Calcretes, Fluviolacustrine Sediments and Subsidence Patterns In

Sedimentology (2012) 59, 1659–1676 doi: 10.1111/j.1365-3091.2012.01320.x

Calcretes, ﬂuviolacustrine sediments and subsidence patterns in Permo-Triassic salt-walled minibasins of the south Urals, Russia

ANDREW J. NEWELL*, MICHAEL J. BENTON, TIMOTHY KEARSEY,GRAEME TAYLOR§, RICHARD J. TWITCHETT§ and VALENTIN P. TVERDOKHLEBOV– *British Geological Survey, Maclean Building, Wallingford OX10 8BB, UK (E-mail: [email protected]) School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK §School of Geography, Earth and Environmental Sciences, University of Plymouth, Plymouth, PL4 8AA, UK –Geology Institute of Saratov State University, Astrakhanskaya 83, 410075 Saratov, Russia

Associate Editor – Xavier Janson

ABSTRACT The south Uralian foreland basin forms part of the giant, yet sparsely documented, PreCaspian salt tectonic province. The basin can potentially add much to the understanding of fluviolacustrine sedimentation within salt- walled minibasins, where the literature has been highly reliant on only a few examples (such as the Paradox Basin of Utah). This paper describes the Late Permian terrestrial fill of the Kul’chumovo salt minibasin near Orenburg in the south Urals in which sediments were deposited in a range of channel, overbank and lacustrine environments. Palaeomagnetic stratigraphy shows that, during the Late Permian, the basin had a relatively slow and uniform subsidence pattern with widespread pedogenesis and calcrete development. Angular unconformities or halokinetic sequence boundaries cannot be recognized within the relatively fine-grained fill, and stratigraphic and spatial variations in facies are therefore critical to understanding the subsidence history of the salt minibasin. Coarse-grained channel belts show evidence for lateral relocation within the minibasin while the development of a thick stack of calcrete hardpans indicates that opposing parts of the minibasin became largely inactive for prolonged periods (possibly in the order of one million years). The regular vertical stacking of calcrete hardpans within floodplain mudstones provides further evidence that halokinetic minibasin growth is inherently episodic and cyclical. Keywords Calcretes, fluviolacustrine, Permian, PreCaspian basin, salt minibasin, salt tectonics, Urals.

INTRODUCTION Inﬁll of the topographic low or minibasin by sediment creates additional loading and drives Salt minibasins are sub-circular, polygonal or subsidence in a process termed passive down- linear pods of sedimentary strata surrounded by building (Jackson & Talbot, 1994). In continental diapiric structures. These basins are characteris- settings salt minibasins can accommodate many tic features of salt tectonic provinces where kilometres of ﬂuviolacustrine sediment and have differential loading of a salt layer by sedimentary the potential to form continuous, high-resolution overburden creates instability and the lateral records of terrestrial environments and eco- movement of salt away from areas of high load systems in addition to having commercial impor- into adjacent diapirs (Hudec & Jackson, 2007). tance as hydrocarbon reservoirs (Barde et al.,

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2002a). There are remarkably few places in the salt-walled minibasins during the latest Permian; world where salt minibasins are exposed at the (ii) their often complex vertical and lateral surface, and relative to other types of sedimentary arrangement; and, in particular; (iii) the impor- basin (for example, extensional or compressional) tance of calcretes in understanding subtle varia- the number of outcrop studies that have exam- tions in the rate and location of subsidence. ined the relationship between their formation and fill is small and mostly concentrated within the La Popa Basin in north-eastern Mexico (Aschoff & GEOLOGICAL SETTING Giles, 2005; Buck et al., 2010) and the Paradox Basin in Utah (Bromley, 1991; Lawton & Buck, The south Uralian foreland basin is a narrow 2006; Prochnow et al., 2006; Matthews et al., flexural trough formed along the western margin 2007). of the Ural fold and thrust belt following collision This paper presents new information on salt of the Siberian plate and Baltica in the Late minibasins from the south Urals foreland basin, Carboniferous (Zonenshain et al., 1984; Puchkov, a giant but sparsely documented salt tectonic 1997). In the Orenburg area, the basin is about province near Orenburg in southern Russia 100 km wide and lies between the fold and thrust (Fig. 1). Here, minibasins that subsided into a belt of the Ural Mountains and the concealed thick layer of Lower Permian (Kungurian) salt can eastern margin of the Russian platform (Fig. 2). contain up to 4 km of synorogenic fluviolacus- The basin is filled with up to 6 km of Late trine sediment derived from the Ural Mountains Carboniferous to Triassic synorogenic sediment during the Late Permian and Triassic. Closely eroded from the Ural orogen (Puchkov, 1997). To analogous minibasins have been described using the south, the narrow Urals foreland basin subsurface data from the hydrocarbon fields of expands into the vast PreCaspian basin (Fig. 1), Temir (Fig. 1), 300 km to the south in Kazakhstan which has a closely comparable tectono-strati- (Barde et al., 2002a,b). This paper uses outcrop graphic history along its eastern margin adjacent sections linked by magnetostratigraphy and, in to the Urals (Barde et al., 2002b; Volozh et al., addition to introducing a new area for salt 2003b). tectonic studies, aims to show: (i) the range of The sedimentary fill of the south Urals foreland fluviolacustrine facies that were deposited in basin readily divides into two major packages

n basi alt n s pia as C re P

Fig. 1 Map showing the location of the study area near Orenburg to the west of the southern Ural Mountains and the generalized tectonic features of the PreCaspian salt basin. The study area is located at the transition between the southern Uralian foreland basin and the PreCaspian Basin, with both basins containing thick Permian salt deposits. See Fig. 2 for a detailed map of the boxed study area.

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Fig. 2 Maps showing the location of the study area relative to Orenburg and the Ural Mountains. (A) Hill-shaded digital model (derived from Shuttle Radar Topography Mission) showing the strong north–south trending fabric of the Urals fold and thrust belt. The studied sections at Sambullak and Tuyembetka are around 20 km west of the main Urals frontal thrust in an area north of Saraktash. See Fig. 4 for detailed geological and topographic maps of the boxed area. (B) Map showing the location of salt walls and domes (based on Orenburg & Saraktash 1 : 200 000 geological maps) in the north–south trending south Ural foreland basin. Minibasins ﬁlled predominantly with Upper Permian and Triassic continental clastics are located between the salt diapirs. See Fig. 3 for a cross-section along the indicated line.

(subsalt and suprasalt) separated by Lower Platform and continental clastics adjacent to the Permian Kungurian evaporites (Fig. 3). The pre- fold belt (Puchkov, 1997). The marine basin was salt succession comprises Late Carboniferous and plugged by a bedded succession of evaporites Early Permian marine shales and turbidites (gypsum, anhydrite, sodium and potassium salts) ﬂanked by platform carbonates along the Russian and ﬁne-grained clastics in the Kungurian when

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Sambullak Tuyembetka (projected) (projected) W Permo-Triassic minibasins Main Urals Ural fold and E Triassic thrust thrust belt 0 –1000 Kungurian –2000 (m) Salt –3000 0 10 20 Distance (km) 30 40 50 Kul’chumovo minibasin Triassic Upper Permian Lower Permian Neogene T1PP P2VZ-KS P1SN-IN Carboniferous Jurassic T1BL P2KS P1TS-SR P2VZ P1US P2SM-GR P2OS-BL P2NZ See Fig. 4 for key to abbreviations Fig. 3 Broadly west–east orientated cross-section in the Saraktash area (the location is shown in Fig. 2B). Minibasins inﬁlled with Upper Permian and Triassic sediments are developed on a thick layer of Kungurian Salt which has been displaced into adjacent salt walls and domes. The approximate locations of the logged section at Sambullak and Tuyembetka are projected onto the section. See Fig. 4 for a key to the Permian and Triassic map codes. Based on the Saraktash 1 : 200 000 geological map M-40-III.

