Genetic units and facies architecture of a Lower Cretaceous fluvial-aeolian succession, São Sebastião Formation, Jatobá Basin, Brazil

Accepted Manuscript Genetic units and facies architecture of a Lower Cretaceous fluvial-aeolian succession, São Sebastião Formation, Jatobá Basin, Brazil João Pedro Formolo Ferronatto, Claiton Marlon dos Santos Scherer, Ezequiel Galvão de Souza, Adriano Domingos dos Reis, Raquel Gewehr de Mello PII:

S0895-9811(18)30212-8

DOI:

https://doi.org/10.1016/j.jsames.2018.11.009

Reference:

SAMES 2036

To appear in:

Journal of South American Earth Sciences

Received Date: 14 May 2018 Revised Date:

13 November 2018

Accepted Date: 13 November 2018

Please cite this article as: Formolo Ferronatto, Joã.Pedro., Scherer, C.M.d.S., de Souza, Ezequiel.Galvã., Domingos dos Reis, A., de Mello, R.G., Genetic units and facies architecture of a Lower Cretaceous fluvial-aeolian succession, São Sebastião Formation, Jatobá Basin, Brazil, Journal of South American Earth Sciences (2018), doi: https://doi.org/10.1016/j.jsames.2018.11.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Genetic Units and facies architecture of a Lower Cretaceous fluvial-aeolian

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succession, São Sebastião Formation, Jatobá Basin, Brazil.

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João Pedro Formolo Ferronatto – Federal University of Rio Grande do Sul1

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Claiton Marlon dos Santos Scherer - Federal University of Rio Grande do Sul 2

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Ezequiel Galvão de Souza - Federal University of Rio Grande do Sul 3

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Adriano Domingos dos Reis - Federal University of Rio Grande do Sul 4

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Raquel Gewehr de Mello - Federal University of Rio Grande do Sul 5

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Federal University of Rio Grande do Sul – Campus do Vale Av. Bento Gonçalves,

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3308-6329

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[email protected]

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claiton.scherer@[email protected]

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[email protected]

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[email protected]

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ACCEPTED MANUSCRIPT Abstract

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The Lower Cretaceous São Sebastião Formation of Jatobá Basin consists of continental

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strata accumulated in an arid environment dominated by aeolian deposits. The best

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outcrops are located in the county of Ibimirim, in the Pernambuco State. Through the

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description of several stratigraphic sections, three aeolian and two fluvial facies

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associations characterize this formation. The aeolian facies represent (a) aeolian

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crescentic dunes, (b) aeolian sand sheets, and (c) blowouts; the fluvial facies record (d)

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sheet floods and (e) channelized ephemeral rivers. These facies associations are

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organized into three genetic units that are bounded by supersurfaces. The unit 1 is

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formed by the intercalation of sheet flood, aeolian dry sand sheets, and aeolian dunes

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facies associations, often with soft sediment structures. The cross-bedded strata ascribed

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to aeolian dunes become larger and more frequent towards the top of the unit, as the soft

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sediment deformation structures become scarcer. The Unit 2 comprises aeolian sand

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sheets facies scoured by fluvial channels, with erosive concave bases, and by blowouts,

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which are subsequently filled by aeolian dunes deposits. Unit 3 consists of cross-bedded

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strata related to medium-to-large, either simple or compound aeolian dunes (draas).

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Supersurfaces bound the genetic units recording depositional gaps, thus defining

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different episodes of accumulation.

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Keywords: Lower Cretaceous; Genetic units; Fluvial-aeolian succession; São Sebastião

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Formation; Jatobá Basin.

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INTRODUCTION Clastic deposits record interaction between fluvial and aeolian systems occur

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frequently in sedimentary basins, both in ancient (e.g. Clemmensen et al., 1989;

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Langford and Chan, 1989; Trewin, 1993; Mountney et al., 1998; Newell, 2001;

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Mountney and Thompson, 2002; Poland and Simms, 2012), as in modern environments

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(e.g. Lancaster and Teller, 1988; Langford, 1989; Stanistreet and Stollhofen, 2002; Al-

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Masrahy and Mountney, 2015). For this reason, numerous works have discussed

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different scales of fluvial-aeolian interactions, from autocyclic phenomena, result of

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years, decades or centuries of contemporary fluvial-aeolian processes (e.g. Loope, 1985;

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Langford and Chan, 1988; Bullard and Livingstone, 2002; Mountney and Jagger, 2004;

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Ulicný, 2004), to allocyclic phenomena that create a large-scale alternation between

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non-contemporary fluvial and aeolian deposits, resulting in regional sedimentation

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patterns due to climatic and/or tectonic alterations (e.g. Clemmensen et al., 1989;

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Herries, 1993; Howell and Mountney, 1997; Kocurek et al., 2001; Veiga et al., 2002;

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Scherer and Lavina, 2005; Veiga and Spalletti, 2007; Scherer et al., 2007).

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This paper characterizes and details the evolution of the fluvial-aeolian deposits

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of São Sebastião Formation, Lower Cretaceous of the Jatobá Basin. The specific

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objectives include understanding the processes and interactions between fluvial and

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aeolian systems through facies characterization, documentation of the facies

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associations and subdivision of the formation into lithostratigraphic unit, using as

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criteria the different stacking patterns of facies associations, and the identification of

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stratigraphically significant surfaces.

