Fluvio-lacustrine Systems Research Articles

Plio-Pleistocene drainage reorganization in the middle Yellow River of China, revealed by provenance and paleocurrent analysis of fluvial sediments

Continental-scale drainage reorganization is generally understood to result from landform evolution forced by the coupling between tectonic activity and climate change, attracting worldwide attention. Planation surfaces and river terraces constitute the most direct geomorphological archives for integrating the history of river systems and for reconstructing processes of drainage reorganization based on provenance analysis of the fluvial sediments. The Middle Yellow River has incised into the Ordos Block, which was previously levelled by a planation surface, creating the Jinshaan Gorge and linking the Hetao Basin to the north and the Fenwei Basin to the south. Multiple lines of evidence, from terraces correlation, sedimentary characteristics, and fluvial provenance, suggest that the Middle Yellow River catchment was occupied by fluvio-lacustrine systems before drainage integration, accumulating a sedimentary sequence derived from surrounding Red Clay and bedrocks during the period between 8.3 and 3.7 Ma. The planation surface was uplifted subsequently, forcing reorganization of the fluvio-lacustrine systems. Their integration resulted in the formation of the Jinshaan Gorge prior to 1.2 Ma. Numerous terraces were created during incision by the Middle Yellow River from north to south along the gorge. Our data, obtained from zircon UPb dating and lithological composition of fluvial gravels, further points to a remarkable discrepancy in provenance between the fluvio-lacustrine systems overlying the planation surface and the terraces sequence formed by the integrated Middle Yellow River, confirming the drainage reorganization process constrained by the geomorphic records.

A Pliocene lacustrine system in the Nellis basin, southern Nevada, USA: implications for the Colorado River drainage system

ABSTRACT Lacustrine basins and the sediments in them provide a critical component of regional tectonic development and climate history. Each sub-basin in the landscape records the interplay between active tectonism and shifting climate. Many of the basins in southern Nevada that contain deposits of the Muddy Creek Formation were closed fluvio-lacustrine systems during the latest Miocene and into the Pliocene. They were subsequently integrated into the regional Colorado River system. Lacustrine deposits of the Muddy Creek Formation are exposed along the trace of the Las Vegas Valley shear zone in the Nellis Dunes Recreation Area (NDRA), northeast of Las Vegas, Nevada. The Muddy Creek Formation in NDRA consists of approximately 130 m of mixed carbonates and clastics, and new tephrochronology suggests an early to late Pliocene age (∼ 4.7 to ∼ 2.6 Ma) for the lowermost lacustrine deposits and an unknown, but younger, age for the uppermost spring-fed lacustrine deposit. These sediments were deposited in an arid to semiarid lake (Lake Nellis) and alluvial floodplain system based on the interpretation of lithologies and facies associations. The stratigraphic succession coarsens upward and reflects establishment of a lacustrine carbonate system overlain by a clastic succession of peripheral lake sediments of alluvial and floodplain origin. The peripheral lake deposits consist of spring-fed and fluvial wetland mudflats of brown claystone and siltstone. This brown claystone is capped by a succession of yellow and red sandstones deposited by fluvial and minor eolian processes. The second, and youngest, freshwater limestone, likely disconformably, overlies the yellow and red sandstones, and is thought to be temporally and depositionally distinct from the underlying Muddy Creek deposits. The transition from the lower lacustrine carbonates to clastic fluvial channel and flood-plain deposits indicates desiccation of Lake Nellis and possibly occurs soon after ∼ 2.7 to ∼ 2.6 Ma based on the ages of tuffs occurring in the uppermost limestone and marl beds. This is consistent with an interpreted increase in regional aridity after ∼ 2.8 Ma (Smith et al. 1993). The second limestone deposit at the top of the section represents an even younger spring-fed lacustrine deposit of unknown age. Throughout the lower carbonate section, three tuffs were identified, and geochemically correlated, using a discriminant-function analysis, to the tuff of Napa (≤ 4.70 ± 0.03 Ma), the Putah Tuff (∼ 3.3 Ma), and the lower tuffs of the Badlands (∼ 2.7 to ∼ 2.6 Ma). The timing of deposition of Lake Nellis sediments postdates the integration of the three major lake systems in this region (Lake Grand Wash, Lake Hualapai, and Lake Las Vegas) into the Colorado River drainage (5.6–4.9 Ma), and also postdates the full integration of the river to sea level (4.8–4.63 Ma) (Howard et al. 2015; Crow et al. 2021). Lake Nellis represents an isolated lake basin that was one, and maybe the last, lacustrine system to be fully integrated into the Colorado River drainage in this region.

