Article

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”

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

ResearchGate has not been able to resolve any citations for this publication.
Article
Full-text available
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.
Thesis
Full-text available
En el presente trabajo se analizan la estratigrafía y los paleoambientes deposicionales de la Formación Lajas (Jurásico medio) a partir del estudio de los afloramientos de esta unidad localizados en el suroeste neuquino, y relacionados a la Dorsal de Charahuilla-Plottier, en un área de 2800 Km2. La Formación Lajas, unidad eminentemente arenosa de ambiente de plataforma, litoral a continental, se dispone mediante un contacto diacrónico sobre pelitas de plataforma de la Formación Los Molles. A su vez es cubierta, mediante un contacto neto, por pelitas rojas de la Formación Challacó. A partir del levantamiento de 12 secciones estratigráficas de detalle se realizó primeramente un análisis de facies sedimentarias y posteriormente un análisis estratigráfico y estratigráfico secuencial de estos depósitos. Esto permitió diferenciar 19 facies sedimentarias, las que mediante su agrupamiento natural definen 13 asociaciones de facies, correspondientes a los siguientes ambientes y subambientes deposicionales: Plataforma costa afuera - prodelta; Barra de desembocadura dominada por ola/Playa; Barra mareal de plataforma a estuarina; Barra de desembocadura fluvio-dominada; Barra de desembocadura de "braid-deltas"; Llanura mareal; Llanura deltaica inferior, pantano interdistributario a bahía interdistributaria; Canal estuarino; Sistema fluvial meandriforme; Sistema fluvial entrelazado arenoso; Sistema fluvial anastomosado; Sistema fluvial entrelazado gravoso; Barreal. Mediante el análisis estratigráfico secuencial de las secciones relevadas se discriminaron 4 secuencias deposicionales de tercer orden (en el sentido de Vail et al., 1977), las que se denominan JC4, JC5, JC6 y JC7. La secuencia JC4 (Toarciano superior alto - Bajociano inferior alto) se reconoce como integrada por 5 secuencias deposicionales de un orden menor, denominadas JC4.1, JC4.2, JC4.3, JC4.4 y JC4.5. La secuencia JC4.1 es la más extendida, y corresponde principalmente a la Fm. Los Molles, en tanto que las secuencias restantes pertenecen a la Fm. Lajas. Las secuencias superiores (JC4.3, JC4.4 y JC4.5) se reconocen únicamente en el sector occidental del área de trabajo debido a problemas tectosedimentarios que se detallan más abajo. La secuencia JC5 (Bajociano inferior alto - Bajociano superior alto) muestra una amplia distribución en la zona de trabajo, correspondiendo al sector medio y superior de la Fm. Lajas. El hallazgo de Stephanoceras (Stemmatoceras) sp. (Zona de Humphriesianum a Rotundum) y de Teloceras aff. crickmayi chacayi (West. & Ricc., Zona de Rotundum) permitió datar bioestatigráficamente por primera vez a la Fm. Lajas en esta región. Las secuencias JC6 (Bajociano superior alto - Bathoniano inferior) y JC7 (Bathoniano inferior alto - Bathoniano superior alto) se integran con facies propias de la Fm. Challacó. La secuencia JC6 es la que presenta la mayor distribución regional. En base a evidencias estratigráficas se reconoce una importante actividad tectónica en la dorsal de Charahuilla-Plottier, la que habría enfatizado la discordancia estratigráfica que separa a las secuencias JC4 y JC5. En el sector occidental del área de estudio (Perfiles Lohan Mahuida y Puesto Pichonleo) esta discordancia está poco desarrollada, en tanto que en el sector centro-oriental se reconoce a partir de esta discordancia la pre¬sencia casi continua de un cuerpo sedimentario de hasta 35 metros de potencia, de la base de la secuencia JC5, el que forma la conspicua cornisa notada por varios autores en el flanco este de la Sierra de Chacaico. Se estima que dicha discordancia habría producido una truncación de unos 200 metros estratigráficos de la Fm. Lajas en este sector, ya que se encuentran ausentes las secuencias JC4.4 y JC4.5. Existen asimismo evidencias de actividad tectónica entre las secuencias JC6 y JC7, reconocida localmente en el Perfil Bosque Petrificado, y expresada mediante el desarrollo de una discordancia angular de unos 40-50º entre ambas secuencias. Cabe destacar que la secuencia JC7 se encuentra a su vez en discordancia angular por debajo de niveles conglomerádicos de la Fm. Quebrada del Sapo (= Fm. Tordillo) o cuando esta está ausente, de pelitas marinas de la Fm. Vaca Muerta.
Conference Paper
Full-text available
Jurassic synsedimentary tectonics in the southern part of the Neuquén Basin. Detailed field studies in the Quebrada del Sapo – Picún Leufú area allowed the recognition of nine unconformity-bounded stratigraphic units, which are partially equivalent to Los Molles, Lajas, Challacó, Lotena, Tordillo, Vaca Muerta and Quintuco formations. These units, ranging in age from Early Bajocian to Late Tithonian, often display folding, truncation and onlap relationships at their boundaries. These characteristics strongly suggest a tectonic control in their origin. The number and significance of different tectonic episodes is consistent with a complex deformational history mainly acting along two different structural axes, which were roughly oriented east-west and north-south. The east-west axis provides the best resolution due to the number of units involved. According to the stratigraphy, this last structure shows along time evidences of both compression and extension, that could be related to the shallow expression of tectonic movements of a deep high-angle (basement-involved?) fault system. Main compressive activity aparently occurred during the Bathonian, which is substantially older that previously considered for the area (Intramalm unconformity). Broadly speaking, compressive activity appears to be occurred during the deposition of overall regressive units (Lajas, Challacó and Tordillo formations), while major transgressions (Lotena and Vaca Muerta formations) apparently coincided with extensional events. These observations provide new insights to discuss the eustasy vs. tectonics imprints in the stratigraphic record of the Neuquén Basin. On the other hand, if the main buried fault responses to regional stresses, then the recorded activity could be used to analyze the progressive geodynamic evolution of the basin during the studied interval.
