Trough cross-bedded conglomerate facies

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.