Climatic and geomorphologic cycles in a semiarid distributive fluvial system, Upper Cretaceous, Bauru Group, SE Brazil

Studies of distributive fluvial systems and their preserved successions envisage the distribution and orientation of architectural elements to be primarily controlled by channels radiating outward from fan apices, in many cases along an elongate basin margin. Conceptual models for such systems account for the downstream dynamics of the fluvial network, but with limited consideration of temporal geomorphic variations, resulting vertical organisation of architectural elements, or of the interplay of factors controlling system dynamics. To understand the external and internal architecture of distributive fluvial systems, and the factors that influence their sequential facies organisation, a sedimentary succession of the proximal portion of an Upper Cretaceous, semiarid, distributive fluvial system, located at the north-eastern margin of the Bauru Basin (Southeast Brazil), has been analysed in detail. Three fining- and thinning-upward fluvial sequences are identified, forming an interval separated at the top and the bottom by two palaeosol profiles. Each sequence is formed of channel and floodplain deposits. Two types of channel deposits are identified. One is composed of stacked sets of small-scale dune deposits, suggesting perennial and steady fluvial regime, associated with more humid climate periods. The other is composed of large-scale sets indicative of flattened dunes associated with unsteady and fast-changing fluvial flow, formed in quasi-supercritical flow regime conditions, associated with drier climate periods. The vertical alternation of these two types of channel deposits records the accumulation of a fluvial succession that responded to high-frequency, climate-induced cyclic change in bounding conditions. Two palaeosol profiles, at the top and at the bottom of the succession, indicate temporary interruptions and cessation of the fluvial sedimentation, likely related to avulsion of the fluvial belt. Thus, the studied succession reveals high-frequency climate-induced allogenic sedimentary cycles that occur within a long-period autogenic geomorphologic-induced sedimentary cycle. This work suggests that the internal architecture of the channel deposits can be used as a climate proxy, and that climate and geomorphology act jointly as notable factors to control the vertical organisation of distributive fluvial systems

Microbial influence on the accumulation of Precambrian aeolian deposits (Neoproterozoic, Venkatpur Sandstone Formation, Southern India)

In many presently active aeolian systems, processes of sediment erosion, transport and deposition are markedly influenced by vegetation, which acts as an important sediment stabilising agent. Current sedimentary models for hyperarid continental settings devoid of vegetation and with adequate sand supply and wind force envisage intense aeolian activity, resulting in the construction of extensive aeolian sand-seas (ergs). During the Proterozoic, the Earth’s surface was devoid of vegetation, yet ergs are not so common in Proterozoic continental successions. High water-table levels, and erosional reworking by fluvial or marine processes are recognised as possible factors that restricted accumulation and preservation of extensive Proterozoic aeolian systems. By contrast, Proterozoic biotic communities have traditionally been considered largely incapable of markedly influencing clastic sedimentary processes in continental settings. However, since it is now known that bacteria have colonised parts of the continental Earth's surface since the Palaeoarchean, a question arises as to whether such organisms might have exercised some control on aeolian processes via substrate stabilisation. The Neoproterozoic Venkatpur Sandstone Formation (maximum age 709 Ma) is an aeolian depositional unit comprising small, isolated barchanoid and transverse dunes, and dry and damp sand sheet palaeoenvironments. Water-table-influenced aeolian sand-sheet deposits composed of thin sand layers that alternate with a suite of microbially induced sedimentary structures (MISS) provide evidence for fossilised bacteria. These strata record the interaction between microbial mats and aeolian depositional processes, which enabled the construction and accumulation of sand deposits. As microbial mats stabilised and provided plasticity to the depositional accumulation surface, they protected underlying (i.e. accumulated) aeolian deposits from possible wind erosion, reducing the possibility that these stored deposits could be recycled as lagged input to a downwind aeolian system. The stabilising influence of the microbial mats enabled the accumulation of the aeolian deposits, resulted in a marked decrease in the availability of sand for aeolian transport, and hindered construction and accumulation of large aeolian bedforms. This depositional model highlights the significance of the microbial mats for controlling the depositional processes in Precambrian aeolian-dominated environment. Probably, this model may be applied to other pre-vegetated Earth continental environments

Variations in water discharge at different temporal scales in a mud-prone alluvial succession: The Paleocene-Eocene of the Tremp-Graus Basin, Spain