Remobilized Cordón Caulle 2011 tephra deposits in north-Patagonian watersheds: Resedimentation at deltaic environments and its implications

The Argentinean side of the Andean North Patagonia is drained by several fluvio-lacustrine systems flowing eastwards from their headwaters at the Andes mountains towards large glacial lakes. These watersheds are recurrently affected by the deposition of tephra erupted from the many active Andean volcanos located in the Southern Volcanic Zone. This research focusses on two fluvial systems (Pireco and Totoral, PT) located in Nahuel Huapi National Park, which were severely impacted by the Cordón Caulle 2011 eruption (CC2011). We investigate particularly the resedimentation of pyroclastic materials on their lacustrine deltas and its potential implications considering 1) the deltas morphology and 2) mass-wasting processes developing at the delta fronts. Original tephra fall deposits were registered at Nahuel Huapi National Park in 2011. Based on the isopach map, we performed a GIS-based volume estimation of the deposit for the studied watersheds. Delta plain progradation from 2011 to 2017 was analyzed from satellite images. We studied the temporal evolution of the subaqueous morphology and mass-wasting processes at the delta fronts using high resolution bathymetric grids registered in 2011, 2014 and 2017. During June 2011 the studied watersheds received a tephra input of ~72 x 106m3. The significant progradation experienced by both deltas especially during the first month of the eruption was favored by 1) abundant precipitation at the watersheds concomitant to heavy tephra deposition and 2) the pre-existing subaqueous morphology characterized by a wide, shallow “terrace” bordering PT delta plains. About six years after the beginning of CC2011, 6% of the tephra fallen over the entire basins had been resedimented at the delta environments (terrace and delta front), totalizing ~3 x 106t. Evidence of pre-eruption and/or co-eruptive mass-wasting processes occurring at PT delta fronts was found on November 2011. Up to 30-m thick pyroclastic deposits were accumulated at PT delta fronts since November 2011, burying mass-wasting features at the upper delta fronts. A suspended-sediment collector installed at the prodelta in 2012 detected hyperpycnal currents transporting pyroclastic materials. These currents probably explain the development of mass-movements at high depths, such as the one that displaced and filled up the bottom sediment trap. This frontally-confined landslide initiated at a depth of >60 m and involved tephra and organic materials deposited at the lower part of Totoral delta front. Because of their recent deposition and loose consolidation, we propose that fresh tephra deposits at PT delta fronts could become particularly unstable and represent a potential hazard to failure, especially if a strong earthquake affects the study area in the next years. A landslide-induced tsunami wave generated at these deltas would impact rural settlements located along the coasts of Brazo Rincón and probably reach the coastal city of Villa La Angostura.

Revisions to the chronostratigraphic framework of the Upper Jurassic Walloon Coal Measures of the Surat Basin, Australia