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GEOLOGICAL CONTEXT The Jatobá Basin is located in the northeast of Brazil, more specifically in the

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southwestern Pernambuco State (Figure 1). The Recôncavo-Tucano-Jatobá Rift System

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is related to the breakup of the Gondwana Supercontinent and the formation of the

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South Atlantic Ocean. This strike-slip basin (Peraro, 1995) extends along an ENE-

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WSW trend approximately 155 km long and 55 km wide, with the main border fault in

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the northern portion (Ibimirim lineament; Magnavita and Cupertino, 1987). The Jatobá

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Basin overlies the Borborema Craton, formed by metamorphic and granitic rocks of

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Proterozoic age near the limit with the São Francisco Craton. The basement structure,

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composed mainly by SW-NE oriented lineaments, strongly controlled the opening of

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this basin, recording change in the initial rift system from S-N (Tucano Basin)

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orientation to a SW-NE orientation (Jatobá Basin; Magnavita and Cupertino, 1987;

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Santos et al., 1990; Costa et al., 2007).

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The Lower Cretaceous (~140 to 125 Ma) São Sebastião Formation accumulated

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in Jatobá Basin reflecting the interaction between aeolian and fluvial systems, over an

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average thickness of about 200 m (Rocha and Leite, 2001; Costa et al., 2007; Rocha and

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Amaral, 2007; Guzmán et al., 2015). The São Sebastião Formation crops out in the

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northern portion of the basin, where the best exposures are located near Ibimirim town

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(Figure 1). As a formation of the Massacará Group it represents the terminal rift tectonic

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stage and correlates to the uppermost part of Salvador Formation at the boundary fault.

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Stratigraphically, the São Sebastião Formation overlays the deltaic systems of the Ilhas

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Group and underlies the alluvial deposits of the Marizal Formation. The contact

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between Ilhas Group and São Sebastião Formation is gradual and undefined, while the

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contact between the São Sebastião Formation and the alluvial deposits of the Marizal

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Formation is sharp and erosive (post-rift unconformity). The Marizal Formation was

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ACCEPTED MANUSCRIPT deposited during the sag stage (thermal subsidence) of the Jatobá Basin, exceeding the

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limits of the rift stage and covering a larger area.

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Figure 1. Geologic and location map of the studied outcrops (adapted from Rocha and Leite, 2001).

METHODOLOGY

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Eight sedimentary sections were logged in detail to define the vertical succession

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of facies and facies associations, summarized in a compound vertical log of

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approximately 100 m (Figure 2). In addition, six architectural panels were constructed

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from the photo-mosaics of the selected outcrops, allowing the boundary surface

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identification and the definition of the two-dimensional (2D) geometries of the facies

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associations and genetic units. The facies nomenclature was based on grain-size and

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ACCEPTED MANUSCRIPT sedimentary structures, modifying the scheme of Miall (1996). Paleocurrent orientations

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were measured from unidirectional sedimentary structures, mainly through cross-

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bedding. To identify and interpret the bounding surfaces was used the hierarchy of

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bounding surfaces in cross-stratified aeolian sandstones proposed by Kocurek (1981).

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Figure 2. Compound vertical log of the São Sebastião Formation in the Jatobá Basin.

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FACIES ASSOCIATIONS The São Sebastião Formation comprises five facies association: (a) aeolian

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crescentic dunes, (b) dry aeolian sand sheets, and (c) blowouts, while the fluvial

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deposits occur as (d) sheet floods, and (e) channelized ephemeral rivers.

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Aeolian facies associations

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Aeolian crescentic dunes

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Description

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This facies association is characterized by fine- to coarse-grained sandstones,

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yellowish brown to light red-colored, moderately to well-sorted, well-rounded grains of

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quartz, organized in trough sets filled with tangential cross stratification (St(e)), 0.2 m

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to 20 m thick (Figure 3). The cross-bedded strata have in the steeper portions (22-30°)

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1–2 mm thick, massive grain fall laminae, intercalated with 1 to 8 cm thick, massive to

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inversely graded grainflow strata. Downdip, grainflow and grain fall strata pinch out,

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intertonguing with low-angle (1-20°), mm-thick, inversely graded, wind-ripple

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lamination (Figure 4). Some thinner sets (<2 m) show gentle dip angle less than 25° and

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are composed entirely by wind-ripple lamination.

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Soft sediment deformations can affects individual or 2 or 3 vertically

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superimposed sets. Deformed sets can range from few tens of centimeters to a few

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meters in thickness (0.2 to 7 m) and laterally the deformation dissipates. Convolute

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laminations (Sc(e)) are one of the most common structures and are characterized by

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different degrees of asymmetrical folding (Figures 3C and 4F) and pass upward into

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undeformated sets. The transition between deformed and undisturbed sets can be

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gradational or marked by a sharp transition.

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ACCEPTED MANUSCRIPT Three hierarchies of surfaces (sensu Kocurek, 1981) can be defined in this facies

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association from smallest to largest. 3rd order surfaces display sigmoidal geometry,

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occurs internally to the sets, truncates the cross-bedded strata with average dip angle of

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22°, and lateral spacing that varies from 1 to 10 m. The dip direction of these surfaces

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and their overlying cross-bedded strata are usually similar, generally less than 15° of

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divergence. 2nd order surfaces occur interally to cosets, consist of downwind dipping

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surfaces (5° to 13°) bounding smaller sets, 0.3 to 1 m in thickness. A divergence larger

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than 30° in dip directions is commonplace between this surfaces and the orientation of

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the overlying cross-bedded strata. 1st order surfaces define the limits of sets and cosets

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of cross-bedded strata, and dip at low angles opposite to the dip direction of the

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overlying cross-bedded strata.

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Interpretation

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The combination of well-rounded grains, well-sorted sandstones, climbing

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aeolian ripples strata forming the cross-bedded sets bases that yield up-dip to grain flow

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and gain fall deposits are best interpreted as aeolian dune deposits (Hunter, 1977;

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Kocurek and Dott, 1981).

