Sedimentary environments through the Late Permian and Early TriassicWe first logged the succession on Sambullak Hill in 1995, and returned in 2004 and 2006. The great crag of red-coloured earliest Triassic conglomerate stands out as a mappable feature along the crests of the hills all around. From the top of the hill, one can see a huge distance, across to the town of Saraktash, perhaps 20 km away in one direction, along the meandering wooded valley of the Sakmara, a broad tributary of the River Ural.
The crest of Sambullak Hill is composed of 10 m or more of well cemented, hard conglomerate, dipping at 10o or so to the east. Walking 1 km along the crest of the ridge takes you from the highest point in the vicinity gently down to the riverside. The same conglomerate can be picked out in neighbouring ranges of hills and it clearly extends some distance, forming part of the base of a vast alluvial fan measuring at least 20 km across, even at this distance of some 50 km west of the Ural Mountains.
Earlier logging in the Vyazovka Ravine (above) had shown that the Upper Permian succession consists of repeated fining-upward cycles. Each cycle begins with a coarse cross-bedded sandstone, fines upwards to siltstones and mudstones and ends with a paleosol. The paleosols are sometimes associated with plant remains and are nearly always invested with carbonate. The broad interpretation is that these are the deposits of cyclical lakes, with occasional influx of sediment (the coarser sands), then finer lake deposits and finally a paleosol when the lake dried out - all perhaps the result of a broadly monsoonal climate.
The Russians had clear biostratigraphic evidence that these finer lake beds were latest Permian in age, belonging to the upper part of the Tatarian, the Vyatkian. They dated the overlying conglomerate as lowermost Triassic, Vokhmian, based partly on mapping evidence and partly on finds of ostracods and of the aquatic tetrapod Tupilakosaurus in associated channel lags. These age assignments are probably correct, but must still be assessed with respect to other units in Russia, more particularly by comparison with the international (marine) time-scale.
Andy Newell had interpreted the PT successions in Russia previously as evidence for a major change in fluvial style (Newell et al. 1999). Below the boundary, in the uppermost Permian, the clastic sediments indicated relatively low-energy styles of deposition and meandering streams. Above the boundary, the sediments pointed to much higher energy flow regimes, with deposition of conglomerates close to the Ural Mountains, and coarse sands at greater distances.
The sedimentary shift: massive run-offValentin Tverdokhlebov had studied these great outpourings of coarse sediment at the beginning of the Triassic and he attributed them to renewed uplift of the Ural Mountains. The Urals had been uplifted primarily in the late Carboniferous and early Permian as the separate Eurasian and Siberian continental plates came into contact. Plate movement more or less ceased, but it would be no surprise if the deep suture zone between the two former continents was still tectonically active.
Tverdokhlebov (1971) had shown, in his PhD work, that the coarse sediments were in the form of vast alluvial fans (see Figure below) that spewed westwards from the west side of the Ural Mountains, each fan spreading for a length of 100-150 km over the low-lying Permian lakes and meandering rivers on the great plain. He had identified all the boulders in the different basal Triassic alluvial fans and found that each fan had its own petrological signature, indicating subtly different sources of the rocks from deep within the Ural Mountains. The conglomerate boulders include blocks of Devonian or Carboniferous limestones, often with fossils, and metamorphic and igneous rocks.
Left: Palaeogeomorphological map of the western margin of the southern Urals, around the Orenburg area and the Ural and Sakmara rivers in Middle Blyumental (Middle Induan) time. Symbols: (1) sediments of lacustrine-alluvial plains; (2) sediments of alluvial plains; (3) sediments of alluvial plains, periodically exposed to denudation; (4) sediments of proluvial plains; (5) low folded hills; (6) medium folded hills; (7) high folded hills; (8) amphibians; (9) reptiles; (10) phyllopods; (11) alluvial fans (a, proximal part; b, distal part); (12) sediments of coarse anticlinal folds and related ridge sediments; (13) coarse synclinal structures and related mountain-edge depressions; (14) directions of flow (a, based on dominant slopes of cross-beds; b, based on orientations of oblique pebbles); (15) main orientation of clastic material; (16) secondary orientation of erosion; (17) conjectured orientations of river flow; (18) boundaries of the Pre-Ural Depression; (19) boundaries of palaeogeomorphological zones; (20) boundaries of areas of uplifted blocks. The five major alluvial fans are indicated: (A) Giryal; (B) Novokulchumov; (C) Dubovskii; (D) Novochebenkov; (E) Nakaz. (Based on information in Tverdokhlebov (1971).)
Independently, Roger Smith - a sedimentologist working in South Africa - and his collaborator Peter Ward from the University of Washington in Seattle, had reached a similar conclusion. The famous Permo-Triassic succession of the Karoo Basin showed a similar sedimentary switch from a low-energy flow regime with meandering streams in the Late Permian to a high-energy flow regime with braided streams and alluvial fans in the Early Triassic (Ward et al. 2000).
Since then, a similar shift in fluvial style has been noted across the PTB in Australia (Michaelsen 2002), India (Sarkar et al. 2003) and Spain (Arche & López-Gómez, 2005). Such a shift does not occur everywhere: in numerous PT sections in Antarctica, for example, there is some evidence of coarsening of the sandstones above the boundary in some sections, but braided streams set in during the latest Permian, and the main change is from sandstones dominated by volcanic clasts in the Permian to sandstones with quartz clasts in the earliest Triassic (Collinson et al. 2006).
Studies of soils and their chemical signatures (Retallack 2005; Sephton et al. 2005) confirm that there was a soil erosion crisis, where soil and organic matter from the land was washed into the sea. If this was a world-wide phenomenon, local-scale tectonism cannot be the cause - but what then?
Perhaps there were global-scale upheavals, with mountains being uplifted in several parts of the world. So far, independent evidence for such global activity has not been found. Perhaps there was a massive increase in rainfall world-wide? Again, there is no clear evidence for such a phenomenon, nor a suggestion of how it might have come about. If anything, the evidence suggests reduced rainfall.
Andy Newell (Newell et al 1999) argued that the abrupt increase in channel size associated with a major influx of gravel around the PTB could be related to climate change. There was a well-documented switch worldwide from a semi-arid/sub-humid climate in the latest Permian toward one of greater aridity in the earliest Triassic, and this can increase sediment yield by reducing vegetation cover. If vegetation is stripped from the surface of the land, rates of erosion can increase perhaps tenfold.
This fits with other evidence that the normal green plants had been temporarily killed off and replaced by an unusual horizon at the boundary, dominated by strands produced either by fungi or algae. Below this horizon, the sediment samples contain spores of ferns, seedferns, horsetails and other plants that grew at low, medium and tree-like levels. Such plants soon return in higher units in the Early Triassic. But the fungal/ algal boundary bed perhaps indicates a dramatic loss of normal vegetation. We know the devastating erosion that can follow the removal of plants today, such as in Bangladesh, where the rate of runoff and erosion has increased hugely after logging higher in the foothills of the Himalayas.