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Soft‐sediment deformation structures from the Alcântara Formation (late Albian to Cenomanian), São Luís Basin, northern Brazil, consist of (1) contorted structures, which include convolute folds, ball‐and‐pillow structures, concave‐up paths with consolidation lamination, recumbently folded cross‐stratification and irregular convolute stratification that grades into massive beds; (2) intruded structures, which include pillars, dykes, cusps and subsidence lobes; and (3) brittle structures, represented by fractures and faults displaying planes with a delicate, ragged morphology and sharp peaks. These structures result from a complex combination of processes, mostly including reverse density gradients, fluidization and liquefaction. Reverse density gradients, promoted by differential liquefaction associated with different degrees of sediment compaction, led to the genesis of convolute folds. More intense deformation promoted the development of ball‐and‐pillow structures, subsidence lobes and sand rolls, which are attributed to denser, and thus more compacted (less liquefied), portions that sank down into less dense, more liquefied sediments. Irregular convolute stratification that grades into massive beds would have formed at periods of maximum deformation. The subsidence of beds was accompanied by lateral current drag and fluid escape from water‐saturated sands. In addition, the fractures and faults record brittle deformation penecontemporaneous with sediment deposition. All these mechanisms were triggered by a seismic agent, as suggested by a combination of criteria, including (1) the position of the study area at the edge of a major strike‐slip fault zone that was reactivated several times from the Albian to the Holocene; (2) a relative increase in the degree of deformation in sites located closer to the fault zone; (3) continuity of the deformed beds over large distances (several kilometres); (4) restriction of soft‐sediment deformation structures to single stratigraphic intervals bounded by entirely undeformed strata; (5) recurrence through time; and (6) similarities to many other earthquake‐induced deformational structures.

ABSTRACT. As the number of phylogeographic studies on Amazonian birds increases, spatially and temporally different scenarios are gradually replacing previous interpretations of Amazonian historical biogeography. At the same time, recent studies have improved significantly our understanding of Amazonian geological history during the late Tertiary and Quaternary, two periods regarded as critical for the recent diversification of the Amazonian avifauna. The notion that geologically older and more stable areas of Amazonia (such as the Brazilian and Guianan shields) functioned as "species-pumps", whereas geologically more dynamic areas (such as the western Amazonian lowlands) mostly "captured" part of the diversity generated nearby, was supported by a recent phylogeographic study focusing on a species complex of the genus Xiphorhynchus (Dendrocolaptidae). Here, I review 14 recent additional molecular datasets to assess whether this historical scenario can be extended to other lineages of Amazonian birds as well. The reviewed datasets indicated, among other things, a clear dichotomy in the diversification histories of species associated with seasonally flooded versus upland forests, and those with high versus low dispersal capabilities. Many "core" lineages of upland species found nowadays in western Amazonia are clearly associated with more basal lineages from the Brazilian shield, Guianan shield, and the Andes, indicating a more recent history in this geologically dynamic region. On the other hand, lineages associated with seasonally flooded forests seem to have an ancient history in western Amazonia, apparently expanding over the geologically more stable areas only recently. Most sister taxa of the reviewed lineages exhibited levels of pairwise sequence divergence consistent with splitting events dating back to a time frame stretching from the late Miocene to the early Pleistocene (late Tertiary and early Quaternary periods), a period when significant physiographic and landscape changes took place in Amazonia. When interpreted together, all reviewed studies provide evidence that geology and landscape evolution are tightly linked with the timing and mode of differentiation of Amazonian birds.

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Morphologic features obtained from SRTM data, integrated with geologic information, are emphasized in this paper in order to provide the basis for understanding the development of the lowest Amazon drainage basin, focusing on the history of one of the largest Amazonian tributaries, the Tocantins River, and on the origin of the Marajó Island, throughout the Quaternary. This approach led to the recognition of a fan morphology related to the record of a tectonically controlled N/NW-S/SE orientated paleovalley cut down into Miocene and older rocks. The incised valley was fed by a paleo Tocantins River, which deposited its sediment load continuously to the north-northwest, reaching the Marajó Island and producing a deposit displaying a fan-morphology during the PlioPleistocene/Pleistocene. As characterized in the SRTM images, this channel system became abandoned due to capture by NE-SW orientated faults and establishment of the Pará River by W-E strike slip movements. This event, which probably took place in the Mid-Holocene, was responsible for detachment of the Marajó Island from the mainland.INPE ePrint: sid.inpe.br/ePrint@80

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