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Hell Creek Formation and Cretaceous–Paleogene extinction

Ben Creisler

A new online paper:

David E. Fastovsky & Antoine Bercovici (2015)
The Hell Creek Formation and its contribution to the
Cretaceous–Paleogene extinction: A short primer.
Cretaceous Research (advance online publication)


The Hell Creek Formation is the global standard for understanding K–Pg

We present a summary of the current paleontological and sedimentological data.

Survivorship patterns concordant with catastrophic inception of
ubiquitous flooding.

Parsimony and data still concordant with impact as single agent for
mass extinction.

Volcanic hypothesis lacks global signature.


Although it represents but one geographic data point, the uppermost
Maastrichtian Hell Creek Formation (HCF), exposed in the upper Great
Plains of the North American craton, remains the most studied source
for understanding the final ∼1.5 Myr of the Mesozoic Era in the
terrestrial realm. Because it lies conformably below the earliest
Paleocene Fort Union Formation, and together these two units preserve
a rich fauna and flora, much of what is understood about the
terrestrial Cretaceous–Paleogene (K–Pg) boundary comes from this

The HCF has been reconstructed as an expansive, fluvially drained, low
coastal plain, built out, to the west, against the Laramide Orogen,
and to the east, against the ultimate transgression (Cannonball) of
the Western Interior Sea. Its meandering rivers and moist soils
supported a multi-tiered angiosperm-dominated flora and rich insect
and vertebrate faunas, including dinosaurs, crocodilians, squamates,
turtles, and mammals. A dramatic facies change representing the
initiation of catastrophic flooding is preserved, within available
levels precision, at the K–Pg boundary.

High-precision stratigraphy has proven difficult in this lenticular
fluvial system. Where present, the boundary can be recognized by the
bipartite boundary claystone; otherwise, palynostratigraphy has proven
a powerful tool. Numerical dates have been successfully obtained from
in tonsteins at the boundary and above, in the Fort Union; however,
these have proven elusive below the boundary within the HCF. The K–Pg
boundary in this region is dated at 66.043 Ma (Renne et al., 2013).
Magnetostratigraphic studies have been carried out in the HCF;
although all but one have lacked numerical dates, these have been used
for correlations of widespread, disjunct exposures and for the
estimation of sedimentation rates.

The palynoflora is largely homogenous through the HCF; at the K–Pg
boundary, it shows an abrupt ∼30% extinction. This makes it a powerful
tool for identification of the K–Pg boundary, although because the
boundary is identified on absence of Cretaceous taxa rather than
presence of earliest Paleocene taxa, several competing methods have
been applied to identifying the K–Pg boundary using pollen.

The macroflora, consisting largely of leaves, consists of three
successive floras, showing increasing diversity through the HCF. The
ultimate of these three floras undergoes an abrupt 57% extinction;
taken as a whole, however, the macroflora undergoes a 78% extinction
at the K–Pg boundary.

The best data available for dinosaurs – including archaic Aves – show
an abrupt extinction. By contrast, salamanders and other
lissamphibians, as well as chelonians, cross the boundary virtually
without perturbation. Squamates appear to have suffered significant
extinctions at the K–Pg boundary, as did euselachians (elasmobranchs)
and insects. Mammals suffered a 75% extinction; however, some of this
figure cannot be shown to have occurred in less than the last 500 kyr
of the Cretaceous, and thus has been potentially attributable to
causes other than a bolide impact. Taken together, the survivorship
patterns are concordant with the catastrophic inception of ubiquitous
flooding characterizing the K–Pg boundary.

While the key K–Pg boundary question in the HCF was once the rate of
the biotic extinction, it has moved to the distinction between
single-cause scenarios, with the Chicxulub bolide as agent of
extinction, and multi-cause scenarios, uniting habitat partitioning,
Deccan flood-basalt volcanism, climate change, competition, and bolide
impact. Not every potential environmental perturbation need be a
mechanism for the extinction: parsimony and the data continue to be
concordant with a bolide impact as the single agent of the terrestrial
K–Pg mass extinction.