Article
Full-text available
El grupo Cuyo (Jurásico inferior a medio) constituye el primer evento de sedimentación marina generalizada en la Cuenca Neuquina. Integra una sección clástica progradante, en la que depósitos peliticos de offshore (Fm Los Molles) con ocasionales niveles de turbiditas, son sucedidos por acumulaciones de areniscas a conglomerados marinos de plataforma a continentales (Fm Lajas). En este trabajo se presenta un esquema estratigráfico secuencial para el Grupo Cuyo basado en estudios de afloramientos localizados en los alrededores de la Sierra de la Vaca Muerta. El relevamiento de ocho secciones estratigráficas de detalle complementado con un estudio fotoestratlgráfico ha permitido identificar cinco secuenoas deposicionales, las que muestran evidencias de actividad tectónica sindeposioonal. La secuencia 1 se reconoce con base cubierta en el sector sur de la zona de estudio, y se integra por areniscas de plataforma a litorales de naturaleza cuarzo-feldespática, con zonas de aporte localizadas principalmente en el sureste Las secuencias 2 a 5 se integran por areniscas y conglomerados de plataforma a litorales de naturaleza lítica, con aportes elásticos desde el suroeste. Dichas secuencias evidencian una progresiva migración hacia el norte del depocentro, el que es acompañado por un aumento general de la textura. El análisis estratigráfico permite aseverar que las secuencias 2 a 5 se habrían acumulado en depocentros elongados, con geometrías y facies controladas por actividad tectonica jurásica. Dicha actividad podría corresponder a un reflejo superficial de la inversión parcial de estructuras distensivas basamentales, como consecuencia de pulsos tectónicos compresivos desde el sureste (Macizo Nordpatagónico) durante el Bathoniano / Caloviano temprano. Los cambios en la zona de aporte permiten especular sobre el creo miento de una barrera física en la zona de la Dorsal de Huíncul, la que habría limitado progresivamente los aportes elásticos desde el sureste. En base a este estudio y anteriores en la subcuenca de Picún Leufú se discute la posición estratigráfica y significado de la Fm Challacó, y la posible pertenencia de la Formación Tábanos (evaporitas) al Grupo Lotena.
Article
Full-text available
A sequence-stratigraphic analysis, based on outcrop data, is presented for the Middle Jurassic Cuyo Group in the southern Neuquen Basin. The studied area is located 40 Km south of the city of Zapala, and comprises Middle Jurassic outcrops placed in the Lohan Mahuida - Picun Leufu - Cerro Lotena area. In these outcrops, the Cuyo Group comprises shallow marine to continental beds up to 1200 m of thickness. Twelve sedimentary sections were measured through the succession, in which facies and sequence stratigraphic analysis have been carried out. The study allowed to recognize eight depositional sequences, related to both third and fourth-order base level cycles. Thirdorder sequences, named JC4, JCS, JC6 and JC7, show an internal predictive succession of systems tracts, indicative of a mainly eustatically driven mechanics. Except for the last two sequences, which are continental deposits all over this area, ammonoid fossils remains indicate an Upper AalenianUpper Bajocian age. The JC4 depositional sequence is internally composed by a progradational fourth order sequence set. These minor sequences are named JC4.l, JC4.2, JC4.3, JC4.4, and JC4.S. Local to regional studies allow to recognize an extensive truncation - non deposition episode (up to 200 meters) between the JC4.1-JC4.2 and JCS sequences, because three fourth order depositional sequences (JC4.3, JC4.4, and JC4.S) are missing in the central-east area. Tectonic evidences have also been found between the JCS-JC6 and JC6-JC7 sequences in the Puesto Bascuii.an and Bosque Petrificado area. In this last position, the JC7 sequence rests over the JC6 sequence with a SO" angular uncorformity. The tectonic vs. eustatic controls of each sequence boundary is discussed. A detailed outcrop map of each recognized depositional sequences in the studied area is also included.
Article
Full-text available
The Middle Jurassic Cuyo Group in the~uthern Neuquen Basin comprises shallow marine to continental beds up to 1200 m of thickness. Twelve sedimentary sections were measured through the succession, in which facies and sequence stratigraphic analysis have been carried out. The study allowed us to recognize eight depositional sequences, related to both third and fourth-order scales (sensu Exxon). Because these outcrops extend from mainly fluvial deposits on the east to coeval shallow marine deposits on the west, they allow to analyze sequence stratigraphic relationships, facies, and paleoenvironmental changes during the evolution of a depositional sequence. (1) In shallow marine environments, third-order sequences start with a major erosive and non-depositional event followed by up to 20 meters of sandy-braided fluvial to high sinuosity estuarine channel deposits linked with a late lowstand systems tract - early transgressive systems tract stage. The transgressive systems tract deposits are characterized by 2-6 meters thick shallowing upward tidal bars with a retrogradational parasequence set. Highstand systems tract deposits starts with open-shelf mudstones followed by 4-6 meters of shallowing upward cycles of input to wave dominated stream-mouth bars, with a progradational parasequence set. (2) In mainly continental areas, third order sequences begin with up to 10 meters of coarse grained braided-river deposits resting over a regional discontinuity. These deposits are interpreted as developed in early transgressive systems tract stage. The Transgressive systems tract deposits are expressed by 12-14 meters of marsh levels, with tidal influence. The highstand systems tract deposits start with off-shore marine mudstones, followed by a strongly prograding input-dominated deltaic systems, and ending with thick high-sinuosity sandy fluvial deposits.
