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Kinneyia structures are among the most typical wrinkle structures observed on ancient siliciclastic sediment surfaces since the Archean. Recently, Kinneyia structures have been grouped together with other microbially induced, crinkly decorations on ancient bedding surfaces as wrinkle structures. They are mainly preserved on upper surfaces of ancient siliciclastic-event deposits and are characterized by millimeter-scale, winding, flat-topped crests separated by equally sized round-bottomed troughs and pits. The structure resembles small-scale interference ripples including crest-dominated linear and pit-dominated honeycomb-like patterns. The steep slopes of the crests, however, exclude their formation at the air or water-sediment interface. Thin sections across Kinneyia structures reveal their formation beneath microbial mats. They formed at an early stage and do not arise from loading and other processes related to burial. Based on the close relationship to event deposits, a genetic model considering the specific hydraulic conditions on siliciclastic tidal flats after storms or floods is proposed. Numerical calculations show that, after microbial mats have been reestablished on the new sediment surface and groundwater is still flowing downslope, the top portion of the sediment confined beneath mats may be liquefied, thus allowing grains to move with the groundwater. Oscillations of groundwater flow owing to periodic reversals of flow direction at rising tides, and a tidal signal of oscillating pore pressure may enhance formation of ripplelike structures along the boundary with the overlying mat. The model applies primarily to Kinneyia structures presumed to be formed beneath cohesive microbial mats in peritidal zones.
A detailed characterization of Rhizocorallium assemblages from the Middle Triassic sediments (Ladinian, Muschelkalk facies) of the Betic Cordillera, Spain, and their paleoenvironmental interpretation are provided. Rhizocorallium jenense shows variable U-shaped structures: straight and short, slightly arcuate or sinuous, and fan-shaped. Spreiten are protrusive, and vertically retrusive forms are frequent. Scratches are very abundant, more or less continuous and parallel on the marginal tubes, and more prominent and shorter on the spreiten. Rhizocorallium shows a major orientation at 121°E. Rhizocorallium irregulare is scarce and occurs as long, straight-to-slightly-sinuous or arcuate U-shaped forms and usually exhibits scratch marks. The dominance of R. jenense and its orientation, similar to that of gutter casts, reveal colonization during high-energy regimes, mainly related to storm phases. The presence of R. irregulare reflects comparatively quieter intervals, probably interstorm phases. Trace-fossil features reveal firm-ground consistency and Glossifungites ichnofacies. The occurrence of dominant, suspension-feeding R. jenense and scarce, deposit-feeding R. irregulare reflects a high concentration of nutrients in the water column and comparatively organic-poor sediments. The high concentration of Rhizocorallium indicates a large population of trace makers and, probably, opportunistic behavior. Size-frequency curves, however, are coherent with steady-state populations (taphonomic bias?) and with a single generation. Scratch-mark casts could suggest crustaceans as producers, with some appendages in continuous contact with the substrate.
Storm-generated event beds are an important source of paleoecological information, especially in Paleozoic strata. Storm deposition and subsequent physical and biological modification can potentially alter the diversity structure of death assemblages significantly. To examine the effects of storm deposition on fossil assemblage composition, storm beds are compared with co-occurring beds representing background sedimentation in 67 samples from six Ordovician mixed carbonate-clastic units deposited above the maximum storm wave base. In the great majority of pairwise comparisons, evenness and sampled richness are higher in storm beds than in background beds. This effect is not explained by differences in lithification, skeletal fragmentation, or in the proportions of aragonitic or multielement skeletons. The elevated diversity of storm beds can result from homogenization of fine-scale faunal patchiness preserved in background beds or may be due to taphonomic feedback. The relative importance of these two end-member scenarios can be evaluated with detrended correspondence analysis. In shallow, carbonate-dominated environments, the former appears to predominate, while the latter is more important in a deeper setting dominated by fine-grained clastics. The disparity between background beds and storm beds suggests that, at least in the Lower Paleozoic, background beds may record a higher-resolution paleoecological signal while storm beds record a more complete census of alpha diversity. Because post–Middle Ordovician increases in the depth and intensity of bioturbation may have diminished the temporal resolution and increase the faunal completeness of background beds, this disparity is not necessarily expected in younger strata.
A semi-quantitative evaluation method has been developed to map integrated fossil plant records (leaf, fruit, and pollen assemblages) in terms of zonal vegetation. It incorporates taxonomy, physiognomy, and autecological features of the fossil plants. To derive vegetation types, the final stage of this method involves a quantitative evaluation. The maps presented here are based on 198 organ assemblages from 173 localities and stratigraphic levels of the Miocene–early Pliocene across Europe. They cover three time intervals: (1) 4–7 Ma (Messinian–middle Zanclean; late Miocene–early Pliocene); (2) 8.5– 12 Ma (latest Serravallian–middle Tortonian); and (3) 14–17 Ma (late Burdigalian–late Langhian). We also look at the interval 13–14 Ma (late Langhian–early Serravallian). They not only show vegetation types but offer more detailed information on the composition of these associations (proportions of major components). The major zonal vegetation types recognized here are broad-leaved deciduous forests, mixed mesophytic forests, broad-leaved evergreen forests, subhumid sclerophyllous forests, open woodlands, and xeric grasslands. Between 14 Ma and 17 Ma, broad-leaved evergreen forests were widespread, and subhumid sclerophyllous forests were more strongly represented in western parts of Central Europe than in eastern parts. In the interval 8.5–12 Ma, broad-leaved deciduous forests largely replaced the evergreen forests, although these were still present in some refugia (e.g., the Lower Rhine embayment and northern Balkan peninsula), and first records of xeric grasslands are found along the northern margin of the Black Sea. The more discontinuous record available from 4 Ma to 7 Ma indicates that broad-leaved evergreen forests still occurred in northern parts of the Mediterranean (northern Italy, Balkan peninsula), while open woodlands increasingly appeared in central and southern parts of Italy and Greece.
The main objective of this contribution is to evaluate the environmental fidelity of dead diatom assemblages along two microtidal estuarine systems from southeastern Buenos Aires Province (Argentina). Living communities (inferred from counting protoplasm-containing cells) were compared to dead diatom assemblages through several fidelity metrics. Gradient analysis (by canonical correspondence analysis) was applied in order to assess the quantitative relationship between diatom assemblages and the environmental gradient. The presence of allochthonous components in diatom assemblages was assessed by analyzing the distribution of the main ecological groups. Results indicated a good agreement between living communities and total assemblages in surface sediments, as well as between total surface and subsurface assemblages from both estuaries. A high percentage of the variance in diatom assemblage composition was explained by the environmental gradient, particularly by sediment composition and salinity, indicating that taphonomic alterations play a minor role in structuring these assemblages. The good preservation of diatom thanatocoenoses in estuarine sediments makes them accurate indicators of the environmental conditions at the point of deposition, providing useful information for paleosalinity reconstructions in coastal settings.