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Figure 3. Facies association of aeolian dunes. (A) and (B) Tangential foresets in trough cross-bedded sets of aeolian dunes deposits (St(e)). Person (1.65 m in height) for scale in B. (C) Overlying sets of aeolian dunes cross-bedded strata (St(e)). The set delimited by solid white lines shows convolute laminations, dashed white lines (Sc(e)). Person (1.70 m in height) for scale. (D) Tangential cross-bedded strata. Rock hammer is 28 cm in length.

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The cross-bedded strata, constituted by grain flows represent deposits of aeolian

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dunes with well-developed, steep lee-faces, while the cross-bedded sets composed

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entirely by wind-ripple laminations represent severely truncated dunes or the absence of

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well-defined lee-faces (Kocurek and Dott, 1981). The presence of tangential cross-

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bedded with a tight unimodal foreset dip direction indicates crescentic aeolian dune

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origin with a sinuous crest line (McKee, 1979; Rubin, 1987).

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ACCEPTED MANUSCRIPT Convolute laminations result from overpressuring of pore space in water-

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saturated sands, permitting the unpacking of grains and consequent deformation of the

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original depositional structures (McKee et al., 1971; Lowe, 1975; Doe and Dott, 1980;

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Horowitz, 1982). One possible mechanism is loading by dune climbing. According the

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model of Bryant et al. (2016) based on Horowitz (1982) successive sets of deformed

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cross-beds with convolute structures that grade upward into undeformed primary

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structures suggest deeper seated deformation mechanism. Deep deformation zones are

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interpreted as result of liquefaction and fluid scape generated by the accumulation

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overload. The small- to medium-scale sets with convolute structures within single sets

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or co-sets and with gradational lateral contacts suggest that the deformation occurred

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within the active dune or draa (Bryant et al., 2016).

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Figure 4. Facies association of aeolian dunes. (A) Detail of the grain flow wedges intercalated with climbing ripple laminations in thinly preserved cross-bedded strata. The grain flow strata show coarser grainsize than the climbing ripple laminations. Pen is 14 cm in length. (B) Inverse grading internally to grain flows. Medium grains at the base and coarse at the top. (C) Grain flow ‘gf’ wedges at the top intercalating with wind ripple laminations ‘wr’ towards the base. (D) Inverse grading in grain flow wedge. Tip of pen is 3.5 cm. (E) Intervals with predominating grain flow deposits separated by whitish thin grain fall deposits ‘gfl’. Pen is 14 cm in length. (F) Two overlying sets of small-sized aeolian cross-bedded strata with soft sediment deformation. Rock hammer is 28 cm in length.

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The low-angle surface 1st order surface that bounds sets or cosets formed by

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erosion in the interdune/draas subcritical climbing of aeolian dunes or draas (Kocurek,

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1981, 1996; Mountney, 2006). The inclined surfaces that divide the cosets and present

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divergent dip direction in more than 30° regarding overlying cross-bedded strata,

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represent 2nd order erosive superposition surfaces (Kocurek, 1981,1996; Mountney,

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2006). The superposition surfaces are formed by the migration of smaller dunes

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obliquely down the lee-face of draas. The sigmoidal, erosive surfaces internal to sets

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ACCEPTED MANUSCRIPT and presenting dip direction similar to those of the foresets are interpreted as 3rd order

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reactivation surfaces (Kocurek, 1996; Mountney, 2006). These 3rd order surfaces have

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genesis related to changes in dune height, changes in dune asymmetry, and/or local

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fluctuations in wind direction and/or speed (Kocurek, 1981, 1996; Hunter and Rubin,

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1983; Kocurek et al., 1991).

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Aeolian sand sheets

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Description

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The deposits are tabular sets, from 1 to 9 m thick, and composed internally by

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two distinct facies. The first facies is characterized by fine- to medium-grained

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sandstone, organized in sets of horizontal to low-angle (<5°) laminations (Sl(e)) that are

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composed of mm-thick, inversely graded laminations (Figures 5A and 5B). The second

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facies consists of medium- to coarse-grained sandstone, organized in sets of low-angle

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(5-10°) inversely graded cross stratification that is limited to the base of planar surfaces

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and to the top by convex surfaces (Figures 5C through 5G). Sets have maximum

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thickness of 2 m and are laterally extensive up to 30 m.

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Interpretation

The presence of tabular sets composed of fine to coarse sandstones that are

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organized in sets of horizontal- to low-angle inversely graded stratifications and have a

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restricted lateral extent, are consistent aeolian sand sheet deposits. These deposits are

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generated by the migration and climbing of wind ripples over a dry depositional surface

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under conditions of net sedimentation (Hunter, 1977; Veiga et al., 2002). The horizontal

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and low-angle strata indicate development of wide, low-amplitude aeolian sand sheets.

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ACCEPTED MANUSCRIPT According to Kocurek & Nielson (1986), aeolian sand sheets occur in a context of low

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availability of dry sand, related to high level of the phreatic water table, cementation

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surface, periodic floods, coarse grainsize and/or presence of vegetation. On the other

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hand, the medium- to coarse-grained sandstones, with low-angle cross-bedded strata

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limited near the top of convex surfaces, indicate taller bedforms without a developed

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slip-face and compatible with a domic dune interpretation (McKee, 1966, 1979;

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Karpeta, 1990) or zibars (Nielson and Kocurek, 1986; Biswas, 2005).

