María Gabriela Mángano

The paleoenvironmental and paleoecological significance of the lacustrine Mermia ichnofacies: An archetypical subaqueous nonmarine trace fossil assemblage

Although nonmarine environments commonly are considered as diverse and varied as marine settings, only a single recurrent and archetypical ichnofacies, the Scoyenia ichnofacies, has been formally defined. However, recent research proposed the terrestrial Termitichnus ichnofacies as a subset of the Scoyenia ichnofacies. The Mermia ichnofacies is formally defined herein to include nonmarine, fully aquatic trace fossil suites characterized by the dominance of horizontal to subhorizontal grazing and feeding traces produced by mobile deposit feeders, the subordinate occurrence of locomotion traces, overall high to moderate ichnodiversity and abundance, and low specialized grazing patterns. The Mermia ichnofacies typifies unconsolidated, fine‐grained, permanent subaqueous substrates, and well‐oxygenated, low‐energy lake bottoms, periodically punctuated by episodic sedimentation. In addition, it is suggested to include in the Scoyenia ichnofacies, floodplain and transitional fluvio‐lacustrine trace fossil assemb...

Invertebrate Ichnology of Continental Freshwater Environments

SUMMARY: The study of continental ichnofaunas has shown an explosive development during the last decade. At present, three continental archetypal ichnofacies are accepted: the Scoyenia, Mermia , and Coprinisphaera ichnofacies. Integration of ichnologic, sedimentologic, and paleobiologic information is very useful in facies analysis and sequence stratigraphy of continental successions.

The ichnotaxonomic status of Plangtichnus and Treptichnus

A new ichnospecies, Treptichnus pollardi, is described from Carboniferous lacustrine rocks of the Paganzo Basin, northwestern Argentina. Accordingly, the ichnotaxonomic status of the ichnogenera Plangtichnus and Treptichnus is reviewed. Because these ichnotaxa represent different parts of the same 3D zigzag structure, they are considered to be synonymous. Considering historical usage, Plangtichnus is regarded as a nomen oblitum and Treptichnus as a nomen conservandum. Four Treptichnus ichnospecies have been recognized in the stratigraphic record: T. bifurcus, T. triplex, T. lublinensis, and T. pollardi isp. nov. Treptichnus pollardi differs from the ichnospecies described hithertoo by the absence of segment projections and the presence of shafts vertical to the zigzag trace, expressed at the bedding plane by pits either at the angle of juncture between burrow segments or within burrow segments. It is also distinguished from T. lublinensis and T. triplex by its thinner burrow segments.

Trace fossils and their palaeoecological significance in Lower Cretaceous lacustrine conservation deposits, El Montsec, Spain

Abstract La Pedrera and La Cabrua at El Montsec, Spain, are two Lower Cretaceous lacustrine conservation deposits (Konservat-Lagerstatten) that have provided a rich, diverse, and well-preserved faunal and floral fossil record. Although the lithographic limestones where the body fossils are found are finely laminated and essentially undisturbed by bioturbation, epigenic trace fossils occur on the surface of certain laminae. Trace fossils at La Pedrera include the fish trail Undichna britannica and the arthropod trackway Hamipes didactylus, while the ichnospecies found at La Cabrua are Steinsfjordichnus brutoni, Gordia arcuata, Cochlichnus anguineus and Undichna britannica, all but the last corresponding to the activity of small invertebrates. The trace fossil occurrences record short-term events in a lake bottom that did not support any benthic life for most of its history. Differences in the trace fossil assemblages for the two localities agree with sedimentological data in attributing a more distal position for La Pedrera.

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

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

Vagorichnus, a new ichnogenus for feeding burrow systems and its occurrence as discrete and compound ichnotaxa in Jurassic lacustrine turbidites of Central China

A new ichnogenus, Vagorichnus, is figured and described from Jurassic lacustrine turbidites of the Anyao Formation, Henan Province, Central China. Vagorichnus consists of complex burrow systems composed of discontinuous curved to meandering segments in which irregular networks are formed. The ichnospecies V. anyao covers burrow systems characterized by constricted and irregularly annulated segments, and ridge‐like knobs. Although traces are preserved as hypichnial ridges on the sole of turbidite sandstones, several lines of evidence indicate that these ichnofossils record the activity of a post‐event infauna. Vagorichnus occurs as discrete and compound ichnotaxa. In the latter case, it grades into Gordia and Tuberculichnus. Vagorichnus is interpreted as invertebrate feeding structures (Fodinichnia).

