> Yes. These blocks grew before the depositional event (preflood).
Unfortunately many, indeed most, of the parental reefs don't rest on bedrock, but
occur high up in thick sedimentary and volcanic successions. The Cambrian reefs I
have studied are underlain by 100's of m of fossiliferous Cambrian sediments, and
many kilometres of Neoproterozoic sediments. The Devonian reefs of the Canning
Basin, although abutting against crystalline bedrock, are underlain by Early
Devonian. Lateral equivalents of the reefs in the basin are underlain by many
kilometres of Silurian and Ordovician sediments.
> But at Capitan reef although there are some sponges, little if any
> organically bound coral are found. And the line between "reef" and "talus"
> is abrupt, unlike in real growing reefs.
Unlike you I have not had the privilege of seeing these great rocks. However the
literature says that the Capitan reef contains abundant evidence of rapid and
extensive early marine cementation which would form a rigid framework and thus
supplant the need for organic binding. However, organic binding is not always
preserved in the rock record and is only one of the criteria by which reef facies
is recognised. Many reefs show little binding but extensive baffling of fine
grained sediments (baffling rocks - Ha!) by branching organisms (sponges, corals,
algae, bryozoans, etc.). Other factors are important. Evidence of organically
constructed topographic relief and a rigid wave resistant structure are the most
important. Evidence of rapid seafloor cementation, organic binding, baffling,
frame construction and marginal talus aprons are some of the features that point to
this. You should take up snorkelling or scuba diving! The line between reefs and
fore-reef/reef margin is generally abrupt in both the geological record and in the
modern environment. The reef-slope transition is very variable. Sometimes it is
vertical for 10's to 100's of metres, sometimes it is more gradual. But there is
generally a sharp boundary between the organically bound (and often cemented) reef
environment and the fore/back reef environment which always has talus derived from
the reef falling or sliding down the slope
>
>
> Thanks for your added info on reefs.
>
> Karen
They are fun systems to work with
God Bless
Jonathan