the PreCaspian basin and south Urals foreland thrust belt, might indicate some control by base- basin became tectonically and eustatically iso- ment structures (Barde et al., 2002a; Volozh et al., lated from the open sea, leading to the deposition 2003a) and lateral tectonic compression may have of several kilometres of salt under an arid climate been important as a means of initiating minibasin and low terrigeneous input (Volozh et al., 2003b). development (Ings & Beaumont, 2010). However, The salt layer covered a vast area of the Pre- salt walls may also be generated or enhanced by Caspian basin and extended northward into the differential linear loading produced by allu- the south Urals foreland basin (Volozh et al., vial wedges prograding away from a mountain 2003a). The suprasalt stratigraphy is dominated front (Prochnow et al., 2006). by Upper Permian and Triassic fluviolacustrine The Upper Permian and Triassic fill of south continental clastics that were deposited onto a Uralian minibasins typically grades upwards thick tabular layer of unstable Kungurian evapo- from marine-influenced mudstones, limestones rite. Differential loading and salt movement and evaporites in the Kungurian and Roadian to produced a complex network of minibasins sur- fully continental environments in the Wordian rounded by salt diapirs and walls (Fig. 2B). In the to Changhsingian (Fig. 4) (Tverdokhlebov et al., south Urals foreland basin, linear or sinuous salt 2005). Palaeogeographical reconstructions place walls, which can exceed 100 km in length, are the the south Urals between 20° and 30° north of predominant diapiric structure (Fig. 2B). The the Equator around the Permo-Triassic boundary diapirs are typically 5 to 10 km wide, spaced at (Zonenshain et al., 1984), suggesting that warm 10 km intervals and reach amplitudes of 4 km climates prevailed. The common occurrence of (Fig. 3). The strong preferred north–south orien- plants, animals, lacustrine facies, well-rooted tation of the salt walls, parallel to the fold and palaeosols and calcretes in predominantly fine-

Period Epoch Stage Gorizont Svita Map code Typical lithologies Continental red beds Thickness (m) Upper Carnian-Norian Suraskaiskaya T3SK Mudstone, sandstone, conglomerate 300 Ladinian Bukobay Bukobayskaya T2BK Mudstone, sandstone, conglomerate 700 Middle

c i Anisian Donguz Donguzkaya T2DN Mudstone, sandstone, conglomerate 400

a Yarenskian Petropavlovskaya T1PP Sandstone, conglomerate, mudstone 400

T Olenekian Sludkian Kzylsaiskaya T1KZ Sandstone, conglomerate, mudstone 350 Lower Rybinskian Staritskaya T1STT1BL Sandstone, conglomerate, mudstone 370 Induan Vokhmian Kopanskaya T1KP Pebble-boulder grade conglomerates, pebbly sandstone 400-800 Upper Wuc.-Cha. Vyatkian Kul'chumovskaya P2KS Sandstone, conglomerate, mudstones, limestones, calcretes 280-500 P2VZ-KS Capitanian Tatarian Severodvinian Vyazovskaya P2VZ Sandstone, conglomerate, mudstones, limestones 336-550 Middle Wordian Urzhumian Salmyshskaya/Grebenskaya P2SM-GR Sandstone, conglomerate, mudstones, limestones 200-500

a Belebeyskaya P2BL Sandstone, conglomerate, mudstones, limestones 400-700 i Roadian Kazanian P2OS-BL

r Osnovskaya P2OS Sandstone, mudstone, limestone 300

P Nezhinskaya PSNZ Sandstone, mudstone, anhydrite, gypsum. 0-500 Kungurian Ufimian Lower Kammenaya P1SN-IN Anhydrite, rare mudstone 2000 Sakmarian-Artinskian P1TS-SR Sandstone, mudstone, limestone, dolomite 100-2000 Asselian P1HL-SH P1US Sandstone, mudstone, limestone 50-350 Fig. 4 Stratigraphic subdivision of Permo-Triassic strata in the Saraktash area. Gorizonts are biostratigraphic divisions and Svitas are broadly equivalent to formations. Generalized lithology, thickness and the transition from marine- inﬂuenced to fully continental sedimentation are shown. The diagram also shows the approximate correspondence between the international Permian stages and the Russian stages (Uﬁmian, Kazanian and Tatarian). See Newell et al. (2010) for a full explanation of the correlations. Based on the Saraktash 1 : 200 000 geological map M-40-III.

Ó 2012 The Authors. Journal compilation Ó 2012 International Association of Sedimentologists, Sedimentology, 59, 1659–1676 Calcretes in salt-walled minibasins 1663 grained Late Permian (Tatarian) fluviolacustrine The present structural configuration of the strata suggest that the climate was at least minibasin is an asymmetrical anticline which is seasonally humid (Newell et al., 1999). The cored by Late Permian age rocks of the Sever- Permo-Triassic boundary (PTB) is marked by a odvinian Gorizont and Vyazovskaya Svita in the major sedimentary event in the south Urals with area around Kul’chumovo (Fig. 4). Gorizonts are the abrupt introduction of stacked, coarse- the main regional biostratigraphic units identified grained fluvial conglomerates into the basin primarily from their palaeontological characteris- (Fig. 4). Given the evidence for low levels of tics, and may unite several svitas from different tectonic activity in the south Ural orogen basins which are largely lithostratigraphic units around the PTB (Puchkov, 1997) the timing of given a locality name that is close to their type this event, coincident with the eruption of the section. Newell et al. (2010) and Benton et al. Siberian Traps and the end-Permian mass (2012) provide a detailed discussion of Russian extinction (Twitchett et al., 2001; Benton et al., stratigraphic terminology and how the local 2004), suggests a climate-driven change in flu- Russian stages (Fig. 4) equate to the International vial regime, with extreme climate fluctuations Permian time scale. and a much reduced vegetation cover increasing Westwards from Kul’chumovo, Upper Permian the erosion and transport rate of coarse-grained rocks dip and young through the uppermost sediment (Newell et al., 1999). Analogous Vyatkian Gorizont before they are sharply over- abrupt changes in fluvial regime at the PTB lain by multi-storey Triassic conglomerates and have been described from many parts of the gravelly sandstones of the Vokhmian Gorizont world (Retallack, 1999; Retallack & Krull, 1999; (Fig. 4). These largely coarse-grained Triassic Ward et al., 2000) and provide support for this deposits crop out over a large area to the north- concept. This PTB event had a major impact on west of Sambullak where they extend up to the the tectono-stratigraphic evolution and hydro- Middle Triassic Donguz Gorizont (Fig. 5A). Hill- carbon reservoir properties of salt-walled mini- shaded digital terrain models (derived from basins in the south Urals (Barde et al., 2002a), Shuttle Radar Topography Mission) show dis- but the focus of this paper is on the relatively tinctive north-east to ENE striking ridges (Fig. 5B) fine-grained Late Permian fluviolacustrine sedi- formed by packages of Triassic conglomerate, ments and, in particular, the importance of pebbly sandstone and mudstone. The topographic calcretes in understanding subtle variations in features formed by these sedimentary packages the rate and location of subsidence. (Fig. 5B) show divergence caused by lateral thickness variation and truncation related to the development of angular unconformities or halo- KUL’CHUMOVO MINIBASIN kinetic sequence boundaries (Giles & Lawton, 2002; Matthews et al., 2007). Halokinetic se- The minibasin under investigation is located near quence boundaries occur both at the base and Kul’chumovo (Fig. 5), 11Æ5 km north-west of within the Triassic basin fill and probably result Saraktash and around 20 km from the main from localized high rates of subsidence caused by frontal thrust of the Ural Mountains (Fig. 2). The the abrupt loading of the salt by fluvial conglo- minibasin is around 15 km across and has the merates and pebbly sandstones (Newell et al., form of a sinuous trough surrounded by five major 1999). salt diapirs (Fig. 2) whose position is marked On the opposing limb of the anticline, Upper by low gravity anomalies (Fig. 5C) and more Permian rocks dip and young eastwards from generally by outcrops of Jurassic and Neogene Kul’chumovo toward Tuyembetka where a rela- strata preserved within salt wall basins (Fig. 5A) tively thin veneer of Triassic conglomerates form The floor of the Kul’chumovo minibasin is a cap to this range of low hills (Fig. 5). Triassic around 2 km below the surface in the east, but rocks do not extend beyond the Vokhmian Gori- to the west lies at a depth of over 3Æ5 km below zont in this shallow, eastern part of the mini- the surface in the area to the north-west of basin. Salt is exposed in the core of the anticline Sambullak (Fig. 3), where Middle and Upper just to the north of Kul’chumovo (Fig. 5A). Hill- Triassic rocks are preserved (Fig. 5A) and the shaded digital terrain models show Permian gravity anomaly is relatively high (Fig. 5C). The sandstones deformed into curved ridges around basin has an irregular planform marked by out- these probable salt chimneys (Fig. 5B); these may cropping Permo-Triassic rock and is linked to be derived from the salt dome to the south as the adjacent minibasins. gravity anomaly map (Fig. 5C) shows no evidence