The Upper Jurassic Walloon Coal Measures (WCM) in the Surat Basin host the largest coal seam gas (CSG) resource in Australia. Despite this, a poorly defined lithostratigraphic framework hinders the development of reservoir models and groundwater flow simulations. Correlations in the WCM are challenging, owing to the complex arrangement of facies over short distances and the absence of a reliable regional stratigraphic datum. To better correlate the strata, 26 tuff beds were dated using the U–Pb chemical abrasion thermal ionisation mass spectrometry methodology across the Surat Basin CSG fairway. This initially suggested that coal-bearing strata in the basin were diachronous. However, the acquisition of a new date from the Surat Basin has identified a five million year time gap between dated tuffs ~20 m apart. This suggests the presence of an unconformity and that there were two independent episodes of coal accumulation in the basin. Above the unconformity, there are incised valleys with a sedimentary infill that transitions from fluvial- to tidal-influenced facies, as indicated by dinoflagellate cysts and tidal sedimentary structures, including double mud drapes. The cause of the unconformity is likely to be tectonic, as eustatic sea-level was rising during the Kimmeridigian. The marine incursion into the basin is the consequence of a highstand of sea-level during the early Tithonian. The application of the new chronostratigraphic framework should elucidate the evolution of fluviolacustrine systems in the basin and aid in resource prediction. Further dating of tuffs in the basin could refine the stratigraphic framework.

Evolution of the depositional environments of the Jurassic Walloon Coal Measures, Surat Basin, Queensland, Australia

AbstractThe Jurassic Walloon Coal Measures of the Surat Basin in eastern Australia host the continent's most significant coal bed methane resources. Previous studies have interpreted the Walloon Coal Measures within a single depositional facies model encompassing a wholly terrestrial setting. Using a multidisciplinary approach (facies analysis, palynology and wireline logs), the evolution of the Walloon Coal Measures is described within a new chronostratigraphic framework defined by accurate and precise U–Pb tuff dates. Analysis of sedimentary facies indicates that the majority of the Walloon Coal Measures was deposited by relatively small (<300 m wide), low gradient rivers on a poorly‐drained floodplain with numerous small lakes and mires. However, this study also identified some marine‐influenced facies with brackish palynomorphs (notably dinoflagellate cysts) and tidal sedimentary structures. These facies appear to have been deposited in estuaries during times of transgression. The evidence for base level shifts suggests that the coals may not have coevally accumulated with at least some of the thicker sandstones. Palaeogeographic maps for eleven time intervals suggest that rivers drained towards to the south/south‐west and south‐east, as indicated by sandstone percentage and gross unit isopach maps, presumably into proximal estuarine complexes. Marine incursions into the continent probably came from the north and east during times of high eustatic sea level and as precursors to those of the more persistent and extensive transgressions of the Early Cretaceous. A similar multidisciplinary approach should help to elucidate the evolution of other fluviolacustrine systems in other basins and aid in resource prediction.

The Functioning of a Water Body Within a Fluvio-Lacustrine System as an Effect of Excessive Nitrogen Loading—The Case of Lake Symsar and its Drainage Area (Northeastern Poland)

Generally, in water ecosystems, it is assumed that rivers play a transport role. In turn, lakes have accumulation properties. However, in fluvio-lacustrine systems, each water body located on a river track can disrupt naturally occurring processes. One such process is the nitrogen cycle. An analysis of the nitrogen cycle, at both the global and local levels, is of extreme significance in view of the progressive degradation of aquatic ecosystems. In this study, we attempted to show that the specific properties of reservoirs located in river–lake systems contribute to an adequate reaction of these reservoirs to situations involving an excessive pollution load. Despite the intensive exchange of water in lakes, they were mainly shown to have an accumulation function. In particular, in those located in the lower part of the system, the total nitrogen load transported outside the example reservoir decreased by 4.3%. The role of these reservoirs depends on the morphometric, hydrologic, and meteorological conditions. The actual loading of the water body was shown to be more than double the permitted critical loading. The creation of conditions similar to those occurring in river–lake systems by, for example, delaying the outflow of water, may favor the protection of surface water from the last element of the system, because this limits the transport of pollutants. This study of the functioning and evolution of lakes’ fluvio-lacustrine systems, including the balance of the nutrient load, enables the prediction of the aquatic ecosystem’s responses in the future and their changes.

