GEO
308 In keeping with the Princeton tradition of small classes lead by senior
professors, four students in Professor Gerta Keller’s GEO 308 class
this term had a unique opportunity to learn how the science of geology
is conducted. The four students traveled to Mexico during spring break
to participate in a field excursion lead by Prof. Keller and Prof. Stinnesbeck
(University of Karlsruhe, Germany) two of the world’s leading experts
and Prof. Lopez-Oliva (Princeton Ph.D. l996) from the University of
Nuevo Leon, Linares. Joining with students from Germany and Mexico the
Princeton undergraduates reviewed the geological evidence for one of
the most controversial theories in earth sciences (Figures 1A, 1B).
This is the theory that a giant extraterrestrial object smashed into
the Earth 65 million years ago and wiped out the dinosaurs. But did
it?
This theory has captivated the public’s imagination because of
the spectacular images it evokes. The impact bolid, thought to be either
a meteor or a comet, is posited to have been about 10 kilometers in
diameter. The tremendous force of such an impact is estimated to have
caused an earthquake with a Richter scale reading of 12 (roughly 10,000
times as powerful as the largest recorded earthquake), and a mega tsunami
five kilometers tall. Such an event would have thrown a tremendous amount
of debris into the atmosphere and the resulting reduction in sunlight
could have cut off photosynthesis long enough to have caused the mass
extinction of dinosaurs and up to 80% of all other life forms (Alvarez
et al., l980).
Paleontologists have long known that a mass extinction occurred at
the end of the Cretaceous about 65 million years ago and they refer
to this event as the “K/T Boundary” mass extinction, where
K refers to Kreide, the German term for Cretaceous and T refers to Tertiary.
The idea that species could become extinct due to extraterrestrial causes
was promoted by Cuvier as early as the end of the 18th century to explain
the disappearance of mammalian fossils in the Paris Basin and the first
quantitative assessment of the K/T boundary mass extinction was published
by Phillips in 1860. In 1980 Alvarez (* Princeton Ph.D) and coworkers
proposed their impact theory as the cause of the K/T boundary mass extinction.
They became interested in this topic after the discovery of a thin layer
of iridium at the K/T boundary at Gubbio, Italy. Since iridium is rarely
found in high concentrations on earth, but is common in extraterrestrial
rocks, its subsequent detection at locations worldwide made an extraterrestrial
source seem plausible. By the early 1990’s the site of a likely
K/T impact crater was discovered in subsurface sediments near the village
of Chicxulub on the Yucatan Peninsula of Mexico. Sixty five million
years ago Yucatan was a shallow tropical sea.
Armed with the location of the crater, investigators were able to
begin looking for geological evidence of the event. Since l990 glass
spherule layers have been found throughout northeastern and central
Mexico, Guatemala, Belize, Haiti, Cuba and the southern United States.
These glass spherules are explained as the result of the immense force
of the impact that vaporized the bolid and melted the sediments at the
impact site. Melted rock (termed ejecta) was then spread ballistically
throughout the region.
The most distinctive spherule deposits have been found in northeastern
Mexico where they are associated with thick beds of sandstone and shales
that are popularly interpreted as having formed as impact generated
megatsunami waves. These deposits are easily recognized and occur in
outcrops extending over 500 kilometers. At the time of the K/T boundary
these deposits formed on the ocean floor of the continental slope at
a depth of about 500 m and over 100 miles off the coast. Today this
area forms the eastern edge of the Sierra Madre Oriental.
These sandstone and shales, labeled siliciclastic deposits, generally
occur in three distinct layers called Units I, II, and III. Supporters
of the impact theory interpret these units as clear evidence of the
Chicxulub impact and the resulting megatsunami (Smit et al., l992, l996).
Other scientists are not convinced. For impact supporters the spherule
layer that marks the lowest Unit I is the result of airborne ejecta
settling through the water column within a matter of hours after the
impact. The overlying massive sandstone of Unit II is deposited by the
megatsunami, and the alternating sand, shale and rippled sandy limestone
layers of Unit III are deposited by the back and forth of the seiche
and the slow settling through the water column of the finer sediments.
All this is supposed to have happened within a matter of hours to days.
Supporting evidence for this scenario includes bi-directional current
ripples said to record the ebb and flow of the dissipating seiche waves
caused by the rebounding of the megatsunami (Smit et al., l996). An
iridium anomaly at the top of Unit III and thin clay layer is taken
to mark the final fallout of dust particles, and the K/T boundary.
This beautiful sexy scenario has one major problem – it is not
supported by current evidence based on field observations and laboratory
analyses by a team of workers (Stinnesbeck et al., l996, 2001; Keller
et al., l997, in press), including four MA thesis that together investigated
over 42 outcrops containing the siliciclastic deposits and spherule
layers (Schulte, l998; Lindenmayer, l998; Affolter, 2000; Schilli, 2000).
These studies show that the evidence uncovered can not be explained
by this impact-tsunami scenario and that many questions remain unresolved.
For example, are the rock layers of Units I, II and II of precisely
K/T boundary age? Is it possible that they were depositd by an impact
generated megatsunami within a matter of hours to days? Is there one
spherule-producing event? or multiple spherule-producing events?
