Mesoproterozoic erg and sand sheet system: Architecture and controlling factors (Galho do Miguel Formation, SE Brazil)

Highlights

• 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.

Abstract

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.

Introduction

Precambrian aeolian deposits were formed under atmospheric-biospheric, palaeoenvironmental and palaeogeographic, conditions different than those of nowadays, becoming more abundant only after 1.8 Ga (Eriksson et al., 1998, Eriksson and Simpson, 1998, Tirsgaard and Øxnevad, 1998, Rodríguez-López et al., 2014). The absence of vegetation during this period would suggest a widespread development of aeolian systems, due to a greater amount of sediment available for the wind transport (Tirsgaard and Øxnevad, 1998, Bristow and Mountney, 2013). However, in the geologic record, aeolian deposits do not occur frequently in the Precambrian time. The poor-preservation or non-occurrence of the aeolian successions may be associated with (i) the restricted extension of Precambrian continental terrains, (ii) reworking of the aeolian sediments by sheet-braided rivers and (iii) strandline processes or non-recognition due to low preservation of the textural aspects of sedimentary rocks, usually obliterated by high-grade metamorphism and/or intense shearing (Eriksson et al., 1998, Eriksson and Simpson, 1998, Rodríguez-López et al., 2014, Ielpi and Rainbird, 2016). When preserved, the accumulation of these strata was conditioned mainly by tectonic and climate settings, which controlled the groundwater level and the geographic distribution of sediment sources that fed the aeolian systems (Tirsgaard and Øxnevad, 1998). Fluctuations in the groundwater level are responsible for the removal or preservation of the aeolian deposits, with preservation conditioned to increased precipitation and rise of the groundwater, whereas the falling stage results in a deflation of the strata (Kocurek and Havholm, 1993).

The preserved Precambrian aeolian deposits are dominantly interpreted as dune/draa, dune-plinth, interdune and sand sheet areas, exhibiting simple dune ridges that do not present high morphological complexity, consisting mainly of transverse, barchan and barchanoid bedforms (Jackson et al., 1990, Patterson and Heaman, 1991, Simpson and Eriksson, 1991, Simpson and Eriksson, 1993, Eriksson and Simpson, 1998, Martin et al., 2002, Simpson et al., 2012, Simplicio and Basilici, 2015, Bállico et al., 2017a). In general, aeolian deposits characterised by cross-stratifications are interpreted as crescent-shaped dune systems (Ross, 1983, Jackson et al., 1984, Pulvertaft, 1985). Despite the widespread variety of forms and the large expression of linear, oblique and star dunes in present ergs, the recognition of these dune morphologies is scarce in the literature (McKee, 1979, Lancaster, 1982, Mountney, 2006, Rodríguez-López et al., 2014). For example, the non-recognition of linear dunes in the geological record may be related to lateral migration of this morphological type of bedform, which generates deposits relatively similar to crescent-shaped dune deposits (Rubin and Hunter, 1985, Clemmensen, 1989, Rubin, 1990, Chrintz and Clemmensen, 1993, Rubin et al., 2008, Rubin and Rubin, 2013). In theory, the main feature for the identification of linear dune deposits would be the recognition of a nearly opposite dip direction of cross-bed sets, related to straight dune ridges with slipfaces in both sides of the crest, migration parallel to the wind direction and aggradation of aeolian sediments (Bagnold, 1941, Tsoar, 1978, McKee, 1979, Lancaster, 1982, Bose et al., 1999, Scotti and Veiga, 2019). Instead, some authors have demonstrated that the migration of linear dunes is mainly lateral with preferential migration of one slipface (Rubin and Hunter, 1985, Rubin, 1990, Bristow et al., 2000, Rubin et al., 2008, Rubin and Rubin, 2013, Scherer, 2002, Scotti and Veiga, 2019). Lateral migration and aggradation of linear dunes/draas, as well as the oblique advance of compound/complex draas, can originate a great variety of strata geometries and peculiarities that allow to differing them from other aeolian bedforms (Clemmensen, 1989, Clemmensen and Blakey, 1989, Rubin, 1990, Scherer, 2000, Scherer, 2002, Scotti and Veiga, 2019).

Were aeolian dune systems simple throughout all the Precambrian time? What are the characteristic architectural elements of compound dunes and complex draas? What factors controlled the development and evolution of the aeolian systems in a pre-vegetational setting? The Mesoproterozoic deposits of the Galho do Miguel Formation, southeast of São Francisco Craton (eastern Brazil; Fig. 1), present excellent exposures with a horizontal and vertical variety of facies geometry that may contribute to understand the sedimentary complexity of these aeolian systems.

The present work focuses on: (i) description of the architectural elements from a Mesoproterozoic aeolian succession, (ii) determination of forming processes and geometry of simple and complex aeolian dunes/draas, and (iii) reconstruction of spatial and temporal evolution of this Precambrian aeolian system. The argumentation of these topics may contribute to understand the recognition of complex dune strata in ancient deposits, the sedimentation processes in pre-vegetation aeolian systems and the evolution of Precambrian deserts.