowing both sedimentary structures and large-scale architectural information to be documented in detail. The extensive event bed stratification that characterizes the delta front and interdistributary bay deposits is interpreted to be the result of river floods because of the sharp base, fining-upward grain size trend, and seaward-directed paleocurrent indicators within each bed. The absence of wave-generated structures precludes a storm wave origin for these beds. Interflood deposits have two forms. The first is fine-grained sandstone and siltstone that is highly bioturbated with no tidal structures and is indicative of an overall river-dominated system. The second is also finer grained than the river flood beds but shows distinctive tidal features, indicative of a tide-influenced system during times when river influence was least. Mouth bars can be classified as purely river-dominated or river-dominated with tidal influence, depending on the type of interflood beds present. Tide-influenced mouth bars tend to be muddier, with more internal flow baffles or barriers and a lower-angle dip than their river-dominated counterparts. This study suggests that heterolithic successions containing pervasive tidal structures need not be tide-dominated because not all tidal structures are of equal importance in environmental interpretations. Their location (in river flood layers versus interflood layers) can be used as a methodology for determining the dominant and subordinate processes responsible for shaping the delta.

Hyperpycnal delivery of sand to the continental shelf: Insights from the Jurassic Lajas Formation, Neuquén Basin, Argentina

AbstractHyperpycnal currents are river‐derived turbidity currents capable of transporting significant volumes of sediment from the shoreline onto the shelf and potentially further to deep ocean basins. However, their capacity to deposit sand bodies on the continental shelf is poorly understood. Shelf hyperpycnites remain an overlooked depositional element in source to sink systems, primarily due to their limited recognition in the rock record. Recent discoveries of modern shelf hyperpycnites, and previous work describing hyperpycnites deposited in slope or deep‐water settings, provide a valuable framework for understanding and recognizing shelf hyperpycnites in the rock record. This article describes well‐sorted lobate sand bodies on the continental shelf of the Neuquén Basin, Argentina, interpreted to have been deposited by hyperpycnal currents. These hyperpycnites of the Jurassic Lajas Formation are characterized by well‐sorted, medium‐grained, parallel‐laminated sandstones with hundreds of metre extensive, decimetre thick beds encased by organic‐rich, thinly laminated sandstone and siltstone. These deposits represent slightly obliquely‐migrating sand lobes fed by small rivers and deposited on the continental shelf. Hyperpycnites of the Lajas Formation highlight several unique characteristics of hyperpycnal deposits, including their distinctively thick horizontal laminae attributed to pulsing of the hyperpycnal currents, the extraction of coarse gravel due to low flow competence, and the extraction of mud due to lofting of light interstitial fluid. Recognition of shelf hyperpycnites in the Lajas Formation of the Neuquén Basin allows for a broader understanding of shelf processes and adds to the developing facies models of hyperpycnites. Recognizing and understanding the geometry and internal architecture of shelf hyperpycnites will improve current understanding of sediment transfer from rivers to deeper water, will improve palaeoenvironmental interpretations of sediment gravity‐flow deposits, and has implications for modelling potentially high‐quality hydrocarbon reservoirs.

Refining palaeoenvironmental analysis using integrated quantitative granulometry and palynology

Accurate palaeoenvironmental analysis is at the heart of producing reliable interpretations and depositional models. This study demonstrates a multivariate statistical approach to facies analysis based on relationships between grain size and quantitative palynology. Our methodology has the advantage that it can be used on small amounts of sample, such as core or well cuttings, as the basis for facies analysis. Proof of concept studies involving the collection of grain-size and palynological datasets from well-exposed outcrops of the Middle Jurassic, Lajas Formation of the Neuquén Basin, Argentina, demonstrate that canonical correspondence analysis can be used to consistently recognize facies and aid in the determination of depositional environments. This study demonstrates the link between depositional facies, grain-size distribution, palynomorph hydrodynamics and assemblage taphonomy of palynomorphs. This knowledge can be transferred into a semi-automated statistical facies prediction technique for the subsurface in complex depositional settings, particularly when calibrated against conventional sedimentary facies analysis. Supplementary material: The full set of grain-size data and statistical scores are available at: https://doi.org/10.6084/m9.figshare.c.3745481.v1

Recognition criteria, characteristics and implications of the fluvial to marine transition zone in ancient deltaic deposits (Lajas Formation, Argentina)