Article
Full-text available
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.
Article
Full-text available
The 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 (FMTZ), which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the FMTZ 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 FMTZ. Heterolithic deposits with dm-scale interbedding of coarser- 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 FMTZ 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 FMTZs, is fundamental to refining interpretations of ancient deltaic successions.
Article
Full-text available
Discontinuity bounded units: their application to the Andean Jurassic. Sequences as originally defined are equivalent to Unconformity-Bounded Units (UBU). Differences bctween UBU and depositional sequences, as defined in seismic or sequential stratigraphy, based on conformities vvhich include "insignificant" hiatus, are not enough to accept two different types of stratigraphic units. Therefore, in order to simplify the stratigraphic classification and nomenclature UBU´s should be broadened to accommodate "depositional sequences". UBU's name should then be changed to "Discontinuity-Bounded Units" (DBU). Discontinuities including unconformities, disconformities, paraconformities, and sedimentary discontinuities (i.e. unconformities and their "correlative conformities") bounding depositional sequences. UBU, depositional sequences and DBU are objective stratigraphic units, and all them have chronostratigraphic significance because they were formied during a given interval of time, as defined by the ages of the oldest and youngest rocks included in them. Groeber's Jurassic cycle and subcycles referred both to the sedimentary events and the resulting depositional units. As proposed by subsequent authors they may be understood as southamerican chronostratigraphic units, or local lithostratigraphic units. As part of Groeber's "subcycles" refer to transgressive-regressive successions they could also be considercd as cyclothems or hemicyclothems. However, as defined by Groeber (1946) all features, i.e. magnitude, boundaries and content, place these "cycle" and "subcycles" within the DBU. Thus, the "Jurassic" of west central Argentina is here named "Araucanian Synthem". It comprises the Sañico, Cuyo and Lotena-Chacay subsynthcms, each of them including several miosynthems.
Article
Full-text available
Ichnologic and sedimentologic studies of the Lajas Formation (Middle Jurassic) in Sierra de la Vaca Muerta allowed the recognition of two different types of deltaic mouth bars, each of them showing trace fossil suites with different characteristics. Type I deltaic mouth bars consist of fine to coarse sandstones and fine conglomerates completely reworked by fair-weather and storm wave action, revealing a predominance of basinal hydraulic processes (e.g., waves) during bar construction and progradation. Trace fossil assemblages are composed of Ophiomorpha and Haentzschelinia in the foreset beds, and Polykladichnus, Skolithos, and Arenicolites in the topset beds. Type II deltaic mouth bars comprise sandstones that are fine to coarse and massive or present high angle cross-stratification and current ripples migrating in the opposite direction to the inclination of the foresets. These bars are interpreted to have been deposited during intervals of extraordinary fluvial discharge when wave action was restricted to the topset part of the bars. Whereas equilibrium trace fossils occur in the bottomset beds, escape trace fossils and Ophiomorpha are recorded in the distal foreset beds. In the topset beds, Skolithos and Polykladichnus specimens are very abundant. In general, the two types of mouth bars show low diversity, low abundance of trace fossils and a simple tiering structure. Such traits reflect environmental stresses mainly produced by fluctuating hydraulic energy, salinity, sediment input and high mobility of the substrate.
Article
Full-text available
Fluvial discharge fluctuations are a fundamental characteristic of almost all modern rivers and can produce distinctive deposits that are rarely described from ancient fluvial or mixed-energy successions. Large-scale outcrops from the Middle Jurassic Lajas Formation (Argentina) expose a well-constrained stratigraphic succession of marginal-marine deposits with a strong fluvial influence and well-known tidal indicators. The studied deposits show decimetre-scale interbedding of coarser- and finer-grained facies with mixed fluvial and tidal affinities. The alternation of these two types of beds forms non-cyclic successions that are interpreted to be the result of seasonal variation in river discharge, rather than regular and predictable changes in current velocity caused by tides. Seasonal bedding is present in bar deposits that form within or at the mouth of minor and major channels. Seasonal bedding is not preserved in channel thalweg deposits, where river flood processes were too powerful, or in floodplain, muddy interdistributary bay, prodelta and transgressive deposits, where the river signal was weak and sporadic. The identification of sedimentary facies characteristic of seasonal river discharge variations is important for accurate interpretation of ancient deltaic process regime.
Conference Paper
Full-text available
DISCONTINUITY—BOUNDED UNITS. APPLICATION TO THE ANDEAN JURASSIC.- Sequences as originally defined are equivalent to Unconformity—Bounded Units (UBU), and depositional sequences as defined in seismic or sequential stratigraphy could be considered as identical to cyclothems. Differences between UBU and depositional sequences, based on conformities which include “insignificant" hiatus are not clear cut. Therefore, in order to simplify the stratigraphic nomenclature UBU should be broadened to accommodate “depositional sequences". UBU's name should then be changed to "Discontinuity—Bounded Units" (DSU). Discontinuities, including unconformities, disconformities, paraconformities, and sedimentary discontinuities (i.e. unconformities and their "correlative conformities”) bound depositional sequences. UBU, DBU and depositional sequences are objective stratigraphic units, and all them have chronostratiqraphic significance because they were formed during a given interval of time, as defined by the ages of the oldest and youngest rocks included in them. Groeber's Jurassic cycle and subcycles referred both to the sedimentary events and the resulting depositional units. As proposed by subsequent authors they may be understood as southamerican chronostratigraphic units, or local lithostratigraphic units. As part of Groeber's "subcycles" refer to transgressive-regressive successions they could also be considered as cyclothems or hemicyclothems. However, as defined by Groeber (194ó) all features, i.e. magnitude, boundaries and content, place these “cycle” and “subcycles” within DBU. Thus, the “Jurassic” of west central Argentina is here named “Araucanian Synthem”. It includes the Sañico, Cuyo and Lotena-Chacay Subsynthems.
