ALBERT

Description: At present, three continental archetypal ichnofacies are widely accepted: the Scoyenia, Mermia and Coprinisphaera ichnofacies. The last is present in palaeosols, and the first two occur in fluvio-lacustrine environments. Additionally, the Skolithos ichnofacies may be present in relatively high-energy fluvio-lacustrine deposits. The ichnofauna from active fluvial channels is characterised by low-diversity assemblages of simple vertical burrows and escape traces, referred to the Skolithos ichnofacies. Abandoned or inactive channel deposits characteristically contain low-diversity assemblages dominated by meniscate traces. Floodplain water bodies that experienced progressive drying (desiccated overbank deposits) may contain abundant arthropod and vertebrate trackways, backfilled meniscate traces, ornamented burrows and bilobate traces with scratch marks, which allow recognition of the Scoyenia ichnofacies. Floodplain water bodies that are filled by overbank vertical accretion without experiencing desiccation (overfilled overbank deposits) include simple grazing trails, locomotion trails and horizontal dwelling burrows, representing impoverished occurrences of the Mermia ichnofacies. Hydrologically closed lakes are very stressful environments in which subaqueous ichnofaunas are rare. The richest ichnofaunas in closed lakes are present at the lake margins, and record the activity of terrestrial rather than aquatic faunas (Scoyenia ichnofacies). Hydrologically open lakes host relatively diverse and abundant ichnofaunas, comprising the Scoyenia ichnofacies in low-energy, lake-margin areas, and the Mermia ichnofacies in permanent subaqueous lacustrine zones. Sediments deposited in relatively high-energy lacustrine environments, such as wave-dominated shorelines and delta mouth-bars, commonly are represented by the Skolithos ichnofacies. Although continental trace fossils have not been extensively used in sequence stratigraphy, they have potential for future integrated study. Softground trace fossils are commonly well developed in overfilled lake basins and are useful to delineate parasequences and parasequence sets. In balanced-fill and underfilled lake basins, softground ichnofaunas are poorly developed because of stressful conditions. In contrast, firmground suites are rare in overfilled lake basins, but widespread in lowstand deposits of balanced-fill and underfilled lake basins. Early lowstand amalgamated incised fluvial channels are usually unbioturbated, but palaeosol ichnofaunas (e.g. the Coprinisphaera ichnofacies) may delineate sequence boundaries in interfluve areas. Increasingly isolated fluvial channels encased in overbank deposits develop during late lowstand, and lacustrine deposits accumulate during transgressions, favouring preservation of biogenic structures.

Description: Trace fossils are sensitive indicators of environmental fluctuations and, accordingly, ichnological studies have the potential to improve facies characterization of marginal-marine systems. Carboniferous intertidal deposits in eastern Kansas and western Missouri accumulated under contrasting palaeoenvironmental conditions, ranging from the open shoreline to fluvio-estuarine transitions. Comparative analysis of these exposures illustrates lateral variations in trace-fossil content and allows characterization of the intertidal ichnofaunas developed in three sub-environments: open marine, restricted bays and fluvio-estuarine transitions. Openmarine tidal flat ichnofaunas are characterized by (1) high ichnodiversity, (2) marine ichnotaxa produced by both euryhaline and stenohaline forms, (3) the presence of both infaunal and epifaunal traces, (4) the presence of simple and complex structures produced by presumed trophic generalists and specialists respectively, (5) dominance of horizontal trace fossils of the Cruziana ichnofacies, (6) presence of multispecific associations, (7) high density, and (8) wide size range. This ichnofauna is present in heterolithic deposits and reflects the activity of a biota that inhabited tidal flats dominated by normal-marine salinities and connected directly to the open sea. Restricted-bay ichnofaunas display (1) low ichnodiversity, (2) ichnotaxa commonly found in marine environments, but produced by euryhaline organisms, (3) dominance of infaunal traces rather than epifaunal trails, (4) simple structures produced by opportunistic trophic generalists, (5) a combination of vertical and horizontal trace fossils from the Skolithos and Cruziana ichnofacies, (6) the presence of monospecific associations, (7) variable density, and (8) small size. This assemblage occurs in heterolithic facies and records the activity of a brackish-water benthic fauna inhabiting intertidal areas of estuarine basins and embayments. Fluvio-estuarine ichnofaunas are characterized by (1) moderate to relatively high diversity, (2) forms typically present in continental environments, (3) the dominance of surface trails and absence of burrows, (4) temporary structures produced by a mobile deposit-feeding fauna, (5) a mixture of trace fossils belonging to the Scoyenia and Mermia ichnofacies, (6) moderate density of individual ichnotaxa, (7) absence of monospecific suites, and (8) small size. This assemblage occurs in tidal rhythmites and records the activity of a typical freshwater/terrestrial benthos inhabiting tidal flats that were developed in the most proximal zone of the inner estuary under freshwater conditions. Through integration of ichnological and sedimentological data, conventional sedimentological interpretations of marginal-marine depositional systems can be refined and enhanced.