The Microbial Community of a Terrestrial Anoxic Inter-Tidal Zone: A Model for Laboratory-Based Studies of Potentially Habitable Ancient Lacustrine Systems on Mars.

Evidence indicates that Gale crater on Mars harboured a fluvio-lacustrine environment that was subjected to physio-chemical variations such as changes in redox conditions and evaporation with salinity changes, over time. Microbial communities from terrestrial environmental analogues sites are important for studying such potential habitability environments on early Mars, especially in laboratory-based simulation experiments. Traditionally, such studies have predominantly focused on microorganisms from extreme terrestrial environments. These are applicable to a range of Martian environments; however, they lack relevance to the lacustrine systems. In this study, we characterise an anoxic inter-tidal zone as a terrestrial analogue for the Gale crater lake system according to its chemical and physical properties, and its microbiological community. The sub-surface inter-tidal environment of the River Dee estuary, United Kingdom (53°21′15.40″ N, 3°10′24.95″ W) was selected and compared with available data from Early Hesperian-time Gale crater, and temperature, redox, and pH were similar. Compared to subsurface ‘groundwater’-type fluids invoked for the Gale subsurface, salinity was higher at the River Dee site, which are more comparable to increases in salinity that likely occurred as the Gale crater lake evolved. Similarities in clay abundance indicated similar access to, specifically, the bio-essential elements Mg, Fe and K. The River Dee microbial community consisted of taxa that were known to have members that could utilise chemolithoautotrophic and chemoorganoheterotrophic metabolism and such a mixed metabolic capability would potentially have been feasible on Mars. Microorganisms isolated from the site were able to grow under environment conditions that, based on mineralogical data, were similar to that of the Gale crater’s aqueous environment at Yellowknife Bay. Thus, the results from this study suggest that the microbial community from an anoxic inter-tidal zone is a plausible terrestrial analogue for studying habitability of fluvio-lacustrine systems on early Mars, using laboratory-based simulation experiments.

Chemical and mineralogical composition of fluvial sediments (Bistrita River, Romania): Geogenic vs. anthropogenic input into rivers on its way through mining areas

The upper reaches of the Bistrita drainage system were selected as a natural test site to determine the geogenic and anthropogenic input into fluvio-lacustrine systems in humid mid-latitude morphoclimatic zones. The reason for this selection lies in the complex geology and its metallogenic evolution leading to a great variety of Fe, Mn, U, and polymetallic sulfides ore deposits. It sparked an intense mining activity during the past centuries with a strong impact on the drainage system similar to many mineralized sites in the world which are still under exploitation. Sediment samples from Bistrita River were analyzed by means of X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), near-infrared (NIR) and micro-Raman spectroscopy (μ-Raman).Our results revealed that the chemical and mineralogical built-up of the stream sediments is mainly geogenic, with most of the trace elements accumulated in the river sediments derived from the source rocks exposed in catchment area of the River Bistrita. A strong input by man has been detected in the drainage system near abandoned mining sites. The trace elements are mainly accommodated in the structure of detrital minerals representative of the clastic aureole around the source rocks, and to a lesser extent adsorbed onto the surface of clay minerals. The REE incorporated into muscovite furnish evidence of having derived from the source rocks, prevalently mica schists exposed by supergene processes in the provenance area and rule out a neoformation of clay minerals on transport and deposition.

Pillow lavas and fluvio-lacustrine deposits in the northeast of Paraná Continental Magmatic Province, Brazil

The northeast edge of the Paraná Continental Magmatic Province was marked by a topographical barrier that gave rise to a predominantly peridesertic environment. This high allowed the recurrent of above-average rainfall in this area of the Province, in contrast with other areas where many evidences indicate that lava flows have settled in an arid climate, in a desert environment. As a result, alluvial-fan debris flows and fluvio-lacustrine systems developed occasionally. This paper presents one of these sequences, which is remarkable for the presence of sedimentary rocks and two sets of pillow lavas. Both sets of pillow lavas show interaction with the fluvio-lacustrine sediments with distinctive grain size (from pebbly sand to clay) that locally preserve the original sedimentary structures such as bedding and lamination. This interaction gives rise to peperites, which show a diversity of textures for different sedimentary granulometries. Hyaloclastites and fragments of lava also occur in the interpillow material and may be intricately mixed with sediment. These features, based mainly on detailed field observations in the northeastern region, demonstrate that the palaeorelief, sedimentary environment, stratigraphy and morphology of sedimentary units and basaltic flows are contrasting from the central and southern regions of the Paraná Continental Magmatic Province.

