Clastic Dike in a Kame-Esker Complex


Clastic dikes in unconsolidated sands and gravel are enigmatic features because they are supposed to have been formed by the injection of material from either above or below. The formation of cracks in unconsolidated material such as coarse gravel is implausible.

The photos show a clastic dike, about four feet wide, in the side of a gravel pit. The dike is shown in more detail in the second photo. The dike contains fine cross-stratified sand with a network of diagonally cross-cutting fault planes. Conventional interpretation suggests the gravel was produced during the melting of an ice sheet in the glacial period. Gravel was deposited by meltwater streams, and a wide crack was formed, that was infilled by sandy sediments. An alternative explanation is provided below.  
 
The location is in what was once a large kame or esker complex located 6 km west of St. Jacobs, Ontario.
Overview of clastic dike in gravel pit
Close up view of clastic dike

In the in situ disintegration interpretation the term "clastic dike" is retained but the materials are considered to have non-clastic origins. The gravel is explained by a disintegration process. The original material was sedimentary rock, which disintegrated when overburden was removed by fast currents. The surface of disintegration penetrated down from the surface, forming the successive layers of sand and gravel. No "stoss and lee" effects in the vicinity of pebbles, or turbulence effects are expected  as no currents were involved. Joints were formed in the rock, that are represented by the walls of the dike. The sandy strata contained in the dike was formed by disintegration of the section between two parallel joints. This probably occurred before the surrounding rock was also disintegrated. Subsequently the disintegration of the enclosing rock produced coarser gravel layers. The  kame complex was formed by the thrusting up of the drift as a pressure ridge, due to expansion effects, as thick drift layers were thrust towards each other. Expansion due to disintegration can account for the evidence of faulting, folding and thrusting which is one of the characteristic internal features of kames and eskers.


Copyright © 2006 by Douglas E. Cox
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