Of course, the earth does not get covered in oysters every 10,000 years. Not all calcium carbonate production in the oceans comes from oysters for one thing, and the production of shells, corals and calcium carbonate sands is mainly restricted to narrow shorelines and ocean shelves with significantly more production in tropical seas than in arctic or cold seas. A large amount of calcium carbonate is preserved as reef build ups such as Australia's Great Barrier Reef or around carbonate rich shelves such as those surrounding the Bahamas.
If we imagine our 5cm thick layer of oysters suddenly concentrated in a few geographical regions, we can imagine that these smaller regions must be very productive. In a period of 10,000 years at least several metres of calcium carbonate must be produced. Exactly how much is created in any given area will depend on how rapidly the sea floor is subsiding relative to sea level. If it is subsiding slowly, the calcium carbonate production will be limited since the water depth will become too shallow. If it is subsiding too fast, the calcium carbonate production will not be able to 'keep up' and the habitat will become 'drowned'. Most organisms that produce calcium carbonate depend in some way on remaining in the photic zone where light is able to penetrate the water.
The production of calcium carbonate therefore occurs in areas where the balance of subsidence and accumulation is within certain given limits. If subsidence becomes very slow, the seafloor will become 'fossilised', a sort of ghost town littered by the remains of creatures that once flourished. Occasionally a storm will pass over their remains, hoist them into the water column and drop them down again in piles which might show some sort of sorting. Sponges will etch holes in everything. Worms will dig over and over through the same sediment. Unusual chemical reactions associated with the sediment-water interface that barely affect sediment that has been rapidly deposited and quickly buried, start to make their mark on the sediment. Shells become phosphatised, each crystal in their shell being replaced by a phospate crystal. Water in the sediment becomes calcium carbonate enriched and starts to precipitate crystal between the grains, cementing them together. Unusual minerals like green glauconite start to form and change the appearance of the rock.
What is being created here is a 'condensed bed' overlain by a hiatal surface (or a 'time gap' surface). The condensed bed will continue to form until another rise in sea level introduces a new breath of life into the area. The condensed bed is a physical record of stasis, of slowness, of corruption and of transformation. Condensed beds are both rotten and bejewelled. They contain the decayed remnants or what could not be preserved side-by-side with the bejewelled ornaments of what could. They bear witness to endless reworkings, struggling or failed efforts at making a living from an inch of dirt. Here lived the adaptors, the make-doers, the outcasts, the deliberators.
Whenever I look at sedimentary rocks such as limestone or sandstone I am reminded how much like a book the earth is. The vertical accumulation of beds are pages that are 'written on' by physical processes, by organisms and by chemical processes. Sometimes these beds are even divided into laminae so fine that they might be the pages of a book. An example of these thin laminae are the slates used to roof many houses. It is not often that you see the surfaces of these beds, but when you do, the meanings of these 'writings' become very clear. They say: 'a storm passed here rolling these clods of clay to produce tracks in the sand' or 'a dinosaur ran across this mud flat' or 'an ammonite died and dropped to the sea floor where it turned to stone'.
If the layers of rock that develop on the sea floor were the pages of a book, then the layers of rock laid down in a rapidly subsiding basin area would be the pages of a novel, a great epic story where each bed, each 'page' was an event, a new twist in a never-ending story. The layers of the condensed bed that I have just described, on the other hand, would be poetry.
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