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Blowouts

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Description

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This facies association is linked to zibar-type sand sheet deposits. The lenticular

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bodies range from 0.5 to 2 m thick, approximately 15 m in length, and delimited by

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gentle concave-upward basal surface (Figure 6A). Internally, these sedimentary bodies

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are composed by medium- to coarse-grained sandstones and consist of well-sorted and

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well-rounded grains, organized in sets of tangential cross-stratifications (St(e); Figure

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6B). The cross-stratified sets are composed by grain flow, grain fall and wind-ripple

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laminations (Figures 6C and 6D).

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Figure 5. Aeolian sand sheets facies association. (A) Tabular packages of sandstones with horizontal to low-angle aeolian laminations (<5°). Rock hammer is 28 cm long. (B) Detail of the climbing ripple laminations. Tip of pen is 5.5 cm. (C) Limit of the low-angle cross-bedded sets. Set boundary is marked by white line. Rock hammer is 28 cm long. (D) Detail of the low-angle laminations. Pen is 14 cm. (E) Detail of an aeolian ripple with preserved ripple bedform above tip of pen. Tip of pen is 4 cm. (F) Climbing inversely graded wind ripple laminations. Inverse grading consists of medium sand at the base and coarse sand at the top. Tip of pen is 1.5 cm. (G) Low-angle aeolian stratification with coarse-grained wind ripple laminations. Pen is 14 cm long.

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ACCEPTED MANUSCRIPT Interpretation

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Gentle, concave-upward bounding surfaces lacking intra or extraclasts adjacent

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to the surfaces suggest erosion from aeolian processes similar to modern blowouts

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(Jungerius and Van der Meulen, 1988, 1989; Gradzinski, 1992; Hesp, 1996, 2002;

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González-Villanueva et al., 2011; Hesp and Walker, 2012). The gentle concave

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geometry of the base surface argues that they reflect saucer-type blowouts (Cooper,

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1958).

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Blowouts are erosive structures formed by wind acceleration in topographically

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irregular sandy terrains, which create turbulent flows characterized by spiral vortexes

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near the lee-face of the dunes (Hesp, 2002). They are often restricted coastal dune

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fields, however an exception occurs in inland eolian dune fields, when wind energy is

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extreme. The presence of well-sorted bimodal sandstones, with well-rounded grains and

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tangential cross-stratifications (St(e)) formed by grain flow and grain fall, indicates that

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the filling of these depressions occurred by migration sinuous-crested crescent dunes

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with steep lee facies that had flow separation (McKee, 1979). The absence of 2nd order

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surfaces indicates that the crescentic dunes were simple, not forming draas.

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Figure 6. Blowout facies association. (A) Overall view of a blowout showing lenticular geometry with regular concave base marked by white line. Person (1.65 m in height) for scale. (B) Sandstones with tangential cross-bedded sets recording aeolian dunes that fill the blowouts. Rock hammer is 28 m. (C) and (D) Detail of grain flow wedges ‘gf’ and wind ripple laminations ‘wr’ that form the trough cross-bedded sets. Rock hammer in D is 28 cm.

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Sheet floods Description

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This facies association comprises tabular bodies ranging from 0.2 to 1 m thick,

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gently basal erosive surfaces that are sometimes capped by pebble and granule lags

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(Figure 7A and 7B). Well-rounded to sub-angular fine- to medium-grained sandstones

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are moderately to poorly sorted, and usually contain scattered granules and pebbles

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(Figure 7D). Stratification types include low-angle cross stratification (Sl), horizontal

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lamination (Sh), ripple cross stratification (Sr) and massive (Sm) (Figure 7). Tangential

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cross-stratifications (St) are less frequent. Millimeter to 2 cm thick stratification formed

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ACCEPTED MANUSCRIPT by granules or coarse sand occurs as concentrations at the base of the sets or are parallel

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to stratification. Internally the bodies are often fining-upward (Figure 7A) characterized

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by medium-grained sandstones at the base (Sh, Sl, Sm and/or St) that vertically grade to

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very fine and fine-grained sandstones at the top (Sr).

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Interpretation

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Thin, tabular packages with gently erosive surfaces, fining-upward cycles,

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moderately to poorly sorted sandstone, combined with massive (Sm) sandstones,

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tractive structures generated in transitional (Sl) to upper (Sh) regime flow, necessitates

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that this facies association be interpreted as sheet floods deposits, formed during short

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rapid sedimentation events (Picard and High, 1973; Tunbridge, 1984; Hampton and

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Horton, 2007). The massive sandstone are deposited by high concentration of suspended

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sand (hyperconcentrated flows), which cause turbulence suppression by precluding the

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development of flow separation and dune development (Allen and Leeder, 1980). The

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occurrence of medium-grained sandstones, massive, with horizontal laminations or low-

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angle cross-stratifications, succeeded by fine-grained sandstones with ripple cross

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lamination, represents waning flows associated with flash floods (McKee et al., 1967;

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Miall, 1977, 1996). The presence of grains with roundness varying from sub-angular to

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well-rounded indicates that these floods reworked aeolian deposits.

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Figure 7. Sheet floods facies association. (A) Tabular packages with fining-upward cycles marked by medium sandstones at the base and fine sandstones at the top. (B) Massive sandstone with pebbles and granules at the base. Pen is 14 cm long. (C) Detail of poorly sorted sandstones with well-rounded grains, low-angle cross stratifications at the base and ripple cross stratifications at the top. Pen tip is 8 cm. (D) Poorly sorted sandstone with well-rounded quartz grains. Pen tip is 3.5 cm. (E) Massive sandstone bed with some scattered granules.