Trace fossils and sedimentary environments of the upper cretaceous in the Xixia Basin, Southwestern Henan Province, China

The Upper Cretaceous Gaogou, Majiachun and Sigou formations in the Xixia Basin are composed of terrestrial clastic deposits, which are dominated by red-violet mudstones, muddy siltstones, siltstones, grayish fine-grained sandstones, intercalated with grayish, off-white medium- and coarse-grained sandstones or conglomerates. They contain 10 ichnogenera and 22 ichnospecies and Rhizoliths, including Arenicolites isp., Archaeonassa isp., Beaconites antarcticus, B. capronus, B. coronus, Cylindricum isp., Gastruichnus xixiaensis, Palaeophycus megas, P. tubularis, P. heberti, P. sulcatus, P. cf. sulcatus, P. striatus, Planolites beverleyensis, P. isp., Scoyenia gracilis, Skolithos verticalis, S. linearis, S. isp., Taenidium irregularis, T. serpentinum and T. barretti. From north to south across the basin, alluvial fan, braided and meandering fluvial and fluvio-lacustrine systems are recognised on the basis of sedimentary features. In reference to them, four ichnoassemblages are recognised: (1) Skolithos-Gastruichnus ichnoassemblage in the channel environment of braided and meandering river, (2) Beaconites-Taenidium ichnoassemblage in the natural levee and crevasse splays, (3) Beaconites-Palaeophycus ichnoassemblage in the flood plain and (4) Taenidium-Scoyenia ichnoassemblage in the periodically exposed fluvio-lacustrine (pond or very shallow lacustrine) sedimentary environments. The present work not only enrich the trace fossils database of terrestrial origin but also offer new data to complete further continental ichnofacies mode and also provide the constraints on palaeoenvironments of the Xixia Basin.

Development of Fluvio-Lacustrine Systems in the Young Glacial Area in Poland Based on the Research in the Valleys of the Rivers Wierzyca and WDA

Development of Fluvio-Lacustrine Systems in the Young Glacial Area in Poland Based on the Research in the Valleys of the Rivers Wierzyca and WDA Both rivers presented in the article, the Wierzyca River and the Wda River, flow along typical polygenic valleys in young glacial areas. Their valleys include a number of postglacial basins, mainly subglacial channels connected by gap sections. The course of fluvial processes in postglacial basins was determined by the Late Glacial melting and the development of lakes. In Holocene, on the lacustrine and delta deposits, which were filling up the lake basins, wide meander zones with meandering river beds developed. In Late Glacial erosive processes dominated at the gap sections, which ended up with the development of slide meanders. Since the beginning of Holocene, lateral erosion has been dominant due to stabilization of the river bed. This has lead to the widening of the floodplain. Despite this, along the major course of both valleys the width of the floodplains is insufficient for free development of river channels. This is the case of constrained meandering.

