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Geological and paleobiological attributes described in a carbonate unit from the upper Silurian of the Middle Atlas (Morocco) are interpreted as the result of chemosynthetic processes fueled by fluid seepage. These attributes include: the presence of authigenic carbonate accumulation embedded in siliciclastic rocks; tightly packed, monospecific megafaunal assemblages (atrypid brachiopods); microbial-derived products; precipitation of bio-induced minerals (especially hematite); and seep-derived carbonate phases. Evidence of in-situ growth of the dense brachiopod communities is that their shells are all articulated, and show a complete range of growth sizes. Products of microbial activity are shared with other fossil seep deposits and include clotted micrite concentrations, crusts, and laminated structures, microtufts, biofilms, and endolithic filaments. The best-developed microbial features were recovered in reddish mudstone beds below and within the brachiopod concentrations, where they consist of complex microbial frames morphologically comparable to the ones produced by Beggiatoa mats in modern chemosynthesis-based environments. These remnant mats are associated with early hematite precipitation, which has permitted their extensive mineralization and, consequently, the preservation of their stratigraphic relationships with the shell accumulations. Carbonate phases such as botryoids, splayed calcite, and stromatactoids form the infilling of cavity systems and microconduits. Repeated events of mineral precipitation and corrosion are documented in botryoids and stromatactoids, as well as in more extensive corrosion surfaces, as a result of changes in the ambient chemistry related to type and changing intensity of the seeping fluids. This carbonate unit is the oldest-known seep-derived unit, with a well preserved, voluminous, and diverse microbial component.
New evidence of fossil macroborings in the Lower Ordovician (Ibexian) of western Utah demonstrates that the macroboring behavioral strategy was firmly established in the earliest stages of the great Ordovician diversification of the marine biosphere. In Utah, borings were excavated in hardgrounds that had developed on sponge-algal mounds and flat-pebble conglomerates in the Fillmore Formation (Ibexian). The most complete specimens possess a neck up to 1 cm in length that opens into a teardrop-shaped chamber with a maximum diameter of 1 cm. The chamber terminates at a depth of 3–4 cm below the hardground surface. These borings belong to the ichnogenus Gastrochaenolites. The organisms responsible for creating the borings are unknown. Sedimentologically, the effect of boring on hardgrounds was to break them into pebble- and cobble-sized clasts. The endolithic lifestyle represented by the borings may have evolved in response to ecologic pressures such as predation or competition for food resources. The macroborings from the Fillmore Formation represent an innovative strategy that may have resulted in the later development of new body plans and the early establishment of endolithic macroinvertebrates.
The end-Permian mass extinction left an indelible mark on trace-fossil assemblages. This is evident during the lag phase of the biotic-recovery interval that occurred in the Early Triassic. Research on the Spathian Virgin Limestone Member of the Moenkopi Formation, western United States, has yielded a mixed carbonate–siliciclastic trace fossil assemblage. The presence of such traces as Thalassinoides, Laevicyclus, and Gyrochorte indicate that previously unreported metazoan behaviors had reappeared in equatorial regions by the close of the Early Triassic. Whereas diversity of trace-fossil assemblages increased from earliest to late Early Triassic time, persistent small size, low average ichnofabric index, low bedding-plane coverage, and reduced tiering point to prolonged stressful environmental conditions following the end-Permian mass extinction—conditions for which there is an abundance of global sedimentological evidence. This trace-fossil assemblage provides a record of soft-bodied organisms that might not otherwise be detected from the study of body fossils alone; it therefore acts as a constraint on the timing of the biotic recovery. This assemblage from the Virgin Limestone Member also serves as an indicator of environmental conditions that might not otherwise be gleaned, illustrating the utility of using trace fossils as environmental proxies during the lag phase of a biotic recovery. The presence of some of these traces (i.e., Thalassinoides) in lowermost Triassic strata of western Canada suggests that the recovery of trace-makers after the end-Permian mass extinction was asynchronous, and that northerly latitudes may have experienced a less-protracted biotic recovery than in equatorial regions.
Bivalve borings are described for the first time in coprolites. They occur along with bored bone from Cretaceous through Eocene phosphatic conglomerates in the Taoudeni and Iullemmeden Basins of northeastern Mali. Coprolites are extensively penetrated by flask-shaped borings (Gastrochaenolites): the oldest known occurrence of the ichnospecies G. ornatus preserves mechanical scratch traces at the base of some early Eocene coprolite borings. The alleged tracemaker, a pholad bivalve of the Subfamily Martesiinae, is preserved as an external mold in one of the early Eocene coprolites. It is the first occurrence of rock-borers in this subfamily from the Paleocene of West Africa. Bored coprolites were very firm to fully lithified (paleocoprolites) when bored by the pholads, indicating early-diagenetic phosphogenesis of the clasts. The intense and repeated bioerosion of coprolites and bone by pholads suggests a long residence time in shallow-marine waters for the clasts. Therefore, the vertebrate fossil assemblages in these Mali conglomerates are considered highly time-averaged. Recognition of Gastrochaenolites borings in vertebrate remains provides useful taphonomic and paleoenvironmental information, despite their destructive impact on the fossils.