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Ó 2012 The Authors. Journal compilation Ó 2012 International Association of Sedimentologists, Sedimentology, 59, 1659–1676 Calcretes in salt-walled minibasins 1665 that a salt wall or major salt dome is located in the is a gap of uncertain thickness between the two sinuous trough-like basin between Sambullak and Tuyembetka sections which, based on relative Tuyembetka. elevations and structural dip, certainly does not The morphology of the Kul’chumovo minibasin exceed the 38 m allowed by Taylor et al. (2009) in the Late Permian will not have been the same and is probably much less. At all locations trench- as that seen today because the salt has had a long ing was used extensively to reveal fresh bedrock history of deformation from the Permian to the and maintain continuity of the logs. Rock samples present day. Cross-sections on the Saraktash from Sambullak and Tuyembetka were collected geological map (Fig. 3) show relatively uniform for petrographic and isotopic work on the calcretes thickness trends in Late Permian strata suggesting (Kearsey et al., 2012) and for palaeomagnetic that the basin may have been a simple vertical dating (Taylor et al., 2009). trough, with the subsequent folding and strongly asymmetrical, down to the west, subsidence a consequence of the rapid loading of Triassic MAGNETOSTRATIGRAPHY fluvial conglomerates. A full account of the sampling strategy, method- ology and results of palaeomagnetic dating of the LOCATION OF SECTIONS AND METHODS Sambullak and Tuyembetka sections discussed in this paper can be found elsewhere (Taylor et al., The availability of rock exposure in the grassy 2009). Both sections yielded a mixture of normal steppe terrain limited the study of the minibasin to and reverse polarities. The Tuyembetka section two main sections. The first section is located was more comprehensively sampled (104 samples) at Sambullak, a prominent escarpment on the because of the presence of well-cemented litho- western side of the minibasin, which reaches an logies. The remanence is dominated by reverse elevation of 191 m adjacent to the Sakmara River polarity directions throughout the Vyatkian sec- (Fig. 5). A 140 m logged section was made through tion with only relatively narrow normal polarity Late Permian (Vyatkian Gorizont) mudstones, zones (Fig. 6). In contrast, the conglomerates, sandstones and thin conglomerates up to the mapped as Triassic (Vokhmian), are predomi- well-cemented Triassic conglomerates which cap nantly of normal polarity with a short-lived reverse the escarpment (Fig. 6). On the eastern side of the event toward the top. Sampling at Sambullak was minibasin, a composite section of two main parts less comprehensive (43 samples) because of diffi- totalling around 150 m was made at Tuyembetka culties with coring friable mudstones, and only the which reaches an elevation of 377 m (Fig. 6). upper part of the section could be sampled. The Tuyembetka 1 provided a 110 m section through sampled Late Permian (Vyatkian) part of the sec- Vyatkian fluviolacustrine strata, which in the tion shows a well-defined reverse–normal–reverse upper parts are distinguished by many calcrete sequence of polarity zones with the Permian/ horizons creating a striking red and grey coloured Triassic boundary falling within the next higher zonation of the exposed slope (Fig. 7A). normal polarity zone. The uppermost normal Tuyembetka 2 was located 640 m to the east of polarity zone in the Vyatkian at Sambullak Section 1 (Fig. 5) and provided a 40 m log through (n1R3P in the composite magnetostratigraphy of Triassic conglomerates which cap the hill and dip Taylor et al., 2009) provides an important tie to the at 20° to the east. The underlying contact with uppermost normal polarity zone in the Vyatkian at Vyatkian strata is covered by thick scree and there Tuyembetka (Fig. 6).

Fig. 5 (A) Geological map of the study area. The minibasin presently has the form of an asymmetrical anticline with the logged section at Sambullak located on the western limb of the fold and the Tuyembetka sections located on the eastern limb. Hatched areas are where gravity anomalies are below )20 milligals and indicate the position of salt diapirs. See Fig. 4 for an explanation of map codes. Based on the Saraktash 1 : 200 000 geological map M-40-III. (B) Hill-shaded digital terrain model (derived from Shuttle Radar Topography Mission) showing the relationship of geology and logged sections to topography. Note the divergent and truncated stratal packages associated with Triassic conglomerates near Sambullak and Permian bed curvature around salt chimneys near Kul’chumovo. (C) Inset map shows gravity anomalies in the study area. Low values shaded in blue indicate the position of salt diapirs while red and yellow shading shows the sinuous trough-like basin between Tuyembetka and Sambullak. The gravity anomaly map is taken from the Saraktash 1 : 200 000 geological map M-40-III. Grid coordinates shown on the maps are UTM Zone 40N.

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Grain size Calcrete

Tuyembetka maturity .

o u

a Calcrete r

Grain size n

r 8.6 km l

S c M

F association

u a l

. a .

. Sambullak maturity .

F Mud 1 L L P Gravel B Med-coa. sand N 3 2 4

n . 0.0

y b

u o

r l

M N B

u l

i a

. . .

association

P B

L Mud F Gravel Med-coa. sand L N 1 P 2 3 4

0.0 t

Stacked, multi-storey 10.0 e

fluvial conglomerates m

Local PTB o

a 10.0 i

20.0 f

20.0 30.0

Local PTB ?

30.0 n

t Scree covered section: 40.0

a i gap is estimated

Not sampled

40.0 c

e 50.0

- Calcrete

n i Cluster 5

50.0 o

- l Palaeomag. 60.0

n tie lines Calcrete

h cluster 3

a 60.0 i

70.0 s

? e 70.0 t

80.0 l

Calcrete d

cluster 2 d ? d

80.0 t

o 90.0

90.0 b

g 100.0 a Calcrete

o cluster 1

t 100.0 o

d c 110.0

N 110.0

120.0 8

120.0 c

130.0 s

130.0 l

140.0 i

140 0 n

a Conglomerate Heterolithic sandstone and mudstone 150.0 h

Not sampled Pebbly sandstone Blocky, rooted reddish brown mudstone

Sandstone 8 Laminated mudstone

g Muddy sandstone Carbonates (calcrete) i

F 160.0 Palaeomagnetic zone: normal/reverse/uncertain Halokinetic sequence boundary

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Fig. 6 Summary logged sections at Sambullak and Tuyembetka showing the main vertical changes in lithology and grain size. Columns indicate the position and maturity of calcretes [see Table 1 for the relationship to the Machette (1985) maturity stages] and the vertical location of lacustrine facies, bioturbation and plant debris. Palaeomagnetic stratigraphy is shown in the left column and is used as a tie between the sections.

