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• Precambrian linear megadunes; a unique process of development an accumulation. • Precambrian linear megadunes underook a component of lateral migration. • The water table played a key role on aeolian accumulation, even in dry environments. • Precambrian ergs did not offer optimal conditions for linear megadune construction. Recognizing the deposits of linear megadunes in ancient aeolian successions is challenging when the original bedform topography is not preserved. The present study provides a detailed analysis of the depositional architecture of an ∼ 80 m-thick interval of the Mesoproterozoic Galho do Miguel Formation (SE Brazil) to reconstruct the original morphologies of linear megadunes and to understand their dynamics. The depositional architecture reveals compound cross-bedded sets, which transition laterally into thick low-angle cross-stratified sets and planar-parallel sandstone beds. The depositional features indicate the presence of complex megadunes separated by adjoining wide dry inter-megadune areas, which were themselves flooded seasonally by a water table that rose above the depositional surface. Linear megadunes experienced two phases of development: vertical accretion and subsequent episode of lateral migration. During the vertical accretion phase, high sediment supply, typical of Precambrian aeolian systems, and the convergence of winds from two distinct directions promoted high rates of megadune growth, bedform elongation, and the formation of superimposed dunes. The lateral migration phase was characterised by a component of lateral bedform shift relative to the main along-crest sediment transport direction; this prevented preservation of a bimodal pattern of dune cross-strata azimuths. The accumulation of these deposits occurred via a combination of megadune climbing that occurred with progressive rise of the water-table level. The water-table rise hindered cannibalization of the lower parts of the migrating megadunes, allowing the accumulation and subsequent preservation of megadune sandstone packages. The rise of the water table was also responsible for the accumulation of thick dry inter-megadune strata. The unusual thickness of linear megadune and inter-megadune deposits of the Galho do Miguel Formation, as compared with Phanerozoic examples, are attributed to the interplay between the barren conditions of the early Earth, the morphodynamics of the linear megadunes, and the seasonal impact of the water table at the accumulation surface. Without a water-table control, opportunities for megadune accumulation and preservation were likely limited in Precambrian ergs. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • Precambrian aeolian deposits can be accumulated in hybrid dry-wet environments. • Proterozoic interdune strata are of greater thickness than Phanerozoic equivalents. • Topographic relief controlled interaction of water table and accumulation surface. • Draas, simple dunes and interdunes accumulated coevally. [ABSTRACT FROM AUTHOR]

• Barren Proterozoic aeolian systems should comprise well-developed megadunes (ergs). • Yet, Proterozoic successions show few megadunes but rather wet/damp aeolian deposits. • Groundwater inhibited large aeolian bedforms by restricting sand availability. • But, relative groundwater rise allowed accumulation and preservation of megadunes. • Proterozoic hybrid (dry/wet) aeolian systems had improved preservation potential. Based on Phanerozoic and present-day cases, aeolian systems are categorised into dry, wet and stabilising types. It is questioned here whether these models are applicable to Proterozoic systems when environmental conditions on the Earth's surface were markedly different. Facies and architectural-element analyses have been applied to the Mesoproterozoic aeolian succession of the Galho do Miguel Formation, SE Brazil. The aim is to identify and discuss what controlling factors govern the construction and preservation of Proterozoic aeolian systems, and to explain how these differ from Phanerozoic models. In the metaquartzarenites of the Galho do Miguel Formation four aeolian subenvironments - megadunes (draas), large-scale isolated dunes with dry interdunes, small-scale isolated dunes with damp or wet interdunes and salt flats - coexisted and alternated temporally and spatially. The construction of megadunes, large-scale dunes and dry interdunes occurred in topographically elevated areas, usually above the water table, but that were occasionally flooded; isolated dunes with damp and wet interdunes, and salt flats formed in low-lying areas with water table at or close to the surface. A long-lived sediment supply combined with ongoing tectonic subsidence enabled the accumulation of a thick aeolian succession (1000–1500 m) that covered a large area (4000 km2). The water table controlled the