Matti E. Räsänen

Late Miocene Tidal Deposits in the Amazonian Foreland Basin

Late Miocene tidal sediments of Acre, Brazilian Amazonia, were deposited in an embayment or interior seaway located in the sub-Andean zone. This late Tertiary embayment system may once have connected the Caribbean with the South Atlantic. The tidal coasts of the embayment-seaway have provided an avenue for the earliest waif (over water) dispersal phases of the great American biotic interchange in the late Miocene. The subsequent change from semimarine to terrestrial environments is of value in assessing the importance of earlier hypotheses on the evolution of the westem Amazonian landscape and gives insight into the formation of several observed biogeographic patterns, especially of aquatic biota.

Fluvial Perturbance in the Western Amazon Basin: Regulation by Long-Term Sub-Andean Tectonics

Haffers refuge theory proposes that during the arid climatic phases of the late Pleistocene, tropical lowland forests of Amazonia were reduced to isolated patches contributing to the high species richness of the present-day forest. The theory was developed because no obvious historic or modern geomorphic isolation barriers were recorded in Amazonia. Analyses of radar images combined with stratigraphical data show that in the basinal forelands of the tectonically active Andes the geological setting causes long-term fluvial perturbance. This leads to a temporally structured highly complex mosaic of fossil and present floodplains. These dynamics have been present with varying activity and geographic range during the Tertiary and Quaternary, providing site-turnover that has not been recognized by the biogeographic tradition of the Amazon basin.

Recent and ancient fluvial deposition systems in the Amazonian foreland basin, Peru

Still active Sub-Andean foreland deformation is suggested to have syndepositionally modified the fluvial depositional environments in the Peruvian Amazonian foreland basin throughout Neogene-Quaternary time. Modern fluvial aggradation continues to proceed on a large scale ( c . 120 000 km 2 ) in two differing depositional systems. Firstly, various multistoried floodbasin deposits are derived from the meandering and anastomosing rivers within the subsiding intraforeland basins. Secondly, in the northern part of the Pastaza-Maranon basin the largest known Holocene alluvial fan-like formation ( c . 60 000 km 2 ) composed of reworked, volcaniclastic debris derived from active Ecuadorian volcanoes, has been identified. The widespread, poorly known, dissected surface alluvium ( terra firme ) which covers the main part of the Peruvian Amazonian foreland basin shows further evidence of long-term foreland deformation, and terraces indicate both the effects of tectonism and Pleistocene climatic oscillations. In northern Peru, the surface alluvium was deposited by a Tertiary fluvial system with palaeocurrents to the west and northwest into the Andean foreland basin. In southern Peru, the respective surficial alluvium was part of a post-Miocene fluvial system flowing northeast into the main Amazon basin. Both systems were gradually abandoned when the eastward migrating Andean foreland deformation led to the more distinctive partitioning of the intraforeland basins, and the modern drainage system was created.

The Last Interglacial-Glacial cycle in NE Fennoscandia: a nearly continuous record from Sokli (Finnish Lapland)

Abstract Boreholes from the Sokli basin in Finnish Lapland have recovered a unique sequence of diamicts interlayered with fine-grained sediments that are rich in fossils. The diamicts are interpreted as till beds deposited by the Fennoscandian Ice-Sheet. The fine-grained intercalations seem to represent a series of warm stages that include one interglacial and three interstadials. The interglacial is characterized by a mixed taiga pollen assemblage and corresponds to a distinct diatom gyttja bed. The interstadials show progressively colder floras from open birch forest to arctic forest limit to shrub tundra. Their sediments comprise gyttja interlayered with wood and sand (interstadial 1); sands grading into sandy gyttja (interstadial 2); and sands overlain by laminated silt and clay (interstadial 3). The interstadial 2 and 3 deposits are underlain by diamict and are interpreted as deglaciation sequences. Silts with a tundra pollen assemblage separate the interglacial and interstadial 1 deposits. Luminescence dating evidence indicates that the interglacial bed most probably represents the NW European mainland Eemian Interglacial (marine isotope stage (MIS) 5e). The interstadial 1, 2 and 3 deposits are tentatively correlated with the Early Weichselian Brorup and Odderade Interstadials and an interstadial of Middle Weichselian age, respectively (MIS 5c, 5a and 3, respectively). The correlation implies that the easternmost part of the ice-divide zone of the Fennoscandian continental ice-sheet was deglaciated during part of MIS 3.

