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Recurrent Emplacement of Non-Glacial Diamictite During the Late Paleozoic Ice Age

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Recurrent Emplacement of Non-Glacial Diamictite During the Late Paleozoic Ice Age Recurrent emplacement of non-glacial diamictite during the late Paleozoic ice age F.F. Vesely1, M.C.N.L. Rodrigues1, E.L.M. da Rosa1, J.A. Amato2, B. Trzaskos1, J.L. Isbell2, and N.D. Fedorchuk2 1Departamento de Geologia, Universidade Federal do Paraná, Curitiba, PR, Caixa Postal 19001, CEP 81531-980, Brazil 2Department of Geosciences, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53211, USA ABSTRACT Most ice-age diamictites were emplaced in basinal settings by non-subglacial processes. Nonetheless, the occurrence of diamictite in rock successions is widely employed to estimate ice extent and delimit glacial growth-decay cycles that serve as background for paleoclimate model- ing. We report a cyclic diamictite-mudstone succession from the Bashkirian-Moscovian Campo do Tenente Formation in southern Brazil to investigate the timing of diamictite emplacement with respect to glaciation. Glacial cycles can be recognized, in which mudstones with ice-rafted debris record deposition in a marine-infuenced water body during intervals of maximum ice advance. Diamictites, on the other hand, lack striated and faceted clasts, have deformed blocks of sandstones, and are enveloped by IRD-free mudstones. They are interpreted as non-glacial mass-transport deposits derived from delta collapse during intervals of maximum ice retreat, in which dispersed gravel derives from block assimilation and not from ice rafting. INTRODUCTION diamictite are used to delimit and correlate thick Discriminating subglacial diamictites (til- (tens to hundreds of meters) sequences record- lites) from those formed in glacioaquatic set- ing glacier advance-retreat cycles (e.g., França tings or via non-glacial landslides is an old, but and Potter, 1991; Visser, 1997; Fielding et al., still controversial, issue on the reconstruction 2008). We investigate the timing of diamictite of deep-time glaciations (e.g., Deynoux, 1985; deposition within a higher-resolution (several Eyles and Januszczak, 2007). Glacially related meters thick) diamictite-mudstone stratigraphy and non-glacial diamictites may intercalate in of a Bashkirian-Moscovian succession of the the same succession (e.g., Le Heron et al., 2017), Campo do Tenente Formation (southern Bra- which may lead to overestimations of glacial zil). The Campo do Tenente Formation crops out extent based on diamictite distribution (e.g., in the eastern Paraná Basin (Fig. 1A), a region González-Bonorino and Eyles, 1995). Diverse that was glaciated by the presumed westward interpretations of diamictite can result in dis- advance of ice lobes from an ice sheet in south- crepant ice-volume calculations ranging from west Africa (Windhoek Ice Sheet). Figure 1. Geological setting. A: Location of the 7 supercontinent-scale ice sheets up to 3 × 10 study area in eastern Paraná Basin (southern km2 large (e.g., Cao et al., 2018), to isolated ice DATA DESCRIPTION AND Brazil) according to its former position during centers on highlands that together are one order INTERPRETATION the late Paleozoic, with blue arrows indicat- of magnitude smaller (e.g., Isbell et al., 2012). The 140-m-thick Campo do Tenente Forma- ing paleo-ice flow directions. Black crosses indicate localities from which subglacial It has been long recognized that most ice- tion (lower Itararé Group; Fig. 1B) is consid- erosional features have been reinterpreted age diamictites accumulated in basinal settings ered as the upper half of a long-term (~5 m.y.) as products of iceberg scouring (1: Rosa et via non-subglacial processes (Eyles, 1993). glacigenic sequence (França and Potter, 1991). al., 2016; 2: Vesely and Assine, 2014) or sub- Evidence includes their association with tur- It covers ~700 km2 of the eastern Paraná Basin, aerial erosion (3: Fedorchuk et al., 2018). B: bidites, ice-rafted debris (IRD), and slump- onlaps the basement to the east-southeast and Schematic subdivision of the Itararé Group in the southeast Paraná Basin, with black arrow generated structures. Nonetheless, diamictite drapes a glacially scoured and striated substrate pointing to the examined interval. Formation is often taken as a paleoclimate proxy, and its (Barbosa, 1940). Previously reported Botryoc- names: CT—Campo do Tenente; MF—Mafra; recurrence in rock successions is widely used cocus sp.,Tasmanites spp., and Leiosphaeridia RS—Rio do Sul. to delimit cycles of ice-sheet growth and decay spp. (Kipper et al., 2017) indicate a brackish (e.g., Fielding et al., 2008). Concerning mass- marginal/restricted marine environment for this and F2-A– F2-D; see the GSA Data Repository1). transport diamictites, these assumptions are formation. The unit comprises multiple