>At 11:09 PM 11/17/97 -0800, Arthur V. Chadwick wrote:
>
>>You raise an interesting question... Why is the geologic record a
>>singularity?
>>Why are different periods successively different lithologies (and different
>>paleocurrent patterns?)
>
>Obviously no one can explain every single depositional system in the
>geologic column. However, I can offer some explanations for some of the
>features of the earth. I will explain this simply, not for Art's sake (he
>is a better geologist than I) but for those who are unfamiliar with geology.
>For instance, the early Paleozoic (Cambrian through Mississipian) have a
>lot of carbonates. In order for the creatures to live in the sea which
>deposit carbonate in their shells, there has to be a lack of clastics(sand
>and shale). And these carbonates have very little clastic input. Here is how
>I think the widespread lower paleozoic carbonates were deposited.
>
>A look at the paleogeographic maps in Dott and Batton Evolution of the
>Earth, 1972 show that much of the continent was under the sea at this time
>with only a few regions emergent. Without much land surface there can not
>be erosion and thus the deposition of sand and shale. This leaves the
>continent covered by clear water, the perfect environment for crinoids,
>brachs, etc. Their shells left piles of carbonate.
If there is not much erosion and deposition of sand and shale, how did the
vast sand and shale beds of the Lower Paleozic get deposited? I'm thinking
particularly of the Ordovician St. Peter Sandstone, which is a pure quartz
sand, very well sorted, that extends over much of the continental United
States. Dott & Batten mention it briefly in their 1976 edition, p. 221 &
224; other texts show pictures of the grains and especially look for a map
of its extent. It is only about 300 ft. thick in the eastern US, thinning
westward, yet is recognizable over all those thousands of square miles,
very well sorted and unfossiliferous. Not a calm seaside scene....
Karen