Comparison with regional and global composite lar blocks. The faces of the multifaceted, equant palaeomagnetic sections suggests that there are no blocks sometimes have clay coatings that invari- major unconformities within the Late Permian or ably are slickensided. Downward branching root across the major lithological change at the Permo- traces, typically with a diameter of 1 mm or less, Triassic boundary (Taylor et al., 2009). The are common and are highlighted by a pale grey locally postulated Permo-Triassic boundary falls calcite or clay infill (Fig. 7C). The mudstones are close to a polarity transition from reverse to generally a uniform pale reddish brown colour normal but always within the lower part of the (10R5/4) but are locally mottled greenish grey normal interval, which is consistent with present (10Y8/1) or dark reddish purple (10RP5/4). global correlations (Taylor et al., 2009). Recent global analyses by Hounslow et al. (2008) demon- Interpretation strate that, in marine strata, the Permian/Triassic Massive red mudstones probably were deposited boundary falls within the lower part of a normal from suspension in temporary bodies of standing polarity magnetozone (the LT1 zone), prior to the water (ephemeral lakes or floodplains) but they first short-lived Triassic reversal. The normal retain little of their primary structure because of polarity magnetozone containing the Vokhmian destratification by rooting and other pedogenic conglomerates is the equivalent of the LT1 processes during frequent episodes of subaerial magnetozone, and the position of the Vyatkian/ exposure. The common development of angular Vokhmian boundary, within the lower part of this interlocking blocks or peds with slickensided clay normal magnetozone, is thus consistent with the coatings may have resulted from cracking around position of the conodont-defined Permian/ roots and shrink-swell on wetting and drying of the Triassic boundary in marine sections worldwide muds (Retallack, 1997). The destratified and rooted (Taylor et al., 2009). The Vyatkian strata in the muds represent weakly developed palaeosols that studied sections are therefore upper Changhsin- could be classified as Inceptisols under the scheme gian in age, and the Vokhmian conglomerates are of Retallack (1997). The weakly developed palae- lowermost Induan. osols suggest relatively short breaks between sediment accretion events, with an aggrading soil profile that prevented the development of well- LITHOFACIES differentiated horizons (Wright, 1990). The close association of the mudstones with coarse-grained Upper Permian (Vyatkian) strata include a range fluvial channel fills suggests that most of the fine- of conglomerates, sandstones, mudstones and grained sediment is likely to have been transported carbonates that can be described in terms of five and dispersed across floodplains and into ephem- main facies. Figure 6 shows the relative abun- eral lakes by water. However, some of the silt dance and vertical stacking pattern of these facies fraction may have been contributed as wind-blown at the two main sections. dust, an important process in many modern dry- land floodplain and playa lacustrine basins (Hesse & McTainsh, 2003). Blocky, rooted reddish brown mudstone Description Carbonates Massive reddish brown mudstone is the core Vyatkian facies and can occur in uniform inter- Description vals up to 5Æ1 m thick, although the average Nodules and beds of grey and pinkish grey carbon- thickness is 0Æ86 m. The mudstones range in ate occur within both logged sections, with 39 texture from silty clays to clayey silts, and carbonate intervals identified at Sambullak and generally contain a variable proportion of fine- 102 intervals at Tuyembetka (Fig. 6). The carbon- grained sand. Primary sedimentary structure, ate shows a range of morphologies that, in most such as bedding or lamination, is rare or absent cases, can be classified into one of four types. and the mudstones are mostly massive and Type 1: In its least developed form, macro- composed of centimetre-scale interlocking angu- scopic carbonate occurs as sub-millimetre-thick

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A B

C D

Fig. 7 (A) Hillslope at Tuyembetka looking south–west showing alternating clusters of pale grey calcretes and red brown mudstone, circled person for scale (ca 1Æ8 m tall). (B) Vertically coalesced calcrete nodules at Sambullak erosively overlain by cross-bedded pebbly sandstone. (C) Intensely rooted reddish brown mudstone. (D) Calcrete hardpan showing tepee structure at base. films or shells around angular fractured blocks to 0Æ5 m in length, which can merge and weld (peds) of reddish brown mudstone or as weakly laterally into a discontinuous nodular bed (Fig. 7B). indurated powdery nodules less than 1 cm in These densely nodular horizons are typically 1 to diameter. Root traces are common in both nod- 2 m thick and tend to have diffuse lower and upper ules and host mudstone. boundaries, although in some cases are sharply Type 2: Carbonate is present within reddish truncated by fluvial conglomerates (Fig. 7B). brown mudstones as hard, indurated spherical or Type 4: In its maximum development carbonate sub-spherical nodules generally less than 2 cm in occurs as highly indurated, grey and pink mottled diameter. The nodules have sharp boundaries beds. These beds were only seen at Tuyembetka, and are dispersed in a matrix of mudstone, where they average 0Æ43 m thick but range up to generally over an interval of 1 to 2 m. The 1Æ20 m thick. Bedded carbonate is concentrated abundance of nodules may increase upwards within the central part of the Tuyembetka logged within a profile and root traces are common in section where it occurs in a set of distinct clusters both nodules and host mudstone. approximately 5 to 10 m thick separated by 10 to Type 3: Carbonate nodules have increased in 15 m of reddish brown mudstone (Fig. 6). Car- size (up to 20 cm) and abundance so that, while bonate beds within each cluster are separated by some are still dispersed within a reddish brown continuous or discontinuous mudstone beds up mudstone matrix, others are touching and have to 1 m thick and they also contain thin, highly welded contacts. Nodules commonly coalesce deformed, often vertically orientated, sinuous into vertically orientated pipe-like structures up lenses and patches of the host mudstone. The