Article
Full-text available
The present monograph deals with the description of 47 species Trigoniidae from the Jurassic and Early Cretaceous of the Neuquén province, Argentina. The treated genera, distributed amongst sufamilies Trigoniinae, Myophorelliinae, Vaugoniiae, Megatrigoniinae, Pterotrigoniinae, Laevitrigoniinae and Minetrogoniinae are: Trigonia, Neuquenitrigonia nov., Frenguelliella, Jaworskiella, Myophorella, Scaphotrigonia, Steinmanella, Vaugonia, Maputrigonia, Anditrigonia, Antutrigonia nov., Rutitrigonia, Iotrigonia, Pterotrigonia, Quoiecchia and Myophorigonia, Neuquenitrigonia gen. nov. is established with Trigonia hünickeni H. Leanza & Garate as type species, and Antutrigonia gen. nov. is erected on the basis of Trigonia opistolophora Lambert, and also includes T. frenguellii and T. groeberi by showing transverse ribbing on the area. The genus Quoiecchia is reported for the first time in the Andean. Early Cretaceous. Four new sp. And Quolecchia sigeli n. sp. The bioestratigraphical analysis allowed to distinguish the following trigoniid associations: 1) Jaworskiella burckhardti and Frenguelliella tapiai association (Pliensbaquian); 2) Trigonia corderoi association (Bajocian to early Callovia); 3) Trigonia mirandaensis association (Kimmeridigian); 4) Anditrigonia carrincurensis association (Tithonian); 5) Maputrigonia hugoi association (Beriasian); 6) Steinmanella quintucoensis association (Valaginian) and 7) Steinmanella vacaensis association (Hauterivian).
Article
Full-text available
The genus Iotrigonia van Hoepen (1929) is based on a poorly-known species which has not been re-described since its inception. Treatise interpretation of the genus (Cox 1969) rested largely on an older referred species, I. haughtoni (Rennie), which differs significantly from the type, thus changing the concept of the genus. Here the genus is re-diagnosed and evolutionary relationships discussed. Of the 44 species assessed, 9 are incorrectly allocated and the remainder are distributed among 8 genera of which 5 are new.
Article
Full-text available
The Triassic red beds of the Caturrita Formation (Paraná Basin, southern Brazil) contain abundant Skolithos, common Taenidium, and rare Arenicolites in low-angle, trough cross-bedded fine-grained sandstones interpreted as crevasse-splay deposits, forming a Skolithosdominated composite piperock and providing easily mappable horizons. Skolithos cf. serratus is the dominant trace fossil, being closely spaced (∼ 160 burrows/m2), slightly inclined, and having sharp, irregular boundaries with delicate scratch ornament. The average burrow length is approximately 0.2 m, but some may extend as deep as 0.4 m into the substrate. They crosscut a previous monospecific Taenidium barretti ichnocoenosis, formed by vertical meniscate burrows, which is superimposed on a previous Skolithos linearis ichnocoenosis, formed by abundant S. linearis, subordinate horizontal T. barretti, and rare Arenicolites isp. Colonization was firstly generated in softgrounds, maybe under subaqueous conditions, and reveals gradual substrate desiccation, the S. cf. serratus being produced in firmgrounds. The analyzed specimens of Skolithos resemble modern tiger beetle and midge larva burrows, while Taenidium is similar to modern oligochaete burrows. The dense concentration of vertical burrows reveals opportunistic colonization and suggests important environmental (climatic?) changes opening the colonization window to insects in floodplains. This highlights the stratigraphic significance of the colonized surfaces, which represent nondepositional hiatuses.
Conference Paper
Full-text available
MARINE HETTANGIAN AND SINEMURIAN IN ARGENTINA.- On the northern bank of the Atuel river, Mendoza province, Argentina, the occurrence of Hettangian and Lovver Sinemurian marine fossiliferous levels has been documented for the first time. Up to now, it was usually accepted that the marine Jurassic sequence in the area started with Upper Sinemurian strata (Epophioceras Faunule). Representatives of the genera Psiloceras Hyatt, Caloceras Hyatt, Alsatites Haug, Waehneroceras Hyatt, Schlotheimia Buyle, Sulciferites Spath, Badouxia Guex y Taylor, Vermiceras Hyatt, Coroniceras Hyatt, Agassiceras Hyatt, Euagassiceras Spath and Arnioceras Hyatt, thus indícate the presence of beds equivalent to the Planorbis, Liasicus, Angulata, Bucklandi, Semicostatum, (and Turneri) Zones of tho Standard International Chronostratigraphic Scale. Hence, in Argentina, the marine Hettangian, Sinemurian and lowermost Pliensbachian have only been proved in the neighborhoods of the Atuel river region, vvhere marino Triassic could also be represented. Elsevvhere in Argentina the oldest Jurassic ammonite bearing levels belong to the upper Lower and Upper Pliensbachian. Regionally, these findings imply a substantial modification to previous paleogeographic reconstructions for the Early Jurassic of South America.