An early Cambrian shallow-marine ichnofauna from the Puncoviscana Formation of northwest Argentina: the interplay between sophisticated feeding behaviors, matgrounds and sea-level changes

Abstract An early Cambrian ichnofauna consisting of Helminthoidichnites tenuis, Helminthopsis tenuis, Multina isp., Oldhamia alata, and Pilichnus cf. dichotomus is documented from shallow-marine deposits ranging from the upper offshore to the offshore transition in the Puncoviscana Formation of northwest Argentina. Although the ichnogenus Oldhamia is more common in Cambrian deep-marine environments, this occurrence provides further evidence that it is also present in shallow-marine environments. The burrow network Multina (senior synonym of Olenichnus) is preserved at the base of tempestites, representing the activity of post-storm colonizers. A drowning surface separating offshore-transition deposits below from upper-offshore deposits above contains widespread evidence of trace fossils in direct association with matgrounds. The undermat miners Oldhamia alata and Pilichnus cf. P. dichotomus occur on this surface, revealing exploitation of organic matter in the biomat. Low sediment rate during drowning and paucity of bioturbation by sediment bulldozers may have promoted the establishment of the matground. In comparison with the simpler animal-matground interactions characteristic of the Ediacaran, the combination of Cambrian evolutionary innovations and the presence of microbial mats promoted more sophisticated interactions. Complex feeding trace fossils revealing that systematic undermat mining, as displayed by Oldamia alata and Pilichnus cf. dichotomus, is a product of the Cambrian explosion.

Decoupled evolution of soft and hard substrate communities during the Cambrian Explosion and Great Ordovician Biodiversification Event

Significance The majority of body plans were established during the Cambrian Explosion (CE), whereas the significant taxonomic increases during the Great Ordovician Biodiversification Event (GOBE) were manifest at lower taxonomic levels. Data on the diversity and disparity of bioturbation and bioerosion indicate that soft and hard substrate communities experienced decoupled evolution. Ichnofossil data indicate that rapid diversification of bioturbation occurred during the early early Cambrian (Fortunian) rather than during the late early Cambrian as indicated by shelly fossils. The first rapid increase in bioerosion took place during the GOBE approximately 80 My after the CE in bioturbation. Contrasts between the Cambrian Explosion (CE) and the Great Ordovician Biodiversification Event (GOBE) have long been recognized. Whereas the vast majority of body plans were established as a result of the CE, taxonomic increases during the GOBE were manifested at lower taxonomic levels. Assessing changes of ichnodiversity and ichnodisparity as a result of these two evolutionary events may shed light on the dynamics of both radiations. The early Cambrian (series 1 and 2) displayed a dramatic increase in ichnodiversity and ichnodisparity in softground communities. In contrast to this evolutionary explosion in bioturbation structures, only a few Cambrian bioerosion structures are known. After the middle to late Cambrian diversity plateau, ichnodiversity in softground communities shows a continuous increase during the Ordovician in both shallow- and deep-marine environments. This Ordovician increase in bioturbation diversity was not paralleled by an equally significant increase in ichnodisparity as it was during the CE. However, hard substrate communities were significantly different during the GOBE, with an increase in ichnodiversity and ichnodisparity. Innovations in macrobioerosion clearly lagged behind animal–substrate interactions in unconsolidated sediment. The underlying causes of this evolutionary decoupling are unclear but may have involved three interrelated factors: (i) a Middle to Late Ordovician increase in available hard substrates for bioerosion, (ii) increased predation, and (iii) higher energetic requirements for bioerosion compared with bioturbation.

