about the water street park boulders
About the Seafloor Image
1 = Stromatolites (formed by sediment trapped by layers of cyanobacteria)
2 & 3 = Ancient Sea Sponges
4 = Brachiopod
5 & 6 = Crawling Arthropods
7 & 8 = Swimming, Predatory Arthropods
9-17 = Trilobites
18-22 = Crustaceans (early ancestors of crabs and lobsters)
23 & 24 = Mollusks (early clams)
25 = Segmented Worms
26 = Echinoderm (early starfish)
27 = Chordate (earliest animals with a spinal column)
28-30 = Extinct animals with no modern affinities
Location
[Return to top]Stromatolites
Stromatolites are formed by communities of photosynthetic cyanobacteria that trap calcium carbonate (CaCO3) sediment in microbial mats. Over time, the layers of trapped calcium carbonate accumulates, is buried, and solidifies into rock called limestone, preserving the stromatolites as fossils. Stromatolites form in the photic zone of warm shallow marine continental shelves. They are important to the ecology of both early and modern life on Earth because cyanobacteria are responsible for emitting large amounts of oxygen into the atmosphere and stabilizing the carbon cycle. High emission of atmospheric oxygen in Earth's ancient oceans increased the metabolism of primitive organisms supplying the energy they needed to evolve, diversify, and create complex ecosystem relationships.Several stromatolite forms and textures are present in the boulders. These include large, convex mounds (Fig. 2), wavy beds (Fig. 3), and small, concentric, round heads (Fig. 4).
Fig. 2. Convex mound stromatolites.
Fig. 3. Wavy bed stromatolites.
Fig. 4. Round head stromatolites.
Fig. 5. Modern stromatolites at low tide, Sharks Bay, Australia. Source.
Wave Ripples
Ripples (Figs. 6 and 7) are decorations preserved on the tops of sedimentary beds that are formed by the water or wind currents that move sediment. Ripples contain information about the environment of sediment deposition and they are present on the faces of the boulders at Water Street Park. There are two main types of ripples: asymmetric current and symmetric wave ripples. Wave ripples are the kind preserved on The Water Street Park bounders. They formed in shallow water environments by the constant back and forth motion of waves. By measuring the ripple height (amplitude), spacing (wavelength), and grain size, the ancient depth of the water and height of the waves can be calculated. These calculations are explained below and indicate that the water could not have been deeper than ~0.75 m ( 2.5 ft) with waves that were about 0.1 m (4 inches) high. Consistent with other regional geologic evidence, the wave ripples confirm that this area of Pennsylvania were under the waters of a shallow sea during the Late Cambrian, 515-500 million years ago.
Fig. 6. Symmetric wave ripples as shown on a boulder at Water Street Park. Source.
Fig. 7. Close-up of some eroded wave ripples at Water Street Park. Source.
Mudcracks
Mudcracks (Figs. 8 and 9) form when sea level falls exposing saturated fine-grained sediment to evaporation. As the sediment dries it contracts to form distinct polygonal patterns. Mudcracks are preserved when a rising sea floods and quickly deposit new sediment on top of the dried mud. Mudcracks are typically found in tidal zones or wetlands and are great at preserving fossils.
Fig. 8. Mudcracks on boulder at Water Street Park. Source.
Fig. 9. Large mudcracks in lime mud at a modern shoreline exposed at low tide.
Stratigraphy
Fig. 10. Stratigraphic column of eastern Pennsylvania and central New Jersey.
The Allentown Formation is one part of a very thick package of sedimentary rock that underlies eastern Pennsylvania (Fig. 10). These rocks were originally sediments deposited in a series of basin that formed as 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 carbonate 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 found here
[Return to top]Geology
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How do we know?
Evidence of Water Depth
Geologists can calculate the depth of the environment in which the sediments of the Water Street Park boulders were deposited in by measuring the wave ripple height , ripple wave length, and sedimentary grain size of the ripple marks (Fig. 13). From these variables geologists can use simple mathematical equations to reconstruct the water depth.
Figure 13. This diagram shows what components of a wave ripple are measured to determine water depth.
Figure 14. Lehigh University Earth and Environmental Science student, Jocelin Gregorio, takes ripple height measurements of wave ripples on one of the Water Street Park boulders.
After taking field measurements of the wave ripples and obtaining samples of the carbonate sedimentary rock, the rock grains are then sifted through a series of sieves that sort the grains based on size. From there the average of the masses (weight) of all grain sizes are taken to produce an average grain size. The average grain size is used as a variable in the calculations to determine the range of water depths the wave ripple could form.
Data
of Wave Ripple Formation and Water Depth
Figure 15. This graph shows the ocean wave height and ocean water depth needed to create the wave ripples seen on the Water Street Park wave ripples. Evidence indicates that wave ripples formed at ocean depths less than 0.75 meters (~2.5 feet).
Frequently asked questions include:
How do we know that the Water Street Park Boulders
was originally deposited in a warm shallow ocean?
The features found in the Water Street Park Boulders provide evidence that the Allentown Formation dolostone was originally deposited as a limy mud in a warm, shallow sea. Among these features are the oolites (not visible on the boulders), wave ripples, mudcracks, and stromatolites. All four 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 Water Street Park boulders are of the type that form in tidal regions, like Shark Bay, Australia, where they are regularly exposed during low tide. Furthermore, as seen in Figure 14, measurements from the Water Street park wave ripples and the grain size of carbonate sediments provide evidence that the water depth was no greater than 0.75 meters during the Late Cambrian Period.
More Information
More information about Pennsylvania geology can be found at the Pennsylvania Geological Survey website: dcnr.state.pa.us/topogeo/
Also, check out the online web mapping app.
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