Mazon Creek - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 109

S TRATIGRAPHIC SETTING AND
TAPHONOMY OF THE M AZON
C REEK BIOTA
The Mazon Creek fossils occur in siderite
(FeCO 3 ) concretions in the Francis Creek
Shale Member of the Carbondale
Formation, of Westphalian D age. The
Francis Creek Shale overlies the
Colchester N˚ 2 Coal Member, and is itself
overlain by the Mecca Quarry Shale
Member ( 107, 108 ). The Colchester Coal
is generally about 1 m (3.3 ft) thick; the
Francis Creek Shale is typically a grey,
muddy siltstone with minor sandstones
and varies from complete absence up to
25 m (82 ft) or more in thickness. The
siderite concretions occur only in the
lower 3-5 m (10-16 ft) of the member,
and only where the shale is more than
15 m (50 ft) thick. The shale is coarser
and bears sandstones near its top. The
Mecca Quarry Shale is a typical
Pennsylvanian black shale that peels easily
into sheets, and contains a rich fauna of
sharks and their coprolites, exhaustively
monographed by Zangerl and Richardson
(1963). It is generally about 0.5 m (18 in)
thick but is absent over areas where the
Francis Creek Shale is more than about
10 m (33 ft) thick, and thus also over the
nodule-bearing parts of the Francis Creek
Shale.
The fossils are found almost only
within the siderite concretions. When
these are broken open they reveal a nearly
three-dimensionally preserved organism,
though the fossil becomes more flattened
towards the edge of the nodule. Fossils are
generally preserved as external molds,
commonly with a carbonaceous film (if a
plant). There may be crystals such as
pyrite, calcite, or sphalerite on the mold
surfaces. The commonest mineral on
these surfaces, however, is kaolinite, a
white, soapy, clay mineral. This is often
found completely infilling the space
between the molds (i.e. forms a cast). It is
soft and therefore easy to remove

mechanically. Fossils with few hard
parts or little rigidity, such as jellyfish,
may collapse completely and be preserved
as composite molds; even arthropods
may show dorsal and ventral structures
superimposed.