FA 3: Overbank Deposits

Deposits of FA 3 are mainly sandy deposits in the immediate proximity to the stream channel, in which architectural elements of levee and crevasse channel are included. Though proximal crevasse splays also can be incorporated into FA 3, the crevasse splay element is included in FA 2, though occasionally interbedded with FA 3.

7.3.1 FA 3.1: Levee Deposits

Description

Levee deposits, or levees (Fig.7.7), are composed of decimetre thick, 0.1-1 meter, beds of tabular planar cross-stratified- and through cross-stratified sandstone (facies B and A), plane parallel stratified sandstone (facies E), ripple laminated sandstone (facies F), apparent massive sandstone (facies G), and sandstone with mudstone laminae, which are frequently bioturbated. Occasionally these facies are interbedded with mudstone (facies J) and siltstone (facies K). The levee elements appear rather planar in their geometry but are slightly wedge shaped and thin away from the channel they are associated with. Levee strata compose bed sets with a thickness of ~13 meters and are laterally extensive within distances of ~300 meters in outcrop. The lower boundary of levees is slightly erosive towards underlying floodplain fines (FA 2.1), architectural elements of FA 1, or elements of same facies association (FA 3.2). The upper boundary may be conformable when overlain by the depositional units of the facies associations just mentioned above. Small crevasse channels, with concave up erosion surfaces of ~3 meters width and < 1 meter thick, commonly cut

through the levee deposits thus appear embedded within. Laterally, levee deposits will be associated with abandoned channel fill (FA 1.1.) at one end and pinch out into floodplain fines (FA 2.1) at the other.

Fig. 7.7: Thick levee complex (FA 3.1) on top which extends hundreds of meters. The red line marks a concave-up erosional base which is interpreted to be a crevasse splay cutting through the levee complex.

Smaller levees deposits are also present in this figure and are marked with red arrows.

Interpretation

The depositional units interpreted as levees have characteristics of thin beds of siltstone to sandstone with some mudstone in between. These beds have near planar geometry and compose compound bedsets. The interpretation of levee deposits is also highly associated with the lateral relationship to an abandoned channel. Levee bedsets insinuate a highly aggradational succession with near planar beds with little erosion at base. The bedsets are locally cut by crevasse events which is thought to be the case in Fig. 7.7

Levee deposits can have a great variety of facies incorporated and differ greatly from site to site. Levees grow during submergence by major flood where suspended material will be deposited when the turbulent flow ceases. Minor floods may not cover the levees or only locally. Finer material will be deposited further away from the channel. Crevasse events will break through the levees giving rise to crevasse channels and splays (Collinson, 1996).

The preservation potential for levee deposits depends greatly on migration- and avulsion history of the river channels in which it is generated from, in addition to the valley

confinements. A wide meander belt can lead to well preserved levees. Low velocity currents in the stream channel will also be reflected in slow-moving, little erosive waters spreading out in the floodbasin and thus generate fine-grained levees and floodplain deposits

7.3.2 FA 3.2: Crevasse Channel Sandstone Deposits Description

Crevasse channel sandstone deposits, or crevasse channels (Fig. 7.6), contain plane parallel stratified sandstone with local mudstone or coal laminae (facies D), through or tabular cross-stratified sandstone with little or no mud drape in foresets (facies A and B), and ripple laminated sandstone (facies F). Larger coal fragments may be found at base. Crevasse channels are characterized by their small cross sectional size and shallow depth of up to ~50 and 5 meters, respectively, as well as their association with crevasse splays and levee deposits.

Crevasse channels have channel geometry defined by their concave erosive lower

boundaries. Crevasse channels differ from other channels with their winged sides composed by crevasse splays. They overlie floodplain fines (FA 2.1), crevasse splays (FA 2.3), or architectural elements of same facies association (FA 3). The upper boundary of FA 3.2 units is conformable to floodplain fines (FA 2.1), abandoned channel fill (FA 1), or elements of the same facies association (FA 3).

Interpretation

Crevasse channels are mostly recognized by their association with crevasse splays which gives the units “stearhead” geometry in cross section (Fig. 7.6), and their relatively small sizes compared to the stream river cross sections. When viewed from an oblique angle or along the axis of the crevasse channels, they are more difficult to identify. Sand-sand or sand-mud amalgamated surfaces may be present where a crevasse channel sustained for some time, thus recording multiple crevasse events. Thickness variations are not expected to be great over short distances (tens to few hundred meters) along the axis, but will thicken and thin towards channel and floodplain respectively on a larger scale.

Crevasse channels are ephemeral channels which break through the levees during flooding and branches out on the floodplain with an associated crevasse splay lobe. Crevasse channel deposits normally represent a waning flow, characterized by ripple lamination superimposed on cross-stratified sandstone. Slow avulsion processes of the stream channel through the crevasse channel may occur and thus give rise to continuous active growth of a crevasse

channel, forming a reversed facies development, from current ripple lamination at the base to cross-stratified sand dunes on top. Intervals of sand and mud laminae (facies D) or mudstone (facies J) interbedded with cleaner sandstone may indicate several minor crevasse events superimposed where sandstone-mudstone laminae represent the waning flow on top of each event with a continuous sedimentation. Where mud is deposited between high energy flow events, amalgamated surfaces may have been formed with sandstone to sandstone contacts with discontinuous mudstone laminae between.