A quantitative analysis of environmental associations in sauropod dinosaurs

Both the body fossils and trackways of sauropod dinosaurs indicate that they inhabited a range of inland and coastal environments during their 160-Myr evolutionary history. Quantitative paleoecological analyses of a large data set of sauropod occurrences reveal a statistically significant positive association between non-titanosaurs and coastal environments, and between titanosaurs and inland environments. Similarly, “narrow-gauge” trackways are positively associated with coastal environments and “wide-gauge” trackways are associated with inland environments. The statistical support for these associations suggests that this is a genuine ecological signal: non-titanosaur sauropods preferred coastal environments such as carbonate platforms, whereas titanosaurs preferred inland environments such as fluvio-lacustrine systems. These results remain robust when the data set is time sliced and jackknifed in various ways. When the analyses are repeated using the more inclusive groupings of titanosauriforms and Macronaria, the signal is weakened or lost. These results reinforce the hypothesis that “wide-gauge” trackways were produced by titanosaurs. It is commonly assumed that the trackway and body fossil records will give different results, with the former providing a more reliable guide to the habitats occupied by extinct organisms because footprints are produced during life, whereas carcasses can be transported to different environments prior to burial. However, this view is challenged by our observation that separate body fossil and trackway data sets independently support the same conclusions regarding environmental preferences in sauropod dinosaurs. Similarly, analyzing localities and individuals independently results in the same environmental associations. We demonstrate that conclusions about environmental patterns among fossil taxa can be highly sensitive to an investigator's choices regarding analytical protocols. In particular, decisions regarding the taxonomic groupings used for comparison, the time range represented by the data set, and the criteria used to identify the number of localities can all have a marked effect on conclusions regarding the existence and nature of putative environmental associations. We recommend that large data sets be explored for such associations at a variety of different taxonomic and temporal scales.

Late Cenozoic development of the Strouma and Mesta fluviolacustrine systems, SW Bulgaria and northern Greece

The Strouma and Mesta are two of the largest rivers that drain across SW Bulgaria and northern Greece into the northern Aegean Sea. Their modern valleys, flanked by Quaternary river terraces, are incised into a diverse landscape, which records the region's complex tectonic history. A network of lacustrine basins existed in the region in the Late Oligocene to earliest Miocene, but was disrupted by thrusting and folding related to Early Miocene transpression. This deformation was followed by a period of erosion, covering most of the Early and Middle Miocene, which probably marked the initiation of the Strouma and Mesta fluviolacustrine system, with geometries unrelated to the older systems. The first clear evidence of these river systems dates from the Middle Miocene (late Badenian to Sarmatian). The systems evolved in the Late Miocene (Maeotian to early Pontian), when lakes existed, characterized by diatomaceous algae and by occasional burial of abundant plant fragments and coal formation. Areas in the south, south of the Kerkini fault, were intermittently submerged beneath the Aegean Sea at this time. Intense localized uplift of horst blocks in late Pontian and Pliocene, associated with crustal extension, resulted in deposition of thick alluvial fans, with tilting of sedimentary successions in adjacent grabens evident by the end of the Pliocene. The highest horsts (Osogovo, Rila, Pirin, and Belasitsa) experienced additional uplift in the Pleistocene, in part as a result of regional uplift and in part through continued normal faulting. Pleistocene climate change also resulted in influxes of glacial and fluvioglacial systems. The present form of the Strouma and Mesta fluviolacustrine systems is thus the result of interplay between crustal extension, regional uplift, and global climate change.

Algological studies of fluvio-lacustrine systems in the northern European part of Russia

Algological studies of fluvio-lacustrine systems in the northern European part of Russia

Neogene fluviolacustrine systems in the northern Peri-Aegean Region

Neogene fluviolacustrine systems in the northern Peri-Aegean Region

Increase in mercury contamination recorded in lacustrine sediments following deforestation in the central Amazon