AbstractThe seaward end of modern rivers is characterized by the interactions of marine and fluvial processes, a tract known as the fluvial to marine transition zone, which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the fluvial to marine transition zone are preserved in the rock record, large‐scale outcrops of deltaic deposits of the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) have been investigated. Fluvial–tidal indicators consist of cyclically distributed carbonaceous drapes in unidirectional, seaward‐oriented cross‐stratifications, which are interpreted as the result of tidal modulation of the fluvial current in the inner part of the fluvial to marine transition zone. Heterolithic deposits with decimetre‐scale interbedding of coarser‐grained and finer‐grained facies with mixed fluvial and tidal affinities are interpreted to indicate fluvial discharge fluctuations (seasonality) and subordinate tidal influence. Many other potential tidal indicators are argued to be the result of fluvial–tidal interactions with overall fluvial dominance or of purely fluvial processes. No purely tidal or tide‐dominated facies were recognized in the studied deposits. Moreover, fluvial–tidal features are found mainly in deposits interpreted as interflood (forming during low river stage) in distal (delta front) or off‐axis (interdistributary) parts of the system. Along major channel axes, the interpreted fluvial to marine transition zone is mainly represented by the fluvial‐dominated section, whereas little or no tide‐dominated section is identified. The system is interpreted to have been hyposynchronous with a poorly developed turbidity maximum. These conditions and the architectural elements described, including major and minor distributary channels, terminal distributary channels, mouth bars and crevasse mouth bars, are consistent with an interpretation of a fluvial‐dominated, tide‐influenced delta system and with an estimated short backwater length and inferred microtidal conditions. The improved identification of process interactions, and their preservation in ancient fluvial to marine transition zones, is fundamental to refining interpretations of ancient deltaic successions.

Ichnology of Deltaic Mouth-Bar Systems of the Lajas Formation (Middle Jurassic) in the Sierra de la Vaca Muerta, Neuquén Basin, Argentina

Fil: Canale, Nerina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Patagonia Norte. Instituto de Investigacion en Paleobiologia y Geologia; Argentina

The role of tidal, wave and river currents in the evolution of mixed‐energy deltas: Example from the Lajas Formation (Argentina)

AbstractMany modern deltas show complex morphologies and architectures related to the interplay of river, wave and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed‐energy systems and their reservoir heterogeneities. The Lajas Formation, a sand‐rich, shallow‐marine, mixed‐energy deltaic system in the Neuquén Basin of Argentina, previously defined as a tide‐dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross‐stratified, sheet‐like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean‐water (i.e. very little mud is present in the system), sand‐rich deltaic system, very different from the majority of mud‐rich, modern tide‐influenced examples. The sand‐rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal‐current reworking, which produced amalgamated sandstone bodies in the delta‐front area, and a final stratigraphic record very different from the simple coarsening‐upward trends of river‐dominated and wave‐dominated delta fronts.

Sedimentological and Paleoclimate Modeling Evidence for Preservation of Jurassic Annual Cycles in Sedimentation, Western Gondwana

Abstract The Lajas Formation in the Neuquén Basin, Argentina, consists of a succession of mainly deltaic deposits. In the Middle Jurassic (170 million years ago), the basin was in western Gondwana roughly at the same paleolatitude as its present location (32°–40°S). Decimeter-scale, interbedded, coarser-grained and finer-grained beds in channelized and nonchannelized deltaic deposits have been interpreted as a product of variability in river discharge. The coarser-grained sandstone beds have erosional bases and contain mudstone clasts; internal cross bedding is commonly directed paleoseawards. These beds are interpreted as deposition during river-flood conditions. In contrast, the finer-grained beds are composed of interlaminated sandstone and mudstone, deposited during interflood periods. Bidirectional ripples and millimeter-scale sand–mud laminae suggest the influence of tides. This sedimentological evidence raises the question of whether these cycles represent annual variability in fluvial input. To answer this question, a simulation using the Fast Ocean Atmosphere Model for the Middle Jurassic was run to equilibrium. The model shows that the paleoclimate of the Neuquén Basin was characterized by a strong seasonal cycle, with a wet winter and a dry summer. Model runs suggest that February mean temperatures were 28°C with 4-mm precipitation (±4 mm standard deviation) per month, whereas August mean temperatures were 8°C with 34-mm precipitation (±17 mm standard deviation) per month. The strong seasonal cycles in the simulation, representing 24% of the variance in the precipitation time series, suggest that the sedimentological cycles represent annual variations. The simulation also suggests a Middle Jurassic climate where increased seasonality of precipitation occurred farther poleward than previously thought.