This Earthcache will take you along a section of the Saucon Rail Trail to an informational sign describing the Allentown Formation Dolostone.
Please use the parking coordinates provided.
About the Outcrop
The Allentown Formation is a dolostone formation [CaMg(CO3)2] originally deposited as limy mud and sand in warm, shallow seas that once covered all of the eastern United States during the Late Cambrian Period (515-500 Ma). Underlying much of the southern Lehigh Valley and Saucon Valley, the Allentown Formation outcrops along the Saucon Rail Trail (SRT) near the border of Hellertown and Lower Saucon Township. The dolostone beds in the SRT outcrop dip gently to the south and exhibit characteristics consistent with numerous shallowing upward cycles, beginning as thick-bedded mudstone originally deposited in deep water, locally overlain by thin-bedded oolite-bearing calcarenite originally deposited in wave- and tide-dominated shallow water, overlain by intertidal, stromatolite-bearing mudstone. Among the most ancient organisms preserved in the fossil record, stromatolites are simple ecosystems of cyano-bacteria bio-film mats that trap carbonate mud and grow upwards in the photic zone. The stromatolites are numerous and range in size from < 1 cm to 50 cm in diameter. They are found today only in sheltered, saline environments but were much more common in the Cambrian oceans.
Stromatolites
Several stromatolite forms and textures are present in the SRT outcrop. These include 1. large, convex mounds, 2. wavy beds, and 3. small, concentric, round heads.
1. Convex
2. Wavy
3. Round
Stratigraphy
The Allentown Formation is one part of a very thick package of sedimentary rock that underlies eastern Pennsylvania. 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.
Geology
The Allentown Formation was deposited in a passive margin basin that existed on the east coast of North America before the rise of the Appalachian Mountains. At this time, North America was near the equator, and actually facing south. Note that most of Pennsylvania was under a shallow ocean at this time.
The subsequent rise of the Appalachain Mountains changed the geography of eastern North America and deformed the sedimentary rocks that had been deposited there, including the Allentown Formation. The results of that mountain building and deformation is evident in the geologic map of Pennsylvania, the Saucon Creek watershed portion of which is presented on the sign at GZ.
Limestone and dolostone are important economic resources in the state of Pennsylvania. They are used for building stone, as a flux in steel making, as an essential ingredient in Portland Cement, and as aggregate in road and building construction.
The above information is taken from: (visit link)
Logging Requirements:
Please email the answers to the following questions. Once you've sent the answers, you may log the find. I may delete logs if answers are not received within a few days of logging.
- What geological age does this formation date from?
- Looking at the map showing the Paleogeographic Reconstruction of North America on the informational sign, what was the topography of most of Pennsylvania at this time?
- Look at the rock formation. Based on the information provided in the cache description and the informational sign at this location, what types of Stromatolites can you see?
- Which formation occurred between 250 and 286 Ma?
- Where do most modern limestone and dolostone deposits occur? What does this tell us about the Allentown Formation dolostone?
This cache was placed with permission from Lower Saucon Township. Geocaching Permit #17.