The Gunflint Chert - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 4

from the middle Devonian of Scotland, UK
(see Selden and Nudds, 2004). This was
effectively the first documentation of well-
preserved fossils deep in the Precambrian,
but the paper was greeted with skepticism
from both geologists and botanists alike. In
the middle of the twentieth century,
paleontologists were firmly rooted in the
opinion that the Precambrian was barren of
such fossils, and the Gunflint forms were
dismissed as being geofacts.
A decade later, after Tyler's death,
Barghoorn published a more compre-
hensive paper on the Gunflint biota in
which he eventually applied Linnaean
nomenclature to eight of the most
common forms, such as Gunflintia,
Kakabekia , and Eoastrion (Barghoorn and
Tyler, 1965). This description of actual
genera caught the imagination of both the
scientific and popular press and was the
catalyst for a blossoming of research
programs through the 1960s.

to the Gunflint Iron Formation by Leith
et al . (1935). The name comes from the
former use of the Gunflint cherts as flints
for early muzzle-loaders, their high silica
content making them ideal for producing
sparks.
The Formation has recently been dated
at 1,878 million years and thus falls in the
middle of the Paleoproterozoic Era
(which together with the Mesoproterozoic
and Neoproterozoic eras make up the
Proterozoic Eon which extends from the
base of the Cambrian, at 542 million years,
back to 2,500 million years; Table 1).
It outcrops almost continuously from
Thunder Bay on Lake Superior to
Gunflint Lake on the Minnesota-Ontario
border and continues into Minnesota, a
total distance of approximately 180 km
(110 miles), and averages 122 m (400 ft)
in thickness ( 1 ).
Together with the overlying and
conformable Rove Shale Formation, the
Gunflint Iron Formation constitutes the
Animikie Group. These rocks are relatively
undeformed and exhibit low-grade
metamorphism, ranging from diagenetic to
subgreenschist. Goodwin (1956) divided
the Gunflint Iron Formation into four
members ( 4 ), the Basal Conglomerate
Member (or Kakabeka Conglomerate), the

S TRATIGRAPHIC SETTING AND
TAPHONOMY OF THE G UNFLINT
C HERT
The Gunflint Formation was originally
named by Van Hise and Clements (1901)
for iron-rich sediments in the Thunder
Bay region of Ontario, and later changed

4

Member Facies Thickness (feet)

Upper Limestone 5-20

taconite
tuffaceous shale
algal chert and jasper

150-180
5-16 Total 200-280
48-86

Upper Gunflint

taconite
tuffaceous shale
algal chert

150-210
4-22 Total 210-240
2-15

Lower Gunflint

Basal Conglomerate 1-5

4 Diagram to show the stratigraphy of the Gunflint Iron Formation after Goodwin, 1956.