Description: 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.

Description: Evidence for macroscopic life in the Paleoproterozoic Era comes from 1.8 billion-year-old (Ga) compression fossils [Han TM, Runnegar B (1992) Science 257:232–235; Knoll et al. (2006) Philos Trans R Soc Lond B 361:1023–1038], Stirling biota [Bengtson S et al. (2007) Paleobiology 33:351–381], and large colonial organisms exhibiting signs of coordinated growth from the 2.1-Ga Francevillian series, Gabon. Here we report on pyritized string-shaped structures from the Francevillian Basin. Combined microscopic, microtomographic, geochemical, and sedimentologic analyses provide evidence for biogenicity, and syngenicity and suggest that the structures underwent fossilization during early diagenesis close to the sediment–water interface. The string-shaped structures are up to 6 mm across and extend up to 170 mm through the strata. Morphological and 3D tomographic reconstructions suggest that the producer may have been a multicellular or syncytial organism able to migrate laterally and vertically to reach food resources. A possible modern analog is the aggregation of amoeboid cells into a migratory slug phase in cellular slime molds at times of starvation. This unique ecologic window established in an oxygenated, shallow-marine environment represents an exceptional record of the biosphere following the crucial changes that occurred in the atmosphere and ocean in the aftermath of the great oxidation event (GOE).

Description: The early Late Carboniferous rocks of the Guandacol Formation in western Argentina preserve the glacial to postglacial transition. In the study area, this unit has been divided in three intervals: 1) a lower diamictitic interval; 2) a middle interval chiefly composed of mudstone, and 3) an upper sandstone-dominated interval. The lower interval records infill of a fjord incised into the underlying Ordovician limestone. The middle and upper intervals reflect postglacial sedimentation. Four ichnotaxa, occurring as both discrete and compound trace fossils, are documented from the lower and middle intervals of the Guandacol Formation. Diplopodichnus biformis and Cruziana diplopoda n. isp. occur in the thinly bedded stratified diamictite in the upper section of the lower interval. These deposits record sedimentation from debris flows with dropstones reflecting overprinting of ice-rafting and rain-out processes. Cruziana cf. problematica and Rusophycus carbonarius are present in very-fine to fine-grained sandstone layers interbedded with dropstone-bearing mudstone in the lower section of the middle interval. These deposits record the interplay of suspension fall-out sedimentation, ice-rafting, rain-out processes, and storm waves. The presence of linguliformean brachiopods in coeval beds nearby strongly suggests marine influence and that brackish-water conditions prevailed during the early phase of the transgression. Harsh paleoenvironmental conditions may explain the small size of the trace fossils and the low ichnodiversity in comparison to that expected in fully marine environments. The morphology of the trace fossils as bilobate ridges and furrows ornamented with scratch marks indicates that the structures were produced by arthropods, most likely trilobites and/or notostracans. Although the possibility that different ichnotaxa have resulted from changes in burrowing behaviors can not be completely disregarded, the fact that distinct Cruziana ichnospecies display non-overlapping facies distribution may suggest their production by different arthropods.

Description: Although the notion of recurrent trace-fossil assemblages through geologic time as a result of similar sets of environmental conditions has a long history in ichnology, the idea that recurrent ichnofaunas may have occurred in connection with macroevolutionary events has not yet been put forward. We refer to this phenomenon as "the Deja vu effect," and suggest that understanding its significance may shed light on how organisms colonize underutilized or empty ecospace, the establishment of the mixed layer, and the temporal and spatial distribution of matgrounds. These recurrent trace-fossil assemblages record the activity of the epifauna and a very shallow tier infauna, in the absence of mid- to deep-tier trace fossils and mottled textures. They typically consist of grazing trails, very shallow feeding burrows, and arthropod trackways. Shallow-tier burrows and trails with well-developed scratch marks may be present. These ichnofaunas suggest that the initial exploitation of underutilized or empty ecospace is linked to a set of recurrent behavioral strategies to obtain food from matgrounds and firmgrounds in the absence of a well-developed mixed layer. Preservation of these behaviors in the form of trace fossils is mediated by a set of taphonomic conditions. These taphonomic windows occur during the initial colonization of empty or underutilized ecospace, such as the Ediacaran-Cambrian transition or the subsequent colonization of the land, but also after the end-Permian mass extinction, which may have had a negative impact on the mixed layer in marine environments, producing a return to the ecologic conditions of the early Paleozoic.