Ó 2012 The Authors. Journal compilation Ó 2012 International Association of Sedimentologists, Sedimentology, 59, 1659–1676 Calcretes in salt-walled minibasins 1669 lower half of each limestone bed is commonly a subaerial pedogenic origin. This interpretation laminated or bedded on a scale of 1 to 5 cm. is supported by the dense micritic texture of the Bedding generally is irregular, discontinuous and carbonates and evidence of shrinkage, brecciation wavy and occasionally is deformed in solitary, and host-sediment displacement. The carbonates sharp-crested anticlines (Fig. 7D). In contrast, the typically have a d13C value between 0Æ00 and upper half of each limestone bed is commonly a )8Æ24 which is too positive to be formed by massive, well-cemented breccia composed of stagnant ground water but is a similar value to centimetre-scale angular clasts set in a matrix of modern pedogenic carbonates (Kearsey et al., micrite or microspar. Branching root structures 2012). The four main types of carbonate described (generally less than 1 mm in diameter) are com- here are also very similar to the development mon throughout the carbonate beds and are stages of calcrete seen in many modern and infilled by clear sparry calcite. The upper and ancient soils (Machette, 1985; Alonso-Zarza & lower margins of carbonate beds can be sharp or Wright, 2010). Table 1 provides a key to how the gradational and sometimes show a transition into calcrete types in the Kul’chumovo minibasin isolated or coalesced carbonate nodules. The equate to the calcrete stages of Machette (1985) precise lateral extent of carbonate beds cannot for soils with a low gravel content. Types 1 to 3 be determined because of exposure constraints are a direct equivalent to the Machette (1985) but, during sedimentary logging, many were Stages I to III while the indurated carbonate tracked around the Tuyembetka hillslope (Fig. hardpans (Type 4) at Tuyembetka may include 6A) for several hundred metres without showing features of Stage IV to VI calcretes, with much evidence of lateral pinch out. lateral variation in maturity within a single bed. The mineralogy, microfabrics and stable isotope Soil carbonates or calcretes result from the geochemistry of the carbonates are described in displacive and/or replacive introduction of car- Kearsey et al. (2012) who show that all of the bonate (calcite or dolomite) into the soil profile. Permian Vyatkian carbonates are composed of Precipitation generally occurs above the ground admixtures of cryptocrystalline calcite and dolo- water table but within the lower B horizon of the mite, much of which has a clotted fabric with soil profile and typically progresses through a patches of micrite surrounded by a coarser micro- series of stages, which starts with a few filaments cystalline matrix. The dense micrite may enclose or faint carbonate coatings on ped surfaces and floating grains of quartz and is often cross-cut by a ends with continuous bedded horizons, which secondary phase of sparry calcite that infills sharply defined curved or irregular veins. Table 1. Relationship between calcrete types in the Kul’chumovo minibasin (see text for details) and the Interpretation calcrete maturity stages of Machette (1985), as defined Carbonates in continental environments can form for sediments with a low (<20%) gravel content. in a range of settings from deep permanent lakes Type Machette Description to well-drained soils and in a continuum of stage (Machette, 1985) intermediate settings, such as ephemeral lakes, palustrine environments and in areas of shallow 1 I Few carbonate filaments or faint or discharging ground water (Alonso-Zarza, coatings on ped surfaces 2003). Shifts in the water table can easily trans- 2 II Soft carbonate nodules typically form one type of depositional system into 5 to 40 mm in diameter another, so the determination of the primary 3 III Many coalesced nodules in a origin of terrestrial carbonates is often problem- matrix that is moderately to atic. A lacustrine origin, however, seems unlikely firmly cemented for the bulk of the carbonates discussed here IV Carbonate laminae 2 to 10 mm given the general absence of primary depositional thick capping an indurated features, such as lamination, ooids or a shelly hard pan fauna. Ground water calcretes are mostly associ- 4 V Indurated sheets of carbonate with ated with shallow, permeable aquifers (Alonso- laminae greater than 10 mm thick Zarza & Wright, 2010) rather than the mudstones on the upper surface described here. Following the criteria outlined by VI Indurated carbonate sheets with Mack et al. (2000) the close association of the complex laminated, brecciated, carbonates with mudstones showing rootlets, recemented and pisolithic fabric blocky ped structure and colour mottling suggests

Ó 2012 The Authors. Journal compilation Ó 2012 International Association of Sedimentologists, Sedimentology, 59, 1659–1676 1670 A. J. Newell et al. result from progressive nodule growth and coa- Thinly bedded sandstone and mudstone lescence. Once a continuous carbonate layer or hardpan is formed the downward percolation of Description water within the soil profile is restricted and may This facies typically comprises metre-thick inter- promote the development of crude horizontal vals of fine-grained sandstone and mudstone. lamination. Deformation of the laminae by fold- Sandstone beds are 1 to 10 cm thick and are ing and brecciation may result from expansion massive, horizontally laminated or show small- and contraction related to wetting and drying, scale cross-lamination. The sandstone can con- daily or seasonal temperature fluctuations, crystal tain much finely comminuted plant debris and growth, rooting or animal activity (Watts, 1977; mudstone intraclasts. Bedding planes may show Klappa, 1980; Eren, 2007). Calcrete development extensive bioturbation with vertical, cylindrical, is favoured by a range of conditions including a mud-lined burrows up to 9 mm in diameter. The semi-arid to sub-humid climate, an abundant sands have sharp bases and, while they do intraformational or extraformational source of not show evidence for deep basal scour or carbonate (for example, wind-blown dust), micro- channellization, may show minor gutter casts bial and plant activity, which releases CO2, and mudcracks. Interbedded mudstones are gen- relatively well-drained soils and, in particular, erally 1 to 2 cm thick, reddish brown, massive geomorphic stability, with sufficiently low rates and sometimes rooted. Heterolithic sandstones of sedimentation or erosion that permit carbonate and mudstones generally occur within intervals build up within the B soil horizon. If the accre- of well-rooted reddish brown mudstone, but tion rate is high this level will become isolated occasionally lie between laminated lacustrine from the zone of carbonate precipitation, and mudstones and rooted palaeosols (Fig. 8). calcrete development will cease and move to a higher level in the aggrading soil profile. Studies Interpretation on reliably dated Quaternary calcretes show that The lack of evidence for channellization and the time required to develop a mature bedded unidirectional current ripples suggests that these calcrete horizon can range from 3 ka to over 1 Ma thin tabular sands may have been deposited (Wright, 1990).

Fig. 8 Selected intervals of the Tuyembetka logged section showing typical vertical alternation of pebbly fluvial channel fills, weakly pedified and lacustrine muds and calcretes. See Fig. 6 for stratigraphic position of logs.

Ó 2012 The Authors. Journal compilation Ó 2012 International Association of Sedimentologists, Sedimentology, 59, 1659–1676 Calcretes in salt-walled minibasins 1671 from unconfined sheetfloods on a low-gradient channel, but there is insufficient evidence to floodplain or dry lake bed. Comparable thin- determine whether bars were mid-channel, giving bedded sands have been described from the the river a braided pattern, or bank attached. distal parts of terminal splays in Lake Eyre, Semi-arid rivers (as suggested by the presence of central Australia where channellized flow from calcretes) are commonly hybrid and alternate incoming channels becomes unconfined as it between braided and single-thread meandering enters a dry lake basin (Fisher et al., 2008). planforms depending on flow stage (Baker et al., Heterolithic sandstone units overlying parallel- 1988). Intraclasts and silicified woody debris laminated mudstones may have formed minor suggest laterally mobile channels cutting into deltas prograding into standing water bodies and vegetated floodplains. Thin massive sands be- a relatively wet environment is suggested by the tween cross-sets were probably deposited from abundance of reworked plant material and bio- waning flows and indicate multiple flood events turbation. within channels.

Cross-bedded conglomerate and pebbly Laminated greyish brown or reddish brown sandstone mudstone Description Description Units of conglomerate and pebbly sandstone This facies includes greyish brown or reddish range up to 5 m thick and are typically encased brown mudstones that show a horizontal, with reddish brown rooted mudstones. Conglo- continuous or discontinuous parallel lamination. merates are common toward the base of the Vy- Greyish brown laminated mudstones often atkian section at Tuyembetka and toward the top contain plant material, including cordaitalean at Sambullak (Fig. 6). The conglomerate bodies leaves (Taylor et al., 2009), and are usually the usually have sharp erosive bases cut into mud- target facies where ostracods are found, which are stones or calcrete horizons (Fig. 7B) and gener- central to the biostratigraphy (Newell et al., ally fine upwards through pebbly sandstone and 2010). The laminated mudstones may contain sandstone into overlying mudstone. Clasts are thin cross-laminated silts or fine-grained sand- generally less than 5 cm in diameter, are stones and are characterized by an absence of sub-rounded to sub-angular and include a mix- roots or calcretes, although they may be moder- ture of Uralian-derived quartzites and cherts, ately calcareous overall. Intervals of laminated together with intraformational calcrete, mud- mudstone generally do not exceed 2 m in thick- stone and sandstone clasts. Toward the base of ness and interbed with channel conglomerates the section at Tuyembetka conglomerates con- and rooted floodplain mudstones. tain large silicified stems and branches of arbo- rescent gymnosperms (Taylor et al., 2009) Interpretation (Fig. 6). Conglomerate bodies are typically made Laminated muds were probably deposited from from several sets of trough or tabular cross- suspension in shallow perennial lakes that bedded gravel that are generally less than 1 m formed habitats for invertebrates and reducing thick and may be separated by thin beds of environments for the preservation of plant massive sandstone (Fig. 6). Where cross-bed debris. Some of the lake deposits alternate with azimuths can be measured, these are generally well-drained floodplain or mudflat facies and directed toward the south-west or west. Expo- these may have developed periodically in parts sure constraints limit the available information of the basin, depending on the balance between on the lateral extent and internal architecture of water input from surrounding river systems and conglomerate bodies, but the pebbly surface evaporative loss, analogous to the alternations residue and sharp ridge-like features formed by of dry mudflat and shallow lake environments some of the thicker bodies can be traced laterally that are seen in the modern Lake Eyre basin for several hundred metres. of central Australia (Magee et al., 2004). Others are closely associated with channel deposits Interpretation and may have formed in abandoned channel Conglomerate bodies probably represent the fill of reaches. Laminated mudstones that underlie wide, shallow gravelly river channels. Sets of channel deposits may have resulted from flood- trough or tabular cross-beds represent the migra- ing of a topographic low prior to the avulsion of tion of subaqueous dunes or bars within the a new channel.

