Gravel-bed river deposits contain a range of facies that includes planar cr
oss-bedded gravel (Gp), massive gravel (Gm) and trough cross-bedded gravel
(Gt). Of these, the Gt facies is commonly interpreted to represent deposits
of channel-fills and/or three-dimensional (3D) gravel dunes and/or scours
or pools. In this study a calcrete-conglomerate sheet built up of Gt facies
is found to consist of trough cross-beds of 3D dune origin (Gt(d)) and of
bedload sheet origin (Gt(b)). Whereas Gt(d) occurs as sets, Gt(b) is found
to occur as isolated large troughs. The Gt(d) facies is characterised by hi
gh dips, low width/height ratios and foreset bed thickness of about 100-140
mm. In contrast the Gt(b) facies has low dips, high width/height ratios an
d foreset bed thickness usually less than 50 mm. The bivariate plot of widt
h/height (of troughs) vs. maximum dip effectively discriminates between the
se varieties of troughs. A third variety of Gt may be recognised by the pre
sence of a scoop- or spoon-shaped lower bounding surface as seen in the dep
ositional dip direction. This facies has been demonstrated in other works t
o represent filling of scours or pools on the streambed and is designated a
s Gt(c). For the preservation of 3D gravel dunes in the stratigraphic recor
d, special flow conditions are necessitated. Of prime importance is the rap
id recession of flow subsequent to a short duration of peak discharge. Such
discharge patterns are critical to the preservation of gravel dunes in the
stratigraphic record and appear to have been met with in the case of glaci
al-outburst floods or floods in the seasonal tropics. (C) 1999 Elsevier Sci
ence B.V. All rights reserved.