Re: Widespread depositional systems (was re: inference)

Steven Schimmrich (
Tue, 18 Nov 1997 07:38:11 -0500

Glenn Morton wrote:

>>>Each age had a dominant to predominant type of deposition. There is the
>>>widespread quartzites of the lowest Cambrian, the mottled, algal dolomites
>>>of the lower Ordovician (Beekmantown, Red River etc), the red-beds and
>>>of the Silurian, the black shales of the Devonian, the Crinoidal
>>>of the Mississipian, the coals and cyclicity of the Pennsylvanian, the
>>>redbeds and salt of the Permian and Triassic, the oolites of the Jurassic,
>>>the chalk and greensands of the Cretaceous, and the diatomites of the
>>>Tertiary. Today we have widespread clastic deposition. This is merely a
>>>continuation of what we have seen in previous geologic periods, only a
>>>different lithology

Art Chadwick replied:

>>You raise an interesting question... Why is the geologic record a
>>Why are different periods successively different lithologies (and different
>>paleocurrent patterns?)

David B. Fenske asked:

>Why are there layers in the geological column that correspond to different
>ages and that look the same? Let's say we have geological process #1
>forming part of the layer 2.3 billion years ago in Canada (oh, upper left
>BC, let's say). Over in Africa, or Asia, or even the Southern States,
>geological processes are happening. Different landscapes, different
>conditions, presumably different rocks(?). Why is there any similarity in
>the part of the column that is deposited? Why is there a certain kind of
>rock or layered pattern that forms in such different regions? Why are
>there boundaries? If because of changing conditions, why would the
>boundary be the same somewhere else where conditions haven't changed? Or
>is that layer thicker because conditions continued longer?

Rock strata from different ages look the same because the processes that
produced them are the same. The mechanical and chemical weathering of
rocks, the transport and sorting of sediments, the burial and lithification
of sediments are all processes controlled by the laws of chemistry and
physics and don't change throughout geologic time (a fundamental assumption
of all sciences). A quartz sand beach deposit is the same in the
Precambrian as it is today (except, of course, for evidence of life found
in most Phanerozoic rocks).

You see different types of rocks in the geologic record for a single
locality because conditions change. The Earth is not a static place,
especially on the geologic time scale (why else would you find coral reefs
in the middle of Texas?).

- Steve.

      Steven H. Schimmrich             KB9LCG
      Department of Physical Sciences               Kutztown University
      217 Grim Science Building, Kutztown, PA 19530      (610) 683-4437     Fides quaerens intellectum