Article
Full-text available
El estudio de matas microbianas en ambientes marinos costeros silicoclásticos actuales constituye una importante fuente de información para el análisis de análogos fósiles. En el estuario de Bahía Blanca se estudiaron planicies de marea cubiertas por matas desde un enfoque geobiológico, con el objetivo de identificar y describir estructuras inducidas por actividad microbiana (ESIAM), y distinguir rasgos presentes en los sistemas actuales que permitan reconocer estas comunidades de microorganismos en depósitos fósiles. Se realizaron muestreos biológicos y sedimentológicos, y se implementaron técnicas de microscopía óptica y electrónica para la determinación de microorganismos y la identificación de texturas características y minerales autigénicos. Las matas microbianas resultaron estar dominadas por cianobacterias y, en menor proporción, diatomeas; microorganismos que generan sustancias exopoliméricas que bioestabilizan la superficie sedimentaria. Se reconocieron texturas típicas de matas epibentónicas, tales como granos orientados y granos pequeños unidos por la mata, pirita framboidal y ceolitas. En la zona intermareal-supramareal se identificaron domos de gas, fábrica porosa esponjosa y superficies tipo colador. Además se observaron grietas de contracción, dobleces, fragmentos de mata, depresiones y remanentes erosivos, pliegues y arrugas, así como también ondulitas multidireccionales. Las estructuras halladas están relacionadas con condiciones de calma/latencia, exposición prolongada (desecación) y eventos de tormenta con alta energía erosiva sobre la planicie (olas y corrientes fuertes). En este sentido, las ESIAM resultan importantes indicadoras de condiciones ambientales particulares que presentan un elevado potencial de preservación. Su reconocimiento en el registro fósil, a través de rasgos macro y microscópicos, permiten refinar las interpretaciones paleoambientales en sucesiones marino-costeras.
Article
Full-text available
Crevasse subdeltas develop on modern river-dominated delta plains, and may be affected by the interaction of river currents and marine processes. However, their sedimentology and stratigraphic architecture is poorly constrained, leading to simplistic depositional models of delta-plain systems in the ancient record. Extensive exposures of the Middle Jurassic Lajas Formation permit the architecture, main stratigraphic surfaces, and lateral and vertical facies variations of crevasse subdelta deposits to be constrained. Lower-delta-plain successions studied in the Lajas Formation consist of up to 5-m-thick distributary channels and interdistributary-bay deposits, interpreted as crevasse subdeltas. Crevasse subdelta deposits consist of small-scale lenticular units (∼ 1–2 m thick) interpreted as crevasse channels and upward-coarsening and upward-thickening packages (∼ 2 m thick) with clinothems interpreted as crevasse mouth bars. These deposits preserve interbedding of coarser and finer sediments that are interpreted as river flood and interflood couplets associated with variations in river discharge. River flood beds are commonly structureless and erosionally based, and show little evidence of tidal action and brackish-water conditions. Interflood deposits show rhythmically distributed mudstone drapes, bimodality, and brackish trace fossils. This study highlights an important but largely undocumented component of interdistributary deposits consisting of tide-influenced, but strongly river-dominated, prograding depositional bodies. An implication is that some coarsening-upward, forward-accreting units previously interpreted from the rock record as interchannel “tidal bars” may instead represent minor mouth bars of tide-influenced crevasse subdeltas. Furthermore, present-day crevasse subdeltas are restricted to river-dominated delta systems that flow into semi-enclosed or enclosed seas and lakes with microtidal conditions and limited wave action, which is comparable to paleogeographic reconstructions for the Neuquén Basin during the Middle Jurassic.
Article
Full-text available
Jurassic clastic wedges sourced from the Huíncul Arch. A case study in the Picún Leufú area. Neuquén Basin, Argentina. The Huíncul Arch constitutes an extensive and complex east west oriented mesozoic tectonic structure located in the southern part of the Neuquén Basin. The presence of structural and stratigraphic barriers all along this ancient structure controlled the migration and trapping of hydrocarbons, resulting in the occurrence of many oil fields at both sides of the uplifted area. Although evidences of ancient subaerial exposure and erosion of Early to Middle Jurassic strata in the axes of the Huíncul Arch seem to be clear at some outcrops, deposits related to these erosional surfaces were not previously been identified and mapping in the field. This paper also documents for the first time the existence of deposits supplied from the erosion of uplifted Jurassic strata along the Huíncul Arch. These clastic wedges (displaying "abnormal" paleocurrents from the northeast) occurred in the "Challacó" (hereafter called Bosque Petrificado Fm), Lotena and Tordillo formations thus suggesting uplift episodes older than those previously considered. In the case of the Tordillo Formation, in the area comprise two clearly differentiable sub-units (sequences), bounded by an unconformity: a basal sub-unit (sequence 1), composed of conglomerates and red mudstones of fluvio-lacustrine origin, and an upper sub-unit (sequence 2), integrated by sandstones of aeolian origin (dunes and dry interdunes). Facies changes and paleocurrents indicate that the conglomerates of the sequence 1 were supplied from the northeast, and probably derived from the erosion of coarse-grained deposits of the Cuyo Group. Sequence 2 is disposed over a deflation surface. This last sequence is in turn unconformably covered by offshore marine mudstones of the Vaca Muerta Formation. INTRODUCCIÓN La Dorsal de Huíncul constituye un elemento morfoestructural positivo antiguo, el cual se habría configurado de forma compleja en varias etapas especialmente a partir del Jurásico medio (Orchuela et al. 1981; Bettini 1984; Ploszkiewicz et al. 1984; Vergani et al. 1995; Zavala y González 2001; Freije et al. este congreso). Las evidencias de campo y subsuelo sugieren que esta estructura habría tenido su principal evolución con anterioridad al Cretácico tardío, ya que las capas rojas del Grupo Neuquén que sellan el relieve antiguo no han sufrido importantes modificaciones en tiempos posteriores. De esta manera, esta estructura compleja habría actuado como una barrera estructural y estratigráfica efectiva para la migración de los hidrocarburos, resultando en la localización de numerosos yacimientos a ambos lados de la dorsal antigua (Cruz et al. 2000). De modo particular, las etapas evolutivas de la Dorsal de Huíncul enunciadas precedentemente involucraron períodos de marcada deformación con generación de relieve subaéreo, el cual habría sido objeto de erosión y redepositación. En este sentido Vergani et al. (1995) citan una erosión de más de 2000 metros en las zonas axiales. Conceptualmente, los materiales clásticos derivados de dicha erosión se habrían acumulado al pie del relieve antiguo a ambos lados de la Dorsal, constituyendo cuñas clásticas. En virtud de su geometría lenticular y su empaquetamiento relativamente abierto, estas cuñas clásticas constituyen potenciales reservorios de relativo interés. En este trabajo se presenta un estudio de detalle de los afloramientos del Jurásico medio-tardío localizados cerca del hundimiento Este del anticlinal de Picún Leufú, en las vecindades del puente de la ruta 40 sobre el arroyo del mismo nombre (Fig. 1). En esta sucesión se reconocen distintas unidades clásticas que permiten caracterizar de manera más precisa la geometría, extensión y cambios de facies reconocibles dentro de las mencionadas cuñas clásticas. Cabe destacar que esta contribución representa la versión extendida del trabajo presentado en la Hedberg Conference desarrollada en Mendoza en el año 2001 (Zavala y Freije 2001).
