FA 4: Inclined Heterolithic Strata (IHS)

Inclined heterolithic stratification (IHS) (Thomas et al. 1987) occurs on millimetre to meter scale, but differ from Visser’s (1980) bundles. The IHS facies association represents the more heterolithic units in the study area. The IHS units include e.g. lateral accretionary- (FA 1.2), levee- (FA 3.1) and channel infill elements (FA 1). IHS has, however, been put into its own facies association because it is thought to represent a somewhat different depositional environment than the previous mentioned facies associations.

The elements of FA 4 can be separated into general characteristics of IHS elements with sporadically rhythmic sandstone-mudstone laminae and elements with centimetre to decimetre thick inclined heterolithic strata.

7.4.1 FA 4.1: IHS Elements with Sporadically Rhythmic Sandstone-mudstone Laminae

Description

Sandstone-mudstone lamination (facies D) is present within 2-3 meters thick and ca 50 meters wide sandstone beds of low dip to slightly concave-up geometry beds with undulating base. Sandstone-mudstone laminae are also present intermittently where otherwise

centimetres to decimetre scale heterolithic strata prevail. A rhythmic trend is commonly present in the laminae.

The rhythmically stratified sandstone-mudstone laminae are arranged in up to 10 cm thick

(Fig.7.8). On a larger scale of some decimetres, <0,5 meters, there are variation in the mud/sand ratio in the beds. These variations generate intervals of grouped sandstone-mudstone laminae interbedded with 5-15 cm thick sandstone of most commonly massive (facies G) or plane parallel stratified (facies D) with no or little mud.

Interpretation

The sandstone-mudstone variation creating the laminae is thought to have resulted from fluctuations in the energy levels, though continuous deposition, where the sand is deposited by traction currents or suspension, whereas the mud settled from suspension. The rhythmic variation is probably due to tidal cyclicity where the grouped sandstone-mudstone laminae are produced by the daily tidal fluctuations and the alternation with sand is due to the monthly variation in the tides of neap- and spring flood (Rebata et al. 2006). The higher order of variability in the sand/mud ratio may represent seasonal variations of e.g. wet and dry periods.

Fig.7.8: Sandstone-mudstone lamina (facies D) of with cm thick sporadically rhythmic variations which can be interpreted to represent neap- and spring tides where higher amount of sand would represent spring floods.

These deposits are found in elements of concave-up channel geometry with locally thin dune structures at base and a lateral association with levee deposits. The deposits may thus represent abandoned channel fill or very low energy current channel fill.

Where FA 4.1 is present within the sandier beds of FA 4.2, it may signify periods of lower energy currents and fluctuations with continuous deposition. Intertidal flats with sandstone-mudstone laminae have been recorded within mixed flats, commonly the middle region of the intertidal flats (Boggs, 2001).

7.4.2 FA 4.2: Elements with Centimetre to Decimetre thick Inclined Heterolithic Strata

Description

Centimetres to few decimetres (up to 0,3 m) thick heterolithic strata can be composed of a great variety of facies where the sandstone mainly has structure of parallel stratification (facies E or facies D), tangential or through cross-stratification (facies A, B or C), apparently massive (facies G) and occasionally ripple lamination (facies F) and soft sediment

deformation (facies M). Interbedded with sandstone are mudstone (facies J), siltstone (facies K) and intrabasinal conglomerate (facies I) (Fig. 7.9).

Fig. 7.9: Element on top with centimetres to decimetres thick Inclined Heterolithic Stratification (FA 4.2).

(Photo: M. Nyrud).

Elements of FA 4.2 are generally found at the uppermost stratigraphic level in the study area.

These elements show a high degree of amalgamation and aggradation laterally and vertically, which occasionally makes it difficult to separate the geometry of one element. However;

several channel downcuts and infill and sigmoidal accretionary surfaces are distinguishable.

Moreover, a subhorisontal element of thin alternating sandstone, siltstone and mudstone beds are included in this facies association.

Interpretation

The elements of this facies association are interpreted to be e.g. point bars/lateral

accretionary elements, abandoned channel fill and levees, associated with channel infill and overbank deposits. These elements and facies associations are already defined above in their own facies associations, but due to their strong heterolithic character they are distinguished from previously described facies associations and included in this new facies association;

IHS.

7.4.3 Discussion on Formation of IHS Elements

Inclined heterolithic stratification (IHS) is traditionally interpreted to represent depositional elements related to tidal channels or strong tidally influenced deposits within fluvial-deltaic systems, e.g. point bars of distributary rivers and tidal bars (Rebata et al. 2006; Thomas et al.

1987). Tidal channels are marine sourced and will spread out on the intertidal flats of the delta- or coastal plain with increased branching and reduced power inland. Tidal channel related deposits indicate a position within the intertidal flat, i.e. very close to the coastline.

The tidal channels can spread onto the intertidal flat directly from the coastline or diverge from the fluvial distributary channel onto the flat. Evaporites are expected to form on an intertidal flat in an arid climate. The nature of the intertidal flat also varies with amount of sand present. The amount of sand depends firstly on the presence of a marine source of sand, though channels which diverge from a distributary channel may be sourced partially

fluvially. Further the amount of sand on the intertidal flat tends to increase from the

supratidal area towards the lower intertidal region. Intertidal flats have thin planar beds often with sedimentary structures like ripple laminated sand, tidal rhythmites, bidirectional current indicators, mud drape and wavy-, lenticular- and flaser bedding. Additionally shell fragments and intraclasts may be present in higher energy areas, e.g. channels (Boggs, 2001).

However; the IHS deposits do not necessarily origin from tidal channels, but can also form in fluvial distributary channels which record high tidal influence. The tides reach further inland in fluvial channels since they have lower gradients than the surrounding areas. The IHS units may thus be deposited above the intertidal flat.

FA 4 may therefore represent either tidal channels or intertidal flat, or highly tidally

influenced fluvial distributary channels and floodplain. The thin subhorisontal beds and the bed containing abundant sandstone-mudstone laminae may be interpreted as intertidal flat sediments; though have not been due to the concave-up geometry of the latter and the lack of other intertidal indicators like shell fragments, abundant peat/coal and evaporites.