Page 22 - Sedimentology and Stratigraphy

02
02
Final
Final
9781405193795_4_0
Stratigraphy
and
9781405193795_4_0
Stratigraphy
9
9
26.2.2009
26.2.2009 8:14pm Compositor Name: ARaju
page
Proof
26.2.2009
page
Proof
and
Compositor
Compositor
8:14pm
ARaju
ARaju
Name:
Name:
Nichols/Sedimentology
8:14pm
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 9
Nichols/Sedimentology
Gravel and Conglomerate 9
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Fig. 2.5 Nomenclature used for mixtures
of gravel, sand and mud in sediments and
sedimentary rock. & ' &) $ & ' &&) & &
Rocks with equally spaced fracture planes in all direc- When discoid clasts are moved in a flow of water
tions form cubic or equant blocks that form spherical they are preferentially oriented and may stack up in a
clasts when the edges are rounded off (Fig. 2.8). Bed- form known as imbrication (Figs 2.9 & 2.10). These
rock lithologies that break up into slabs, such as a stacks are arranged in positions that offer the least
well-bedded limestone or sandstone, form clasts with resistance to flow, which is with the discoid clasts
one axis shorter than the other two (Krumbein & dipping upstream. In this orientation, the water can
Sloss 1951). This is termed an oblate or discoid flow most easily up the upstream side of the clast,
form. Rod-shaped or prolate clasts are less common, whereas when clasts are oriented dipping down
forming mainly from metamorphic rocks with a stream, flow at the edge of the clast causes it to be
strong linear fabric. reoriented. The direction of imbrication of discoid

Fig. 2.6 A clast-supported conglomerate: the pebbles are all Fig. 2.7 A matrix-supported conglomerate: each pebble is
in contact with each other. surrounded by matrix.