Article
Full-text available
INTRODUCCIÓN La Cuenca Neuquina se localiza en el centro-oeste de la Argentina, con un relleno sedimentario de más de 7000 metros dispuesto sobre corteza continental. Estudios regionales sugieren la existencia de un arco volcánico algo elevado localizado al oeste, el que en algunas ocasiones habría aislado la cuenca del Océano Pacífico (Mutti et al. 1994). Dentro de este contexto, el límite entre los grupos Cuyo y Lotena representaría la primera desconexión con el mar abierto. En la presente contribución se discuten algunas observaciones de campo de este contacto efectuadas en Mallín del Rubio, Sierra de la Vaca Muerta y algo más al sur, en la zona de Los Catutos (Fig 1). Figura 1: Mapa de ubicación de las localidades analizadas; 1) Los catutos; 2) Mallín del Rubio Dichas observaciones de campo se enmarcan dentro de un estudio regional que comprendió además el levantamiento de más de 6000 metros de secciones estratigráficas de detalle de ambos grupos, complementadas con un mapeo de fotohorizontes (Zavala y González 2001). SOBRE EL SIGNIFICADO DE LA DISCORDANCIA INTRACALOVIANA En la Sierra de la Vaca Muerta existen claras evidencias de actividad tectónica compresiva sinsedimentaria, relacionada posiblemente a la inversión parcial de bloques de basamento, durante la depositación del Grupo Cuyo, más precisamente la Formación Lajas (Zavala y González 2001). Dicha inversión tectónica habría producido una migración progresiva del depocentro de la Formación Lajas hacia el norte, condicionando substancialmente la depositación.
Article
Full-text available
This work is based on the comparison between facies tracts of flood-dominated fluvio-deltaic systems and basin-plain turbidites to the main combined flow experimental data available in literature; it discusses the possibility of a genetic link between sigmoidal and hummocky structures and their significance in facies analysis. Sigmoidal and hummocky-cross stratifications are large-scale sedimentary structures, usually considered indicative of tidal and storm deposits, respectively. However, facies analysis of flood-dominated fluvio-deltaic systems in tectonically active settings shows that these two types of structures are also typical of these depositional systems. In flood-dominated river-delta systems, coarse-grained mouth bars, which can be characterized by different types of sigmoidal-cross stratifications deposited by sediment-laden stream flows entering seawater, pass down-current into fine-grained delta-front sandstone lobes. That is to say, sharp based normally graded beds with hummocky-cross stratifications, deposited by flood-related hyperpycnal flows characterized by an oscillatory component, whose origin can be related to different processes. At a small scale, biconvex and rounded ripples and megaripples with sigmoidal-cross laminae are related to small- and medium-scale hummocky structures in basin plain turbidites, where ponding and rebound processes can transform the turbidity currents into combined flows. These field observations suggest a genetic link, at different scales, between these two types of structures, especially in terms of combined flows. This study, therefore, has prompted a re-examination of the combined-flow sedimentary structures produced in laboratory experiments, and has led to the proposal and discussion of some facies schemes (small- and large-scale) based not only on the ratio of Uu (unidirectional velocity) to Uo (oscillatory velocity) but also upon grain sizes, rates of fallout and frequency of oscillatory component (i.e. the period T).
Article
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.
Article
Hyperpycnal 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 deepwater 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. This article is protected by copyright. All rights reserved.
Book
Ichnology is the study of traces created in the substrate by living organisms. This is the first book to systematically cover basic concepts and applications in both paleobiology and sedimentology, bridging the gap between the two main facets of the field. It emphasizes the importance of understanding ecologic controls on benthic fauna distribution and the role of burrowing organisms in changing their environments. A detailed analysis of the ichnology of a range of depositional environments is presented using examples from the Precambrian to the recent, and the use of trace fossils in facies analysis and sequence stratigraphy is discussed. The potential for biogenic structures to provide valuable information and solve problems in a wide range of fields is also highlighted. An invaluable resource for researchers and graduate students in paleontology, sedimentology and sequence stratigraphy, this book will also be of interest to industry professionals working in petroleum geoscience.
Article
Taphonomic analysis was performed on palynological samples of marine successions from the Cuyo Group of the Neuquén Basin. The group comprises the Los Molles and Lajas Formations (Middle Jurassic) and represents the first major marine depositional episode after the configuration of the basin. A total of 21 outcrop samples from five sections, located in the central-western part of the basin, were analyzed. The palynological counts of each sample were carried out taking into account the preservation state of the palynomorphs, which was mainly influenced by two parameters: their intrinsic properties and the environmental depositional conditions. Four main deterioration types (corrosion, degradation, mechanical damage, and concealment by authigenic minerals) and their six combinations were identified. The growth of pyrite crystals within the palynomorphs was strongly influenced by the exine structure, being dominant in bisaccate pollen grains. Mechanical damage appears to be the most important deterioration type, reflecting the highly dynamic conditions of shelfal marine environments. The term Palynotaphofacies (Pt) was proposed to characterize a sedimentary rock yielding palynological associations with distinctive preservational features. Three palynotaphofacies have been recognized: Pt-1 (tidal-influenced environments) is characterized by the highest percentage of deteriorated forms, reflecting either intermittent exposure to weathering or intense reworking. Pt-2 shows the highest percentage of well-preserved palynomorphs associated with rapid burial due to a high sedimentation rate and it comes from the delta-front facies of fluvial-dominated deltas. Finally, Pt-3 was identified in samples of prodelta hyperpycnites with a wide predominance of terrigenous components and it is characterized by the coexistence of well-preserved and deteriorated palynomorphs.