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FACIES ASSOCIATIONS opment of mature bedded calcretes implies long breaks in sedimentation, which could be in the The five facies recognised in the Permian sections order of tens to hundreds of thousands of years at Sambullak and Tuyembetka can be grouped (Machette, 1985), but is difficult to predict due to into the following three facies associations. (i) the many variables that influence calcrete The channel–floodplain–lake association in- development (Wright, 1990; Alonso-Zarza & cludes fluvial channel conglomerates, weakly Wright, 2010). (iii) The floodplain association is pedified overbank muds, horizons of nodular dominated by weakly pedified reddish brown calcrete and laminated lacustrine deposits. This sandstones with only occasional thin sheetflood relatively wet association contains most of the sandstones and sparse nodular calcrete breaking bioturbation and plant fossils found within the the monotony. Plants and burrows are relatively logged sections (Fig. 6) and represents deposition sparse and it probably represents a floodplain, within an active channel belt with relatively floodbasin or mudflat environment that was frequent overbank flood events, and ponding of relatively distal to the active channel belt and flood waters in topographic lows on the flood- received periodic fine-grained sediment from plain and inactive channel reaches. (ii) The overland flows. floodplain-bedded calcrete association includes rooted, weakly pedified reddish brown mud- Vertical changes in facies association stones alternating with regularly spaced clusters of laminar, highly brecciated calcrete hardpan. The sections at Sambullak and Tuyembetka show Plants and burrows are absent from this associa- markedly different vertical stacking patterns in tion (Fig. 6) and it represents a relatively dry facies association (Fig. 9). At Tuyembetka, the depositional system that was distal to, or elevated channel–floodplain–lake association is found at above, the active channel belt. The regular devel- the base and passes vertically into the floodplain-

8 km (facies widths schematic) Sambullak Tuyembetka

Abrupt loading by Triassic fluvial conglo mer ate Halokinetic seque s nce boundary

Not exposed

Stacked L L

calcrete A hardpans W Channel belt Palmag

Palm T Shift in channel belt locatio L A

Rooted n Channel belt with overbank overbank lacustrine muds Increase in channel frequency and size muds and plant debris

10 m

Mud

sand

gravel

Mud Gravelly/sandy channel fills Correlative sand palaeomagnetic gravel Calcrete zone Mudstone, massive, red, rooted Palmag Mudstone, laminated lacustrine Temporary sediment cover Fig. 9 Semi-schematic stratigraphic architecture of the minibasin showing the upward-coarsening stratigraphic trend at Sambullak and the contrasting switch from channel belt facies to stacked calcrete hardpans at Tuyembetka. The vertical distribution of facies is based on logs but the lateral extent of channels and calcretes is schematic and for visualization purposes only. Palaeomagnetic dating provides correlation between the sections.

Ó 2012 The Authors. Journal compilation Ó 2012 International Association of Sedimentologists, Sedimentology, 59, 1659–1676 Calcretes in salt-walled minibasins 1673 bedded calcrete association. At Sambullak, the Significance for salt-walled minibasin Vyatkian section starts with a thick interval of the subsidence and stratigraphy floodplain association and progressively coars- ens-upwards into the channel–floodplain–lake In the latest Permian (Vyatkian) the Kul’chumovo association. minibasin was nearly full having already accumulated around 2 km of sediment since the start of subsidence in the Ufimian (Fig. 4). While Lateral changes in facies association compressional tectonics may have played a part Anticlinal folding and erosion mean that it is not in the initiation of basin subsidence (Ings & possible to correlate beds physically between the Beaumont, 2010), it seems probable that in the two logged sections (Fig. 5) but palaeomagnetic Vyatkian basin subsidence and salt diapir growth work has provided five tie-lines between the were driven by passive downbuilding (Hudec & sections (Taylor et al., 2009). Short normal palae- Jackson, 2007). In this process, a salt diapir rises omagnetic intervals are particularly useful within continually with respect to surrounding strata the predominantly reversed Vyatkian interval and remains exposed, possibly with a thin on- (Fig. 6). The uppermost and most reliable of these lapping sediment cover, at the level of the normal zones (n1R3P in the composite magneto- depositional surface while sediments accumulate stratigraphy of Taylor et al., 2009) shows that around it in steep-sided, trough-shaped bedded calcrete hardpans at Tuyembetka are coe- minibasins (Fig. 10). The added weight of sedi- val with active channel belts at Sambullak 8Æ6km ment in the minibasins displaces the underlying to the south-west (Fig. 9). Significantly, this nor- salt and feeds the growth of the surrounding salt mal palaeomagnetic zone reveals only very minor structures (Jackson & Talbot, 1994; Jackson et al., thickening of the active channel belt deposits at 1994). The temporary sedimentary carapace that Sambullak relative to the cluster of calcrete hard- may conceal the diapir roof is progressively pans at Tuyembetka (Fig. 6). To some extent, this uplifted and reworked as the structures grow. will reflect the reduced compaction of muds Since the crests of salt diapirs remain at or close cemented and replaced by early-diagenetic car- to the level of the depositional surface, they do bonate, but nonetheless implies very minor differ- not necessarily form significant topographic bar- ences in basin subsidence between the two riers as shown by some reconstructions (Barde locations. There is no magnetostratigraphic control et al., 2002a). Positive topography can develop if on lateral correlations at the base of sections but, salt wall growth exceeds the subsidence rate of given the evidence for relatively minor lateral adjacent basins (Hudec & Jackson, 2007) but thickness change, it seems probable that the rivers of sufficient power may be capable of channel–floodplain–lake association at the base eroding and dissolving the growing topographic of Tuyembetka correlates with the fine-grained highs rather than being forced to deflect around distal floodplain deposits at Sambullak (Fig. 9). them (Matthews et al., 2007).

Gravelly channel fill Temporary sediment cover on salt diapir

Sandy delta/sheet floods Calcrete hardpan Shallow lake

Fig. 10 Block diagram showing a

Static m possible reconstruction of Late k Permian depositional systems in the 1 south Urals with channel belts and lacustrine facies locating in rapidly subsiding parts of the minibasin and Salt calcrete hardpans forming in static Subsiding areas. Areas of rapid subsidence 10 km migrate over time. The dimensions of the block are approximate.