The geochemical study of surface sediments (vertical profiles of 30–80 cm) from lentic ecosystems of the Tapajós River permit the observation of environmental changes responsible for the mercury contamination of aquatic systems exploited by the human riverine population. The Arapiuns and Amazon rivers are compared. Measurements of mercury, textural indicators (water content and dry density), mineralogic indicators (iron and aluminum associated with oxyhydroxides and aluminosilicates), and organic indicators (carbon, nitrogen, atomic C/N ratio) were performed over the full length of the cores. The results demonstrate that soil erosion is responsible for an increase in surficial sediment mercury concentrations in the different aquatic systems of the Tapajós and Arapiuns rivers. This increase is the result of the relative enrichment of the sedimentary deposits in fine particulates rich in aluminosilicates, oxyhydroxides and mercury, transported in suspension in the water column. The oxyhydroxides of iron and aluminum associated with fine, clayey particles seem to control the accumulation of heavy metals in the sediments of the Tapajós, Arapiuns and Amazon rivers. Overall, the mercury levels in the sediments studied have the same relationship with the aluminosilicates and the texture of the sediments. The quantity of aluminosilicates permits the evaluation of diagenetic effects, the influence of the clay content and the matrix effect on the levels of mercury in sediments. The activity of lead-210 measured in two cores suggests that the superficial sediments originate from eroded soils. A preliminary dating using the constant initial concentration model indicates that the environmental changes recorded in the Tapajós River sediments would have been initiated some time between the 1950s and 1970s. They would then coincide with the important colonization of the Brazilian Amazon during this period. The results presented have important implications for the geochemical interpretation of anthropogenic disturbances in the Amazon. They demonstrate that the recent colonization of the drainage basins and the growing exploitation of new parcels of land in the central Amazon disturb the mineral and organic matter cycles, as well as that of mercury. The ensuing result is increased exportation of fine particulate mercury from the surface of soils to drainage waters that transport them to fluvio-lacustrine systems where they finally settle out.

Distribution, Classification, and Ages of Martian Impact Crater Lakes

Paleolakes in impact craters on Mars are characterized at global scale using the Viking Orbiter data. We identified 179 paleolakes in impact structures formed by the influx of water and sediment derived from valley networks and channels that can be classified into three different fluviolacustrine systems: closed, open, and lake-chain systems. We show the hydrogeologic implications for each of the three systems and their significance in terms of duration of fluviolacustrine activity. This study provides a catalog of areographic, physical, and physiographic data for each of the studied impact crater lakes, associated with the description of the sedimentary structures observed. It also identifies environments where life and/or precursors to life could have found favorable conditions to evolve.

Sedimentation in a tectonically partitioned, nonmarine foreland basin: The Lower Cretaceous Kootenai Formation, southwestern Montana

The Lower Cretaceous Kootenai Formation in southwestern Montana was deposited in the nonmarine, Cordilleran foreland basin in the United States during a period of intensified uplift in the westward adjacent, but increasingly impingent, Sevier fold-thrust belt. Concurrently, the foreland basin was partitioned by uplift of intra-foreland structural elements and incipient plutonism. Kootenai fluvial and fluviolacustrine depositional systems developed and evolved in response to changing basin architecture. The Kootenai thus provides a case study of nonmarine sedimentary responses to tectonic partitioning in a foreland basin. The following criteria for recognition of tectonic partitioning of a nonmarine foreland basin are demonstrated by the Kootenai: (1) complex, internalized drainage patterns which locally diverge from the over-all regional paleoslope direction and a typical longitudinal pattern; (2) intraformational and interformational unconformities, both proximal to the fold-thrust belt and within the foreland basin; (3) evidence for cannibalization and redistribution of foreland-basin sediments; (4) abrupt deflections of paleodrainage axes in response to initiation of intraforeland uplifts; and (5) dramatic base-level alterations and consequent changes of fluvial geomorphology as a result of tectonic partitioning and segmentation of the foreland basin. Kootenai sedimentation can be divided into six phases. 1. A tectonic pulse in the Sevier fold-thrust belt produced a thin but widespread sheet of gravel alluvium, referred to as the “Basal Conglomerate Member,” which was deposited by a shallow, perhaps ephemeral, gravel-bed, braided-stream system. Drainage was predominantly eastward, across the axis of the foreland basin. 2. Subsequently, initial activation of intra-foreland structures over the present Tobacco Root, Madison, Gravelly, and Beartooth Archean blocks and the Boulder batholith caused reworking of the Basal Conglomerate in all but the westernmost foreland basin. Although the topographic manifestations of these uplifts were subtle, the foreland basin was effectively partitioned into at least four distinct drainage basins. The resulting deposit, commonly called the “Pryor Member,” but here referred to as the “First Sandstone Member,” was deposited by sand- and gravel-dominant, braided-stream systems in the west and east, respectively. Paleodrainage directions in the western foreland basin were southward, but over-all drainage was northward. 3. A period of mud-dominant sedimentation followed deposition of the First Sandstone, probably resulting from a combination of decreased tectonism, influx of siliceous volcanic ash, and change in source lithology. 4. Renewed tectonic activity in the fold-thrust belt generated a second episode of coarse-grained alluvial sedimentation, producing the Kootenai Second Sandstone Member. Basin partitioning was accentuated, but drainage directions remained essentially unchanged. Initial activation of the Blacktail-Snowcrest uplift dammed the southern portion of the Second Sandstone fluvial system. Base level was elevated, and a major anastomosed-stream system developed upstream from the structural dam. Downstream from the axis of the uplift, a transport-efficient, probably entrenched, meandering system developed. 5. The remainder of Kootenai siliciclastic deposition was dominated by muddy fluvial and fluviolacustrine systems. The foreland-basin configuration remained essentially unchanged as drainage became ponded in response to further elevation of base level as basin partitioning continued. An extensive carbonate lacustrine interval punctuated this period of sedimentation. 6. Gradual southward transgression of the Cretaceous sea left a transitional fluviolacustrine to marine imprint on the sedimentary record of the uppermost Kootenai and overlying Blackleaf Formation. Rejuvenation of orogenesis in the fold-thrust belt, augmented by cratonic and major intraforeland volcanic and sedimentary lithic sources, renewed the supply of coarse-grained detritus to the foreland basin during deposition of the Blackleaf.