The late Paleocene to earliest Eocene sedimentary record in the eastern Tremp-Graus Basin (southern Pyrenees, Spain) consists of a mud-prone fluvial succession informally known as ‘Upper Red Garumnian’, which is dominantly composed of non-channelized deposits interspersed with fluvial channel bodies. The succession records evidence of variations in water discharge at multiple temporal scales. One-hundred-and-eighty sand-prone and conglomeratic bodies present within otherwise fine-grained deposits have been examined by facies and architectural-element analyses. These have been assigned to four distinctive types: (i) simple and (ii) compound channelized deposits, (iii) multilateral and multistorey elements, and (iv) sediment gravity-flow deposits. Simple and compound channel bodies represent deposits of channelized high-energy flows associated respectively with single floods or multiple events occurring in the same channel belt. Multilateral and multistorey elements are associated with sedimentation in longer-lived channel belts. An increase in the occurrence of these latter elements through the stratigraphy can be related to changes in tectonic and/or climatic drivers, heralding a change in facies architecture that signifies a shift to perennial discharge conditions and incised-valley backfilling. This is itself followed by a marked change in architectural style in a part of stratigraphy recording the Paleocene-Eocene Thermal Maximum (PETM), when variations in precipitation regime and/or intrabasinal characteristics (e.g., vegetation) related to this event favoured the lateral migration of channels, causing the development of laterally extensive coarse-grained bodies. Floodplain sediments comprise (i) heterolithic deposits accumulated in the vicinities of the fluvial channels where riparian ecosystems were developed, and (ii) overbank mudstones with variable degree of pedogenesis, especially associated with wetting and drying cycles in areas of the floodplain located distally away from fluvial channels. This succession records the importance of water-discharge variations in the evolution of alluvial systems: it displays facies arrangements recording the effects of highly variable discharge in ephemeral to perennial channels, as well as variations in stratigraphic architecture testifying to longer-term hydrological changes in the late Paleocene and earliest Eocene

Hybrid dry-wet interdune deposition in Precambrian aeolian systems: Galho do Miguel Formation, SE Brazil

It is currently not known whether the construction and accumulation of Precambrian aeolian interdunes occurred in the same way as for their Phanerozoic counterparts. To investigate this, a Mesoproterozoic aeolian succession, the Galho do Miguel Formation (SE Brazil), was studied using facies and architectural-element analyses. The sedimentary succession comprises deposits of simple dunes, draas (megadunes), and dry and wet interdunes. Dry and wet aeolian sub-environments were coeval and their development was likely controlled by local topographic relief that governed a hybrid dry-wet aeolian system. The dry sub-environment was composed of interconnected interdune corridors between large and well-developed bedforms (simple dunes and draas). The water table did not influence the construction of climbing dunes and dry interdunes, but this was fundamental for long-term aeolian accumulation. Due to the proximity of the water table to the depositional surface, the dry interdune flats were eroded only up to the groundwater level, where the wetness inhibited sand removal during dune migration. This condition enabled the accumulation of thick packages of dry interdune deposits (up to 3 m thick). The wet sub-environment is represented by extensive interdune flats, non-climbing dunes, and a continuously near-surface water table. In this context, the interdune deposits underwent only minimal reworking associated with dune migration. This setting allowed the progressive rise of the interdune substrate and the amalgamation of interdune deposits of different generations, thereby producing thick wet-interdune stratal packages (up to 8 m thick). The accumulation of thick packages of interdune strata in both sub-environments was generated by: (i) high rates of vertical accumulation of the interdune substrate due to high rates of sediment input for aeolian construction in Precambrian systems, and (ii) progressive relative rise of the water table. These conditions enabled the long-term accumulation and successive preservation of Precambrian aeolian systems, in which the stabilising effects of vegetation did not operate. Therefore, the aeolian architecture of the Galho do Miguel Formation suggests that: (i) Precambrian aeolian systems probably produced thicker dry and wet interdune deposits than their Phanerozoic counterparts, and (ii) hybrid dry-wet aeolian systems likely provided the most favourable conditions for long-term accumulation and successive preservation of these types of deposit in the Proterozoic

Variations from dry to aquic conditions in Vertisols (Esplugafreda Formation, Eastern Pyrenees, Spain): Implications for late Paleocene climate change