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Channelized ephemeral rivers Description

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This facies association occurs as lenticular sand bodies, varying from 0.3 to 3 m

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thick, limited at the base by erosive and irregular concave-shaped surfaces that

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commonly at high angles in the center with the angle decreases towards the edge

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(Figure 8A, 8B and 8C). Internally the sand bodies are composed by medium- to coarse-

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grained sandstones, reddish, moderately to poorly sorted, with sub-angular to rounded

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grains (Figure 8F). The sandstones are massive (Sm) or low-angle (Sl), tangential (St),

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or sigmoidal (Ssg) cross-stratification. Ssg is uncommon (Figure 8G). Sandstone

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intraclasts, up to 70 cm, and granules and pebbles of quartz are found at the base of sand

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bodies on or near the erosive contact (Figures 8C and 8D). Sandstone intraclasts, are

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composed by well-sorted and well-rounded coarse-grained sand, preserving the primary

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low-angle cross-stratification, present soft sediment deformation (Figure 8E),

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sometimes partially connected to underlying substrate, or partially ripped out (Figure

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8C).

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Interpretation

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The presence of lenticular sand bodies, with erosive bases, internally composed

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of structures indicative of transitional conditions between low to upper flow regime,

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suggests fluvial channels characterized by high energy currents (Miall, 1977; Langford

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and Braken, 1987; Alexander and Fielding, 1997; Blair, 2000; Fielding, 2006). Sm

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indicates a rapid variation in discharge, during which transcritical and supercritical

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bedforms cannot stabilize during flow deceleration, preventing a restructuration to

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subcritical bedforms, like 2D and 3D dunes (Alexander and Fielding, 1997). Hence, Sm

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is interpreted as the product of hyperconcentrated flows (Lawson, 1982; Lavigne and

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Suwa, 2004). The presence of hyperconcentrated flow deposits is consistent with

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interpretation of ephemeral high energy fluvial channels. The textural similarity between the channel fill sandstones and the underlying

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aeolian sandsheets shows that windblown deposits were subject to erosion by flood

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water as demonstrated by aeolian sand clasts along the channel margins. Aeolian sand

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intraclasts with well-preserved wind ripples lamination and occasionally even with soft-

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sediment deformed clasts indicates some cementation or pores were water saturated,

334

generating adhesion between the grains (Mountney and Howel, 2000; Rodríguez-López

335

et al., 2012).

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337 338 339 340 341 342 343 344 345

Figure 8. Channelized ephemeral rivers deposits facies association. (A), (B) and (C) fluvial channels with irregular concave-shaped erosive bases marked by white line. The white arrows indicate intraclasts and the yellow arrow points to a intraclast partially ripped out from the substrate. Hammer is 28 cm, in A and C. Person (1.85 m in height) for scale in B. (D) Detail of a channel base with sand intraclasts concentration. White arrows point to the varying angle of erosion. Note the steepness in the center and the gentile angle on the margin. Hammer is 28 cm. (E) Intraclasts with preserved aeolian climbing ripple laminations and tails formed by soft sediment deformation. (F) Massive coarse sandstone, with subangular to well-rounded grains and rare scattered granules. Pen tip os 4 cm. (G) Sets of medium to coarse sandstones with sigmoidal cross stratifications ‘Ssg’ and low-angle cross stratifications Sl. Pen is 14 cm long.

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346 347

GENETIC UNITS

348

In this paper the São Sebastião Formation is subdivided in three genetic units,

349

each one with distinct depositional features and separated by supersurfaces (Kocurek,

23

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1988; Kocurek et al., 1991; Craubaugh and Kocurek, 1993; Havholm et al., 1993;

351

Scherer, 2002; Veiga et al., 2002; Bourquin et al., 2009).

352

Genetic Unit 1 (U-1)

RI PT

353

This unit is restricted to the base of the São Sebastião Formation in the Jatobá

355

Basin. The contact relation with the underlying deltaic deposits of the Ilhas Group is

356

obscured. As a consequence, U-1 had a minimum thickness of 57 m, as measured from

357

the bounding upper supersurface.

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The basal portion of U-1 is characterized by intercalation of sheet floods, aeolian

359

sand sheets and aeolian dunes facies associations (Figure 9A). The bulk abundance is

360

60% of aeolian deposits and 40% fluvial deposits. Due to the limited outcrops, it is not

361

possible to define the lateral relation between the contained fluvial and aeolian systems.

362

The sheet floods sand bodies, marked by fining-upward cycles (Figure 7), with

363

thicknesses up to 1 m, sometimes form up to 5 m-thick amalgamated packages. In some

364

cases, only gravelly lags mark the 1st order surface between aeolian packages, indicating

365

aeolian reworking of the fluvial beds. In turn, aeolian dunes and aeolian sand sheets

366

packages alternate vertically. The sets of aeolian dunes show maximum thickness of 1.5

367

m, constituting of wind-ripple strata only (Figure 3D), while the aeolian sand sheets

368

reach up to 2 m thick, forming together packages up to 10 m (Figure 5A and 5B). The

369

aeolian dunes cross-bedded strata commonly have soft sediment deformation (Figure

370

9A). On the other hand, the top contacts of the aeolian deposits are characterized by a

371

plane to gently erosive surface, over which sheet floods strata rest.

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ACCEPTED MANUSCRIPT In the upper half of U-1 the sheet flood deposits are less abundant, in the

373

dominantly aeolian dunes and aeolian sand sheets deposits. The ratio between the

374

aeolian dunes and aeolian sand sheets facies associations increases progressively

375

towards the top, followed by the increase in thickness of aeolian dunes cross-bedded

376

strata bounded by 1st order surfaces (Figure 9B). The individual sets can reach up to 12

377

m in thickness, being constituted dominantly by grain flows (Figure 4C and 4E). The

378

deformational features of the sets become less frequent vertically.