alternat- IRD (dropstones and pellets) amount was esti- problematic because previously inferred gla- ing packages (Fig. 1B) of (1) thick diamictite, mated visually each 10 cm, and thin sections of cial infuence on mass fows vary from strong and (2) thinly bedded shale/ rhythmite/ diamic- macroscopically IRD-free intervals were exam- (e.g., Visser, 1997) to absent (e.g., Eyles and tite, which we defne as two facies associations ined to look for fner IRD fractions. Januszczak, 2007), which unfortunately renders (F1 and F2, respectively; Fig. 2). A sedimento- these diamictites non-unique within glacigenic logical log assembled in the Campo do Tenente 1 GSA Data Repository item 2018211, lithofacies stratigraphic schemes. municipal quarry (Fig. 3A) allowed a detailed of the Campo do Tenente Formation, is available In the mid- to high-latitude stratigraphic description of the two associations and the iden- online at http://www.geosociety.org/datarepository record of the late Paleozoic ice age, recurrent tifcation of six individual facies (F1-A, F1-B, /2018/ or on request from [email protected]. GEOLOGY, July 2018; v. 46; no. 7; p. 615–618 | GSA Data Repository item 2018211 | https://doi.org/10.1130/G45011.1 | Published online 7 June 2018 ©GEOLOGY 2018 Geological | Volume Society 46 | ofNumber America. 7 For| www.gsapubs.org permission to copy, contact [email protected]. 615 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/46/7/615/4225413/615.pdf by University of Wisconsin Milwaukee user on 18 January 2019 Figure 2. Facies characteristics. Diamictite (facies F1) can be homogeneous (A) or with bands derived from sandstone block assimilation (B). Blocks exhibit original sedimentary structures (C) and gravel-rich layers (D). E: Photomicrograph of thinly laminated mudstone (F2-D) lacking ice-rafted debris (IRD) (note small-scale soft-sediment deformation). F–H: F2-B rhythmites full of IRD in the form of poorly sorted sediment pellets (F) and dropstones (G); the latter often striated (H). I: Stratified diamictite (F2-C) with shale interbeds and imbricated rip-up clasts (Rip). Thick Diamictite Facies Association (F1) settings due to current-generated sedimentary decreases upward giving rise to a silt-mud rhyth- structures and heterolithic bedding character- mite (F2-B). Within F2-B, the amount of IRD Description istic of fuvial or tide-infuenced environments. increases from near zero at the base to an inter- F1 diamictites are sheet-like, 8-m-thick, The absence of these facies in underlying strata val full of IRD in which rhythmicity is hardly sandy-muddy and clast-poor facies (Fig. 2A) rules out block incorporation by basal plucking recognizable. IRD increase-decrease cycles containing less than 5% gravel, with a maximum and, instead, indicates entrainment via collapse repeat several times upsection (Fig. 3A). The size of 7 cm. They lack faceted/polished/stri- from areas upslope (e.g., Nemec et al., 1988). uppermost interval is a 2-m-thick shale lacking ated clasts. Heterogeneous (F1-B) diamictites The fact that all grain sizes in F1-A (mud to IRD (Fig. 2E). When present, IRD is in both show up as meter-scale blocks of fne to gravelly pebbles) were also observed in blocks suggests dark and pale layers, and comprise extrabasinal sandstones (Figs. 2B–2D). Blocks are plastically that the matrix is a product of disintegration and lonestones and sediment pellets (Figs. 2F and deformed and disrupted, and primary sedimen- homogenization of a protolith (e.g., Eyles and 2G). Lonestones are up to 40 cm in diameter, tary structures are often preserved (Fig. 2C). Eyles, 2000) and that gravel in the diamictite is commonly striated (Fig. 2H), often bend and derived from block assimilation. The absence of penetrate subjacent layers, and are onlapped Interpretation faceted/striated/oversized clasts precludes inter- by overlying beds, suggesting deposition by A subaquatic origin for F1 is suggested by preting F1 as re-sedimented IRD or glacigenic foating ice (dropstones; Thomas and Connell, its intercalation with shale and rhythmite facies. debrites, and points to a non-glacial origin. 1984). Pellets are more abundant and comprise The presence of allochthonous blocks and the aggregates (from a few millimeters to 5 cm) strongly aligned magnetic fabrics reported by Thinly-Bedded Rhythmite-Shale-Diamictite of mud, sand, and gravel (Fig. 2F). Some are Gravenor and Von Brunn (1987) and Amato Facies Association (F2) stretched parallel to bedding and partially dis- (2017) favors mobilization by slumps and debris rupted, giving rise to nodular layers of poorly fows, denoting a mass-fow origin for these Description sorted sediment. diamictites. Allochthonous blocks are inter- The base of F2 includes a 20-cm-thick, sand- The F2 succession is punctuated by discrete preted as from proximal, shallower depositional mud, IRD-free rhythmite
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