The study of Upper Cretaceous and Lower Paleogene benthic foraminifera from the Bidart section (SW France) provides detailed data on the paleobathymetry as well as paleoenvironmental conditions across the Cretaceous/Paleogene (K/Pg) boundary. A quantitative analysis of benthic foraminiferal assemblages from the Upper Maastrichtian Abathomphalus mayaroensis Biozone and the Danian Guembelitria cretacea, Parvularugoglobigerina eugubina, and Parasubbotina pseudobulloides Biozones was performed. Benthic foraminifera indicate that the upper Maastrichtian and lower Danian sediments at Bidart were deposited in the upper–middle part of the slope.
Benthic foraminiferal assemblages indicate mesotrophic conditions during the late Maastrichtian and a strong decrease in the food supply to the sea floor coincident with the K/Pg boundary. This change in the trophic regime was related to the collapse of the food web triggered by the mass extinction of calcareous primary producers. Benthic assemblages in the lower Danian are strongly dominated by few species, and suggest that primary productivity was dominated by blooms of non-calcareous primary producers, creating a stressful environment for the benthic fauna. The faunal turnover, together with the geochemical evidence, is compatible with an asteroid impact scenario. Benthic foraminiferal assemblages suggest that primary productivity had not completely recovered more than 200 kyr after the K/Pg boundary event.
Isolated logs preserved in the Älterer Flußsande of the Weißelster basin, Germany, are examined relative to their subjacent bedform. Paleocurrent analyses conducted on both the log orientation and its underlying bedform are used to determine whether or not any relationship exists between these two structures. Two patterns emerge from this investigation. When the data set is taken collectively, the mean vector of the log orientation is in a subperpendicular relationship to the mean vector of their bedforms. This coarse woody detritus is not oriented preferentially parallel to paleocurrent direction as has been previously hypothesized. In addition, when individual log orientations are compared with their underlying bedforms, logs may be oriented parallel, subperpendicular, or perpendicular to the bedform. There is no statistical preference for any particular orientation in the data set. Comparison is made with an actualistic data set from the Lassa Distributary of the Rajang River delta where a similar trend is documented, and with previously published experimental and field data. This comparison indicates that there is no statistical preferred orientation of wood clasts in fluvial systems. Hence, these results caution against the use of woody phytoclasts as independent indicators of paleocurrent trends in fluvial systems.
Three cruises surveying the eastern Australian shelf revealed the occurrence of coralline algal buildups at depths between 80 and 120 m in tropical (Capricorn area) and subtropical (off Fraser Island) settings. The buildups, decimeters to several meters high, started to grow on an erosion surface during the Holocene transgression in water depths within ∼ 30 m of their present-day depth, and continued to present, as indicated by living covers of coralline algae. The buildup framework is a boundstone of encrusting coralline algae growing one over the other. The result is an open structure partially filled by mudstone to packstone internal sediment and minor marine cements, and affected by several phases of bioerosion. Mesophyllum is the main algal builder. Other melobesioids and Sporolithon appear in most samples. The tropical Capricorn buildups comprise algal assemblages slightly more diverse than the ones in the subtropical examples off Fraser Island. The buildup accretion involves many phases of framework growth, bioerosion, and sediment infilling at low average rates (maximum 2 to 3 cm/k.y.). These modern examples demonstrate that deep-water algal mounds can be coeval with shallow-water coral reefs and can be found in outer-platform and platform-edge deposits in ancient tropical platforms (e.g., Huon Gulf, Papua New Guinea). Upper Paleozoic phylloid algal mounds built by an open framework of Archaeolithophyllum crusts are similar to the northeastern Australian Mesophyllum-dominated boundstones, indicating they may have developed in similar deep-water, open-platform settings.
Taphonomic studies are scarce for all fossil foraminifera, especially for fusulinids, which have been studied rarely, due to their traditional use primarily for biostratigraphy. The analysis of 900 well-preserved fusulinids, notably Triticites ventricosus, indicates both biostratinomy and diagenesis of the assemblage in the Hughes Creek Shale Member of the Foraker Limestone (Pennsylvanian, Virgilian) in east-central Kansas. The excellent preservation of these specimens provides valuable insight into the taphonomic process.
The biostratinomy has involved both encrustation and corrasion. The encrusting foraminifer Tolypammina is pervasive, with 63 percent of the fusulinids having at least one encrusting foraminifer. Corrasion is not as common: only 23 percent of the specimens have significant corrasion. Diagenesis of the fusulinids occurred during compaction of the shale and is manifest as compression that has resulted in pressure solution and flattening. Pressure solution formed pits on 47 percent of the specimens, and another 3 percent have other fusulinids or fossil fragments embedded into them. Flattened or cracked specimens were not abundant, with only 8 percent of the fusulinids having such features.