In an attempt to resolve these questions we have conducted a review
of the literature, examined and sampled a series of outcrops in northeastern
Mexico during a one- week field trip, and examined the samples in the
laboratory. All of the outcrops we examined have been previously studied
with regards to the above questions. In particular, we studied outcrops
of the Mesa Juan Perez, El Peñon, and Marie de los Angeles areas
(Figure 2). At each location, all major outcrops were first examined
to obtain an overview of the lithological transitions, the presence
or absence of the three units, their lateral continuity, the exposure
of sediments below it, and structural disturbances, such as slumps,
faults and folds. The sections were measured, the lithologies examined
for bioturbation, erosive contacts, reworked clasts, and the information
sketched and noted in field books. Samples were taken for subsequent
laboratory analyses.
Review of Literature This theory has several fatal problems.
1) At the particularly well-studied outcrop of El Peñon there
is a very conspicuous 20 centimeters thick sandy limestone that separates
the spherule layer of Unit I into two layers (Fig. 3, Keller et al.,
l997). This sandy limestone layer represents normal sedimentation over
a long time period and therefore could not have accumulated during the
time it took for the glass spherules to settle through the 500 m water
column. An alternative explanation for this spherule Unit I is that
it represents two spherule depositional events separated by some interval
of time.
2) The presence of several discrete horizons of bioturbation within
Units II and III in several localities, including El Peñon, pose
more difficulties to the theory of instantaneous deposition. The term
bioturbation describes the feeding burrows left by marine organisms
living on the ocean floor. The presence of these burrows in several
layers is evidence that these organisms repeatedly colonized the seabed
and established living communities during the time Units II and III
were deposited. For the impact-generated tsunami theory to be valid,
no bioturbation (no living organisms) can be present in the nearly instantaneous
deposition. During a 1994 field trip to northeastern Mexico (lead by
Gerta Keller and Wolfgang Stinnesbeck) an international team of 55 geologists
agreed that the presence of burrows within these deposits would constitute
the “smoking gun’ that proved long-term deposition and invalidated
the short-term tsunami interpretation. This evidence has since been
documented in Units II and III in various outcrops in northeastern Mexico
(Keller et al., l997; Ekdale and Stinnesbeck, l998, Fig. 4A-F). For
example, near the base of Unit II at El Penon we observed a 10 cm-long
J-shaped burrow infilled with glass spherules (Fig. 4B, C). In Unit
III there are several discrete horizons with small burrows formed by
worms (Chondrites, Figs. 4A, D, E), and larger burrows formed by crabs
(Ophiomorpha, Fig. 4F). The presence of large burrows has sometimes
been explained as downward burrowing (up to 2 m) by organisms after
the tsunami event (Smit et al., l996). However, sediment infilling the
burrows is of the same age as the surrounding sediments, rather than
from overlying Tertiary strata and indicates that the bioturbation could
not have occurred after deposition of Units II and III, or after the
K/T boundary (Ekdale and Stinnesbeck, l998).
3) The presence of bi-directional ripple marks in Unit III as reported
by Smit et al. (l993, l996) has been offered as evidence of seiche waves
following the tsunami. However, a rigorous investigation of ripple marks
and measurements of current directions by Lindenmayer (2000) over a
large area reveals only one current direction parallel to the paleocoast,
and hence provides no support for an impact-generated tsunami event.
The sandy limestone within the spherule layer and the various horizons
of burrows within Units II and III indicate that the siliciclastic deposits
found throughout northeastern Mexico could not have been deposited within
hours or days. Even strong supporters of the impact theory have had
trouble accepting the impact-tsunami scenario. Bohor (l996) demonstrated
that a tsunami wave would have been an impossible mode of transport
for the sediments due to the shallow depth of the marine shelf. He also
rejected the bi-directional current claims and pointed out that the
sediment was transported from a shallower area to the west, and hence
from the opposite direction of Chicxulub and the movement of the tsunami.
Apart from the difficulty in justifying instantaneous impact-generated
tsunami deposition, there are several other problems with the original
impact theory.
a) The most serious problem is that in several localities the siliciclastic
deposits do not extend to the K/T boundary. At four localities (Lajilla,
Mulato, Parida and La Sierrita, Lopez-Oliva and Keller, l996), a thin
layer of marls from the late Maastrichtian Mendez Formation overlies
the siliciclastic deposits and the K/T boundary is above this marl layer.
The Maastrichtian is the last period of the Cretaceous, and in northeastern
Mexico, where the Mendez Marl Formation is the signature formation for
the late Maastrichtian, this rock unit represents normal deposition
in an open marine environment. The planktic foraminiferal assemblages
within these sediments indicate that deposition occurred during the
last 300,000 years of the Maastrichtian (Plummerita hantkeninoides zone,
Lopez-Oliva and Keller, l996). This indicates that Units I-III must
predate the K/T boundary.
b) The spherule layer of Unit I
Student
sleuths hone rock-solid reasoning skills, by Stephen Schultz, Princeton
Weekly Bulletin, September 22, 2003, Vol. 93, No. 3
Impact at 65 Million Years Ago
Undergraduates review geological evidence
Terrence A. McCloskey, Brian Wilson, Alexa Chew and Ana I. Garcia
Integral to the impact theory and its success in explaining the siliciclastic
deposits and tie these to the mass extinction is the idea that the deposition
of all three units (spherule, sandstone and alternating sand-silt-shale
layers) took place within a matter of hours or days. The spherule layer
is at the bottom of the deposit and must have been deposited first,
whereas the K/T boundary should be at the top and represent the fallout
of fine airborne material of the impact including iridium. If the mass
extinction was caused by the spherule-producing event, then the time
interval represented by this deposit must be extremely short –
hours, days or weeks.