The Gunflint Chert - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 5

Lower and Upper Gunflint members, and
the Upper Limestone Member. The Lower
and Upper Gunflint members both exhibit
a similar sedimentary cyclicity going in
ascending order from algal chert, to shale,
to chert-taconite, and finally to chert-
carbonate. The Lower Gunflint Member
thus begins with the Lower Algal Chert
Facies, and it is this unit that has yielded
most of the microbiota, and in many
locations is characterized by stromatolitic
bioherms ( 5, 6, 7 ).
Taphonomically, the Gunflint cherts
and stromatolites differ from those nor-
mally encountered in the Precambrian.
Most cherts with microfossils occur as
lenses or as thin beds which formed within
carbonate sediments after deposition. The
Gunflint cherts, however, are thought by
many to have formed by silica precipitation
directly onto the sea floor. Likewise,
stromatolites are normally found in
carbonates, but at Gunflint they actually
occur in the cherts; Knoll (2003) suggested
that the Gunflint 'stromatolites' may
actually be sinters - laminated build-ups of
silica precipitated from SiO 2 springs as in
Yellowstone Park today and not necessarily
produced by microorganisms.

Large populations of microfossils lay
jumbled and disorientated on successive
laminae of the stromatolitic structures, but
rather than these being the organisms that
built the laminae, perhaps they merely
fell onto the accreting surfaces to be
entombed and preserved in silica, just as
in the Devonian Rhynie Chert of Scotland
in the UK (see Selden and Nudds, 2004).
Other scientists do not agree that the chert
is a primary silica chemical sediment
and have presented evidence that the
initial mineral was a carbonate, possibly
aragonite, which was later silicified
(Sommers et al ., 2000).

D ESCRIPTION OF THE G UNFLINT
C HERT BIOTA
The first eight genera described here
correspond to those originally described
by Barghoorn and Tyler (1965), while the
last one was described subsequently by
Knoll et al . (1978); these are the most
common and best known of the Gunflint
fossils. Several other taxa have been
described (see Awramik and Barghoorn,
1977), but most are either of doubtful
biological origin or are very rare and are
not included here.