Re: Widespread depositional systems (was re: inference)
David Campbell (bivalve@mailserv0.isis.unc.edu)
Tue, 18 Nov 1997 13:30:08 -0400There are certain temporal shifts in depositional patterns, in addition to
the widespread occurrence of certain kinds of deposition throughout the
fossil record. Dolomite is increasingly abundant in older deposits because
it generally forms from limestone over long periods of time. Bacterial and
algal mat deposits (stromatolites, etc.) are abundant in the Precambrian
and decrease dramatically after that, with occasional minor rebounds after
major extinctions wiped out most of the animals that would feed on them.
Deep-water carbonate deposits are common after the Triassic, when some of
the major groups of calcareous plankton evolved; diatomites do not appear
until the Cretaceous, when the first unquestioned diatoms appear. Glacial
deposits are present when the continents are positioned so as to allow
parts of the world to get very cold. Subduction has destroyed all ocean
floor older than the Jurassic except for pieces that got stuck onto a
continent, so abundant seafloor-associated rocks are limited in age.