Terrestrial Palaeoecology and Global Change

Chapter 2. Taphonomy
27
Triassic sequence at Nedubrovo, Vologda Region of European Russia, begins with grey
gravels and coarse sands deposited over the eroded surface of variegate Permian marls.
These basal deposits contain reworked rhizocretions as well as scattered bone fragments
of terrestrial tetrapods. They are overlain, sequentially, by purple clay, black silty
clay, greyish green siltstones, pink siltstones and brown clay, cut by conglomerates of the
next erosion cycle (Krassilov et al., 1999a, 2000). Plant debris is found at a few levels in
the green horizon. The black silty clay is mm-lamellate, smectitic, with pyrite nodules,
charcoal, wood fragments and abundant leaf cuticles (also occasional insect cuticles)
scattered over the lamellae or amassed as coaly lenticles. The palynological assemblage
of predominantly terrestrial origin contains few planktic unicells, as well as Tympanicysta,
the abundant filaments of cylindrical to barrel-shaped cells previously assigned to
fungi but reinterpreted (Krassilov et al., 1999b) as green algae related to Spirogyra
(Zygnematophyceae). This latter forms dense mats in fresh-water ponds, occurring also
in brackish waters and mineral springs. In Tympanicysta, a brackish environment might
have fostered formation of thick-walled akinetes that were preferentially preseved in
estuarine deposits.
While the redeposited rhizocretions of the basal gravels suggest a caliche over the
adjacent drylands, the abundant plant debris of the black clay horizon and above is evidence
of a dense plant cover (xeromorphic conifers and peltasperms) exporting an abundant
dead mass. The purple clay – black clay succession reflects a switch from lagoonal
(with inflowing surface waters – outflowing bottom waters) to estuarine (reverse) circulation
(green silt horizon) and back (pink horizon). Anoxy of the black clay facies might
have been related to the following factors (or their combination): (1) ponding of estuaries
by the rising sea; (2) influx of terrestrial organic material enhancing eutrophication; (3)
ashfall or redeposited ash from a nearby volcanic area (the Timano-Petchorian volcanic
province simultaneous with Siberian traps) as a source of smectite, Fe ++ enrichment
enchancing a release of phosphorus from sediment fertilizing surface waters.
The smectitic clay with Tympanicysta is a characteristic feature of the Permian/
Triassic transboundary sequences (Jin et al., 2000) indicating that, at the moment of
frontal river ponding and eutrophication at an early stage of the end-Permian transgression,
the organic-rich waters were splayed offshore generating an estuarine-type circulation
in the advancing epeiric seas (more on this in IX.2-3).
Variegate cyclothems commonly contain primary or redeposited fossil soil (palaeosol)
horizons. A considerable depth of calcification in the redbed palaeosols abounding in
stacked carbonate rhizocretions indicates a dry to seasonally dry/wet climate. However,
the excessive abundance of carbonate nodules, as in the Lower Eocene of Bighorn
Basin, Wyoming (VII.5), indicates reworked, rather than authentical, palaeosols. In such
cases, the associated plant assemblages are poor guides to redbed climates.
The in situ root beds of variegate cyclothems contain either horizontal or vertical
roots suggesting repetitious developments from waterlogged to well-drained soil and the
corresponding vegetational succesion (of Eospermatopteris wetlands to Archaeopter-