Grebenka Section

Yelisseev Section

The section at the Yelisseev locality is approximately 100 meters thick. Each bed is typically traceable laterally for a few meters, at most 50 metres. The following lithofacies observations and architectural elements are the work of Anders Ahlberg (Lund University , Sweden) as reported in Spicer et al. (2002).

The lithofacies are coded in accordance with Miall (1978, 1996), and the alluvial architectural elements of Miall (1996) are used for facies subdivision, although there are inevitable limitations imposed by restricted lateral and three dimensional exposure. General sandstone classification is according to Pettijohn et al., (1982), and the nomenclatural schemes of Fischer and Schmincke (1984) and McPhie et al. (1993) are used for pyroclastic sediment classification. The sorting scheme of Folk (1974) is also utilized.

Conglomeratic Stream Deposits

Description, lithofacies

These beds are comprised of tabular, at places slightly trough-shaped, crudely stratified multistorey bodies of clast-supported, matrix-rich conglomerates (lithofacies Gh). Horizontal poorly developed bedding dominate. Maximum clast diameters vary, up to 25 cm. Clast imbrication can be observed, but only in very few beds. Any imbrication is concealed by the pronounced sphaericity of most clasts; an equant shape is typical of volcanic epiclastic debris (Fischer and Schmincke 1984). In rare cases intercalations of very angular pebbles occur. The conglomerates are typically massively bedded, normal graded, and in places amalgamated with low-angle scours into the substrate. Horizontally stratified lenses of coarse-grained sandstone (lithofacies Sm-Sh) occur infrequently within the conglomerates. The same sandstones also form the matrix of the conglomerates. The lenses are 1 - 3 m in width and less than 20 cm in thickness. Lateral exposure did not allow measurements of the widths of the conglomerate units, but they appear to be in order of 100 m in width.

Interpretation, architectural element

The channel conglomerates are referred to architectural element CH, but it was not possible to break down these conglomerates into downstream or lateral accretion units (i.e., elements DA, LA). The conglomerates were deposited on flat pavements as channel lag deposits of laterally mobile braided channels. Occasional normal grading may be attributed to waning flow due to gradual channel abandonment. Channel bar deposits are probably represented within these conglomerate sheets, but no apparent leeside foresets are apparent in the section to support this assumption. The sand lenses probably filled less active channels of the braidplain.

Although sheetformed conglomerates may be produced by repeated catastrophic floods, the Grebenka conglomerate beds show a lack of matrix supported beds, and no observed linear correlation is apparent between maximum clast size and bed thickness; features which have been held typical of such ephemeral events (Nemec and Steel, 1984).

Pebbly Stream Channel Sandstones

Description, lithofacies

Coarse- to very coarse-grained sandstone in the section make up only very few tabular and trough cross-bedded cosets (lithofacies Sp and Sr). These occur in isolated sandy channels and within channel conglomerates. Moderately sorted, faintly laminated, coarse-grained sandstones with pebble lags along scour-and-fill surfaces occur at several levels (lithofacies Ss), and commonly contain lag surfaces with logs and leaf mats. These sandstones repeatedly grade into floodplain fines and paleosols (see below). At least one sandy channel was rather narrow, approximately 50 - 100 m in width. Channel-fills (Ss units) typically exhibit thicknesses of 1 - 2 m, at most 5 m.

Interpretation, architectural element

Tabular and trough cross-bedding (lithofacies Sp, St) is interpreted as indicative of downstream foreset migration of 2D and 3D-dunes within channels and crevasse channels (architectural element SB), although bank migration cannot be excluded. Limited paleocurrent data indicate mainly southward river flow.

Scour-and-fill-sandstones (lithofacies Ss) represent narrow, <5 m deep, channels which typically were abandoned and subsequently filled with floodplain fines and vegetation (grading into lithofacies Fl, P; see below). Channel size and the intimate interfingering with floodplain fines suggest that these sandstones may represent crevasse channels (architectural element CR).

Overbank Hyperconcentrated Flow Deposits

Description, lithofacies

These are made up of only two one-meter-thick, sheetformed, massively to crudely bedded, sandstone beds with normal grading. They show flat basal scouring surfaces with pebble lags, followed by massively bedded normal graded coarse-graned sandstones with faint scouring surfaces (lithofacies Sm), trough cross-bedded fine to medium sandstones (St) and final gradation into floodplain fines (lithofacies Fl).  The coarse-grained sandstones are very rich in subangular to angular broken crystals and splinters.
         
Interpretation, architectural element

These beds are interpreted as aqueous hyperconcentrated crevasse splay sheets (CS), being the result of stream channel choking and aggradation by syn-eruptive volcanic ash influx.

Levee and Crevasse Sheet Sandstone

Lithofacies, description

At places within overbank fines, thin, horizontally stratified (Sh) to trough cross-bedded (St) normal graded beds of pebbly sandstone occur, occasionally with tree trunks and roots in growth position. These deposits are, in their proximal parts, commonly stacked and slightly amalgamated into the substrate. Occasionally they fine and thin away distally, interfingering conformably with floodplain fines.

Interpretation, architectural elements

Proximal overbank floods appear to have passed between mature trees growing on the river bank or levee (architectural element LV). These trees probably slowed down the overbank flow profoundly, and induced sand deposition. They also hindered erosion of the levee. Overbank flow which continued to lower, more distal, parts of the floodplain lost even more energy and consequently laid down crevasse splay sand sheets (architectural element CS) in otherwise undisturbed overbank areas.

Floodplain Pond and Microdelta Deposits

Description, lithofacies

Within overbank deposits, 10 - 20 cm thick, well-sorted siltstones  (lithofacies Fl) contrast against the predominating coarse-grained tuffaceous sandstones. The siltstone beds are typically horizontally laminated, and within them leaf mats are well preserved. In some places, branches and in situ tree trunks occur together with the leaves. Occasionally the laminated siltstones (Fl) coarsen upwards into trough cross-bedded sandstone (lithofacies St), followed by incipient rooting (Fr) and paleosol development (P).

Interpretation, architectural elements

The siltstones are referred to shallow, poorly oxygenated ponds of standing water beyond the levees (architectural element FF), which trapped aeolian (?) silt and preserved fallen and wind-transported leaves. The coarsening up trend of the cross-bedded sandstone is regarded as representative of lacustrine microdelta progradation, i.e., pond infilling, which introduced stream transported leaves and provided shallow water and sub-aerial surfaces for pioneer plants and the onset of pedogenic processes.

Floodplain Fines and Paleosols

Description and interpretation

The lower and upper parts of the Yelisseev succession contain mudstone beds, horizontally laminated (Fl) or massively bedded (Fm). These are clearly spatially related to in situ plant fossils (lithofacies Fr; rhizomes, rootlets, tree trunks in growth position). The floral remains within these deposits indicate pioneer to mature forest communities. Typically, however, the pedogenic impact on the substrate is slight, without well-developed horizonation and destratification, i.e., very weak to weak stages of paleosol development sensu Retallack (1988).