Article
Experiments were conducted to define equilibrium combined-flow bed configurations developed under a wide range of oscillatory and unidirectional velocity components, with a constant oscillation period of 8.5sec and median grain size of 0.09mm. Experiments support the idea that the cross-stratification is produced by combined flow: the large 3D ripples produced at high oscillatory velocities and small to moderate unidirectional velocities would presumably generate what might be called anisotropic hummocky cross-stratification. -from Authors
Article
The Middle Jurassic Lajas Formation of the Neuquén Basin, Argentina, is interpreted as a 400–600 meter thick aggradational, macrotidal tide-dominated deltaic system. Tide-dominated systems are most commonly found in estuarine settings, and thus the biota found in such settings are generally influenced by lowered salinities. This paper documents the ichnology of the tide-dominated Lajas Formation, which includes a diverse ichnofauna, much of which is subject to tidal conditions, though not necessarily to reduced salinity. The ichnogenera recorded include: Asteriacites, Arenicolites, Asterosoma, Dactyloidites, Didymaulichnus, Diplocraterion, Chondrites, Cruziana, Helminthorhaphe, Macaronichnus, Ophiomorpha, Parahaentzschelinia, Palaeophycus, Planolites, Polykladichnus, Protovirgularia, Rhizocorallium, Rosselia, Schaubcylindrichnus, Scolicia, Siphonichnus, Taenidium, Teichichnus, and Thalassinoides. The range of tidal environments from which the trace fossils were collected encompasses tidal flats, tidal channels, and tide-dominated delta fronts, passing offshore into shelf mudstones of the Los Molles Formation and landward into the fluvial deposits of the Challaco Formation. The paleoenvironmental controls on the distribution of ichnotaxa are discussed in relation to Lajas Formation ichnology and sedimentology.
Book
This book describes microbially induced sedimentary structures as new group in the Classification of Primary Sedimentary Structures. Those structures rise from the interaction of benthic microorganisms with physical sediment dynamics caused by waves and currents. We can observe the formation of those features today in modern coastal settings, where the structures are much more common than stromatolites. However, fossil structures occur since the earliest Archean, and are abundant throughout Earth history. The structures constitute a significant archive for the understanding of Earth's earliest worlds, and are important biosignatures for the detection of life on other planets. This book is for sedimentologists, paleontologists, microbiologists, and geobiologists likewise.
Article
Many modern deltas show complex morphologies and architectures related to the interplay of river, waves 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.This article is protected by copyright. All rights reserved.
Article
There are few recognized examples of fluvial-dominated deltaic systems affected by hyperpycnal discharges. The development of predictive models in those systems are essential to understand the distribution of sedimentary facies and for determining the location of the main sandy bodies, which constitute the potential hidrocarbon reservoirs. In Portada Covunco and Sierra de la Vaca Muerta areas (Neuquén, Argentina), the Lajas Formation constitutes an excellent example of a deltaic fluvio-dominated succession consisting of prodelta and deltaic front deposits, affected by hyperpycnal discharges and reworked by wave action (storm and normal wave action). Prodelta deposits show a strongly tabular geometry alternating between mudstones and fine sandstones with abundant organic matter and development of an impoverished Cruziana ichnofacies. Deltaic-front deposits are integrated by mudstones, fine to coarse sandstones and fine conglomerates showing tabular geometries and development of the Skolithos and Glossifungites ichnofacies, the later related to deltaic lobe avulsión. Normal progradation of this deltaic system shows stratigraphic intervals consisting of tabular and lenticular bodies of variable thicknesses assigned to hyperpycnal-channel and lobe systems. Internally, these deposits show transitional and recurrent passages between different sedimentary structures with marked textural variations, multiple internal reactivation surfaces and abundant organic matter content. Hyperpycnal channels systems have different dimensions and fill patterns. At the foot of the areas with the greatest gradients, hyperpycnal discharges produced mayor channels with aggradational fill, whereas in areas with lower gradients, high sinuosity channels developed. Hyperpycnal lobes were accumulated in areas where hyperpycnal flows lost confinement. The presence of conglomeratic levels affected by wave action on top of the lobe deposits indicates pauses in sedimentation during which colonization windows could develope, allowing the establishment of the benthic fauna.
