Several stromatolite forms and textures are present
in the SRT outcrop. These include large, convex
mounds (Fig. 2), wavy beds (Fig. 3), and small,
concentric, round heads (Fig. 4).
Fig. 2. Convex mound
Fig. 3. Wavy bed stromatolites.
Fig. 4. Round head stromatolites.
[Return to top]
Fig. 5. Modern stromatolites at low tide, Sharks
Bay, Australia. Source.
Fig. 8. Stratigraphic column of eastern
Pennsyvania and central New Jersey.
The Allentown Formation is one part of a very thick
package of sedimentary rock that underlies eastern
Pennsylvania (Fig. 8). These rocks were originally
sediments deposited in a series of basin that formed
as the the Appalachian Mountains were being built
from ~500 to 250 Ma. When the Appalachians were
being rapidly uplifted by the collision of
landmasses and volcanic island arcs, siliciclastic
detritus was shed into the basins by rivers, forming
deltas and reworked into beaches and marine deposits
by currents and waves. When mountain building waned,
the basins were flooded by the ocean that
accumulated thick deposits of carbonte mud. Most
carbonate sediments were originally deposited as the
minerals calcite or aragonite [CaCO3] forming
limestone. Later, during burial and the movement of
fluids, the limestone was transformed into dolostone
[CaMg(CO3)2]. The age of the Allentown
Formation, like all of the rocks in this
stratigraphic column, is known from their fossils,
the study of biostratigraphy, anchored by
radiometric numeric ages on volcanic ash beds that
occur at several places in the column.
The shallowing up cycles common in the Allentown
Formation are indicative of the complex interplay
between carbonate production, subsidence of the
basin collecting the sediment, and eustasy, the
rising and falling sea level. An excellent
description of limestone forming environments can be
[Return to top]
How do we know?
Geologists understand the history of Planet Earth
through observation, comparison of past processes to
those operating in the present, and by the rigorous,
repeatable testing of hypotheses following the
scientific method. These principles of knowing and
understanding have been applied to the rocks exposed
in Pennsylvania and collectively they are the
foundation of the State's geologic evolution that
guides us in our use of its economic resources.
Frequently asked questions include:
How do we know that the Allentown Formation was
originally deposited in a shallow ocean?
There are many features in the SRT outcrop that
lead to the clear inference that the Allentown
Formation dolostone was originally deposited as a
limy mud in a warm, shallow sea. Among these are the
oolites, wave ripples (visible on the blocks used to
make the steps), and the stromatolites. All three of
these features are common to modern limy mud-bottom
shallow seas in warm, tropical areas. We know that
the water must have been shallow because the
stromatolites we observe at the SRT outcrop are of
the type that form in tidal regions, like Shark Bay,
Australia, where they are regularly exposed during
low tide. With the exception of some lakes,
virtually all of the modern limestone and dolostone
deposition occurs in the oceans.
How do we know the age of the rocks exposed at
the SRT outcrop?
The enormous depth of geologic time is a difficult
concept for us to comprehend, given that human
lifetimes are measured in tens of years, rather than
the hundreds of millions of years that represented
by the sedimentary rocks exposed in the Saucon and
Lehigh valleys. Considering the crystalline rocks of
South Mountain, the geologic history of eastern
Pennsylvania extends back past 1 billion years.
Geologists use two key tools to document deep
geologic time and develop a time-stratigraphic
framework that can be extended and used globally.
These two tools are (1) direct radiometric dating of
minerals using decay of radioactive elements and (2)
biostratigraphy, which is the study of the history
of life, including extinctions.
Radiometric dating of minerals is grounded in the
firm physical laws that govern radioactive decay of
long half-lived elements such as uranium, rubidium,
or potassium, to their corresponding daughter
products of lead, strontium, and argon. In closed
minerals where the amount of radioactive parent and
corresponding daughter element are conserved,
radioactive decay is accurate clock of the age of
mineral formation and incorporation in a geologic
horizon. For eastern Pennsylvania, there are several
geologic horizons formed of ancient volcanic ash
that contain the mineral zircon, formed as a product
of the volcanic eruption. Zircon is a good mineral
to date because it contains uranium that can take
advantage of the uranium-lead radiometric method.
These ashes lie in the Marcellus and Jacksonburg
formations in the stratigraphic column of Fig. 7
above. Because these sedimentary rocks are
stratigraphically above the Allentown Formation, we
know that they must be younger than the Allentown
Formation, so the Allentown Formation cannot be
younger than ~460 million years. Similarly, zircons
in the crystalline rocks of South Mountain that are
stratigraphically below the Allentown Formation have
been dated to ~1 billion years old. Because these
rocks are older than the Allentown Formation, we
know from direct radiometric dating in eastern
Pennsylvania that the Allentown Formation dolostone
must be between ~ 1 billion and 460 million years
[Return to top]