Description: Neogene marine strata crop out in the present Coastal Cordillera and Longitudinal Depression of south-central Chile between 38°30′S and 41°45′S, indicating the onset of a major marine transgression that covered most of the forearc in this area. In order to determine the sedimentary environment, paleobathymetry, and age of these deposits, we carried out integrated sedimentologic, ichnologic, and micropaleontologic studies on samples from oil wells and outcrops in the region. Our results indicate that these successions were deposited at lower-bathyal depths (〉2000 m) during the middle to late Miocene. Shallow-marine deposition followed in the southwestern part of the study area during the Pliocene(?). We attribute deep-marine sedimentation in this area to a major event of subsidence in the Miocene that affected the entire forearc and that was caused by basal subduction erosion. We suggest that the anomalously thin crust that characterizes this area may have facilitated forearc subsidence and allowed the Miocene transgression to advance much farther inland here than in other regions of Chile. Subsequent uplift of the forearc is ascribed to basal accretion or underplating of sediments. Our conclusions contradict previous studies that favor a stable margin at these latitudes since the Jurassic. Deep-marine sedimentation in this area during the Miocene implies that the present Coastal Cordillera and Longitudinal Depression were probably submerged during that epoch.

Description: The Upper Devonian–Lower Carboniferous Bakken Formation is a widespread siliciclastic unit in the subsurface of the Williston Basin that is subdivided into three members: lower and upper organic-rich shale members and a dolomitic, silty, and sandy middle member. Although the unit has become one of the most active oil plays in North America and numerous sedimentologic studies have been made, no consensus about the depositional environments of the middle member has been achieved. Previous studies suggested several depositional and sequence-stratigraphic scenarios, including lowstand offshore-shoreface, normal-regressive offshore-shoreface, incised estuary, and falling-stage shoreface complexes for the middle member. We propose a new depositional and sequence-stratigraphic model and compare it with some previous interpretations. This new model includes a basal transgressive systems tract (TST) embracing shelf deposits, a highstand systems tract comprising shelf to lower shoreface environments, and an upper TST encompassing a brackish-water embayment complex and offshore to shelf settings. Petrophysical characterization of the sedimentary facies reveals that bay-mouth cross-stratified fine-grained sandstone, flaser-bedded very fine grained sandstone formed in wave-dominated tidal flats, offshore-transition highly bioturbated interbedded very fine grained sandstone and siltstone, and tidal-flat very fine grained sandstone with common mud drapes possess the best reservoir qualities. Recognition of a restricted embayment within the Bakken middle member has major implications for both exploration and production. Embayment facies with good reservoir quality constitute good oil prospects in localized areas, whereas fully marine facies may represent good oil prospects of more regional extent.

Description: 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.

Description: During forced regression, the coastline advances seaward irrespective of sediment supply and relocates to a topographically lower position. Forced-regressive deposits are a component of the falling-stage systems tract. They are recognized by a basal unconformity which records erosion during the seaward facies shift, and are in turn capped by another unconformity due to subaerial exposure or wave ravinement during subsequent transgression. In wave-dominated settings, the basal “regressive surface of marine erosion” is a scoured surface cut by wave action, because erosion is needed to maintain the seaward-sloping bathymetric profile in equilibrium with the wave energy. The response to falling sea level in tide-dominated settings has not been as thoroughly discussed. The stratal architecture of the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains reveals a new mechanism for the formation of this surface landward of the lever point of balance between sedimentation and erosion in the subtidal environment. Subtidal parasequences of the Lake O'Hara Member are typically composed of thin-bedded mudstone and ripple cross-laminated sandstone, followed by Skolithos pipe rock and thin- to medium-bedded cross-stratified sandstone overlain by thick-bedded cross-stratified sandstone. However, at the top of the Lake O'Hara Member, an erosional surface truncates a pipe-rock interval and the expected capping cross-stratified sandstones are absent. Overlying this surface, the Lake Oesa Member comprises tidal-flat deposits composed mainly of thinly interbedded mudstone and sandstone. This jump in facies at the erosional surface can be explained as a response to a fall in sea level. As the shoreline is forced to regress, the laterally continuous tidal flats advance and the pre-existing shallow-subtidal compound dunes are scoured by strong tidal currents, along with waves, that gradually carve a new equilibrium profile. We argue that the accretion of intertidal flats on top of subtidal sands is an overlooked yet predictable component of falling-stage systems tracts in tide-dominated settings.