RI PT

372

The paleocurrent measures, measured from aeolian cross-bedded strata, present a

380

mean vector towards 319° (NW; see Figure 2) for U-1, with low variance. The measures

381

acquired in the sheet floods deposits indicate a mean paleoflow to 282° (WNW; see

382

Figure 2).

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384

Genetic Unit 2 (U-2)

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The U-2 is characterized by a tabular package, 10 m thick, composed of

386

predominantly zibar-type sand sheet deposits (Figure 5C, through 5G) that are

387

intercalated laterally and vertically, in smaller proportion, with isolated sand bodies of

388

channelized ephemeral rivers (Figure 8) and blowouts (Figure 6). Blowouts and fluvial

389

channel deposits comprise approximately 25%, while aeolian sand sheets represent 75%

390

of U-2. The fluvial channel sandbodies can overlay zibar-type sand sheets or blowouts

391

deposits, always marked by a concave, erosive, sharp-relief basal surface, sometimes

392

with sand intraclasts. The blowouts mainly overly aeolian sand sheets, with a sharp

393

regular and concave basal contact. In some cases, aeolian sand sheets are laterally

394

continuous into aeolian dunes that fill in the blowouts. This relation is marked by an

395

incremental increase in dip angle of the cross-bedded strata from the blowout flanks

AC C

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25

ACCEPTED MANUSCRIPT (Figure 10). Trough cross stratification of aeolian crescentic dunes and domic dunes

397

show a pattern of foreset dip towards WNW (Figure 2). Paleocurrent dispersion is

398

greater in crescentic dunes (1800) than in domic dunes (900). This pattern reflects the

399

filling of crescentic dunes into the circular structures of the blowout, generating a

400

greater spread in the foreset dip direction.

RI PT

396

In U-2 has some inflexion towards west in the aeolian paleocurrents comparing

402

to those from U-1 (Figure 2). The paleocurrent measured on cross-bedded strata of

403

dome dunes indicate a main vector WSW, while the tangential cross-bedded strata

404

filling the blowouts show paleoflow to WNW (Figure 2).

M AN U

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401

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26

ACCEPTED MANUSCRIPT 406

408 409 410 411 412 413 414

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Figure 9. Lateral panels of Unit 1. (A) Interpreted photomosaic and vertical log of the U-1 base. Outcrop rd with predominance of small-sized aeolian dunes cross-bedded strata with reactivation surfaces (3 order surfaces) ‘R’ and interdune ‘I’. Profuse features of soft sediment deformation (convolute lamination). Between the aeolian packages, a massive sandstone bed is evident, related to the sheet flood deposits. Person (1.70 m in height) for scale. (B) Photomosaic of the U-1 top. Sets of trough cross-bedded strata from aeolian dunes limited by interdune surfaces ‘I’.

27

ACCEPTED MANUSCRIPT

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415

416

Figure 10. Interpreted photomosaic and vertical log of a representative outcrop from U-2. Zibar-type aeolian sand sheets are cut by blowouts and ephemeral fluvial channels. Person (1.65 m in height) for scale.

M AN U

417 418 419 420 421

Genetic Unit 3 (U-3)

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U-3 is the most homogenous unit from São Sebastião Formation in the Jatobá

424

Basin, showing minimum thickness of 25 m, the top surface has not been identified. U-3

425

is composed by cross-bedded sets varying from 1 to 20 m thick, simple and compound

426

crescentic dunes (Figure 11). The simple crescentic dunes are characterized by cross-

427

bedded sets and composed predominantly by grain flow. These grain flows are

428

intercalated at the base with wind-ripple lamination (Figure 3 and 4), displaying a

429

pattern of foreset main dip towards 257° (WSW) and dispersion approximately 180°.

430

The largest cross-bedded strata set of simple dunes is 20 m thick and occurs filling a

431

large scoured depression. The crescentic dunes superimposed on draas are characterized

432

by cross-strata cosets, up to 5 m thick, where the 2nd order surfaces shows a mean vector

433

towards 290° (WNW), while the cross-bedded sets of the superimposed dunes show

434

mean values towards 322° (NWN). The divergent orientation between the cross-bedding

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28

ACCEPTED MANUSCRIPT and the 2nd order surfaces indicate that the superimposed dunes migrated obliquely

436

relative to the main draa crestlines (Rubin, 1987; Scherer, 2000, 2002; Rodríguez-López

437

et al., 2008). Combined paleocurrents of U-3, shows a mean vector towards 278°

438

(WNW; Figure 2). In U-3 only one lenticular sand body of ephemeral fluvial channel

439

was identified, with a maximum thickness of 0.5 m.

444 445

446

447

448

449

Figure 11. Interpreted lateral panel of U-3. Cross-bedded strata of simple and compound (draas) aeolian st nd dunes separated by interdune surfaces ‘I’, 1 order surfaces. Superposition surfaces ‘S’, 2 order surface, limit sets composing the cosets of the draa. Person (1.65 m in height) for scale.

EP

441 442 443

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435

29

ACCEPTED MANUSCRIPT 450

Supersurfaces The U-1, U-2 and U-3 are bounded by flat to slightly undulating surfaces that

452

truncate interdune surfaces (1st order) and mark a change in the fluvial-aeolian

453

depositional model (Figure 12). Therefore, these surfaces are interpreted as

454

supersurfaces and represent regional-scale surfaces that delineate the end of an aeolian

455

accumulation event (Kocurek, 1981, 1988).

RI PT

451

Between U-1 and U-2 is planar and subhorizontal (<20) supersurface (Figure

457

12), whereas the supersurface separating U-2 from U-3 may be planar, severely erosive

458

or concave-shaped. The concave portion is high angle, with a minimum relief of 20 m,

459

without intra- or extraclasts, overlying the supersurface. This supersurface can erode the

460

U-2 and upper portion of the U-1(Figure 12B).

