Geology Reference
In-Depth Information
(a)
(b)
CO
2
H
2
O
Accretion
low
tide
(c)
(d)
Erosion
Suns
heat
(e)
(f)
Land
Land
Sea
Sea
Fig. 3.
Schematic drawings of beachrock formation that can lead to formation of a road-like feature. (a) Cross-section
through a beach shows formation of beachrock beneath intertidal beach sands. Waves and tides serve to pump huge
volumes of seawater through intertidal sands. Evaporation of seawater, which may aid in the cementation process, is
enhanced by tidal fl uctuation and waves. Circular inset shows microscope view of acicular aragonite growing on and fus-
ing beach-sand grains together to form rock. (b) Beachrock forms beneath a seaward-prograding beach during intermittent
beach accretion. Pulses of sedimentation cause progradation of beach sand beyond area of most active cementation. During
this process, the forming beachrock lies hidden beneath soft sand. (c) Transgression of the sea over the beach occurs when
supply of sand is halted or when sea level rises. Erosion of overlying sand exposes the previously formed beachrock while
the intertidal (and submarine) process of aragonite cementation continues. Good examples of this process can be seen along
a swimming beach at Bimini. Such quickly formed rock often contains broken bottles, pottery and other human artefacts.
(d) Exposure to the sun and to constant wetting and drying causes the rock to crack into separate slabs in much the same
way as a concrete road. The size of individual slabs is controlled by rock thickness. Uncemented sand underlying the rock
also promotes cracking as the rock settles in much the same manner as ice on a frozen pond. Constant abrasion by mov-
ing beach sand rounds off corners to form pillow shapes. (e) Viewed from above, the straight row of broken rock slabs can
resemble a road. (f) When sea level rises and erosion of underlying sand takes place, the stones are submerged to various
depths (see Figs 5 and 6). Multiple parallel 'roads' may be produced in this manner. The 'road' may curve when beachrock
forms on a hook-shape sand spit.
on a transgressive beach. In the Persian Gulf, both
in the intertidal and subtidal zone, the rapidly
forming rock is horizontal and thin. Although
this rock cracks and buckles extensively, it tends
to form polygonal shapes separated by expansion
ridges (Shinn, 1969; Evamy, 1973; Assereto &
Kendall, 1977).
The process of intertidal cementation on fi sh-
hook sedimentary spits is relevant to the Bimini
Road beachrock. Off Bimini, the area of curved
