Ichnology of the Alfarcito Member (Santa Rosita Formation) of northwestern Argentina: animal-substrate interactions in a lower Paleozoic wave-dominated shallow sea
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Bibliographic record
Abstract
Abstract. The lower fine-grained interval of the Alfarcito Member of the Upper Cambrian-Tremadocian Santa Rosita Formation in northwestern Argentina represents an overall transgressive-regressive cycle, recording the establishment of an open-marine, shallow, low-gradient platform. Four main sedimentary facies have been recognized, environmentally ranging from the lower offshore to the lower/middle shoreface. Alternating and contrasting energy conditions due to repeated storm events superimposed on fairweather sediments were among the key controlling factors for trace fossil distribution and preservation. Fairweather and storm-related trace fossil assemblages are recognized. The first is the most diverse and includes the more varied behavioral strategies, such as locomotion (Archaeonassa fossulata, Cruziana semiplicata, C. problematica, Cruziana isp. and Diplichnites isp.); resting (Rusophycus moyensis, R. carbonarius, Rusophycus issp. and Bergaueria aff. B. hemispherica); pascichnia (Dimorphichnus aff. D. quadrifidus); feeding (Arthrophycus minimus, ?Gyrolithes isp., Gyrophyllites isp., ?Phycodes isp. and Planolites reinecki); and dwelling (Palaeophycus tubularis, P. striatus). This assemblage represents the Cruziana ichnofacies. The storm-related assemblage is monospecific and includes vertical dwelling traces (Skolithos linearis), and is attributed to the Skolithos ichnofacies. Integration of ichnologic and sedimentologic data allows proximal-distal trends in shallow-marine trace fossil assemblages along a nearshore-offshore transect to be reconstructed. High-energy conditions prevailed in lower and middle shoreface environments and bioturbation is restricted to vertical burrows (Skolithos linearis), recording colonization after storm events. Environmental conditions in the offshore transition and the upper offshore are more variable and reflect the alternation of high-energy storm events and low-energy fairweather mudstone deposition. The storm-related Skolithos ichnofacies is present, but alternates with the faiweather assemblage (Cruziana ichnofacies) that reaches a diversity maximum in the upper offshore. Trace fossils are scarce in lower offshore deposits, being restricted to a few non-descript burrows, mostly Palaeophycus tubularis. Shoreface deposits from the Alfarcito Member compare favorably with the strongly stormdominated type. All these deposits are stacked to form regional coarsening-upward parasequences that record short-term progradational episodes separated by drowning surfaces. Vertical distribution of trace fossils parallels changes in stratal stacking patterns. Several ichnotaxa (especially Cruziana semiplicata, Rusophycus moyensis and Arthrophycus minimus) have biostratigraphic implications. Resumen. ICNOLOGIA DELMIEMBRO ALFARCITO (FORMACION SANTA ROSITA) DEL NOROESTE DE ARGENTINA: INTERACCIONES SUSTRATO-ANIMAL EN UN MAR SOMERO DEL PALEOZOICO INFERIOR DOMINADO POR OLEAJE. El intervalo inferior de grano fino del Miembro Alfarcito de la Formacion Santa Rosita (Cambrico Superior-Tremadociano) representa un ciclo general transgresivo-regresivo, registrando el establecimiento de una plataforma marina somera de bajo gradiente. Se reconocieron cuatro facies sedimentarias principales, cuyos ambientes varian entre costa-afuera bajo y shoreface bajo a medio. Entre los factores claves que controlaron la distribucion de los icnofosiles y su preservacion se encuentran las condiciones de energia alternantes y contrastantes debidas a eventos de tormenta repetidos superpuestos sobre sedimentos de buen tiempo. Se reconocieron asociaciones de trazas fosiles de buen tiempo y de tormenta. La primera es la mas diversa e incluye la mayor variacion de estrategias de comportamiento, tales como locomocion (Archaeonassa fossulata, Cruziana semiplicata, C. problematica, Cruziana isp. y Diplichnites isp.); reposo (Rusophycus moyensis, R. carbonarius, Rusophycus issp. y Bergaueria aff. B. hemispherica); pascichnia (Dimorphichnus aff. D. quadrifidus); alimentacion (Arthrophycus minimus, ?Gyrolithes isp., Gyrophyllites isp., ?Phycodes isp. y Planolites reinecki); y habitacion en dos ocasiones (Palaeophycus tubularis, P. striatus). Esta asociacion representa la icnofacies de Cruziana. La asociacion relacionada con tormentas es monoespecifica e incluye trazas de alojamiento vertical (Skolithos linearis), y se atribuye a la icnofacies de Skolithos. La integracion de los datos icnologicos y sedimentologicos permite reconstruir las tendencias proximal-distales en las asociaciones de trazas fosiles marinas a lo largo de una transecta nearshore-offshore. En los ambientes de shoreface inferior y medio prevalecieron condiciones de alta energia y la bioturbacion se halla restringida a excavaciones verticales (Skolithos linearis), registrando la colonizacion luego de los eventos de tormenta. Las condiciones ambientales en la transicion a costa afuera y en costa afuera superior son mas variables y reflejan la alternancia de eventos de tormenta de alta energia y depositacion de fangolitas de buen tiempo y baja energia. Se halla presente la icnofacies de Skolithos relacionada con tormentas, la que alterna con la asociacion de buen tiempo (icnofacies de Cruziana) que alcanza su maxima diversidad en el costa-afuera superior. Los icnofosiles son escasos en el offshore bajo, estando restringidos a unas pocas excavaciones, mayormente Palaeophycus tubularis. Los depositos de shoreface del Miembro Alfarcito se pueden comparar con los tipos fuertemente dominados por tormentas. Todos estos depositos forman parasecuencias granocrecientes que registran los cortos episodios progradacionales separados por superficies de inundacion. La distribucion vertical de las trazas fosiles acompana los cambios en los patrones de depositacion. Varios icnotaxones (especialmente Cruziana semiplicata, Rusophycus moyensis y Arthrophycus minimus) poseen interes bioestratigrafico.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it