Sedimentological studies of ancient fluvial systems commonly do not consider in detail palaeosols in floodplain deposits. This is the case of the upper Paleocene Esplugafreda Formation (Pyrenees, Spain), a 100–340 m thick alluvial succession, in which palaeosols represent more than 80% of its thickness. This unit closely predates the climate crisis of the Paleocene/Eocene transition, the PETM (Paleocene/Eocene Thermal Maximum). The purpose of this paper is to define the palaeoenvironmental conditions of formation of these palaeopedogenic conditions, and to establish possible changes during the late Paleocene directly prior to the onset of the PETM. This paper analyses the palaeosols in terms of variations in ‘aquic conditions’, concerning to water saturation, reduction and formation of redoximorphic features in soils. The uppermost part of the Esplugafreda Formation consists of cumulative sandy mudstone palaeosols, laterally associated with sparse channel deposits. Two pedotypes were recognised: Pont d'Orrit and Areny. A mud content >60% (mainly montmorillonite), pedogenic slickensides, wedge-shaped aggregates, mukkara and blow structures indicate that both pedotypes belong to the Vertisol order. They reveal a climate characterised by strong seasonal moisture variations. The Pont d'Orrit pedotype, which dominates the lower portion of the succession, shows reddish brown horizons (Bss), calcareous nodules concentration (Bssk horizon) and scarce redoximorphic features, which indicate a semi-arid climate. The Areny pedotype, which prevails in the upper part, shows a yellow horizon (Bssg) containing goethite and abundant redoximorphic features, which reveal aquic conditions. There is no clear sedimentological evidence that river flooding became relatively more frequent in the upper part of the succession, suggesting that the aquic conditions were caused by an increase in precipitation. This study demonstrates that (i) changes in some pedogenic features within the Vertisols enable interpretation of palaeoenvironmental variations and (ii) this region experienced a changing climate in the late Paleocene, before the onset of the PETM

Landscape and depositional controls on palaeosols of a distributive fluvial system (Upper Cretaceous, Brazil)

The stratigraphic record of distributive fluvial systems is commonly characterised by frequent and complex interstratification of palaeosols among channel and overbank deposits. However, current models focus primarily on sedimentation and pay only limited attention to palaeopedogenesis, thereby failing to incorporate important palaeoenvironmental and stratigraphic information. This study proposes a pedosedimentary model for distributive fluvial systems that depicts and accounts for two palaeopedogenetic trends: one downdip, in relation to distality from the fan apex, and one along-strike, in relation to distance from active channel belts. Palaeosols are reported in detail from an Upper Cretaceous succession of the Bauru Basin, southeastern Brazil, through the application of macro-, micromorphological and geochemical studies, combined with facies and architectural-element analyses of sediments. In the downdip palaeopedogenetic trend, the proximal zone of the depositional system is characterised by a dominance of well-drained Inceptisols that develop on amalgamated channel fills; in the medial zone, Inceptisols occur interlayered with overbank deposits containing Entisols and poorly drained Vertisols. The distal zone preserves more mature and poorly drained Inceptisols developed on deposits of overbank and sporadic distal channel fills. These pedotypes show an increase in maturity and hydromorphism, moving away from the apex to the fan toe. This is likely linked to (i) the progressive approach of the topographic surface to the water table, and (ii) the average increase in distance to an active channel belt in distal locations. The along-strike palaeopedogenetic trend culminates in poorly developed palaeosols in floodplain regions that correspond to topographic depressions located between channel belts and which were subject to recurrent floods. Because palaeopedogenesis in the floodplain region is penecontemporaneous to sedimentation, pedotypes show an increase in maturity, bioinduced calcification and hydromorphism with distance from the active channels; they pass laterally from Entisols and Inceptisols near active channels, to Vertisols away from active channels. Conversely, following avulsion, abandoned channel belts remain as topographically elevated alluvial ridges located at some distance from the newly active channels and positioned above the water table and this leads to the development of better drained and better developed Inceptisols relative to pedotypes of the floodplain region. Overall, both palaeopedogenetic trends demonstrate the overriding controls of topography, sedimentation rate and parent material on pedogenesis, with only minor climatic influence. This work offers a novel pedosedimentary model for distributive fluvial systems and highlights the palaeoenvironmental significance of palaeosol trends, providing new constraints for the recognition of distributive fluvial systems in the rock record