Geological control of floristic composition in Amazonian forests

Aim Conservation and land-use planning require accurate maps of patterns in species composition and an understanding of the factors that control them. Substantial doubt exists, however, about the existence and determinants of large-area floristic divisions in Amazonia. Here we ask whether Amazonian forests are partitioned into broad-scale floristic units on the basis of geological formations and their edaphic properties. Location Western and central Amazonia. Methods We used Landsat imagery and Shuttle Radar Topography Mission (SRTM) digital elevation data to identify a possible floristic and geological discontinuity of over 300 km in northern Peru. We then used plant inventories and soil sampling to document changes in species composition and soil properties across this boundary. Data were obtained from 138 sites distributed along more than 450 km of road and river. On the basis of our findings, we used broad-scale Landsat and SRTM mosaics to identify similar patterns across western and central Amazonia. Results The discontinuity identified in Landsat and SRTM data corresponded to a 15-fold change in soil cation concentrations and an almost total change in plant species composition. This discontinuity appears to be caused by the widespread removal of cation-poor surface sediments by river incision to expose cation-rich sediments beneath. Examination of broad-scale Landsat and SRTM mosaics indicated that equivalent processes have generated a north–south discontinuity of over 1500 km in western Brazil. Due to similarities with our study area, we suggest that this discontinuity represents a chemical and ecological limit between western and central Amazonia. Main conclusions Our findings suggest that Amazonian forests are partitioned into large-area units on the basis of geological formations and their edaphic properties. The evolution of these units through geological time may provide a general mechanism for biotic diversification in Amazonia. These compositional units, moreover, may correspond to broad-scale functional units. The existence of large-area compositional and functional units would suggest that protected-area, carbon sequestration, and other land-use strategies in Amazonia be implemented on a region-by-region basis. The methods described here can be used to map these patterns, and thus enable effective conservation and management of Amazonian forests.

Ichnology and Sedimentology Reveal Depositional Characteristics of Bay-Margin Parasequences in the Miocene Amazonian Foreland Basin

ABSTRACT Proposed depositional models for Miocene Amazon foreland basin strata (Pebas Formation, Peru) are controversial. Recent depositional models include lacustrine and tidally influenced, brackish-water embayment. This paper presents data that support tidally influenced, brackish-water deposition for at least part of Pebas time (10-14 Ma). Two parasequences are presented (Santa Julia and Tamshiyacu). Both crop out along the Amazon River in Upper Amazonia near Iquitos, Peru. At these locations, abundant evidence of brackish-water, tidally influenced deposition is documented, including marginal marine bioturbation, sedimentary couplets, semidiurnal couplets (preserved in burrows), and pinstripe lamination. The deposits are locally highly bioturbated. At both locations ichnogenera normally associated with marine to brackish-water depositional environments are common. Three normally marginal-marine ichnofabrics are reported: (1) a Chondrites-reburrowed, Planolites ichnofabric resident only in massive-appearing muds; (2) a Scolicia (Laminites), Thalassinoides, Ophiomorpha ichnofabric that is manifested as intensely bioturbated silty sands; and (3) a Thalassinoides-generated ichnofabric that is interpreted to have descended into consolidated substrates and is thus representative of the Glossifungites Ichnofacies. Several trace fossils contain laminated infills organized into distinct sedimentary couplets that are best interpreted as resulting from semidiurnal processes. Six conclusions are arrived at: (1) sedimentological and ichnological data consistently indicate that sediment accumulation dominantly occurred in sporadically dysaerobic, marine to brackish water, under a tidal influence; (2) sediment accumulation occurred in bay-margin environments that prograded into a shallow, quiescent bay; (3) a stratified water column is indicated by the ichnofauna; (4) low accommodation space, repetitive and rapid adjustments of relative sea level, shallow wave base, and a stratified water column combined to generate an atypical parasequence architecture; (5) previously published isotopic data are consistent with sediment accumulation in brackish to marine water; and (6) marine incursion into Amazonia occurred during the Middle Miocene.

Miocene semidiurnal tidal rhythmites in Madre de Dios, Peru

New data from upper Miocene deposits in the Madre de Dios region, southern Peru, allow the delineation of tidal regime for the first time in western Amazonia and provide strong evidence of elevated tidal range and brackish-water influence. The results point out the insufficiency of the current depositional models and support the earlier hypothesis that western Amazonia was also connected to the Paranan Sea during the late Miocene. In this paper we present sedimentological, ichnological, and statistical (Fourier transformation) data from two selected outcrops containing rhythmite successions from an area that is traditionally considered as continental. The sediments are interpreted to represent tide-dominated, inner-middle estuarine deposits. The cyclic rhythmites display semidiurnal cyclicity. The results are significant because (1) they contradict recent interpretations of the areas paleoenvironmental history; (2) the evidence for tidal processes is persuasive; and (3) the delineated tidal regime and range provide a unique insight into the depositional dynamics of a system having many important paleogeographic implications.

