Pf. Friend et al., Intermediate-scale architectural features of the fluvial Chinji Formation (Miocene), Siwalik Group, northern Pakistan, J GEOL SOC, 158, 2001, pp. 163-177
The Lower Siwalik, Chinji Formation (Late Miocene) of the Chinji Village ar
ea, northern Pakistan has provided remarkable material for the study of ter
restrial fossil faunas, magnetic reversal stratigraphy and fluvial sediment
ology. This paper considers patterns in the sedimentary stratigraphy, using
magnetic reversals to constrain the time framework, and focusing on an int
ermediate (kilometre) horizontal length-scale. The project aimed to determi
ne the architecture and time relationships of the channel sandstone bodies
in a panel 300 m in stratigraphic thickness, and 11 km in horizontal length
(along stratigraphic strike). This panel (the 1990 Fence) trends at a high
angle to the flow direction of the Late Miocene river channels, and repres
ents about 2 Ma of sediment accumulation. There is a continuous range in th
ickness of the sandstone bodies, but they can be usefully classified into (
i) microbodies, (ii) minor sheets, (iii) thin mega-sheets and (iv) thick me
ga-sheets. The microbodies are probably mainly marginal features of the thi
cker bodies. The minor sheets were formed by small river channels, and the
mega-sheets were formed as the deposits of the largest, generally braided,
channel belts. Two aspects of the intermediate length-scale architecture of
the Chinji Formation are analysed: (1) the presence in the Fence of three
thick mega-sheets separated by two mudstone-dominated intervals that lasted
for about 0.5 and 1.0 Ma, respectively and (2) the abrupt upwards increase
in sandstone/mudstone proportions that defines the upper stratigraphic bou
ndary of the Chinji Formation. We suggest that each of the three thick mega
-sheet episodes resulted from avulsions into the area of large-channel belt
complexes that formed central features of the Chinji river network and wer
e each constrained by scarps or valley-side slopes during episodes of net d
eposition that lasted for about 100 ka, and may have resulted from climate
and/or sea-level changes. The regional upward change from mudstone- to sand
stone-domination at the top of the Chinji Formation resulted from a similar
, but one-off, and more widespread, change in plan-view style of the river
network, produced either by tectonic change in the mountain source area, or
by climatic change.