The Hell Creek Formation - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 161

boundary at the top of this packet of
sediments rather than at its base ( 166 ).
There followed years of dispute as to
whether Brown's Hell Creek Beds were
Cretaceous or Tertiary, and even though
it was clear that they contained dinosaurs,
some authorities argued for survival of
the dinosaurs into the Tertiary (see
Hartman, 2002 for discussion).
This dispute over the precise positioning
of the K/T boundary continued through
the pre-war years until Roland Brown, a
USGS paleontologist, concluded in 1938
that: “the Hell Creek Formation was
uppermost Cretaceous in age with a
distinctive fossil record including the
dinosaur Triceratops , ammonites, and . . .
plants”, that: “the Fort Union Formation
was a mappable unit with its own distinctive
flora and fauna that should be assigned to
the Paleocene” and, finally, that: “the thin
zone of interfingering beds at the upper
limit of the Hell Creek Formation and the
base of the . . . overlying Fort Union . . .
marks the boundary between the Mesozoic
and the Cenozoic”.

deposited in fluvial channel systems and
associated floodplains that developed on
the resulting coastal plain during the last
1.8-2.2 million years of the Cretaceous
Period (Murphy et al ., 2002). Rapid lateral
facies changes within the formation do not
allow its further subdivision, other than the
recognition of the Breien Member, which
represents a brief return of marine
conditions. The overlying Fort Union
Formation begins with the continental
Ludlow Member at its base, overlain by
the Cannonball Member representing a
return once more to fully marine
conditions ( 166 ).
Following Brown (1938) the K/T
boundary came to be recognized as
occurring below the lowest lignite coal
bed in the Ludlow Member, and above
the highest dinosaur beds of the Hell
Creek Formation. This rule of thumb
worked remarkably well until 1980,
when the meteorite impact theory of
Alvarez et al . (1980) allowed the exact
geochronological boundary (or 'time
line') to be defined with considerably
more precision. Various workers have
since been able to demonstrate that
the K/T boundary, as now precisely
defined by the iridium-rich level (and
by palynology and the presence of
impact debris), is not always exactly
coincident with the Hell Creek/Fort
Union formational boundary. The
lithostratigraphic formational boundary
is thus diachronous and in some areas
may be as much as 3 m (10 ft) below the
geochronological K/T boundary. In other
words, an interval of Fort Union strata of
Cretaceous age, and characterized by
typical Maastrichtian palynomorphs, is
often recorded below the K/T boundary
(Nicholls and Johnson, 2002).
This small stratigraphic interval
between the highest dinosaur beds of
Hell Creek and the iridium-rich level
has become controversially known as
'the 3 m gap'. It represents a regional
paleoenvironmental change that is
independent of the K/T extinction event
and has led some authorities to argue

S TRATIGRAPHIC SETTING AND
TAPHONOMY OF THE H ELL
C REEK F ORMATION
The Hell Creek Formation is thus a
lithostratigraphical unit, bounded below by
the Fox Hills Formation, and above by the
Fort Union Formation ( 166 ) and dates
from the late Cretaceous Maastrichtian
Stage (approximately 65 million years ago).
It outcrops in Montana, North Dakota, and
South Dakota (while the equivalent strata
in Wyoming are known as the Lance
Formation) and the total thickness has
been estimated to vary between about 40 m
(132 ft) up to 170 m (560 ft) (Johnson et
al ., 2002).
The underlying Fox Hills Formation
represents the near-shore and beach
deposits at the edge of the Western Interior
Seaway as it made its retreat from the
Western Interior Basin. The succeeding
Hell Creek Formation, consisting of poorly
cemented fine-grained sandstones (with
siltstones and carbonaceous shales), was