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This study has shown that a relatively complex the development of calcrete hardpans at stratigraphy can develop within fine-grained Tuyembetka is related to low subsidence rates fluviolacustrine salt-walled minibasins of the and extended periods of pedogenesis (Fig. 10). south Urals. First, active channel belts in the Analogous catenary relationships between cal- Kul’chomovo minibasin did not remain fixed cretes and palustrine or lacustrine carbonates within one part of the basin but switched from a have been widely reported (Alonso-Zarza, 2003). position on the eastern margin of the basin Salt tectonics has also been recognized as an (Tuyembetka) to one on the west (Sambullak) important control on palaeosol development in over the course of the Vyatkian (Fig. 9); this the Chinle Formation of the Paradox Basin contrasts with other types of basin, such as where mature soils develop on topographic extensional half grabens, where fluvial channels highs flanking salt walls and above inflating tend to become localized within a persistent, segments of salt pillows (Prochnow et al., 2006; narrow zone of high subsidence (Leeder & Matthews et al., 2007). Similarly, in marine Gawthorpe, 1987). In salt-walled minibasins com- environments salt domes may be marked by plex patterns of shifting subsidence are possible condensed mudstones or limestones (Giles & as salt is expelled from under different parts of Lawton, 2002). the aggrading sediment pile. The apparent cyclic development of clusters of Salt minibasins are commonly assumed to be bedded calcrete at Tuyembetka is of particular areas of rapid subsidence, but magnetostrati- interest (Fig. 6). While these cycles could be graphy has shown that the Late Permian palaeo- modulated to some degree by long-term climate magnetic zones at Tuyembetka and Sambullak fluctuations, it is also possible that they record an are actually quite condensed relative to other inherent episodic and cyclical pattern to haloki- parts of the south Urals, suggesting low rates of netic minibasin growth, similar to that postulated subsidence (Taylor et al., 2009). This observation for palaeosol sequences in the Chinle Formation may indicate relatively slow and progressive of the Paradox Basin (Prochnow et al., 2006). loading of the salt by predominantly fine-grained Magnetostratigraphy indicates that the 150 m Upper Permian sediments. Major angular uncon- thick Vyatkian section at Tuyembetka probably formities or halokinetic sequence boundaries equates to the Wuchiapingian and Changhsingian (Giles & Lawton, 2002) and large lateral thick- on the global time scale (Taylor et al., 2009), a ness changes in the basin fill are not seen until time period of around nine million years. In very the Triassic when there was rapid loading by approximate terms, it is therefore possible that coarse-grained fluvial conglomerates (Fig. 5C); the 10 to 15 m of compacted mudstone that this compares with the development of haloki- occurs between calcrete clusters might represent netic sequence boundaries at the base of fluvial relatively continuous subsidence, over an interval sand-dominated units in the Paradox Salt Basin that is in the order of one million years. Calcrete (Matthews et al., 2007). By contrast, palaeomag- clusters are typically made from four to five netic evidence shows that lateral thickness calcrete beds which, together with their thin changes in Late Permian sediments of the intervening mudstones, have a total compacted Kul’chumovo minibasin were small and cer- thickness of 5 to 10 m. This thickness, together tainly well below seismic resolution. However, with the understanding of the time required to this study shows that even very minor lateral develop laminar and brecciated calcretes in the changes in accommodation within a salt-walled Quaternary (Machette, 1985), suggests a lengthy minibasin can induce major lateral changes in period of very slow subsidence which may also be facies. For example, a stacked sequence of in the order of one million years. Calcretes, which calcrete hardpans at Tuyembetka is coeval with require geomorphic stability to develop to ad- active channel belt deposits at Sambullak vanced stages, thus provide further evidence for (Fig. 9). Viewed in isolation, the upward change an episodic pattern to salt-withdrawal basin from conglomeratic channel fills to a thick subsidence. succession of stacked calcretes at Tuyembetka could be interpreted as a long-term switch in climate toward greater aridity. However, this CONCLUSIONS interpretation is not supported by the Sambullak section where the upward change is towards The south Uralian foreland basin is a giant yet conglomeratic channel fills and thick-bedded sparsely documented (outside of Russian litera- calcretes are absent; this tends to suggest that ture) salt tectonic province that has the potential

Ó 2012 The Authors. Journal compilation Ó 2012 International Association of Sedimentologists, Sedimentology, 59, 1659–1676 Calcretes in salt-walled minibasins 1675 to add much to the understanding of fluviolacus- REFERENCES trine sedimentation within salt-walled minibasins. Thus far the literature has been highly Alonso-Zarza, A.M. (2003) Palaeoenvironmental significance reliant on the Paradox Basin of Utah for outcrop of palustrine carbonates and calcretes in the geological record. Earth-Sci. Rev., 60, 261–298. analogues of non-marine salt-walled minibasins. Alonso-Zarza, A.M. and Wright, V.P.. (2010) Chapter 5: This study has concentrated on the relatively Calcretes. In: Developments in Sedimentology (Eds A.M. fine-grained fluviolacustrine fill of the Kul’chu- Alonso-Zarza and L.H. Tanner), 61, pp. 225–267. Elsevier, movo minibasin which, during the latest Permian Amsterdam. (Vyatkian), appears to have had a relatively slow Aschoff, J.L. and Giles, K.A. (2005) Salt diapir-influenced, shallow-marine sediment dispersal patterns: insights from and uniform subsidence pattern based on the outcrop analogs. AAPG Bull., 89, 447–469. relative thickness of palaeomagnetic zones and Baker, V.R., Kockel, C.R. and Patton, P.C. (1988) Flood Geo- the widespread evidence of pedogenesis and morphology. Wiley Interscience, New York. calcrete development. Sediments were deposited Barde, J.-P., Chamberlain, P., Galavazi, M., Gralla, P., in a range of channel, overbank and lacustrine Harwijanto, J., Marsky, J. and van den Belt, F. (2002a) Sedimentation during halokinesis: Permo-Triassic reser- environments which, despite the evidence for voirs of the Saigak Field, Precaspian Basin, Kazakhstan. relatively slow and uniform subsidence, show Petrol. Geosci., 8, 177–187. major vertical and lateral facies changes. Coarse- Barde, J.-P., Gralla, P., Harwijanto, J. and Marsky, J. (2002b) grained channel belts show evidence for lateral Exploration at the eastern edge of the Precaspian Basin: relocation within the minibasin while the devel- impact of data integration on Upper Permian and Triassic prospectivity. AAPG Bull., 86, 399–415. opment of a thick stack of calcrete hardpans Benton, M.J., Tverdokhlebov, V.P. and Surkov, M.V. (2004) indicates that opposing parts of the minibasin Ecosystem remodelling among vertebrates at the Permian- became largely inactive for prolonged periods Triassic boundary in Russia. Nature, 432, 97–100. (possibly in the order of one million years). An Benton, M.J., Newell, A.J., Khlyupin, A.Yu., Shumov, I.S., important conclusion for hydrocarbon explora- Price, G.D. and Kurkin, A.A. (2012) Preservation of exceptional vertebrate assemblages in Middle Permian tion in comparable minibasins (e.g. Barde et al., fluviolacustrine mudstones of Kotel’nich, Russia: strati- 2002a) is that sand fairways therefore will not graphy, sedimentology, and taphonomy. Palaeogeogr. always be basin-centred but can shift laterally as Palaeoclimatol. Palaeoecol. doi: 10.1016/j.palaeo.2012.01. salt is expelled from under different parts of the 005. aggrading sediment pile. Bromley, M.H. (1991) Architectural features of the Kayenta formation (Lower Jurassic), Colorado Plateau, USA: Under conditions of relatively slow and uniform relationship to salt tectonics in the Paradox Basin. Sed. subsidence, fine-grained fluviolacustrine mini- Geol., 73, 77–99. basin fills may not show well-developed or clear Buck, B.J., Lawton, T.F. and Brock, A.L. (2010) Evaporitic angular unconformities or halokinetic sequence paleosols in continental strata of the Carroza Formation, La boundaries (Giles & Lawton, 2002), and strati- Popa Basin, Mexico: record of Paleogene climate and salt tectonics. Geol. Soc. Am. Bull., 122, 1011–1026. graphic features such as calcrete distribution Eren, M. (2007) Genesis of tepees in the quaternary hardpan become central to understanding the subsidence calcretes, Mersin, S Turkey. Carbonates Evaporites, 22, history of the basin. The regular vertical stacking of 123–134. calcrete hardpans within floodplain mudstones Fisher, J.A., Krapf, C.B.E., Lang, S.C., Nichols, G.J. and provides further evidence that halokinetic mini- Payenberg, T.H.D. (2008) Sedimentology and architecture of basin growth is inherently episodic and cyclical the Douglas Creek terminal splay, Lake Eyre, central Aus- tralia. Sedimentology, 55, 1915–1930. (Prochnow et al., 2006). Giles, K.A. and Lawton, T.F. (2002) Halokinetic Sequence Stratigraphy Adjacent to the El Papalote Diapir, Northeast- ern Mexico. AAPG Bull., 86, 823–840. ACKNOWLEDGEMENTS Hesse, P.P. and McTainsh, G.H. (2003) Australian dust deposits: modern processes and the quaternary record. Fieldwork in Russia was supported by Natural Quat. Sci. Rev., 22, 2007–2035. Environment Research Council (NERC) grant NE/ Hounslow, M.W., Peters, C., Mørk, A., Weitschat, W. and Vigran, J.O. (2008) Biomagnetostratigraphy of the C518973/1 to MJB and RJT. We thank the many Vikinghøgda Formation, Svalbard (Arctic Norway), and the geologists and support staff from Saratov and geomagnetic polarity timescale for the Lower Triassic. Geol. Moscow who made fieldwork in Orenburg possi- Soc. Am. Bull., 120, 1305–1325. ble. Kate Giles and Greg Retallack are thanked for Hudec, M.R. and Jackson, M.P.A. (2007) Terra infirma: their constructive reviews of the manuscript. AJN understanding salt tectonics. Earth-Sci. Rev., 82, 1–28. Ings, S.J. and Beaumont, C. (2010) Shortening viscous pres- and TK publish with the permission of the sure ridges, a solution to the enigma of initiating salt Executive Director of the British Geological Sur- ‘withdrawal’ minibasins. Geology, 38, 339–342. vey (NERC).