M AN U

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464

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463

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30

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465 Figure 12. (A) Photomosaic illustrating the plane boundaries (sub-horizontal) of the supersurface separating the genetic units. Observe that the supersurface limiting U-1 and U-2 truncates an interdune surface. (B) Interpreted lateral panel showing the plane boundary between U-1 and U-2, and the severely erosive concave-shaped surface separating U-2 and U-3.

31

ACCEPTED MANUSCRIPT 466

STRATIGRAPHIC EVOLUTION The São Sebastião Formation of Jatobá Basin is characterized by fluvial-aeolian

468

succession and can be subdivided in three genetic units (U-1, U-2 and U-3) bounded by

469

supersurfaces, defining distinct accumulation events in the basin. The predominant

470

occurrence of aeolian dunes and aeolian sand sheet strata, associated with sheet floods

471

and channelized ephemeral rivers deposits suggest a semi-arid to arid climate for the

472

Lower Cretaceous succession of the Jatobá Basin (Figure 2 and 13).

SC

RI PT

467

U-1 represents a drying-upward succession characterized by aeolian sand sheets,

474

sheet floods and aeolian crescentic dunes at the base, that pass vertically into aeolian

475

dunes (Figure 2, 13A and B). The small scale fluvial-aeolian intercalation at the base of

476

U-1 demonstrates the contemporaneity between aeolian sand sheets, small-scale aeolian

477

crescentic dunes and sheet flood deposits (Bullard and Livingstone, 2002; Heness et al.,

478

2014; Herries, 1993). The paleocorrent pattern indicates that ephemeral flows cut

479

orthogonally the dune field, but the direction dispersion display significant deflection of

480

the flows caused by dunes crests. This pattern of paleocurrents indicates that sheet

481

floods entered an active aeolian system, without record of surface that suggests

482

deflation. The architecture of this interval represents episodes of high-water discharge at

483

the erg outer margin, generating sporadic flash floods that invade the dune field.

TE D

EP

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484

M AN U

473

The dominant occurrence of aeolian dune cross-strata at the top of the U-1,

485

indicate an increase in dry sand availability, causing the construction and climbing of

486

large aeolian dunes (Figure 13B; Kocurek, 1981). The absence of damp and dry

487

interdune deposits indicates that water table is placed below the interdune surface,

488

defining a dry aeolian system to the upper part of U-1. Accumulation in dry aeolian

489

systems requires a high sand availability and does not occur until the depositional

32

ACCEPTED MANUSCRIPT surface reaches sand saturation. This condition is marked by the aeolian dunes climbing

491

without the development of laterally extensive adjoining interdune flat areas (Wilson,

492

1971; Rubin and Hunter, 1982; Kocurek et al., 1992). The occurrence of a dry aeolian

493

system, characterized by absence of fluvial stream deposits, indicates a central-erg

494

context.

RI PT

490

The vertical facies sucession of U-1 (Figure 2), from an erg-margin to a center

496

erg setting, represents the expansion of the erg. The expansion of the aeolian dune fields

497

during the accumulation of the U-1 must be related to climatic variation and a shift at

498

the top of U-1 to more arid setting.

M AN U

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495

The horizontal geometry of the supersurface bounding U-1 and U-2 (Figure 12

500

and 13C) suggest a genesis associated to aeolian deflation (Fryberger, 1993; Mountney

501

and Jagger, 2004). However, the lack of features indicative of humidity associated to

502

this supersurface (e.g.polygonal fractures, corrugated Stoke’s surface, calcretes,

503

silcretes and paleosols) indicate that aeolian deflaction did not reach the water table

504

(Havholm et al., 1993). The absence of a deflation lag is due the aeolian origin of the

505

underlying setting that is composed of well-sorted fine to coarse sand, without gravel

506

clasts.

EP

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507

TE D

499

The U-2 deposited above the supersurface is characterized by vertical and lateral

508

interfingering of domic dunes, sand sheets, ephemeral fluvial channels and blowouts

509

(Figure 2 and 13D). The occurrence of blowout and domic dunes indicates strong winds

510

during deposition of the U-2. The blowouts are formed due to high-velocity winds in the

511

absence of vegetative cover (Hesp, 2002). Domic dunes developed when winds were

512

strong and unidirectional delaying normal upward growth of dune crest (Pye and Tsoar,

513

2009). The lateral transition from low-angle cross stratifications related to aeolian sand

33

ACCEPTED MANUSCRIPT sheets and dome dunes to tangential cross-stratifications generated by crescentic dunes

515

filling blowouts, indicates increasing accommodation space. The increase of

516

accommodation space initiates a reduction of wind velocity in a zone where air flow

517

divides (Kocurek & Havholm, 1993) and generates favorable conditions for crescentic

518

aeolian dunes development. Sharp-edged channels, formed in high energy flow records

519

periods of increase of runoff. The frequent occurrence of fluvial deposits indicates a

520

wetter climatic context compared to the top of U-1 (Figure 2).

RI PT

514

The supersurface bounding U-2 and U-3 has both horizontal and concave-up

522

aspects (Figure 12B, 13E and 13F). The U2-U3 supersurface has no features that

523

support wind deflation to the water table. The concave-shape surface represents

524

scouring into the previously deposited aeolian accumulation of the U-2 and U-1,

525

generated by large aeolian blowout features, similar to superscoops described by Blakey

526

(1988) or formed by migrating scour pits related to basal large bedforms of U-3, similar

527

to scour-fill features described by Kocurek and Day (2018).