The Significance of Bioturbated Inclined Heterolithic Stratification in the Southern Part of the Miocene Solimoes Formation, Rio Acre, Amazonia Brazil

Abstract Sedimentary models that apply to the Middle Miocene succession in Amazonia are controversial. Although tidally-influenced sedimentary deposits have been described from several locations, the identification of brackish-water or marine facies has been hampered by limited outcrop exposure. Also, ichnological data largely have been ignored. This study focuses on ichnological and sedimentological relationships observed in outcropping strata of the Solimoes Formation (Middle Miocene) along the Acre River in western Brazil and northern Bolivia. The studied strata comprise a fine-grained lower unit that is sharply overlain by dipping, interbedded sands and muds, known as inclined heterolithic stratification (IHS). The IHS is present throughout the length of the outcrop, about 80m. The outcropping strata are interpreted to represent two depositional subenvironments: (1) A lower unit that resulted from sediment accumulation in a shallow, restricted, subaqueous depositional environment. The deposit ultimately became emergent with subsequent paleosol development. (2) An upper unit dominated by marginal marine point-bar deposits that developed in a channel. Trace fossils observed in the upper unit provide evidence that mesohaline waters occupied the channel at the time of sediment accumulation. This is supported most strongly by the presence of Scolicia, a common marine trace fossil, and reburrowed (composite) Ophiomorpha. The resultant ichnofabric represents a response to sedimentary events that demonstrates the IHS beds reflect seasonal or annual cyclicity. The analysis of the river-exposed outcrop at Boca de Santa Pedro, Brazil, leads to four conclusions: (1) the IHS exposed in the upper portion of this deposit are possibly tidally influenced and almost certainly accumulated in a brackish-water channel; (2) if IHS are bioturbated, their temporal significance can be assessed; (3) seasonal fluctuations in discharge were significant enough to alter depositional and biological processes in this paleochannel, and; (4) brackish-water incursion into Amazonia during the Middle Miocene can be traced as far south as northern Bolivia.

Paleogeography of Miocene Western Amazonia: Isotopic composition of molluscan shells constrains the influence of marine incursions

Strontium, oxygen, and carbon isotope compositions of well-preserved mollusks (bivalves) indicate a dominantly freshwater depositional setting for the lower Miocene–upper Miocene Pebas Formation in Western Amazonia. Molluscan 87Sr/86Sr ratios identify different freshwater sources. Andean runoff was the dominant water source in Miocene Western Amazonia, though there was occasional influx of waters from cratonic catchments. At only one stratigraphic level, isotope signals indicate increased (mesohaline) aquatic salinities, in concert with a clearly more saline molluscan faunal assemblage. Strontium isotope–based salinity estimates are surprisingly low when compared to other paleosalinity estimates based on the interpretation of (ichno)faunal assemblages and sedimentological structures. We propose that these seemingly contrasting observations can be unified if Miocene Western Amazonia was occupied by a long-lived (lacustrine) wetland system with a restricted connection, via the Los Llanos Basin, to the Caribbean Sea. Abundant runoff supplied fresh water to this system, which effectively blocked the influx of saline waters through the restricted marine connection to the north. Much like modern Lake Maracaibo, such a system could have been the site of microtidal currents and thus could have hosted brackish-water fauna in a dominantly freshwater depositional system.

A shift from lithostratigraphic to allostratigraphic classification of Quaternary glacial deposits

The bedrock of the northern halves of North America and Europe is covered by Quaternary glacial deposits, forming a surficial overburden that is relatively thin, nonlithified, litho logically variable on a small scale (in terms of grain-size, mineralogy, texture, fabric, structure, and color), and often has a well-preserved depositional topography. These geologically unique characteristics and the fact that the glacial overburden was long considered to be of only restricted economic value have caused it to be treated differently in geological research from the older, regularly lithified strata. Due to the striking geomorphology of these glacial deposits, their investigation has also been incorporated into physical geography research. Thus, the segregation of the Quaternary research community into different schools of geology and geography has created multiple classification approaches and has caused the formal stratigraphic classifications successfully applied in pre-Quater nary geology to be applied less regularly to Quaternary glacial strata. This has led to inefficient use of Quaternary geological data for scientific and socio-economic purposes. The few currently existing national Quaternary stratigraphic frameworks are based on lithostratigraphy. These are poorly suited for describing deposits in glaciated shield areas in particular; we propose a classification for such areas based on the combined use of allostratigraphic and lithostratigraphic data, with alloformations as the fundamental units and lithostratigraphic units filling out the framework where appropriate. This classification would provide a hierarchical framework for gla ciogenic deposits that could potentially support stratigraphic information systems, databases, and digital spatial models more effectively than the traditional lithostratigraphic frameworks.