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Jackson, M.P.A. and Talbot, C.J. (1994) Advances in salt in the Upper Triassic Chinle Formation: Castle Valley, Utah. tectonics. In: Continental Deformation (Ed P.L. Hancock), Sedimentology, 53, 1319–1345. pp. 159–179. Pergamon Press, Oxford, United Kingdom. Puchkov, V.N. (1997) Structure and geodynamics of the Ura- Jackson, M.P.A., Vendeville, B.C. and Schultz-Ela, D.D. (1994) lian orogen. Geol. Soc. Lond. Spec. Publ., 121, 201–236. Salt-related structures in the Gulf of Mexico; a field guide Retallack, G.J.. (1997) A Colour Guide to Paleosols. Wiley, for geophysicists. Lead. Edge, 13, 837–842. Chichester, 175 pp. Kearsey, T., Twitchett, R.J. and Newell, A.J.. (2012) The origin Retallack, G.J. (1999) Postapocalyptic greenhouse paleocli- and significance of pedogenic dolomite from the Upper mate revealed by earliest Triassic paleosols in the Sydney Permian of the South Urals of Russia. Geol. Mag., 149, 291– Basin, Australia. Geol. Soc. Am. Bull., 111, 52–70. 307. Retallack, G.J. and Krull, E.S. (1999) Landscape ecological Klappa, C.F. (1980) Brecciation textures and tepee shift at the Permian–Triassic boundary in Antarctica. Aust. structures in Quaternary calcrete (caliche) profiles from J. Earth Sci., 46, 785–812. eastern Spain: the plant factor in their formation. Geol. J., Taylor, G.K., Tucker, C., Twitchett, R.J., Kearsey, T., Benton, 15, 81–89. M.J., Newell, A.J., Surkov, M.V. and Tverdokhlebov, V.P. Lawton, T.F. and Buck, B.J. (2006) Implications of diapir- (2009) Magnetostratigraphy of Permian/Triassic boundary derived detritus and gypsic paleosols in Lower Triassic sequences in the Cis-Urals, Russia: no evidence for a major strata near the Castle Valley salt wall, Paradox Basin, Utah. temporal hiatus. Earth Planet. Sci. Lett., 281, 36–47. Geology, 34, 885–888. Tverdokhlebov, V.P., Tverdokhlebova, G.I., Minikh, A.V., Leeder, M.R. and Gawthorpe, R.L. (1987) Sedimentary models Surkov, M.V. and Benton, M.J. (2005) Upper Permian ver- for extensional tilt-block/half-graben basins. Geol. Soc. tebrates and their sedimentological context in the South Lond. Spec. Publ., 28, 139–152. Urals, Russia. Earth-Sci. Rev., 69, 27–77. Machette, M.N. (1985) Calcic soils of the southwestern United Twitchett, R.J., Looy, C.V., Morante, R., Visscher, H. and States. Geol. Soc. Am. Spec. Pap., 203, 1–21. Wignall, P.B. (2001) Rapid and synchronous collapse of Mack, G.H., Cole, D.R. and Trevin˜ o, L. (2000) The distribution marine and terrestrial ecosystems during the end-Permian and discrimination of shallow, authigenic carbonate in the biotic crisis. Geology, 29, 351–354. Pliocene–Pleistocene Palomas Basin, southern Rio Grande Volozh, Y., Talbot, C. and Ismail-Zadeh, A. (2003a) Salt rift. Geol. Soc. Am. Bull., 112, 643–656. structures and hydrocarbons in the Pricaspian basin. AAPG Magee, J.W., Miller, G.H., Spooner, N.A. and Questiaux, D. Bull., 87, 313–334. (2004) Continuous 150 k.y. monsoon record from Lake Eyre, Volozh, Y.A., Antipov, M.P., Brunet, M.F., Garagash, I.A., Australia: insolation-forcing implications and unexpected Lobkovskii, L.I. and Cadet, J.P. (2003b) Pre-Mesozoic geo- Holocene failure. Geology, 32, 885–888. dynamics of the Precaspian Basin (Kazakhstan). Sed. Geol., Matthews, W.J., Hampson, G.J., Trudgill, B.D. and Underhill, 156, 35–58. J.R. (2007) Controls on fluviolacustrine reservoir distribu- Ward, P.D., Montgomery, D.R. and Smith, R. (2000) Altered tion and architecture in passive salt-diapir provinces: river morphology in South Africa Related to the Permian– insights from outcrop analogs. AAPG Bull., 91, 1367– Triassic Extinction. Science, 289, 1740–1743. 1403. Watts, N.L. (1977) Pseudo-anticlines and other structures in Newell, A.J., Tverdokhlebov, V.P. and Benton, M.J. (1999) some calcretes of Botswana and South Africa. Earth Surf. Interplay of tectonics and climate on a transverse fluvial Process., 2, 63–74. system, Upper Permian, Southern Uralian Foreland Basin, Wright, V.P. (1990) Estimating rates of calcrete formation and Russia. Sed. Geol., 127, 11–29. sediment accretion in ancient alluvial deposits. Geol. Mag., Newell, A.J., Sennikov, A.G., Benton, M.J., Molostovskaya, I.I., 127, 273–276. Golubev, V.K., Minikh, A.V. and Minikh, M.G. (2010) Zonenshain, L.P., Korinevsky, V.G., Kazmin, V.G., Pechersky, Disruption of playa-lacustrine depositional systems at the D.M., Khain, V.V. and Matveenkov, V.V. (1984) Plate tec- Permo-Triassic boundary: evidence from Vyazniki and tonic model of the South Urals development. Tectono- Gorokhovets on the Russian Platform. J. Geol. Soc. Lond., 167, physics, 109, 95–135. 695–716. Prochnow, S.J., Atchley, S.C., Boucher, T.E., Nordt, L.C. and Manuscript received 11 January 2011; revision Hudec, M.R. (2006) The influence of salt withdrawal sub- accepted 17 January 2012 sidence on palaeosol maturity and cyclic fluvial deposition

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