Book
1 SUMMARY - In peripheral foreland basins, turbidite systems are characterized by relatively simple depositional settings. The fundamental expression of these systems are huge accumulations of basinal, cyclically-stacked sandstone lobes with an impressive lateral extent. These accumulations, representing potential reservoirs for hydrocarbons, record the main depositional zone of large-volume turbidity currents originated from hyperpycnal flows emanating from flood-dominated deltaic systems along basin margins. Large-scale submarine erosional features, cut into shelfal and slope sediments, acted as conduits of turbidity currents during their basinward motion. Treating turbidite systems in terms of transfer and depositional zones of turbidity currents may avoid much confusion, both conceptually and in practice. Whatever the depositional model, transfer and depositional zones of turbidity currents will exist in each turbidite basin considered. Like in a fluvial system (in the sense of Schumm, 1977), turbidity currents originate (source zone), flow (transfer zone) and will eventually decelerate to the point that all their sediment load will be deposited (depositional zone). Regardless of their size, plan-view and cross-sectional geometry, and facies types, these deposits record the depositional zone of the currents considered. - Turbidite sandstone beds appear to be the deposit of bipartite turbidity currents consisting of a basal, high-density and overpressured granular flow overlain by a low-density, fully turbulent flow. These two flows are the basic components of a turbidity current whose sediment load includes coarse-grained sediment. This concept is supported by both theoretical and experimental work (e.g., Sanders 1965; Ravenne and Beghin, 1983; Norem et al., 1990). Conglomerate, pebbly-sandstone and relatively coarse-grained sandstone facies are the typical deposit of granular flows. The genuine Bouma sequence, as intended in these notes, forms farther basinward and is primarily the deposit of relatively low-density and fully turbulent flows which have outdistanced their parental inertia flows in basinal regions. - A basal, overpressured granular flow, mainly driven by inertial forces and excess pore pressures, is needed to form laterally extensive beds of relatively coarse-grained sediment, i.e. the turbidite reservoir facies. This kind of flow has been described and termed in many different ways (e.g., high-density turbidity current, modified grain flow, debris flow, hyperconcentrated flow, flow-slide), thus generating considerable confusion among sedimentologists and stratigraphers. - Coarse-grained granular flows, probably originated directly from flood-generated subaerial flowslides and their bulking through erosion and sediment failures in delta-front regions, are funneled in relatively steep submarine conduits along which they progressively accelerate until reaching a phase of catastrophic bed erosion. As a consequence, a large amount of fine-grained sediment is incorporated within the upper, turbulent part of each flow, thus increasing its density, thickness and velocity. At this point a fully-developed, bipartite turbidity current is formed. - All other things being equal, the processes controlling sediment transport and deposition within a turbidity current during its basinward motion are essentially related to the ability of the basal granular flow to maintain its excess pore pressure and to the amount of turbulent energy developed in the upper and more dilute part of the flow. The first factor controls the runout distance of the granular flow and therefore the lateral extent of coarse-grained facies. The second factor controls how much finegrained sediment (fine sand and mud) can be resuspended from the basal granular layer and transported farther basinward, and how much traction the turbulent flow can exert on the residual coarser-grained sediment of the granular layer. These two factors combine in determining the degree of efficiency of a turbidity current, i.e. its ability to carry its sediment load in abasinward direction and segregate the original grain populations of this load into distinct and relatively well-sorted facies types with distance. Flow efficiency is therefore of fundamental importance in controlling geometry and reservoir characteristics of turbidite sandstone beds. - Highly efficient turbidity currents dominate transport and deposition in large and elongate foreland basins. Facies tracts reconstructed from these basin fills through careful stratigraphic correlations permit the recognition of the many different processes (erosion, water escape, bypass, reworking, deposition) that characterize a turbidity current during its basinward motion. Facies and processes can thus be framed into models of fairly good predictive value. - Particularly during their earlier stages of development, foreland basins have apparently a relatively simple topography. Turbidity currents accelerate when moving along their steeply sloping conduits and decelerate at the exit of these conduits due to the lateral spreading of the flow. After moving across substantially flat basin-proximal regions (lobe region), where they progressively outdistance their parental granular flows, turbulent flows may reach sufficient thickness to experience reflection from bounding slopes and ponding in terminal basin-plain regions. Structurally complex settings require that facies distribution pattern be considered only on a case-by-case basis. - Turbidity currents may be forced to deposit much of their sediment load without developing a significant segregation of their load if: (1) local topographic obstacles decelerate the currents; (2) parental granular flows cannot reach the catastrophically erosive phase either because they are too thin (small volume) or move on insufficiently steep slopes; (3) parental granular flows cannot maintain their excess pore pressure long enough because of their low mud content. In all these cases, turbidity currents will be poorly efficient, generating facies tracts composed of a limited numberof facies, each characterized by relatively poor sorting and, therefore, poor reservoir quality. - Sequence stratigraphy and sedimentary cyclicity of turbidite basin fills has to be treated with great caution. As amply discussed in these notes, turbidite systems of foreland basins are invariably associated with tectonically-induced angular unconformities along basin margins, suggesting that tectonic uplift must play a major role in producing large sediment availability and high-gradient fluvial systems. As suggested by Milliman and Syvitski (1992), Mulder and Syvitski (1995) and Mutti et al. (1996), sediment flux to the sea is dramatically increased in settings of this type through flooding. Sedimentary cyclicity of turbidite systems of foreland basins strongly suggests that climatically-induced high-frequency ciclicity (Milankowich cyclicity) punctuated low-frequency cycles governed by tectonic uplift, subsidence and denudation.
Article
The arrangement of sediment couplets preserved in Thalassinoides shafts suggests that tides regulated the passive filling of these trace fossils and, thus, represent tubular tidalites. The thickness variation of individual layers and couplets implies a mixed diurnal, semi-diurnal tidal signature where packages of either thick-layered or thin-layered couplets alternate. Calcarenitic sediment accumulated when tidal current velocity was too high to allow deposition of mud, whereas a marly mud layer is interpreted to have formed during more tranquil times of a tidal cycle (in particular, low-tide slack water). The tidal record within the burrows covers a few weeks and the corresponding spring–neap cycles. The fill of the Thalassinoides shafts is the only known record to decipher the tidal signature from otherwise totally bioturbated sediments. These deposits accumulated in a lower-shoreface to upper-offshore setting during the late Miocene on a shallow shelf extending from the Atlantic Ocean to the west into northern Patagonia. The fill of all investigated burrows started around spring tide and, thus, the behaviour of the burrow producers – probably crustaceans – is speculated to have been affected by tides or the high water level because all studied burrows became abandoned around the same period of a tidal cycle. This article is protected by copyright. All rights reserved.