TE D

M AN U

SC

521

The U-3 is characterized by the migration and climbing of simple and compound

529

crescent aeolian dunes (Figure 2 and 13F). The absence of interdune deposits indicates

530

the U-3 as dry aeolian systems, similar to the top of the U-1. The occurrence of dry

531

aeolain dunes field of U-3 suggests a more arid climatic condition compared to U-2. As

532

the climate becomes more arid, the water table falls increasing the amount of sediment

533

available for aeolian transport, and accumulation creating conditions favorable for the

534

development of a large aeolian dune field.

AC C

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528

535

The paleocurrent measures show a general direction dune migration to WNW

536

(Figure 2), indicating winds blowing from ESE. During deposition of São Sebastião

537

Formation the surface wind circulation pattern was zonal and the studied area was

538

placed in middle latitudes of Gondwana (200 S). Zonal pattern winds are established by

34

ACCEPTED MANUSCRIPT thermal gradient between poles and the equator, and the Earth’s rotational speed

540

breaking the major wind cell into three minor cells in each hemisphere (Compagnucci

541

2011). The aeolian dune migration of the São Sebastião Formation to WNW is

542

compatible with the model proposed by Hay and Flogel (2012) for the Lower

543

Cretaceous that indicate wind blowing to WNW over latitudes 20° to 30° in the

544

Gondwana Supercontinent.

RI PT

539

AC C

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545

35

ACCEPTED MANUSCRIPT

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546

547 548 549 550

Figure 13. Depositional models for the genetic units of São Sebastião Formation in Jatobá Basin. (A) Basal portion of U-1. (B) Upper part of U-1. (C) Formation of supersurface bounding U-1 and U-2. (D) U-2 deposition above U-1. (E) Planar supersurface bounding U-2 and U-3. (F) U-3 erosion and deposition.

36

ACCEPTED MANUSCRIPT CONCLUSIONS

552

• The presence of supersurfaces divides the formation in different genetic units, each

553

with different sedimentary evolution. The contact between each unit is sharp and

554

indicates climate changes in the basin.

RI PT

551

• Interactions between aeolian and fluvial systems occur at the base of U-1 and all over

556

U-2. The features described in the small-scale interaction in U-1 suggest

557

contemporaneously related systems. In U-2 these deposits also occur simultaneously.

558

In both cases these interactions are due to autocyclic influences related to seasonal

559

climatic oscillations.

M AN U

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555

• The U-1 defines a drying upward cycle. This cycle of increasing aridity of the system

561

is marked by the reduction of fluvial sheet floods deposits and increasing amount of

562

aeolian dunes deposits towards the top of the unit. The U-2 deposits - zibars cut by

563

channelized ephemeral rivers and blowouts - indicate less arid climate conditions

564

similar to the base of U-1. The U-3 characterizes a new period, when the climate

565

became more arid, with high sediment supply available for the aeolian transport. This

566

period is marked by the appearance of large aeolian dunes, of simple and compound

567

(draa) types.

EP

AC C

568 569 570 571 572 573

TE D

560

ACKNOWLEDGEMENTS This work was supported by RIFTE project and Human Resources Training Program

574

– PETROBRAS. The authors would like to acknowledge the Federal University of

575

Rio Grande do Sul (UFRGS) for the infrastructure. We thank all colleagues in the

576

stratigraphy and sedimentology laboratory. We are also grateful for the constructive

577

reviews by Reinhardt Fuck (Editor), Ed Simpson and anonymous referee.

37

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Kocurek, G., Nielson, J., 1986. Conditions favourable for the formation of warmclimate aeolian sand sheets. Sedimentology 33, 795-816.

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ACCEPTED MANUSCRIPT Kocurek, G., Robinson, N. I., Sharp Jr, J. M., 2001. The response of the water table in coastal aeolian systems to changes in sea level. Sedimentary Geology 139, 1-13.

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Kocurek, G., Townsley, M., Yeh, E., Havholm, K. G., Sweet, M. L., 1992. Dune and dune-field development on Pardre Island, Texas, with implications for interdune deposition and water-table-controlled accumulation. Journal of Sedimentary Petrology 62-4, 622-635.

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Langford, R. P., Chan, M. A., 1989. Fuvial-eolian interactions: Part II, ancient systems. Sedimentology 36, 1037-1051.

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ACCEPTED MANUSCRIPT McKee, E. D., Crosby, E. J., Berryhill, H. L., 1967. Flood deposits, Bijou Creek, Colorado, June 1965. Journal of Sedimentary Research 37, 829-851.

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Miall, A. D., 1996. The Geology of Fluvial Deposits: Sedimentary facies, Basin Analysis, and Petroleum Geology. Springer-Verlag, Germany.

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ACCEPTED MANUSCRIPT Rocha, D. E. G. A., Leite, J. F., 2001. Mapa Geológico da Bacia de Jatobá - Geologia. Escala 1:250.000. Recife: Serviço Geológico do Brasil – CPRM.

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Trewin, N. H., 1993. Controls on fluvial deposits in mixed fluvial and aeolian facies within the Tumbladooda Sandstone (Late Silurian) of Western Australia. Sedimentary Geology 85, 387-400.

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ACCEPTED MANUSCRIPT Ulicný, D., 2004. A drying-upward aeolian system of the Bohdasín Formation (Early Triassic), Sudetes of NE Czech Republic: record of seasonality and long-term palaeoclimate change. Sedimentary Geology 167, 17-39.

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ACCEPTED MANUSCRIPT Research Highlights:

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• The formation is compound by aeolian dunes and sand sheets, blowouts and ephemeral fluvial deposits. • Supersurfaces divides the São Sebastião Formation in three genetic units. • Interactions between aeolian and fluvial systems occur simultaneously. • The main factor that influenced the dynamics of the system was the climate.