The Green River Formation - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 179

(200-260 ft) thick, and the overlying
Angelo Member, about 60 m (200 ft)
thick (Grande, 1984). It is the Fossil
Butte Member which has produced the
most numerous fossil fish from two units,
the organic-rich '18-inch' layer, and the
organic-poor 'split-fish' layer ( 190 ). The
18-inch layer, near the top of the Fossil
Butte Member, is a buff-coloured, varved
limestone containing about 4,000
couplets of light and dark laminae,
possibly representing annual deposition
over 4,000 years ( 190A ). By far the best-
preserved fossil fish are from this layer.
The slightly older, 2 m (6 ft) thick, split-
fish layer, on the other hand, is a poorly
laminated limestone with less well-
preserved fish ( 190B ). It may be that
these two layers represent midlake and
near-shore conditions respectively
(Grande and Buchheim, 1994).

Lake Gosiute had its greatest extent in
the middle Eocene ( 189 ) and the Laney
Member, deposited at this time, has
produced great numbers of fossil fish
from this lake from three distinct rock
types (Grande, 1984). The 'Farson' type
is an orange or red siltstone with fish
preserved only as external moulds or
impressions; the 'Fontenelle' rock type is
a light coloured dolomite with fine
laminations; the 'Fish-Cut' rock type is
similar to Fontenelle, but also contains
many dark layers of kerogen. Plant- and
insect-beds are also known.
Lake Uinta existed for the longest time
span, for more than 17 million years in
total. During its Paleocene history it is
often known as 'Lake Flagstaff' and the
deposits from this time comprise the
Flagstaff Member which Fouch (1976)
placed within the Green River Formation

190A 190B

190 Green River Formation lithologies, Kemmerer, Wyoming. A: the 18-inch layer, Craigs
Quarry, Fossil Butte National Monument; B: the split-fish layer, Warfield Springs Quarry.