Download STRATIGRAPHY AND ROCK FORMATION

STRATIGRAPHY AND ROCK FORMATION
From sediment to sedimentary rock
Sedimentary rocks form by consolidation of sediment,
which is either fragmented rock particles transported
and deposited by air, water or ice, or derived through
chemical or biological processes.
Sedimentary rocks are classified by the processes the
sediment is derived. The former type of sedimentary
rock is called terrigenous sediment; the latter types
are chemical and biogenic sediment.
The conversion of sediment into sedimentary rocks
invokes the following processes:
Compaction – increase in density and removal of
water from the sedimentary mass
Cementation – binding of loose particles by growth of
new minerals. The new minerals often precipitate from pore water in between particle grains.
Diagenesis – growth of new minerals, or secondary minerals, through various chemical replacement
processes.
source: goodrichscience.com
Messages in the sedimentary layers
The grain size, color and composition of the sedimentary rock are indicative of the processes by which
the sediment was formed as well as the physical and climatic conditions of the depositional
environment. Remember the rocks are tens of millions years old. The geography and landscape back
then were totally different from that of today. The sedimentary rock has nothing to do with the sea you
see by the rocks.
The field of study of layered rocks is called stratigraphy. In a layered sequence of rocks, the younger
layers are at the top and older at the bottom.
Rocks of Ping Chau
The sedimentary rocks are fine-grained, brownish, reddish to yellowish in color, and form continuous
layers with planar bedding surfaces. Rock types include mudstone, siltstone and dolomitic siltstone. The
sedimentary characteristics suggest a quiet depositional environment with restricted water circulation.
It is possible the sediments were deposited in a near-shore lake or lagoon.
The rocks were originally laid down in a horizontal manner. The rock layers at ping Chau are tilted in
earlier crustal movement.
Biogenic sediment
The rocks by the pier is a good example of biogenic sedimentary rock. The rock layers show scaly
lamination and contorted layers. These are formed by deposits of algae or marine micro-organisms.
(Note: the inclined layers are NOT cross bedding as shown in the display sign!)
Some of the biogenic sediment on ping Chau forms mushroom-shaped mounds which are believe to be
remnants of a sedimentary form called stromatolite.
Stromatolites are deposit from probably cyanobacteria in water.
Rhythmic bedding
The differently colored sedimentary layers form rhythms suggesting cyclical changes in the depositional
environment. These cycles are on the order of thousands of years.