Shallow marine event sedimentation in a volcanic arc-related setting: the Ordovician Suri Formation, Famatina Range, northwest Argentina

Abstract The Loma del Kilometro Member of the Lower Ordovician Suri Formation records arc-related shelf sedimentation in the Famatina Basin of northwest Argentina. Nine facies, grouped into three facies assemblages, are recognized. Facies assemblage 1 [massive and parallel-laminated mudstones (facies A) locally punctuated by normally graded or parallel-laminated silty sandstones (facies B] records deposition from suspension fall-out and episodic storm-induced turbidity currents in an outer shelf setting. Facies assemblage 2 [massive and parallel-laminated mudstones (facies A) interbedded with rippled-top very fine-grained sandstones (facies D)] is interpreted as the product of background sedimentation alternating with distal storm events in a middle shelf environment. Facies assemblage 3 [normally graded coarse to fine-grained sandstones (facies C); parallel-laminated to low angle cross-stratified sandstones (facies E); hummocky cross-stratified sandstones and siltstones (facies F); interstratified fine-grained sandstones and mudstones (facies G); massive muddy siltstones and sandstones (facies H); tuffaceous sandstones (facies 1); and interbedded thin units of massive and parallel-laminated mudstones (facies A)] is thought to represent volcaniclastic mass flow and storm deposition coupled with subordinated suspension fall-out in an inner-shelf to lower-shoreface setting. The Loma del Kilometro Member records regress ive-transgressive sedimentation in a storm- and mass flow-dominated high-gradient shelf. Volcano-tectonic activity was the important control on shelf morphology, while relative sea-level change influenced sedimentation. The lower part of the succession is attributed to mud blanketing during high stand and volcanic quiescence. Progradation of the inner shelf to lower shoreface facies assemblage in the middle part represents an abrupt basinward shoreline migration. An erosive-based, non-volcaniclastic, turbidite unit at the base of this package suggests a sea level fall. Pyroclastic detritus, andesites, and a non-volcanic terrain were eroded and their detritus was transported basinward and redeposited by sediment gravity flows during the low stand. The local coexistence of juvenile pyroclastic detritus and fossils suggests reworking of rare ash-falls. The upper part of the Loma del Kilometro Member records a transgression with no evidence of contemporaneous volcanism. Biostratinomic, paleoecologic, and ichnologic analyses support this paleoenvironmental interpretations and provide independent evidence for the dominance of episodic sedimentation in an arc-related shallow marine setting. Fossil concentrations were mainly formed by event processes, such as storms and volcaniclastic mass flows. High depositional rates inhibited formation of sediment-starved biogenic concentrations. Collectively, trace fossils belong to the Cruziana ichnofacies. Low diversity, scarcity, and presence of relatively simple forms indicate benthic activity under stressful conditions, most probably linked to high sedimentation rates. Contrasting sedimentary dynamics between ‘normal shelves’ and their volcaniclastic counterparts produce distinct and particular signatures in the stratigraphic record. Arc-related shelves are typified by event deposition with significant participation of sediment gravity flows, relatively high sedimentation rates, textural and mineralogical immaturity of sediments, scarcity and low diversity of trace fossils, and dominance of transported and reworked faunal assemblages genetically related to episodic processes.

Sedimentary facies, depositional processes and climatic controls in a Triassic Lake, Tanzhuang Formation, western Henan Province, China

The Middle to Upper Triassic Tanzhuang Formation represents part of the infill of the early Mesozoic Jiyuan-Yima Basin. The upper part of this stratigraphic unit records deposition within prevailing shallow lake conditions. Well-developed sequences crop out near Jiyuan, western Henan Province, central China. Six sedimentary facies clustered into two facies assemblages were recognized in the lacustrine section. Facies assemblage 1 consists of stacked coarsening-upward sequences composed, from base to top, of organic-rich shales (facies E, type I), laminated siltstones (facies A) and current-rippled laminated sandstones (facies B). Units of assemblage 1 record progradation of small mouth-bar deltas within a perennial open lacustrine system under temperate and humid conditions. Facies assemblage 2 lacks a clear vertical pattern and consists of interbedded fine-grained carbonates and siltstones (facies C); deformed and wave-reworked sandstones (facies D); organic-rich shales (facies E, type II) and clayey mudstones (facies F). The assemblage also represents a perennial, hydrologically-open, shallow lacustrine system, but characterized by strong seasonal climatic control. Water stratification probably occurred in several periods of the lake history. Pangaean megamonsoonal influence is envisaged to explain the strong seasonality imprint evidenced toward the upper part of the Tanzhuang lacustrine column.