accumulation of this unit. Where it was close to the accumulation surface, it acted to limit the availability of the wind-blown sand, hampering the construction of large and compound bedforms and allowing the deposition of damp and wet interdunes and salt flats as a wet aeolian system. Where large and compound bedforms with dry interdunes developed as a dry aeolian system, slow but progressive subsidence-driven water-table rise provided accumulation space that enabled system preservation. The Galho do Miguel Formation constitutes a hybrid aeolian system, in which both dry and wet environmental conditions were coeval. In the Mesoproterozoic, the absence of rooted-vegetation capable of acting as a sand stabilising agent allowed the widespread generation of aeolian systems in humid as well as arid environments. In humid environmental settings, water played a significant role in the accumulation and preservation of aeolian deposits, preventing their reworking by the wind or other exogenous agents. [ABSTRACT FROM AUTHOR]

Proterozoic aeolian successions tend to exhibit a low complexity in the architectural organisation, generally related to simple dunes with low morphological diversity. Although most palaeoenvironmental reconstructions of ancient ergs confirm this scenario, the general conditions in the Precambrian landmass should have allowed the construction of larger and more complex sedimentary aeolian systems than those recorded in the Phanerozoic successions. To address this paradigm, the present work proposes to discuss the development and morphodynamics of ancient draa-scale bedforms and to understand which controlling factors produced the apparent absence of draa architecture in the Precambrian sedimentary record. For this purpose, this study examines an interval of c. 80 m of the Galho do Miguel Formation, which is a Mesoproterozoic aeolian succession composed of metasandstones showing well-preserved primary sedimentary structures and geometries with excellent vertical and horizontal exposures. A detailed sedimentological analysis allowed the recognition of six main architectural elements: (i) simple transverse dune, (ii) simple linear dune; (iii) compound transverse draa; (iv) complex linear draa, (v) dry interdune and (vi) sand sheet. The lateral and vertical relationships between these architectural elements support the evidence of coeval spatial coexistence of distinct aeolian morphologies (transverse and linear bedforms), probably controlled by lateral variations in sand saturation and in circulation pattern of the palaeowind. The stratigraphic organisation indicates different intervals of draa construction and destruction, which appear to be associated with different sedimentary pulses. The intervals of draa construction represent the vertical succession between simple and compound dunes of similar morphologies, representing temporal variation in sedimentary availability and changes in angle of dune climbing. The erosional and irregular surface at the bottom of compound sets of cross-strata can represent periods of irregular accumulation, associated with partial draa destruction (dune cannibalisation), probably due to seasonal variations in sedimentary availability throughout construction stages. Evidence of a near-surface water table level, as small wave ripples, indicates that the position of the water table was directly linked to the sedimentation pattern throughout these intervals. This relationship implies that ground water was the main controlling factor on sand availability and wind saturation during aeolian morphodynamics, as well as on accumulation and preservation processes. Moreover, in conditions of high rates of subsidence, the thickness accumulated and preserved via water table fluctuation is incorporated in the sedimentary record; otherwise, these deposits would have eroded due to aeolian deflation or fluvial reworking. Apparently, the absence of large and complex aeolian systems in the Precambrian record can be related to the absence of continuous near-surface water table during the draa development, directly decreasing their preservation potential. • Draa-scale bedforms were commonly constructed in Mesoproterozoic aeolian systems. • Pre-vegetational aeolian systems showed a large morphological complexity. • Water table controlled the accumulation of Precambrian draa deposits. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image [ABSTRACT FROM AUTHOR]

