LAB
8
Snyder
Hill Field Trip: fossils in the field
While in the field:
1. Snakes and scorpions are in the area. Kick a loose rock over before picking it up!
2. The quarry on the west side of Snyder Hill is still occasionally used for target practice. If you hear nearby gunshots, carefully make yourself known to the shooters.
3. When climbing up the slope, avoid kicking rocks downslope: there may be people below you.
4. When using your rock hammer, always wear eye protection and strike with the blunt end. The pointy end is for prying objects from cracks in the rock.
Snyder Hill is an outcrop of Permian limestones and dolomites consisting of the Concha Limestone (the lower formation) and the Rainvalley Formation (the upper, tan-colored formation). Snyder Hill is located in the southern part of the Tucson Mountains, on the north side of State Highway 86, between Kinney Road and Ryan Field.
The purpose of this trip to acquaint you with what fossils most often look like in the field. They don’t all occur in hand specimens with identification labels.
The trip will also illustrate how the type of fossils and mode of their preservation changes in a stratigraphic sequence and how those changes can be used to make interpretations about the original environment of deposition.
Rock types you will see here:
Limestone – most limestones today form in shallow water in subtropical to tropical latitudes. Most of the calcium carbonate that makes up limestones today is biogenic in origin; calcareous algae, foraminifera, shell debris, etc. Some of the limstone is very fine-grained, while other limestones may be coarse-grained.
Dolomite -- dolomites often form today in tropical, shallow water, tidal flat
environments. The environment of deposition is often characterized by high evaporation rates and high salinity.
Chert (sometimes bedded, mostly in nodules of varying size) The chert is largely “secondary” in origin. It probably formed after burial and during early cementation when silica-rich fluid crystallized around nuclei made of siliceous sponge spicules.
These rock types are easily distinguished by how they react to dilute acid –limestone (CaCO3) fizzes vigorously; dolomite (CaMgCO3) fizzes weakly or when powdered; chert (SiO2) doesn’t fizz.
The “bedding” characteristics of the rocks vary at Snyder Hill. Some are thickly or massively bedded (bed thicknesses equal to or greater than 1 m); some medium-bedded (decimenter scale) and others are thinly bedded (cm-scale or less).
Although you are likely to see a lot of fossils, they are not especially well-preserved and are usually quite difficult to remove from the outcrop. Most fossils will appear in cross-section rather than as whole specimens.
You can expect to see fossils of brachiopods, bryozoans, echinoids, crinoids, gastropods, bivalves, sponges, and corals.
Murphy’s Laws of fossil collecting:
1. The best fossils are on top of the hill (not true here)
2. When trying to break a fossil loose from the rock, the fossil will break before the rock does.
3. The fossils you collect at the end of the trip will look better than the ones you collect at the beginning.
4. Everyone else’s fossils will look better than your’s.
Walk up the outcrop, noting how the lithology, bedding, fossils, and quality of preservation change.
1. Now, describe the lower, middle and upper parts of the outcrop. How do the lithologies, bedding, fossils, and quality of preservation differ?
a. Lower:
b. Middle
c. Upper
2. Make a sketch of three fossils that you find, identifying each one to the extent possible
a.
b.
c.
2. Paleoenvironmental intepretation.
What environments of deposition are represented by the lower, middle and upper portions of the stratigraphic section? What is the evidence for you interpretation?