Neogene Paleogeography of Western Snake River Plain, Idaho and Oregon: ABSTRACT

Analysis of Miocene through Pleistocene siliciclastic and volcaniclastic sequences in the western Snake River Plain of Idaho and Oregon allows detailed paleogeographic reconstruction of sedimentation associated with the development of a rapidly subsiding continental basin. Extensional tectonism was accompanied by voluminous outpourings of basaltic and silicic volcanic material. These in turn were reworked basinward by marginal alluvial fan-braided stream networks into basin-center fluviolacustrine systems. Episodic influxes of both felsic and basaltic tephra are recorded in fossiliferous lacustrine silt and claystones of the Poison Creek and Chalk Hills Formations, radiometrically bracketed between 12.5 to 5 m.y.B.P. Basinward-fining facies indicate deposition in a large acustrine complex fed by at least 2 major fluvial systems. Complex interfingering of coarse-grained strandline deposits with offshore fine-grained sediments suggests repeated expansion and contraction of the lake system and record the dynamic interplay between basin tectonism and sedimentation. A major unconformity and presence of extensive interbasinal basaltic volcanism (Banbury Basalt) reflects draining of this lacustrine system and renewed extensional tectonism in the basin approximately 4 m.y.B.P. Reestablishment of a major fluviolacustrine complex is represented by deposits of the Pliocene-Pleistocene Glenns Ferry Formation (4-1.8 m.y.B.P.). The fossiliferous mudstone, sandstone, and conglomerate represent alluvial fan, meandering stream, flood-basin, and lake margin systems. Inter edded with these are a series of basaltic tuffs and flows reflecting continuing volcanism along the basin margin and basin center. Continued structural adjustments along high-angle normal faults that bound the basin are indicated by stratigraphic relationships as well as the style of fluvial sedimentation. Periods of rapid basin subsidence are recorded by thin, discontinuous channel sandstone encased by thick overbank siltstone and claystone. More stable periods allowed development of sheet-like channel sandstone with subordinate amounts of overbank fines. Basin-wide structural adjustments have continued to the present, as indicated by the incisement of these Neogene deposits by the modern Snake River. End_of_Article - Last_Page 518------------