• Oblique dunes were developed during the Mesoproterozoic time. • The architectural element analysis suggests simple transverse dune to oblique draa system. • Climate oscillations control the development of sand sheet areas. • Water table variations were the main controlling factor in this Precambrian aeolian system. Aeolian systems of the Precambrian time were established in different palaeoclimatic and palaeogeographic conditions than the present. Different morphologies of dunes/draas, as linear, oblique and star forms, are widespread in the recent aeolian systems. However, the ancient aeolian deposits are rarely interpreted as these types of bedforms and usually referred to as transverse dunes. The nature and complexity of bedform morphology in Precambrian aeolian systems, as well as the factors that allow the construction, accumulation and preservation of these environments are still not well defined. Extensive exposures of Galho do Miguel Formation in southeastern Brazil represent a Mesoproterozoic Erg succession and exhibit a distinct facies architecture, which contributed to the understanding of these deposits. Three architectural elements were identified: (i) tangential cross-stratified sandstone with superimposed trough cross-bedded sandstone, (ii) planar-laminated sandstone, and (iii) extensive flat-bedded sandstone dominated by wrinkly laminae. These elements are organized into three succession groups representative of simple transverse dune, oblique compound draa and sand sheet systems. The simple to compound dune/draa system corresponds to the most central parts of the erg, associated with a higher sediment availability and lower water-table level. Conversely, the sand sheet system occurs under low sediment availability and higher water-table level in the marginal portions of the erg. Precambrian climatic oscillations may have changed the aeolian system morphology, allowing the alternation of simple and complex aeolian dunes, resulting in high-frequency changes in sand sheet system, and the remobilization of previously accumulated sediments over large geographic areas. [ABSTRACT FROM AUTHOR]

• A quantitative comparison of the architectures of 87 Precambrian and Phanerozoic aeolian successions is presented. • Precambrian aeolian successions are characterised by thinner aeolian dune sets (formed by small barchan dunes). • Precambrian aeolian successions are associated with a greater occurrence of sandsheet, zibar, and wet interdune deposits. • The Precambrian aeolian stratigraphic record partly reflects the absence of sediment-stabilizing terrestrial vegetation. • The Precambrian aeolian sedimentary record is geologically distinct from that of the majority of the Phanerozoic. The Precambrian is associated with a set of geologically unique palaeoenvironmental conditions arising from differences in the Earth's atmosphere, biosphere and geosphere. The extent to which these conditions operated to generate an aeolian stratigraphic record that is geologically distinct from that of the Phanerozoic remains poorly understood. This study presents a quantitative comparison of the preserved sedimentary architectures of 87 globally distributed aeolian successions from the Precambrian (30 case studies) and the Phanerozoic (57 case studies). Compared to those of the Phanerozoic, Precambrian aeolian successions are characterised by: (1) thinner aeolian dune sets, mostly documenting the accumulation of small, rapidly migrating barchan dunes; and (2) a greater occurrence of sandsheet, zibar, and damp and wet interdune deposits, many occurring interdigitated with non-aeolian strata. The nature of the Precambrian aeolian stratigraphic record reflects in part the absence of sediment-stabilizing terrestrial vegetation: compared to Phanerozoic systems developed since the evolution of land plants, aeolian processes (erosion, transport and deposition) were more active, even under markedly more humid climates. Precambrian aeolian successions record greater evidence of fluvial and marine incursions into contiguous aeolian dune fields. Water tables were commonly elevated close to the level of the accumulation surface for protracted episodes; extensive damp and wet substrates restricted the availability of dry sand for aeolian dune construction, thereby limiting dune size. In some Precambrian aeolian systems, sediment supply was additionally restricted by the development of cryptobiotic films and crusts that preferentially developed on damp substrates in the absence of vegetation. Common fluvial and marine aqueous interactions promoted the reworking of aeolian dunes into sandsheet deposits; many such accumulations likely represent eroded remnants of larger aeolian dune fields. Given the absence of vegetation-induced sediment binding and baffling, and wind-buffering effects, coarser sand fractions were more readily transported by the wind in the Precambrian; this led to the preferential development of coarse-grained zibar deposits. This study demonstrates how the Precambrian aeolian sedimentary record is geologically distinct from that of the majority of the Phanerozoic, providing new insights into the controlling factors that governed aeolian sedimentation during the Precambrian through interactions between the geosphere, biosphere and atmosphere. [ABSTRACT FROM AUTHOR]